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 (α and β). Integrins αVβ3 and αVβ5 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 αVβ3, αVβ5, αVβ6, αVβ8 and α5β1. Based on these observations, a few conformationally constrained cyclopeptides containing the isoDGR sequence were synthesized. However, these ligands showed a moderate affinity for αVβ3 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 αVβ3 and αVβ5 receptors (Figure 1b). Interestingly, these tests showed that compound 2 is a potent αVβ3 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 αVβ3 integrin receptor were performed starting from the representative conformations obtained from the MC/SD simulations.