Unusually long-term responses to vemurafenib in BRAF V600E mutated colon and thyroid cancers followed by the development of rare RAS activating mutations.

INTRODUCTION: V600E BRAF mutation is an established driver mutation in a variety of tumors. Vemurafenib is a selective inhibitor of the BRAF V600E kinase, known to be highly effective in BRAF V600E-positive metastatic melanoma. As a single agent, vemurafenib is relatively ineffective in other V600E-positive malignancies. Case 1: A 72 year old man with metastatic CRC who failed several previous lines of chemotherapy. Genetic analysis of 315 cancer-related genes (Foundation Medicine, FMI) revealed a BRAF V600E mutation. The patient was treated with vemurafenib resulting in a partial response of 18 months. Genetic analysis following development of resistance revealed a new mutation in KRAS-G12R. Case 2: V600E mutation was identified in a 59 year old woman with metastatic PTC refractory to radioiodine therapy. The patient was treated with vemurafenib resulting in a partial response lasting 43 months. Genetic analysis following development of resistance revealed a new mutation in NRAS-Q61K. The presented cases demonstrated the development of rare RAS mutations as a genetic mechanism of acquired BRAF inhibitor resistance. This observation is strongly supported by the analysis of a large database consisting of 712 BRAF V600E-positive melanoma samples showing higher rates of BRAF V600E and RAS mutations co-occurrence in metastatic lesions compared to local tumors (OR = 3.8, p = 0.035). This enrichment is likely a result of the development of RAS mutations following treatment with BRAF inhibitors. DISCUSSION: We report two cases showing extreme response to vemurafenib, which could not be predicted prior to treatment commencement. Genetic testing demonstrated a resistant mechanism not previously reported in CRC or PTC patients, namely an acquired mutation of RAS. This is supported by an analysis of a large cohort of BRAF V600E-positive melanomas. Further studies are needed in order to identify predictive markers for response to vemurafenib and to explore novel strategies to overcome RAS-mediated resistance.