Osama E. Rahma and Samir N. Khleif
Molecular targeted therapy (MTT) is a new approach to cancer treatment that resulted from the plethora of molecular and biologic discoveries into the etiology of cancer, which took place over the last quarter of a century. Several agents have already been approved by the U.S. Food and Drug Administration (FDA) for clinical use. Many more are currently being tested in clinical trials, and their widespread integration into the mainstream for cancer treatment is expected to increase at an accelerated pace during the next decade.
Agents in this type of therapy are vastly different from the traditional chemotherapeutic agents that constitute the majority of therapy described throughout the chapters of this book. These new drugs are designed with the intention to specifically target molecules that are uniquely or abnormally expressed within cancer cells while sparing normal cells. Within this chapter, we will discuss drugs that are already available for clinical use; provide a brief description of the mechanism of action of these agents, the pathways they target, and some of their clinical uses; also address promising agents that are currently in clinical trials and may be coming soon to the clinic.
A. Characteristics of MTT
An ideal molecule for targeted therapy should have the following characteristics:
The molecule is uniquely expressed in cancer cells; hence the therapeutic agent will specifically target the cancer and not the normal cells.
The molecule is important for the maintenance of the malignant phenotype; therefore, once the targeted molecule has been effectively disabled, the cancer cell will not be able to develop resistance against the therapeutic agent by suppressing its function or expelling the targeted molecule from the cell.
The degree to which target molecules do not embody these characteristics coupled with nonspecificity of the therapeutic agent determines, in part, the limitations of current targets and agents.
B. Classification and type of MTT
The classification of MTT is a moving target. In this chapter, we will classify MTT based on the targeting strategy of the molecule. There are two targeting strategies for MTT:
1. Function-directed therapy. This therapeutic strategy is intended to restore the normal function or abrogate the abnormal function of the defective molecule or a pathway in the tumor cell.
This is accomplished by:
Reconstituting the normal molecule
Inhibiting the production of a defective molecule
Aborting, altering, or reversing a newly acquired function by targeting the defective molecule, its function, and its downstream effect.
Agents under this category will be classified based on the mechanism of action and subclassified based on the known affected targeted pathway.
2. Phenotype-directed therapy. This is a therapeutic strategy that is intended to target the unique phenotype of the cancer cell where killing the cell is more dependent on nonspecific mechanisms rather than targeting a specific pathway. Such agents include monoclonal antibodies (MoAbs)—including immune conjugates—immunotoxins, and vaccine therapy. Accordingly, agents under this category will be classified based on the type of therapy and subclassified based on the targeted pathway or molecule.
Table 2.1 summarizes the classification and FDA-approved indications of molecular-targeted agents.
II. FUNCTION-DIRECTED THERAPY
Agents under this category target specific cellular pathways (e.g., signal transduction pathways, angiogenesis, protein degradation, and immune modulators).
A. Cell signaling targeted therapy
Signal transduction pathways are crucial for delivering messages from the extracellular environment into the nucleus and enabling the cell to carry on cellular processes including survival, cell proliferation, and differentiation. These signals are initiated from the cell surface by the interaction of molecules (ligands) such as hormones, cytokines, and growth factors with cell receptors. Cell receptors, in turn, transfer the signal through a network of molecules to the nucleus, which leads to the transcription of new molecules responsible for engineering the desired outcome.
In cancer cells, these pathways are found to be altered through the mutation of some of their components. This leads to the functional dysregulation of the affected pathways resulting in uncontrolled proliferation and inhibition of apoptosis. Accordingly, targeting the components of these pathways is a prime goal for the development of MTT. The components of these pathways include the following:
The receptors for these ligands—the majority of which are kinase receptors
The cascade of proteins that form these pathways, which are mainly protein kinases; other classes of proteins are also involved.
Accordingly, strategies targeting signal transduction pathways include the following:
Blocking of the ligand-receptor binding. This leads to the prevention of the initiation of the signal and can be accomplished by either blocking circulating ligands or blocking ligand binding to the extracellular domain of the cellular receptor.
Inhibition of receptor protein kinases. This leads to the prevention of phosphorylation of the intracellular kinase domain of the receptor, hence, aborting the cascade of proteins reactions in the cell signaling pathways. Blocking adenosine triphosphate (ATP) binding to the receptor is one example to achieve this inhibition.
Inhibition of intracellular signaling proteins.
1. Blocking of the ligand-receptor binding. Blocking receptors and ligand-receptor interaction is currently achieved by utilizing specific MoAbs directed against the ligand or the receptor. MoAbs are biologic agents designed with the intention to specifically target soluble proteins or membrane proteins with an extracellular domain. The MoAbs can exert their antitumor effect through multiple potential mechanisms including blocking the targeted receptor or ligand and preventing its function in transmitting signals to the nucleus, activating antibody-dependent cellular cytotoxicity, or helping to internalize the receptor and hence deliver toxic agents into the cells. The MoAb technology has been very much improved in the last decade by humanizing these agents partially in chimeric or fully humanized constructs. Substituting the murine Fc portion of the MoAb with a human equivalent leads to a significant decrease in the generation of a human antimouse antibody (HAMA) immune reaction. Although generation of human antichimera antibodies (HACAs) may still occur for those MoAbs, it does not occur with fully humanized MoAbs. This technology to humanize MoAbs has made these molecules more usable in the treatment of cancer, particularly when repetitive dosing is required. In this section, we will discuss MoAbs generated against specific membrane receptors. MoAbs that are generated against membrane nonreceptor antigens will be discussed later in the chapter (Section III.A).
a. Epidermal growth factor receptor (EGFR) family. The EGFRs are a small family of proteins belonging to the larger receptor tyrosine kinase (RTK) family. The EGFR family includes at least four described receptors: EGFR1, Her-2-neu (erbB2), Her3 (erbB3), and Her4 (erbB4). These receptors are glycoproteins consisting of three domains: an extracellular ligand-binding domain, a transmembrane domain, and an intracellular domain with a tyrosine kinase activity. Binding of the ligands to the receptor leads to the activation of the intracellular tyrosine kinase and the phosphorylation of the receptor, which in turn leads to activation of the downstream signal transduction pathway. The activation of this pathway promotes cell activation, proliferation, and enhanced survival. Agents have been developed against the receptors EGFR1 and Her-2-neu.
(1) EGFR1-targeted therapy. EGFR1 is the first member of the EGFR family to be identified. It is activated by binding to epidermal growth factor (EGF) and to transforming growth factor alpha (TGF-α). EGFR1 is found to be over-expressed in many cancers including 50% to 70% of colon, lung, and breast cancers. Several antibodies targeting EGFR have been approved by the FDA for clinical use in patients with cancer:
Cetuximab (Erbitux) is a humanized immunoglobulin-G (IgG1) chimeric MoAb that binds to the external ligand-binding domain of EGFR1. It also binds with much lower affinity to EGF and TGF-α. The combination of cetuximab and irinotecan can improve disease response and progression-free survival (PFS) over the use of cetuximab alone in patients with advanced colorectal carcinoma who express EGFR on their tumors and have previously failed irinotecan therapy. Recent studies have suggested that better PFS and overall survival (OS) can be achieved when cetuximab is combined with FOLFIRI (a combination made up of folinic acid, fluorouracil, and irinotecan) or FOLFOX-4 (a combination made up of folinic acid, fluorouracil, and oxaliplatin) in advanced colon cancer (see Chapter 7 for a definition of these regimens and further discussion). The increased response rate as a result of adding cetuximab was higher in patients with tumors expressing the wild type KRAS gene. Currently, cetuximab in combination with irinotecan is approved by the FDA to treat patients with advanced colon cancer expressing EGFR who failed irinotecan treatment or as a single agent in patients who cannot tolerate irinotecan. It is also approved in combination with radiation or as monotherapy in patients who failed prior platinum-based therapy in unresectable head and neck cancers. Recently, a phase III trial demonstrated that patients with advanced EGFR-positive non-small-cell lung cancer (NSCLC) treated with cetuximab combined with cisplatin/vinorelbine had superior survival compared to chemotherapy alone. It has also been found that, in this group of patients, KRAS mutation correlates with progressive disease and shorter median time to progression, but not with survival. Similar to other antibodies, common side effects include rash and diarrhea, and, although very uncommon, cardiac arrest and myocardial infarction (MI) were reported among the serious side effects.
