Abstract 6218: Discovery and preclinical evaluation of AMG 397, a potent, selective and orally bioavailable MCL1 inhibitor

MCL1 inhibitors are an emerging class of therapeutics targeting key protein-protein interactions between MCL1 and BH3 domain containing pro-apoptotic BCL-2 family members. Several MCL1 inhibitors have entered clinical trials including AMG 176, currently in Phase I clinical development for hematologic malignancies. However, all existing clinical stage MCL1 inhibitors are being administered intravenously. Here we describe the discovery and preclinical evaluation of AMG 397, the first orally administered MCL1 inhibitor in clinical development. The pursuit of an orally bioavailable MCL1 inhibitor with an acceptable human clinical dose required improvements to AMG 176 in both potency and pharmacokinetic properties. The chemistry effort to identify AMG 397 began with the observation that there were two conformations in AMG 176 bound to MCL1 with a difference of 2.8 kcal/mol in energy. A structure-guided approach in combination with ligand-based design to attempt to favor the more active conformation provided the advanced lead AM-3106 (Ki=100 pM) with significantly improved cell based potency (OPM2 viability IC50 = 19 nM) compared to AMG 176. Further refinements in potency and PK properties of AM-3106 culminated in the discovery of AMG 397. AMG 397 exhibited picomolar affinities for MCL1 (Ki=15 pM), selectively competing for binding to the BH3-binding groove of MCL1 with pro-apoptotic BCL-2 family members (e.g. BIM). In cells, AMG 397 potently disrupted the interaction between MCL1 and BIM. Treatment of OPM2 cells, an MCL1 dependent multiple myeloma cell line, induced clear increases in Caspase-3/7 activity within one hour. Subsequent effects on viability were observed across a treatment time course with maximal effects observed following 24 hours of continuous exposure (IC50=50 nM). Time course washout viability studies demonstrated that AMG 397 induced a committed step towards apoptosis and suggested that discontinuous dosing schedules would be sufficient to achieve antitumor effects in vivo. Cell lines from hematologic malignancies including AML, multiple myeloma and DLBCL exhibited greatest sensitivity to AMG 397 in a large tumor cell line profiling screen (n=300). In vivo, oral administration of AMG 397 demonstrated rapid and sustained dose dependent increases in activated BAK, cleaved Caspase-3 and cleaved PARP in established OPM2 xenografts. Discontinuous dosing schedules of once or twice weekly at 25 or 50 mg/kg of AMG 397 exhibited significant tumor regressions in mice bearing OPM2 xenografts, with 9 of 10 mice tumor free at the end of the study in both 50 mg/kg groups. AMG 397 was also tested in the MOLM-13 orthotopic model of AML where twice weekly administration at 10, 30 and 60 mg/kg achieved 47% tumor growth inhibition (TGI), 99% TGI and 75% regression respectively. Combination of AMG 397 at 10 mg/kg twice weekly and 50 mg/kg of venetoclax daily achieved 45% regression in the same model. Citation Format: Sean Caenepeel, Rex Karen, Brian Belmontes, Alla Verlinsky, Hong Tan, Yajing Yang, Xiaoyue Chen, Kexue Li, Jennifer Allen, Jan Wahlstrom, Jude Canon, Angela Coxon, Paul Hughes. Discovery and preclinical evaluation of AMG 397, a potent, selective and orally bioavailable MCL1 inhibitor [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6218.