Ivana K. Kim,
Joan W. Miller
OVERVIEW
The identification of vascular endothelial growth factor (VEGF) as a key mediator of ocular angiogenesis has led to the development of several pharmacologic agents designed for targeted inhibition of this molecule. Two of these agents, pegabtanib and ranibizumab, have been proven efficacious in the treatment of neovascular age-related macular degeneration (AMD) and are currently available for clinical use. Additional drugs targeting the VEGF pathway as well as other antiangiogenic compounds are under evaluation in clinical trials and appear promising. As options for pharmacologic therapy multiply, combination therapy using various agents and modalities may provide improved visual outcomes for patients with choroidal neovascularization due to AMD.
Recent progress in the treatment of neovascular AMD has enabled an evolution from nonspecific, ablative laser therapy to targeted pharmacologic approaches. These new pharmacologic agents represent the culmination of over a decade of investigations into the mechanisms of ocular angiogenesis.
Early observations of increased vascularity associated with the growth of tumors gave rise to the field of angiogenesis.[1]Ide and colleagues first proposed the idea of secreted factors which could promote growth of blood vessels in 1939, basedon studies of vascular development accompanying the growthof transplanted rabbit epithelioma.[2] Similarly, in 1948, Michaelson suggested that a diffusible 'Factor' from the retina stimulated the retinal and iris neovascularization seen in diabetic retinopathy.[3] However, many years passed before such proposed angiogenic factors were identified.
THE ROLE OF VEGF IN OCULAR ANGIOGENESIS
The idea of inhibiting angiogenesis was first proposed as a strategy for treating cancer in 1971 by Judah Folkman, who spearheaded the efforts to discover tumor angiogenesis factors.[4] In 1983, Senger and colleagues discovered a protein secreted from a guinea pig tumor cell line which was a potent inducer of vascular leakage and named it vascular permeability factor (VPF).[5] In 1989, Napoleon Ferrara and colleagues at Genentech identified a molecule in the conditioned media from bovine pituitary follicular cells which promoted the proliferation of endothelial cells and called it VEGF.[6] The subsequent cloningof these two factors demonstrated that they were the same protein.[7,8]
This molecule is also known as VEGF-A, as it belongs toa family of genes including placental growth factor (PlGF), VEGF-B, VEGF-C, VEGF-D, and VEGF-E, a viral-derived homolog.[9] VEGF-C and -D mediate lymphangiogenesis. Human VEGF-A is alternatively spliced into at least four active human isoforms, consisting of 121, 165, 189, and 206 amino acids. VEGF121 and VEGF165 are diffusible forms, although VEGF165 retains heparin-binding ability. The other longer isoforms remain bound to extracellular matrix. VEGF165 is the predominant form and possesses greater mitogenic activity. Some recent work in rodent models suggests that this isoform may be the primary mediator of pathologic neovascularization.[10] Two tyrosine kinases serve as VEGF-A receptors, VEGFR-1 (fms-like tyrosine kinase 1, flt-1) and VEGFR-2 (kinase domain region, KDR or fetal liver kinase, flk-1). VEGFR-2 appears to play the major role in signaling the mitogenic, angiogenic, and permeability effects of VEGF.[9]
VEGF AS A MEDIATOR OF OCULAR NEOVASCULARIZATION
It had long been appreciated that neovascularization of the retina and iris was temporally and spatially correlated with retinal ischemia due to various etiologies. The discovery that hypoxia could upregulate VEGF expression made VEGF a plausible candidate for the factor responsible for abnormal blood vessel growth in the eye.[11,12] Evidence from both clinical and animal studies then accumulated to support the critical role of VEGF in ocular neovascularization. VEGF expression was shown to be correlated with iris neovascularization in a primate model of ischemic retinal vein occlusion,[13] and a similar correlation was demonstrated in a neonatal mouse model of retinal neovascularization.[14] Additionally, injection of VEGF in normal primate eyes produced iris neovascularization, neovascular glaucoma, and retinal microangiopathy.[15,16] Suppression of neovascularization was also achieved by VEGF inhibition through the use of neutralizing antibodies in the primate model of iris neovascularization and chimeric proteins acting as soluble VEGF receptors in the mouse model of retinal neovascularization.[17,18]
Human clinical studies confirmed the association of VEGF expression with pathologic ocular neovascularization. Measurements of vitreous VEGF levels in patients with active proliferative diabetic retinopathy demonstrated significantly higher VEGF concentrations when compared to patients with other retinal disorders not characterized by abnormal blood vessel growth.[19] Another study which analyzed both aqueous and vitreous levels of VEGF in a variety of conditions characterized by ocular neovascularization correlated elevated ocular VEGF concentrations to the presence of active neovascularization. This study also demonstrated reduction of VEGF levels after panretinal photocoagulation for retinal neovascularization.[20]
VEGF IN CHOROIDAL NEOVASCULARIZATION
Although hypoxia is not clearly a stimulus for the development of choroidal neovascularization (CNV), VEGF does appear to function in CNV formation.
