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.

An MSK trial of neoadjuvant dostarlumab for MSI-high locally advanced rectal cancer showed a 100% complete clinical response rate. This groundbreaking result allows for non-operative management, sparing patients from chemotherapy, radiation, and life-altering surgery, potentially representing a cure with immunotherapy alone.

Unlike the 100% complete clinical response seen in some rectal cancer trials, studies like NICHE-2 for MSI-high colon cancer show a lower pathologic complete response rate (around 68%). This crucial difference suggests non-operative management is far riskier in colon cancer and requires a distinct clinical approach.

In a study of neoadjuvant Dostarlamab for MSI-high tumors, 100% of rectal cancer patients achieved a clinical complete response, compared to 82% of colon cancer patients. Experts find this high degree of discordance surprising and currently lack a clear biological explanation, as such differences are not typically observed in the metastatic setting.

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.

Retrospective data suggests patients with MSI-high rectal cancer might not just respond poorly to standard neoadjuvant chemoradiation (TNT), but their disease could actually progress. This makes immunotherapy a potentially safer and more effective first-line neoadjuvant choice, not just an alternative.

Data from trials like CheckMate 816 shows that achieving a Pathologic Complete Response (PCR) after neoadjuvant chemo-immunotherapy is a powerful early surrogate endpoint. Patients with PCR demonstrate markedly improved overall and event-free survival.

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 failure of the Checkmate 914 adjuvant trial, which used a six-month duration of nivolumab plus ipilimumab, suggests this shorter treatment window may be inadequate. In contrast to positive trials with one year of therapy, this outcome indicates that treatment duration is a critical variable for achieving a disease-free survival benefit in the adjuvant RCC setting.