A new peptidic vector for molecular imaging of apoptosis, identified by phage display technology

Phosphatidylserine (PS) exposure on the cell surface is an early marker of apoptosis. To select PS binding peptides as vectors of contrast agents to image apoptosis, a phage library has been exposed to perfused mouse livers. Phages not retained on control livers during the first perfusions were used for selections on apoptotic livers in a second series of perfusions. Four selected phages were further evaluated for binding to PS-coated enzyme-linked immunosorbent assay (ELISA) plates. They presented an apparent affinity constant (Ka (app)) for PS ranging from 6.08 x 10(10) M to 1.62 x 10(11) M. These phages did not bind to phosphatidylcholine, and competition with annexin V confirmed their specific interaction with PS. The phage with the highest affinity-bound PS in ELISA with a Ka (app) = (1.6 +/- 0.2) x 10(11) M. It carried the TLVSSL peptide that was synthesized. Specific competition with annexin V and with the synthetic peptide was performed and confirms the specificity of the interaction.