Abstract C137: Rapid assessment of TORC1 suppression as a functional biomarker predicting responsiveness to RAF and MEK inhibitors in BRAF-mutant melanoma patients.

Abstract

Abstract The clinical development of selective RAF and MEK inhibitors has transformed the treatment of the ∼50% of melanoma patients whose tumors harbor BRAF mutations. However, a substantial percentage of these patients fail to respond to therapy, and most responses are partial and short-lived. Although multiple mechanisms of resistance have been identified in BRAF-mutant melanoma, no clinically useful biomarkers have been established to predict which patients are most likely to demonstrate sensitivity or resistance to RAF or MEK inhibitors. We found that suppression of TORC1 activity in response to RAF or MEK inhibitors, as measured by decreased phosphorylation of ribosomal protein-S6 (P-S6), was a functional biomarker that effectively predicted sensitivity in BRAF-mutant melanoma cell lines in vitro and in mouse tumor xenografts. In sensitive melanomas, TORC1 and P-S6 were suppressed in response to RAF or MEK inhibitors, but in resistant melanomas, TORC1 activity was maintained, in some cases despite robust suppression of MAPK signaling by these inhibitors. In mouse models, suppression of TORC1 after MAPK inhibition was necessary for induction of apoptosis and tumor response in vivo. Notably, in paired biopsies obtained from patients with BRAF-mutant melanoma before treatment and after initiation of RAF inhibitor therapy, P-S6 suppression was associated with significantly improved progression-free survival [HR 0.19, 95% CI 0.01-0.84, p=0.03]. Finally, we found that changes in P-S6 in patients’ tumor cells could be readily monitored in real-time by multiplexed, quantitative immunofluorescence microscopy of serial fine-needle aspiration biopsies obtained from patients before and during the first 2 weeks of RAF inhibitor therapy. This approach provides a minimally-invasive means of rapidly monitoring the efficacy of treatment, before changes in tumor volume are apparent by traditional radiographic imaging. Together, these results establish suppression of P-S6 after initiation of RAF inhibitor therapy as a robust potential functional biomarker to guide the treatment of patients with BRAF-mutant melanoma, and present a powerful methodology for monitoring changes in potentially any signaling pathway in response to targeted therapies in patients. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C137. Citation Format: Ryan B. Corcoran, S. Michael Rothenberg, Aaron N. Hata, Anthony C. Faber, Adriano Piris, Rosalynn M. Nazarian, Ronald D. Brown, Jason T. Godfrey, Daniel Winokur, John Walsh, Mari Mino-Kenudson, Shyamala Maheswaran, Jeffrey Settleman, Jennifer A. Wargo, Keith T. Flaherty, Daniel A. Haber, Jeffrey A. Engelman. Rapid assessment of TORC1 suppression as a functional biomarker predicting responsiveness to RAF and MEK inhibitors in BRAF-mutant melanoma patients. [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 C137.