Simple SummaryThe sensitivity to chemotherapy of KRASG12C-mutated colorectal cancer has been investigated to verify whether the combination of chemotherapy plus a KRASG12C-inhibitor might become the standard of care in the near future. To this aim, the present retrospective study was designed to assess the performance of irinotecan vs. oxaliplatin in the first-line treatment of KRASG12C-mutated mCRC patients and provide support for first-line decision making. In this setting of patients treated with FOLFIRI or FOLFOX +/− bevacizumab, irinotecan and oxaliplatin were compared using a propensity-score-matched analysis. The survival superiority of irinotecan was demonstrated over oxaliplatin in KRASG12C-mutated patients, while no differences were observed in a control cohort of KRASG12D-mutated patients. This should be considered when investigating chemotherapy plus targeted agent combinations.AbstractBackground: KRASG12C-mutated metastatic colorectal cancer (mCRC) has recently been recognized as a distinct druggable molecular entity; however, there are limited data on its sensitivity to standard chemotherapy. In the near future, the combination of chemotherapy plus a KRASG12C-inhibitor might become the standard of care; however, the optimal chemotherapy backbone is unknown. Methods: A multicentre retrospective analysis was conducted including KRASG12C-mutated mCRC patients treated with first-line FOLFIRI or FOLFOX +/− bevacizumab. Both unmatched and propensity-score-matched analysis (PSMA) were conducted, with PSMA controlling for: previous adjuvant chemotherapy, ECOG PS, use of bevacizumab in first line, timing of metastasis appearance, time from diagnosis to first-line start, number of metastatic sites, presence of mucinous component, gender, and age. Subgroup analyses were also performed to investigate subgroup treatment–effect interactions. KRASG12D-mutated patients were analysed as control. Results: One hundred and four patients treated with irinotecan-(N = 47) or oxaliplatin-based (N = 57) chemotherapy were included. In the unmatched population, objective response rate (ORR) and median (m) progression-free and overall survival (mPFS and mOS) were comparable between the treatment arms. However, a late (>12 months) PFS advantage was observed with irinotecan (HR 0.62,p= 0.02). In the PSMA-derived cohort, a significant improvement with irinotecan vs. oxaliplatin was observed for both PFS and OS: 12- and 24-month PFS rates of 55% vs. 31% and 40% vs. 0% (HR 0.40,p= 0.01) and mOS 37.9 vs. 21.7 months (HR 0.45,p= 0.045), respectively. According to the subgroup analysis, interaction effects between the presence of lung metastases and treatment groups were found in terms of PFS (pfor interaction = 0.08) and OS (pfor interaction = 0.03), with a higher benefit from irinotecan in patients without lung metastases. No difference between treatment groups was observed in the KRASG12D-mutated cohort (N = 153). Conclusions: First-line irinotecan-based regimens provided better survival results in KRASG12C-mutated mCRC patients and should be preferred over oxaliplatin. These findings should also be considered when investigating chemotherapy plus targeted agent combinations.