Clinical complete response to serplulimab plus bevacizumab and chemotherapy in MSS/KRAS-mutant metastatic sigmoid colon adenocarcinoma: a case report and literature review

Mismatched-repair-proficient/microsatellite-stable (pMMR/MSS) metastatic colorectal cancer (mCRC) is typically refractory to immune checkpoint inhibitors. Early randomized data suggest that adding PD-1 blockade to anti-angiogenic therapy and oxaliplatin-based chemotherapy may improve outcomes, but real-world evidence remains limited. A 62-year-old woman presented with advanced sigmoid colon adenocarcinoma and liver, lung, and nodal metastases (cT4aN2bM1b). Molecular profiling showed KRAS p.G12D and pMMR/MSS status. First-line treatment with serplulimab, bevacizumab, and XELOX was initiated. After two cycles, carcinoembryonic antigen (CEA) declined from 3458.2 ng/mL to 1812.84 ng/mL. Over eight induction cycles, cross-sectional imaging demonstrated sustained regression of hepatic and pulmonary metastases without new lesions, representing a near-complete response. Following six cycles of maintenance therapy with serplulimab, bevacizumab, and capecitabine, restaging CT and endoscopy showed no visible tumor, consistent with a clinical complete response (cCR), accompanied by resolution of abdominal pain. The patient remains progression-free beyond 12 months at last follow-up. This case illustrates the feasibility and potential of combining PD-1 blockade, anti-VEGF therapy, and oxaliplatin-based chemotherapy for pMMR/MSS mCRC, aligning with efficacy signals from ASTRUM-015 (phase 2 median PFS 17.2 vs 10.7 months; HR 0.60; MSS subgroup 17.2 vs 10.1 months; HR 0.58). Prospective, biomarker-informed trials are warranted to validate patient selection and durability of benefit.