Process Development and Synthesis of Birinapant: Large Scale Preparation and Acid-Mediated Dimerization of the Key Indole Intermediate

Birinapant/TL32711 (<b>1</b>) is a novel bivalent antagonist of the inhibitor of apoptosis (IAP) family of proteins which is currently in clinical development for the treatment of cancer and hepatitis B virus (HBV) infection. In this report, we present a detailed description of the <b>1</b> drug substance synthesis used to support our ongoing clinical studies. Key transformations in this process included the development of a scalable, high-yielding route to acyl indole <b>14</b> as well as a two-step dimerization/oxidation of indole <b>19</b> that afforded biindole <b>21</b> in excellent yield and purity (70% yield, 2 steps; >95 area% purity by HPLC analysis). In addition, partial defluorination of <b>21</b> was observed following hydrogen-mediated benzyloxycarbonyl (Cbz) protective group removal which was obviated by the use of HBr/HOAc for this transformation. The use of commercially available amino acid derivatives afforded related impurities which proved difficult to purge in subsequent steps. Thus, defining the impurity specification for these reagents was critical to providing <b>1</b> drug substance of >99 area% chemical purity. Using this process, we have successfully prepared <b>1</b> drug substance multiple times on >500-g-scale in support of our clinical development program

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