Analysis of molecular mechanisms of response and resistance to vemurafenib (vem) in <i>BRAF^V600E</i>melanoma.

Abstract

8503^ Background: Vem induces frequent clinical responses (RR &gt;50%) and improved survival in patients (pts) with BRAF-mutated metastatic melanoma. Multiple mechanisms of escape from vem have been proposed. We performed a centralized analysis of pretreatment, cycle 1, day 15 (D15), and progression (DP) tumor samples collected during the phase II BRIM-2 trial (Sosman et al, NEJM 2012). Methods: Of 132 pts enrolled, archival tumor samples, biopsies taken at baseline (BL), D15, and DP were obtained from 84, 38, 26, and 22 pts, respectively, and analyzed by immunohistochemistry (IHC) for signaling molecules in the MAPK, PI3K/AKT, and cell cycle pathways. Genetic analyses of signaling genes were performed using Sequenom MASSarray and direct DNA sequencing. Results: High levels of ERK phosphorylation were seen at BL indicating constitutive MAPK signaling due to the BRAF mutation. In 19/22 paired samples, pERK levels were reduced following vem (BL vs D15). Mean absolute pERK reduction in pts with clinical response (RECIST criteria) to vem (n=14) was significantly greater than in non-responders (n=8; p=0.013). Baseline cytoplasmic PTEN H-score was higher in responders vs non-responders (H-score=88 vs 68; p=0.042). At progression, upregulation of pERK (H-score=181) was frequently, but not uniformly found vs D15 tumors (H-score=48.5). At progression, increases in Cyclin D1 and Ki67 were seen, without obvious changes in PTEN or pAKT. NRAS mutations occurred in 3/13 DP; 2 had paired BL samples without NRAS mutations. Only 1/82 pts had a concomitant NRAS and BRAF mutation at BL, and did not respond to vem. MAP2K1 (MEK1) codon 124 mutations occurred in 7/92 BL and 1/20 DP samples. 2 pts with BL MEK1 mutations had partial responses. Conclusions: MAPK signaling was effectively inhibited by vem early in treatment, and in the subset of patients with matched tumor samples the degree of pathway inhibition correlated with clinical response. MAPK signaling is upregulated in many lesions at progression. NRAS mutations appear to be a mechanism of acquired resistance in a few tumors (3/13), while the role of MEK1 mutations in resistance is less clear.