Panitumumab (Vectibix) is a fully humanized MoAb that has been developed against EGFR. Panitumumab binds to EGFR1 with higher affinity than cetuximab. A randomized phase III study demonstrated that patients with refractory EGFR-expressing metastatic colorectal cancer treated with panitumumab plus best supportive care had a better PFS compared to patients who received best supportive care alone. The patients who benefit from the treatment were those with tumors that did not contain KRAS mutations. Therefore, panitumumab was approved by the FDA as monotherapy for chemotherapy-refractory EGFR-expressing metastatic colon cancer. Other diseases with promising results using panitumumab include NSCLC and renal cancer. Common adverse effects include rash, peripheral edema, fatigue, and diarrhea. Serious toxicity, including bronchospasm, has been reported only rarely, and as a consequence does not require premedication for human use.
Other anti-EGFR MoAbs currently being evaluated in phase II trials include the following:
Matuzumab is a humanized anti-EGFR IgG1 MoAb. The agent has been tested in a phase I trial followed by paclitaxel in EGFR-expressing advanced NSCLC with a partial response achieved in 3 of 18 patients and a complete response reported in 1 treated patient. An ongoing trial is evaluating matuzumab in combination with pemetrexed in advanced NSCLC.
Nimotuzumab is a recombinant humanized IgG1 MoAb against EGFR that is approved for squamous cell carcinoma in head and neck in other countries and has been granted orphan drug status for glioma in the United States. Currently, it is being tested in combination with external radiotherapy in patients with NSCLC.
(2) Her-2-neu (HER2, erbB2)-targeted therapy. HER2 is the second member of the EGFR family. This receptor has the same basic structure as the other family members; however, no conjugate ligand has been identified for HER2. There have been no mutations identified in the HER2 gene in human cancers, yet it is overexpressed in many epithelial cancers including colon, pancreas, genitourinary, and breast cancers. HER2 signals via the phosphoinositide-3 kinase (PI3K)/Akt and mitogen-activated protein (MAP) kinase pathways, and HER2 overexpression leads to inhibition of apoptosis and increase in cell proliferation.
Trastuzumab (Herceptin) was one ofthe first MTTs to be introduced in clinical use. It is a humanized (chimeric) MoAb that binds the HER2. While the mechanism of action of trastuzumab is not entirely clear, it is believed to act through one or more of the following mechanisms: inhibiting the tyrosine kinase signaling of the receptor; activating antibody-dependent cellular cytotoxicity; induction of apoptosis; inducing G1 arrest by modulating the cyclin-dependent kinases; inhibition ofangiogenesis; and enhancing chemotherapy-induced cytotoxicity. The FDA approved trastuzumab in 1998 for use in patients with metastatic breast cancer overexpressing HER2 protein. In a large, multicenter phase III study in patients with metastatic breast cancer overexpressing HER2, it was demonstrated that trastuzumab, when used as first-line therapy in combination with chemotherapy (with either the combination of anthracyclines and cyclophosphamide or paclitaxel as a single agent), can significantly increase both the duration of response and the OS. Trastuzumab is currently used in three settings for patients with breast cancers overexpressing HER2: as a first-line therapy in combination with paclitaxel; as a second-line monotherapy in patients who have received at least one prior chemotherapy regimen; or in an adjuvant setting. Common adverse effects are asthenia, rash, and diarrhea. Serious side effects are ventricular dysrhythmia, cardiomyopathy, and thromboembolism.
Pertuzumab is a fully humanized MoAb directed against the extracellular domain of HER2. When pertuzumab was added together with trastuzumab in patients with metastatic breast cancer who did not response to trastuzumab alone, the combination to the two MoAbs showed a significant efficacy. A phase III trial is currently being conducted to evaluate the efficacy of pertuzumab and trastuzumab together in combination with docetaxel in HER2 metastatic breast cancer.
Ertumaxomab is a MoAb that binds HER2 and CD3. Ertumaxomab was evaluated in patients with HER2-positive metastatic breast cancer previously treated with trastuzumab and showed promising results.
b. Vascular endothelial growth factor (VEGF). The VEGF family of proteins is one of the specific positive regulators of angiogenesis. It is comprised of five different growth factors: VEGF-A, VEGF-B, VEGF-C, VEGF-D, and placental growth factor. Of these, VEGF-A exerts the most influence on the angiogenesis process. The VEGF proteins bind to three tyrosine-kinase receptors: VEGF receptor 1 (VEGFR1/Flt-1), VEGFR2 (kinase insult domain receptor/fetal liver kinase 1, Flk-1), and VEGFR3 (Flt-4). VEGFR2, through its interaction with VEGF, is thought to be the main mediator of tumor-associated angiogenesis and metastatic processes, while VEGFR1 plays a role in hematopoiesis. The VEGF-A is expressed or overexpressed in many tumors including lung, breast, and ovarian cancer, as well as gastrointestinal stromal tumors (GISTs) and in particular renal cell carcinoma (RCC), where the expression has been found to be high. Accordingly, targeting these molecules to abrogate their ability to stimulate tumor-associated angiogenesis constitutes a logical therapeutic strategy to control cancer. Both antibodies and small molecules have been developed as targeted therapies utilizing this pathway. Here, we will discuss the antibodies. The small molecules will be discussed later in the chapter.
Bevacizumab (Avastin) is a humanized murine anti-VEGF MoAb. It functions by blocking VEGF binding to its receptors (VEGFR), thereby inhibiting the tumor-induced angiogenesis process. When combined with fluorouracil-based chemotherapy regimens in advanced colon cancer, bevacizumab demonstrated improvement of both PFS and OS. Bevacizumab has also been shown to be effective in other tumors including NSCLC (nonsquamous). When combined with paclitaxel and carboplatin, it showed higher response rates and longer disease-free survival and median survival. In untreated metastatic RCC, the addition of bevacizumab to interferon alpha resulted in an increased PFS of 5 months compared with interferon-alpha (IFNα) alone. Currently, bevacizumab is approved by the FDA for use as first-line treatment in advanced colon cancer in combination with fluorouracil-based chemotherapy; in combination with platinum-based chemotherapy as a first-line treatment in patients with locally advanced, metastatic, or recurrent NSCLC (nonsquamous); and in combination with IFNα for the treatment of patients with metastatic RCC. It is also approved as monotherapy in recurrent glioblastoma.
