pathways simultaneously, and may prove more efficacious than monotherapy. Further, anticipated improvements in drug delivery will likely decrease treatment burden. The effect on the cost of treatment remains to be seen – a decrease in treatment burden could reduce the cost of AMD treatment, but the development of new agents, and their use either as monotherapy, or more likely, in combination with other agents, could result in increased costs of therapy.

Emerging and Future Therapies

Anti-vascular Endothelial Growth

Factor Agents

VEGF plays a key role in normal and pathologic angiogenesis. The VEGF family consists of VEGF A through F and placental growth factor

(PlGF), which share structural domains but have different biological properties [1]. VEGF has multiple isoforms formed through alternative splicing and named based on the number of amino acids [1].

Multiple pharmacologic agents target VEGF along various pathways in an effort to inhibit CNV (see Fig. 9.1). Pegaptanib, ranibizumab, bevacizumab, and VEGF Trap-Eye bind and inhibit VEGF, preventing it from activating VEGF receptor. VEGF receptor tyrosine kinase inhibitors prevent transduction of the VEGFbinding signal. Small interfering RNA molecules, or siRNAs, silence mRNA to prevent the translation of VEGF (bevasiranib) or VEGF receptor-1 (Sirna-027).

Ranibizumab

Ranibizumab (Lucentis®, Genentech, Inc., South San Francisco, CA, USA) is a recombinant,

Fig. 9.1 Mechanisms of inhibition of vascular endothelial growth factor-A (VEGFR). Pegaptanib, ranibizumab, bevacizumab and VEGFR-Trap all bind VEGFR, inhibiting it from binding and activating VEGFR receptor. Bevasiranib is a small interfering RNA (siRNA) that

prevents translation of VEGFR. Likewise, Sirna-027 is a siRNA that prevents the translation of VEGFR receptor. VEGFR receptor tyrosine kinase inhibitors, like Vatalanib, prevent transduction of the VEGFR binding signal (Vascular Endothelial Growth Factor Receptor).

humanized, monoclonal antibody Fab fragment against VEGF-A, capable of binding and inhibiting all isoforms. Ranibizumab’s VEGF binding site was affinity-matured, allowing the drug to be five–to twenty-fold more potent in binding VEGF-A than bevacizumab on a molar basis [2].

Intravitreal ranibizumab revolutionized the treatment paradigm of neovascular AMD by actually improving visual acuity on average, as demonstrated in two phase III studies. The MARINA (Minimally Classic/Occult Trial of the Anti-VEGF Antibody Ranibizumab in the Treatment of Neovascular AMD) Study Group evaluated 716 patients with minimally classic or occult neovascular AMD. The study reported a mean gain of 6.6 letters at two years in the group receiving 0.5 mg of ranibizumab monthly, compared to a mean loss of 14.9 letters in the group receiving sham injections [3]. Likewise, ANCHOR (Anti-VEGF Antibody for the Treatment of Predominantly Classic CNV in AMD) evaluated 423 patients with predominantly classic neovascular AMD. The trial demonstrated a mean gain of 10.7 letters in the 0.5 mg ranibizumab group at two years compared to a mean loss of 9.8 letters in the verteporfin photodynamic therapy group [4, 5]. Please refer to Chapter 6 for more details regarding ranibizumab.

Though ranibizumab represented a major breakthrough for neovascular AMD when compared to all previous therapies – including the first approved anti-VEGF medication, pegaptanib – the success of regular intravitreal injections is coupled with several disadvantages. These include the burden of monthly office visits of patients and their caregivers, the direct and indirect costs of treatment, and the risks and discomfort associated with frequent intravitreal injections. Anti-VEGF therapy will likely continue to be part of the neovascular AMD arsenal

Pearl

Anti-VEGF agents will likely continue to be part of the neovascular AMD arsenal in the future, if it is not replaced by more potent, longer acting, and/or less expensive alternative.

in the future, if it is not replaced by more potent, longer acting, and/or less expensive alternatives.

Bevacizumab

Bevacizumab (Avastin®, Genentech, Inc., South San Francisco, CA, USA) is a full-length antiVEGF monoclonal antibody that binds and inhibits all VEGF-A isoforms. Intravitreally, the half-life of bevacizumab is roughly twice that of ranibizumab, thought due to bevacizumab’s greater size (149 kDa vs. 48 kDa) and presence of the Fc portion.

The Comparison of Age-Related Macular Degeneration Treatments Trials [6] (CATT) is a phase III trial assessing the efficacy of bevacizumab versus ranibizumab. The trial will also evaluate an as-needed dosing regimen. If noninferior to ranibizumab, bevacizumab would be a proven and far cheaper anti-VEGF agent in our armamentarium against neovascular AMD. Please refer to Chapter 6 for more details regarding bevacizumab.

