Structure-activity studies in a family of β-hairpin protein epitope mimetic inhibitors of the p53-HDM2 protein-protein interaction

Inhibitors of the interaction between the p53 tumor-suppressor protein and its natural human inhibitor HDM2 are attractive as potential anticancer agents. In earlier work we explored designing beta-hairpin peptidomimetics of the alpha-helical epitope on p53 that would bind tightly to the p53-binding site on HDM2. The beta-hairpin is used as a scaffold to display energetically hot residues in an optimal array for interaction with HDM2. The initial lead beta-hairpin mimetic, with a weak inhibitory activity (IC50 = 125 mu M), was optimized to afford cyclo-(L-Pro-Phe-Glu-6ClTrp-Leu-Asp-Trp-Glu-Phe-D-Pro) (where 6ClTrp = L-6-chlorotryptophan), which has an affinity almost 1000 times higher (IC50 = 140 nM), In this work, insights into the origins of this affinity maturation based on structure-activity studies and an X-ray crystal structure of the inhibitor/HDM2(residues 17-125) complex at 1.4 angstrom resolution are described, The crystal structure confirms the beta-hairpin conformation of the bound ligand, and also reveals that a significant component of the affinity increase arises through new aromatic/aromatic stacking interactions between side chains around the hairpin and groups on the surface of HDM2.