Cancer Chemotherapy & Biotherapy: Principles & Practices, 4th Edition

Antimicrotubule Agents

Eric K. Rowinsky

Microtubules are highly strategic subcellular targets of anticancer therapies, and antimicrotubule agents are mainstay constituents of both curative and palliative therapeutic regimens. Over the last several decades, an increasing number of structurally complex, naturally occurring alkaloids or synthetic compounds that disrupt microtubules have also been identified.1, 2, 3, 4, 5 The natural product antimicrotubule agents have wide chemical diversity, and it is notable from an evolutionary standpoint that the microtubule seems to be a preferred, self-protective target of many marine and plant species alike. These organisms produce highly potent, structurally diverse compounds that are capable of binding to nearly identical sites on microtubules and induce almost identical actions. Despite their early promise and diversity, only two antimicrotubule agents, vincristine (VCR) and vinblastine (VBL), were widely used until the late 1980s. However, the identification of other classes of antimicrotubule agents with novel mechanisms of cytotoxic action and spectra of antitumor activity, such as the taxanes, epothilones, semisynthetic vinca analogs, and estramustine phosphate, has resulted in a resurgence of interest in the microtubule as an important target in cancer chemotherapy.

MICROTUBULE STRUCTURE

Microtubules are composed of molecules of tubulin, each of which is a heterodimer consisting of two tightly linked, closely related globular polypeptide subunits.1, 2, 3, 4, 5 These protein subunits, α-tubulin and β-tubulin, each consist of approximately 450 amino acids with a molecular weight of 50,000 daltons and are encoded by small families of related genes.1, 2, 4, 6, 7 When tubulin molecules assemble into microtubules, they form linear “protofilaments” with the dimers aligned side by side around a hollow central core, and the β subunit of one dimer in contact with the α-tubulin subunit of the next, as shown in Figure 11.1. The protofilaments are aligned in parallel with the same “polarity,” that is, one end, termed the plus end, at which assembly is rapid and one end, termed the minus end, at which growth is slow or net disassembly occurs. A third, less abundant, member of the tubulin superfamily, γ-tubulin, comprises the microtubule-organizing center (MTOC) or centrosome.8

The functional diversity of microtubules is achieved through the binding of various regulatory proteins such as microtubule-associated proteins (MAPs), expression of various tubulin isotopes, and post-translational modifications of tubulin. There are at least six isotypes of α-tubulin and seven isotypes of β-tubulin, which are distinguished by slightly different amino acid sequences, encoded by different genes, and expressed at varying degrees in different cells and tissues.9, 10 Nevertheless, they are similar proteins from a functional standpoint and copolymerize in vitro.9, 10 The intrinsic dynamicity of microtubules is also influenced by the isotypic composition of tubulin, and the sensitivity of microtubules to both microtubule depolymerizing and polymerizing agents relates, in part, to the composition of both tubulin isotypes, post-translation modifications on tubulin, and MAPs.10, 11, 12 Both α-tubulin and β-tubulin can be modified post-translationally by acetylation, detyrosination/tyrosination, and removal of the penultimate glutamic-acid residue of α-tubulin, which occur only on microtubule polymers, modify the behavior and stability of microtubules, and account, in part, for the distinct functional differences of microtubules in various tissues.9, 13 The C-terminal amino acid sequence of β-tubulin is the most variable in terms of both amino acid composition and post-translational modifications, which may also partially account for tissue-dependent functional diversity. Modified regions of polymerized tubulin provide sites for the binding of MAPs, which regulate the dynamic behavior and stability of cytoplasmic microtubules.12 The major classes of MAPs, which can be isolated with microtubules from tubulin-rich brain tissue, include tau (molecular weights, 40,000 to 60,000 daltons) and high-molecular weight (200,000 to 300,000 daltons) proteins whose members include MAP1, MAP1c (an ATPase), MAP2, MAP4, and the motor proteins dynein (a GTPase) and kinesin (an ATPase). Both classes of MAPs have two binding domains, one of which binds to microtubules. Because this domain also binds to free tubulin molecules simultaneously, MAPs facilitate the initial nucleation step of tubulin polymerization. The other domain appears to be involved in linking the microtubule to other cellular components. Some MAPs, such as the dyneins and kinesins, function as microtubule motors, transmitting chemical energy to mechanical sliding force and moving various solutes and subcellular organelles along the microtubule.1, 7, 10, 11, 12, 14 Motor protein function is critical to many types of dynamic cellular processes such as mitosis, premeiosis, and organelle transport. In addition to the MAPs, other regulatory proteins, such as survivin, stathmin, TOG, MCAK, MAP4, EB1, dynactin I, RAC1, and RHIT regulate microtubule function.7, 12

Figure 11.1 Heterdimers of α-tubulin and β-tubulin assemble to form a short microtubule nucleus. Nucleation is followed by elongation of the microtubule at both ends to form a cylinder that is composed of tubulin heterodimers arranged head-to-tail in 13 protofilaments. Each microtubule has an end where net addition of heterodimers takes place, called the plus (+) end, with β-tubulin facing the solvent, and a minus end (–) with α-tubulin facing the solvent.

MICROTUBULE FUNCTION

Although microtubules are primarily recognized as being principal components of the mitotic spindle apparatus that separates the duplicate set of chromosomes, they also play critical roles in many interphase functions such as maintenance of cell shape and scaffolding, intracellular transport, secretion, neurotransmission, and possibly the relay of signals between cell surface receptors and the nucleus.15, 16, 17, 18 Furthermore, the integrity of microtubule structures is required for cells to pass through various cell cycle checkpoints, and the lack of integrity appears to trigger programmed cell death or apoptosis.19

The unique functions of microtubules are related to their polymerization dynamics, which involve an equilibrium between α-β-tubulin heterodimer subunits and the microtubule polymer.1, 2, 7, 15, 16 Tubulin polymerization occurs by a nucleation-elongation mechanism, in which the slow formation of a short microtubule “nucleus” is followed by rapid elongation of the microtubule at its ends by the reversible, noncovalent addition of α-β-tubulin heterodimers (Fig. 11.1). In essence, the microtubule polymer is in a complex and dynamic equilibrium with the intracellular pool of α-β-tubulin heterodimers, which incorporates free heterodimers into the polymerized structure and simultaneously releases heterodimers into the soluble tubulin pool. Microtubule assembly and disassembly occur simultaneously at both ends of the microtubule and these processes are in dynamic equilibrium, the direction of which is determined by several factors, including the concentration of free tubulin and various chemical mediators that promote assembly (e.g., Mg2+, guanosine triphosphate [GTP]) and inhibition of assembly (Ca2+).12, 13, 20, 21, 22, 23 The assembly process uses energy provided by the hydrolysis of GTP. Tubulin binds GTP with high affinity, and as tubulin-GTP is added to the ends of growing microtubules, GTP is gradually hydrolyzed to guanosine diphosphate (GDP) and Pi. The Pi ultimately dissociates from the microtubule, leaving a microtubule core that consists of tubulin bound to GDP. The GDP nucleotide remains nonexchangeable until the tubulin subunit dissociates from the microtubule. Although tubulin polymerization and dissociation, and consequently microtubule elongation and shortening, occur simultaneously at each end, the net changes in length at the more kinetically dynamic plus end are much larger over time than those at the minus end. If the polymerization reaction is followed in vitro, an initial lag phase is noted, after which microtubules form rapidly until a plateau phase is reached. In the intact cell, microtubules usually grow from a specific nucleating site or the MTOC, which, in most cases, is the centrosome.

Dynamic instability gives rise to a dynamically changing cytoplasm. At any instant, cytoplasmic microtubules are either rapidly growing or catastrophically dissociating. Microtubules undergo long periods of slow lengthening, short periods of rapid shortening, and periods of attenuated dynamics. During rapid polymerization, the high concentration of free tubulin results in net assembly until a plateau phase is reached, at which time a critical concentration of tubulin is attained and the rates of both polymerization and depolymerization are balanced. Two fundamental processes govern microtubule dynamics in vivo. The first, known as treadmilling, is the net growth at one end of the microtubule and the net shortening at the opposite end.21, 24 Treadmilling plays a role in many microtubule functions, most notably the polar movement of the chromosomes during the anaphase stage of mitosis. The second dynamic behavior, termed dynamic instability, is a process in which the plus ends of individual microtubules switch spontaneously between states of slow sustained growth and rapid shortening.25, 26 The transition between microtubule growth and shortening is regulated, in part, by the presence or absence of the region of GTP-containing-tubulin at the microtubule end. A microtubule can grow as long as it maintains a stabilizing “cap” of tubulin-GTP or tubulin-GDP-Pi. Once the GTP cap is lost from the plus end, the end loses subunits more rapidly.27 Depolymerizatin occurs approximately 100-fold faster at a GDP cap than a GTP cap, and therefore, once rapid depolymerization occurs, the GTP cap is difficult to regain. On the other hand, the minus end is bound tightly to the MTOC, which interferes with both assembly and disassembly of the subunits. In essence, the capping process represents an adaptation that results in microtubule stability at the capped end. The rate of dynamic instability is accelerated during some processes, such as mitosis, which results in the formation and attachment of the mitotic spindles to the chromosomes. The rate and magnitude of both dynamic instability and treadmilling are much slower in purified tubulin than in cells, and it is clear that these mechanisms can be altered by MAPs and other regulatory proteins, variable expression of tubulin isotypes, post-translational tubulin modifications, and the expression of tubulin mutations.2, 28

In the nonmitotic phases of the cell cycle, microtubules radiate from the MTOC, which is located centrally near the nucleus and consists of a centrosome, a lattice of MAPs, γ-tubulin, and a pair of centrioles. The minus ends of the microtubules are positioned in or near the centrosome, whereas the plus ends extend out toward the cell periphery. The centrosome duplicates before mitosis and the two centrosomes then separate into the poles of the forming mitotic spindle. The microtubules of the interphase array depolymerize and, as the nuclear envelope breaks down and releases the now condensed chromosomes, a spindle-shaped array of newly assembled microtubules is organized. In essence, the interphase microtubule network disassembles at the onset of mitosis and is replaced by a new population of spindle microtubules that are much more dynamic than the microtubules that comprise the interphase cytoskeleton.28, 29, 30 In most cells, mitosis progresses rapidly and the highly dynamic microtubules that comprise the mitotic spindle render them sensitive to the vinca alkaloids, taxanes, and other antimicrotubule agents.2, 29, 30, 31

Dynamic instability and treadmilling are vital to the assembly and function of the mitotic spindle, and the high dynamaticity of mitotic spindle microtubules is required for the precise alignment of the chromosomes and their attachment to the spindle during metaphase, as well as chromosome separation during anaphase. These processes enable microtubules, which emanate from each of the two spindle poles to make vast growing and shortening excursions, essentially probing the cytoplasm, until they become attached to a chromosome at the kinetochore. Attachment of the plus end of the microtubules to the kinetochore of the chromosomes selectively “caps” or stabilizes this end of the mitotic spindle microtubules that emanate from the centrosomes. If even a single chromosome is unable to achieve a bipolar attachment to the spindle, perhaps from drug-induced suppression of microtubule dynamics, the cell will not traverse beyond aprometaphase/ metaphase-like state, which eventually triggers apoptosis. Although mitotic spindles form in the presence of low concentrations of antimicrotubule agents, mitosis cannot progress beyond the mitotic cell cycle checkpoint at the metaphase/anaphase transition or is delayed in this stage.27, 31 Such perturbations in mitotic spindle dynamics may delay cell cycle progression at critical mitotic checkpoints, ultimately triggering apoptosis.19, 28, 29, 30, 31 In the unperturbed normal state, oscillations of the duplicated chromosomes, dynamic instability, and microtubule treadmilling, in which there is addition of tubulin to the spindle at the kinetochore and loss of tubulin at the spindle poles, exert considerable tension on the chromosomes in metaphase.22 Both tension and oscillations are required for the proper function of the mitotic spindle and progression from metaphase to anaphase. In the next mitotic stage, anaphase, microtubules that are attached to the chromosomes undergo shortening, while another subpopulation of microtubules called interpolar microtubules lengthen, resulting in polar movement of the chromosomes. Suppression of spindle-microtubule treadmilling and dynamic instability by antimicrotubule agents reduce spindle tension and impedes progression from metaphase to anaphase, triggering cell death.19, 28, 29, 30, 32

VINCA ALKALOIDS

The vinca alkaloids are naturally occurring or semisynthetic nitrogenous bases that are present in minute quantities in the pink periwinkle plant Catharanthus roseus G. Don (formerly Vinca rosea Linn). The early medicinal uses of C. roseus for controlling hemorrhage, scurvy, toothaches, and diabetes and for the healing of chronic wounds led to the screening of these compounds for their hypoglycemic activity, which turned out to be of little importance compared with their anticancer properties.33, 34 Although many vinca alkaloids have been investigated clinically, only VCR, VBL, and vinorelbine (VRL) are approved for use in the United States. A third widely studied vinca alkaloid, vindesine (VDS, desacetyl VBL carboxyamide), a semisynthetic derivative and human metabolite of VBL, was introduced in the 1970s. It has been used in combination with other agents, particularly the platinating agents and/or mitomycin C (or both), to treat non–small cell lung cancer, but it is also active in several hematologic and solid malignancies.17, 33, 34, 35, 36 Although VDS demonstrated notable activity against several tumor types, particularly non–small cell lung cancer, it has been available only for investigational purposes in the United States and has not demonstrated a unique role in cancer therapeutics. The semisynthetic VBL derivative VRL (5′-norhydro-VBL), which is structurally modified on its catharanthine nucleus, is approved in the United States as either a single agent or in combination with cisplatin to treat non–small cell lung cancer and has been also registered for advanced breast cancer in many other countries. 17, 33, 35, 36, 37, 38, 39, 40, 41 In addition to demonstrating broad antitumor activity as a single agent and the possibility that it is not completely cross-resistant with VCR and VBL, VRL can be administered orally, in contrast to other available vinca alkaloids. The key features of these vinca alkaloids are given in Tables 11.1 and 11.2. Although the clinical development of other vinca alkaloids, such as vinleurosine and vinrosidine, have been abandoned because of unpredictable toxicity, a novel bifluorinated vinca analog, vinflunine, appears to have unique antitumor and toxicologic profiles and has demonstrated impressive activity in bladder and other cancer.17, 33, 35, 36, 37, 38, 39, 40, 41, 42

TABLE 11.1 KEY FEATURES OF THE VINCA ALKALOIDS

 

Vincristine Sulfate

Vinblastine Sulfate

Vindesine Sulfate

Vinorelbine Tartrate

Mechanism of action

Low concentrations inhibit microtubule dynamics (dynamic instability and treadmilling) High concentrations inhibit polymerization of tubulin

Standard dosage (mg/m2)

1–1.4 every 3 weeks

6–8 every week

3–4 every 1–2 weeks

15–30 every 1–2 weeks

Pharmacokinetics and disposition

See Table 11.2

Principal toxicity

Peripheral neuropathy

Neutropenia

Neutropenia

Neutropenia

Other common toxicities

Constipation, SIADH

Thrombocytopenia
Alopecia
Peripheral neuropathy (mild)
SIADH

Peripheral neuropathy (moderate)
Alopecia

Peripheral neuropathy (moderate)
Constipation
Nausea and vomiting
Diarrhea

Precautions

Patients with abnormal liver function should be treated with caution. See section on dosage and schedule for specific dosing guidelines.

SIADH, syndrome of inappropriate antidiuretic hormone secretion.

Despite the minor structural differences between VCR and VBL, their antitumor and toxicologic profiles differ vastly. VCR is used more commonly in pediatric oncology than in adults with cancer, most likely owing to the higher level of sensitivity of pediatric malignancies and better tolerance of therapeutic VCR doses in children. VCR is an essential part of the combination chemotherapeutic regimens used for acute lymphocytic leukemia and plays an important role in the treatment of both Hodgkin's and non-Hodgkin's lymphomas. VCR-based combination regimens, particularly those in which the agent is administered as a protracted infusion or as daily bolus injections in combination with doxorubucin and dexamethasone (known as VAD) and occasionally other agents, are commonly used to treat multiple myeloma.43 VCR also plays a role in the treatment of Wilms' tumor, Ewing's sarcoma, neuroblastoma, oligodendroglioma, medulloblastoma, and rhabdomyosarcoma in children, and in treating small cell lung cancer in adults.17 VBL has been an integral component of therapeutic regimens for germ cell malignancies and advanced lymphoma and has been used in combination with other agents to treat Kaposi's sarcoma and bladder, brain, and non–small cell lung and breast cancers.17, 33 In addition to the clinically relevant antitumor activity of VRL in non–small cell and breast cancers, VRL has demonstrated activity in advanced ovarian carcinoma and lymphoma, but a unique role in the treatment of these cancers has not been defined. It has also been reported that VRL as single-agent treatment confers high therapeutic indices to older patients with advanced breast and lung cancers.38, 40

TABLE 11.2 PHARMACOKINETIC PARAMETERS OF THE VINCA ALKALOIDS124,125,126,132,134,154

 

Vincristine Sulfate

Vinblastine Sulfate

Vindesine Sulfate

Vinorelbine Tartrate

Standard adult dosage range (mg/m2/week)

1.0–1.4

6–8

3–4

15–30

Optimal pharmacokinetic model

Triexponential

Triexponential

Triexponential

Triexponential

Elimination half-lives

   α(min)

<5

<5

<5

<5

   β(min)

50–155

53–99

55–99

49–168

   γ(h)

23–85

20–64

20–24

18–49

Clearance (L/hr/kg)

0.16

0.74

0.25

0.4–1.29

Primary mechanism of disposition

Hepatic metabolism and biliary excretion

Hepatic metabolism and biliary excretion

Hepatic metabolism and biliary excretion

Hepatic metabolism and biliary excretion

Structures

The vinca alkaloids have a large dimeric asymmetric structure composed of a dihydroindole nucleus (vindoline), which is the major alkaloid in the periwinkle, linked by a carbon-carbon bond to an indole nucleus (catharanthine), which is found in much lower quantities in the plant (Fig. 11.2). VCR and VBL are structurally identical except for the substituent (R1) attached to the nitrogen of the vindoline nucleus, where VCR possesses a formyl group and VBL has a methyl group. These small structural differences impart major clinical differences. VBL and VDS differ in two substituents (R2 and R3) attached to the vindoline nucleus, whereas the catharanthine ring of VRL is modified.

Mechanism of Action

The vinca alkaloids induce cytotoxicity by interacting with tubulin. 6, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54 However, they are also capable of many other biochemical and biologic actions that may or may not be related to their effects on microtubules, including competition for transport of amino acids into cells;inhibition of purine, RNA, DNA, and protein syntheses;disruption of lipid metabolism;elevation of oxidized glutathione;inhibition of glycolysis;alterations in the release of antidiuretic hormone;inhibition of release of histamine by mast cells and enhanced release of epinephrine;inhibition of calcium-calmodulin–regulated cyclic adenosine monophosphate phosphodiesterase;and disruption in the integrity of the cell membrane and membrane function. 15, 16, 17, 33, 38

Despite their diverse biologic properties, the cytotoxic activity of the vinca alkaloids is primarily the result of their ability to disrupt microtubules, particularly microtubules comprising the mitotic spindle apparatus. In support of this mechanism of action, there is a strong relationship between cytotoxicity and the dissolution of the mitotic spindle and accumulation of mitotic figures.47 Furthermore, the accumulation of mitotic figures correlates with both drug concentration and duration of treatment. Although the vinca alkaloids are generally classified as “antimitotics,” this mechanism may not be the sole mechanism of cytotoxicity in vivo because they also disrupt interphase microtubules involved in chemotaxis, migration, intracellular transport, movement of organelles, secretory processes, membrane trafficking, and transmission of growth factor signals from the cell surface receptor to the nucleus.15, 17 In addition, the vinca alkaloids disrupt the structural integrity of platelets and other cells, which are rich in tubulin and depend on microtubules for structure.48Therefore, the fact that the vinca alkaloids induce morphologic changes and cytotoxicity in normal and malignant cells in both mitosis and interphase is not surprising.15, 49, 50, 51

Figure 11.2 Structural modifications of the vindoline nucleus and catharanthine nucleus in various vinca alkaloids. (Reprinted with permission from Rahmani R, Zhou XJ. Pharmacokinetics and metabolism of vinca alkaloids. In: Workman P, Graham MA, eds. Cancer Surveys, Pharmacokinetics and Cancer Chemotherapy, vol 17. Plainview, NY: Cold Spring Harbor Laboratory Press, 1993:269.)

