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FDA approves atezolizumab and chemotherapy for stage III dMMR/MSI-H colon cancer

On October 8, 2026, the U.S. Food and Drug Administration (FDA) approved atezolizumab in combination with chemotherapy for patients with stage III deficient mismatch repair (dMMR) colon cancer.1 Whilst the standard adjuvant treatment in this setting consists of fluoropyrimidine- and oxaliplatin-based chemotherapy, the benefit of adding an immune checkpoint inhibitor in resected stage III dMMR disease prior to this approval remained uncertain.2

The approval is based on findings from the Phase III ATOMIC trial (NCT02912559), which enrolled 712 patients with resected stage III dMMR colon cancer. Patients received either atezolizumab plus mFOLFOX6 (a modified FOLFOX6 regimen of fluorouracil, oxaliplatin, and leucovorin) for 6 months followed by atezolizumab monotherapy for an additional 6 months, or mFOLFOX6 alone for 6 months. At a median follow-up of 40.9 months, 3-year disease-free survival was 86.3% with atezolizumab plus mFOLFOX6 compared with 76.2% with mFOLFOX alone (HR, 0.50, 95% CI: 0.35–0.73; P<0.001). The safety profile was consistent with the known effects of atezolizumab and chemotherapy, although grade 3 or 4 adverse events occurred more frequently with the combination than with mFOLFOX6 alone, at 84.1% and 71.9%, respectively.2

J. Randolph Hecht, MD, UCLA Jonsson Comprehensive Cancer Center, Los Angeles, CA, briefly provides his perspectives on the pivotal trial data, stating:

“[ATOMIC] showed about a 10% improvement that was across the curve. So about a 10% improvement in relapse-free survival and I think that will be the standard of care. It was generally well-tolerated. We all, as oncologists, we all know about immune-related adverse events, but it seemed to be very well-tolerated.”

Approval of atezolizumab and mFOLFOX6 represents an important advance in the adjuvant treatment of stage III dMMR colon cancer, offering an immunotherapy-based approach with the potential to reduce recurrence risk and improve long-term disease control following curative-intent surgery.

References:

  1. U.S. Food and Drug Administration. FDA approves atezolizumab in combination with chemotherapy for Stage III mismatch repair deficient colon cancer. Available here. (Last Accessed 09/10/2026).
  2. Sinicrope FA, Ou FS, Arnold D, et al. Atezolizumab plus FOLFOX for stage III mismatch repair-deficient colon cancer. N Engl J Med . 2026;394(12):1155–66.