Overexpression of VEGF through gene transfer using viral vectors induces choroidal neovascularization in rat and primate models.[21,22] Implantation of microspheres containing VEGF into the subretinal space also promotes choroidal neovascularization in primates.[23] Additionally, increased VEGF expression has been demonstrated in rodent and primate models of laser-induced CNV.[24,25] Conversely, blockade of the VEGF pathway using monoclonal antibodies in the primate model of laser-induced CNV[26] and kinase inhibitors in a mouse model[27] prevents CNV formation. This type of experimental data is also supported by clinical evidence demonstrating the presence of VEGF in choroidal neovascular membranes removed from patients with neovascular AMD.[28,29]
ANTI-VEGF AGENTS
Given such weighty evidence for VEGF as a key mediator of ocular neovascularization, it became a prime therapeutic target in the search for more efficacious treatments for neovascular AMD (Table 148.1).
TABLE 148.1 -- Agents Approved (*) or in Clinical Trials for Treatment of Neovascular Macular Degeneration
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Agent |
Target |
Delivery |
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Pegaptanib* |
VEGF |
Intravitreal |
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Ranibizumab* |
VEGF |
Intravitreal |
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Bevacizumab |
VEGF |
Intravitreal |
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Bevasiranib (Cand5) |
VEGF |
Intravitreal |
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Sirna-027 |
VEGFR-1 |
Intravitreal |
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VEGF Trap |
VEGF, PIGF |
Intravitreal |
|
AdPEDF |
PEDF (gene transfer) |
Intravitreal |
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Anecortave acetate |
Proteolytic cascade leading to extracellular matrix degradation |
Juxtascleral |
|
Squalamine lactate |
Intracellular signaling |
Systemic |
PEGAPTANIB
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Key Features |
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Pegaptanib sodium (Macugen) became the first anti-VEGF agent approved by the United States Federal Drug Administration (FDA) for ophthalmic use (0.3 mg intravitreal injection) in December 2004. It is a modified 28-base RNA aptamer which binds with high affinity to the 165-amino-acid isoform of VEGF. The efficacy of pegaptanib in preventing moderatevision loss (?3 lines) from subfoveal CNV was demonstrated by the VEGF Inhibition Study in Ocular Neovascularization (VISION), two concurrent, randomized, double-masked, sham-controlled studies involving 1186 patients, which showed a treatment benefit for all subtypes and sizes (up to 12 disc areas) of CNV lesions due to AMD[30] (Figs 148.1 and 148.2). Patients were randomized to receive 0.3,1.0, or 3.0 mg pegpatanib intravitreally or sham injection once every 6 weeks. With pegaptanib (0.3 mg) treatment, 70% of patients avoided moderate vision loss at 1 year versus 55% of patients in the control group. The proportion who maintained or gained vision was 33% in the 0.3 mg pegaptanib group versus 23% of those receiving sham. However, the number of patients who gained three or more lines of vision was small: 6% in the pegaptanib (0.3 mg) group versus 2% in the control group (clinical example, Fig. 148.3).
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FIGURE 148.1 Mean change in visual acuity for all dose groups in the first year of the VISION trials of pegaptanib (Macugen) for neovascular AMD. |
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FIGURE 148.2 Mean changes in visual acuity at week 54 of the VISION trials according to baseline characteristics of a angiographic subtype, b visual acuity, and c lesion size. *, p<0.05 for pegaptanib vs sham injection; ?, p<0.001; ?, p<0.01. |
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FIGURE 148.3 Color photo and fluorescein angiogram of patient with occult choroidal neovascular membrane. Vision was 20/50. Patient received pegaptanib therapy. b OCT scans of same patient shown in a. The left scan was taken prior to treatment and reveals a pigment epithelial detachment and subretinal fluid. The right scan was taken after 4 injections of pegaptanib (6 months of therapy). The subretinal fluid had resolved and vision had improved to 20/30. |
Two-year results of this study suggested that there is a benefit to continued treatment for a second year[31] (Fig. 148.4). Patients who received pegaptanib for 2 years were less likely to lose ?15 letters than those who discontinued treatment after 1 year. Ten percent of patients treated with pegaptanib for 2 years gained ?3 lines of vision. Additionally, the proportion of patients who gained vision was higher in the group that received 2 years of treatment than those that received usual care (Fig. 148.5).
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FIGURE 148.4 Mean visual acuity from week 54 to 102 in the VISION trials. |
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FIGURE 148.5 Proportion of patients gaining vision after 2 years in the VISION trials. |
The safety profile of pegaptanib appears quite favorable.[32] The most common ocular adverse events noted during the trials included eye pain, vitreous floaters, punctate keratitis, and increased intraocular pressure. These symptoms were mainly attributed to the injection procedure rather than the drug. The serious ocular adverse events were also injection-related and consisted of endophthalmitis (1.3%/patient in year 1), retinal detachment (0.7%/patient in year 1), and traumatic cataract (0.6%/patient in year 1). The incidence of these events was similar for the second year across all cohorts. Only one of the cases of endophthalmitis in the first year resulted in severe vision loss, and none of the patients who developed endophthalmitis in the second year experienced severe vision loss. Clinical studies of systemically administered bevacizumab(a monoclonal antibody directed against all VEGF-A isoforms) in oncology patients identified hypertension, hemorrhage, and thromboembolic events as serious systemic adverse events.[33,34] There was no suggestion that intravitreal administration of pegaptanib was associated with such systemic side effects through year 2 of the VISION trials.