The vinca alkaloids bind rapidly, avidly, and reversibly to sites on tubulin (known as the vinca domain), which appear to be the same binding sites for other agents such as the complex plant alkaloid maytansine.6, 33, 44, 45, 53, 54 However, the binding sites are distinct from those of the taxanes, GTP and GDP, and the site on the tubulin heterodimer shared by colchicine, podophyllotoxin, steganacin, combretastatin, and many synthetic compounds.6 Unlike colchicine, the vinca alkaloids bind directly to microtubules without first forming a complex with soluble tubulin, and they do not copolymerize with the tubulin lattice of the microtubule.3, 6, 55, 56

The vinca alkaloids bind to microtubules at two binding sites, each with different affinities. These agents bind to tubulin at the microtubule ends with high affinity (Kd, 1 to 2 µmol) and considerably lower affinity (Kd, 0.25 to 0.3 mmol) to tubulin sites located along the sides of the microtubule surface.3, 21 There are approximately 1 to 17 high-affinity binding sites per microtubule located at the ends of each microtubule in bovine brain (from a potential number of 17,000 tubulin dimers per average 10-µm microtubule), whereas the density of the low-affinity, high-capacity binding site is 1.4 to 1.7 sites per heterodimer.54, 57 The binding of the vinca alkaloids to high-affinity sites is responsible for the substoichiometric and potent suppression of tubulin exchange that occur at low drug concentrations (<1 µmol). At low concentrations, treadmilling is disrupted, but microtubule mass is not affected. Furthermore, low concentrations of the vinca alkaloids perturb dynamic instability in an “end-dependent” fashion. Dynamic instability is strongly enhanced at the minus ends (kinetic destabilization), whereas dynamic instability is inhibited at the plus end. In essence, these actions increase the time that microtubules spend in a state of attenuated activity, neither growing nor shortening, the end result of which is a potent block at the metaphase/anaphase transition in mitosis.2

At high stoichiometric concentrations (µmol), these actions are accompanied by microtubule depolymerization as the result of effects on low affinity sites. Binding of the vinca alkaloids to the low affinity sites induces tubulin to self-associate into nonmicrotubule tubulin polymers and ordered aggregates through a self-propagation pathway. Self-propagation occurs as vinca alkaloid binding progressively weakens the lateral interactions between protofilaments, induces conformation changes in tubulin, and exposes new sites. The exposure of new sites further increases the binding affinity of the vinca alkaloids and, in turn, likely results in the formation of vinca alkaloid-tubulin spiral aggregates, protofilaments, and paracrystalline structures, ultimately leading to the disintegration of microtubules. The proposal has been made that MAPs stabilize the longitudinal interactions between dimers in the protofilaments as they splay apart after binding the vinca alkaloid, as illustrated in Figure 11.3.58

Figure 11.3 Model of the vinca alkaloid-induced disassembly of microtubules containing microtubule-associated protein into spiraled protofilaments composed of one or two spirals. (Reprinted with permission from Donoso JA, Haskins KM, Himes RH. Effect of microtubule proteins on the interaction of vincristine with microtubules and tubulin. Cancer Res 1979;39:1604.)

The vinca alkaloids potently block mitosis at the metaphase/anaphase transition. Following nuclear envelop breakdown, the vinca alkaloids block mitotic spindle formation and reduce the tension at the kinetochores of the chromosomes. Although chromosomes may condense, they remain scattered in the cells. The chromosomes separate along their lengths, but still remain attached at their centromeres.55, 59 Mitotic progress is delayed in a metaphase-like state with chromosomes “stuck” at the spindle poles, unable to move to the spindle equator. The cell-cycle signal to the anaphase-promoting complex, which is required for the cell to transition from metaphase to anaphase is blocked and the cells eventually undergo apoptosis. However, cyclin B concentrations may remain high and cell cycle progression to interphase in the absence of anaphase or cytokinesis may occur, resulting in chromatin decondensation and formation of multilobed nuclei.55 At low concentration, the vinca alkaloids may induce mitotic arrest, which does not involve microtubule depolymerization. Nevertheless, the disruption of spindle microtubule dynamics without microtubule depolymerization may ultimately lead to apoptosis, which involves the inactivation of antiapoptotic proteins and induction of apoptotic genes (see “Mechanism of Action” and “Mechanism of Resistance” in “Taxanes” section).19, 54 The induction of apoptosis, however, does not depend on the presence of an intact p53 checkpoint;sensitivity of isogenic cell lines differing only in p53 status are the same.60The loss of p21, a protein that controls entry into mitosis at the G2-M checkpoint, enhances sensitivity of tumor cells to both vinca alkaloids and taxanes, possibly by hastening entry of drug-damaged cells into mitosis.19, 61

The relationships between the inhibitory effects of vinca alkaloids on cell proliferation, mitotic arrest, mitotic spindle disruption, and depolymerization of microtubules have been characterized in a series of elegant studies.44, 62 Although the antiproliferative effects of the vinca alkaloids are noted over a wide range of drug concentrations, the concentration that inhibits cell proliferation is directly related to the concentration that induces metaphase arrest. The inhibition of proliferation and blockage of cells in metaphase at the lowest effective drug concentrations occur with little or no microtubule depolymerization or disorganization of the mitotic spindle apparatus. With increasing drug concentrations, the organization of microtubules and chromosomes in arrested mitotic spindles deteriorates in a manner that is common to all derivatives. The cumulative body of data indicates that the antiproliferative effects of the vinca alkaloids at their lowest effective concentrations are caused by alterations in the dynamics of tubulin addition and loss at the ends of mitotic spindle microtubules rather than by simple depolymerization of the microtubules. Similar effects have been demonstrated with nocodazole, podophyllotoxin, and the taxanes.32, 62, 63

In addition to their direct cytotoxic effects on tumor cells, the vinca alkaloids, taxanes, and other antimicrotubule agents inhibit angiogenesis with surprising potency. In vitro, 0.1 to 1.0 pmol/L VBL blocked endothelial proliferation, chemotaxis, and spreading on fibronectin, all essential steps in angiogenesis.64 In combination with antivascular endothelial growth factor receptor antibodies, low doses of VBL significantly augment antitumor activity, even in tumors resistant to direct cytotoxic effects of the drug.65 In these experiments, the combination of drug and antibody produced early and marked endothelial necrosis and tumor regression. However, the relative contribution of these antiangiogenic effects to the clinical antitumor activity of the vinca alkaloids is unclear.

Mechanistic and Functional Differences

With regard to effects on microtubule dynamics, the naturally occurring vinca alkaloids VCR and VBL, the semisynthetic analog VRL, and the bifluorinated analog vinflunine impart similar actions, but they have distinguishing features as well.66, 67 For example, vinflunine appears to be more active than the other vinca alkaloids against several murine and human tumor xenografts even though it has a significantly lower affinity to tubulin and a lower potential to induce vinca alkaloid-tubulin spiral polymers.42, 68 However, the effects of vinflunine and VRL on microtubule dynamics differ from than those induced by VBL and VCR in that they decrease the growth rate and duration of time growing, but decrease the time spent in attenuation to a greater extent. In contrast, VBL and VCR decrease the shortening rate and increase the time spent in attenuation.

The explanation for the differential effects of the vinca alkaloids on both normal tissues and tumors is not clear. VCR, the most potent of the analogs in humans and the most neurotoxic, has the greatest affinity for tubulin.69 However, although the vinca alkaloids may demonstrate similar potencies against preparations of tubulin derived from any given tissue, the differential sensitivities of various tissues to the vinca alkaloids are the result of many factors. 5, 46,58, 67, 70, 71, 72, 73, 74, 75, 76 One possible contributing factor is tubulin isotype composition, which varies among tissues. Differential tubulin isotype expression may influence the intracellular accumulation of the vinca alkaloids and other antimicrotubule agents that avidly bind tubulin.9, 10, 77 In addition, differences in the type and concentration of MAPs, which may influence drug interactions with tubulin, and variability in cellular permeation and retention may influence the formation and stability of complexes formed between the vinca alkaloids and tubulin.15, 46, 66, 69, 70, 71, 78, 79 For example, the higher cellular retention of VCR compared with VBL in cultured leukemia cells may explain why VCR is more potent than VBL during short treatment periods, whereas the drugs are equitoxic with more prolonged exposures.70, 75, 76, 79, 80, 81 The magnitude of intracellular GTP concentrations may also influence the type of interactions between the vinca alkaloids and tubulin, and variability in VCR retention among tumors and normal tissues may be related to differences in GTP hydrolysis.76, 79, 80, 81Other factors that might account for differences in vinca alkaloid sensitivities between various tissues include differences in cellular pharmacology and pharmacokinetics, which will be discussed in subsequent sections.

Cellular Pharmacology

Although the vinca alkaloids are rapidly taken up into cells and then accumulate intracellularly, intracellular/extracellular concentration ratios range from 5- to 500-fold depending on the cell type.70, 72, 78, 81, 82 In murine leukemia cells, the intracellular concentrations of VCR are 5- to 20-fold higher than the extracellular concentrations, and this ratio has been reported to range from 150- to 500-fold for other vinca alkaloids in human and murine leukemia cell lines.75, 83 In isolated human hepatocytes, VRL is more rapidly taken up and metabolized than other vinca alkaloids.71, 82, 83, 84, 85 Although the vinca alkaloids are retained in cells for long periods of time and thus may have prolonged cellular effects, there are marked differences in cellular retention between these agents.85, 86, 87, 88, 89 For example, VBL is retained to a much greater degree than either VCR or VDS. Overall, the most important determinant of drug accumulation and retention is lipophilicity, although a number of factors undoubtedly play a role.82, 83 Drug uptake and retention also appear to be determined by tissue-specific and drug-specific factors, as illustrated by studies indicating that the accumulation and retention of VRL in neurons are much less than other vinca alkaloids.80 The mechanisms responsible for the intracellular accumulation of the vinca alkaloids and other antimicrotubule agents are not fully known but likely involve binding to cellular tubulin.44 Differential uptake of drugs in different tumor types may also result from diverse expression of tubulin isotypes with different binding characteristics, different uptake, efflux pump mechanisms, and intracellular reservoirs for drug accumulation.

It was originally believed that the vinca alkaloids entered cells by both energy-dependent and temperature-dependent transport processes; however, temperature-independent, nonsaturable mechanisms, analogous to simple diffusion, most likely account for most transport, and temperature-dependent saturable processes are less important.15, 18, 82, 90 Although the drug concentration and duration of treatment are important determinants of both drug accumulation and cytotoxicity, the duration of drug exposure above a critical threshold concentration is perhaps the most important determinant of cytotoxicity.71, 84 Cytotoxicity is directly related to the extracellular concentration of drug when the duration of treatment is kept constant;for prolonged exposure to VCR, the concentration yielding 50% inhibition lies in the range of 1 to 5 nmol/L (Fig. 11.4).84, 91

Figure 11.4 Cytotoxicity of vincristine (VCR) for L1210 murine leukemia cells as measured by cloning efficiency. Cells were placed in soft agar containing VCR at the specified concentration;the number of colonies was counted 14 days later and expressed as a percentage of the number of colonies that developed from unexposed (control) cells. (Reprinted with permission from Jackson DV, Bender RA. Cytotoxic thresholds of vincristine in a murine and human leukemia cell line in vitro. Cancer Res 1979;39:4346.)

 

Mechanisms of Resistance

Resistance to the vinca alkaloids develops rapidly in vitro in the presence of the agents and arises by at least two different mechanisms. In most experimental models, drug resistance is associated with decreased drug accumulation and retention, but the clinical relevance of this phenomenon is not known. The first mechanism is pleiotropic or multidrug resistance (MDR), which can be innate or acquired. Although a large number of proteins that mediate MDR, the best-characterized ones are the ATP-binding cassette (ABC) transporters that belong to the largest known transporter gene family and translocate a variety of substrates across cellular compartments. These intracellular and extracellular membrane-spanning proteins transport endobiotics and xenobiotics and confer resistance to the vinca alkaloids and other structurally bulky, natural product chemotherapeutic agents in vitro.92 The best studied ABC transporters with respect to conferring resistance to the vinca alkaloids are the permeability glycoprotein (P-gp), or the MDR1 encoded gene product MDR1 (ABC Subfamily B1;ABCB1), and the multidrug resistance protein (MRP) (ABC Subfamily C2;ABCB1).85, 92, 93, 94, 95, 96, 97, 98, 99

MDR1 is a 170-kD P-gp energy-dependent transmembrane transport pump that regulates the efflux of a large range of amphiphatic hydrophobic substances, resulting in decreased drug accumulation. Pgp forms a channel in the membrane through which drugs are transported, and drug resistance is proportional to the amount of Pgp.93 Pgp is constitutively overexpressed by various normal tissues, including renal tubular epithelium, colonic mucosa, adrenal medulla, and other epithelial tissues.86 The efflux protein is also commonly expressed in human cancers, particularly those derived from tissues in which it is constitutively expressed (e.g., kidney and colon cancers). It is also found in post-treatment lymphomas, leukemias, and multiple myeloma.

MDR1 confers varying degrees of cross-resistance to other structurally bulky natural products, such as the taxanes, anthracyclines, epipodophyllotoxins, dactinomycin (actinomycin D), and colchicine.92, 93, 94, 95, 96, 97, 98, 99 These cells may have homogeneously stained chromosomal regions or double-minute chromosomes, which indicates the presence of an amplified gene that codes for P-gp.87, 88 The specific Pgp associated with resistance to the vinca alkaloids shows slight antigenic and amino acid sequence differences and a different peptide map after digestion than does Pgp from cells selected for resistance to colchicine or paclitaxel.96, 97 In fact, two forms of the protein are produced by a single clone of VCR-resistant cells, and these forms undergo post-translational N-glycosylation and phosphorylation, which leads to further structural diversity. This diversity may explain the greater degree of resistance for the specific agent used compared with the resistance to other drugs conferred by MDR, and it also may explain the variable patterns of resistance among cells of the MDR type. The composition of membrane gangliosides in cancer cells resistant to the vinca alkaloids has also been shown to differ from that of wild-type cells. The clinical ramifications of this resistance mechanism are not known. However, in one study in childhood acute lymphoblastic leukemia, VCR resistance measured in vitro did not correlate with P-gp overexpression.99

Resistance to the vinca alkaloids is also conferred by MRP1, which is a 190-kD membrane-spanning protein that shares 15% amino acid homology with MDR1.89, 92, 100, 101, 102 MRP1 expression has been found in many types of tumors and has been implicated as a component of the MDR phenotype in cancers of the lung, colon, breast, bladder, and prostate, as well as leukemia.87, 88, 89, 92, 100, 101, 102 MRP1 has been shown to transport glutathione conjugates of several types of compounds, including alkylating agents, as well as etoposide and doxorubicin but only confers resistance to the latter agents. The MRP1 resistance profile also includes the vinca alkaloids and methotrexate.89, 92, 100, 101, 102 The clinical significance of the role of MRP1 in transporting conjugated forms of certain chemotherapy agents has not been determined. Although many other ABC transporters have been characterized in vitro and several enhance cellular resistance to the vinca alkaloids, their roles in conferring inherent or acquired resistance to the vinca alkaloids in the clinic are even less clear than those of MDR1 and MRP1.

Another important feature of MDR1 and MRP in vitro is that drug resistance is reversible after treatment with various agents that have distinctly different structural and functional characteristics, such as the calcium-channel blockers, calmodulin inhibitors, detergents, progestational and antiestrogenic agents, antibiotics, antihypertensives, antiarrhythmics, antimalarials, and immunosuppressives.92 These agents bind directly to Pgp, thereby blocking the efflux of the cytotoxic drugs and increasing intracellular drug concentrations. Therefore, the role of MDR modulators has been a source of great contemporary interest, but the interpretation of clinical studies of resistance modulation has been confounded by the fact that MDR modulators, particularly MDR1 reversal agents, also enhance drug uptake in normal cells, decrease biliary elimination and drug clearance, and lead to enhanced toxicity.103, 104, 105 Overall, strategies aimed at reversing resistance to the vinca alkaloids in the clinic with pharmacologic modulators of both MDR1 and MRP, have been disappointing, most likely because of the placticity of MDR1, which is capable of producing a large number of alternate resistance proteins in response to environmental stress.92 Nevertheless, the characterization of the genetics and role of the ABC transporters in normal organ function and the disposition of chemotherapeutic agents have led to the delineation of genetic polyporphisms that may impact upon pharmacokinetics and drug toxicity.

Structural alterations in α- or β-tubulin, resulting from either genetic mutations and consequential amino acid substitutions or posttranslational modifications, including phosphorylation and acetylation, have been identified in tumor cells with acquired resistance to the vinca alkaloids.10, 44, 57, 58, 106, 107 The consequences of functionally significant differences in α- and β-tubulins are “hyperstable” microtubules that are collaterally sensitive to the taxanes and similar tubulin stabilizing natural products (see “Mechanisms of Resistance, Taxanes”). Although the means by which tubulin alterations confer resistance to the vinca alkaloids is not clear, the phenomenon is not apparently due to decreased drug-binding affinity of the altered tubulin.108, 109, 110, 111 Instead, alterations in α- and β-tubulins promote resistance to agents that inhibit microtubule assembly by increasing microtubule stability, perhaps by promoting longitudinal interdimer and intradimer interactions and/or lateral interactions between protofilaments.112

Resistance to the vinca alkaloids has also been demonstrated to be related to overexpression of the β-III isotype of β-tubulin.113 Some have also speculated that changes in the GTP- binding domain of tubulin may be the structural basis for this type of resistance.109 Another important feature of this type of resistance to the vinca alkaloids is that collateral sensitivity is conferred to the taxanes, which inhibit microtubule disassembly.

Clinical Pharmacology

Analytical Assays

Information about the pharmacology of the vinca alkaloids in humans has been limited in the past by lack of sensitive, specific, and reliable analytic assays capable of measuring the minute plasma concentrations that result from the administration of milligram quantity doses of these agents. Pharmacologic studies were performed initially with radiolabeled drugs;however, interpretation of the results has been confounded by the chemical instability of these agents. Several vinca alkaloids, particularly VCR and VBL, may undergo spontaneous degradation under mild conditions, forming degradative products that can be separated using high-pressure liquid chromatography (HPLC).114 Therefore, investigators have used radiolabeled compounds coupled to HPLC for further separation to define the plasma disposition of the vinca alkaloids.115, 116, 117, 118, 119, 120 The extent to which the formation of degradative products occurs in vivo, however, is not known.

Radioimmunoassay and enzyme-linked immunosorbent assay (ELISA) methods using specific antisera are capable of detecting picomolar drug concentrations.17Because polyclonal antisera raised against the vinca alkaloids cannot distinguish between the parent compounds and related derivatives, these assays may not provide sufficient quantitative information about degradation products and metabolites. However, more refined radioimmunoassay and ELISA methods using monoclonal antibodies have considerably greater sensitivity and specificity.17

Technical advances in extraction and chromatographic detection (electrochemical and fluorescence) have made HPLC and gas chromatography the most feasible means of separating the vinca alkaloids from their metabolites. Tandem mass spectrometry used in conjunction with HPLC has enhanced the sensitivity of chromatographic methods.121, 122, 123

Pharmacokinetics

General

The vinca alkaloids are most commonly administered intravenously as a bolus injection or brief infusion, and their pharmacokinetic behavior in plasma optimally fits open three-compartment models.17 Pharmacokinetic characteristics include large volumes of distribution, high clearance rates, long terminal half-lives (t½), hepatic metabolism, and biliary/fecal excretion. At conventional adult dosages, peak plasma concentrations (Cpeak), which persist for only a few minutes, range from 100 to 500 nmol, and plasma levels remain above 1 to 2 nmol for long durations.124, 125 Pertinent pharmacokinetic parameters are summarized in Table 2. There is also large interindividual and intraindividual variability in their pharmacologic behavior, which has been attributed to many factors, including differences in protein and tissue binding, hepatic metabolism, and biliary clearance.91 In comparative studies of VCR, VBL, and VDS, VCR has had the longest terminal t½ and the lowest clearance rate, VBL has had the shortest terminal t½ and the highest clearance rate, and VDS has had intermediate values.125, 126 The proposal has been made that the longer terminal t½ and lower clearance rate of VCR account in part for its greater propensity to induce neurotoxicity, but there appear to be other determinants of tissue sensitivity as discussed in “Mechanistic and Functional Differences.”125

Although prolonged infusion schedules may avoid excessively toxic Cpeak values and increase the duration of drug exposure in plasma above biologically relevant threshold concentrations for any given tumor, there is little, if any, evidence to support the notion that prolonged infusion schedules are more effective than bolus schedules. This approach has primarily been directed at achieving plasma concentrations for relevant periods, since the duration of exposure to relevant concentrations is a principal determinant of cytotoxicity in vitro;however, rapid, high, and avid distribution and binding of the vinca alkaloids to peripheral tissues, owing to the ubiquitous nature of tubulin, is likely responsible for the efficacy of short administration schedules.

