An MDM2 inhibitor (IC50 = 0.018 μM); inhibits proliferation in cancer cell lines expressing wild-type p53 (IC50s = 0.18-2.2 μM) and cell lines expressing mutant p53 (IC50s = 5.7-20.3 μM); prevents and reduces tumor growth in an SJSA-1 mouse xenograft model at 50 and 100 mg/kg per day, respectively; decreases platelet counts in rats at 50 and 100 mg/kg and in cynomolgus monkeys at 10 and 20 mg/kg