Bevacizumab was approved by the FDA in 2008 for use as first-line therapy in combination with paclitaxel in patients with metastatic HER2-negative breast cancer, based on an improvement in PFS of 5.9 months in patients receiving the combination compared to those receiving paclitaxel alone. However, the FDA Oncology Drugs Advisory Committee recommended that approval be withdrawn based on new trials that did not show any improvement in OS and minimal improvement in PFS. Based on the new data and the increased risk of death due to bevacizumab in the new trials (0.8% to 1.2%), the FDA is reviewing the approval of bevacizumabas first-line therapy in metastatic breast cancer.
On the other hand, while the addition of bevacizumab to gemcitabine-erlotinib did not improve OS in patients with metastatic pancreatic cancer, the PFS was significantly longer in the bevacizumab group compared with placebo. Major serious side effects include arterial thrombosis, where bevacizumab has been shown to double the incidence of this complication. The drug has also been shown to increase the incidence of hemorrhage and hypertension in certain cases. Hemoptysis seems to be a particular risk in squamous cell lung cancer.
c. Insulin-like growth factor type I receptor (IGF1R). IGF1R is an RTK belonging to the insulin-like growth factor (IGF) receptor family which is comprised of three transmembrane proteins and binds to the IGF-1 and IGF-2. It is overexpressed in many tumors including melanoma, colon, pancreas, prostate, and kidney cancers. IGF1R overexpression in cancer cells is an important factor for their proliferation, transformation, and metastasis. Therefore, IGF1R became an attractive target for cancer therapy.
Figitumumab is a new humanized MoAb against IGF1R. The combination of figitumumab, paclitaxel, and carboplatin demonstrated safety and efficacy in patients with advanced NSCLC. Other phase III trials of figitumumab in advanced NSCLC are ongoing.
2. Inhibition of RTKs. Kinases are enzymes that have the ability of attaching a phosphate moiety to another protein. This occurs on a side chain of a serine, threonine, or tyrosine moiety, and the side chain that becomes phosphorylated is used to classify these kinases. The phosphorylation of proteins regulates the behavior of the molecules including protein binding activity, enzymatic activity, trafficking within the cell, or degradation. As a consequence, the phosphorylation process is a crucial biochemical reaction involved with controlling the behavior of a cell. Their critical role in cancer is shown by the observation that mutations in these kinases may lead to drastic outcomes, including uncontrolled proliferation. Receptor serine/threonine kinases will be discussed in another section; here, we will discuss the RTKs. RTKs are a combination of protein families sharing several structural and functional features. These kinases are glycoprotein receptors with extracellular, transmembrane, and intracellular domains. While the transmembrane domain acts as an anchor for the receptor within the membrane of the cell, the extracellular domain contains a binding site for a specific multipeptide ligand. On receptor-ligand binding, signaling events specific to the receptor are initiated. The cytoplasmic domain contains a catalytic tyrosine kinase region and a regulatory region, which are integral to the transmission of downstream signals to the nucleus. Autophosphorylation of the receptor's kinase region initiates a signal transduction cascade leading to cell proliferation, survival/apoptosis, migration, adhesion, and promotion of angiogenesis. Some of the subfamilies in this group of RTKs include the platelet-derived growth factor receptor (PDGFR), EGFR, VEGFR, and fibroblast growth factor receptor. These RTKs are overexpressed or mutated in many human cancers. Therefore, targeting RTK activity is an attractive strategy for cancer therapy and is currently achieved by small molecules. A few small molecules have already been introduced into the clinical practice and many other are currently in clinical trials. Here we will discuss some of these molecules.
Erlotinib (Tarceva) is an orally available small molecule with the N-(-3-ethynylphenyl)-6,7-bis (2-methoxyethoxy)-4-quinazolinamine. This compound is a reversible kinase inhibitor of EGFR and acts by competing with ATP in binding the intracellular domain of the tyrosine kinase region. It blocks the signal transduction of the EGFR, leading to the inhibition of the downstream effect of the pathway including cell propagation and survival, as well as angiogenesis. Erlotinib is a highly selective inhibitor for the EGFR tyrosine kinase region, as concentrations of more than 1,000-fold are required for the inhibition of other tyrosine kinases. In phase III placebocontrolled clinical trials in patients with locally advanced or metastatic NSCLC, the efficacy of erlotinib was assessed after the failure of at least one chemotherapy regimen. Erlotinib resulted in a median survival of 6.7 months versus 4.7 months when compared to placebo. On the other hand, no major benefit was observed when erlotinib was used as a first-line therapy in combination with platinum-based chemotherapy. In a recent randomized, multinational trial, the administration of erlotinib after standard platinum-based chemotherapy resulted in improved PFS when compared to placebo. For patients with pancreatic cancer, the addition of erlotinib to gemcitabine was found to improve median survival by 13.8 days over gemcitabine alone, with an increase in 1-year survival from 17% to 24%. Ac-cordingly, erlotinib was approved by the FDA in November 2005 for the treatment of patients with locally advanced or metastatic NSCLC as a second- or third-line therapy. The FDA has recently approved erlotinib for maintenance treatment of patients with locally advanced or metastatic NSCLC whose disease has not progressed after four cycles of platinum-based first-line chemotherapy. Erlotinib is also approved as a first-line therapy in combination with gemcitabine for locally advanced or metastatic pancreatic carcinoma. Although the trend in patients carrying the wild type RAS is to benefit more from erlotinib therapy, no significant correlation were found between KRAS mutations and outcome in patients enrolled in erlotinib trials. Clinical trials are currently being conducted to test erlotinib in combination with other agents as first-line therapy for advanced NSCLC, and as an adjuvant or neoadjuvant in patients with bladder cancer. The most common toxicities include skin rash (12%) and diarrhea (5%). MI and interstitial lung disease are reported among the serious side effects.
Gefitinib (Iressa) is a small molecule designed to effectively inhibit the tyrosine kinase activity of the EGFR. This compound initially showed an effect in randomized phase II trials with symptomatic improvement in advanced NSCLC. However, further placebo-controlled phase III studies as frontline showed no survival benefit. Therefore, the FDA changed the labeling to limit its use to patients with locally advanced or metastatic NSCLC who have previously benefited from the drug or for patients who are already receiving the agent and have demonstrated benefit. As a first-line therapy in NSCLC, gefitinib in combination with platinum-based chemotherapy showed no benefits. Gefitinib can cause rash and diarrhea. Serious side effects include interstitial lung disease and hemorrhage.
Sunitinib (Sutent) is an ATP competitive inhibitor that leads to the inhibition of the phosphorylation of the kinase and inhibition of further downstream signal transduction in multiple RTKs. It functions as an inhibitor to a closely related family of RTKs including PDGFR α and β VEGFR, stem cell factor receptor KIT, FMS-like tyrosine kinase-3 receptor, and the Ret oncoprotein. Accordingly, the sunitinib antitumor effect is multifactorial. It inhibits cell proliferation and has an antiangiogenesis effect. The antiangiogenesis effect of sunitinib is through the inhibition of both the VEGFR and PDGFR, which is important for the recruitment of pericytes. By inhibiting both VEGFR and PDGFR, sunitinib possesses a stronger inhibiting effect on angiogenesis cells than those agents targeting VEGF alone. Angiogenesis is the hallmark of RCC, and RCC has been demonstrated to overexpress VEGF and PDGF. Sunitinib would be expected to play a therapeutic role in this disease. A multinational phase III clinical trial comparing sunitinib to IFNα as a first-line treatment in advanced RCC showed a major advantage in overall survival of 11 months versus 5 months and has been approved by the FDA as first-line therapy for this indication. Kit and PDGFR play an important role in the development of the GISTs. More than 85% of GISTs possess activating mutations of the Kit kinase, and another 5% are associated with mutation in the PDGFR. Based on the mechanism of action of sunitinib, it is expected to play a role in the inhibition of such tumors and is a natural candidate for the treatment of GIST. Sunitinib showed a delay in tumor growth in patients with advanced GIST who failed imatinib compared with placebo in another phase III trial. As a result, sunitinib has been approved for patients with GIST whose disease has progressed or are unable to tolerate treatment with imatinib. Sunitinib is also currently being tested in other cancers including breast cancer and neuroendocrine tumors with promising results. Common side effects are rash, neutropenia, lymphopenia, thrombocytopenia, and increased transaminases. Serious side effects are hypertension, left ventricular dysfunction, prolonged QT, and severe hypothyroidism.
