Following positive Phase III data for adjuvant atezolizumab plus chemotherapy in Stage III MSI-high colon cancer, clinicians are extrapolating this approach to high-risk Stage II patients. For some, they favor using immunotherapy alone, omitting chemotherapy due to its perceived limited additional benefit in the Stage II setting.

The STELLAR-303 trial is the first to show an immunotherapy-based regimen provides an overall survival benefit in microsatellite-stable CRC. Crucially, this benefit extends to patients with liver metastases, a subgroup that has historically shown profound resistance to immunotherapy, highlighting the drug's novel mechanism.

While immunotherapy is largely ineffective in metastatic microsatellite stable (MSS) colorectal cancer, emerging data suggests it may have surprising efficacy in the early-stage (neoadjuvant) setting. This differential response is likely due to a more favorable tumor microenvironment in earlier disease, suggesting a new therapeutic window.

In late-stage metastatic colorectal cancer, the goal shifts from achieving significant tumor shrinkage to stabilizing the disease. This recalibration of 'success' focuses on maintaining quality of life and managing symptoms for patients who have undergone multiple prior therapies.

The COMET study found combining chemotherapy with atezolizumab did not improve overall survival versus atezolizumab alone. However, it nearly eliminated early progressive disease (2.8% vs. 32.4%), suggesting a critical role for patients with high tumor burden who cannot risk initial progression on monotherapy.

Despite the ATOMIC trial (adjuvant FOLFOX + atezolizumab) being practice-changing and included in NCCN guidelines for stage 3 MSI-high colon cancer, experts from major academic centers would not use it. They cite the high toxicity of chemotherapy and superior data from neoadjuvant immunotherapy trials like NICHE2, which achieve excellent outcomes without any chemotherapy.

A meta-analysis of over 3,000 patients shows that neoadjuvant chemotherapy for MSS colon cancer provides a 5% improvement in survival. This benefit is clinically meaningful and equivalent in magnitude to the landmark addition of oxaliplatin to 5-FU in the original MOSAIC trial.

The LEAP-010 trial showed a combination therapy improved tumor response and progression-free survival but failed to improve overall survival, the ultimate measure of benefit. This highlights the risk of relying on surrogate endpoints, which can be misleading, especially when a treatment adds significant toxicity.

The Skyscraper 07 trial failed its primary endpoint for a TIGIT/PD-L1 inhibitor combo in esophageal cancer. However, a secondary analysis of the Atezolizumab-only arm revealed significant survival benefits. This unexpected positive signal from a technically "negative" study may lead to a new standard of care, pending regulatory interpretation.