Abstract 1115: BET bromodomain inhibition synergizes with MEK inhibitors in uveal melanoma

Abstract Uveal melanoma (UM) is the most common primary intraocular malignancy in adults. Approximately 50% of the patients develop hepatic metastasis and the median survival rate is 12 months. Currently available drugs have shown limited clinical activity in patients with UM and there is an urgent need for new effective therapies. UM is characterized by activating mutations of G protein q/11, mutations in BAP1, and amplification of the oncogene MYC. Epigenetic dysregulation plays a critical role in UM pathogenesis, and we have demonstrated that Gnaq/11 mutant UM cells are addicted to BRD4 activity. BRD4 is a key regulator of transcriptional elongation by recruiting positive transcriptional elongation factor complex (PTEFb) to chromatin and activating RNA polymerase II-dependent transcription. While BET inhibitors have shown potent activity in pre-clinical studies, single-agent BET inhibitors exhibited limited efficacy in advanced UM clinical trials. MAPK signaling in constitutively active in Gnaq/11 mutant UM cells and previous studies have shown that the MAPK pathway mediates resistance to BET inhibition. Therefore, we sought to combine MEK and BET inhibitors in a panel of UM cells. We found synergistic activities with a variety of structurally distinct MEK inhibitors including trametinib and binimetinib in combination with BET inhibitors, such as I-BET and PLX2853. These combinations induced a greater reduction in cell viability and significant apoptosis compared to either monotherapy, in both parental and BET inhibitor-resistant cells. Western blot studies were performed to evaluate the mechanism of the combination treatment with BET and MEK inhibition and demonstrated a marked downregulation of p-ERK, MYC, Bcl-xL and upregulation of BIM with the combination therapy. Similarly, the combination treatment resulted in greater inhibition of tumor growth compared to either BET or MEK inhibition alone in a Gnaq mutant xenograft model. In conclusion, our studies suggest that co-targeting of MEK and BET proteins may be required to maximize the responses of UM cells to BET inhibitors. Clinical trials with the combination of BET and MEK inhibitors are warranted. Citation Format: Grazia Ambrosini, Elgilda Musi, Gary K. Schwartz. BET bromodomain inhibition synergizes with MEK inhibitors in uveal melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1115.