RANIBIZUMAB
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Key Features |
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More recently, results of Phase III trials also confirmed the efficacy of ranibizumab (Lucentis) in neovascular AMD, with rates of visual improvement not previously achieved. This drug was approved by the US FDA for the treatment of neovascular AMD in June 2006 at a dose of 0.5 mg given intravitreally. Ranibizumab is a recombinant, humanized monoclonal antibody fragment (48kDa) which binds all isoforms of the VEGF-A molecule (Fig. 148.6). The benefit of ranibizumab for patients with minimally classic and occult CNV was demonstrated in the MARINA (minimally classic/occult trial of the anti-VEGF antibody ranibizumab) study, a randomized, multicenter, double-masked, sham-controlled trial with an enrollment of 716 patients.[35] Patients in this study were randomized to monthly injections of either 0.3 or 0.5 mg ranibizumab or sham injections. After 1 year, 95% of patients who were treated with ranibizumab avoided moderate vision loss, compared to 62% of patients who were in the control group. Thirty-four percent of patients treated with ranibizumab 0.5 mg experienced visual improvement of three or more lines compared to 5% of patients receiving sham injections. The mean change in visual acuity at the end of the first year of treatment was a gain of 7.2 letters in the 0.5 mg ranibizumab group compared to a loss of 10.4 letters in the sham group. Approximately 40% of patients treated with ranibizumab achieved visual acuity of 20/40 or better at theend of the first year versus 11% of the sham-treated group(Fig. 148.7) The treatment benefit was sustained through 2 years. At month 24, 90% of treated patients (0.5 mg) avoided moderate vision loss compared to 53% in the control group. Similarly, 33% of treated patients gained three or more lines of vision after 2 years versus 4% of the control group, and the mean change in visual acuity was +6.6 letters in the treated patients versus 14.9 in the control group (Fig. 148.8).
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FIGURE 148.6 Ranibizumab (Lucentis ®) is a recombinant, humanized, monoclonal antibody fragment with high affinity for all isoforms of VEGF-A. |
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FIGURE 148.7 Mean change in visual acuity over 2 years of the MARINA trial of ranibizumab for minimally classic/occult CNV due to AMD. Vertical lines are one standard error of the means. *, p<0.001 vs sham at each month and p = 0.006 (for 0.3 mg) and 0.003 (for 0.5 mg) at day 7. |
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FIGURE 148.8 Percentage of patient achieving 20/40 or better in the MARINA trial. Vertical lines are 95% confidence intervals. ?, p<0.001 vs sham. |
The results of the ANCHOR (anti-VEGF antibody for the treatment of predominantly classic choroidal neovascularization in AMD) trial demonstrated that ranibizumab is superior to PDT for predominantly classic CNV. This study randomized 423 patients to receive verteporfin PDT and sham injection or sham PDT and ranibizumab 0.3 or 0.5 mg. The patients received ranibizumab or sham injections monthly. Verteporfin or sham PDT was administered upon study entry and could be repeated every 3 months based on evidence of leakage on angiography. The results at month 12 revealed that 96% of patients treated 0.5 mg ranibizumab avoided moderate vision loss compared to 64% of patients treated with PDT.[36] Furthermore, 40% of patients treated with ranibizumab 0.5 mg, gained three or more lines of vision versus 6% in the PDT group. The mean changes in visual acuity in the higher-dose ranibizumab group was a gain of 11.3 letters versus a mean loss of 9.5 letters in the PDT group (Fig. 148.9). Results of a subgroup analysis of this data suggested a consistent benefit of ranibizumab compared to PDT regardless of age, visual acuity, lesion size, or presence of occult CNV at baseline.[37]
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FIGURE 148.9 Mean change in visual acuity over 1 year of the ANCHOR trial of ranibizumab compared to verteporfin PDT for predominantly classic CNV due to AMD. Vertical bars represent one standard error of the mean. P<0.001 for all comparisons vs PDT at each month. |
The safety profile of ranibizumab appears similar to that of pegaptanib. The rates of serious ocular adverse events at month 24 of the MARINA study for ranibizumab 0.5 mg were endophthalmitis 1.3%, uveitis 1.3%, retinal tear 0.4%, and lens damage 0.4%.[35] Similarly, at month 12 in the ANCHOR study for the 0.5 mg ranibizumab group, the rate of endophthalmitis was 1.4% and uveitis was seen in 0.7% of patients.[36] There did not appear to be a significant increase in systemic adverse events such as hypertension or arterial thromboembolic events with ranibizumab treatment in either study.