Vincristine

After conventional doses of VCR (1.4 mg/m2) given as brief infusions, peak plasma levels approach 400 nmol/L.17, 36, 115, 116, 125, 126, 127, 128 VCR binds extensively to both plasma proteins (reported values in the range of 48 to 75%) and formed blood elements, particularly platelets, which contain high concentrations of tubulin and led, in the past, to the use of VCR-loaded platelets for treating disorders of platelet consumption, such as idiopathic thrombocytopenia purpura.17, 113 The platelet count has been inversely related to drug exposure.17, 128 In dogs and rodents, the spleen accumulates VCR to a greater extent than any other tissue.17, 115, 130 Poor drug penetration across the blood-brain barrier has been documented in most studies.17 The low penetration of VCR across the blood-brain barrier and other tumor sanctuary sites can be attributed to its large size and the fact that it is an avid substrate for the ABC transporters, which maintain the integrity of these blood-tissue barriers.17, 36, 74, 115, 116, 119, 125, 126, 127, 128 Pharmacologic inhibition of MDR, however, may allow entry of VCR into the brain.115, 119, 130, 131 In humans, VCR concentrations in cerebrospinal fluid are 20- to 30- fold lower than in plasma and do not exceed 1.1 nmol/L.131

After standard doses of VCR administered intravenously as a bolus injection, plasma disposition is triphasic, with t½α values of less than 5 minutes because of extensive and rapid tissue binding. Consequently, the apparent volumes of distribution (Vd) are high (mean central Vd, 0.328 ± 0.1061 L/kg and V[Vd for the terminal γ phase] of 8.42 ± 3.17 L/kg), which indicates extensive tissue binding.125 Beta t½ (t½β) values range from 50 to 155 minutes, and gamma t½ (t½γ) values are even more variable, ranging from 23 to 85 hours, which suggests slow clearance from the tissue compartment.17, 118, 125, 128 Considerable interest has arisen in using protracted VCR administration schedules, because prolonged infusions may closely simulate the optimal in vitro conditions required for cytotoxicity.84, 91, 128, 131 For example, VCR concentrations of 100 to 400 nmol/L are achieved only briefly after bolus injection, and levels generally decline to less than 10 nmol/L in 2 to 4 hours. Exposure to 100 nmol/L VCR for 3 hours is required to kill 50% of L1210 murine or CEM human lymphoblastic leukemia cells, whereas treatment durations of 6 to 12 hours are required to achieve this degree of cytotoxicity at 10 nmol/L, and no lethal effects occur at VCR concentrations below 2 nmol/L.84 A 0.5-mg intravenous bolus injection of VCR followed by a continuous infusion at dosages of 0.5 to 1.0 mg/m2 per day for 5 days results in steady-state VCR concentrations ranging from 1 to 10 nmol/L, and terminal t½ after discontinuation of the infusions ranging from 10.5 hours (1.0 mg/m2) to 21.7 hours (0.5 mg/m2).128 Although peak VCR plasma concentrations achieved with prolonged infusions are lower than levels achieved with bolus injections, more prolonged schedules are associated with a greater duration of drug exposure above a critical threshold concentration.128 However, this reasoning involves relating drug exposure in plasma to cytotoxicity and the drug exposure in tumors is likely disproportionately higher because of extensive tissue distribution, avid tissue binding, and slow clearance from tissue compartments.

VCR is metabolized and excreted primarily by the hepatobiliary system.73, 74, 115, 116 Within 72 hours after the administration of radiolabeled VCR, approximately 12% of the radioactivity is excreted in the urine (at least 50% of which consists of metabolites), and approximately 70 to 80% is excreted in the feces (40% of which consists of metabolites).36, 83, 115, 116, 119, 120, 124, 125, 126, 128, 131, 132 VCR is rapidly excreted into bile with an initial bile to plasma concentration ratio of 100:1 that declines to 20:1 at 72 hours posttreatment.115 Metabolites accumulate rapidly in the bile, so that only 46.5% of the total biliary product is the parent compound.115 As many as 6 to 11 metabolites have been detected in both humans and animals.130, 133, 134, 135 The structures of most, however, most have not been identified. The metabolites 4-deacetylVCR and N-deformylVCR have been isolated from human bile, whereas 4-deacetylVCR, and both 4′-deoxy-3′-hydroxyVCR and 3′,4′- epoxyVCR N-oxide have been identified after incubation of VCR with bile.114, 120, 127, 133 The nature of the metabolites identified to date, as well as the results of metabolic studies in vitro, indicate that is VCR is metabolized principally by the hepatic cytochrome P-450 mixed function oxidase CYP3A.36, 83, 126, 133, 134, 135, 136 The importance of CYP3A in drug disposition is also supported by observations of enhanced clearance with phenytoin and carbamazepine that induce CYP3A4, and increased toxicity with CYP3A inhibitors, particularly itraconazole.134, 135 In addition, transfection of tumor cells with CYP3A4 increases resistance to VBL, whereas cancer cells selected for VBL resistance may show increased CYP3A4 activity.136 There has been conflicting, albeit sparse, evidence indicating that VCR Cpeak values or systemic exposure are directly related to the degree of neurotoxicity.33

Vinblastine

The pharmacologic behavior of VBL also reflects its extensive tissue binding and resembles that of VCR. Although plasma protein binding has been reported to range from 43 to 99.7%, it most likely approaches the high end of this range.118, 137 VBL binds extensively to formed blood elements, with 50% of radiolabeled drug bound to platelets, red blood cells, and white blood cells within 20 minutes after an intravenous injection.140 Extensive platelet binding is most likely the result of the high concentrations of tubulin in platelets.

Plasma disappearance fits a triexponential model with a rapid distribution phase (t½α <5 minutes) from rapid tissue binding.118 VBL is more avidly sequestered in tissues than VCR, as demonstrated by retention of 73% of radioactivity in the body 6 days after an injection of the radiolabeled agent.118 Values for t½α and t½β have been reported to range from 53 to 99 minutes and 20 to 24 hours, respectively.17, 118, 125 High steady-state levels and long terminal t½ values have been reported after 5-day infusions of VBL: 1.1 nmol/L at 1 mg/m2 per day (t½, 28 days);3.3 nmol/L at 1.7 mg/m2 per day (t½, 3 days);and 6.6 nmol/L at 2 mg/m2 per day (t½, 6 days).17, 141

The principal mode of VBL disposition is hepatic metabolism and biliary excretion. Over a 9-day period after treatment of dogs with radiolabeled VBL, 30 to 36% of radioactivity is recovered in bile and 12 to 17% is found in urine.142 Fecal excretion of the parent compound is relatively low, which indicates that metabolism is significant. In vitro studies indicate that the cytochrome P-450 CYP3A isoform is primarily responsible for drug biotransformation.143 At least one metabolite, desacetylvinblastine (VDS), which may be as active as the parent compound, has been identified in both dogs and humans.118, 142 Small quantities of VDS also have been detected in both urine and feces.

Vindesine

Similar to the other vinca alkaloids, plasma disposition of VDS is characterized by a triexponential process.17, 118, 119, 125, 144, 145 VDS is also rapidly distributed to peripheral tissues except for sanctuary sites that are protected by ABC transporters (e.g., brain and testes). T½α values for VDS are less than 5 minutes and values for t½β and t½γ range from 55 to 99 minutes and 20 to 24 hours, respectively. Clearance is low, which indicates that drug accumulation may occur with short-interval, repetitive dosing schedules. The large Vd, low renal clearance rate, and long terminal t½ of VDS also suggest that it undergoes extensive tissue binding and delayed elimination. Although peak plasma VDS concentrations that range from 0.1 to 1.0 µmol/L are achieved with bolus injections, levels typically decline to <0.1 µmol/L in 1 to 2 hours after treatment. Plasma levels achieved with bolus injection are approximately 16-fold higher than levels achieved with prolonged infusions;however, optimal steady-state VDS concentrations for cytotoxicity (0.01 to 0.1 µmol/L) are readily achieved with prolonged infusion schedules (1.2 to 2.0 mg/m2 per day for 2 to 5 days).128, 134, 140, 144, 146, 147, 148

The liver is the main organ involved in VDS metabolism and disposition, and CYP3A appears to be the principal P-450 CYP isoform involved in drug biotransformation.83, 117, 130, 149 VDS concentrations in bile are much higher than simultaneously measured plasma levels, and biliary and renal clearance rates have been reported to be 29 and 12 mL/minute, respectively.130 Renal excretion accounts for only 1 to 13% of drug disposition.17, 134, 147

Vinorelbine

The pharmacologic behavior of VRL is similar to that of the other vinca alkaloids, and the decline of plasma concentrations following rapid injection have been characterized by biexponential and triexponential models.36, 41, 53, 83, 132, 150 After intravenous administration, there is a rapid decay of VRL concentrations followed by a much slower elimination phase (t½γ, 18 to 49 hours). Plasma protein binding has been reported to range from 80 to 91%, with binding primarily to α1-acid glycoprotein, albumin, and lipoproteins, and platelet binding is also extensive.17, 53, 44, 83, 151 The unbound fraction has been reported to range from 0.09 to 0.20.53

VRL is widely distributed, and high concentrations are found in virtually all tissues (tissue to plasma ratios of 20 to 80), except brain.53, 54, 83, 135, 152 The wide distribution of VRL reflects the agent's lipophilicity, which is among the highest of the vinca alkaloids. In fact, drug concentrations in human lung have been demonstrated to be 300-fold greater than plasma levels and 3.4- to 13.8-fold higher than lung concentrations achieved with VDS and VCR, respectively. Plasma protein binding in the range of 80 to 90% has been reported. As with other vinca alkaloids, drug disposition principally occurs in the liver, and 33 to 80% of the drug is excreted in the feces, whereas urinary excretion represents only 16 to 30% of total drug disposition, most of which is unmetabolized VLR.39, 53, 83, 132, 153, 154 Studies in humans indicate that 4-O-deacetyl-VRL and 3,6- epoxy-VRL are the principal metabolites, and several minor hydroxy-VRL metabolites have been identified.53, 44, 154 Although most metabolites are inactive, the deacetyl-VRL metabolite may be as active as VRL but this finding is of minor clinical significance because concentrations of this metabolite are minute. The CYP3A isoenzyme appears to be principally involved in biotransformation.53, 54

In one study the total body clearance of VRL (1.2 L/hour per kilogram) and t½γ values of approximately 26 hours were found to be the same in older and younger patients, provided that patients have normal hepatic function.155 Clearance has been found to be adversely affected in patients who have liver metastases that replace more than 75% of the organ;clearance can be predicted in such patients by the monoethylglycinexylidide clearance test, which assesses CYP3A4 function.156 Although VRL clearance is not accurately predicted by bilirubin concentrations in serum, markedly elevated levels have been associated with significant reductions in clearance in the few patients studied.

VRL is active when given orally. In animal studies,100% of total radioactivity is absorbed after the ingestion of tritium-labeled VRL, whereas human studies using powder-filled, liquid-filled, and gelatin-filled capsules have shown that the bioavailability of the parent compound is 43% for the powder-filled and 27% for the liquid-filled capsules; the bioavailability of the gel-filled capsule was negligibly affected by food.41, 157, 158 Cpeak values are achieved within 1 to 2 hours after oral treatment, and interindividual variability is moderate.

Drug Interactions

Pharmacokinetic interactions of the vinca alkaloids and other drugs have not been studied in detail. Methotrexate accumulation in tumor cells is enhanced in vitro by the presence of VCR or VBL, an effect mediated by a vinca alkaloid-induced blockade of drug efflux;however, the minimal concentrations of VCR required to achieve this effect in myeloblasts (0.1 µmol/L) is realized only momentarily during clinical treatment, and even higher concentrations are needed to enhance MTX uptake in lymphoblasts.159, 160, 161 The schedule of VCR followed by MTX has not demonstrated synergism in the L1210 murine leukemia cells.162 Cytotoxic synergy is noted with the sequence of MTX followed by VCR; however, but this interaction is not likely the result of the enhancement of MTX uptake. Thus, very little justification exists for routine use of VCR pretreatment in high-dose MTX protocols. The vinca alkaloids also inhibit the cellular influx of the epipodophyllotoxins in vitro, resulting in less cytotoxicity, but the clinical ramifications are unknown.163 L-Asparaginase may reduce the hepatic clearance of the vinca alkaloids, particularly VCR, which may result in increased toxicity. To minimize the possibility of this interaction, VCR should be given 12 to 24 hours before L-asparaginase. The use of mitomycin C in combination with the vinca alkaloids has been associated with pulmonary toxicity as described in “Toxicity, Pulmonary.”

Treatment with the vinca alkaloids has precipitated seizures associated with subtherapeutic plasma phenytoin concentrations, most likely due to induction of CYP3A.164, 165 Reduced plasma phenytoin levels have been noted from 24 hours to 10 days after treatment with both VCR and VBL. As previously discussed, administration of the vinca alkaloids with erythromycin, itraconazole, and other inhibitors of CYP3A may also lead to severe toxicity.133, 134, 135, 137, 138, 166,167, 168 Concomitantly administered drugs, such as pentobarbital and H2-receptor antagonists, may also influence VCR clearance by modulating hepatic cytochrome P-450 metabolic processes.161 Another potential drug interaction may occur in patients who have Kaposi's sarcoma related to acquired immunodeficiency syndrome and are receiving concurrent treatment with 3′ azido-3′-deoxythymidine (AZT) and the vinca alkaloids, as the vinca alkaloids may inhibit glucuronidation of AZT to its 5′-O-glucuronide metabolite.167 Based on a report of a constellation of severe toxicities, including inappropriate secretion of antidiuretic hormone (SIADH), bilateral cranial nerve palsies, peripheral neuropathy, cranial nerve palsies, heart failure and cardiovascular effects following VCR treatment in pediatric patient with acute lymphocytic leukemia who had been receiving treatment with nifedipine and itraconazole, it is possible that these medications may enhance the neurologic and cardiovascular effects of the vinca alkaloids.133, 168 Lastly, the significant interindividual and intraindividual variability of VCR pharmacokinetics in children has been attributed to the variable induction of P-450 metabolism because of concurrent use of P-450-inducing corticosteroids.168

Doses and Schedules

The vinca alkaloids are most commonly administered by direct intravenous injection or through the side-arm tubing of a running intravenous infusion. Experienced oncology personnel should administer these agents because drug extravasation causes severe soft tissue injury.

Vincristine

VCR is routinely administered to children weighing more than 10 kg (body surface area ≥1 m2) as a rapid (bolus) intravenous injection at a dose of 1.5 to 2.0 mg/m2 weekly, whereas 0.05 to 0.065 mg/kg weekly is commonly used in smaller children (<10 kg or body surface area <1 m2). For adults, the conventional weekly dose is 1.4 mg/m2 weekly. A restriction of the absolute single dose of VCR to 2.0 mg/m2, which is commonly referred to as capping, has been adopted, based on early reports of substantial neurotoxicity at higher doses. This restriction is largely empirical, and available evidence suggests that the practice of capping should be reconsidered.164 The fact that the cumulative dose may be a more critical factor than single dose has readily been appreciated;however, significant interpatient variability exists, and some patients are able to tolerate much higher VCR doses with little or no toxicity.171, 172 This may be because of large interindividual differences in drug exposure, which may vary as much as 11-fold.173, 174 Moreover, the safety and efficacy of treatment regimens that do not employ capping at 2.0 mg have been documented in adults.164, 175 In any case, VCR dosage modifications should be based on toxicity, particularly peripheral and autonomic neuropathy. However, dosage should not be reduced for mild peripheral neurotoxicity, particularly if the agent is being used in a potentially curative setting. Instead, doses should be modified for manifestations indicative of more serious neurotoxicity, including severe symptomatic sensory changes, motor and cranial nerve deficits, and ileus, until toxicity resolves. In clearly palliative situations, dose reductions, lengthened dosing intervals, or selection of an alternative agent may be justified in the event of moderate neurotoxicity. A routine prophylactic regimen, consisting of stool softeners, dietary bulk, and laxatives, to prevent the consequences of severe autonomic toxicity, particularly severe constipation, is also recommended.

Based on in vitro data indicating that the duration of VCR exposure above a critical threshold concentration is an important determinant for cytotoxicity, prolonged infusion schedules have been evaluated.17, 128, 134 After a 0.5- mg/m2 intravenous injection of VCR, total daily VCR doses of 0.25 to 0.50 mg/m2 as a 5-day infusion are generally well tolerated.17 In children, the administration of VCR as a 5-day infusion has permitted a twofold increase in the dose that can be safely administered without major toxicity compared with bolus schedules.

VCR is a potent vesicant and should not be administered intramuscularly, subcutaneously, or intraperitoneally. Direct intrathecal injection of VCR or other vinca alkaloids, which has occurred as an inadvertent clinical mishap, induces a severe myeloencephalopathy characterized by ascending motor and sensory neuropathies, encephalopathy, and rapid death (see “Toxicity, Miscellaneous”).176, 177 Administration of VCR 0.4 mg/day as a 5-day infusion by the hepatic intraarterial route also has been associated with profound toxicity, including disorientation and diarrhea.178

Although the issue has not been evaluated carefully, the major role of the liver in the disposition of VCR implies that dose modifications should be considered for patients with hepatic dysfunction.174 However, firm guidelines for dose modifications have not been established. A 50% dosage reduction is recommended for patients with plasma total bilirubin levels between 1.5 and 3.0 mg/dL and at least a 75% dosage reduction for serum total bilirubin levels above 3.0 mg/dL. Dosage reductions for renal dysfunction are not indicated.179

Vinblastine

Although VBL has been administered intravenously on various schedules, the most commonly used schedule administers a bolus injection at a dose of 6 mg/m2per day in cyclic combination-chemotherapy regimens. Approved initial dose recommendations for weekly dosing are 2.5 and 3.7 mg/m2 for children and adults, respectively, followed by gradual dose escalation in increments of 1.8 and 1.25 mg/m2, respectively, each week based on hematologic tolerance. The recommendation is also that maximal weekly doses of 18.5 and 12.5 mg/m2 in adults and children, respectively, should not be exceeded;however, these doses are substantially higher than most patients can tolerate because of myelosuppression, even on less frequent treatment schedules. Because the severity of the leukopenia that may occur with identical VBL doses varies widely, VBL probably should not be given more frequently than once each week. Oral administration may result in unpredictable toxicity.180

Five-day continuous infusions of VBL have been used at dosages ranging from 1.5 to 2.0 mg/m2 per day, which achieve plasma concentrations of approximately 2 nmol/L. In one study of VBL on this administration schedule, patients who were more likely to respond to treatment had longer terminal-phase t½ values.181 Little, if any, evidence exists, however, to support the notion that prolonged infusion schedules are more effective than bolus schedules.

Although specific guidelines have not been established, VBL dosages should be modified for patients with hepatic dysfunction, especially biliary obstruction, because of the importance of the liver in drug disposition (see “Doses and Schedules, Vincristine”). Dosage reductions in patients with renal dysfunction are not indicated.179

Vindesine

VDS has been administered intravenously on many schedules, including weekly and biweekly bolus and prolonged infusion schedules. The agent also has been given in fractionated doses as either an intermittent or a continuous infusion over 1 to 5 days. VDS is most commonly administered as a single intravenous dose of 2 to 4 mg/m2 every 7 to 14 days, which is associated with antitumor activity and a tolerable toxicity profile.17 Intermittent or continuous- infusion schedules usually administer VDS dosages of 1 to 2 mg/m2 per day for 1 to 2 days or 1.2 mg/m2 per day for 5 days every 3 to 4 weeks.17, 124 More prolonged schedules (up to 21 days) also have been evaluated.

Specific dosing guidelines have not been established for patients with hepatic or renal dysfunction; however, the pharmacologic similarities of VDS and other vinca alkaloids and the increased toxicity of VDS noted in patients with abnormal liver function mandate dosage reduction for patients with severe hepatic dysfunction, especially biliary obstruction (see “Doses and Schedules, Vincristine”). Dosage modifications are not indicated for renal dysfunction.179

Vinorelbine

VRL is most commonly administered intravenously at a dose of 30 mg/m2 on a weekly or biweekly schedule as a slow injection through a side-arm port into a running infusion (alternatively, a slow bolus injection followed by flushing the vein with 5% dextrose or 0.9% sodium chloride solutions) or as a short infusion over 20 minutes.182 It appears that the more rapid infusions produce less local venous toxicity.182 An acceptable oral formulation, however, is not yet available. Other dosing schedules that have been evaluated include long-term oral administration of low doses and intermittent high-dose and prolonged intravenous infusion schedules.182 Like the other vinca alkaloids, VRL clearance is impaired in patients with hepatic dysfunction, and dosage reductions should be considered in this setting.156 Recommendations include a 50% dosage reduction for serum total bilirubin concentrations between 1.5 and 3 mg/dL and a 75% dosage reduction for patients with plasma total bilirubin concentrations above 3.0 mg/dL. Dosage reductions are not recommended for patients with renal insufficiency.

