SUMMARY: Colorectal cancer (CRC) is the third most common cancer diagnosed in both men and women in the United States. The American Cancer Society estimates that approximately 154,270 new cases of CRC will be diagnosed in the United States in 2025 and about 52,900 patients will die of the disease. The lifetime risk of developing CRC is about 1 in 23. The majority of CRC cases (about 75 %) are sporadic whereas the remaining 25 % of the patients have a family history of the disease. Only 5-6 % of patients with CRC with a family history background are due to inherited mutations in major CRC genes, while the rest are the result of accumulation of both genetic mutations and epigenetic modifications of several genes. Colorectal cancer is a heterogeneous disease classified by its genetics, and even though the diagnosis of CRC in the US is dropping among people 65 years and older, the incidence has been rising in the younger age groups, with 12% of CRC cases diagnosed in people under age 50.
The DNA MisMatchRepair (MMR) system is responsible for molecular surveillance and works as an editing tool that identifies errors within the microsatellite regions of DNA and removes them. Approximately 10% to 15% of nonmetastatic CRCs exhibit deficient mismatch repair (dMMR), accounting for an estimated 330,000 cases annually worldwide. Defective MMR system leads to MSI (Micro Satellite Instability) and hypermutation, with the expression of tumor-specific neoantigens at the surface of cancer cells, triggering an enhanced antitumor immune response. These tumors respond poorly to Fluoropyrimidine-based chemotherapy alone, especially in the adjuvant setting. While immune checkpoint inhibitors are approved for dMMR colorectal cancer in the metastatic setting, their benefit in earlier stages, particularly post-resection, had not been previously established in a prospective trial.
Atezolizumab (TECENTRIQ®) is an anti PD-L1 monoclonal antibody, designed to directly bind to PD-L1 expressed on tumor cells and tumor-infiltrating immune cells, thereby blocking its interactions with PD-1 and B7.1 receptors expressed on activated T cells. PD-L1 inhibition may prevent T-cell deactivation and further enable the activation of T cells.
The Phase 3 ATOMIC trial (NCT02912559), sponsored by the National Cancer Institute and conducted across multiple centers including NCTN sites and the German AIO group investigated whether the addition of Atezolizumab, a PD-L1 checkpoint inhibitor, to standard adjuvant chemotherapy could improve Disease-Free Survival (DFS) in patients with resected Stage III dMMR colon adenocarcinoma.
Study Design and Population
The trial enrolled 712 patients with surgically resected Stage III colon cancer confirmed to have dMMR. Eligibility included patients aged 12 years and older (one pediatric patient was enrolled). MMR status was initially determined locally by immunohistochemistry and subsequently confirmed centrally. Participants were randomized 1:1 to receive:
- Control arm: mFOLFOX6 (5-Fluorouracil, Leucovorin, and Oxaliplatin) for 6 months (N=357)
- Experimental arm: mFOLFOX6 plus Atezolizumab (840 mg IV every 2 weeks) for 6 months, followed by maintenance Atezolizumab monotherapy for an additional 6 months (N=355)
Median patient age was 64 yr. 55.1% were female, 84% of tumors were proximal, 46% were clinical low risk (T1-3N1) and 54% were high risk (T4 and/or N2). Stratification was based on nodal status (N1/N1c vs N2), tumor depth (T1-T3 vs T4), and tumor location (proximal vs distal colon). The Primary endpoint was Disease-Free Survival (DFS). Secondary endpoints included Overall Survival (OS) and Adverse Event (AE) profile. At the second interim analysis, median patient follow-up was 37.2 months and 124 DFS events were observed.
Results and Efficacy
After a median follow-up of 37.2 months, the Primary endpoint of DFS was significantly improved in the Atezolizumab arm. The 3-year DFS was 86.4% in the combination arm vs 76.6% in the mFOLFOX6-only arm (Hazard Ratio (HR)=0.50; P< 0.0001, crossing the prespecified efficacy boundary. This represents a 50% relative reduction in the risk of recurrence or death with the addition of Atezolizumab. Importantly, the benefit was consistent across predefined subgroups, including patients over 70 years old and those with both low and high-risk disease (based on T and N-stage). Tumor location, patient sex, and race did not impact the observed treatment benefit.
Safety and Tolerability
Grade 3 or more treatment-related adverse events occurred in 71.7% of patients receiving Atezolizumab plus chemotherapy, compared to 62.1% in those receiving chemotherapy alone. Although the addition of Atezolizumab resulted in a modest increase in toxicity, the side effect profile was consistent with prior experience with checkpoint inhibitors and considered manageable.
Clinical Implications
The ATOMIC trial is the first large, prospective, randomized Phase 3 study to demonstrate a clear benefit from adding immunotherapy to adjuvant chemotherapy in Stage III dMMR colon cancer. As highlighted by the investigators, current adjuvant treatment recommendations for dMMR tumors have historically been extrapolated from studies in mismatch repair–proficient populations or based on retrospective analyses. The robust DFS improvement observed here provides definitive evidence supporting a new treatment paradigm for this molecularly defined subgroup.
Although Overall Survival (OS) data are not yet mature with a median OS follow-up of 42.5 months, early signs are promising. However, future OS analyses may be complicated by the use of subsequent checkpoint inhibitors in patients who recur. The researchers emphasized the clinical relevance of these findings, noting their applicability to both sporadic dMMR cancers and Lynch syndrome associated tumors.
Future Directions
The ATOMIC trial sets a new benchmark for adjuvant therapy in dMMR colon cancer. However, important questions remain. Chief among them is the optimal duration of immunotherapy in this setting. Atezolizumab was administered for nearly a year, including maintenance. Ongoing research should clarify whether such prolonged treatment is necessary or if shorter regimens could maintain efficacy while reducing toxicity.
Moreover, while this study confirms benefit in the postoperative setting, parallel efforts are warranted to evaluate checkpoint inhibition in the neoadjuvant context. Encouraging responses such as those seen in small studies of neoadjuvant immunotherapy in dMMR rectal cancer highlight the need to explore earlier immunotherapeutic intervention in colon cancer as well.
Conclusion
The ATOMIC trial provides compelling evidence that incorporating Atezolizumab into adjuvant therapy improves Disease-free survival in patients with Stage III dMMR colon cancer, marking a major advancement in the management of this biologically distinct subset. Given these results, the combination of Atezolizumab and mFOLFOX6 should be considered the new standard of care in this setting. This trial also exemplifies the power of cooperative group studies in driving progress for biomarker-defined subsets within common malignancies.
Randomized trial of standard chemotherapy alone or combined with atezolizumab as adjuvant therapy for patients with stage III deficient DNA mismatch repair (dMMR) colon cancer (Alliance A021502; ATOMIC). Sinicrope F, Ou F-S, Arnold D, et al. J Clin Oncol 43, 2025 (suppl 17; abstr LBA1)
