Birinapant, a Smac-Mimetic with Improved Tolerability for the Treatment of Solid Tumors and Hematological Malignancies

Birinapant (<b>1</b>) is a second-generation bivalent antagonist of IAP proteins that is currently undergoing clinical development for the treatment of cancer. Using a range of assays that evaluated cIAP1 stability and oligomeric state, we demonstrated that <b>1</b> stabilized the cIAP1-BUCR (BIR3-UBA-CARD-RING) dimer and promoted autoubiquitylation of cIAP1 in vitro. Smac-mimetic <b>1</b>-induced loss of cIAPs correlated with inhibition of TNF-mediated NF-κB activation, caspase activation, and tumor cell killing. Many first-generation Smac-mimetics such as compound <b>A</b> (<b>2</b>) were poorly tolerated. Notably, animals that lack functional cIAP1, cIAP2, and XIAP are not viable, and <b>2</b> mimicked features of triple IAP knockout cells in vitro. The improved tolerability of <b>1</b> was associated with (i) decreased potency against cIAP2 and affinity for XIAP BIR3 and (ii) decreased ability to inhibit XIAP-dependent signaling pathways. The P₂′ position of <b>1</b> was critical to this differential activity, and this improved tolerability has allowed <b>1</b> to proceed into clinical studies