Alternative dosing regiments for ranibizumab remain under investigation. The PIER (A Phase IIIb, multicenter, randomized, double-masked, sham injection-controlled study of the efficacy and safety of ranibizumab in subjects with subfoveal choroidal neovascularization with or without classic CNV secondary to AMD) study was designed to examine the effect of a modified dosing regimen for ranibizumab consisting of three initial injections at monthly intervals followed by mandated doses at three month intervals.[38] A mean gain in visual acuity was seen in the treatment groups at month 3 (+4.3 letters in 0.5 mg group vs 8.7 in sham group), but was not sustained through month 12. One-year results revealed that the mean change in visual acuity was a loss of 1.6 and 0.2 letters in patients treated with 0.3-, and -0.5 mg of ranibizumab respectively, comparedto a loss of 16.3 letters in the control group (Fig. 148.10). More patients treated with ranibizumab avoided moderate vision loss (90% in 0.5 mg group versus 49% in control group), confirming the benefit of ranibizumab treatment. However, fewer patients gained 3 or more lines of vision (13% in 0.5 mg group vs 10% in sham group) as compared to the pivotal Phase III trials MARINA and ANCHOR. These results suggest that quarterly dosing may not be as effective as monthly injections. The PIER study remained ongoing at the time of this writing.
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FIGURE 148.10 Mean change in visual acuity over 1 year of the PIER trial of ranibizumab for neovascular AMD which involved quarterly dosing. |
The Phase I/II, open-label, single-center PrONTO (Prospective Optical Coherence Tomography Imaging Patients with Neovascular AMD Treated with Intraocular Lucentis) Study conducted at the Bascom Palmer Eye Institute was also ongoing at the time of this writing and involved a modified dosing protocol using three initial injections of 0.5 mg ranbizumab at monthly intervals followed by additional doses as indicated.[39] Strict criteria for additional dosing in this study were predetermined and included visual acuity, ocular coherence tomography (OCT), and fluorescein angiography findings. At 1 year, patients in this study gained an average of 9.3 letters. The mean number of injections was 5.5, although there was significant variability among patients, and the mean time to first retreatment was 4.3 months. Although there is no control group for comparison, these findings suggest that individually tailored dosing may be similarly efficacious to monthly dosing for ranibizumab.
BEVACIZUMAB
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Key Features |
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Bevacizumab (Avastin) became the first anti-VEGF drug licensed for clinical use when it was approved for the treatment of metastatic colorectal cancer in February 2004. It is a humanized, full-length monoclonal antibody (149 ?Da) which binds all isoforms of VEGF-A and was developed for intravenous administration. Primate studies of retinal penetration after intravitreal injection using radiolabeled antibody preparations demonstrated that full-length antibody failed to penetrate the internal limiting membrane while an antibody fragment diffused through all layers of the retina[40] (Fig. 148.11). Such data indicating a molecular weight limitation for retinal penetration as well as concerns regarding potential cytotoxicity related to the Fc portion of antibodies provided the rationale for the development of ranibizumab for neovascular AMD. However, in light of positive data from clinical trials of pegaptanib and ranibizumab for neovascular AMD, the off-label use of bevacizumab for this condition was initiated by clinicians prior to the approval of ranibizumab.
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FIGURE 148.11 Microautoradiographic images of the distribution of a 125I-rhuMab HER2 (full-length antibody against HER2) and b 125I-rhuMab VEGF Fab (antibody fragment against VEGF) after intravitreal injection in rhesus monkey. In a, the full-length antibody remains at the level of the inner limiting membrane. In b, the antibody fragment penetrates through the entire neural retina to the level of the RPE. |
An early open-label, single-center study of systemically administered bevacizumab in nine patients with neovascular AMD suggested visual benefit with mean increase in vision at 12 weeks as well as a mean decrease in central retinal thickness by OCT.[41] There was also some improvement in these measures noted in the fellow eyes of the study patients. A mild increase in systolic blood pressure that was felt to be drug-related was observed at 6 weeks.
Due to concerns regarding potential systemic toxicities of intravenously administered bevacizumab, intravitreal use was adopted. Several short term reports of intravitreal bevacizumab for the treatment of patients with neovascular AMD suggest a beneficial effect on visual acuity and retinal thickness.[42-45] The largest of these series was a retrospective study of 266 eyes of 266 patients, the majority (69.7%) of whom were considered to have had an inadequate response to other treatments.[44] These patients were treated with three injections of 1.25 mg bevacizumab at monthly intervals. Three-month follow-updata was available for 141 eyes and revealed visual acuity improvement (halving of the visual angle) in 38.3% of patients as well as progressive decrease in mean central macular thickness measurements by OCT (340 ?m baseline to 213 ?m at month 3).
No apparent safety concerns have been detected in these studies with intravitreal doses up to 2.5 mg. Additionally, histologic and electrophysiologic studies in rabbits have shown no evidence of toxicity after intravitreal bevacizumab in doses upto 2.5 mg.[46,47] However, longer-term, prospective, randomized studies are necessary to fully evaluate the safety and efficacyof bevacizumab as a treatment for neovascular AMD.
OTHER ANTI-VEGF AGENTS
RNA Interference
A powerful new technology for inhibiting the function of selected genes known as RNA inference (RNAi) promises anew generation of anti-VEGF drugs. This strategy exploits a cellular phenomenon in which a small, double-stranded RNA molecule (short interfering RNA, siRNA) associates with a RNA-protein complex called RISC (RNA-induced silencing complex), resulting in selective degradation of an mRNA transcript containing a sequence homologous to the siRNA[49] (Fig. 148.12). Synthesis of siRNA molecules tailored to silence expression of chosen genes is possible and offers the potential for a new class of therapeutic agents. Various companies have developed siRNAs targeting VEGF or its receptors and shown efficacy in preclinical models.