Toxicity

The principal toxicities of the vinca alkaloids differ dramatically despite their structural and pharmacologic similarities. Peripheral neurotoxicity is the predominant toxicity of VCR, whereas myelosuppression predominates with VBL, VDS, and VRL. Nevertheless, peripheral neurotoxicity is often noted following cumulative treatment with VBL, VDS, and VRL, inadvertent high-dose treatment, and settings or patients who are inordinately susceptible (see “Neurologic”). On the other hand, VCR can cause myelosuppression under the similar conditions. Several potential explanations for the selective effects in various normal and neoplastic tissues are discussed in “Mechanism of Action, Vinca Alkaloids”, in this chapter.

Neurologic

The vinca alkaloids, particularly VCR, induce neurotoxicity characterized by a peripheral, symmetric mixed sensory-motor, and autonomic polyneuropathy.33, 36,53, 170, 183, 184, 185, 186 The primary neuropathologic effects are axonal degeneration and decreased axonal transport as a result of the interference with axonal microtubule function. Initially, only symmetric sensory impairment and paresthesias in a length-dependent manner (distal extremities first) usually are encountered. Neuritic pain and loss of deep tendon reflexes may develop with continued treatment, which may be followed by foot drop, wrist drop, motor dysfunction, ataxia, and paralysis. Back, bone, and limb pains occasionally occur. Nerve conduction velocities are usually normal, although diminished amplitude of sensory and motor nerve action potentials and prolonged distal latencies, suggesting axonal degeneration, may be noted.33, 36, 53, 183, 184, 185 Cranial nerves may be affected rarely, resulting in hoarseness, diplopia, jaw pain, and facial palsies. The uptake of VCR into the brain is low, and central nervous system effects, such as confusion, mental status changes, depression, hallucinations, agitation, insomnia, seizures, coma, SIADH, ataxia, athetosis, and visual disturbances, are rare.33, 74, 119, 120, 187, 188 Acute, severe autonomic neurotoxicity is uncommon but may arise as a consequence of high-dose therapy (>2 mg/m2) or in patients with altered hepatic function. Autonomic toxicities include abdominal cramping, paralytic ileus (see “Gastrointestinal, Toxicity, Vinca Alkaloids”), urinary retention (see “Genitourinary”), cardiac autonomic dysfunction, orthostatic hypotension, and arterial hypotension and hypertension (see “Cardiovascular”).170, 185, 186, 187, 188, 189, 190, 191Laryngeal paralysis may also occur.192

In adults, the neurotoxic effects of VCR may begin with cumulative doses as little as 5 to 6 mg, and manifestations may be profound after cumulative doses of 15 to 20 mg. Children appear to be less susceptible than adults, but older persons are particularly prone. However, the apparent influence of age may, in fact, be the result of previously inadequate dose calculation by body weight in children and adults and by body surface area in infants.193, 194 In infants, VCR doses are calculated now according to body weight. Patients with antecedent neurologic disorders, such as Charcot-Marie-Tooth disease, hereditary and sensory neuropathy type 1, Guillain-Barrá syndrome, and childhood poliomyelitis, are highly predisposed.195, 196, 197 Hepatic dysfunction or obstructive liver disease increases the risk of developing severe neuropathy because of impaired drug metabolism and delayed biliary excretion.

The only known treatment for VCR neurotoxicity is discontinuation of the drug or reduction of the dose or frequency of treatment.170, 198, 199 Although a number of antidotes, including thiamine, vitamin B12, folinic acid, and pyridoxine, have been used, these treatments have not been clearly shown to be effective.33, 126, 170, 198, 199 Folinic acid (not folic acid) has been shown to protect mice against an otherwise lethal dose of VCR and has been used successfully in several cases of VCR overdosage in humans;however, it has not been studied prospectively.200, 201 Concurrent administration of a mixture of gangliosides with VCR also has been reported to reduce the peripheral neurotoxicity produced with standard dosages of VCR.202 Another agent used to prevent neurotoxicity for which results are encouraging, is glutamic acid on the basis of its ability to enhance microtubule formation in vitro, as well as its possible competition with VCR for carrier-mediated membrane transport.199, 203 In a randomized clinical trial, coadministration of glutamic acid and VCR reduced the incidence of paresthesia and loss of the Achilles tendon reflex.203 However, glutamic acid has not been shown to ameliorate VCR-related gastrointestinal and hematologic toxicities. The adrenocorticotropic hormone analogue ORG 2766 has also been shown to protect against VCR-induced neuropathy, both in an animal model and in cancer patients in a placebo-controlled pilot study, but the relative younger age of the patients in the experimental arm as compared with that of the the placebo group may have accounted for this result.33 Nerve growth factor, insulin-like growth factor I, and amifostine have been anecdotally reported to alter the natural course of drug-induced neurotoxicity.37

The manifestations of neurotoxicity are similar for the other vinca alkaloids;however, they are typically less common and severe.17, 33, 36, 53, 126 Severe neurotoxicity is observed infrequently with both VBL and VDS. VRL has been shown to have a lower affinity for axonal microtubules than either VCR or VBL, which seems to be confirmed by clinical observations.68, 204 Mild-to-moderate peripheral neuropathy, principally characterized by sensory effects, occurs in 7 to 31% of patients, and constipation and other autonomic effects are noted in 30% of subjects, whereas severe toxicity occurs in 2 to 3%. Muscle weakness and discomfort at tumor sites may also occur. In a study in patients with non–small cell lung cancer who were treated with either VRL alone, VRL plus cisplatin, or VDS plus cisplatin, the rate of severe neurotoxicity was lower in both the single-agent VRL and VRL plus cisplatin arms than in the VDS plus cisplatin arm.205 Furthermore, the addition of cisplatin did not increase the incidence of severe toxicity in excess of that observed with VRL alone.

Hematologic

Neutropenia is the principal dose-limiting toxicity of VBL, VDS, and VRL. Thrombocytopenia and anemia are usually less common and less severe. The onset of neutropenia is usually 7 to 11 days after treatment, and recovery is generally by days 14 to 21. Myelosuppression is not typically cumulative. Hematologic toxicity of clinical relevance is uncommon after VCR treatment but may be a major manifestation following inadvertent administration of high dosages. VCR also may increase circulating platelets because of the endoreduplication of megakaryocytes.206

Gastrointestinal

Gastrointestinal toxicities, aside from those caused by autonomic dysfunction, may be caused by all the vinca alkaloids.17, 33, 36, 53, 126, 207 Gastrointestinal autonomic dysfunction, as manifested by bloating, constipation, ileus, and abdominal pain, occur most commonly with VCR or high doses of the other vinca alkaloids. Paralytic ileus, intestinal necrosis, and perforation have been reported.208 Poor intestinal transit may result in the impaction of stool in the upper colon. An empty rectum may be noted on digital examination, and an abdominal radiograph may be useful in diagnosing this condition. This condition may be responsive to high enemas and laxatives. A routine prophylactic regimen to prevent constipation is therefore recommended for all patients receiving VCR. Paralytic ileus also may occur, particularly in pediatric patients. The ileus, which may mimic a “surgical abdomen,” usually resolves with conservative therapy alone after termination of treatment. Patients who receive high dosages of VCR or have hepatic dysfunction may be especially prone to develop severe gastrointestinal complications as a result of autonomic neurotoxicity. Although success with drugs used prophylactically to minimize toxicity, including lactulose, caeruluin, metaclopramide, and the cholecystokinin analog sincalide, has been reported anecdotally, these agents also may alter the pharmacokinetic behavior of the vinca alkaloids by affecting biliary excretion and/or enterohepatic recirculation, which may ultimately result in increased drug clearance.17 Mucositis, stomatitis, and pharyngitis occur more frequently with VBL than with VRL or VDS and is least common with VCR. Nausea, vomiting, and diarrhea may also occur to a lesser extent. Asymptomatic and transient elevations in liver function test results, particularly alkaline phosphatase levels, have been noted. Pancreatitis has also been reported with VRL.207

Cardiovascular

Hypertension and hypotension, presumably resulting from autonomic neurotoxicity, have been observed.189, 190 The vinca alkaoids alone or in combination-chemotherapy regimens, particularly those that also include cisplatin and bleomycin, have been implicated rarely in causing acute cardiac ischemia and massive myocardial infarctions.209 The underlying mechanism for these effects is not known. Hypertension is the most common cardiovascular effect of VBL. Raynaud's phenomenon may be a lingering effect, especially in patients treated with the combination of VBL, cisplatin, and bleomycin (PVB).210 In one study, symptomatic Raynaud's phenomenon developed in 44% of patients with germ cell malignancies who were treated with PVB, and an even higher percentage developed abnormal vasoconstrictive responses to cold stimuli.210 This toxicity occurs less frequently when etoposide is substituted for VBL. The calcium-channel–blocking agent nifedipine has been reported to ameliorate the symptoms of Raynaud's phenomenon induced by VBL.211

Pulmonary

Respiratory reactions, characterized by dyspnea, have been reported in approximately 5% of patients, particularly when vinca alkaloids are combined with mitomycin C.212, 213 These respiratory reactions may be classified into two types. One type is an acute reaction with bronchospasm, resembling an allergic reaction. The second type is a subacute reversible reaction associated with cough and dyspnea and occasionally with interstitial infiltrates. This reaction typically occurs within 1 hour after treatment. The use of steroids has been felt to be beneficial in severe cases, and several patients have been retreated without sequelae. No evidence exists that VRL causes chronic pulmonary toxicity.

Genitourinary

VCR-induced autonomic neurotoxicity may produce bladder atony, thereby causing polyuria, dysuria, incontinence, and urinary retention.190 Therefore, the suggestion has been made that other drugs that are known to cause urinary retention, particularly in the older population, should be discontinued if possible for several days after treatment with VCR.

Extravasation

All vinca alkaloids are potent vesicants and may cause severe tissue damage if extravasation occurs. Injection-site reactions, including erythema, pain, and venous discoloration, are common;however, severe local toxicity is uncommon (< 2%). The risk of phlebitis increases if veins are not adequately flushed after treatment. If extravasation is suspected, treatment should be discontinued, and aspiration of any residual drug remaining in the tissues should be attempted.51, 214, 215, 216, 217 Animal experiments have demonstrated that cold packs may increase toxicity, and hot packs may limit damage. The application of local heat immediately for 1 hour four times daily for 3 to 5 days and the injection of hyaluronidase, 150 to 1500 units (15 units/mL in 6 mL of 0.9% sodium chloride solution) subcutaneously, through six clockwise injections in a circumferential manner using a 25-gauge needle (changing the needle with each injection) into surrounding tissues is the treatment of choice in minimizing both discomfort and latent cellulitis. The use of calcium leucovorin, diphenydramine, hydrocortisone, isoproterenol, sodium bicarbonate, and vitamin A cream have been ineffective in animal models.216 An immediate surgical consultation to consider early debridement is recommended. Discomfort and signs of phlebitis may also occur along the course of an injected vein, with resultant sclerosis. The risk of phlebitis may increase if the vein is not adequately flushed after treatment.

Endocrine

All the vinca alkaloids have been implicated as a cause of SIADH by directly affecting the hypothalamus, neurohypophyseal tract, or posterior pituitary. Patients who are receiving intensive hydration are particularly prone to severe hyponatremia secondary to SIADH, which may result in generalized seizures.17, 33, 36, 53, 126 This entity has been associated with elevated plasma levels of antidiuretic hormone and usual remits in 2 to 3 days. Hyponatremia generally responds to fluid restriction, as with hyponatremia associated with SIADH from other causes.

Miscellaneous

Alopecia occurs in a small proportion of patients. An acute necrotizing myopathy has also been observed.218 Hand-foot syndrome is a rare toxicity of VRL.219VBL may cause photosensitivity reactions, possibly as a result of corneal irritation.

The inadvertent intrathecal administration of the vinca alkaloids causes an ascending myeloencephalopathy that is usually fatal. Reports of immediate cerebrospinal fluid withdrawal and lavage with Ringer's lactate solution supplemented with fresh-frozen plasma (15 mL/L) at a rate of 55 mL/hour for 24 hours has provided somewhat encouraging results, in that two affected patients survived with significant paraplegia, but intact cerebral function.176, 177 To prevent this mistake, pharmacy, nursing, and physicians should be trained not to administer intrathecal methotrexate and intravenous VCR in a single setting, and the drugs should not be delivered together to staff. 220

TAXANES

The taxanes are perhaps the most important additions to the chemotherapeutic arsenal in the late twentieth century. The prototypical taxane, paclitaxel, and docetaxel, a potent semisynthetic analog, have demonstrated antitumor activity of major impact. Paclitaxel was discovered as part of a National Cancer Institute program in which extracts of thousands of plants were screened for anticancer activity.221 In 1963, a crude extract with antitumor activity was isolated from the bark of the Pacific yew, Taxus brevifolia, a slowly growing evergreen found in the old-growth forests of the Pacific Northwest, and paclitaxel was identified as the active constituent of the extract by Wall and Wani in 1971.222, 223, 224, 225 Interest in the agent accelerated in 1979 after researchers described its unique mechanism of action on microtubules.222, 223, 224, 225 Paclitaxel is also isolated from other members of the Taxus genus and produced by Taxomyces andreanae, a fungal endophyte isolated from the inner bark of the Pacific yew. Although paclitaxel originally came from the bark of the scarce Pacific yew, alternate sources, including nonbark biomass, ornamental species, and, most importantly, partial synthesis from a readily available precursor, 10-deacetylbaccatin III, derived from the needles of more abundant yew species such as the European yew, Taxus baccata, are producing sufficient quantities of the drug to meet commercial demand. Docetaxel, which is also derived semisynthetically from 10-deacetylbaccatin III, is slightly more water soluble than paclitaxel and a more potent antimicrotubule agent in vitro.226 The key features of the taxanes are displayed in Tables 11.3 and 11.4, respectively.

TABLE 11.3 KEY FEATURES OF THE TAXANES

 

Paclitaxel

Docetaxel

Mechanism of action

Low concentrations inhibit microtubule dynamics (dynamic instability and treadmilling).
High concentrations inhibit depolymerization of tubulin

Standard dosage (mg/m2)

175 over 3 hours every 3 weeks
135–175 over 24 hours every 3 weeks
80 over 1 hour weekly

60–100 over 1 hour every 3 weeks
(75 is the most common dose used)
36 over 1 hour weekly

Pharmacokinetics and disposition

See Table 11.4

Principal toxicity

Myelosuppression

Myelosuppression

Other common toxicities

Alopecia
Peripheral neurotoxiicty
HSRs

Alopecia
Peripheral neurotoxicity
Rashes and nail disorders
HSRs (mild to moderate)

Premedication

Corticosteroids, H1- and H2-histamine antagonists before each treatment to prevent HSR (see “Administration”)

Corticosteroids with each treatment to prevent fluid retention;H1-histamine antagonists recomemended to HSRs.

Precautions:

Patients with abnormal liver function should be treated with caution. See section on dosage and schedule for specific dosing guidelines.

HSRs, hypersensitivity reactions.

The most impressive clinical activity of the taxanes has been in patients with ovarian and breast cancers.221, 226, 227, 228, 229 Paclitaxel initially received regulatory approval for the treatment of patients with ovarian cancer after failure of first-line or subsequent chemotherapy.221 It was next incorporated into first-line treatment with a platinum compound in patients with suboptimally debulked stage III or IV ovarian cancer;the regimen demonstrated a survival advantage over standard treatment and received regulatory approval for this induction.221, 228 Regulatory approval was subsequently granted for patients with advanced breast cancer after failure of combination chemotherapy or at relapse within 6 months of adjuvant chemotherapy.221, 227 Recently, the combination of gemicitabine and paclitaxel demonstrated superior survival to paclitaxel alone in the first-line metastatic breast cancer setting, and received regulatory approval.230 Additionally, regulatory approval for treating patients with stage II breast cancer following standard doxorubicin-based adjuvant chemotherapy after the early results of a phase 3 study suggested that the addition of the taxane conferred superior progression-free and overall survival;however, a survival advantage was not apparent with longer follow-up.227, 231 Intriguing results have been noted following treatment of patients with stage II breast cancer with alternative taxane-containing regimens, particularly “dose-dense” regimens, but randomized studies to determine the effects of these approaches on survival are being carried out.227, 231 Paclitaxel also received approval in the United States for second-line treatment of Kaposi's sarcoma associated with AIDS and in combination with cisplatin as primary treatment of non–small cell lung cancer.233, 234

TABLE 11.4 PHARMACOKINETIC PARAMETERS OF THE TAXANES331,337,353,357

 

Paclitaxel

Docetaxel

 

1-hour infusion

3-hour infusion

1-hour infusion

1-hour infusion

Dosage (mg/m2/week)

100 (weekly)

175 (every 3 weeks)

35 (weekly)

75 (every 3 weeks)

Optimal pharmacokinetic model

Triexponential

Triexponential;saturable elimination and distribution above 175 mg/m2

Triexponential

Triexponential

Cpeak (_mol/L)

3.37

4.3

1.9

2.3

Clearance (L/hr/m2)

244

12.7

29.1

25.8

Vdss (L/m2)

99

67

T½γ (hr)

18.8

61.3

91.7

Protein binding (%)

>95%;albumin and α1-acid glycoprotein

>80to 95%;α1–acid glycoprotein, albumin, and lipoproteins

Primary mechanism of disposition

Hepatic metabolism and biliary excretion

Hepatic metabolism and biliary excretion

Cpeak, peak plasma concentration; T½γ, terminal phase half-life;Vdss, volume of distribution at steady-state.

Although docetaxel initially received regulatory approval for the treatment of patients with metastatic breast cancer that progressed on or relapsed after anthracycline-based chemotherapy, the indication was next broadened to a general second-line indication and, more recently, as first-line chemotherapy for locally advanced or metastatic breast cancer in combination with capecitabine.226, 227 More recently, regulatory approval was granted for docetaxel's use in combination with cyclophosphamide and doxorubicin in the adjuvant treatment of patients with local breast cancer following definitive local treatment.227 In non–small cell lung cancer, docetaxel initially received regulatory approval for treatment of unresectable, locally advanced or metastatic disease after demonstrating increased survival after failure of cisplatin-based therapy, and, more recently, regulatory approval was granted for docetaxel in combination with cisplatin as first-line treatment for such patients. Both paclitaxel and docetaxel have demonstrated notable activity in patients with hormone-refractory prostate cancer (HRPC) and regulatory approval was recently granted for docetaxel in combination with prednisone for this indication as the regimen has a survival advantage compared to mitoxantrone and prednisone.235, 236, 237, 239 It is important to note that the antitumor spectra for paclitaxel and docetaxel are identical, with activity noted in many other diverse cancers including head and neck, esophageal, gastric, endometrial, bladder, small cell lung, and germ cell carcinomas, and lymphoma and melanoma. The extent to which apparent differences in response rates, other end points, and regulatory indications granted between docetaxel and paclitaxel reflect differences in study design, dose, schedule, or inherent drug activities cannot be determined at this juncture.

Structures

The structures of paclitaxel, docetaxel, and their precursor 10-deacetylbaccatin III are shown in Figure 11.5. The taxanes are complex alkaloid esters, consisting of a 15-member taxane ring system linked to an unsualy four-member oxetane ring at positions C-4 and C-5.238, 239 The taxane rings of both paclitaxel and docetaxel, but not 10-deacetylbaccatin III, are linked to an ester at the C-13 position. Structure-function studies suggest that taxane analogs without this ester linkage interact minimally with mammalian tubulin, although they still stabilize microtubules of the amoeba Physarum polycephalum. Furthermore, the moieties at the C-2′ and C-3′ positions are essential for the unqiue antimicrotubule action of the taxanes. Acetyl substitution at C-2′ results in a substantial loss of activity. Neither the acetyl group at C-10 nor the phenyl group at C-5′ are required for in vitro activity, and the structures of paclitaxel and docetaxel differ in linkages at these positions.238, 240

Figure 11.5 Structures of the taxanes: paclitaxel (A) and docetaxel (B).

