Pembrolizumab's Approval for MSI-High Tumors
Pembrolizumab (Keytruda), a PD-1 inhibitor, is FDA-approved for unresectable or metastatic MSI-H/dMMR solid tumors in adults and children after prior treatment, based on pooled data from five KEYNOTE trials showing an objective response rate (ORR) of 39.6% (95% CI: 33-46.5), with 12% complete responses.[1][2] Approval came in 2017 as the first tumor-agnostic therapy, covering cancers like colorectal, endometrial, and gastric with microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) status.[1]
How Pembrolizumab Works in MSI-High Tumors
MSI-H tumors have high mutation burdens from defective DNA mismatch repair, producing neoantigens that trigger immune responses. Pembrolizumab blocks PD-1 to unleash T-cells against these tumors, explaining higher efficacy here than in MSS tumors.[3] In KEYNOTE-177, first-line MSI-H metastatic colorectal cancer patients had median progression-free survival (PFS) of 16.5 months versus 8.2 months with chemotherapy.[4]
Key Clinical Trial Results
- KEYNOTE-158 (various solid tumors): ORR 34% (95% CI: 27.5-40.9); median duration of response (DOR) not reached at 14 months.[2]
- KEYNOTE-177 (colorectal): Superior PFS (HR 0.60); 43.8% ORR vs. 33.1% for chemo.[4]
- KEYNOTE-164 (colorectal post-chemo): ORR 33%; median DOR 35.3 months.[5]
Responses persist in 70-80% of cases beyond 2 years, outperforming many chemotherapies.[2][4]
Which MSI-High Cancers Show Strongest Response?
Endometrial (48% ORR), gastric/small intestinal (46-52%), and colorectal (33-44%) respond best; biliary and others around 25-30%.[2] Not all MSI-H tumors respond equally—PD-L1 status and tumor burden influence outcomes.[3]
Limitations and Resistance Risks
About 60% of patients don't respond initially; acquired resistance occurs in 20-30% via PD-L1 upregulation or beta-catenin mutations.[6] No overall survival benefit in some arms (e.g., KEYNOTE-177 immature data).[4] Hyperprogression affects <5%.[3]
Compared to Other Immunotherapies
Pembrolizumab outperforms nivolumab (Opdivo) in MSI-H colorectal (ORR 31% vs. 55% in some head-to-heads, but similar DOR).[7] Dual PD-1/CTLA-4 (nivo+ipi) yields 60% ORR in refractory cases but with higher toxicity.[8] Use alone first unless progressing.[1]
Testing and Patient Selection
Confirm MSI-H via IHC (dMMR) or PCR/NGS; FDA recommends both.[1] Best for PD-L1+ or high tumor mutational burden subsets.[3]
[1]: FDA Label for Keytruda
[2]: KEYNOTE-158 NEJM 2019
[3]: MSI-H Review, Nature Reviews 2021
[4]: KEYNOTE-177 NEJM 2020
[5]: KEYNOTE-164 Lancet Oncology 2020
[6]: Resistance Mechanisms, Cancer Discovery 2019
[7]: Nivolumab in MSI-H, JCO 2018
[8]: Nivo+Ipi CheckMate-142, JCO 2021