Lapatinib ditosylate (Tykerb) is an HER2 RTK inhibitor. It is FDA-approved for patients with advanced, refractory HER2-positive breast cancer who failed trastuzumab, as a single agent or in combination with letrozole or capecitabine. When lapatinib was combined with capecitabine, anthracyclines, taxanes, and trastuzumab in a phase III, open-label, randomized trial, patients with HER2-positive refractory locally advanced or metastatic breast cancer had a longer time to disease progression compared with capecitabine alone and a nonsignificant trend toward longer OS. On the other hand, lapatinib ditosylate showed no clinical efficacy in patients with HER2-negative metastatic breast cancer. Common side effects are diarrhea, anemia, and rash. Severe side effects are hand-foot syndrome and severe hepatotoxicity.
Pazopanib (Votrient) is a tyrosine kinase inhibitor of VEGFR, PDGFR, and c-Kit. Although pazopanib was found to increase PFS by 5 months compared to placebo in patients with advanced RCC who were previously untreated or who only received cytokine therapy, the increase in OS was not significant. Pazopanib is FDA-approved for patients with advanced RCC. Side effects of the drug include diarrhea, hypertension, and nausea. Noted serious side effects were hepatotoxicity, hemorrhage, MI, and QT prolongation.
Vandetanib (Zactima) is a multityrosine kinase inhibitor of EGFR, VEGFR2, and the RET gene, which is associated with hereditary and sporadic medullary thyroid cancer. Vandetanib demonstrated an improvement in median PFS compared to placebo in unresectable locally advanced or metastatic medullary thyroid cancer. In patients with advanced NSCLC, the addition of vandetanib to docetaxel resulted in a statistically significant improvement in PFS. Common side effects are fatigue, headache, anorexia, nausea, vomiting, diarrhea, and myelosuppression. Hypertension and corrected QT interval prolongation are occasional.
3. Inhibition of intracellular signaling proteins and protein kinases. This therapeutic strategy is directed against a group of proteins that function in a network of communicating cascades to transfer the signal from receptors into the nucleus to produce the intended biologic effect including cell proliferation, apoptosis, angiogenesis, etc. When mutated, these proteins produce deregulated pathways contributing to the malignant transformation of the cell. These proteins are either nonreceptor tyrosine or serine/ threonine kinases. The non-RTKs are cytoplasmic kinases. Many of these are attached to and closely linked to membrane receptors. They are usually activated by the binding of ligand to their associated receptors. Some of these kinases include src, abl, and JAK. The serine/threonine kinases are intracellular kinases and some play crucial roles in carcinogenesis. These kinases include raf, kinases from the PI3K/Akt/mammalian target of rapamycin (mTOR) pathway, and the MAP kinases. Small molecules have been designed to block or reverse the effect of these pathways and some are in clinical use. In general, these molecules that inhibit the intracellular signal proteins and protein kinases can target multiple targets, including receptor kinases. They can, therefore, be classified as receptor kinase inhibitors. For the sake of simplicity, this chapter will classify these kinases based on their primary kinases or pathway effect and will allude to their other roles within the description of the drug.
a. Bcr-Abl tyrosine kinase. The Bcr-Abl fusion protein is the resultant product of the translocation between the Bcr and Abl-1 genes. The Abl-1 gene encodes a nonreceptor tyrosine kinase, while the Bcr encodes a serine/threonine kinase. The product of the translocation encodes for a phosphorylated fusion protein that activates many pathways including the RAS, PI3K, and STAT pathways and results in malignant transformation. Drugs designed to target this molecule include:
Imatinib mesylate (Gleevec) was one of the first targeted therapy small molecules to enter into clinical practice. It is primarily a protein kinase inhibitor that is designed to inhibit Bcr-Abl tyrosine kinase. By inhibiting the Bcr-Abl fusion tyrosine kinase, imatinib mesylate induces apoptosis in Bcr-Abl–positive cells through binding to Abl-1 and competing with ATP, leading to inhibition of the active tyrosine kinase of the fusion protein. It is active against Bcr-Abl–positive chronic myelogenous leukemia (CML) and Bcr-Abl–positive acute lymphocytic leukemia (ALL). Imatinib mesylate produces hematologic, cytogenetic, and molecular remissions that are often long-term and sustainable. Imatinib mesylate also inhibits the receptor kinases PDGF, stem cell factor, and c-Kit. As mentioned earlier, c-Kit is mutated in 85% of GISTs. Imatinib was tested in and found to be effective in GISTs. The current FDA-approved indications for imatinib mesylate include CML that is Philadelphia chromosome–positive (Ph+) whether newly diagnosed, in chronic phase, in accelerated phase or blast crisis, after failure of IFNα therapy, or recurrence after stem cell transplant. It is also indicated in malignant c-Kit–positive GISTs that are unresectable or metastatic. Three large international phase III trials are currently ongoing to evaluate the role of imatinib mesylate in the adjuvant setting with patients with GIST. Imatinib mesylate is also FDA-approved for patients with myelodysplastic syndrome with PDGFR gene rearrangement and in patients with chronic eosinophilic leukemia. Common side effects are edema, rash, diarrhea, vomiting, and night sweats. Serious side effects are congestive heart failure (CHF), cardiogenic shock, and cardiac tamponade. Imatinib mesylate can also cause severe anemia, thrombocytopenia, and febrile neutropenia. Clinically significant resistance to imatinib mesylate is increasingly seen and has been found to occur in patients who develop mutations within the kinase domain in the Bcr-Abl proteins. Therefore, the need to develop alternative Bcr-Abl inhibitors is very important. Some of these alternative kinase inhibitors are discussed subsequently.
Dasatinib (Sprycel) is an oral inhibitor of multiple tyrosine kinases including Bcr-Abl, c-Kit, and PDGFR. Clinical data with dasatinib showed that 31% to 38% of imatinib-resistant and 75% of imatinib-intolerant patients with chronic phase CML reached major cytogenetic response. In addition, 30% to 59% of patients with advanced CML and Ph+ ALL showed major hematologic response. Therefore, dasatinib was initially approved for patients with chronic, accelerated, or blast phase of CML who are intolerant or resistant to prior therapy with imatinib. Subsequently, dasatinib was also approved as first-line therapy in CML based on a study that showed a superiority of dasatinib compared to imatinib in major molecular response rate and complete cytogenetic response rate at 12 months (46% versus 28% and 77% versus 66%, respectively). Dasatinib is also indicated in patients with Ph+ ALL who failed prior therapy. Ongoing trials are testing dasatinib as treatment for patients with castration-resistant progressive prostate cancer. Dasatinib can cause edema and rash. Serious side effects include CHF, prolonged QT interval, anemia, thrombocytopenia, and neutropenia.
