Abstract A27: The combination of navitoclax and PLX4720 in melanoma in vitro and in vivo.

Abstract

Abstract Introduction: Approximately 40–50% of patients with melanoma have a tumor which harbors a BRAF mutation (BRAF+). Vemurafenib is a highly selective inhibitor of mutant BRAF which has recently been FDA-approved for the treatment of patients with BRAF+ advanced melanoma. While response rates to vemurafenib are high, the vast majority of patients will develop resistance. Identification of therapeutic strategies to enhance the efficacy of vemurafenib is a high priority. Navitoclax is a BH3-only mimetic which has single agent activity in tumors with low Mcl-1 and enhances the toxicity of agents which inhibit or downregulate MCL-1. As recent data suggests that selective BRAF inhibitors inactivate Mcl-1 through upregulation of BIM, we therefore studied the effects of combining PLX4720, a selective BRAF inhibitor very similar to vemurafenib, with navitoclax in vitro and in vivo. Methods: Three BRAF mutant melanoma cell lines (A2058, A375, SKMel5) were selected for in vitro analysis and treated for 6 hours with DMSO, navitoclax (1 uM), PLX4720 (1 uM), or the combination. Cells were then lysed after 6 hours of treatment and western blot analysis was performed probing for vinculin, PARP, total and phosphorylated Erk, Mcl-1, BIM, BID, AKT, pS6, and NOXA. Xenografts were generated from the A375 and A2058 cell lines in nude beige mice and grown to 10 mm in size. Mice were then treated with vehicle, navitoclax (100 mg/kg/day), PLX4720 (100 mg/kg/day), or the combination once daily by gavage for 14 days. Tumor measurements were taken daily. Results: Treatment of melanoma cell lines with PLX4720 resulted in downmodulation of Mcl-1 and induction of BIM both in vitro and in vivo. In vitro, the combination of navitoclax and PLX4720 led to greater PARP cleavage than either agent alone. In vivo, the navitoclax enhanced the efficacy of PLX4720 in both xenograft models. Interestingly, the two xenograft models showed differential sensitivity to treatment with the respective single agents. Conclusion: The combination of a selective BRAF inhibitor with a BH3-mimetic may be a promising therapeutic strategy to enhance the clinical efficacy of BRAF inhibitors. Efforts are underway to assess such a combination clinically. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A27.