Liver metastases and peritoneal metastases and response to checkpoint inhibitors in metastatic colorectal cancer with microsatellite instability

Background: There have been conflicting reports on the predictive impact of metastatic disease sites on the response to checkpoint inhibitors (CPI) in microsatellite instability (MSI) metastatic colorectal cancers (mCRC). Recent studies have highlighted peritoneal metastases, ascites, and liver metastases as possible indicators of resistance to CPI. Methods: We performed a detailed analysis of high microsatellite instability (MSI-H) mCRC treated with programmed cell death (PD-1) or PD-1/cytotoxic T-lymphocyte-associated protein 4 CPI in a single center. Overall response rate (ORR), progression-free survival (PFS), overall survival (OS), and stable disease but with complete pathological response upon resection (SDcPR) were analyzed by the presence of liver metastases, peritoneal metastases, or absence of either. The impact of number and size of liver metastases on clinical outcomes were also interrogated. Results: Thirty-five patients with MSI mCRC were included in the analysis. Patients with peritoneal metastatic disease had lower ORR and shorter PFS compared to patients without liver and peritoneal metastases. Contrary to recent reports, ORR and ORR + SDcPR rates were high in patients with liver metastases, at 58% and 66%, respectively. In the liver metastases category, a better response rate was noted for patients with<5 lesions compared to patients with more than 5 lesions. Patients who responded had a higher median tumor mutation burden than patients with progressive disease. Conclusions: In MSI mCRC, no single clinical characteristic was sufficient to preclude CPI response. Peritoneal metastatic disease was associated with numerically lower ORR and shorter PFS. In contrast, liver metastases do not predict poor outcome.