Electrophilic Oxidation and [1,2]-Rearrangement of the Biindole Core of Birinapant

Birinapant/TL32711 (1) is a bivalent antagonist of the inhibitor of apoptosis (IAP) family of proteins and was designed to mimic AVPI, the N-terminal tetrapeptide of the second mitochondria-derived activator of caspases (Smac/DIABLO). Birinapant bound to the BIR3 domains of cIAP1, cIAP2, and XIAP with K i values of 1, 36, and 45 nM, respectively. Birinapant-mediated activation of cIAP1 resulted in cIAP1 autoubiquitylation and degradation and correlated with inhibition of TNF-mediated NF-κB activation, induction of tumor cell death in vitro, and tumor regression in vivo. Birinapant is being evaluated in Phase 1/2 trials for the treatment of cancer and hepatitis B virus (HBV) infection. After one year at accelerated storage conditions, a formulation of 1 afforded four degradants in >0.1% abundance by HPLC analysis. The primary degradants (2 and 3) were formed via oxidation of the biindole core, while the secondary degradants (5 and 6) arose via [1,2]-rearrangement of 3 and 2, respectively. Forced degradation conditions were developed, which allowed the isolation of 2 and 3 in multigram quantities. Novel deuterated analogues of 1 were prepared to determine the site of oxidation, and NMR experiments confirmed the chemical structures of 5 and 6. The de novo synthesis of 2, 3, 5, and 6 confirmed these experimental findings.