VEGF Trap-Eye

Aflibercept (VEGF Trap-Eye or Eyelea Regeneron, Terrytown, NY, USA, and Bayer HealthCare, Leverkusen, Germany) is an antagonist that binds and inactivates VEGF. It is comprised of portions of the extracellular domains of two different VEGF receptors (VEGFR). Specifically, it is a 110 kDa recombinant protein with binding portions of VEGFR-1 and VEGFR-2 fused to the Fc region of human IgG. VEGF Trap-Eye has a much higher affinity for VEGF than humanized monoclonal antibodies, about 140 times that of ranibizumab, and thus has the potential for greater efficacy and duration. Furthermore, VEGF Trap-Eye binds all isoforms of VEGF-A, as well as placental growth factor 1 and 2 (PlGF1 and PlGF2), which may be helpful in treating CNV [7].

Intravenous VEGF Trap was evaluated in a phase I study (CLEAR) [8]. It demonstrated a dose-dependent improvement in retinal thickness, as well as a dose-dependent increase in systemic blood pressure with a maximum tolerated dose of 1 mg/kg. Subsequently, efforts were directed towards intravitreal drug delivery. A phase I study (CLEAR IT-1) [9] evaluated

intravitreal VEGF Trap-Eye, finding injections up to 4 mg were well tolerated. A phase II trial (CLEAR IT-2) [10] randomized 157 patients to various doses on fixed monthly or quarterly regimens for the first 12 weeks, followed by asneeded dosing for the following 40 weeks. At one year, patients receiving four monthly injections of 2 mg VEGF Trap-Eye followed by asneeded dosing improved an average of nine letters from baseline (p < 0.0001), correlating with a decrease in retinal thickness of 143 mm (p < 0.0001). During the as-needed phase, patients required an average of only 1.6 additional VEGF Trap-Eye injections. The results for the 2 mg group were more favorable than those for the 0.5 mg group. Patients initially receiving four monthly doses boasted better visual acuity gains than those receiving quarterly injections, suggesting the value of a loading dose. Overall, the medication was well tolerated without any serious adverse events.

These promising results prompted two large phase III trials. VIEW 1 [11], conducted in the United States and Canada, and VIEW 2 [12], conducted in Europe, Asia, Japan, Australia, and South America, have each enrolled over 1,200 patients. The trials compare 2 and 0.5 mg of VEGF Trap-Eye dosed monthly, as well as 2 mg dose every other month following a loading dose, with ranibizumab. Results are expected in late 2010 or early 2011. See Chapter 6 for preliminary one year results.

Bevasiranib

Small interfering RNA (siRNA) molecules work upstream from VEGF. They inactivate specific messenger RNAs and thus inhibit translation of particular proteins, such as VEGF. Clinically, siRNA drugs are delivered as double stranded RNA molecules that are transported across cellular membrane. An enzyme, Dicer, shortens the siRNA to 21–24 nucleotides, which is then incorporated into a RNA-induced silencing complex (RISC). When activated, RISC binds and digests complementary mRNA, allowing a single molecule of siRNA to degrade multiple copies of mRNA.

Bevasiranib [13] (formerly Cand5, OPKO Health, Inc., Miami, FL, USA) was the first siRNA

directed towards intraocular VEGF production. It has no effect on existing VEGF in the eye, and thus may have synergy when used in combination with conventional anti-VEGF agents to impact the disease in the short term. The phase II Cand5 Anti-VEGF RNA Evaluation (CARE) study evaluated three doses of intravitreal bevasiranib (0.2, 1.5, and 3.0 mg) administered every six weeks (n =127 eyes). At 12 weeks, all groups lost vision, with a mean letter loss of 4.1, 6.9, and 5.8 in each group, respectively [14, 15]. Given these lackluster results compared to the visual improvement experienced with anti-VEGF antibody treatment, researchers hypothesized that bevasiranib may have a delayed benefit because it works upstream from existing VEGF. Monotherapy with bevasiranib was abandoned. Subsequently, a phase III trial evaluated the combination of 2.5 mg bevasiranib and ranibizumab. The COBALT trail (Combining Bevasiranib And Lucentis Therapy) completed enrollment, but was terminated in March 2009 because the Independent Data Monitoring Committee felt the combination was unlikely to reduce vision loss [16, 17].

