Random peptide libraries for target definition

Several reports have shown that panning a random peptide library on a purified cellular receptor identified novel ligands for the target molecule. Many novel ligands for integrins, a class of trans-membrane glycoproteins involved in cell migration and tumor invasion, have been identified in this way. Peptides binding to integrins contained the RGD recognition motif flanked by residues which determine their specificity for differing integrins. Such peptides have been successfully used for targeting various gene and drug delivery vehicles to cell expressing the target integrin. When the target receptor needed to be expressed on the cell surface to retain its native conformation, peptide libraries have been panned on whole cells and different positive selection protocols have been adopted to identify ligands for the receptor of interest. More recently, selections using whole cells have been directed at identifying ligands for unknown receptors with a particular property, for example tissue specificity or internalization. Protocols have been reported that improve the selection efficiency of specific ligands. An innovative approach to selecting ligands for receptors selectively expressed in the organ vasculature has been to inject a library into the circulation in mice. Harvesting peptides that distributed to different organs has identified ligands selectively homing to mouse brain or kidney blood vessels with no previous knowledge of the target receptor. Using the same in vivo panning approach, a library has been injected into the circulation of nude mice bearing human breast carcinoma xenograft, resulting in the identification of ligands for tumor-specific vascular markers. These peptides, previously shown to have high affinity for α(v) integrins, have been combined with a chemotherapeutic drug and shown to act more effectively than the drug alone in tumor-bearing mice.