Research ArticleCancer

Synergistic action of the MCL-1 inhibitor S63845 with current therapies in preclinical models of triple-negative and HER2-amplified breast cancer

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Science Translational Medicine  02 Aug 2017:
Vol. 9, Issue 401, eaam7049
DOI: 10.1126/scitranslmed.aam7049

Cutting off another tumor lifeline

BH3 mimetics are drugs that inhibit the BCL-2 family of prosurvival proteins in cancer cells and thereby promote cancer cell death. Unfortunately, MCL-1, a member of this prosurvival family, can interfere with treatment because it is not sensitive to currently available BH3 mimetics. The MCL-1 inhibitor S63845 was developed to overcome this mechanism of treatment resistance, and Merino et al. examined the effectiveness of this drug in cell lines and xenografts derived from breast cancer patients. The authors demonstrated the drug’s efficacy in combination with several drugs that are already in clinical use and also identified a protein that can promote treatment resistance, which may help predict which patients are more likely to benefit from the new treatment.


The development of BH3 mimetics, which antagonize prosurvival proteins of the BCL-2 family, represents a potential breakthrough in cancer therapy. Targeting the prosurvival member MCL-1 has been an area of intense interest because it is frequently deregulated in cancer. In breast cancer, MCL-1 is often amplified, and high expression predicts poor patient outcome. We tested the MCL-1 inhibitor S63845 in breast cancer cell lines and patient-derived xenografts with high expression of MCL-1. S63845 displayed synergistic activity with docetaxel in triple-negative breast cancer and with trastuzumab or lapatinib in HER2-amplified breast cancer. Using S63845-resistant cells combined with CRISPR (clustered regularly interspaced short palindromic repeats)–Cas9 (CRISPR-associated 9) technology, we identified deletion of BAK and up-regulation of prosurvival proteins as potential mechanisms that confer resistance to S63845 in breast cancer. Collectively, our findings provide a strong rationale for the clinical evaluation of MCL-1 inhibitors in breast cancer.

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