Cyclic isoDGR peptidomimetics as low-nanomolar αvβ3 integrin ligands

Integrins are a large family of heterodimeric transmembrane glycoprotein receptors, composed of two noncovalently associated subunits (\u3b1 and \u3b2). Integrins \u3b1V\u3b23 and \u3b1V\u3b25 have been found to be overexpressed on blood vessels in human tumors, but not on vessels in normal human tissues. For this reason, these integrins have become attractive targets for pharmacological studies mainly in the oncology area. Besides the well known RGD, it has been proposed that the NGR and DGR sequences might also have a role in the interaction between integrins and their ligands. The Corti research group (S.Raffaele/MolMed) discovered that the isoDGR sequence can mimic RGD and interact with the RGD binding site of integrins such as \u3b1V\u3b23, \u3b1V\u3b25, \u3b1V\u3b26, \u3b1V\u3b28 and \u3b15\u3b21. Based on these observations, a few conformationally constrained cyclopeptides containing the isoDGR sequence were synthesized. However, these ligands showed a moderate affinity for \u3b1V\u3b23 integrin in competitive binding assays. Recently, our research group synthesized two cyclic isoDGR-containing peptidomimetics (1 and 2, Figure 1a), bearing a bifunctional diketopiperazine scaffold. Cyclic isoDGR peptidomimetics 1 and 2 were examined in vitro for their abilities to compete with biotinylated vitronectin for binding to the purified \u3b1V\u3b23 and \u3b1V\u3b25 receptors (Figure 1b). Interestingly, these tests showed that compound 2 is a potent \u3b1V\u3b23 integrin ligand (low nanomolar IC50 value, Figure 1b). The preferred conformations of 1 and 2 were investigated by 1H-NMR spectroscopy and by computational methods (MC/SD simulations). In order to rationalize - on a molecular basis - the binding affinity of compounds 1 and 2, docking studies in the active site of the \u3b1V\u3b23 integrin receptor were performed starting from the representative conformations obtained from the MC/SD simulations