Vascular endothelial growth factor (VEGF) is an important signaling molecule for the growth and differentiation of blood and lymphatic vessels and has been implicated in a variety of disease states. Most notable are VEGF's roles in the recruitment of blood vessels to tumor sites and in the increased blood flow and ocular pressure that are hallmarks of macular degeneration. Currently, FDA-approved anti-VEGF treatments include the monoclonal antibodies bevacizumab (Avastin, Genentech) and ranibizumab (Lucentis, Genentech) and the RNA aptamer pegaptanib (Mucagen, Eyetech Pharmaceuticals/Pfizer). Both antibodies and aptamers effectively prohibit angiogenesis by binding VEGF with high affinity, thus blocking VEGF-receptor interactions. Peptides that similarly bind VEGF might provide a useful alternative treatment or perform other functions for drug delivery or detection. For example, therapeutics coupled to VEGF-binding peptides could be utilized for channeling drugs or imaging dyes to sites of increased VEGF secretion like tumors. The goal of designing such a peptide ligand is, therefore, to assure that it binds specifically to its target protein and with high affinity. Toward that end, a screening process was designed to isolate ligands for VEGF from a combinatorial library displayed on the surface of bacterial cells. The best sequences were identified based on their ability to bind VEGF in the presence of a competing mixture of proteins (i.e. serum). A four-amino acid consensus sequence (W-E/D-W-E/D) from this first library screen conferred both affinity and some degree of specificity for VEGF and was thus was used as a starting point for affinity maturation of a VEGF-binding peptide. A new library of peptides was created that incorporated this motif and screened to favor high-affinity binding to VEGF with slow dissociation kinetics. These engineered peptide ligands might be useful alone as therapeutics or in conjunction with other disease-specific agents or drug-delivery mechanisms, and the screening method can be extended to a wide variety of other druggable targets to increase ligand affinity, specificity, and functionality.

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