1. ↵* Corresponding Author:
   Olivier Michielin, Centre Hospitalier Universitaire Vaudois, Pluridisciplinary Center for Clinical Oncology, Rue du Bugnon 21, Lausanne, 1011, SwitzerlandOlivier.michielin@chuv.ch

+Author Affiliations

Identification of multiple mechanisms of resistance to vemurafenib in a patient with BRAFV600E-mutated cutaneous melanoma successfully rechallenged after progression

Abstract

Purpose: To investigate the mechanism(s) of resistance to the RAF-inhibitor vemurafenib, we performed a comprehensive analysis of the genetic alterations occurring in metastatic lesions from a patient with a BRAFV600E-mutant cutaneous melanoma who, after a first response, underwent subsequent rechallenge with this drug. Experimental Design: We obtained blood and tissue samples from a patient diagnosed with a BRAFV600E-mutant cutaneous melanoma that was treated with vemurafenib and achieved a near-complete response. At progression, he received additional lines of chemo/immunotherapy and was successfully re-challenged with vemurafenib. Exome and RNA sequencing were performed on a pre-treatment tumor and two subcutaneous resistant metastases, one that was present at baseline and previously responded to vemurafenib (PV1), and one that appeared de novo after reintroduction of the drug (PV2). A culture established from PV1 was also analyzed. Results: We identified two NRAS activating somatic mutations, Q61R and Q61K, affecting two main subpopulations in the metastasis PV1, and a BRAF alternative splicing, involving exons 4-10, in the metastasis PV2. These alterations, known to confer resistance to RAF inhibitors, were tumor-specific, mutually exclusive, and were not detected in pre-treatment tumor samples. In addition, the oncogenic PIK3CAH1047R mutation was detected in a subpopulation of PV1, but this mutation did not appear to play a major role in vemurafenib resistance in this metastasis. Conclusions: This work describes the co-existence within the same patient of different molecular mechanisms of resistance to vemurafenib affecting different metastatic sites. These findings have direct implications for the clinical management of BRAF-mutant melanoma.