Trials like the dostarlimab study in rectal cancer and NICHE in colon cancer show neoadjuvant immunotherapy can induce profound responses in MSI-high tumors. This is creating a new paradigm where major surgery might be avoided entirely for some patients, marking a significant shift in treatment strategy.

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 the STELR-303 trial of zanzalidinib plus atezolizumab in MSS colorectal cancer was technically positive, showing a 1.5-month median overall survival benefit, experts are not enthusiastic. They view the magnitude of benefit as slight and are concerned about increased toxicity, questioning its clinical meaningfulness and potential adoption into practice.

While the STELLAR-303 trial was positive, experts are concerned about the toxicity of the zenzalintinib/atezolizumab combination. The side effect profile and high rate of dose modifications raise questions about the real-world risk-benefit ratio for what is still an incremental, non-curative improvement in survival for refractory metastatic colorectal cancer.

While the ATOMIC trial established FOLFOX plus atezolizumab as a new standard for adjuvant therapy in MSI-high colon cancer, its design lacked an immunotherapy-only arm. This leaves a critical, unanswered question about the actual contribution and necessity of the chemotherapy component.

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.

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.

The STELLAR-303 trial is the first Phase III study to show a significant overall survival benefit for an immunotherapy-based combination (zanzalintinib + atezolizumab) in refractory microsatellite stable (MSS) metastatic colorectal cancer. This validates the IO+TKI approach in a notoriously "cold" tumor type where prior IO trials failed.