Identification of Lethal Inhibitors and Inhibitor Combinations for Mono-Driver versus Multi-Driver Triple-Negative Breast Cancer Cells

Simple Summary Triple-negative breast cancers lack estrogen and progesterone receptors and HER2. They lack effective targeted therapies and tend to be more aggressive, with the worst five-year overall survival and a higher rate of recurrence. This study investigates the oncogenic signaling mechanisms of two model cell lines, DU-4475 and MDA-MB-231. The former is a mono-driver cancer cell line relying on a BRAF V600E mutation and the latter is a multi-driver cancer dependent on a KRAS-mutation-activated MAP kinase pathway and Src kinase. The results of this study reveal that while the mono-driver cancer cells can be effectively killed by one drug blocking their predominant driver, multi-driver cancer cells can only be killed by drug combinations blocking all drivers. The drug combination in MDA-MB-231 achieves strong synergy and potent synthetic lethality. This study suggests pharmacological synthetic lethality as a foundation for combination targeted therapy for multi-driver cancers. There are no signaling-based targeted therapies for triple-negative breast cancer. The development of targeted cancer therapy relies on identifying oncogenic signaling drivers, understanding their contributions to oncogenesis and developing inhibitors to block such drivers. In this study, we determine that DU-4475 is a mono-driver cancer cell line relying on BRAF and the mitogen-activated protein kinase pathway for viability and proliferation. It is fully and lethally inhibited by BRAF or Mek inhibitors at low nM concentrations, but it is resistant to inhibitors targeting other signaling pathways. The inhibitory lethality caused by blocking Mek or BRAF is through apoptosis. In contrast, MDA-MB-231 is a multi-driver triple-negative breast cancer cell line dependent on both Src and the KRAS-activated mitogen-activated kinase pathway for proliferation and viability. Blocking each pathway alone only partially inhibits cell proliferation without killing them, but the combination of dasatinib, an Src inhibitor, and trametinib, a Mek inhibitor, achieves synthetic lethality. The combination is highly potent, with an IC50 of 8.2 nM each, and strikingly synergistic, with a combination index of less than 0.003 for 70% inhibition. The synthetic lethality of the drug combination is achieved by apoptosis. These results reveal a crucial difference between mono-driver and multi-driver cancer cells and suggest that pharmacological synthetic lethality may provide a basis for effectively inhibiting multi-driver cancers.