Nilotinib (Tasigna) is another Abl kinase inhibitor. Similar to imatinib, it acts by competing with the ATP-binding site of Bcr-Abl. Nilotinib differs from imatinib by having a higher binding activity to the Abl kinase site with higher inhibitory activity in imatinib-sensitive cell lines. Nilotinib was found to induce both hematologic and cytogenetic responses in patients with Ph + CML in chronic or accelerated phase who are resistant or intolerant to imatinib. Nilotinib was initially approved by the FDA in chronic- or accelerated-phase CML that is resistant or intolerant to imatinib. Similar to dasatinib, nilotinib was also subsequently approved by the FDA as first-line therapy in CML based on data demonstrating superiority over imatinib in complete cytogenetic response at 12 months (80% versus 65%) and the time of progression to accelerated phase or blast crisis. Edema, rash, nausea, diarrhea, thrombocytopenia, and anemia are among the common side effects. Prolonged QT, torsade de pointes, and sudden death are among the serious side effects.
b. The Raf/MAP kinase pathway. The Raf is a family of serine/ threonine kinases, including A-Raf, B-Raf, and C-Raf, and is part of the RAS pathway. Raf is activated when RAS, in response to the activation of a RTK, recruits and phosphorylates Raf kinase at the membrane site. Raf, in turn, phosphorylates MEK that activates and phosphorylates ERK. Activated ERK enters the nucleus to activate other transcription factors, leading to cellular proliferation. Aberration in this pathway leads to deregulation of proliferation, resulting in transformation of the cell. B-Raf has been found to be mutated in many tumors such as melanoma, thyroid, and colorectal cancers. Therefore, inhibition of this kinase is a reasonable target in cancer treatment.
Sorafenib (Nexavar) is a small molecular inhibitor of C-Raf kinase that leads to the inhibition of the Raf/MEK/ERK signaling pathway. Sorafenib has also been found to be a strong inhibitor of both VEGFR2 and PDGF kinase. A large phase III study showed that sorafenib can reduce the risk of death by 23% compared to placebo in patients with advanced RCC. Therefore, sorafenib was originally approved in 2005 for patients with advanced RCC. In addition, when sorafenib was compared to IFNα-2a in untreated RCC patients, while PFS was similar in both arms, greater rates of tumor size reduction, better quality of life, and tolerability were achieved in the sorafenib arm. Sorafenib was also approved in 2007 for the treatment of patients with unresectable hepatocellular carcinoma (HCC) after it was found to prolong the median survival and the time to radiologic progression compared to placebo. Other clinical trials are ongoing to test the efficacy of sorafenib in other cancers. Common side effects are hypertension (in 9% to 17% of cases), alopecia, hypophos-phatemia, and diarrhea. Severe side effects are hand-foot syndrome, chronic heart failure, and myocardial infarction.
Other RAS/RAF/MEK/ERK signaling pathway inhibitors are currently being evaluated in clinical trials:
GSK1120212 is an inhibitor of MAP kinase (MEK MAPK/ERK kinase). MEK 1 and 2 are upregulated in different cancers and they are involved in the activation of the RAS/RAF/MEK/ERK signaling pathway. GSK1120212 specifically inhibits MEK 1 and 2, resulting in an inhibition of growth factor-mediated cell signaling and cellular proliferation. The safety and efficacy of GSK1120212 is currently being studied in two phase II clinical trials: the first trial is being conducted in patients with BRAF mutation–positive melanoma who were previously treated with a BRAF inhibitor, and the second trial is being conducted in relapsed or refractory acute myeloid leukemia.
GSK2118436/SB-590885 is a selective inhibitor of RAF kinases. It has more potency toward BRAF than CRAF. SB-590885 inhibits BRAF kinase activity 100-fold more potently than sorafenib, and it may be promising in overcoming the generated resistance to inhibitors that bind to the inactive conformation of BRAF. There is also a potential of combining BRAF inhibitor (SB-590885) with a GSK MEK inhibitor (GSK1120212).
PLX4032/RG7204 is a selective inhibitor of the oncogenic V600E mutant BRAF kinase. PLX4032/RG7204 was studied in a phase I trial and showed 10 partial responses and 1 complete response in patients with melanoma, with 9 patients having regression of liver, lung, and bone metastases. Currently, PLX4032/RG7204 is being studied in a phase II trial in patients with V600E BRAF-mutations and a phase III randomized trial comparing PLX4032/RG7204 with dacarbazine chemotherapy in V600E-mutated melanoma.
c. PI3K, AKT, and mTOR pathway inhibitors. The PI3Ks are a family of lipid kinases divided into three classes based on their protein structure. Class I PI3K has been studied more closely due to the role it plays as a regulator of cell survival, proliferation, and differentiation. Class IA PI3Ks, comprised of four subunits (p110 α, β, γ, and δ), is recruited to the membrane on the activation of RTKs. This leads to a signaling cascade that activates multiple downstream signaling pathways including the AKT pathway. The mTOR pathway is downstream of the PI3K/Akt pathway and plays an important role in cell growth regulation and proliferation. The mTOR pathway is regulated by the PTEN tumor suppressor gene. The protein encoded by the PTEN gene is a phosphatase that works as an on/off switch. The switch moves to “on” position when PI3K deposits a phosphate group on the D3 position of the inositol ring; when PTEN removes the phosphate group from the same position, the switch moves to the “off” position. It has being found that genetic alterations in the PI3K pathways play an important role in different cancers including breast, colon, and ovarian cancer. Therefore, a plethora of novel agents targeting the PI3K/Akt/mTOR pathways have recently been developed for treatment of cancer. Two of these agents are already approved by the FDA and the rest are still under clinical trials and expected to reach the clinic in the next decade. They are as follows:
Everolimus (Afinitor) is a kinase mTOR inhibitor. It was approved by the FDA in 2009 for the treatment of advanced kidney cancer in patients who failed sunitinib or sorafenib. This was based on clinical trials that showed the median PFS of 4.0 months in patients who received everolimus versus 1.9 months in the placebo group. Common side effects include rash, edema, and diarrhea. Serious side effects include pneumonitis, anemia, leukopenia, and neutropenia.
Temsirolimus (Torisel) is a competitive inhibitor of mTOR kinase. It is indicated for the treatment of patients with advanced RCC based on the interim analysis of a phase III, randomized, clinical trial demonstrating temsirolimus significantly increased OS compared to IFNα-2a in patients with previously untreated advanced RCC. However, the combination therapy with IFNα-2a plus temsirolimus did not significantly improve OS as compared to IFNα-2a alone. Common side effects include anemia and hyperlipidemia. Serious anaphylaxis was also reported.
XL147 is a selective inhibitor of Class I PI3K isoforms. It was studied in a phase I trial in patients with solid tumors, the majority of whom had NSCLC. The dose limiting toxicity was related to rash, and the most common side effects were rash and fatigue. XL147 is currently being studied in two phase II trials. The first trial is evaluating the safety and efficacy of XL147 in patients with advanced or recurrent endometrial cancer. The second trial is evaluating the combination of XL147 with trastuzumab or paclitaxel and trastuzumab in patients with metastatic breast cancer who have progressed on a previous trastuzumab-based regimen.