Mechanisms of Action

The unique mechanism of action for paclitaxel was initially defined by Schif and colleagues223, 224 and Manfredi and colleagues225 in 1979. The taxanes bind poorly to soluble tubulin, however, these agents bind directly and with high affinity to tubulin along the length of the microtubule. The binding sites are distinct from exchangeable GTP, colchicine, podophyllotoxin, and the vinca alkaloids, and the taxanes do not inhibit the binding of these agents to their respective sites. Photffinity studies have indicated that paclitaxel binds to the N-terminal 1-31 amino acids and residues 217-233 of the β-tubulin subunit, and the paclitaxel pharmacophore has been characterized.240, 241, 242 Cystallographic models of the β-tubulin N-terminus indicate that His 227 and Asp 224 are critical to binding the C-2 benzoyl side chain of paclitaxel, and modeling data also indicate that both paclitaxel and docetaxel bind to the interior surface of the microtubule lumen.7, 243, 244 Other antimicrotubule natural products with similar mechanisms of action, such as the epothilones and eleutherobins, occupy the same binding sites, albeit with an altered core and side chain.242, 244 Paclitaxel binds reversibly to microtubules reassembled in vitro with high affinity (Kd, 10 nmol), whereas the binding affinity for docetaxel, which is slightly more water soluble, is approximately 1.9-fold higher.2, 3, 4, 7, 225, 246, 247 It has been reported that tubulin assembly induced by docetaxel also proceeds with a critical protein concentration that is 2.1-fold lower than that of paclitaxel.246However, these differences, along with the higher potency of docetaxel, do not necessarily mean that docetaxel has a higher therapeutic index as greater potency may also portend more severe toxicity at identical drug concentrations in vivo. Furthermore, preclinical and clinical studies have been inconsistent about whether the taxanes are completely cross-resistant, possibly because these studies used dose schedules that are not equivalent.249, 250, 251

In contrast to the vinca alkaloids, the taxanes disrupt microtubule dynamics by reducing the critical tubulin concentration required for microtubule assembly and promoting both the nucleation and elongation phases of the polymerization reaction, which, in essence, stabilizes the microtubule against depolymerization and enhances polymerization.2, 3, 5, 18, 30, 32, 223, 224, 225, 251, 252, 253, 254, 255, 257, 258 Nevertheless, the vinca alkaloids and taxanes seem to produce similar disruptive effects on the spindle apparatus. Binding of the taxanes to their binding site on the inside of the microtubule stabilizes the microtubules and enhances tubulin polymerization, presumably by inducing a conformation change in tubulin, that, by an unknown mechanism, increases its affinity for neighboring tubulin molecules.2 In essence, these actions profoundly alter the tubulin dissociation rate constants at each end of the microtubule without affecting the association rate constants, thereby suppressing both treadmilling and dynamic instability. There is one paclitaxel binding site on each tubulin molecule of the microtubule, and the ability of paclitaxel to enhance polymerization is associated with nearly 1:1 stoichiometric binding of paclitaxel to tubulin in microtubules. At submicromolar concentrations that are readily achieved in the clinic, binding is stoichiometric and tubulin polymerization is enhanced. However, substoichiometric concentrations suppress microtubule dynamics without increasing the amount of polymerized tubulin.225, 253 The taxanes induce tubulin self-assembly into microtubules in the cold and in the absence of exogenous GTP and MAPs, which are normally required for these processes 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252 Furthermore, taxane-treated microtubules are highly stable, resisting depolymerization by cold, calcium ions, dilution, and depolymerizing agents. This stability inhibits the dynamic reorganization of the microtubule network, which is essential for many vital cell functions in mitosis and interphase.

The stoichiometry of taxane binding to microtubules in vitro greatly influences the nature of the perturbations of these agents on tubulin dynamics. Both stoichiometric and substoichiometric drug binding inhibit the proliferation of cells, principally by inducing a sustained mitotic block at the metaphase/anaphase boundary. At low concentrations (10 to 50 nmol/L), the binding of small numbers of paclitaxel molecules to microtubules reduce the rate and extent of microtubule shortening at their assembly (plus) ends.2, 3, 18, 27, 252 At higher concentrations (10 to 100 nmol/L), however, paclitaxel preferentially suppresses tubulin dynamics and induces a modest increase in microtubule length at the plus ends with negligible effect on dynamics at the minus ends.236 At paclitaxel concentrations ranging from 100 nmol/L to 1 µmol/L, which are readily achieved in cancer patients, growing and shortening rates are suppressed to the same extent, and microtubules remain in a state of attenuation. At very high concentrations (1 to 20 µmol/L), which are likely achieved intracellularly following administration of standard doses, the binding of paclitaxel to microtubules is saturated at a stoichiometry of 1 mole drug/mole tubulin, and the mass of microtubule polymer increases sharply as tubulin is recruited into the microtubules. In HeLa cells, mitosis is half-maximally blocked at 8 nmol/L paclitaxel, whereas polymer mass is half-maximally increased at 80 nmol/L, and there is no increase in microtubule polymer mass below 10 nmol/L.251 The taxanes inhibit tubulin dissociation at both microtubule ends, but the ends remain free for tubulin addition.208, 256 The taxanes also inhibit microtubule treadmilling.3, 52 Most studies with docetaxel indicate that it suppresses tubulin dynamics similar to paclitaxel, but the structural aspects of abnormal microtubules induced by paclitaxel and docetaxel may differ. In one report, for example, paclitaxel induced the formation of microtubules with predominantly 12 protofilaments, whereas 13 protofilaments are usually evident in docetaxel-induced microtubules.246

The taxanes delay or block mitosis at the metaphase/ anaphase boundary similar to the vinca alkaloids. At low concentrations (<10 nmol/L), mitosis is blocked with no concomitant increase in microtubule mass. Alterations in spindle organization also resemble those induced by the vinca alkaloids, suggesting that mitotic arrest is principally due to perturbations in microtubule dynamics. At higher concentrations (>100 nmol/L), microtubule mass is increased, mitosis is blocked, and large and dense spindle asters containing prominent bundles of stabilized microtubules are formed. With increasing taxane concentrations, the spindles become monopolar and the chromosomes condense, but do not congress.18, 224 Similar to the vinca alkaloids, even substoichometric taxane concentrations, which are sufficient to induce mitotic arrest without increasing microtubule mass, may induce apoptosis (see “Drug Resistance, Taxanes”).219,32, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269

Although the precise mechanisms by which microtubule disturbances lead to apoptosis are not clear, the taxanes interact with numerous substances and regulatory molecules. Microtubule disruption induces the tumor suppressor gene p53 and inhibitors of cyclin-dependent kinases (e.g., p21/Waf-1), and modulates several protein kinases.19, 32, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269 As a consequence, cells are arrested in G2/M, after which time they may either undergo apoptosis or traverse through G2/M and divide.270 Several different mechanisms that potentially link the mitotic arrest induced by the taxanes and other antimicrotubule agents to the initiating event in the intrinsic pathway of apoptosis have been characterized. These initiating events include activation of the pro-apoptotic molecules Bax and Bad and inactivation of the antiapoptotic regulators Bcl-2 and BclxL.271, 272, 273 Various kinases have been implicated in the phosphorylation of Bcl-2 induced by the taxanes and other antimicrotubule agents, including Jun N-terminal kinase (JNK) and its proapoptitic effector Bim, c-Raf, extracellular signal regulated kinase (ERK) 1/2, cyclin-dependent kinase (CDK)-1, cAMP-dependent protein kinase A, and protein kinase Cα.19, 272 Phosphorylation (inactivation) of Bcl-2 family members and phosphorylation of pro-apoptotic molecules (activation) stimulate the intrinsic pathway of apoptosis and downstream effector caspases.270, 274 Although the precise mechanism by which Bcl-2 is inactivated following drug treatment has not been elucidated, paclitaxel has been shown to bind to the ‘loop domain’ of Bcl-2, but it does not appear that Bcl-2 phosphorylation plays a pre-eminent role in inducing apoptosis in all types of cancer.275 The antimitotic effects of the taxanes and other antimicrotubule agents may be linked to apoptosis though other modulatory events such as the phosphorylation of the proapoptotic protein Bad by activating CDK1.273

The taxanes also perturb interphase microtubules in nonproliferating cells. Taxanes induce the formation of microtubule bundles, which resemble hoops and ribbons.274 Paclitaxel has also been reported to induce transcription factors and enzymes that mediate proliferation, apoptosis, and inflammation.262, 273, 274,275 The taxanes enhance the effects of ionizing radiation in vitro at clinically achievable concentrations (<50 nmol/L) and in vivo, which may related to the inhibition of cell-cycle progression in the G2 phase, which is the most radiosensitive phase of the cell cycle.276, 277, 278, 279, 280, 281, 282 In angiogenesis inhibition assays, they also inhibit parameters indicative of angiogenesis at concentrations below those that induce cytotoxicity, but the contribution of these effects on malignant angiogenesis to the overall antitumor actions of the taxanes is not clear.283, 284, 285

The taxanes have been demonstrated to induce many other cellular effects that may or may not relate to their disruptive effects on microtubule dynamics. Although the taxanes primarily block cell-cycle traverse in the mitotic phases, the agents prevents G0 to S phase transition in both normal and malignant cells.276, 286 Explanations that have been proposed to account for the nonmitotic actions of the taxanes include disruptive effects on tubulin in the cell membrane, the interphase cytoskeleton, and microtubules that are involved in growth factor signaling.17, 287

The taxanes also inhibit specific functions in nonmalignant tissues, which may be mediated through their disruptive effects on microtubule dynamics.20 For example, paclitaxel inhibits relevant morphologic and biochemical processes in human neutrophils, including chemotaxis, migration, spreading, polarization, hydrogen peroxide generation, and killing of phagocytized microorganisms.17 In addition, paclitaxel antagonizes the effects of microtubule-disrupting drugs on lymphocyte function and cAMP metabolism, and inhibits the proliferation of stimulated human lymphocytes.17 Paclitaxel mimics the effects of endotoxic bacterial lipopolysaccharide on macrophages, which results in a rapid decrement in tumor necrosis factor-α (TNF-α) receptors and TNF-α release.277, 288 The agent also induces expression of the gene for TNF-α, but these activities are not related to paclitaxel's disruptive effects on microtubule assembly, which raises the issue of the role of cytokines in the antitumor activities of the taxanes.277 Additionally, paclitaxel has been demonstrated to inhibit chorioretinal fibroblast proliferation and contractility in an in vitro model of proliferative vitreoretinopathy, as well as neointimal smooth muscle cell proliferation after angioplasty in a rat model.289, 290 Cardiac arterial stents coated with paclitaxel received regulatory approval in the United States and elsewhere in 2003 because of a significantly decreased incidence of restenosis from fibroblast proliferation and intimal hyperplasia.291 Finally, paclitaxel inhibits secretory functions in many specialized cells, such as insulin secretion in isolated rat islets of Langerhans, protein secretion in rat hepatocytes, and the nicotinic receptor-stimulated release of catecholamines from chromaffin cells of the adrenal medulla.17

Mechanisms of Resistance

The MDR phenotype, which is mediated by several members of the ABC transporter family and confers cross-resistance to a wide range of xenobiotics (as discussed previously in “Mechanisms of Resistance, Vinca Alkaloids”) is the best characterized mechanism of resistance to the taxanes. The most important ABC transporters with respect to conferring taxane resistance is P-gp or the MDR1 encoded gene product MDR1 (ABC Subfamily B1;ABCB1) and MDR2 (ABC Subfamily ABCB4).92, 254, 292 In contrast to the vinca alkaloids, ABCC1 (MRP1) and ABCC2 (MRP2) do not appear to be involved in taxane transport.293, 294Low-level taxane resistance also appears to be conferred by the bile salt export protein (BSEP, also known as ABCC11).92 Early clinical observations of the antitumor profile of the taxanes, particularly in women with breast cancer who respond to the taxanes following the development of progressive disease while receiving treatment with the anthracyclines, indicate that cross-resistance to the taxanes and anthracycline is incomplete, but the role of MDR as a major cause of anthracycline resistance is not clear. Similar to the vinca alkaloids, cells with taxane resistance and the MDR phenotype can be reversed by many classes of drugs, including the calcium channel blockers, tamoxifen, cyclosporine A, and antiarrhythmic agents.92, 295, 296 In fact, plasma concentrations of the principal component of the vehicles used to formulate paclitaxel and docetaxel, polyoxyethylated castor oil and polysorbate-80, respectively, can also reverse taxane resistance.297, 298 However, the plasma concentrations of polyoxyethylated castor oil achieved with paclitaxel on clinically relevant dose schedules are sufficient to reverse MDR, whereas sufficient modulatory concentrations of polysorbate-80 are not achieved with docetaxel. Strategies aimed at reversing taxane resistance with various transporter substrates in the clinic have resulted in low impact at best; however, the interpretation of these results is confounded by the effects of these agents, particularly those that are MDR substrates, on taxane clearance and toxicity.295, 296, 298, 299 Nevertheless, MDR modulators, including verapamil, cyclosporine A, VX-710, the nonimmunomodulatory cyclosporine analogue PSC 833, and other agents that do not affect taxane pharmacokinetics and toxicity, do not appear to significantly enhance antitumor activity.92, 296, 299

Several taxane-resistant mutant cell lines that have structurally altered α-tubulin and β-tubulin proteins and an impaired ability to polymerize into microtubules have been identified (discussed previously in “Mechanisms of Resistance, Vinca Alkaloids”).9, 109, 110, 111, 129, 262, 300, 301, 302, 303, 304 Mutants with “hypostable” microtubules exhibit collateral sensitivity to the vinca alkaloids. Paclitaxel-resistant Chinese hamster ovary cells with mutated β-tubulin alleles that encode the putative taxane binding sites, specifically, leucine moieties at positions 215, 217, and 228 mutated to histidine, arginine, or phenylalanine, have been described.305, 306 Low-level expression led to resistance, whereas high-level expression of any of these mutations caused impairment of assembly, cell-cycle arrest, and failure to proliferate.306

A number of cell lines resistant to tubulin-binding agents, including the taxanes, have been shown to have alterations in tubulin content, tubulin isotype profiles, and tubulin polymerization dynamics.262, 302, 304, 307, 308 For example, paclitaxel-resistant tumors have been demonstrated to have significantly higher levels of class I, III, and IVa isotypes of β-tubulin.19, 112, 304, 309 Higher intratumoral levels of the β-III isotype, which is a minor component of cellular β-tubulin that increases the dynamic instability of microtubules, impedes microtubule assembly, and confers resistance to taxanes, has been demonstrated in tumor biopsies sampled from patients with taxane-resistant malignancies and cell lines with acquired drug resistance.129, 310 Further proof that β-III tubulin levels relate to taxane resistance is provided by experiments that demonstrate that antisense oligonucleotides to β-tubulin class III RNA decrease protein expression and increase drug sensitivity in taxane resistant cells.311 Others have found that mutations affecting the β-tubulin class I genes lead to drug resistance.19

Mutations of tubulin isotype genes, gene amplifications, and isotype switching have also been reported in taxane-resistant cell lines.109, 262, 263, 303, 306, 308,312, 313, 314 Although clinical data from patients with non–small cell lung cancer indicate that mutations in β-tubulin are associated with taxane resistance, these observations have not been confirmed, and several reports have failed to demonstrate that β-tubulin to be a clinically relevant determinant of paclitaxel resistance in breast and ovarian cancers.19, 312, 313 Furthermore, the positive findings may have been caused by amplification of pseudogenes.19, 312, 313 Higher levels of class III β-tubulin RNA levels have also been reported in non–small cell lung cancers of patients who did not respond to taxance-based treatment, which is in line with in vitro findings.315, 316

Growth factor signaling may contribute to taxane resistance by raising the cell's threshold for apoptosis induced by the taxanes and other antimicrotubule agents. For example, insulin-like growth factor I has been demonstrated to protect responsive breast cancer cell lines from anthracyclines and taxanes, possibly by activating the phosphatidylinositol 3-kinase (PI3K) pathway and inducing phosphorylation (inactivation) of antiapoptotic factors.317 Other mediators that may influence the cell's threshold for drug-induced apoptosis include p53, erbB2, auora 2-kinase, survivin, and BRAC1. The centromere-assocated serine/threonine kinase, aurora 2-kinase, which is involved in centrosome separation, biopolar spindle formation, and chromosomal kinetochore attachment to the mitotic spindle, appears to override the mitotitc assembly checkpoint and induce taxane resistance.19 In addition, the overexpresssion of survivin, a member of the inhibitor of apoptosis family of proteins, inhibits caspase activity and apoptosis induced by antimicrotubule agents.19 The disruption of the tumor-suppressor gene, BRAC1, which is implicated in maintaining genomic stability through DNA repair and involved in hereditary breast and ovarian cancers, appears to play a role in conferring resistance to paclitaxel and the inducible expression of BRAC1 may enhance paclitaxel-induced apoptosis.19 The mutational loss of the p53 tumor suppressor does not confer resistance to paclitaxel and other microtubule polymerizing agents in contrast to DNA disruptive agents.19 Also, there was no relationship evident between paclitaxel sensitivity and p53 status in the National Cancer Institute's 60-tumor type-specific drug screening panel.19 Additionally, cells lacking wild-type p53 display increased sensitivity to paclitaxel.19 MAPs have been implicated in mechanisms of resistance to apoptosis induced by the taxanes and other antimicrotubule agents, as illustrated by the observation that MAP4, which is negatively regulated by wild-type p53, increases sensitivity to paclitaxel.318, 319 The suppression of dynamic instability by low concentrations of microtubule polymerizing agents may also enhance the nuclear accumulation of p53 and the induction of proapoptotic p53–up-regulated modulator of apoptosis.19 This may represent a p53-dependent mechanism of apoptosis induced by antimicrotubule agents in cells that harbor functional p53.19 Finally, overexpression of p21, a downstream effector of p53, appears to impede cell cycle traverse in G2, thereby blocking progression into the more drug-vulnerable mitotic phase and decreasing taxane sensitivity.320, 321

Transfection of cells with erbB-2, a member of the epidermal growth factor receptor family that is amplified and overexpressed in approximately 30% of breast cancers, increases taxane resistance, and high expression of erbB-2 in vitro relates to taxane resistance.322, 323 Overexpression of erbB2 can also inhibit CDK1 either by inducing p21, which participates in the G2/M checkpoint and contributes to resistance to apoptosis induced by antimicrotubule agents, or directly phosphorylating (inactivating) CDK1, which may block taxane-mediated entry into mitosis and apoptosis.19 Consistent with this relationship, down-regulation of erbB2 by the anti–erbB2 antibody trastuzimab sensitizes breast cancer cells to the taxanes, and the treatment of women with erbB2 overexpressing breast cancer with traztuzimab combined with paclitaxel increases survival compared to paclitaxel alone.322, 324 Nevertheless, the presence of erbB2 amplification was demonstrated not to adversely influence response to first-line chemotherapy with either epirubicin-paclitaxel or epirubicin-cyclophosphamide.325Furthermore, the taxane-containing regimen may preferentially benefit women with erbB2-expressing breast cancer.

Clinical Pharmacology

Analytical Assays

The earliest analytical assays used to measure paclitaxel concentrations in biologic samples were biochemical assays that exploited the ability of paclitaxel to induce tubulin to form cold-resistant polymers that hydrolyze GTP at 0°C;however, such as assays lacked requisite sensitivitiy (0.1 µmol/L) to measure low plasma concentrations achieved in clinical trials and were too cumbersome for monitoring large numbers of clinical samples.326 Immunologic assays, including indirect competitive inhibition enzyme immunoassays and ELISAs that were developed for detecting taxanes in plant extracts were highly sensitive (0.3 nmol/L) and amenable to high-throughput procedures, but the degree of cross-reactivity of the antibodies to the taxanes, their metabolites, and other moieties are not known.327 The earliest chromatographic separation methods, including HPLC with ultraviolet detection, had variable extraction efficiencies, suboptimal lower limits of sensitivity (≥50 nmol/L), and other assay performance characteristics, which rendered them inadequate for monitoring plasma levels in patients receiving low doses or prolonged infusions. More sensitive HPLC assays, particularly those using tandem mass spectroscopy and solid phase extraction, can detect paclitaxel and docetaxel concentrations in the low nanomolar to picomolar range in minute quantities of plasma ((0.05 mL) and several are capable of simultaneously measuring metabolites.298, 328, 329

Pharmacokinetics

The oral bioavailability of both paclitaxel and docetaxel is poor, owing in part to the constitutive overexpression of P-gp and other ABC transporters by enterocytes and/or first-pass metabolism in the liver and/or intestines. Nevertheless, biologically relevant plasma concentrations can be achieved if the taxanes are administered orally with oral modulators of ABC transporters and/or cytochrome P-450 mixed-function oxidases such as cyclosporin.298, 330 Rapid, avid, and protracted drug distribution and binding to all tissues except for central nervous system tissue result in large volumes of distribution, high clearance rates, short distribution t½ values, and long terminal t½ values.

