First‑Line Immunotherapy for dMMR/MSI‑H Metastatic Colorectal Cancer

Evidence Supporting Pembrolizumab as Standard First‑Line Therapy

The National Comprehensive Cancer Network (NCCN) recommends pembrolizumab monotherapy as the preferred first‑line treatment for treatment‑naïve stage IV dMMR/MSI‑H colorectal cancer, based on the phase III KEYNOTE‑177 trial and current guideline updates. Level 1 evidence. [1][2]3
Pembrolizumab monotherapy provides superior progression‑free survival (PFS) and overall response rates compared with chemotherapy ± targeted agents in dMMR/MSI‑H metastatic colorectal cancer. Level 1 evidence. [1][2]3
In KEYNOTE‑177, the chemotherapy control arm (fluoropyrimidine‑based doublet ± bevacizumab or anti‑EGFR) achieved only a 33.1 % response rate and a median PFS of 8.2 months, whereas pembrolizumab markedly outperformed these outcomes. Level 1 evidence. 4

First‑line chemotherapy (with or without monoclonal antibodies) yields a response rate of 5 % in dMMR patients versus 44 % in pMMR patients, and overall survival of 16.0 months versus 23.6 months respectively. Level 2 evidence. 4
In the CAIRO and FOCUS trials, dMMR patients receiving irinotecan‑containing doublet therapy had comparable PFS to 5‑FU monotherapy (4.0 vs 4.2 months), whereas pMMR patients derived clear benefit from the doublet (8.3 vs 5.8 months). Level 2 evidence. [5][4]
The CALGB bevacizumab trial demonstrated that dMMR patients treated with cetuximab experienced a markedly inferior overall survival (11.9 months) compared with pMMR patients (30.7 months, p = 0.0014), indicating poor efficacy of anti‑EGFR therapy in this subgroup. Level 2 evidence. 5

Recommended pembrolizumab regimen: 200 mg IV every 3 weeks or 400 mg IV every 6 weeks as monotherapy for dMMR/MSI‑H metastatic colorectal cancer. Level 1 evidence. [1][2]3

Nivolumab monotherapy or the combination of nivolumab + ipilimumab are viable alternatives, but pembrolizumab possesses the strongest evidence base from KEYNOTE‑177. Level 1–2 evidence. [1][6]
The CheckMate‑649 trial showed that MSI‑H patients receiving nivolumab + ipilimumab achieved a median overall survival not reached versus 10 months with chemotherapy (hazard ratio 0.28), supporting dual immunotherapy as an effective alternative. Level 1 evidence. 7

After attaining response or stable disease with pembrolizumab, clinicians should consider local ablative therapy (surgical resection or stereotactic radiotherapy) to the solitary para‑aortic node when technically feasible, to maximize long‑term control. Level 2 evidence. 1
Restaging imaging (CT or MRI) is advised every 6–9 weeks during the initial treatment phase to evaluate response and to time local‑therapy interventions appropriately. Level 2 evidence. 1

NCCN and ASCO guidelines advise against initiating chemotherapy ± bevacizumab or anti‑EGFR antibodies as first‑line therapy in confirmed dMMR/MSI‑H disease, due to lack of survival benefit, unnecessary toxicity, and delay of effective immunotherapy. Level 1 evidence. [1][2][3][4]
Chemotherapy should be reserved for patients who progress on immunotherapy or for rare cases where immunotherapy is contraindicated. Level 2 evidence. 1

FOLFOX (leucovorin, 5‑fluorouracil, oxaliplatin) or FOLFIRI (leucovorin, 5‑fluorouracil, irinotecan) doublet chemotherapy are recommended second‑line regimens following pembrolizumab progression in dMMR/MSI‑H disease. Level 2 evidence. [1][2]
If not previously employed, nivolumab + ipilimumab remains an option, leveraging the CheckMate‑649 survival advantage (HR 0.28 versus chemotherapy). Level 1 evidence. [7][6]
Adding bevacizumab to chemotherapy doublets can be considered in the second‑line setting, although specific benefit data for dMMR tumors are limited. Level 2 evidence. [1][5]

Median PFS with pembrolizumab in dMMR/MSI‑H metastatic colorectal cancer exceeds 16 months, substantially longer than the 8.2 months observed with chemotherapy in the KEYNOTE‑177 control arm. Level 1 evidence. 4