Hitting the right targets : a study of molecular mediators of sensitivity to novel potential targeted therapies in cutaneous malignant melanoma

Abstract

Cutaneous malignant melanoma (CMM) is the deadliest form of skin cancer with a 5-year overall survival (OS) rate of less than 10% among patients diagnosed with disseminated disease. During the past decade the emergence and use of novel targeted therapy and immunotherapy has significantly increased the 5-year OS rate (34%). There are some CMM patients who harbor primary resistance to current therapeutic regimes and hence never respond to treatment, whereas a major subset of the patients with objective response ultimately develops a refractory disease due to acquired resistance. Moreover, clinical biomarkers that can be used to find patient subgroups who are most likely to benefit from a particular therapy remains elusive.

In paper I, we found that the combination treatment of ERBB family inhibitor afatinib and MET/ALK inhibitor crizotinib was cytotoxic to CMM cells independent of BRAF/NRAS mutation status. The observations were validated both in vitro and in vivo. In paper II, we explored the molecular mechanisms behind the combination treatment effect of afatinib and crizotinib. We found that the combination treatment downregulated IRS-1, RPS6KB1 and RPS6 protein expression. PMEL/Melanoma gp-100 and PI3K-p85 were upregulated in cells with induced resistance to the combination. We also showed that the resistance to the combination was reversible after a drug holiday. In paper III, we found that the efficacy of MTH1 inhibitor TH1579 is independent of BRAF/NRAS mutation status. TH1579 abrogated cell growth both in vitro and in vivo. We showed that AXL and CAV-1 play a role in mediating sensitivity to TH1579. Moreover, combination treatment of BRAF inhibitors with TH1579 further potentiated cell death in BRAF mutant CMM cells, including BRAF inhibitor resistant CMM cells. In paper IV, we showed that co-expression of MTH1 and PMS2 mRNA, key players in the DNA damage response pathway, is associated with shorter progression free survival after immunotherapy in CMM. We also found that these proteins were increased in refractory tumors. Finally, we showed that co-silencing of MTH1 and PMS2 further induced apoptosis compared to silencing of either gene alone in patient-derived short-term CMM cell cultures.

Overall this thesis highlights potential novel targeted therapeutic strategies for patients with CMM and suggests possible alternate molecular markers that can be targeted to overcome resistance mechanisms that currently pose as a serious clinical challenge in the systemic treatment of CMM.