XL765 is selective inhibitor of mTOR and Class I PI3K isoforms. A phase II trial is ongoing to evaluate the safety and clinical efficacy of either XL147 or XL765 in combination with letrozole in patients with breast cancer that is ER+/ PGR+ and HER2 and refractory to a nonsteroidal aromatase inhibitor.
GDC-0941 is another potent and selective oral inhibitor of the class I PI3K. GDC-0941 demonstrated significant efficacy in preclinical studies in breast, ovarian, lung, and prostate cancer models. It was studied in a phase I trial in solid tumors in 38 patients (10 of whom had sarcoma) and found to have good tolerability.
GSK2141795 is an oral AKT inhibitor currently being studied in a phase I, open-label, two-stage study of lymphomas and solid tumors.
Ridaforolimus (formerly known as deforolimus) is a novel small-molecule inhibitor of mTOR. In a multicenter phase II trial of ridaforolimus in patients with progressive advanced sarcomas, ridaforolimus more than doubled PFS when compared with historical data. It is currently being studied in endometrial, prostate, breast, and NSCLCs.
AZD8055 is a selective, orally active inhibitor of mTOR kinase. It is currently in phase I/II trials in patients with advanced solid tumors.
Perifosine is a synthetic alkylphospholipid that inhibits or modifies different signal transduction pathways (AKT, MAPK, and JNK). Perifosine was studied in a phase II exploratory randomized double-blind, placebo-controlled study where patients with metastatic colorectal cancer received perifosine in combination with capecitabine or placebo. Perifosine in combination with capecitabine more than doubled median time to progression over capecitabine.
B. Angiogenesis-targeted therapy
Angiogenesis is a biologic process that is crucial for the development of tumors. Tumors have exploited this physiologic process to provide the milieu to permit the growth of both primary and metastatic cancers. The process of angiogenesis starts by the release of VEGF by the tumor. VEGF binds to receptors on the blood vessels' endothelial cells, leading to their proliferation and immigration toward the source of the angiogenic signal. Although the antineoplastic effect of antiangiogenesis therapy is mediated through the effect on the environment for the cancer cell growth, the initial mechanism of current therapies is based on molecular targeting, which was previously described.
C. Protein degradation–targeted therapy
Protein degradation is one of the mechanisms by which cell function is regulated. The ubiquitin-proteosome pathway plays a very important role in this regard. The proteosome is a large complex of proteins that degrades other ubiquitinated proteins. It exerts its degradation capability through coordinated catalytic activities of its three proteolytic sites that leads to chymotryptic, tryptic, and postglutamyl peptide hydrolyticlike activities. Many key proteins in the cell cycle, apoptotic, and angiogenesis pathways are regulated by degradation, including the p53, p21, p27 (cell cycle regulatory) proteins; NF-κB, a key transcription factor that is activated by the proteosomes; and ICAM-1, VCAM, and E selectin (cell adhesion molecules). Drugs targeting degradation machinery in the cell include:
Bortezomib (Velcade) is a dipeptidyl boronic acid derivative that inhibits the 26S proteosome. The 26S proteosome is the principal regulator of the intracellular protein degradation. Bortezomib is the first of its class to be approved for clinical use. Bortezomib can selectively inhibit the chymotryptic site of the proteosome. This leads to a selective inhibition of the degradation of proteins involved in cell proliferation and survival regulation; as a consequence, apoptosis is induced. Bortezomib has been found to be particularly effective in myeloma. A phase III trial that randomized patients with myeloma, who failed one to three previous therapies, to bortezomib versus high-dose dexamethasone demonstrated that bortezomib resulted in a superior outcome with respect to response frequency, time to progression, and OS. Bortezomib was approved in 2005 for the treatment of patients with refractory multiple myeloma who had received at least one prior therapy and, in 2008, as an initial treatment for patients with multiple myeloma based on the results of a phase III trial showing the superior efficacy of bortezomib plus melphalan and prednisone in delaying time to progression compared to melphalan-prednisone therapy alone. Bortezomib is also FDA-approved for patients with mantle cell lymphoma who failed at least one prior therapy. Bortezomib is currently being studied in patients with relapsed or refractory peripheral T-cell lymphoma. The observed side effects of bortezomib are asthenia, hypertension, rash, and diarrhea. Serious side effects include CHF, anemia, neutropenia, and thrombocytopenia.
Carfilzomib is the next generation of proteasome inhibitor with higher selectivity and specificity. It binds selectively to the N-terminal threonine active sites within the proteasome and is currently being studied in phase IIb clinical trial in patients with relapsed and refractory multiple myeloma. A phase III international randomized trial will also be evaluating the efficacy of carfilzomib in combination with lenalidomide and dexamethasone versus lenalidomide and dexamethasone in patients with relapsed multiple myeloma.
D. Immune modulation–targeted therapy
1. Specific immune modulators. Immune homeostasis is the function of balancing the effector and inhibitory arms of the immune system. This balance prevents the overreaction of the effector immune response that can lead to the generation of harmful immune reactions against normal tissues. The inhibitory arm of the immune system is composed of cytokines/ligands (e.g., interleukin [IL]-10; TGF-β), T-cell inhibitory molecules/receptors (e.g., CTLA-4 and PD1), or immune cells (e.g., T-regulatory cells and myeloid suppressor cells). Unfortunately, the effect of the inhibitory arm of the immune system can lead to the inability of the immune system to mount a proper response against cancer. Recently, there has been significant scientific progress in the understanding of these inhibitory mechanisms and accordingly, targeting strategies against these inhibitory mechanisms have been developed with the intention of enhancing immune response against tumors. Some are aimed to interrupt the inhibitory signals of the receptors (e.g., CTLA4 or PD1) and others to neutralize the effect of inhibitory cytokines (e.g., TGF-β). Accordingly, most of the specific immune modulators are designed to target immune inhibitory molecules.
a. CTLA-4 inhibitors. CTLA4 is a coinhibitory molecule that is expressed on T-cells. It is a homolog of CD28 with higher binding affinity to B7.1 (CD80) and B7.2 (CD86). CTLA4 binding to B7.1 or B7.2 ligands on antigen presenting cells sends an inhibitory signal into the T-cell leading to reduction in T-cell activation and cytokine production, creating an immunosuppressant environment. Therefore, blocking CTLA4 by using anti-CTLA4 antibodies should prevent this inhibitory signal and maintain and enhance the immune response against tumors. Currently, there are two anti-CTLA4 antibodies in clinical trials.
Ipilimumab (MDX-010) is the first humanized anti-CTLA4 IgG1 MoAb. Ipilimumab demonstrated clinical efficacy in multiple phase II clinical trials in patients with stage III or IV unresectable melanoma. Ipilimumab was studied in patients with metastatic RCC in a phase II study: 1 of 21 patients at the lower dose and 5 of 40 patients at the higher dose had partial responses. Adverse events were mainly autoimmune toxicities in the skin and gastrointestinal tract (grade III to IV immune-related adverse effects reported in up to 33% to 40% of patients). Ipilimumab is also currently being tested in hormone-refractory prostate cancer.