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FIGURE 148.12 In the process of RNA interference, short interfering RNAs (siRNAs) associate with RISC (RNA-induced silencing complex) to trigger selective degradation of particular RNA transcripts. |
Bevasiranib sodium (formerly Cand5) is an siRNA directed against VEGF developed by Acuity Pharmaceuticals. The Phase I clinical trial in patients with neovascular AMD indicated that the drug was safe and well tolerated when delivered intravitreally at doses up to 3.0 mg (Prenner JL, Thompson JT, Miller DG, et al: An open label study for the evaluation and tolerability of five dose levels of intravitreous VEGF siRNA (Cand5) in patients with wet age-related macular degeneration. Paper presented at: American Academy of Ophthalmology Annual Meeting, Chicago, IL, 14-18 Oct 2005.). The Phase II CARE Study (Cand5 anti-VEGF RNAi Evaluation) tested three doses of bevasiranib in 129 patients. Preliminary results suggested safety as well as efficacy at all doses.[49] Phase III trials are expected to begin in 2007.
Sirna-027 is another siRNA molecule which targets vascular endothelial growth factor receptor-1 (VEGFR-1) developed by Sirna Therapeutics. The Phase I dose escalation trial involved 26 patients with neovascular AMD. At 8 weeks after a single injection, 96% of these patients showed stabilization of vision with 23% experiencing visual improvement of three or more lines.[50] Decreases in central retinal thickness measured by OCT were also observed. Phase II studies are anticipated.
VEGF Trap
Another novel molecule designed to block VEGF-mediated angiogenesis is VEGF Trap (Regeneron Pharmaceuticals), which contains immunoglobulin domains of both VEGFR-1 and VEGFR-2 fused to the constant region of human IgG.[51] It functions as a high-affinity soluble receptor that binds and neutralizes both VEGF and PlGF. The Phase I study involved doses ranging from 0.05 to 4 mg which were administered as a single intravitreal injection to 21 patients with neovascular AMD. At 6 weeks, 95% of the patients had stabilization of vision (loss of ?15 letters).[52] The mean change in visual acuity for all patients was a gain of 4.8 letters at 6 weeks. In the highest dose groups (2 and 4 mg), the mean improvement in visual acuity was 13.5 letters. Again, the visual acuity results were accompanied by decreases in OCT measurements of central retinal thickness. A Phase II study is in progress.
OTHER ANTIANGIOGENIC AGENTS
ANECORTAVE ACETATE
Given the complexity of the angiogenic process, drugs which affect an array of factors rather than one molecule may be desirable. The degradation of capillary basement membraneand extracellular matrix is an essential step in the migrationof endothelial cells during neovacularization and occurs through the action of proteases such as urokinase-type plasminogen activator (uPA) and matrix metalloproteinases.[53] The angiostatic cortisene anecortave acetate (Retaane, Alcon, Inc.) is a cortisol derivative which has been chemically modified to eliminate all glucocorticoid activity, thereby eliminating ocular side effects such as glaucoma and cataracts. It has been shown to upregulate the expression of plasminogen activator inhibitor-1 (PAI-1), thereby supressing the action of uPA, which is necessary for the activation of various other proteases.[54] By inhibition of this proteolytic cascade which is downstream of endothelial cell stimulation, anecortave acetate may potentially block angiogenesis initiated by a variety of signals.
In a randomized trial, three different doses (3, 15, 30 mg) of anecortave acetate administered as a posterior juxtascleral injection at 6 month intervals were compared to placebo in 128 patients with subfoveal choroidal neovascularization due to AMD.[55] The primary outcome of change in logMAR visual acuity at 12 months showed a statistically significant difference favoring the 15 mg dose of anecortave acetate compared to placebo. The mean change in the anecortave group was a loss of 1.3 lines versus 3.1 lines for the placebo group (Fig. 148.13). Stabilization of vision (loss of <3 logMAR lines) was seen in 79% of the anecortave (15 mg) treated patients compared to 53% of those receiving placebo. A majority of patients in this study had predominantly classic lesions (78.5% in the anecortave acetate 15 mg group and 86.7% of the placebo group), and the treatment benefit appeared slightly more pronounced when this subgroup of patients was analyzed separately. The difference in visual acuity was 2.4 logMAR lines in favor of anecortave acetate 15 mg versus placebo, and 84% of anecortave-treated patients had stabilization of vision versus 50% of those in the placebo group. The results appeared sustained through the 24 month follow-up with 73% of anecortave-treated patients maintaining vision (<3 lines lost) versus 47% of the placebo group (Kaiser PK, Schaffer HI, Zilliox P, et al: Posterior juxtascleral depot of anecortave acetate for subfoveal CNV in AMD: 24-month outcomes. Paper presented at: American Academy of Ophthalmology Annual Meeting, Anaheim, CA, 17 Nov 2003). No ocular or systemic safety issues were idenitified. However, concerns regarding the results of this trial included the decrease in apparent efficacy with the highest studied dose and the loss of patients between the 6 and 12 month time points. Overall, 41% of patients exited the study between 6 and 12 months, with the highest proportion of patient dropout occurring in the 15 mg group (48.5%).
