Genomic profiles of BRAF inhibitor resistance mechanisms in metastatic colorectal cancer.

e15527 Background: BRAF mutations are present in 5-10% of metastatic colorectal cancers (mCRCs). Patients with mCRC whose tumors harbor BRAF V600 mutation generally respond poorly to standard chemotherapy. Treatment with BRAF inhibitors has been shown to improve outcomes, whereas the resistance develops through undefined mechanisms. Therefore, it is imperative to accurately comprehend the genomic profiling of resistance mechanisms to BRAF inhibitors in mCRC for exploring its clinical treatment strategies. Methods: We analyzed next-generation sequencing results in 520 cancer-associated genes for 22 patients who had BRAF V600E mutant mCRC in order to characterize genomic predictors of treatment outcome and track the acquired resistance (AR) mechanisms during the vemurafenib/dabrafenib/encorafenib treatment in combination with cetuximab +/- trametinib. The median age of the patients was 61 years old, 54.5% were male, 54.5% had right-sided mCRC, and 81.8% received one or more prior chemotherapy lines. Tissue and/or plasma samples were collected at baseline (prior to anti-BRAF treatment) and upon disease progression (PD). Objective tumor responses were radiologically assessed every 6 weeks according to RECIST v1. 1. Results: By Feb 2021, treatment had been discontinued in 15 (63.2%) of the patients due to disease progression, while the other 7 cases were still under treatment. The median PFS (mPFS) for all patients was 4.5 months. The overall response rate was 32%, and the disease control rate was 86%. In addition to BRAF V600E, the most common concomitant mutations were identified in TP53 (20/22), RNF43 (8/22), LRP18 (7/22), APC (7/22) and PIK3CA (5/22). Patients with baseline alterations in RNF43 (n = 8; p = 0.0466), or RECQL4 (n = 4; p = 0.0406) had significantly longer PFS than their respective wild type counterparts. In contrast, baseline alterations in PPP2R2A (n = 3; p = 0.0131), RUNX1T1 (n = 3; p = 0.0147), and receptor tyrosine kinase (RTK) signaling pathway genes (n = 6; p = 0.0057) were each significantly associated with shorter PFS. Among the 15 patients whose disease progressed, 9 were identified with newly developed AR mutations in PD tumor or plasma samples. MET amplification was the most common AR mechanism (n = 4) followed by BRAF amplification, KRAS, NRAS mutations (n = 3 each), and MAP2K1, PIK3R1 mutations (n = 1 each). Q61 substitution was the most dominant form for both KRAS and NRAS AR mutations (83%, 5/6). In terms of signaling pathway, MAPK (67%, 6/9) and RTK (44%, 4/9) pathway alterations were most frequently observed. Conclusions: Our study illustrated the landscape of genetic alterations in patients with BRAF V600E mutant mCRC upon BRAF inhibitor treatment, which could be critical to predict responses to BRAF inhibitors and guide personalized clinical decision-making after disease progression.