New research led by Mayo Clinic Comprehensive Cancer Center has found that adding immunotherapy to chemotherapy after surgery for patients with stage 3 colon cancer and a specific genetic feature called deficient DNA mismatch repair (dMMR) reduces cancer recurrence and mortality by 50% compared to chemotherapy alone. Approximately 15% of colon cancer patients have dMMR, and these tumors have shown less responsiveness to chemotherapy until now. The findings were presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago.
Dr. Frank Sinicrope, an oncologist who led the study, noted that this represents a major advance in treating dMMR stage 3 colon cancer and is expected to change treatment approaches. Until now, standard care after surgery for any stage 3 colon cancer has been chemotherapy, yet about 30% of patients still experience recurrence.
The clinical trial included 712 patients from the U.S. and Germany with dMMR stage 3 colon cancer whose cancer had spread to lymph nodes. Patients received six months of chemotherapy combined with atezolizumab—an immune checkpoint inhibitor that activates the immune system to attack cancer cells—followed by six months of immunotherapy alone.
Dr. Sinicrope explained that dMMR tumors show increased inflammatory cells within the tumor, including those expressing targets of immune checkpoint inhibitors, making them suitable for this combined approach. Based on the results, he recommends this regimen as the new standard of care for this patient group. The research team plans to submit their findings to the National Comprehensive Cancer Network, which includes Mayo Clinic among its 33 leading cancer centers.
The study also included patients with Lynch syndrome, the most common form of hereditary colon cancer, as their tumors often display dMMR. Dr. Sinicrope emphasized that by introducing immunotherapy at an earlier disease stage, meaningful benefits can be delivered to patients.
