Circulating tumor DNA to monitor treatment response and investigate resistance mechanism in BRAF inhibitor treated colorectal cancer patients.

e16112 Background: BRAF mutations are found in about 10% of colorectal cancer (CRC) patients and is a poor prognosis factor in standard chemotherapy. Combination of BRAF and EGFR inhibition exerts a better therapeutic effective, whereas the resistance develops through undefined mechanisms. Circulating tumor DNA (ctDNA) is a non-invasive approach to assess the genetic evolution of tumors and prognosis in response to therapy, which would help better understanding the treatment response and resistance mechanism in BRAF inhibitor treated colorectal cancer patients. Methods: We performed panel next-generation sequencing of 425 cancer-related genes in 39 serial plasma samples collect from 19 patients who have BRAF V600E mutation CRC to track the resistance during the Vemurafenib treatment in combination with Irinotecan and Cetuximab (VIC) and evaluate the treatment response. Tumor responses were assessed radiologically every two months and are used to determine patients with innate or acquired resistance Results: A total of 19 patients were enrolled into the vemurafenib treatment with irinotecan and cetuximab. By January 20, 2020, treatment had been discontinued in 12 (63.2%) of the patients due to disease progression, while the other 7 cases were still under treatment. Among them, four patients with innate resistance (n = 4, 21.1%) were defined as those with PFS of less than 2 months, while patients (n = 15, 78.9%) with acquired resistance were defined as those with PFS of greater than 2 months. The VIC regimen demonstrated efficacy in patients with BRAF-mutant mCRC, with overall response rates was 63.2% (n = 12). Changes in levels of ctDNA at 4 weeks predicts therapeutic responses. CBLB, TP53 and APC baseline mutations were enriched in baseline samples of innate resistant patients. In contrast, MYC and Cell-cycle pathway alterations were enriched in baseline samples of patients with acquired resistance. Acquired RAS and MAPK mutations, known to confer resistance to BRAF inhibitors, were identified in 60% of acquired resistance patients. Moreover, acquired TGFBR2 and SMAD4 loss-of-function mutations were identified as novel resistant mechanism to the combination of BRAF and EGFR inhibition. Experimental validation is ongoing. Conclusions: Longitudinal analysis of ctDNA in BRAF CRC patients provides insights of molecular difference between innate and acquired resistance and gene alternation during resistance requiring. Serial ctDNA monitoring provides early indication of response to BRAF inhibitor in CRC patients.