540 Background: FOLFOX with bevacizumab (BV) is a standard treatment for metastatic colorectal cancer (mCRC); however, its clinical response is around 50% and there is no way to predict responders prior to therapy. In this study, we attempted to identify new biomarkers associated with positive therapeutic responses by means of a comprehensive metabolomic analysis of patient serum. Methods: Serum collected from 68 mCRC patients, who were registered in a phase II study of first-line FOLFOX with BV treatment (Nishina, JJCO 2013) was used to conduct a comprehensive metabolomic quantification analysis using a capillary electrophoresis time-of-flight mass spectrometry. Responders (Rs: n = 37) and non-responders (NRs: n = 31) were defined as those achieving a status of CR/PR and SD/PD, respectively, which were assessed by an extramural review board using RECIST. Statistical analyses were performed using a logistic regression model for treatment response and Cox proportional hazard analysis for overall survival (OS). Results: Among 470 annotated endogenous metabolites, cysteine-glutathione disulphide (CSG), gamma-glutamyl cysteine (gGC) and hypoxanthine (HPX) were identified as significant (p < 0.05) metabolites associated with positive therapeutic responses by both response and survival analyses. Patients were divided into two groups according to cutoff values of pretreatment serum levels of these metabolites. The actual response rate in the high CSG, gGC and HPX group were 86, 69 and 8% whereas those in the low group were 40, 26 and 64%, respectively. The hazard ratio (HR) for OS in CSG, gGC and HPX were 0.28 (p < 0.01), 0.37 (p < 0.01) and 2.39 (p < 0.05), respectively. Using CSG, gGC and HPX, we have developed a multivariate logistic regression model to predict Rs/NRs and survival benefit based on the three metabolite levels in pretreatment serum. Sensitivity, specificity, ROC AUC and HR of OS between Rs and NRs were 89%, 65%, 0.83 and 0.36 (p = 0.002), respectively. Conclusions: We identified three novel pretreatment serum metabolomic markers that are associated with treatment response to FOLFOX with BV in chemotherapy-naive mCRC patients. Clinical trial information: UMIN000001490.