Frontline treatment for patients diagnosed with metastatic melanoma harboring a V600 BRAF mutation is a combination of BRAF/MEK inhibitors. Despite advances in targeted therapies used to treat melanoma, the majority of patients will relapse due to acquired resistance. The intrinsic apoptotic pathway is frequently dysregulated in cancer, promoting cell survival and resistance mechanisms. The anti-apoptotic Mcl-1 protein was identified as a key resistance factor to vemurafenib, a mutant BRAF inhibitor, and likely plays a role in preventing apoptosis in the context of dual BRAF/MEK inhibitor resistance. Activation of apoptosis by using small molecule inhibitors of the anti-apoptotic Bcl-2 proteins has become a validated therapeutic approach and can re-sensitize cancers to standard of care treatments. We found that melanoma cell lines display a heterogeneous dependency on anti-apoptotic proteins. In this study, we systematically investigated the role of Mcl-1 in vemurafenib resistance in melanoma cell lines using genetic and pharmacological inhibition of Mcl-1. Using the functional BH3 profiling assay, we discovered that melanoma cell lines resistant to vemurafenib are increasingly primed for Mcl-1 dependence. Additionally, applying dynamic BH3 profiling we demonstrated that melanoma cell lines subjected to short term treatment with vemurafenib showed greater dependency upon Mcl-1 for survival, thus serving as a selective priming agent. Consistent with these findings, both vemurafenib resistant and short-term vemurafenib pre-treated melanoma cell lines displayed increased sensitivity to Mcl-1 genetic silencing using siRNA and direct Mcl-1 inhibition using small-molecule inhibitors developed by our group. We further demonstrate significant synergy in the induction of apoptosis induced by the combination of vemurafenib and Mcl-1 inhibitors such as UM-101. In a similar way, Mcl-1 priming and increased sensitivity to Mcl-1 inhibitors was shown in melanoma cells pre-treated with MEK inhibitors. We further studied the mechanism of apoptosis induced by Mcl-1 inhibitors and validated their cellular target engagement. These studies provide evidence that targeting Mcl-1 represents a promising therapeutic strategy for treating patients with resistant melanomas and aids in the design of rational combination therapies before resistance is acquired.

Citation Format: Karson J. Kump, Lei Miao, Nurul Ansari, David B. Lombard, Zaneta Nikolovska-Coleska. BRAF/MEK inhibitors selectively prime melanoma for Mcl-1 dependence and sensitize to Mcl-1 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 324.

©2019 American Association for Cancer Research.
2019
American Association for Cancer Research.