Abstract 5601: Overcoming acquired resistance to BRAF inhibitors in melanoma with the HSP90 inhibitor ganetespib
Authors: Acquaviva Jaime , Sang Jim , Smith Don , Zhang Chaohua , Wada Yumiko , Proia David A. 
Published: August 2012

Abstract
Activating BRAF(V600E) kinase mutations occur in 50% of melanoma, 45% of papillary thyroid cancer and 10% of colon cancer. BRAF inhibitors have elicited significant clinical response in melanomas but acquired resistance frequently develops after initial responses. Therefore, consideration of new treatment strategies to block activity of mutant BRAF and deter therapeutic resistance is warranted. Heat shock protein 90 (HSP90) is a molecular chaperone required for the stability of hundreds of client proteins, including the RAF family of kinases, and has emerged as a highly relevant anticancer target. Ganetespib is a second generation, small molecule HSP90 inhibitor currently being evaluated in multiple clinical trials. To investigate the potential for ganetespib in the treatment of melanoma we evaluated its preclinical activity as monotherapy, and in combination with targeted agents, in cells sensitive or resistant to BRAF inhibitors. Ganetespib displayed potent anticancer activity in a large panel of BRAF(V600E) mutant melanoma, thyroid and colon cancer cell lines, with IC50 between 4 and 40 nM. Further analysis showed that ganetespib disrupted oncogenic signaling by inducing the destabilization of mutant BRAF and subsequent inactivation of MEK and ERK. Levels of wild-type BRAF were only slightly reduced in response to ganetespib treatment, implying that the stability of activated BRAF is more reliant on HSP90 and that ganetespib may selectively deplete oncogenic forms of BRAF. Notably, ganetespib treatment also led to depletion of CRAF, which is implicated in driving therapeutic resistance to BRAF inhibitors. In an effort to maximize antitumor activity, we combined low levels of ganetespib with BRAF or MEK inhibitors in vitro and observed increased cytotoxicity compared to single agent treatment. Both the BRAF and MEK inhibitors were cytostatic on their own, which may be due to continued PI3K/mTOR signaling. We subsequently assayed the combination of ganetespib with a PI3K/mTOR inhibitor and observed synergistic increases in cell death. Importantly, ganetespib was capable of blocking the feedback activation on MAPKK induced by a MEK inhibitor as well as the activation of ERK by PI3K/mTOR inhibitors. To determine if these results translated in vivo, similar combinations were performed in BRAF mutant human melanoma xenografts. Co-administration of ganetespib with either a BRAF or PI3K/mTOR inhibitor suppressed in vivo tumor growth to a greater extent compared to monotherapy. Finally, we show that ganetespib effectively blocked MAPK signaling and killed RPMI-7951 melanoma cells (IC50 = 4 nM), which are resistant to BRAF and MEK inhibitors due to the overexpression of COT. In conclusion, targeting HSP90 with ganetespib represents a promising approach for the treatment of BRAF(V600E)-driven cancers and may be especially relevant in the context of acquired resistance to BRAF inhibitors.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5601. doi:1538-7445.AM2012-5601