Efficacy of pembrolizumab in microsatellite-stable, tumor mutational burden-high metastatic colorectal cancer: genomic signatures and clinical outcomes

Background: Pembrolizumab, an immune checkpoint inhibitor (ICI), shows significant survival benefits in patients with microsatellite instability-high (MSI-H) metastatic colorectal cancer (mCRC), but its efficacy in microsatellite-stable (MSS) mCRC is limited. Although ICIs are effective in tumor mutational burden-high (TMB-H) solid tumors, the impact on MSS-TMB-H mCRC, a rare subset within MSS mCRC, remains unclear.

Materials and methods: We conducted a retrospective analysis using clinical and genomic data from the Center for Cancer Genomics and Advanced Therapeutics (C-CAT) repository in Japan. Patients with MSS-TMB-H mCRC who underwent tissue-based comprehensive genomic profiling and were treated with pembrolizumab or other later-line therapies were included. Pembrolizumab's efficacy was compared with that of trifluridine/tipiracil (FTD/TPI) and regorafenib. Genomic profiles of MSS-TMB-H, MSI-H-TMB-H, and MSS-TMB-low (TMB-L) CRCs were analyzed across 71 cancer-related genes.

Results: Among 127 TMB-H mCRC cases treated with pembrolizumab in the C-CAT repository, 77 were MSS and 50 were MSI-H. Pembrolizumab showed significantly shorter time to treatment failure (TTF) and overall survival (OS) in patients with MSS-TMB-H mCRC compared with those with MSI-H-TMB-H mCRC [median TTF 2.0 versus 10.6 months; hazard ratio (HR) 4.79, 95% confidence interval (CI) 2.65-8.64, median OS 4.5 versus 33.6 months; HR 9.86, 95% CI 3.93-24.77, both P < 0.0001]. Among MSS-TMB-H mCRC patients, 19 received pembrolizumab, 73 received FTD/TPI (±bevacizumab), and 18 received regorafenib as their first later-line therapy. Pembrolizumab showed significantly shorter TTF and OS compared with FTD/TPI (median TTF 1.6 versus 4.1 months; HR 2.66, 95% CI 1.41-5.02, P = 0.0017, median OS 5.4 versus 13.8 months; HR 2.42, 95% CI, 1.09-5.38, P = 0.025). Genomic analysis of 6737 CRCs revealed that MSS-TMB-H CRCs harbored fewer pathogenic alterations than MSI-H-TMB-H CRCs but had a profile similar to MSS-TMB-L CRCs.

Conclusions: Pembrolizumab may be less effective than FTD/TPI in later-line treatment of MSS-TMB-H mCRC, potentially due to genomic similarities between MSS-TMB-H and MSS-TMB-L CRC, suggesting the need for alternative therapeutic strategies in this subgroup.