Efficacy of pembrolizumab in microsatellite-stable, tumor mutational burden-high metastatic colorectal cancer: genomic signatures and clinical outcomes
Toshihiro Yamaguchi, Kenji Tsuchihashi, Sayaka Ueno, K. Uehara, Ryo Taguchi, Mamoru Ito, Taichi Isobe, Takashi Imajima, Takanari Kitazono, Kenro Tanoue, Hirofumi Ohmura, Koichi Akashi, Eishi Baba
Abstract
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.