Potential Targeted Therapeutic Strategies for Overcoming Resistance in BRAF Wild Type Melanoma

Melanoma manifests itself from the malignant transformation of melanocytes and represents the deadliest form of skin cancer, being responsible for the disproportionate majority of all skin cancer deaths. The 2002 discovery that 50% of all melanoma patients possess activating BRAF mutations ignited a significant paradigm shift in the way the melanoma field approached research and how patients were treated [1]. The era of targeted therapy had begun and with it came successful targeted BRAF inhibitor therapy regimens, which have accomplished improved clinical benefit (response rate, progression free survival, and overall survival) compared with treatment with chemotherapy in three phase III clinical trials [2]. Although there has been much success in the subgroup of patients whose melanomas harbor activating BRAF mutations, approximately 50% of all melanoma patients do not harbor BRAF mutations. This subgroup of melanoma is composed of ~15-20% of all patients with NRAS mutations and another ~25-30% of patients with neither BRAF nor NRAS mutations. Successful targeted treatment strategies are currently lacking for this subgroup of BRAF-wild type melanomas and therefore novel targeted therapeutic modalities are urgently needed. The work described in this dissertation sheds light on potential approaches for the treatment of BRAF wild type melanoma and will be split into three separate strategies. The first will focus upon the treatment of melanomas without BRAF or NRAS mutations (BRAF/NRAS wild type melanoma) and will expand upon a clinical observation where two melanoma patients were treated with an experimental combination of carboplatin and paclitaxel, with the addition of the AKT inhibitor MK-2206. We demonstrate that the inhibition of AKT significantly enhances the efficacy of chemotherapy in a reactive oxygen species (ROS) mediated fashion, and an induction of autophagy plays a cyto-protective role. The second story focuses upon the treatment of NRAS mutant melanomas by investigating resistance mechanisms to MEK inhibitor treatment. We discovered a MEKi-mediated induction of receptor tyrosine kinase (RTK) signaling to serve as a significant mechanism of escape for NRAS mutant melanomas treated chronically with the MEK inhibitor AZD6244, as well as the recently U.S. Food and Drug Administration (FDA) approved MEK inhibitor trametinib. Novel targeted therapy combinations were then added to overcome the escape from MEK inhibitor therapy. Co-targeting of the receptor tyrosine kinases AXL, PDGFR-β and c-MET with a pan-RTK inhibitor, as well as the mitogen-activated protein kinase (MAPK) pathway with a MEK inhibitor greatly enhanced treatment-induced apoptosis and inhibition of proliferation. The final strategy builds upon the observation that single agent MEK-inhibition is largely ineffective in the treatment of NRAS mutant melanomas. A recovery of MAPK pathway activity in response to MEK inhibition was established to play a significant role in escape of NRAS mutant cells from cell cycle arrest and apoptosis. The combination of a MEK inhibitor with the novel ERK inhibitor VTX-11e prevents the onset of resistant clones and enhances cytotoxicity of the NRAS mutant melanoma cells. This body of work establishes original targeted therapy combinations for the treatment of both NRAS mutant melanomas and BRAF/NRAS wild type melanomas. We propose future clinical investigation with these strategies in the treatment of BRAF wild type melanoma patients in hopes to further extend overall survival