Effect of adding oxaliplatin to adjuvant 5-fluorouracil/leucovorin (5FU/LV) in patients with defective mismatch repair (dMMR) colon cancer stage II and III included in the MOSIAC study

3524 Background: The MOSAIC study (André T, N Engl J Med, 2004) demonstrated that adding oxaliplatin to adjuvant 5FU and LV improved three-year disease-free survival (DFS) in stage II and III resected CC. Efficacy of FOLFOX4 in pts with dMMR stage III was suggested in a retrospective study (Zaanan A, Ann Oncol 2010). Methods: Of the 2,246 pts included in MOSAIC study, formalin-fixed, paraffin-embedded (FFPE) tissue blocks or slides from 1,019 pts were obtained. Thirty-three samples with insufficient tumor tissue were excluded from this translational study. MMR status was determined by immunohistochemistry (IHC) analysis of the protein products of MLH1, MSH2, PMS2, and MSH6 genes. Results: A total of 986 pts (44%) were evaluable for MMR status and MMR status was not evaluable for 1,260 pts (56%). Relapse-free survival (RFS), DFS and overall survival (OS) were similar in both, MMR and MMR not evaluable population. Ninety (9.1%) and 896 (90.9%) pts had dMMR and proficient MMR (pMMR) CC, respectively. Of the patients with 90 dMMR CC, 45 pts had stage II and 45 stage III. Hazard Ratios (HRs) for stage II and III dMMR are 0.52 (0.21–1.28) for RFS, 0.52 (0.24–1.14) for DFS, and 0.45 (0.19–1.05) for OS, respectively. HR for stage III dMMR are 0.56 (0.19–1.61) for RFS, 0.51 (0.18–1.41) for DFS, and 0.44 (0.15–1.34) for OS, respectively. HR for stage II dMMR are 0.64 (0.11–3.70) for RFS, 0.60 (0.17–2.09) for DFS, and 0.52 (0.13–2.10) for OS, respectively. Conclusions: Analyses ofcolon cancerMMR status in pts included in the MOSAIC study support the use of FOLFOX4 in pts with dMMR stage III cancer. Clinical trial information: NCT00275210. [Table: see text]