Abstract 3400: Small molecule biological screens and global analytical approaches reveal the complexity of cancer cell signaling networks and the challenge of personalized medicine.

BRAF is mutationally activated in 50% of melanomas with a variety of data indicating its role as a driver of malignancy. However, while patients with BRAF mutations respond to vemurafenib, most responses are not long lasting. Moreover, the 25% of melanomas harboring an NRAS activating mutation are resistant de novo to BRAF inhibitors. In order to identify effective drug combinations that increase the effectiveness of BRAF inhibition, we used combinatorial small molecule biological screens and global analytical approaches to identify compensatory and redundant pathways. We screened a drug library for combinations with BRAF or MEK inhibitors that cause synergistic cytotoxicity in melanoma cell lines. Although BRAF mutational status predicted sensitivity to inhibitors of BRAF, each BRAF mutant line differed in the combination of drugs that induced synergistic cytotoxicity. One robust and clinically relevant pair identified was the BRAF inhibitor PLX4720 and the EGFR/HER2 inhibitor lapatinib. This drug combination was synergistic in half of the BRAF mutant cell lines tested, including a line that was completely resistant to PLX4720. The combination was also effective in the two NRAS mutant lines tested. Xenograft experiments testing the combination of PLX4720 and lapatinib revealed a significant reduction in tumor volume compared to either single agent alone in both PLX4720 sensitive and resistant BRAF mutant tumors, providing preclinical data supporting the efficacy of this drug combination. Synergy of the PLX4720-lapatinib combination did not correlate with EGFR/HER2 expression, activity state, or mutational status. This further indicates that sensitivity of melanoma to drug combinations is not determined by the targeted oncogenic drivers, but by the existence of unique secondary, compensatory survival responses. We hypothesize that mutationally activated BRAF is wired into the constitutive signaling network differently in each of these cells, presumably a consequence of the diverse secondary mutations that characterize melanomas. Consistent with this, whole exome-sequencing data revealed a diversity of somatic variation among the synergistic and non-synergistic melanomas, while gene expression analysis demonstrated both distinct, line-specific basal transcriptional profiles and the lack of common gene expression signatures in melanomas synergistically inhibited by treatment with PLX4720-lapatinib. Collectively, these data point out the extraordinary robustness of cancer cell signaling networks and the challenges of individualizing therapies. This study has uncovered novel functional drug combinations and suggests that the underlying signaling networks that control responses to targeted agents can vary substantially depending on unexplored components of the cell genotype and cell signaling network. Citation Format: Devin Roller, Brian Capaldo, Aaron J. Mackey, Mark Conaway, Michael J. Weber, Daniel G. Gioeli. Small molecule biological screens and global analytical approaches reveal the complexity of cancer cell signaling networks and the challenge of personalized medicine. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3400. doi:10.1158/1538-7445.AM2013-3400