Paclitaxel

The pharmacokinetics of paclitaxel on both long and short administration schedules have been characterized (Table 3). In early studies that principally evaluated prolonged (6- and 24-hour) schedules, substantial interpatient variability was noted, and nonlinear, dose-dependent behavior was not observed.330In these studies, drug disposition was characterized as a biphasic process, with values for alpha and beta t1/2 values averaging 20 minutes and 6 hours, respectively. However, more recent studies of shorter administration schedules, especially a 3-hour infusion, indicate that the pharmacokinetic behavior of paclitaxel is nonlinear.298, 331, 332, 333, 334 Nonlinearity occurs with all administration schedules, but it is more apparent with shorter infusions that result in higher plasma paclitaxel concentrations that more saturate both drug elimination and tissue distribution processes. Both saturable distribution and elimination may be, in part, responsible for paclitaxel's nonlinear behavior. Tissue distribution becomes saturated at lower drug concentrations (achieved with paclitaxel doses <175 mg/m2 over 3 hours) compared with elimination processes that are effectively saturated at higher concentrations (achieved with paclitaxel doses >175 mg/m2 over 3 hours). The use of shorter infusion schedules also results in higher plasma concentrations of paclitaxel's polyoxyethylated castor oil vehicle, which may be responsible for an appearance of nonlinearity, termed pseudononlinearity.335, 336 A true nonlinear profile may have several important clinical implications, particularly regarding dose modifications at doses associated with nonlinearity because dose escalation may result in a disproportionate increase in drug exposure and hence toxicity, whereas dose reductions may result in a disproportionate decrease in drug exposure, thereby decreasing antitumor activity. Interestingly, shorter paclitaxel infusion schedules are also associated with reduced clearance of the polyoxyethylated castor oil vehicle and reduced exposure to unbound paclitaxel, which may explain the lower incidence of hematologic toxicity and higher incidence of hypersensitivity reactions with shorter infusions.337

The volume of distribution of paclitaxel is much larger than that of total body water, which is likely the result of extensive drug distribution and binding to plasma proteins and other tissue elements, particularly tubulin. In addition, plasma protein binding is high (>95%) and readily reversible.298 At clinically-relevant concentrations (0.1 to 0.6 µmol/L), protein binding is concentration-independent, which may be attributable to nonspecific hydrophobic binding. Despite extensive binding to plasma proteins, paclitaxel is readily eliminated from the plasma compartment, a finding that suggests lower-affinity, reversible binding. Albumin and α1-acid glycoprotein contribute equally to the binding, with a minor contribution from lipoproteins.338, 339 None of the drugs that are commonly administered with paclitaxel, including ranitidine, dexamethasone, diphenhydramine, doxorubicin, 5-fluorouracil, and cisplatin, appear to substantially alter protein binding.338 Drug binding to platelets is extensive and saturable, whereas binding to red blood cells is insignificant.368 Animal distribution studies with radiolabeled paclitaxel indicate extensive drug uptake and retention by virtually all tissues, except “tumor sanctuary sites” such as the central nervous system and testes.340

In addition, clearance is significantly related to body surface area, providing a rationale for dosing based on this measurement.341 In humans, peak plasma concentrations achieved with 3- to 96- hour infusions (>0.05 to 10 µmol/L) and drug concentrations in third-space fluid collections, such as ascites (>0.1 µmol/L), are capable of inducing significant biologic effects in vitro, but drug penetration into the unperturbed central nervous system is negligible.298, 331,332, 333, 334, 336, 337, 341

Paclitaxel disposition occurs predominately by cytochrome P-450 mixed function oxidase metabolism in the liver followed by the excretion of both paclitaxel and metabolites into the bile.330, 331 342, 343, 344, 345, 346, 347 Ninety-eight percent of radioactivity is recovered from feces collected for 6 days after rats are treated with radiolabeled paclitaxel, and approximately 71% of an administered dose of paclitaxel is excreted in the feces over 5 days as either parent compound or metabolites in humans, with 6α- hydroxypaclitaxel being the largest component and accounting for 26% of the dose. Only 5% is unchanged paclitaxel. Renal clearance of paclitaxel and metabolites is minimal, accounting for 14% of the administered dose.298 In humans, cytochrome P-450 mixed-function oxidases, specifically the isoenzymes CYP2C8 and CYP3A4, are responsible for the bulk of drug disposition. All human paclitaxel metabolites that have been identified are hydroxylated derivatives with intact side chains at taxane ring positions C-2 and C-13, whereas low concentrations of baccatin III, which lacks the side chain at position C-13, are found in rat bile.343, 348 The major metabolites in human plasma and bile include 6α- hydroxypaclitaxel, a product of CYP2C8;p-hydroxyphenyl-C3′-paclitaxel, a product of CYP3A4;and a dihydroxymetabolite (6α- and C3′-dihydroxypaclitaxel). There is considerable interindividual variability in the qualitative and quantitative aspects of taxane metabolism, which can be attributed to pharmacogenetic differences in P-450 metabolism and concurrent medications that variably alter metabolism.298, 334, 346, 348 The metabolites are much less active against L1210 leukemia than paclitaxel, but several are as active as paclitaxel in stabilizing microtubules against disassembly in a cell-free system. One possible explanation for this discrepancy is that the cell does not take up these hydroxylated metabolites, which are more polar than paclitaxel.

Several pharmacokinetic parameters indicative of drug exposure have been related to the various principal toxicities of paclitaxel, the most important of which is the relationship between the severity of neutropenia and the duration of drug exposure above biologically relevant plasma concentrations ranging from 0.05 to 0.1 µmol/L.298, 331, 333, 334, 347, 349 However, a prospective analysis of pharmacokinetic determinants of outcome in patients with advanced non–small cell lung cancer treated with cisplatin combined with paclitaxel at either 135 or 250 mg/m2 over 24 hours showed that the magnitude of the steady-state plasma paclitaxel concentration correlated poorly with antitumor activity, disease-free survival, and overall survival.350 In randomized trials evaluating the effects of paclitaxel dose on outcome in patients with advanced ovarian, non–small cell lung, breast, and head and neck, doses above 175 mg/m2 resulted in neither increased progression-free or overall survival

Docetaxel

The pharmacokinetics of docetaxel on a 1-hour schedule are linear at doses of 115 mg/m2 or less and optimally fit a three-compartment model.226, 351, 352,353, 354, 355, 356, 357 Terminal t1/2values ranging from 11.1 to 18.5 hours have been reported. In one population study, plasma concentration data were optimally fit by a three-compartment model, and the following pharmacokinetic parameters were generated: t½γ of 12.4 hours, clearance of 1 L/hr per square meter, and steady-state volume of distribution of 74 L/m2.354, 355, 357 The most important determinants of docetaxel clearance were the body surface area, hepatic function, and plasma α1–acid glycoprotein concentration, whereas age and albumin level had significant influences on clearance. As with paclitaxel, plasma protein binding is high (>85 to 95%), and binding is primarily to α1–acid glycoprotein, albumin, and lipoproteins.226, 351, 352, 353, 354, 355, 356, 357Higher free fraction values relate to low α1–acid glycoprotein concentrations and may portend greater toxicity. As with paclitaxel, docetaxel is widely distributed and avidly bound in all tissues except the central nervous system.355, 357, 358 In both dogs and mice treated with radiolabeled drug, fecal excretion accounts for 70 to 80% of total radioactivity, whereas urinary excretion accounts for 10% or less.385, 387 In rats and dogs, tissue-distribution studies using [14C]docetaxel have demonstrated a rapid initial distribution phase of plasma radioactivity with an apparent t½ of 10 minutes.355 In mice, autoradiographic studies indicate that docetaxel rapidly accumulates in almost all tissues except for the central nervous system.355 Immediately after treatment, tissue uptake of radioactivity is highest in the liver, bile, and intestines, a finding that is consistent with substantial hepatobiliary extraction and excretion. High levels of radioactivity are also found in the stomach, which indicates the possibility of gastric excretion, as well as in the spleen, bone marrow, myocardium, skeletal muscles, and pancreas.

The hepatic cytochrome P-450 mixed-function oxidase isoenzyme CYP3A, the activity of which, in adults, is represented by the combined activities of CYP3A4, CYP3A5, CYP3A7, and CYP3A43 is responsible for the bulk of docetaxel metabolism.386, 387, 390 However, CYP3A4, and CYP3A5, to a lesser extent, confer the highest relative contributions to overall CYP3A activity and are primarily involved in biotransformation that, in contrast to paclitaxel, principally affects the C13 side chain and not the taxane ring.359, 360, 361 CYP2B, and CYP1A also appear to play major roles in biotransformation. The main metabolic pathway consists of oxidation of the tertiary butyl group on the side chain at the C-13 position of the taxane ring, as well as cyclization of the side chain all metabolites appear to maintain their 10-deacetylbaccatin III or 7-epi isomer structural backbones. These metabolites seem to be much less active than docetaxel.

The main pharmacokinetic determinants of toxicity, particularly the principal toxicity neutropenia, are drug exposure and the time that plasma concentrations exceed biologically relevant concentrations.357, 359, 360, 361 A population pharmacodynamic analysis of determinants of outcome in phase 2 trials of docetaxel in patients with metastatic breast cancer revealed that the most important positive determinants of a response and progression-free survival are low pretreatment plasma concentration of α1–acid glycoprotein, number of prior chemotherapeutic regimens, and number of disease sites, whereas both drug exposure and the pretreatment plasma concentration of α1–acid glycoprotein were strong positive determinants of time to progression in patients with advanced lung cancer.352 Conversely, the pretreatment plasma level of α1–acid glycoprotein was negatively, albeit significantly, related to the probability of experiencing both severe neutropenia and febrile neutropenia. α1–Acid glycoprotein has also been demonstrated to be a principal determinant of interindividual variability in docetaxel clearance and one of the main predictors of docetaxel clearance.298, 352

Drug Interactions

Both sequence-dependent pharmacokinetic and toxicologic interactions between paclitaxel and several other chemotherapy agents have been noted, but the number of clinically significant drug-drug interactions has been surprisingly low in light of the importance of cytochrome P-450 pathways in drug disposition.298, 362 The sequence of cisplatin followed by paclitaxel (24-hour schedule) induces more profound neutropenia than the reverse sequence, which is explained by a 33% reduction in the clearance of paclitaxel after cisplatin.362, 363, 364 The least toxic sequence—paclitaxel before cisplatin—was demonstrated to induce more cytotoxicity in vitro;therefore, this drug sequence was selected for clinical development.362, 363, 364 As expected, however, sequence dependence does not appear to be a clinically relevant phenomenon on shorter schedules. Treatment with paclitaxel on either a 3- or 24-hour schedule followed by carboplatin was demonstrated to produce equivalent neutropenia and less thrombocytopenia as compared with carboplatin as a single agent, which is not explained by pharmacokinetic interactions.365, 366 Although sequence dependence has not been noted with combinations of carboplatin and paclitaxel, which induce less thrombocytopenia than comparable single-agent doses of carboplatin, other paclitaxel-based chemotherapy combinations, most notably those involving the anthracyclines, are associated with this phenomena.362, 363, 364, 365, 366, 367 Both neutropenia and mucositis are more severe when paclitaxel on a 24-hour schedule is administered before doxorubicin, compared with the reverse sequence, which is most likely caused by an approximately 32% reduction in the clearance rates of both doxorubicin and doxorubicinol when doxorubicin is administered after paclitaxel.368, 369 Although neither sequence-dependent pharmacologic interactions nor toxicologic interactions between doxorubicin and paclitaxel on a shorter (3-hour) schedule have been noted, pharmacologic interactions occur with both sequences, and combined treatment with paclitaxel (3-hour schedule) and doxorubicin as a bolus infusion has been associated with a higher incidence of cardiotoxicity than would have been expected from an equivalent cumulative doxorubicin dose given without paclitaxel (see “Cardiac”).368, 369, 370 The precise etiology for these interactions is unclear. The pharmacokinetic interactions may not be of sufficient magnitude to account for the enhanced cardiotoxicity of the combination and there are experimental data indicating that paclitaxel enhances the metabolism of doxorubicin to cardiotoxic metabolites, such as doxorubicinol, in cardiomyoctes.370 Docetaxel does not appear to influence doxorubicin pharmacokinetics, but there are experimental data suggesting that, as with paclitaxel, docetaxel can enhance the metabolism of doxorubicin to toxic species in the human heart.370 Similar decrements in the clearance of epirubicin and its metabolites have been noted in studies of paclitaxel combined with epirubicin, but cardiotoxicity does not appear to be enhanced.371 Competition for the hepatic or biliary P-gp transport of the anthracyclines with paclitaxel or its polyoxyethylated castor oil vehicle (or both) is an alternate explanation.297, 349, 362 Interestingly, similar effects have not been noted with docetaxel, which is not formulated in polyoxyethylated castor oil. Hematologic toxicity has also been more profound with the sequence of cyclophosphamide before paclitaxel (24-hour schedule) than the reverse sequence.372 In human tumor xenografts, both paclitaxel and docetaxel have been demonstrated to induce thymidine phosphorylase activity, which may increase the metabolic activation of the oral fluoropyrimidine prodrug capecitabine.373

Drug interactions may also result from the effects of other classes of drugs on the cytochrome P-450–dependent metabolism of the taxanes. Various inducers of cytochrome P-450 mixed-function oxidases, such as the anticonvulsants phenytoin and phenobarbital, accelerate the metabolism of both paclitaxel and docetaxel in human microsomes in vitro and in both children and adults who are concurrently receiving treatment with these anticonvulsants, as manifested by rapid drug clearance and tolerance of high drug doses.277, 298, 343, 344, 345, 346, 359, 360, 362, 374, 375, 376, 377 There is preclinical evidence to suggest that docetaxel has markedly reduced propensity to cause drug interactions that may entail hepatic CYP3A4 induction.378 Conversely, many types of agents that inhibit cytochrome P-450 mixed-function oxidases, such as orphenadrine, erythromycin, cimetidine, testosterone, ketoconazole, fluconazole, midazolam, polyoxyethylated castor oil, and corticosteroids, interfere with the metabolism of paclitaxel and docetaxel in human microsomes in vitro;however, the inhibitory concentrations of these agents exceed those achieved in clinical practice, and the clinical relevance of these findings is not known.27, 298, 359, 343,344, 345, 346, 359, 360, 374, 375, 376, 377 With regard to potential interactions between ketoconazole and the taxanes, inconsistent conclusions have been reached although docetaxel exposure has been demonstrated to be increased in a high proportion of patients receiving concurrent ketoconazole.379 Besides the potent inhibitors of CYP3A listed previously, other well-established inhibitors and inducers of CYP3A, including grapefruit juice and herbal products (e.g., St. John's wort and Echinacea), may potentially induce pharmacokinetic interactions with the taxanes. Although there has been concern that the use of different H2-receptor antagonists with variable cytochrome P-450 inhibitory activities as components of premedication regimens may differentially affect drug clearance and hence toxicity, neither toxicologic nor pharmacologic differences between the agents were noted in a randomized clinical trial.380 As previously discussed (see “Pharmacokinetics”), the considerable interindividual variability in the relative amounts of paclitaxel metabolites may, in part, be the result of isoenzyme activity and induction or inhibition as a result of drug interactions.346, 348, 376 For example, prolonged treatment with corticosteroids induces CYP3A4 and leads to increased dihydroxypaclitaxel, whereas biricodar, an inhibitor of MDR-1 and multidrug resistance protein, inhibits the same enzyme, delays clearance, and significantly lowers the maximum tolerated dosage.296, 348 In addition, interactions between warfarin and the taxanes, possibly because of protein binding displacement effects, have been reported.381

Concern has also arisen that H2-histamine antagonist premedications may be an important source of drug interactions. Use of these agents with the taxanes may produce variable pharmacologic and toxicologic effects because these agents differentially inhibit cytochrome P-450 metabolism, with cimetidine being the most potent inhibitor. However, H2 histamine antagonists do not appear to alter the metabolism and pharmacologic disposition of the taxanes in animal and in vitro studies.382, 383 In addition, the results of a clinical trial in which patients were randomized to receive either cimetidine or famotidine premedication before their first course of paclitaxel and then crossed over to the alternate premedication during their second course have failed to show significant toxicologic and pharmacologic differences between these H2 histamine antagonists.380

Dose and Schedule

Paclitaxel

The development of effective premedication regimens associated with a decreased incidence of major hypersensitivity reactions led to evaluations of paclitaxel on a broad range of schedules. Although paclitaxel, 135 mg/m2 over 24 hours was initially approved for patients with refractory and recurrent ovarian cancer, regulatory approval was subsequently obtained for paclitaxel, 175 mg/m2 on a 3-hour schedule. In patients with advanced breast and ovarian cancers, the cumulative body of randomized study results indicates that both schedules are equivalent, particularly with regard to event-free survival and overall survival, although response rates have occasionally been higher with the 24-hour infusion. However, weekly “dose-dense” regimens have also been associated with intriguing activity.231, 384, 385, 386, 386, 387, 388, 389

Based on in vitro studies, which indicated that the duration of exposure above a biologically relevant threshold is one of the most important determinants of cytotoxicity, more protracted infusion schedules were evaluated.221, 384 Although intriguing results were initially obtained with a 96-hour infusion schedule in patients with advanced breast cancer and non-Hodgkin's lymphoma, there is no clear evidence that protracted schedules are superior to shorter schedules with regard to efficacy, and toxicities, particularly myelosuppression and mucositis, appear to more somewhat greater.384, 386, 388, 389, 390 The lack of clearly superior results with protracted schedules in vivo is likely because of the extensive and rapid distribution of the taxanes to peripheral tissues and, more importantly, the avid and protracted tissue binding of these agents, whereas the agents are washed out from cells in tissue culture. There has also been considerable interest in intermittent schedules, particularly those in which paclitaxel is administered as a 1-hour infusion weekly, which results in substantially less myelosuppression than every 3-week schedules.231, 385, 387, 391, 392 Furthermore, there have been reports of impressive and superior activity of weekly compared with every 3-week schedules in several disease settings, particularly in treatment of metastatic breast cancer patients.387, 393 However, there is no convincing evidence that weekly treatment results in robust activity in tumors unresponsive to the taxanes on every 3-week schedules. Nevertheless, the weekly schedule may be advantageous for patients who are at high risk of developing severe myelosuppression, but there appears to be a higher incidence of neuromuscular effects. Paclitaxel is generally administered every 3 weeks at a dose of 175 mg/m2 over 3 hours. Alternatively, 135 to 175 mg/m2 over 24 hours every 3 weeks is a less common dose-schedule. Several phase 3 studies in patients with advanced lung, head and neck, ovarian, and breast cancers have consistently failed to show that paclitaxel doses greater than 135 to 175 mg/m2 on a 24-hour schedule or greater than 175 mg/m2 on a 3-hour schedule confer superior efficacy than conventional doses.384, 394, 395 The following doses have been recommended on less conventional schedules: 200 mg/m2 over 1 hour as either a single dose or three divided doses every 3 weeks;140 mg/m2 over 96 hours every 3 weeks;and 80 to 100 mg/m2 weekly. The most common schedules evaluated in patients with AIDS-associated Kaposi's sarcoma are paclitaxel, 135 mg/m2 over 3 or 24 hours every 3 weeks, and 100 mg/m2 every 2 weeks.396 Following intracavitary administration, paclitaxel concentrations in the peritoneal and pleural cavities are several orders of magnitude greater than plasma concentrations, which are biologically relevant, and the results of a single randomized trial indicate that the administration of intraperitoneal paclitaxel in conjuction with carboplatin and paclitaxel administered intravenously confers a survival advantage in previously untreated women with optimally debulked advanced ovarian cancer.397, 398, 399

The following premedication is recommended to prevent major hypersensitivity reactions: dexamethasone, 20 mg orally or intravenously, 12 and 6 hours before treatment;an H1-receptor antagonist (such as diphenhydramine, 50 mg intravenously) 30 minutes before treatment;and an H2-receptor antagonist (such as cimetidine, 300 mg;famotidine, 20 mg;or ranitidine, 150 mg intravenously) 30 minutes before treatment. A single dose of a corticosteroid (dexamethasone, 20 mg intravenously) administered 30 minutes before treatment also appears to confer somewhat effective prophylaxis of major hypersensitivity reactions, however, the relative merits of this schedule is not known.400, 401 Contact of paclitaxel with plasticized polyvinyl chloride equipment or devices must be avoided because of the risk of patient exposures to plasticizers that may be leached from polyvinyl chloride infusion bags or sets. Paclitaxel solutions should be diluted and stored in glass or polypropylene bottles or suitable plastic bags (polypropylene or polyolefin) and administered through polyethylene-lined administration sets that include an in-line filter with a microporous membrane not greater than 0.22 µm.

The extensive involvement of hepatic metabolism and biliary excretion in the disposition of paclitaxel, similar to that of other anticancer drugs, such as the vinca alkaloids, in which dose modifications are required indicates that doses should be modified in patients with hepatic dysfunction. Although official recommendations have not been formulated, prospective evaluations indicate that patients with moderate-to-severe elevations in serum concentrations of hepatocellular enzymes or bilirubin (or both) are more likely to develop severe toxicity than patients without hepatic dysfunction.402, 403 Therefore, it is prudent to reduce paclitaxel doses by at least 50% in patients with moderate or severe hepatic excretory dysfunction (hyperbilirubinemia) or significant elevations in hepatic transaminases. Renal clearance contributes minimally to overall clearance (5 to 10%), and even patients with severe renal dysfunction do not appear to require dose modification.404 Based on the pharmacologic behavior, particularly the wide distributive properties of the taxanes, dose modifications are not required solely for peripheral edema and third-space fluid collections.