Tremelimumab is a human IgG2 antibody with high affinity for CTLA-4. Tremelimumab was evaluated in a phase II study in patients with stage III or stage IV recurrent metastatic melanoma and was found to prolong the median survival for 3 months compared to standard therapy. Similar to ipilimumab, autoimmune toxicities including dermatitis (20% to 40%), colitis (44%), uveitis (10%), and thyroiditis (3%) were reported.
b. PD1 inhibitors. PD1 is an inhibitory receptor belonging to the B7-receptor family that interacts with two known ligands: PD-L1 (B7-H1) and PDL2 (B7-DC). Tumors are known to overexpress PDL1. The interaction of PD1 with PDL1 or PDL2 results in downregulation of signals by the T-cell receptor and induction of apoptosis in activated T-lymphocytes leading to immune suppression. Accordingly, anti-PD1 antibodies are being developed in order to overcome the immunosuppression to tumor.
CT-011 is a humanized IgG1 kappa recombinant monoclonal antibody against the PD1 receptor that blocks the interaction of PDL1 with PD1. Clinical responses were observed in six patients within 12 months after initial treatment with CT-011 in a phase I trial. One patient with follicular lymphoma had a complete remission 10 months post single dose administration. CT-011 is currently being evaluated in a number of phase II clinical studies in combination with different agents including autologous stem cell transplantation in diffused large B-cell lymphoma, FOLFOX in colorectal carcinoma, and rituximab in patients with relapsed follicular lymphoma. CT-011 is also being tested in combination with peptide vaccines.
MDX-1106 (ONO-4538) is a fully humanized IgG4 anti-PD1 antibody. MDX-1106 was tested in a phase I trial. One patient with colorectal carcinoma experienced a partial response lasting for more than 6 months. Tumor regressions were observed in four additional patients, including two patients with melanoma, one patient with NSCLC, and one with RCC.
c. TGF-β antibodies. TGF-β is an immunosuppressive cytokine found at the site of most tumors. It inhibits T-cell proliferation and differentiation into cytotoxic or helper T-cells. In addition, it has been found that TGF-β can induce the generation of T-regulatory cells that are known to inhibit antitumor activity. As a result, antibodies targeting TGF-β have being developed for therapeutic purposes to enhance immune response against cancer.
GC-1008 is a TGF-β neutralizing MoAb. It was evaluated in a phase I trial in patients with malignant melanoma and RCC. Five out of twenty-two patients achieved stable disease.
AP-12009 is a TGF-β2-specific phosphorothioate. It has been studied in phase I/II in gliomas and anaplastic astrocytomas, demonstrating a significant increase in survival compared to standard temozolomide therapy.
2. Nonspecific immunomodulators. Nonspecific immunomodulators are a family of medications that are derivatives of thalidomide by minor structural modifications. These modifications lead to the enhancement of drug efficacy and the improvement in the side effect profile, including the neurologic toxicity and prothrombotic effects of thalidomide. The mechanism of action for this group of compounds is not clearly defined. Many pathways have been shown to be triggered by these medications including caspase-8, proteosome, NFκB, and the antiangiogenesis pathways.
Lenalidomide (Revlimid) is one of the new generation of nonspecific immunomodulators. In a randomized phase III, when combined with dexamethasone, lenalidomide was found to be superior in complete response (CR), PFS, and OS to dexamethasone alone in patients with relapsed or refractory multiple myeloma. When lenalidomide is combined with dexamethasone in patients with newly diagnosed multiple myeloma, 91% of patients achieved an objective response, including 11% with CR. Lenalidomide was approved for use in combination with dexamethasone in patients with multiple myeloma who received at least one prior therapy, and in patients with myelodysplastic syndrome with 5q deletion who are transfusion dependent. Side effects of the drug were found to be tolerable and compared to thalidomide, lenalidomide ashowed no significant somnolence, constipation, or neuropathy. Common side effects are edema and rash. Serious side effects include atrial fibrillation, Stevens-Johnson syndrome, neutropenia, anemia, and thrombocytopenia.
III. PHENOTYPE DIRECTED-TARGETED THERAPY
As outlined previously, this is a therapeutic strategy that is intended to target the unique phenotype of the cancer cell where killing the cell is more dependent on direct induction of a cytotoxic effect rather than targeting a specific pathway, as discussed subsequently. Agents under this category will be classified based on the type of therapy and subclassified based on the target pathway or molecule, if applicable.
A. Nonreceptor protein–directed MoAbs
These are a group of antibodies developed to recognize specificantigens expressed on the surface of cancer cells. Their purpose is not for blocking specific pathways or receptor proteins, but rather to induce direct cytotoxic effect. Th ese MoAbs may be used alone (unconjugated) or as a delivery system for cellular toxins, radionuclides, or chemotherapy (conjugated).
1. Unconjugated antibodies
Rituximab (Rituxan) is an IgG1 kappa murine-human chimeric MoAb that is generated against the CD20 antigen. CD20 is expressed on the cell surface of B-cells and hence on the surface of B-cell lymphoma. Rituximab is indicated as a single agent for the treatment of relapsed or refractory B-cell non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL) expressing the CD20 marker. Rituximab was studied in combination with CHOP (Cytoxan, hydroxydaunorubicin, and prednisone) therapy in patients with diffuse large B-cell lymphoma and resulted in better PFS and OS. Thus, rituximab was approved for patients with previously untreated diffuse large B-cell, CD20-positive NHL in combination with CHOP, or CVP (Cytoxan, vincristine, and prednisone), chemotherapy (see Chapter 22). Rituximab is also used in combination with fludarabine and cyclophosphamide in previously untreated and treated patients with CD20-positive CLL. Rituximab can cause hypertension, nausea, vomiting, fever, chills, and lymphopenia. Cardiac arrhythmia, cardiogenic shock, Stevens-Johnson syndrome, toxic epidermal necrolysis, and tumor lysis syndrome are reported as serious side effects.
Alemtuzumab (Campath) is a humanized IgG1 kappa murine–human chimeric MoAb that is directed against CD52-cell surface glycoprotein. CD52 is expressed on the surface of normal and malignant B- and T-cells, natural killer cells, monocytes, and macrophages. Alemtuzumab is indicated for the treatment of patients with B-cell CLL who have failed fludarabine based on demonstrated efficacy of the drug with 2% CR and 31% PR in this patient population. Alemtuzumab is currently being studied in combination with other agents. Common side effects are anemia, neutropenia, thrombocytopenia, rash, and diarrhea. Serious sideeffects are cardiac arrhythmia and cardiomyopathy. Patients who have recently been treated with this MoAb should not receive any live viral vaccines because of the immune suppression effect of the medication.
Ofatumumab (Arzerra) is a humananized IgG1-kappa MoAb that binds to the CD20 molecule on B-lymphocytes, which leads to B-cell lysis. It is FDA-approved for refractory CLL. A recent phase II study comparing the efficacy of of atumumab in patients with fludarabine and alemtuzumab refractory showed better overall response rates in CLL. Ofatumumab can cause rash, neutropenia, anemia, diarrhea, and sepsis.
Epratuzumab is a humanized MoAb that binds to the CD22 glycoprotein. CD22 is expressed on the cell surface of mature B-cells in follicular NHL. An overall response rate of 54% was achieved when epratuzumab was combined with rituximab in patients with relapsed indolent NHL.
