1 Evolving knowledge in pharmacologic treatments

Modern ocular drug therapy is a result of the extraordinary discoveries and challenges achieved through the last decades in different fields of science such as chemistry, biotechnologies, and physics, to mention a few. The understanding of the pathogenesis of ocular diseases underwent significant changes due to discoveries in medicine, genetics, and cell biology. New insights in the pathology of posterior-segment diseases were obtained by the integration of new imaging techniques such as confocal scanning laser angiography and optical coherence tomography. Within the group of the so-called classic therapies, new molecules emerged, such as antibiotics, nonsteroidal antiinflammatory drugs, and steroids. Thereby, ocular therapy of diseases like uveitis, endophthalmitis, and diabetic maculopathy improved markedly. Moreover, new treatment concepts appeared with molecules whose mode of action is more complex and sophisticated.1–3

MEDICAL TREATMENT

VERTEPORFIN

The beginning of the 21st century was marked by the era of the first medical treatment of exudative age-related macular degeneration (AMD). After years of laser photocoagulation as the gold standard in the therapy of this disease, verteporfin became the first medical treatment.4,5 Verteporfin (Visodyne, Novartis Pharmaceuticals) is a twostage process requiring administration of both the photosensitizer for injection and nonthermal red light activation. Verteporfin is transported in the plasma mainly by lipoproteins and appears to accumulate preferentially in proliferating endothelial cells of new vessels, such as in the choroidal neovasculature in the case of the eye. Once verteporfin is activated in the presence of oxygen, highly reactive, short-lived singlet oxygen and reactive oxygen radicals are generated. Light activation of verteporfin results in local damage to neovascular endothelium. Damaged endothelium is known to release procoagulant and vasoactive factors through the lipooxygenase (leukotriene) and cyclooxygenase (eicosanoids such as thromboxane) pathways, resulting in platelet aggregation, fibrin clot formation, and vasoconstriction. The following temporary occlusion of choroidal neovascularization (CNV) has been confirmed in humans by fluorescein angiography and demonstrated a clinical efficacy. However, a collateral damage to retinal structures following photoactivation, including the retinal pigmented epithelium (RPE)andtheouternuclearlayeroftheretina,canbeseen.Unfortunately, photodynamic therapy itself induced a local inflammatory reaction and an increased local production of vascular endothelial growth factor (VEGF), thereby limiting its therapeutic effect. Nevertheless, Visudyne still has some indication as the treatment of choroidal neovessels in myopia.6–8

ANTI-VEGF TREATMENT

A new era of ocular neovascular therapy has started.8,9 Several different approaches regarding anti-VEGF, one of the main factors in ocular neovascularization, are nowadays routine in the treatment of AMD.

The importance of VEGF as a therapeutic target derives from its effect on vascular leakage and neovascularization. Gene-based drugs entered the clinical everyday life only in the recent past. The nucleotide sequence of DNA, RNA, or their modifications is used to induce gene expression (gene therapy), suppress translation of the target mRNA (small interference RNA (siRNA), antisense oligonucleotides, ribozymes), or to bind to a specific protein target (aptamers).10 This approach is interesting as genes express their protein products for prolonged periods, thus a more targeted and long-lasting therapy can be obtained. Currently in ophthalmology clinically used gene-based drugs are Vitravene (Isis Pharmaceuticals) and Macugen (Pfizer). Vitravene is an intravitreally administered phosphorothioate oligonucleotide for the treatment of CMV infection in acquired immunodeficiency syndrome (AIDS) patients. Anti-VEGF aptamers are stable small RNA-like molecules that bind exclusively and with high affinity to the 165-kDa isoform of human VEGF. Sodium pegaptanib (Macugen, Pfizer), can only bind and inhibit the larger VEGF 165 isoform. The aptamer is conjugated to polyethylene glycol to increase its half-life and stability in the vitreous. The same drug is currently in phase II trials for the treatment of diabetic macular edema. Two siRNA molecules (bevasiranib and Sirna-027) that modify the activity of VEGF and its receptor (VEGFR-1) have entered into clinical trials. RNAi is a double-stranded piece of interference RNA that is taken up by chorioretinal cells, activating a protein that breaks down the antisense mRNA. Destruction of VEGF mRNA prevents the production of VEGF protein. The whole process is catalytic, so the RNAi may be a very potent and efficient blockade of VEGF. RNAi may have a long biologic half-life, indicating a much longer interval between intravitreal injections.11

OTHER MEDICAL TREATMENTS

Other antiangiogenic therapy approaches are based on proteins like antibodies or enzymes. In contrast to pegaptanib, bevacizumab (Avastin, Genentech), a full-length, humanized monoclonal antibody against VEGF and ranibizumab (Lucentis, Genentech), a recombinant, humanized, monoclonal antibody antigen-binding fragment (Fab), bind and neutralize all the biologically active forms of VEGF. Both drugs are proteins that were genetically modified from the same murine monoclonal antibody against VEGF. The two proteins differ in their component, size, and affinity for VEGF. Whereas bevacizumab is a humanized, murine full-length antibody with two binding sites for VEGF and an Fc fragment, ranibizumab is a humanized, murine anti- gen-binding fragment (Fab) with only a single affinity-matured binding site for VEGF.

VEGF-Trap (R1R2) is a fusion protein that combines ligand-binding elements taken from the extracellular domains of the receptors VEGFR-1 and VEGFR-2 fused to the Fc portion of immunoglobulin G (IgG). This potent high-affinity VEGF blocker effectively suppresses tumor growth and vascularization in vivo.

Several publications demonstrated the fundamental role of the c-raf kinase in cell proliferation, resistance to apoptosis, and its influence on the VEGF pathway. Although this therapy is still at an experimental level, it appears to be a promising method of treating neovascular tissue in AMD and diabetic retinopathy.