Docetaxel

Docetaxel is most commonly administered at a dose of 75 mg/m2 over 1 hour every 3 weeks but regulatory approval was granted in the United States for a dose range of 60 to 100 mg/m2 over 1 hour in patients with breast and non–small cell lung cancers, respectively;much less data are available for patients treated at 60 mg/m2.226, 405 The most common dose schedule of docetaxel used as a single-agent and in combination regimens is 75 mg/m2 over 1 hour. Although some untreated or minimally pretreated patients generally tolerate docetaxel at a dose of 100 mg/m2 without severe toxicity, tolerance is poorer in more heavily pretreated patients in whom 75 mg/m2 is a much more reasonable from a toxicologic perspective.405 Hematologic toxicity is much less than with conventional dose schedules; however, weekly administration schedules have been associated with a higher incidence of cumulative asthenia and neurotoxicity, particularly with docetaxel doses exceeding 36 mg/m2 per week.391, 392 Despite the use of a polysorbate 80 formulation instead of polyoxyethylated castor oil, which is used to formulate paclitaxel, an unacceptably high rate of major hypersensitivity reactions and profound fluid retention in patients who did not receive premedication has led to the development of several effective premedication regimens, the most popular of which is dexamethasone, 8 mg orally twice daily for 3 or 5 days starting 1 or 2 days, respectively, before docetaxel, with or without both H1-receptor and H2-receptor antagonists given 30 minutes before docetaxel.406, 407

In a retrospective review of docetaxel pharmacokinetics and toxicity in patients without hyperbilirubinemia, clearance was reduced by approximately 25% in patients with elevations in serum concentrations of both hepatic transaminases (1.5-fold or greater) and alkaline phosphatase (2.5-fold or greater), regardless of whether the elevations are the result of hepatic metastases.352, 353 Therefore, dose reductions by at least 25% are recommended for such patients. However, greater dose reductions (50% or greater) may be required in patients who have moderate or severe hepatic excretory dysfunction (hyperbilirubinemia).402 As with paclitaxel (discussed previously in “Dose, and Schedule, Paclitaxel”), there is no rationale for dose modification solely for renal deficiency or third-space fluid accumulation (or both). Also similar to the case with paclitaxel, glass bottles or polypropylene or polyolefin plastic products should be used for preparation and storage, and docetaxel should be administered through polyethylene-lined administration sets.

Toxicity

Despite having similar structural features, the toxicity spectra of paclitaxel and docetaxel do not completely overlap. Myelosuppression, primarily neutropenia, is the principal toxicity of both agents, but the types and frequencies of several nonhematologic side effects are different.

Paclitaxel

Hematologic

Neutropenia is the principal toxicity of paclitaxel. The onset is usually on days 8 to 10, and recovery is generally complete by days 15 to 21 on every 3-week dosing regimens. A critical pharmacologic determinant of the severity of neutropenia is the duration that plasma drug concentrations are maintained above biologically relevant levels (0.05 to 0.1 µmol/L as discussed earlier in “Pharmacokinetics”), which may explain why neutropenia is more severe with more protracted infusions.331, 332, 347 This does not imply that shorter infusions should always be used, because the optimal dose and schedule have not been determined for clinical settings. Instead, the results of randomized clinical studies do not indicate that there is an optimal schedule for any particular tumor, although treatment with higher doses or “equitoxic doses” should be considered if shorter schedules are used.384 Notwithstanding these differences, the main clinical determinant of the severity of neutropenia is the extent of prior myelotoxic therapy.

Neutropenia is noncumulative, and the duration of severe neutropenia, even in heavily pretreated patients, is usually brief. At paclitaxel doses exceeding 175 mg/m2 on a 24-hour schedule and 225 mg/m2 on a 3-hour schedule, nadir neutrophil counts are typically less than 500/µL for fewer than 5 days in most courses, even in untreated patients. Even patients who have received extensive prior therapy can usually tolerate paclitaxel doses of 175 to 200 mg/m2 over 3 or 24 hours. More frequent administration schedules, particularly weekly treatment schedules with doses of 80 to 100 mg/m2, are associated with less severe neutropenia and at least equivalent or greater antitumor activity in a number of tumor types, as compared with single-dose schedules (see “Dose, and Schedule”). Severe effects on platelets and red blood cells are unusual, except in heavily pretreated patients.

Hypersensitivity

The incidence of major hypersensitivity reactions (HSRs) in early trials was approximately 30%, but declined to 1 to 3% following development of effective prophylaxis.221, 408, 409, 410 Major reactions, which are characterized by dyspnea with bronchospasm, urticaria, hypotension, chest, abdominal and backpain, usually occur within the first 10 minutes after the first (and less frequently after the second) treatment and resolve completely after stopping treatment and occasionally occur after treatment with antihistamines, fluids, and vasopressors. Patients who have major reactions have been rechallenged successfully after receiving high doses of corticosteroids, but this approach has not always been successful.411, 412, 413 Rechallenge appears to be most successful in patients who experience severe hypersensitivity manifestations within minutes of starting treatment, if the infusion is immediately discontinued, and if treatment resumes within approximately 30 minutes, which is likely the result of profound and persistent depletion of histamines and other mediators at the time of rechallenge.411, 412, 413 Although the incidence of minor hypersensitivity reactions (HSRs), such as flushing and rash, is about 40%, major reactions do not generally occur after minor HSRs. Based on the resemblence of the HSRs to those caused by radiocontrast dyes, they are probably caused by a nonimmunologically mediated release of histamine or other vasoactive substances, owing to the taxane moiety or, more likely, its polyoxyethylated castor oil vehicle, possibly through complement activation.414 The vehicle is the suspected culprit because it induces histamine release and similar manifestations in dogs, and other drugs formulated in it, such as cyclosporine A and vitamin K, induce similar reactions. Although the incidence of major HSRs is reduced with lower administration rates and longer infusion durations, the rates of major HSRs are low on both 3- and 24-hour schedules when patients are premedicated with corticosteroids and both H1-receptor and H2-receptor antagonists (see “Administration, Dose, and Schedule”).409 In an assessment of the relative safety of two different paclitaxel schedules (3- and 24-hour infusions), the rates of major reactions were low and similar (2.1% versus 1.0%) with premedication.

Peripheral Neurotoxicity

Paclitaxel induces a peripheral neuropathy characterized by sensory symptoms, such as numbness in a symmetric glove-and-stocking distribution.242, 408, 415,416, 417 The most common findings on neurologic examination loss are of sensation and deep tendon reflexes. Neurophysiologic studies support a primary disruption of neuronal microtubules resulting in axonal degeneration and demyelination as the primary pathogenic mechanism;however, manifestations suggestive of microtubule disruption resulting in a neuronopathy may be noted, particularly at higher doses or when combined with other neurotoxic agents.242, 408, 415, 416, 417 Severe neurotoxicity is uncommon when paclitaxel is given alone at doses below 200 mg/m2 on a 3- or 24-hour schedule every 3 weeks or below 100 mg/m2 on a continuous weekly schedule, but almost all “low-risk” patients experience mild or moderate effects. Patients with preexisting neuropathy caused by prior exposure to other neurotoxic agents, diabetes mellitus, congenital conditions, or alcholism, even when manifestations are subclinical, are more prone to paclitaxel-induced neuropathy. Symptoms may begin as soon as 24 to 72 hours after treatment with higher doses (≥250 mg/m2) but usually occur only after multiple courses at 135 to 250 mg/m2 every 3 weeks. Neurotoxicity is generally more pronounced when paclitaxel is administered on short infusion schedules, indicating that peak plasma concentration is a principal determinant. The combination of paclitaxel on a 3-hour schedule and cisplatin is particularly neurotoxic, and regimens consisting of paclitaxel and carboplatin produce less neurotoxicity than paclitaxel-cisplatin regimens. Motor and autonomic dysfunction may occur, especially at high doses and in patients with preexisting neuropathies caused by diabetes mellitus and alcoholism. Glutamate has been reported to reduce the severity of peripheral neuropathy from high doses of paclitaxel. Anecdotal reports, experimental models, and/or insufficiently powered randomized trials have suggested that the sulfahydryl group scavenger drugs, pyridoxine, and anticonvulsants reduce the neurotoxic effects of paclitaxel, but there is no convincing evidence that any specific measure is effective at ameliorating existing manifestations or preventing the development or worsening of neurotoxicity.407, 408, 418, 419 Transient myalgia and arthralgia of uncertain etiology, usually noted 24 to 48 hours after therapy and apparently dose-related, are also common, and a myopathy has been described in patients receiving high doses with cisplatin. Several investigators have reported that treatment with corticosteroids, specifically prednisone 10 mg twice daily for 5 days beginning 24 hours after treatment, is effective at reducing myalgia and arthralgia, and gabapentin, glutamate, and potentially antihistamines could be used for management or prevention.408, 419 Optic nerve disturbances, manifested by scintillating scotoma, may also occur.420, 421 Acute encephalopathy, which can progress to coma and death, has been reported after treatment with high doses (600 mg/m2 or greater).422 A transient acute encephalopathy has also been observed, rarely, within several hours following paclitaxel in patients who received prior cranial irradiation.423

Cardiac

Paclitaxel treatment has been associated with cardiac rhythm disturbances, most of which were identified using cardiac monitoring, the clinical relevance of these effects is not known.407, 408, 424, 425, 426 The most common disturbance, transient, asymptomaic bradycardia, was noted in 29% of patients in one trial in which patients underwent cardiac monitoring, and, in the absence of hemodynamic effects, is not an indication for discontinuing paclitaxel.408, 424, 426 More important bradyarrhythmias, such as Mobitz type I (Wenckeback syndrome), Mobitz type II, and third-degree heart block, have been noted, but the incidence in a large National Cancer Institute database was only 0.1%.426Most episodes have been asymptomatic and almost all documented events involved patients in early trials in which continuous cardiac monitor was routinely performed, indicating that second-degree and third-degree heart block are likely underreported. However, these bradyarrhythmias are probably caused by paclitaxel, as related taxanes affect cardiac automaticity and conduction, and similar disturbances have occurred in humans and animals after ingesting various species of yew plants.426

Myocardial infarction, cardiac ischemia, atrial arrhythmias, and ventricular tachycardia have been noted, but whether there is a causal relationship between paclitaxel and these events is uncertain. There is no evidence that chronic, long-term treatment with paclitaxel causes progressive cardiac dysfunction. Routine cardiac monitoring during paclitaxel therapy is not necessary, but is advisable for patients who may not be able to tolerate bradyarrhythmias, such as those with atrioventricular conduction disturbances or ventricular dysfunction. Although patients with a wide range of cardiac abnormalities and cardiac histories were broadly and empirically restricted from participating in early clinical trials, paclitaxel treatment has been reported to be well tolerated in a small series of patients with gynecologic cancer and with major cardiac risk factors.424 However, repetitive treatment of patients with the combined regimen of paclitaxel on a 3-hour schedule and doxorubicin as a brief infusion is associated with a higher frequency of congestive cardiotoxicity than would be expected to occur with the same cumulative doxorubicin dose given without paclitaxel, as discussed in “Drug Interactions.”349, 369, 370 In one study of previously untreated women with advanced breast cancer who were treated with escalating doses of paclitaxel as a 3-hour infusion and doxorubicin, 60 mg/m2 to a cumulative dose of 480 mg/m2, which would be predicted to result in a less than 5% incidence of congestive cardiotoxicity in patients treated with doxorubicin alone, the incidence of congestive cardiotoxicity was approximately 25%.349 However, the incidence of cardiotoxicity was less than 5% when similar patients received identical schedules of paclitaxel and doxorubicin, but the cumulative doxorubicin dose did not exceed 360 mg/m2. Both experimental and early clinical results suggest that dexrazoxane reduces the cardiotoxicity of the doxorubicin and paclitaxel combination.427, 428 The incidence of congestive heart failure was also significantly higher in patients treated with the combination of trastuzumab and paclitaxel than paclitaxel alone;therefore, cardiac function should be monitored.429

Miscellaneous

Drug-related gastrointestinal effects, such as vomiting and diarrhea, are uncommon. Higher paclitaxel doses or protracted (96-hour) infusional administration may cause mucositis.390, 422, 430 Rare cases of neutropenic enterocolitis and gastrointestinal necrosis have been noted, particularly in patients given high doses of paclitaxel in combination with doxorubicin or cyclophosphamide.431, 432 Severe hepatotoxicity and pancreatitis have also been noted rarely.219, 433Acute bilateral pneumonitis has been reported in fewer than 1% of patients treated on a 3-hour schedule in one series, and both interstitial and parenchymal pulmonary toxicity have been reported, but clinically significant pulmonary effects are uncommon.434, 435 In contrast to the vinca alkaloids, the agent is not a potent vesicant but extravasation of large volumes can cause moderate soft tissue injury. Inflammation at the injection site and along the course of an injected vein may occur. Paclitaxel also induces reversible alopecia of the scalp in a dose-related fashion, and loss of all facial and body hair is usually because of cumulative therapy. Nail disorders have been reported, particularly in patients treated on weekly schedules.436 Recall reactions in previously irradiated sites have also been noted.

Docetaxel

Hematologic

Following treatment with docetaxel administered over 1 hour every 3 weeks, the onset of neutropenia, the principal toxicity of docetaxel, is usually noted by day 8 and complete resolution typically occurs by days 15 to 21.226, 407, 436 At a dose of 100 mg/m2 administered over 1 hour, neutrophil counts are commonly below 500/µL and the incidence of neutropenic sequalae is high.226, 405 Although severe neutropenia at 75 mg/m2 is common, the duration of severe neutropenia and incidence of complications are lower than the higher dose. As with paclitaxel, neutropenia is significantly less when lower doses are administered on a weekly schedule (see “Dose, and Schedule”). The most important determinant of neutropenia is the extent of prior treatment. Significant thrombocytopenia and anemia are uncommon with docetaxel alone.

Hypersensitivity

Despite not being formulated in polyoxyethylated castor oil, HSRs were reported in approximately 31% of patients receiving docetaxel without premedications in early phase 2 studies. 226, 407, 437 As with paclitaxel, major reactions characterized by dyspnea, bronchospasm, and hypotension typically occur during the first two courses and within minutes after the start of treatment. Signs and symptoms generally resolve within 15 minutes after cessation of treatment, and docetaxel is usually able to be reinstituted without sequelae, occasionally after treatment with an H1-receptor antagonist. Fortunately, however, most events are minor and rarely result in discontinuation of treatment.407Both the incidence and severity of HSRs appear to be reduced by premedication with corticosteroids and H1-receptor and H2-receptor antagonists (discussed later in “Administration, Dose, and Schedule”), but the corticosteroid premedication regimen is principally administered to prevent fluid retention. As with paclitaxel, patients who experience major reactions have been retreated successfully after the resolution of symptoms and after treatment with corticosteroids and H1-receptor antagonists. Furthermore, there are several anecdotal reports of patients treated successfully with docetaxel following severe HSRs as the result of taking paclitaxel, but it is not known whether these reactions would have occurred if the patients had been retreated with paclitaxel.407, 438

Fluid Retention

Docetaxel induces a unique fluid retention syndrome characterized by edema, weight gain, and third-space fluid collection.226, 392, 406, 407, 439 Fluid retention is cumulative and does not appear to be caused by hypoalbuminemia or cardiac, renal, or hepatic dysfunction. Instead, increased capillary permeability appears to be responsible for this phenomenon.439 Capillary filtration studies in patients who were not receiving corticosteroid premedication have revealed a two-stage process, with progressive congestion of the interstitial space by proteins and water starting between the second and fourth course, followed by insufficient lymphatic drainage.439 In early studies in which premedication was not used, fluid retention was not usually significant at cumulative docetaxel doses below 400 mg/m2;however, the incidence and severity of fluid retention increased sharply at cumulative doses of 400 mg/m2 or greater and often resulted in the delay or termination of treatment. Premedication with corticosteroids with or without H1-receptor and H2-receptor antagonists has been demonstrated to reduce the overall incidence of fluid retention and increase the number of courses and cumulative docetaxel dose before the onset of this toxicity (see “Dose, and Schedule”).406, 437 Fluid retention typically resolves slowly after docetaxel is stopped, but complete resolution occurs several months after treatment in patients with severe toxicity. Aggressive and early treatment with progressively more potent diuretics starting with potassium-sparing diuretics has been used to manage fluid retention.406, 407, 439 The incidence of fluid retention appears to be lower in studies using lower doses (60 to 75 mg/m2) of docetaxel during each course, but this may be because of the administration of lower overall cumulative doses, and the effects of lower doses on antitumor activity are unknown.

Dermatologic

Skin toxicity may occur in as many as 50 to 75% of patients; however, premedication may reduce the overall incidence of this effect.226, 407, 440, 441 An erythematous pruritic maculopapular rash that affects the forearms, hands, or feet is typical. Other cutaneous effects include desquamation of the hands and feet, that may respond to pyridoxine or cooling, and onychodystrophy characterized by brown discoloration, ridging, onycholysis, soreness, and brittleness and loss of the nail plate.440, 441, 442, 443 Skin and nail changes appear to be most prominent in patients treated with high cumulative doses over long periods, particularly on weekly administration schedules.392

Neurotoxicity

Docetaxel produces neurotoxicity, which is qualitatively similar to that of paclitaxel.226, 407, 444 Patients typically complain of paresthesia and numbness, but peripheral motor effects may also occur. Both neurosensory and neuromuscular effects are generally less frequent and less severe with docetaxel as compared with paclitaxel, and preferential use of docetaxel should be considered in high-risk patients in whom taxane treatment is indicated.226, 407, 444, 445Nevertheless, mild-to-moderate peripheral neurotoxicity occurs in approximately 40% of untreated patients, and patients who had received prior cisplatin appear to be particularly susceptible, with the incidence approaching 74% in one trial.226, 391, 392, 407, 437 Severe toxicity has been unusual after repetitive treatment with docetaxel doses less than 100 mg/m2, except in patients with antecedent neurotoxicity and relavent disorders, such as alcohol abuse and diabetes mellitus. Transient arthralgia and myalgia are occasionally noted within days after treatment. Malaise or asthenia have been prominent complaints in patients who have been treated with large cumulative doses, particularly when docetaxel is administered on a continuous weekly schedule.226, 391, 392, 407, 437

Miscellaneous

Stomatitis is more common with docetaxel than paclitaxel, but still occurs infrequently. Nausea, vomiting, and diarrhea have also been observed infrequently, but severe manifestations are rare. Empiric use of antiemetic premedication does not appear to be warranted. Mild-to-moderate conjunctivitis, which is responsive to topical corticosteroids, and canalicular stenosis causing lacrimation may also occur, particularly with weekly schedules.446 Nausea, vomiting, and diarrhea have also been observed, but severe gastrointestinal toxicity is rare. Similar to paclitaxel, docetaxel is not a potent vesicant and infusion site reactions are uncommon. Other rare events reported that may or may not be drug-related included arrythmias, confusion, erythema multiforme, neutropenic enterocolitis, hepatitis, ileus, interstial pneumonia, seizures, pulmonary fibrosis, hepatitis, radiation recall, and visual disturbances.392 Cardiovascular manifestations, such as angina, arrhythmia, conduction disturbances, congestive heart failure, hypertension, and hypotension, have been noted rarely following treatment, but these events have not been linked convincingly to docetaxel.