Bevasiranib may be considered as part of the neovascular AMD treatment regimen in the future, but with a different approach. Possibilities include different dosing protocols, other combinations with complementary medications, and enhanced drug delivery vehicles [17]. The CARBON trial is underway, a phase III trial evaluating three doses of bevasiranib (1.0, 2.0, and 2.5 mg) used in combination with ranibizumab [18].

VEGF Receptor, Platelet-Derived

Growth Factor, and PDGR Receptor

Inhibition

VEGF must bind to VEGF receptor (VEGFR) to perform its biologic functions. The VEGFR family consists of protein-tyrosine kinases (VEGFR-1, VEGFR-2, and VEGFR-3) and two nonprotein kinase coreceptors (neuropilin-1 and neuropilin-2) [19]. VEGFR-2 mediates almost all of VEGF’s cellular responses, allowing transduction of the VEGF-binding signal. The role of VEGFR-1

is less clear; it may be a decoy receptor, evolved to trap free VEGF-A and prevent VEGFR-2 activation [20].

Platelet-derived growth factor (PDGF) stimulates angiogenesis, as well as pericyte recruitment and maturation [21]. Pericytes play an important role in angiogenesis because they protect endothelial cells against VEGF inhibition [22]. Thus, inhibition of PDGF increases endothelial cell sensitivity to anti-VEGF agents [22]. PDGF binds to PDGF receptor (PDGFR), which is found in two isoforms: PDGFRa and PDGFRb(beta). Inhibition of PDGFR, as well as VEGFR, may be a mechanism to treat CNV in AMD.

Pearl

Platelet-derived growth factor (PDGF) stimulates angiogenesis, as well as pericyte recruitment and maturation. Blockade of PDGF or its receptors can decrease pericyte density in neovascular vessels and improve the effect of anti-VEGF agents. Lesion regression may be possible.

Vatalanib

Vatalanib (formerly PTK-787, Novartis, Basel Switzerland) is a potent tyrosine kinase inhibitor that binds and inhibits all known VEGF receptor tyrosine kinases (VEGFR-1, VEGFR-2, and VEGFR-3), PDGFRb(beta), and c-Kit receptor kinases. C-kit is a proto-oncogene that is a member of the receptor tyrosine kinase family; it is closely related to PDGFR. Preclinical models suggest vatalanib causes dose-dependent regression of VEGF-induced angiogenesis [23]. Vatalanib boasts good oral availability. Oral vatalanib was evaluated in a phase I/II clinical trial in combination with photodynamic therapy (ADVANCE study [24]). This study of 50 patients was completed in November 2007, but not published. Vatalanib demonstrated some biologic activity, but had safety issues. It may reemerge if revised.

TG100801

TG100801 (TargeGen, San Diego, CA), another tyrosine kinase inhibitor, is a prodrug of TG100572, which binds and inhibits VEGFR and PDGFR [25]. In a murine model, systemic delivery of TG100572 induced regression of CNV, but with associated weight loss suggestive of systemic toxicity. Thus, researchers developed a topical inactive prodrug, TG100801, achieving good retinal and choroidal concentrations of TG100572 without detection in plasma [25]. A phase I trial demonstrated safety and tolerability of two concentrations in 42 patients twice a day for 14 days [26, 27]. Unfortunately, toxicity led to discontinuation of the trial although it demonstrated biologic activity [28]. Patients receiving topical TG100801 developed corneal toxicity with deposits in all layers. The corneal changes remained largely irreversible. If refined, the topical route of administration would be a very favorable alternative or adjunct to intravitreal injections.

Pazopanib

Pazopanib (GW786034, GlaxoSmithKline, Middlesex, UK) is a second-generation multitargeted tyrosine kinase inhibitor against all VEGFRs, both PDGFRa and PDGRFb, and c-kit [29]. Pazopanib, administered topically, was evaluated in a phase II trial [30], which was extended with results currently pending [31].

Sirna-027

Sirna-027 (Sirna Therapeutics, San Francisco, CA, USA) is a small interfering RNA (siRNA) structured to silence the gene for VEGFR-1 [32]. A phase I trial of 26 patients exhibited safety of doses ranging from 100 to 1,600 mg. Each patient received one intravitreal injection, with 24 of 26 showingvisualacuitystabilizationatthreemonths. Four of 26 (15%) experienced clinically significant improvement in visual acuity [33]. A phase II trial, however, was terminated because it failed to show visual improvement [34]. The trial contained four arms, three of which evaluated different doses of monthly Sirna-027 versus the control arm of monthly ranibizumab [35]. Sirna-027 may reemerge in the future as part of a combination protocol or, possibly, researchers may alter