Until recently, the treatment of ocular neovascular diseases such as age-related macular degeneration (AMD), diabetic retinopathy (DR), and retinal vein occlusion (RVO) has been restricted to ablative laser-based procedures and surgical interventions. More than a decade of research has identified a central role for vascular endothelial growth factor (VEGF) in the etiology of these diseases, resulting in revolutionary new treatments based on inhibiting VEGF. Two anti-VEGF agents are approved as intravitreal therapies for neovascular AMD. Pegaptanib sodium, the first approved agent, is an RNA aptamer that binds the VEGF₁₆₅ isoform while ranibizumab is a monoclonal antibody antigen-binding fragment that binds all VEGF isoforms. Bevacizumab, a related monoclonal antibody, also is being evaluated for the treatment of ocular neovascular diseases in off-label studies. This chapter will focus on the development and clinical application of pegaptanib.

The importance of VEGF in ocular neovascular diseases stems from its key properties as a regulator of physiological and pathological angiogenesis (1) and as a potent promoter of vascular permeability (2). As such, it contributes to two proximate causes of vision loss: the growth of aberrant retinal vasculature and macular edema. Clinical studies established that vitreous levels of VEGF were elevated in a number of ocular neovascular conditions (3, 4, 5 and 6) while preclinical work with several animal model systems demonstrated that such elevations were both necessary and sufficient for the development of ocular neovascularization (7,8). Studies using animal models of diabetes also were important in establishing the role of VEGF in promoting the increased retinal leukostasis characteristic of DR believed to mediate much of the damage to the retinal vasculature associated with this disease (9).

Alternative splicing of the VEGF gene generates six principal isoforms (1,10) with VEGF₁₆₅ and VEGF₁₂₁ being the most common variants in normal eyes (11). Preclinical evidence suggests that VEGF₁₆₅ is especially pathogenic in that it was dramatically upregulated in a rodent model of ischemic neovascularization (12) and had more potent proinflammatory properties than VEGF₁₂₁ (13). Such properties included chemoattraction of monocytes (13), which amplifies neovascularization induced by ischemia (12) or laser wounding (14, 15 and 16), and upregulation of retinal expression of intercellular cell adhesion molecule-1 (13), a mediator of retinal leukostasis in DR (9). These data suggested that specific inactivation of VEGF₁₆₅ could significantly ameliorate the effects of VEGF in ocular neovascular diseases. This concept was supported by two key preclinical observations in rodent models: not only was intravitreal pegaptanib as effective as a VEGF-receptor (R)-Fc fusion protein that binds all VEGF isoforms in suppressing ischemic ocular neovascularization (12) but it was also capable of reversing the diabetes-induced breakdown of the blood-retinal barrier (17).

VEGF exerts a wide range of actions both in the eye, where it is important in the maintenance of retinal neurons (18) and the choriocapillaris (19), and in the nervous system, kidney, bone, and liver (9,20 and 21). Furthermore, the aberrant neovasculature found in AMD and DR is especially permeable, increasing the potential for systemic exposure to intravitreal anti-VEGF agents (22), as evidenced by the detection of therapeutic effects in the fellow eyes following anti-VEGF therapies in DR (23) and AMD (24). The safety of nonselective versus selective anti-VEGF therapies will be explored in the clinical section of this review.

Pegaptanib is an RNA aptamer that was developed through three applications (25, 26 and 27) of the SELEX [systematic evolution of ligands by exponential enrichment] procedure (28,29) using VEGF₁₆₅ as a target. Chemical modifications of the constituent nucleotides were used to improve its nuclease resistance and affinity, and two 5′ polyethylene glycol substituents were added to increase bioavailability (30). Pegaptanib was found to block the binding of VEGF₁₆₅ to endothelial cell receptors, to inhibit VEGF₁₆₅-mediated cell signaling and proliferation (31), and to reduce VEGF₁₆₅-induced vascular permeability (27,32) while not affecting responses to VEGF₁₂₁ (31). Pegaptanib’s selectivity for VEGF₁₆₅ derives from its interaction with the heparin-binding domain of VEGF₁₆₅ (33), which is not present in VEGF₁₂₁ (10). The predicted secondary structure of pegaptanib and its interaction with the heparin-binding domain of VEGF₁₆₅ are shown in Figure 37-1 (30).

Initial pharmacokinetic studies of pegaptanib performed in monkeys found mean half-lives after intravitreal and intravenous administration of 94 hours and 9.3 hours, respectively, suggesting that clearance from the eye is rate-limiting; pegaptanib remained fully active in VEGF-binding assays 28 days after intravitreal injection (34). A subsequent clinical study of pegaptanib pharmacokinetics following intravitreal injection evaluated 147 patients with neovascular AMD who received either 1 mg or 3 mg every 6 weeks for 54 weeks (35). Mean maximal plasma concentrations in the 1 mg group ranged from 20 to 24 ng/ml; pegaptanib remained above the level of detection (8 ng/ml) for 1 week with a plasma mean terminal half-life of 10 days. Repeat injections did not result in any plasma accumulations nor were there detectable serum antibodies to pegaptanib (35).