Estramustine Phosphate

Estramustine, a conjugate of the alkylating agent nor-nitrogen mustard that is linked to 17β-estradiol by a carbamate ester bridge, is administered as the oral prodrug estramustine phosphate, which is rapidly dephosphorylated by gastrointestinal tract phosphatases to produce estramustine. Estramustine was synthesized so that the 17β-estradiol component would bind to, and accumulate in estrogen receptor–bearing breast cancer cells, and selectively deliver the nor-nitrogen mustard alkylating moiety, after degradation of the carbamate ester. Estramustine was subsequently demonstrated to be capable of inducing cytotoxicity in estrogen receptor–negative cancer cells, and drug binding was not inhibited by 1,000-fold excess concentrations of estradiol.447 Furthermore, no significant anticancer activity was noted in early clinical trials involving patients with breast cancer, and, thereafter, it was determined that DNA alkylation and DNA damage did not occur in vitro at concentrations that induce cytotoxicity.447, 448, 449 This led to radiolabeled drug distribution studies in rats that showed that estramustine did not distribute to tissues that expressed the estrogen receptor.450 Instead, drug accumulated in the ventral prostate, which was mediated by a prostate tissue-specific protein called the estramustine-binding protein (EMBP) that has a heterodimeric structure and a molecular weight of 46,000 daltons.450, 451 This finding, as well as the demonstration of anticancer activity in patients with prostate cancer that were refractory to the estrogen analog diethylsilbesterol, led to evaluations of estramustine in patients with HRPC and regulatory approval for this indication.451, 452 Although estramustine has been associated with activity as a single agent, antitumor activity has been most prominent with estramustine phosphate combined with other antimicrotubule agents including vinca alkaloids, taxanes, and epothilones. The combination of estramustine and docetaxel has demonstrated decrements in prostate specific antigen in approximately 50% of HRPC patients.453, 454 In a randomized phase 3 trial, in which patients with HRPC were treated with either docetaxel plus estramustine phosphate or a standard regimen consisting of mitoxantrone plus prednisone, both overall and progression-free survival were superior in patients treated with the estramustine-based regimen.237, 239, 455

Mechanism of Action

Estramustine's principal mechanism of antineoplastic action involves perturbations in microtubule dynamics. Estramustine depolymerizes both microtubules and microfilaments, binds to and disrupt MAPs, and inhibit cell growth at high concentrations, which ultimately result in mitotic arrest and apoptosis.456, 457,458 The agent binds to β-tubulin (Kd≈23 µmol) at a site distinct from the colchicine and vinca alkaloid binding sites, and its binding affinity to β-tubulin is isotype-dependent.3, 456, 459 Although the phosphorylated form binds to MAPs and estramustine itself binds to other cellular proteins besides tubulin, the drug's interaction with tubulin appears to be principally responsible for its anticancer activity.459, 460, 461 Estramustine reduces the rates of both microtubule lengthening and shortening, inhibits dynamic instability and polymerization of MAP-free microtubules, and modestly increases microtubule mass.459, 462, 463,464 Although estramustine may affect microtubules comprising the interphase cytoskeleton, it principally affects those comprising the mitotic spindle apparatus and induces arrest in G2/M. Similar to the vinca alkaloids and taxanes, mitotic arrest is often followed by apoptosis. The aforementioned antimicrotubule effects of estramustine are mediated by the intact conjugate and not the individual nor-nitrogen or estradiol moieties.459

The tissue-selective accumulation and actions of estramustine and its metabolite, estromustine appear to depend on the presence, distribution, and magnitude of EMBP.465, 466 This is supported by the finding that the magnitude of G2/M arrest directly relates to intracellular concentrations of EMBP after estramustine treatment in vitro.465, 466, 467 In addition to prostate cancer, EMBP and related binding proteins have been identified in malignant brain tumors, but the therapeutic relevance of this finding is not clear.468, 469, 470 Because estramustine phosphate blocks cell cycle traverse in G2/M, crosses the blood-brain barrier, and accumulates in both gliomas and astrocytomas, the potential for estramustine to selectively sensitize brain malignancies to irradiation is being investigated.471, 472

Three distinct mechanisms of resistance have been characterized in cancer cell lines that have been selected for resistance to estramustine including alterations in β-tubulin isotype expression, MAP expression, and ATP-dependent drug efflux. With regard to differential β-tubulin isotype expression, higher ratios of βIII- and βIVa-tubulin relative to other β-tubulin isotypes have been demonstrated in prostate cancer cell lines with acquired resistance to estramustine.3, 304, 473 Estramustine binds less avidly to microtubules with higher βIII-tubulin isotype content compared to other β-tubulin isotypes, and tumor cells with high levels of βIII-tubulin appear to be less prone to the inhibitory and destabilizing effects of estramustine on microtubule dynamics.3, 304, 473However, in studies in which the gene encoding βIII-tubulin has been transfected into prostate cancer cells with resultant overexpression of βIII-tubulin, resistance to estramustine and other antimicrotubule agents is not conferred.473, 474 Overexpression of the MAP tau is another mechanism of acquired resistance to estramustine that has been demonstrated in prostate cancer cell lines.475 However, the extent to which alterations in tau or other altered MAPs contribute to clinical estramustine resistance is not known. Although estramustine is a substrate for the drug efflux pump characterized by the MDR phenotype, P-gp overexpression does are not appear confer resistance to estramustine.473, 476, 477, 478 In fact, estramustine may competitively inhibit P-gp function, reducing the efflux of cytotoxic agents that are substrates for P-gp.473, 476, 478 In addition, cells with amplifications of the ABC2 transporter gene demonstrate a magnitude of estramustine resistance that is proportional to the level of ABC2 gene amplification.473, 479

Pharmacology

The bioavailability of oral estramustine phosphate, which undergoes rapid and complete dephosphorylation to estramustine within the gastrointestinal tract (Fig. 11.6), has been reported to range from 37 to 75%.473, 479, 480, 481 The principal route of estramustine elimination is by rapid oxidative metabolism at C17 to yield estromustine, which is the main metabolite in the plasma.473, 482 Estromustine concentrations in plasma peak within 2 to 4 hours after oral administration, and the mean elimination half-life of estromustine is 14 hours.480, 483 The pharmacokinetic behavior of estromustine in plasma is dose proportional following oral administration of estramustine phosphate in its therapeutic dose range. In patients treated orally with estramustine phosphate 560 mg/day, peak plasma concentrations average 227 ng/mL for estromustine, 23 ng/mL for estramustine, 95 ng/mL for estrone, and 9.3 ng/mL for estradiol.482

Significant first-pass hepatic metabolism occurs after oral administration of estramustine phosphate, and further hydrolysis of estromustine and its carbamate linker in the liver results in the formation of estrone and the release of the alkylating group. Following oral and intravenous administration of radiolabeled estramustine phosphate in humans, estromustine and estramustine are principally excreted in the feces, and only small amounts of conjugated estrone and estradiol are excreted in the urine (<1%).473, 480, 481, 482, 483 Although the pathways responsible for hepatic metabolism of estramustine, estromustine, and the estrogen metabolites have not been fully elucidated, hepatic CYP1A2 and CYP3A4 P-450 isoenzymes are largely responsible for oxidative metabolism of estradiol- and estrone-like steroids in hepatic microsome studies.473, 484

Figure 11.6 Structure of estramustine. Arrows show the bonds that separate the estrogenic portion of the molecule from the nitrogen mustard. (Reprinted with permission from Tew KD, Glusker JP, Hartley-Asp B, et al. Preclinical and clinical perspectives on the use of estramustine as an antimitotic drug. Pharmacol Ther 1992;56:323.)

An intravenous formulation of estramustine phosphate, which is available only for investigational use in the United States, results in 10- to 15-fold higher peak plasma concentrations of estramustine phosphate, estramustine, and estromustine than those achieved following oral administration.455, 473, 480, 481, 482,483, 484 The parenteral formulation is also associated with markedly less interpatient variability in pharmacokinetics.455, 480, 481, 482, 483, 484 The absence of first-pass hepatic metabolism with parenteral administration results in lower clearance rates; the terminal half-lives of elimination for estramustine phosphate, estromustine, and estramustine average 3.7, 110, and 64 hours, respectively, following intravenous administration.483 The reduced clearance of intravenous estramustine results in the accumulation of estromustine, estramustine, estradiol, and estrone in the plasma of patients treated on weekly intravenous dosing regimens.455

Drug Interactions

Coadministration of calcium-rich foods, particularly dairy products, impairs the gastrointestinal absorption of estramustine phosphate as the result of the formation of poorly absorbable calcium complexes.485 Therefore, it is recommended that patients fast for at least 2 hours before oral administration of estramustine phosphate and avoid calcium-rich food and antacids.

The clearance rates of docetaxel and paclitaxel have been reported to be significantly reduced by estramustine phosphate.238 Although the mechanism by which this occurs is not known, the potential inhibitory effects of estramustine on taxane-metabolizing CYP3A4 isoenzyme have been proposed to account for this potential drug-drug interaction.238, 453, 484 Therefore, recommended doses of docetaxel and paclitaxel in combination with estramustine phosphate are less than single agent doses, despite the fact that the taxanes and estramustine phosphate essentially have non-overlapping toxicities.

Toxicity

Because of nausea and vomiting, which are the principal toxicities encountered with oral estramustine phosphate, patients may require modification of dose-schedule or termination of treatment. However, these toxicities are readily managed with standard antiemetic medications. Diarrhea has also been observed in patients treated with estramustine phosphate for protracted periods. Myelosuppression is usually not significant in patients treated with estramustine phosphate as a single agent.

Common estrogenic side effects of treatment with estramustine phosphate include gynecomastia, nipple tenderness, and fluid retention. Physicians should exercise caution in prescribing estramustine phosphate to patients with a history of congestive heart failure because of the risk for fluid retention and edema that can result in a decompensated cardiac state. Thromboembolic complications, including venous thrombosis, pulmonary emboli, and cerebrovascular and coronary thrombotic events, which may occur in up to 10% of patients, represent the most serious toxicities of estramustine phosphate. The cardiovascular effects of oral estramustine phosphate have been attributed to high intrahepatic concentrations of estrogenic metabolites, which result in reduced antithrombin III levels and hypercoagulability.486 Transient elevations in hepatic transaminases has been reported in approximately 33% of patients. In a phase 3 study, in which patients with advanced prostate carcinoma were randomized to treatment with either estramustine phosphate or diethylstilbestrol, the rates of hepatic toxicity were similar on both treatment arms.487 Clinically significant acute hypocalemia, which has been proposed to be the result of an avid uptake of calcium in osteoblastic metastases (i.e., tumor calcium sink), increased uptake of calcium by healing bone lesions and/or the unmasking of a subclinical vitamin D-deficient state, is uncommon.488, 489 Asymptomatic reductions in serum calcium concentrations have been reported in up to 20% of patients.

Patients treated with estramustine phosphate by the intravenous route complain infrequently of the acute onset of perianal and/or perineal pain, which can be minimized by administering the agent over a more protracted infusion duration (60 to 90 minutes).484

Administration, Dose, and Schedule

The recommended daily dose of estramustine phosphate, which is available as a 140-mg capsule, is 14 mg/kg of body weight in three to four divided daily doses; however, patients are usually treated in the daily dosing range of 10 to 16 mg/kg. The agent should be ingested with water at least 1 hour before or 2 hours after meals. Patients are generally treated for 30 to 90 days before assessment of therapeutic benefit. Chronic oral therapy can be maintained for months or even years. Abbreviated 1-, 3-, and 5-day courses of oral estramustine phosphate have been proposed for use in combination the taxanes and other chemotherapeutics. Such schedules appear to reduce the gastrointestinal toxicity associated with chronic oral administration.238, 490, 491 In studies with docetaxel using this abbreviated schedule, the recommended dose of estramustine is 280 mg three times daily (≈600 mg/m2 per day) for 5 days.453

Novel Antimicrotubule Agents in Early Clinical Development

The clinical success of the taxanes has led to a search for other drugs that enhance tubulin polymerization, yielding several promising compounds that may confer a therapeutic advantage over the taxanes, including the epothilones (isolated from myxobacterium Sorangium cellulosum), discodermolide (isolated from the Caribbean sponge Discodermia dissoluta), eleutherobin (isolated from the soft coral Eleutherobia sp), laulimalides (isolated from the marine sponge Cacospongia mycofijiensis), and sarcodictyins (isolated from the Mediterranean stoloniferan coral Sarcodictyon roseum (Fig. 11.7). Some of these compounds compete with paclitaxel for binding to microtubules and appear to bind at or near the taxane site (epothilones, discodermolide, eleutherobins and sarcodictyins), but others, such as laulimalide, seem to bind to unique sites on microtubules.2, 3, 243 Eleutherobin, discodermolide, and laulimalide are especially potent, with Ki values in the 5 to 40 nM range.2, 3, 4 All of the aforementioned compounds possess either low level or no substrate affinity for P-gp and other ABC transporters, and retain various degrees of activity against taxane-resistant cells in vitro, but the clinical implications of these characteristics are not clear.

Of the aforementioned compounds, the epothilones are the furthest along in development. Not only do they promote tubulin polymerization and induce mitotic arrest, but several epothilones possess much greater cytotoxic potency than either paclitaxel or docetaxel, with IC50 values in the submolar or low nanomolar range.2, 3, 4, 492, 493, 494 Similar to the taxanes, they induce tubulin polymerization in the absence of GTP and/or MAPs, resulting in microtubules that are relatively long, rigid, and resistant to destabilization by cold temperature and calcium. In contrast to the taxanes and vinca alkaloids, overexpression of P-gp minimally affects the cytotoxicity of epothilones A and B.2, 3, 492 Furthermore, various point mutations in β-tubulin, which confer resistance to the taxanes in vitro, do not necessarily confer resistance to the epothilones.492, 495 Several epothilone B analogs, including ixabepilone (BMS-247550) and patupiline (EPO906), are currently undergoing clinical evaluation.492, 493, 496, 497, 498 The principal mode of disposition for ixabepilone is via cytochrome P450 metabolism and biliary excretion, whereas patupiline is metabolized via carboxyesterases.492, 496, 497, 498 The pharmacologic characteristics are typified by marked tissue uptake, avid and protracted tissue binding, and long terminal half-lives of elimination.492 Furthermore, their principal toxicities differ, namely diarrhea for patupiline and both myelosuppression and neurotoxicity for ixabepilone.492 In early clinical trials, responses have been noted in patients with advanced carcinomas of the breast, lung, prostate, and ovary, some of whom experienced recurrent disease following or during treatment with the taxanes.492, 496, 498 However, the magnitude of their activity in taxane-refractory malignancies needs to be addressed. Of note, patupiline has shown evidence of antitumor activity in advanced colorectal and renal carcinomas, which are almost always inherently resistant to antimicrotubule agents, but the magnitude of appreciable activity in cancers with primary or acquired resistance to the taxanes is negligible.5, 492, 496, 497, 498 An epotholone D, desoxyepotholone B (KOS862) that has demonstrated at least equivalent potency and less toxicity than the taxanes and epotholone B analogs in preclinical studies, is in early clinical development.494, 499

Figure 11.7 Structures of discodermolide, epotholones A and B, eleutherobin, and laulimalides

Similar to the epothilones A and B, discodermolide-induced tubulin polymers are stable to treatment with calcium and are composed of short microtubules instead of tubulin spirals.500, 501 In addition to low-level to complete cross-resistance to P-gp overexpressing cancer cells, paclitaxel and epothilone-resistant human tumor cells that express mutant β-tubulin, retain sensitivity to discodermolide.500, 501, 502, 503 Furthermore, discodermolide and paclitaxel have demonstrated synergistic cytotoxicity in vitro, suggesting that their tubulin binding sites and microtubule effects are not identical.504, 505 However, unforeseen pulmonary toxicity has been seen in early clinical studies of a completely synthetic discodermolide (XAA296).506 The marine soft coral–derived natural products, sarcotidicytins A and B and eleutherobin, also promote tubulin polymerization in a manner analogous to that of paclitaxel.2, 3, 507, 508 The marine-derived, microtubule-stabilizing cytotoxins laulimalide and isolaulimalide appear to be poor substrates for ABC transporters such as P-gp.2, 3, 4, 508, 509Because eleutherobin, epothilones A and B, and discodermolide, competitively inhibit [3H]paclitaxel binding to microtubules, a common pharmacophore was sought and identified, which may enable the development of hybrid constructs with more desirable biological characteristics.243

Other natural products and semisynthetic antimicrotubule compounds under evaluation interact with tubulin in the vinca alkaloid-binding or colchicine-binding domains. Among the most potent are the dolastatins, which constitute a series of oligopeptides isolated from the sea hare, Dolabela auricularia. Two of the most potent dolastatins, dolastatin-10 and dolastatin-15, noncompetitively inhibit the binding of vinca alkaloids to tubulin, inhibit tubulin polymerization and tubulin-dependent GTP hydrolysis, stabilize the colchicine-binding activity of tubulin, and possess cytotoxic activity in the picomolar to low nanomolar range. Dolastatin-10 and semisynthetic dolastatin analogs (ILX-651 and well as TZT-1027), which binds in the vinca domain, are undergoing preclinical development and clinical evaluation.2, 3, 4, 510

Phomopsin A, halichondrin B, homohalichondrin B, and spongistatin 1, which competitively inhibit vinca alkaloid binding to tubulin are being evaluated in preclinical or early clinical evaluations.511, 512, 513 E7389, a macrocyclic ketone analog of the marine natural product halichondrin B originally isolated from the marine sponge Halicondrin okadai, and two less complex synthetic marocyclic ketone analogs, ER-076349 and ER-086526 are in early clinical evaluations.511These compounds bind to tubulin, inhibit tubulin polymerization, disrupt mitotic spindle formation, induce mitotic arrest, and possess growth inhibitory properties in the subnanomolar range and marked activity in preclinical studies. Several biochemical correlates of apoptosis are also observed following E7389 treatment, including phosphorylation of the antiapoptotic protein Bcl-2, cytochrome c release from mitochondria, proteolytic activation of caspase-3 and -9, and cleavage of the caspase-3 substrate poly(ADP-ribose) polymerase. The agent is currently in phase 1 to 2 evaluations in patients with advanced solid malignancies.

Also in clinical development is HTI-286, a synthetic form of hemiasterlin, which is is a natural product derived from marine sponges.513 Hemiasterlin and its analogs bind to the vinca-peptide site in tubulin, disrupt normal microtubule dynamics, and, at stoichiometric amounts, depolymerize microtubules. HTI-286 is a much weaker substrate for P-gp than the vinca alkaloids and taxanes.513 It has excellent in vivo antitumor activity in human xenograft models, including tumors that express P-gp. The agent is cross-resistant with other vinca peptide-binding agents, including hemiasterlin A, dolastatin-10, and vinblastine (7- to 28-fold), and DNA-damaging drugs, including doxorubicin and mitoxantrone (16- to 57-fold), but is minimally cross-resistant with the taxanes, epothilones, or colchicine (onefold to fourfold). However, resistance appears to be at least partially mediated by mutation of α-tubulin and by an ATP-binding cassette drug pump distinct from P-glycoprotein, ABCG2, MRP1, or MRP3.

Most efforts targeting the tumor vasculature are aimed at the development of agents that inhibit the process of angiogenesis, but recently, several antimicrotubule agents have been demonstrated to rapidly shut down existing tumor vasculature.514 Since the late 1990s, the combretastatins and N-acetylcolchicinol-O-phosphate, compounds that resemble colchicine and bind to the colchicine domain on tubulin, have undergone extensive development as antivascular agents. Several of them (combretastatin-A-43-O-phosphate, combrestatin A-1-phosphate (CA-1-P), ZD6126 and AVE8062A are in clinical trials.515Although objective antitumor activity in preliminary evaluations have been noted, cardiovascular toxicity has been problematic.

Targeting Mitotic Kinesins and Kinases

Although tubulin is the most abundant protein component of the mitotic spindle apparatus, many additional proteins, such as mitotic kinesins, play critical roles in the mechanics of mitosis and in progression through the premitotic cell cycle checkpoint. Kinesins are motor proteins that translate chemical energy released by the hydrolysis of adenosine triphosphate (ATP) into mechanical force for movement along microtubules, transport of a wide variety of cargoes, and the intracellular organization of the mitotic spindle and other microtubule-containing structures.514, 516, 517

The mitotic kinesins are a subgroup of kinesin motor proteins that function exclusively in mitosis in proliferating cells.518 During mitosis, different, highly specialized mitotic kinesins play critical roles in various aspects of mitotic spindle assembly, including the establishment of spindle bipolarity, spindle pole organization, chromosome alignment and segregation, and regulation of microtubule dynamics. The establishment of mitotic spindle bipolarity is among the earliest events in spindle assembly and it requires the function of a specific kinesin motor protein KSP (also known as Eg5), which has no known role outside of mitosis.519 The expression profiles of KSP mRNA in normal tissues are consistent with preferential expression of KSP in proliferating cells relative to normal adjacent tissue and postmitotic neurons. As essential elements in mitotic spindle assembly and function, KSP and mitotic kinesins provide attractive targets for intervention into the cell cycle. CK0106023 (SB-715992) a polycyclic, nitrogen containing heterocycle and allosteric inhibitor of KSP motor domain ATPase with a Ki of 12 nM, is a very potent KSP inhibitor that causes mitotic arrest. It is entering clinical trials.193, 520 The compound, which is 10,000-fold more selective for KSP relative to other members of the kinesin superfamily, has been shown to block assembly of a functional mitotic spindle, thereby causing cell cycle arrest in mitosis and subsequent cell death.519, 521 In tumor-bearing mice, the agent exhibited antitumor activity comparable to or exceeding that of paclitaxel, and caused the formation of monopolar mitotic figures identical to those produced in cultured cells.520

A host of mitotic kinases are also being assessed as strategic targets for anticancer therapeutic development. For example, the aurora kinases (A, B, C) are essential for the regulation of chromosome segregation and cytokinesis during mitosis and aberrant expression and activity of these kinases occur in a wide range of human tumors, and lead to aneuploidy and tumorigenesis.522 Recently, a highly potent and selective small-molecule inhibitor of the aurora kinases, VX-680, that blocks cell-cycle progression and induces apoptosis in a diverse range of human tumor types.522 This compound causes profound inhibition of tumor growth in a variety of in vivo xenograft models, leading to regression of leukemia, colon and pancreatic tumors at well-tolerated doses, and is entering clinical trials.

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