277 The discovery and pre-clinical development of the first clinical stage EZH2-inhibitor, EPZ-6438 (E7438)

genes that govern progression through the cell cycle, the initiation of DNA repair, and apoptosis. The activity of p53 is tightly regulated by the MDM2 protein, in an auto-regulatory feedback loop. Inhibition of the MDM2−p53 protein–protein complex by small molecule inhibitors has been shown to reactivate normal p53 pathways in cells overexpressing MDM2, and so exert an anti-cancer effect. A number of series of potent small-molecule inhibitors have been developed as far as clinical trials. Understanding of the structural basis for inhibition from ligand-bond protein X-ray structures has been important in the optimisation of most of these series. We have developed a class of inhibitors of the MDM2−p53 interaction, based on an isoindolinone scaffold [J. Med. Chem. 2006, 49, 6209–6221, Bioorg. Med. Chem. Letters 2011, 21, 5916– 5919]. NU8406A, (R)-4-chloro-3-(4-chlorophenyl)-3-((1-(hydroxymethyl)cyclopropyl)methoxy)-2-(4-nitrobenzyl)isoindolin-1-one (IC50 = 43.8±6.2 nM) shows comparable in vitro activity to Nutlin-3a. Recently, we reported the MDM2 co-crystal structure of Nutlin-3a using a surface-entropy reduction mutant [Acta Cryst 2013, D69, 1358–1366]. We have applied these MDM2 mutants to solve the co-crystal structure of NU8406A. The structure shows that the binding mode of the isoindolinone is comparable to that for Nutlin-3a, and confirms the (R)-stereochemistry of the ligand. Preliminary structure–activity studies have revealed significant differences between the two series in the way that the Phe19 pocket is occupied.