Abstract CN07-01: Monitoring downstream responses to MAP kinase pathway inhibitor therapy in BRAF mutant melanoma to support rational combinations strategies.
Authors: Flaherty Keith Thomas 
Published: August 2014

Abstract
For half of the advanced melanoma population, selective BRAF inhibitor therapy has transformed the natural history of disease and provided a platform for developing molecularly targeted therapy combinations. Recently, BRAF/MEK combined inhibition has been demonstrated to be superior to BRAF or MEK inhibitor monotherapy, yet limits remain regarding durability of response. It has previously been shown that oncogenic BRAF suppresses MITF expression and that genetic silencing of BRAF or inhibition with selective antagonists can consistently restore MITF expression to a degree that significantly impacts MITF target gene expression. Our preliminary studies analyzing patient tumor samples and performing mechanistic investigations suggest both positive and negative consequences of restoring MITF expression. Of course, BRAF inhibitor therapy has a net positive clinical fact causing tumor regression in approximately 90% of patients treated, conferring disease control that is clearly superior to previously standard cytotoxic chemotherapy, and significantly improving overall survival. Therefore, the hypothetical negative downstream consequences of BRAF inhibition are viewed as potential mechanisms by which tumors partially resistant therapy, preventing complete or more durable responses.
MITF has previously been described as a candidate oncogene in melanoma. Approximately 15-20% of melanomas harbor focal amplifications of MITF, but an even larger fraction have impaired viability in vitro following genetic silencing of MITF. Based on this evidence, one would consider restoration of MITF expression to be potentially deleterious. Our group has demonstrated to pro-survival effects of restored MITF expression following BRAF inhibitor therapy: upregulation of BCL2A1 and induction of oxidative phosphorylation via PGC1alpha. Direct and selective antagonists of BCL2A1 and PGC1alpha are currently lacking, but available investigational agents may at least partly counter them.
MITF has also long been known to regulate the expression of melanocyte lineage antigens such as gp100, Mart-1, TYRP-1, and TYRP-2. Each of these have been shown to be the target of effective antitumor immunity using both active and adoptive immunotherapy strategies. In patient tumor specimens examined before and following 1-2 weeks of BRAF or BRAF/MEK inhibitor therapy, we have demonstrated 2-10 fold increases in MITF expression corresponding to 10-100 fold increased expression of these antigens. Furthermore, we have demonstrated consistently increased numbers of CD8+ T cells in the same tumor biopsy specimens. In characterizing these T cell infiltrates, we have found an increase in clonality in most cases. While a large proportion of these clonal populations were not identified in pretreatment specimens, it was the expansion of pre-existing T cell clones that correlated best with depth of initial response to BRAF inhibitor therapy. Ongoing work seeks to understand the functionality of these T cell infiltrates and the possibility that there antitumor effects remain limited due to immunosuppressive factors in the tumor microenvironment. While we and others explore combination targeted therapy regimens justified on the basis of the somatic genetic profile of an individual patient's tumor, exploration of the downstream consequences of MAP kinase pathway inhibitor therapy has provided additional hypotheses for improving on the efficacy of an already active therapy.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):CN07-01.
Citation Format: Keith Thomas Flaherty. Monitoring downstream responses to MAP kinase pathway inhibitor therapy in BRAF mutant melanoma to support rational combinations strategies. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr CN07-01.