Abstract 3522: BET and BRAF inhibitors act synergistically against BRAF mutant melanoma

Abstract

Abstract Purpose: Despite major advances in the treatment of metastatic melanoma, treatment failure is still inevitable in most cases. Manipulation of key epigenetic regulators, including inhibition of Bromodomain and extra-terminal domain (BET) family members impairs cell proliferation in vitro and tumor growth in vivo in different cancers, including melanoma. BET inhibition is thought to impair downstream transcriptional activity of the MAPK (MAPK) pathway and other key oncogenic signaling pathways, suggesting complementarity with MAPK inhibition. Here, we investigated the effect of combining the BET inhibitor JQ1 with the BRAF inhibitor Vemurafenib in in vitro and in vivo models of BRAF-mutant melanoma. We additionally screened potential synergism of JQ-1 with 27 additional compounds including inhibitors of the proteasome, histone deacetylases (HDACi) and MEK. Methods: We performed cytotoxicity and apoptosis assays, and a xenograft mouse model to determine the in vitro and in vivo efficacy of JQ1 in combination with Vemurafenib against BRAF-mutant melanoma cell lines. Further, to investigate the molecular mechanisms underlying the effects of this combined treatment, we conducted antibody arrays of in vitro drug-treated cell lines and RNA sequencing of drug-treated xenograft tumors. Synergism between JQ1 and other targeted compounds was examined by in vitro cytotoxicity assays. Results: The combination of JQ1 and Vemurafenib acted synergistically in BRAF-mutant cell lines. Additional combinations that were found synergistic included JQ-1 plus proteasome inhibitors (carfilzomib and marizomib, HDAC inhibitors (romidepsin and panobinostat), or anti-Bcl2 small molecules (Obatoclax and TW-37). The combination of JQ-1 and Vemurafenib resulted in marked apoptosis in vitro, with up-regulation of several pro-apoptotic proteins involved in both the intrinsic and extrinsic pathways of apoptosis. In vivo, this combination treatment suppressed tumor growth and significantly improved survival compared to either drug alone. RNA sequencing of tumor tissues revealed almost four thousand genes that were uniquely modulated by the combination. Furthermore, several anti-apoptotic genes such as MCL-1, BCL-XL and BCL-2 were significantly down-regulated. Conclusions: Collectively, our data provide a rationale for combinations of BET inhibitors with additional compounds including BRAF inhibitors, as a novel strategy for the treatment of melanoma. Citation Format: Luca Paoluzzi, Douglas Hanniford, Elena Sokolova, Iman Osman, Farbod Darvishian, Jinhua Wang, James E. Bradner, Eva Hernando. BET and BRAF inhibitors act synergistically against BRAF mutant melanoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3522. doi:10.1158/1538-7445.AM2015-3522