Abstract 765: Multiple signalling pathways are active in BRAF/NRAS wild type melanomas

Abstract In the last five years, the treatment of advanced melanoma has been revolutionised by the introduction of selective kinase inhibitors and immunotherapy. Selective inhibition of the mitogen activated protein kinase (MAPK) pathway has significantly improved the survival of patients with BRAFV600-mutant melanoma, and combination MEK and CDK4 inhibition is currently being trialed in patients with NRAS-mutant melanoma. Almost a quarter of all melanomas, however, are wild type for BRAF and NRAS and have no known actionable mutations. Currently, patients with wild type melanoma are treated with immunotherapies, which produce prolonged responses, but only in a small subset of patients. Overall, the prognosis for patients with wild type melanomas remains poor, and there is an urgent need to establish better therapeutic strategies for these patients. In order to define new therapeutic targets in BRAF/NRAS wild type melanoma, we sought to characterize the key proliferative and survival pathways in wild type melanomas. Wild type (n=14) and BRAFV600 (n=7) melanomas were treated with the MEK inhibitor trametinib at increasing doses from 0.5nM to 5000nM for 72h and sensitivity to MEK inhibition was assessed using viability assays. Signaling pathway activity in response to trametinib (10nM, 24h) was also examined in our panel of melanoma cells using the Human Phospho-Kinase Antibody Array (R&D Systems), which detects 43 different phosphorylated kinases. As expected, BRAFV600 melanomas were exquisitely sensitive to trametinib treatment (IC50=0.8-2.4nM). We also found that although a subset of BRAF/NRAS wild type melanomas (5/14; 36%) were sensitive to trametinib (IC50 0.5-4.9nM), the majority of these melanoma cells (9/14; 64%) were resistant to trametinib (IC50 10.5-264.8nM). Analysis of kinase activation status revealed little difference in signaling pathway activity between the sensitive and resistant wild type melanomas at baseline. Resistant wild type melanomas had higher activity of STAT3 compared to the sensitive cell lines. However, all these wild type melanomas showed variable activation of multiple signaling pathways that were not active in BRAFV600 melanomas. Specifically, at baseline, wild type melanomas showed pronounced activation of the JNK and p38 MAPK pathways, the FAK/Src pathway (i.e. FAK, Lck, Lyn, Fgr and Yes), the epidermal growth factor receptor tyrosine kinase and β-catenin compared to BRAFV600 melanomas. Activation of multiple signaling pathways in wild type melanomas may contribute to innate resistance or limit the efficacy of MEK inhibition, and we are currently exploring the impact of these pathways on the survival and MEK dependence of wild type melanomas. Citation Format: Zizhen Ming, Su Yin Lim, Richard F. Kefford, Helen Rizos. Multiple signalling pathways are active in BRAF/NRAS wild type melanomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 765.