Abstract 3972: Combined inhibition of MCL1 and BCL-2 with AMG 176 and venetoclax induces anti-tumor effects in primary patient samples and models of acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous hematological malignancy involving the clonal expansion of immature myeloid cells. Dysregulated expression of the BCL-2 family of proteins has been implicated in AML pathogenesis. Specifically, the anti-apoptotic family members MCL1 and BCL-2 have been reported to play key roles in AML survival. The selective inhibition of these two proteins represents an emerging strategy in AML treatment. AMG 176 is a potent and selective MCL1 inhibitor currently being tested in an AML Phase I clinical trial. Here we describe the activity of AMG 176 and AM-8621, a structural analog, as single agents and in combination with the BCL-2 inhibitor venetoclax, in models of AML. AM-8621 and venetoclax were profiled as single agents against a panel of AML cell lines to characterize their dependency on MCL1 and BCL-2 for survival. A wide range of sensitivities to both compounds was observed, with several lines exhibiting dependency on both MCL1 and BCL-2 for survival, suggesting functional redundancy and a requirement for combined inhibition to maximize response. To test this hypothesis, we profiled a subset of cell lines with the combination of AM-8621 and venetoclax. A synergistic interaction was detected in each cell line, highlighting their codependence on MCL1 and BCL-2. We also evaluated the synergistic potential of this combination on primary AML patient samples. Here, freshly purified bone marrow aspirates were treated with equimolar concentrations of AM-8621 and venetoclax and compared against single agents in a flow cytometry based viability assay. Marked improvements in activity and potency were observed with the combination over either agent alone. We also tested the combination of AMG 176 and venetoclax in a MOLM-13 orthotopic xenograft model of AML. Mice were treated twice weekly with AMG 176 (60 mg/kg) and daily with venetoclax (100 mg/kg). While both single agents achieved significant reductions in MOLM-13 tumor burden (69% and 33% reduction in BLI respectively), the combination exhibited complete inhibition of tumor growth (100% reduction in BLI) and achieved tumor regression relative to the first day of dosing. We next characterized the effects of this combination on subsets of hematopoietic cells in vivo. The reduced affinity of AMG 176 for murine MCL1 (200-fold) required the use of a human MCL1 knock-in mouse for these studies. Terminal analysis of mice treated with the combination or AMG 176 alone showed significant decreases in peripheral blood B-cells and monocytes, whereas venetoclax alone exhibited significant reductions in B-cells only. Analysis of spleens revealed greater reductions in both cell types following treatment with the combination compared with either single agent. These data highlight the promise of combined MCL1 and BCL2 inhibition as a novel therapeutic strategy for the treatment of AML. Citation Format: Sean R. Caenepeel, Tao Osgood, Brian Belmontes, Jan Sun, Elaina Cajulis, Andrew Wei, Angela Coxon, Jude Canon, Paul Hughes. Combined inhibition of MCL1 and BCL-2 with AMG 176 and venetoclax induces anti-tumor effects in primary patient samples and models of acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3972.