Restoration of p53 activity by inhibition of the p53–MDM2 interaction has been considered an attractive approach for cancer treatment. However, the hydrophobic protein–protein interaction surface represents a significant challenge for the development of small-molecule inhibitors with desirable pharmacological profiles. RG7112 was the first small-molecule p53–MDM2 inhibitor in clinical development. Here, we report the discovery and characterization of a second generation clinical MDM2 inhibitor, RG7388, with superior potency and selectivity.