Dynamic Switching of Apoptosis-Modulating Bim Heterodimers in Response to BH3 Mimetics in Xenograft Models of Hematologic Malignancies.

This study tested the hypothesis that tumor cells can evade apoptosis following BH3 mimetic treatment by utilizing alternative Bim-binding partners. Levels of Bim heterodimers with Mcl-1, Bcl-2, and Bcl-xL were measured in multiple hematologic cell line xenograft models (AMO-1, MV4-11, and RPMI-8226) following single-dose S63845 or venetoclax; Bak-Bax heterodimer and cleaved caspase-3 (cCasp3) levels were measured to demonstrate mitochondrial apoptosis. Antitumor efficacies of these agents were measured in vivo in mice bearing AMO-1 or MV4-11 xenografts and in vitro in patient-derived lymphoblastoid-like cells. The mechanism of the combination activity of the CDC-like kinase inhibitor cirtuvivint with venetoclax was determined in MV4-11 and KG-1a xenografts. S63845 decreased Mcl-1-Bim levels in AMO-1 and MV4-11 tumors by ∼90% while unexpectedly decreasing Bcl-2-Bim and increasing Bcl-xL-Bim levels. Venetoclax decreased Bcl-2-Bim levels while increasing Mcl-1-Bim and Bcl-xL-Bim levels in MV4-11 tumors. The S63845 + venetoclax combination decreased Mcl-1-Bim levels and demonstrated greater cell killing activity and pharmacodynamic effects than either single agent in multiple models, including patient-derived lymphoblastoid-like cells. Cirtuvivint decreased Mcl-1 and Bim levels, combining with venetoclax to induce significantly greater Bak-Bax and cCasp3 responses than either single agent and induce regression of MV4-11 xenograft tumors. Our results elucidate quantitative pharmacodynamics of S63845, venetoclax, and cirtuvivint, an agent that is currently being evaluated with venetoclax to treat acute myeloid leukemia (NCT06484062). Compensatory increases in off-target Bim heterodimer levels in response to either S63845 or venetoclax offer a possible mechanism of clinical drug resistance.