2. Conjugated antibodies
a. Cellular toxin conjugated antibodies
Gemtuzumab ozogamicin (Mylotarg) is a humanized IgG4 kappa antibody directed against the CD33 antigen and conjugated with calicheamicin. Calicheamicin is a cytotoxic agent isolated from fermentation of the bacterium Micromonospora echinospora ssp. calichensis. The CD33 antigen is a sialic acid–dependent adhesion protein expressed on the surface of immature cells of the myelomonocytic lineage and on the surface of leukemic blast cells but not on the surface of normal pluripotent hematopoietic stem cells. When this fusion antibody binds to the CD33 receptors, it is internalized into the cell, and the calicheamicin is cleaved and released. Calicheamicin binds to the minor grooves of the DNA, leading to DNA breaks and apoptosis. Gemtuzumab ozogamicin is indicated for the treatment of older patients (over 60 years old) after the first relapse of myeloid leukemia expressing CD33 who are not candidates for chemotherapy. Clinical trials have shown when gemtuzumab was given as single agent, gemtuzumab may lead to 16% CR and 30% overall response with a median time to remission of 60 days. Gemtuzumab can cause fever, shivering, and nausea. Serious side effects are severe myelosuppression, hemorrhage, disseminated intravascular coagulation, and hepatotoxicity.
b. Radioimmunoconjugate antibodies
Ibritumomab tiuxetan (Zevalin, IDEC-Y2B8) is a murine anti-CD20 MoAb conjugated to tiuxetan that chelates to pure beta-emitting yttrium-90. The mechanism of action includes antibody-mediated cytotoxicity and cellularly targeted radiotherapy (radioimmunotherapy [RIT]). It is indicated for use in patients with rituximab CD20-positive refractory, follicular B-cell NHL. Ibritumomab tiuxetan is also being used in patients with relapsed B-cell NHL following high-dose chemotherapy and autologous stem cell transplantation with promising results. It should be used with caution in patients with 25% or greater marrow involvement with lymphoma, prior external beam radiotherapy to 25% or greater of the bone marrow, or a history of HAMAs or HACAs. Because the drug does not emit gamma radiation, hospitalization is not required. Neutropenia and thrombocytopenia are common and are related to the radionuclide dose. At the higher end of the dosing, 25% of patients will develop nadir neutrophil counts of less than 500/(µL. Low-grade nausea and vomiting are common. Infusion-related fever, chills, dizziness, asthenia, headache, back pain, arthralgia, and hypotension are occasional side effects.
Iodine-131 (131I)-tositumomab (Bexxar) is a murine IgG2a anti-CD20 MoAb radiolabeled with 131I, an emitter of both beta and gamma radiation. The mechanism of action includes antibody-mediated cytotoxicity and cellularly targeted RIT. It is indicated as a monotherapy in patients with rituximab refractory NHL that is chemotherapy refractory, CD20-positive, low grade, or transformed low grade. Furthermore, it was found that the combination of high-dose 131I-tositumomab and autologous hematopoietic stem cell transplantation is effective for relapsed B-NHL. Before dosimetric and therapeutic doses, patients are premedicated with acetaminophen 650 mg and diphenhydramine 50 mg. A saturated solution of potassium iodide, two to three drops orally three times daily, is given beginning 24 hours before the dosimetric dose and continuing for 14 days after the therapeutic dose to prevent uptake of 131I by the thyroid. 131I-tositumomab can cause hypertension, shivering, and diarrhea. It must be used with caution in patients with 25% marrow involvement with lymphoma, prior external beam radiotherapy to 25% of the bone marrow, or a history of HAMAs or HACAs.
These are recombinant proteins that are conjugated to cellular toxins and are designed to bind to specific proteins on the surface of cancer cells, internalized, and induce direct cytotoxic effect by releasing conjugated toxins intracellularly.
Denileukin diftitox (Ontak) is a recombinant construct that includes a fragment of the IL-2 protein (Ala1- Thr133) linked to a fragment of the diphtheria toxin fragment A and B (Met1-Thr387). This construct is designed to bind to the CD25 component of the IL-2 receptor (IL-2R) on the surface of the targeted cells expressing the receptor. The complex becomes internalized into the cytoplasm and releases the toxin to exhibit its damaging effect. The high-affinity IL-2R is normally present on the activated T- and B-lymphocytes and activated macrophages. Cutaneous T-cell lymphoma (CTCL) expresses high-affinity IL-2R and forms an appropriate target. A recent phase III randomized trial in patients with CTCL compared denileukin diftitox to placebo and showed a median PFS over 2 years, and 10% CR and 34% PR in patients who received denileukin diftitox, which is significantly better than placebo (median PFS of 124 days and overall response of 15.9%). Therefore, the indications for this agent include patients with persistent or recurrent CTCL expressing CD25. Denileukin diftitox can cause elevated liver transaminases, fever, nausea, edema, rash, and diarrhea. It can also cause serious capillary leak syndrome.
C. Cancer vaccines
This modality is a type of immunotherapy that is designed to stimulate the patient's own immune system against specific antigens expressed on the cancer cells with the intention to specifically target and destroy cancer cells with minimal side effects. Cancer vaccines are either prophylactic or therapeutic. Prophylactic vaccines are designed to prevent the causative agent of cancer. Accordingly, they generate an immune response against the infectious agent causing cancer. Four vaccines are currently available in the market: two vaccines against the human papillomavirus (Gardasil and Cervarix) and two vaccines against hepatitis B virus (Recombivax HB and Engerix-B).
The second type of cancer vaccines are therapeutic vaccines that can be used for the treatment of advanced disease or the prevention of progression of premalignant lesion or recurrence. This chapter will only be addressing therapeutic vaccines that will be used by oncologists. Therapeutic vaccines are administered to either target specific antigens that have already been identified within the tumor or to target multiple unidentified antigens. The first is usually administered in the form of peptide, protein, DNA, or RNA expressing the specific antigen, and the second is administered in the form of unfractionated whole cell lysate or intact tumor cells with the goal of eliciting T-cell responses against multiple undefined antigens expressed by the tumor. These antigens are either administered directly or pulsed on dendritic cells (antigen presenting cells). The FDA approved the first therapeutic cancer vaccine in April 2010; others are expected to be approved soon.
Sipuleucel-T (Provenge) is an autologous, dendritic cell-based vaccine (CD54+) that is pulsed with a selective prostate antigen: prostatic acid phosphatase. This antigen is expressed on 95% of prostate cancers. A phase III study in patients with metastatic, castration-resistant (hormone-refractory) prostate cancer demonstrated that sipuleucel-T increased 3-year survival by 40% compared to placebo (32.1% versus 23.0%). Based on these studies, the FDA has approved sipuleucel-T in patients with asymptomatic or minimally symptomatic metastatic hormone-refractory prostate cancer. Side effects include chills, fatigue, fever, and joint aches.
M-Vax (DNP-VACC) consists of autologous tumor cells conjugated to a highly immunogenic hapten: dinitrophenyl. Melanoma patients who were immunized with M-Vax had a 60% survival rate over 5 years compared with a survival rate of 20% in a historical control group and a survival rate of 32% in patients treated with high-dose IFNα.
Onco Vax is an autologous tumor cell vaccine. When given as an adjuvant to surgery, it significantly improved recurrence-free interval and recurrence-free survival compared to surgery alone in patients with melanoma.
TroVax is a modified vaccinia Ankara that delivers the tumor antigen 5T4, which is a nonsecreted membrane glycoprotein expressed on clear and papillary RCCs. In a phase II trial, treatment of 25 patients with renal cancer (21 clear cell and 4 papillary) with the combination of high-dose IL2 and TroVax resulted in 52% of patients with stable disease with PFS of 5 months. Survival rate was 80% at 24 months, compared to the historical figure of 50% for IL2 treatment.
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