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FIGURE 148.13 Average change in logMAR visual acuity through month 12 of patients with subfoveal neovascularization due to AMD treated with anecortave acetate vs placebo. b Proportion of patients losing <3 lines of logMAR vision. |
The Phase III clinical trial of anecortave acetate for neovascular AMD compared the 15 mg dose to verteporfin photodynamic therapy in 530 patients with predominantly classic subfoveal choroidal neovascualrization.[56] There was no statistical difference between the percentage of patients losing <3 lines of vision (responders) in the anecortave acetateand PDT groups (45% and 49%, respectively, p = 0.43)(Fig. 148.14). The confidence interval, however, ranged from -13.2% favoring PDT to +5.4% favoring anecortave acetate, and thus the results failed to meet the previously established noninferiority margin of seven percentage points based on the results of the TAP (Treatment of Age-Related Macular Degeneration with Photodynamic Therapy) Study. However, an analysis of the subgroup of patients for whom drug refluxat the time of administration was controlled and who received the second injection within the 6 month treatment window revealed improved outcomes which would have met the noninferiority criterion.
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FIGURE 148.14 Per protocol analysis of percentage of patients losing <15 letters in study of anecortave acetate vs verteporfin PDT in patients with predominantly classic CNV due to AMD (95% confidence limits for difference between PDT and anecortave on right). |
Ongoing clinical trials of anecortave acetate include a study investigating three dosage regimens (15 and 30 mg every 6 months, 15 mg every 3 months) and the Anecortave Acetate Risk Reduction Trial (AART), which will study the efficacy of anecortave acetate 15 mg in preventing wet AMD in patients with high-risk dry AMD over a 4 year period.
SQUALAMINE LACTATE
Another molecule with a unique mechanism of antiangiogenic activity is squalamine lactate (Evizon, Genaera Corp.) an aminosterol which potently inhibits growth factor-induced proliferation and migration of endothelial cells.[57] Squalamine has been shown to cause redistribution of calmodulin within endothelial cells resulting in disruption of various intracellular signaling pathways involving calcium.[58] In addition to blocking growth factor mediated signaling, squalamine may also downregulate integrin expression and induce cytoskeletal rearrangements in endothelial cells[59] (Fig. 148.15). Preclinical studies have suggested the efficacy of squalamine in inhibiting ocular neovascularization in rodent and primate models.[60,61] Several Phase II studies evaluating intravenous squalamine treatment for wet AMD are ongoing. The largest Phase II trial is a randomized, multi-center, double-masked, controlled study involving 108 patients, almost half of whom had active occult lesions.[62] Two different doses of squalamine (40 and 20 mg) are given as weekly intravenous infusions for the first 4 weeks and monthly thereafter. PDT is permitted at the discretion of the investigator. Preliminary results at 24 weeks revealed that 83% of the patients receiving the 40 mg dose lost <15 letters of visual acuity compared to 71% of the control group. Five percent of patients in the 40 mg group and none in the control group gained 15 or more letters. In those subjects with a fellow-affected eye (?60%), 11% of the 40 mg group gained ?15 letters in the fellow eye versus none in the control group. Infusion site reactions were the most common treatment-related adverse events, and no ocular or systemic safety concerns havearisen. A Phase III trial is ongoing, and another Phase II trial investigating higher doses is planned.
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FIGURE 148.15 Mechanism of action of squalamine lactate. |
PIGMENT EPITHELIUM-DERIVED FACTOR
Of the several endogenous inhibitors of angiogenesis thathave been found in ocular tissues, pigment epithelium derived factor (PEDF) is among the most potent inhibitors of endothelial cell migration in vitro.[63] The downregulation of PEDF has been demonstrated in a mouse model of ischemic retinopathy,[63] and patients with proliferative retinopathy from diabetes or retinal vein occlusion have been shown to have decreased vitreous levels of PEDF.[64] Since PEDF also appears to serve a neurotrophic function in the retina,[65] therapeutic use of this growth factor may have additional beneficial effects on photoreceptor survival in AMD. Additionally, some preclinical evidence suggests that PEDF could enhance the effect of verteporfin photodynamic therapy by increasing endothelialcell apoptosis within choroidal neovascular membranes.[66] The efficacy of PEDF gene transfer using an adenoviral vector in inhibiting ocular neovascularization has been demonstrated in animal models.[67-69] Therefore, a Phase I clinical trial of PEDF gene transfer therapy using intravitreal injection of an adenoviral vector expressing human PEDF (AdPEDF.11) was initiated for patients with advanced wet AMD.[70] The initial dose escalation phase has been completed with no evidence of serious adverse events and no dose-limiting toxicities in 28 subjects. Twenty-five percent of patients experienced mild, transient ocular inflammation and 21% had increases in intraocular pressure which responded to topical therapy. Additional treatment of patients with less advanced neovascular AMD is planned.
