Background: Trifluridine (FTD) incorporation into DNA is the main anti-tumor mechanism of action of TAS-102. We performed preclinical analysis and translational validation study to identify the candidate cytokines for TAS-102 efficacy in metastatic colorectal cancer (mCRC) patients (pts). Methods: As a first preclinical process, we selected candidate cytokines according to our transcriptomic and cell biological analysis. We then validated predictive value of the cytokines in mCRC pts receiving TAS-102 (discovery) and regorafenib (control). Blood samples were obtained at baseline (BL), before second cycle (2^nd) and progressive disease (PD), and cytokine levels were measured using ELISA. The change patterns were defined as ‘increased’ or ‘decreased’ from BL. Results: 93 pts were included in this study: 67 received TAS-102 and 26 received regorafenib. Preclinical cDNA microarray analysis with colon cancer cell lines demonstrated marked changes in the RNA expression of interleukin-8, VEGF-A and epiregulin (EREG) after FTD treatment. Particularly, in preclinical model, EREG secretion was highly measured after FTD treatment in FTD-sensitive cells, while not in FTD-resistant cells. In the discovery cohort, increased EREG levels at 2^nd showed trend toward longer PFS and OS than decreased changes (2.6 vs 2.3 mos, P= 0.096; 10.8 vs 6.2 mos, P= 0.091). Meanwhile, decreased EREG levels at PD were marginally associated with longer PFS and significantly with longer OS than increased (4.0 vs 2.3 mos, P= 0.130; 9.8 vs. 6.2 mos, P= 0.016). Combined analysis of EREG changes showed that pts with either increased at 2^nd or decreased at PD had significantly longer PFS and OS compared to those with both decreased at 2^nd and increased at PD (3.0 vs 2.0 mos, HR: 0.47, 95%CI: 0.24-0.93, P= 0.031; 10.8 vs 5.3 mos, HR: 0.30, 95%CI: 0.14-0.61, P= 0.001). The findings were confirmed in the multivariate analysis for OS (HR: 0.311, 95%CI: 0.15-0.66, P= 0.002), and no significant differences were observed in the control cohort. Conclusions: Our preclinical data-based translational validation study suggests that serum EREG levels may predict clinical outcome in mCRC pts receiving TAS-102.