AZD5582 is a potent IAP inhibitor, which is a dimeric compound based on the AVPI motif of Smac. AZD5582 binds potently to the BIR3 domains of cIAP1, cIAP2, and XIAP (IC50 = 15, 21, and 15 nM, respectively). AZD5582 causes cIAP1 degradation and induces apoptosis in the MDA-MB-231 breast cancer cell line at subnanomolar concentrations in vitro. When administered intravenously to MDA-MB-231 xenograft-bearing mice, AZD5582 results in cIAP1 degradation and caspase-3 cleavage within tumor cells and causes substantial tumor regressions following two weekly doses of 3.0 mg/kg. Antiproliferative effects are observed with 14 in only a small subset of the over 200 cancer cell lines examined, consistent with other published IAP inhibitors. As a result of its in vitro and in vivo profile, AZD5582 was nominated as a candidate for clinical development.