COMBINATION THERAPY
With the availability of multiple modalities for the treatment of wet AMD, the use of two or more of these agents in combination is now possible. With goals of improved visual outcomes and a reduction in frequency of treatments, several strategies for combination therapy have been and continue to be explored.
Increasing evidence supports inflammation as a key component in AMD pathogenesis, providing a rationale forthe use of intravitreal triamcinolone acetonide in the treatment of neovascular AMD. The inhibitory effect of steroids on macrophages which secrete angiogenic factors and their ability to reduce leukocyte adhesion and migration might have beneficial effects in AMD.[71] However, the use of intravitreal triamcinolone as monotherapy for AMD has proved suboptimal. In a randomized trial of a single 4 mg dose of intravitreal triamcinolone acetonide in 151 eyes of 139 patients with neovascular AMD, eyes receiving placebo were noted to have significantly more growth of the neovascular membrane compared to those receiving triamcinolone at 3 months after treatment.[72] However, no difference was noted at 12 months, and there was no effect of intra-vitreal triamcinolone on visual acuity loss at 12 months. Another prospective, nonrandomized study using 25 mg of intravitreal triamcinolone in 115 patients and 72 controls with exudative AMD demonstrated a visual acuity benefit at 1 and 3 months follow-up, but not at subsequent follow-ups through a mean of 6 months.[73] These findings reveal that the effect of an intravitreally delivered steroid when used as monotherapy is transient. However, the use of intravitreal steroids in combination with photodynamic therapy might have more sustained benefits.
The combination of photodynamic therapy and intravitreal triamcinolone has shown promising results in early studies. A small pilot study investigated verteporfin PDT immediately followed by 4 mg of intravitreal triamcinolone in 26 patients with neovascular AMD.[74] One year results demonstrated a mean visual acuity improvement of 2.4 lines in patients who had not received any prior treatment, and an improvement of 0.44 lines in those patients who had been previously treated with PDT. Retreatment rates were also low, at about 1.2 for both groups. Ten patients (38.5%) developed an increase in intraocular pressure to greater than >24 mm Hg. A larger, prospective, noncomparative case series of 184 patients with neovascular AMD, the majority with subfoveal CNV, evaluated the results of photodynamic therapy followed by intravitreal injection of 25 mg triamcinolone within 16 h.[75] Over the mean follow-up of 43 weeks, the mean increase in vision was 1.22 Snellen lines and the mean number of required treatments was 1.21. Topical glaucoma therapy was required in 25% of patients and 1% required glaucoma surgery. Cataract progression was observed in 48.73% of phakic eyes. Randomized trials studying various doses and preparations of triamcinolone acetonide in combination with verteporfin photodynamic therapy are ongoing.
Evidence demonstrating increased VEGF expression after PDT provides one explanation for improved results with PDT combined with triamcinolone compared to PDT alone and suggests that combining PDT with anti-VEGF agents should have similar or superior visual results without steroid-related side effects. Trials examining the use of anti-VEGF agents in combination with verteporfin PDT are ongoing. The FOCUS (RhuFab V2 Ocular Treatment Combining the Use of Visudyne to Examine Safety) study is a Phase I/II trial investigatingthe combination of ranibizumab and PDT versus PDT alonefor predominantly classic CNV[76]. One year results revealed that 90% of patients treated with the combination therapy avoided moderate vision loss compared to 68% of patients treated with PDT alone. The proportion of patients gaining 15 or more letters was 24% in the combination group compared to 5% in the PDT-alone group. At month 12, a mean of 0.3 additional PDT treatments was required in the combination group versus a mean of 2.4 additional treatments in those receiving PDT alone.
A lyophilized preparation of ranibizumab was used for this study and initially given at day 7 after PDT. When a higher incidence of uveitis was detected in the combination group, the timing of the intra-vitreal injection was moved to 30 days after PDT. A second Phase II open-label study using the currently approved preparation of ranibizumab 0.5 mg administered the same day as verteprofin PDT showed no safety concerns at the 3 month time point.[77] Given the proven benefit of ranibizumab monotherapy versus PDT for predominantly classic CNV, it possible that the favorable results of combination therapy with ranibizumab and PDT were mainly due to ranibizumab. Additional trials investigating pegaptanib in combination with PDT are ongoing.
A Phase II trial of three doses of squalamine (10, 20, 40 mg) combined with verteporfin PDT compared to PDT alone in45 subjects also suggested an advantage to combination therapy. At 29 weeks, 90% of patients in the 40 mg + PDT group had stable vision (loss of ?15 letters) compared to 71% receiving PDT alone. The mean change in visual acuity was a gain of 0.4 letters in the 40 mg combination group versus a loss of 4.8 letters in those treated with PDT only.[78]
FUTURE DIRECTIONS
NEW TARGETS
In addition to targeting VEGF mRNA or protein, the angiogenic stimulus provided by VEGF may be blocked by interferingwith the signaling events induced by activated VEGF receptors (Fig. 148.16). Various protein kinase C (PKC) isoenzymes are activated by VEGFR binding and are important mediators inthe resulting angiogenic and permeability responses.[79] PKC inhibition has been shown to reduce retinal and CNV in both rodent and porcine models.[80,81] A PKC beta-inhibitor ruboxistaurin mesylate (Eli Lilly and Co.) has been shown to reduce sustained moderate vision loss in patients with moderate to severe nonproliferative diabetic retinopathy.[82] These agents may also prove to be beneficial in neovascular AMD.
