Microsatellite instability status as a predictive marker of clinical outcome from FOLFOX adjuvant chemotherapy in stage III colon cancer.

Abstract

493 Background: The addition of oxaliplatin to 5-fluorouracil (5-FU; FOLFOX regimen) was demonstrated to improve the adjuvant treatment of stage III colon cancer. For patients with microsatellite instability (MSI) tumors, several studies suggested a lack of benefit from 5-FU adjuvant chemotherapy but very little data are available regarding FOLFOX adjuvant therapy. The aim of this study was to further assess the value of MSI status as a marker of clinical outcome from FOLFOX adjuvant chemotherapy in stage III colon cancer. Methods: This multicentric retrospective study included 223 unselected patients with stage III colon cancer treated by FOLFOX adjuvant chemotherapy between 2003 and 2007. MSI status was determined by immunohistochemistry as the absence of MLH1, MSH2 or MSH6 expression. Disease-free survival (DFS) and relapse-free survival (RFS) were analyzed according to the MSI status using Kaplan Meier method and compared by log-rank test. Results: Twenty three tumors (10.3%) were MSI. The rate of 3-year DFS was 88.6% and 76.6% for MSI and MSS groups, respectively (HR=0.64; 95% CI, 0.25 to 1.60; P=0.34). The rate of 3-year RFS was 88.6% and 76.7% for MSI and MSS groups, respectively (HR=0.52; 95% CI, 0.20 to 1.30; P=0.18). Conclusions: A trend toward longer survival was observed for patients with MSI tumors compared with those with MSS tumors but the differences in survival were not significant. Interestingly, DFS at 3-years of patients with stage III MSI tumors treated by FOLFOX was higher in our series (88.6%) than in the largest study published for patients treated by 5-FU-based adjuvant chemotherapy (around 67.5%) or surgery alone (around 62.5%) (Sargent et al, JCO 2010). These observations suggest that adding oxaliplatin to 5-FU re-establishes a benefit of adjuvant treatment in the stage III MSI population. These results should be confirmed by analyzing materials of previously completed trials comparing FOLFOX to 5-FU such as the MOSAIC study. [Table: see text]