Integrin antagonists promising for anti-angiogenesis therapy

Miami-Integrin antagonists may offer a potent alternative strategy for the treatment of sight-threatening eye diseases associated with angiogenesis, said Martin Friedlander, MD, PhD, at Angiogenesis 2004.

"Furthermore, integrin antagonists would interfere with angiogenesis at a final common pathway, and so might be effective regardless of the specific angiogenic stimulus," he continued.

Integrins are heterodimeric proteins that are ubiquitous on all cells. However, binding specificity is conferred by the specific combination of their alpha and beta subunits. While more than 20 integrin subunit combinations exist, research indicates that the integrin α_vβ₃ is highly and relatively selectively expressed on actively proliferating vascular endothelial cells.

Early studies in various animal models showed that antagonists binding to that integrin specifically and potently inhibited cytokine- and tumor-mediated angiogenesis, Dr. Friedlander said.

To investigate integrin expression in ocular neovascular diseases, Dr. Friedlander and colleagues performed animal studies using a corneal membrane model in which neovascularization is induced by implantation of growth factor-containing pellets. They found that basic fibroblast growth factor (bFGF) and tissue necrosis factor-α stimulated angiogenesis via α_vβ₃ whereas vascular endothelial growth factor (VEGF) and transforming growth factor-α stimulated α_vβ₅-mediated angiogenesis. An antibody to α_vβ₃ suppressed angiogenesis induced by bFGF, with complete inhibition achieved in 30% to 40% of animals. However, the same antagonist had virtually no efficacy in preventing VEGF-induced vessel growth.

The reverse situation occurred using an antibody to α_vβ₅ whereas a cyclic peptide antagonist of both integrins could block angiogenesis induced by both growth factors.

"Those results led us to the conclusion that there are at least two angiogenic pathways that are distinguished by their dependency on different integrins," Dr. Fried- lander said.

To examine the clinical relevance of those findings, ocular neovascular specimens were collected from patients with choroidal neovascularization associated with age-related macular degeneration or presumed ocular histoplasmosis syndrome and from individuals with proliferative diabetic retinopathy.

Immunofluorescent staining was performed using antibodies to Factor VIII to identify vessels and monoclonal antibodies specific to each integrin to localize their expression.

Those studies confirmed earlier research indicating that the integrin α_vβ₃ is a specific marker of actively proliferating vascular endothelial cells and demonstrated that only α_vβ₃ was expressed on blood vessels in choroidal neovascular membranes, whereas both α_vβ₃ and α_vβ₅ were present on vascular cells in tissues from patients with proliferative diabetic retinopathy. The integrins were absent in the vasculature of tissue from normal eyes as well as in nonvascular, fibroproliferative ocular tissues and epiretinal membranes from patients with proliferative vitreoretinopathy.

"From that study, we concluded that the choroidal and retinal neovascularization develop as distinct pathologic processes driven by different cytokines," Dr. Friedlander said.

Subsequent studies were performed using a model of newborn mice to identify the potential efficacy of integrin antagonists for preventing retinal neovascularization.

The animals were treated subcutaneously for 4 days with a cyclic peptide antagonist to both α_vβ₃ and α_vβ₅ and almost 50% of animals had complete inhibition of normal postnatal retinal vessel formation without any adverse effects on established vasculature.

Studies are being done by Merck KGaA (Darmstadt) using a potent cyclic RGD peptide integrin antagonist. Investigations have begun in oncology patients, and their early results show encouraging efficacy and safety.

Investigations in patients with ocular neovascular diseases are now being planned.

However, Dr. Friedlander said that, ultimately, a combination of therapies likely will prove most useful for the treatment of ocular neovascular diseases, and preliminary laboratory data show that approach has exciting potential.

"Many treatment approaches are focusing on VEGF, but VEGF is probably not the whole story in ocular neovascularization, nor are the integrins," he said.

"Furthermore, Mother Nature is extremely resilient so that angiogenesis might develop via alternate pathways if one is blocked," Dr. Friedlander said. "Therefore, cocktails of different therapies representing multimodal mechanisms will likely be the most effective solution."