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FIGURE 148.16 Schematic showing process of angiogenesis which involves extracellular matrix degradation, endothelial cell proliferation and migration, tube formation, elongation, and remodeling. Numerous mediators are involved in this cascade providing various targets for pharmacotherapy. |
Similarly, VEGFRs belong to the family of tyrosine kinases and inhibition of this enzymatic function disrupts the downstream signaling cascade. Numerous pharmacologic inhibitors of tyrosine kinases have been synthesized and this class ofdrugs has been used with great success in oncology as aberrant tyrosine kinase activity appears to be pathogenic in certain malignancies.[83,84] Preclinical data suggest the efficacy of tyrosine kinase inhibitors in models of ocular neovascularization.[85] Vatalanib (PTK787, Novartis Pharmaceuticals) is an orally administered tyrosine kinase inhibitor with activity against all three VEGF receptors as well as PDGFR-b, andc-kit.[86] A Phase I/II trial of vatalanib in combination with verteporfin PDT in patients with subfoveal choroidal neovascularization due to AMD is underway.
Although VEGF certainly plays a causal role in ocular neovascularization, other molecules also modulate the angiogenic stimulus in the eye. The Tie (Tyrosine kinase with Immunoglobulin and Epidermal growth factor homology) receptors are also tyrosine kinase receptors expressed on endothelial cells which play a role in vascular development.[87] Their ligandsare the angiopoietins, of which there are at least four.[57] The interaction of Tie-2 and angiopoietin-1 (Ang-1) is thought to induce maturation of vessels by promoting the association of pericytes with endothelial cells. Ang-2 serves as an antagonist, destabilizing vessels and allowing endothelial cells to respond to angiogenic stimuli.[88] Increased expression of Ang-2 has been demonstrated in mouse models of retinal neovascularization[89] and blockade of Tie-2 can inhibit experimental retinal neovascularization.[90] These molecules may serve as future targets in the development of antiangiogenic drugs.
Another factor important for vessel maturation is platelet-derived growth factor-B (PDGF-B), which has been shown to play a critical role in the recruitment of mural cells to endothelial cells.[91] Preclinical studies in several models of ocular angiogenesis have demonstrated that inhibition of PDGF-B in addition to VEGF-A appears to be more efficacious at preventing and regression ocular neovascularization than inhibition of VEGF-A alone[92] (Fig. 148.17). These data suggest that pericyte loss induced by PDGF-B blockade destabilizes neovasculature and increases susceptibility to anti-VEGF treatment. Such combination therapy may lead to improved visual outcomes for a broader spectrum of patients.
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FIGURE 148.17 Results of combination therapy with anti-VEGF aptamer (pegaptanib) and anti-PDGFR-? antibody (APB5) in mouse model of laser-induced CNV. Mice were treated starting at day 7 post-injury. The combination of VEGF and PDGF pathway inhibition resulted in the smallest area of CNV. |
DRUG DELIVERY
The challenge of developing new pharmacologic agents forthe treatment of AMD lies not only in identifying efficacious strategies but also in creating improved delivery methods. Repeated intra-vitreal injections present a disadvantage in terms of risk of endophthalmitis and high frequency oftreatments. Systemic delivery of agents allows for bilateral treatment but also substantially increases the potential for nonocular toxicities. The juxtascleral administration of anecortave acetate given at 6 month intervals offers an advantage with regard to safety and convenience, and thereby justifies its potential use in the prevention as well as treatment of advanced AMD (Fig. 148.18).
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FIGURE 148.18 The posterior juxtascleral injection procedure used for delivery of anaecortave acetate. |
The development of sustained-release delivery platforms for intra vitreal or transcleral administration will provide an increased level of safety and perhaps also efficacy as long-term, continuous levels of therapeutic agents will be achieved. A fluocinolone acetonide implant (Retisert, Bausch and Lomb, Inc.) is currently available for the treatment of chronic, noninfectious posterior uveitis,[93] and injectable forms of both fluocinolone and dexamethasone intravitreal implants are currently being investigated in clinical trials. The transcleral delivery of pegaptanib via poly(lactic-co-glycolic)acid (PLGA) microspheres has been studied in a rabbit model.[94] Biomechanical devicesfor transscleral delivery are also under development. These types of delivery methods may also allow for multiple pharmacologic agents to be delivered simultaneously, easily enabling combination therapy.
The future of pharmacologic therapy for AMD appears promising. An array of drugs is in development and in ongoing clinical trials. Our therapeutic approach will likely become multifaceted as treatment options increase, and combinations of various agents lead to offer improved visual outcomes. The identification and treatment of patients at highest risk for vision loss early in the disease process is a goal that may be realized soon. Continued progress in the understanding and management of macular degeneration will one day reduce the blinding impact of this disease.
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