575P - RAS mutant allele fraction in plasma predicts benefit to anti-angiogenic based first-line treatment in metastatic colorectal cancer

So far, no biomarkers of response to anti-angiogenic drugs are available in colorectal cancer (CRC) treatment. Liquid biopsy technique identifies actionable targets in CRC patients (pts), tracking dynamic mutational changes. We and others described RAS mutant allele fraction in plasma (plMAF) as an independent prognostic marker in metastatic CRC (mCRC). Here, we explored the predictive value of plMAF in RAS mutant pts treated in first line with chemotherapy +/- bevacizumab (bev). A multicentric prospective/retrospective analysis was conducted. We collected data from 226 mCRC pts and selected the subset not eligible for metastasis resection that had basal plMAF sample evaluable for RAS mutant MAF quantification using digital PCR (BEAMing). Pts were stratified as high (≥ 5.8%) or low (< 5.8%) plMAF based on previously defined cut-off (Sanz-Garcia E, ESMO GI, 2017). We investigated associations between different clinicopathological variables (gender, n° and site of metastases, CEA levels, primary site location) and progression-free survival (PFS) stratified by plMAF RAS levels using Cox regression models and survival data were calculated by the Kaplan-Meier method. From October 2017 to May 2019, BEAMing analysis from 62 basal plasma samples was performed. 42 pts (67.7%) were classified as high and 20 pts (32,3%) as low plMAF. Among high RAS plMAF, 24 pts received FOLFOX+bev (57%) and 20 pts FOLFOX alone (43%). In this high plMAF subgroup, a statistically significant longer PFS favouring FOLFOX+bev was observed when compared to FOLFOX alone (10.7 versus 6.9 months, respectively; HR: 0.30; p=0.002). In the low RAS plMAF subgroup, no differences in terms of PFS were observed in either arm (8.9 versus 8.7 months, respectively; HR: 0.70; p=0.6). Multivariate PFS model showed no association between RAS plMAF and clinicopathological variables, except for high RAS plMAF and treatment benefit with FOLFOX+bev. Our results indicate that tumor-borne RAS plMAFs may constitute a potential predictive biomarker of efficacy for anti-angiogenic agents in mCRC. Confirmatory studies in randomized cohorts will be performed. VHIO (Vall d’Hebron Institute of Oncology). AECC (Asociación Española Contra el Cáncer). G. Martini: Research grant / Funding (self), Research Project supported by ESMO with the aid of a grant from Amgen: ESMO-AMGEN. E. Elez: Honoraria (self): Merck Serono; Honoraria (self): Sanofi; Honoraria (self): MSD; Honoraria (self): Roche; Honoraria (self): Servier; Hon