TrialLineage Lineage
FDA Approved · 2017Pembrolizumab for MSI-H/dMMR solid tumors
In May 2017, the FDA granted accelerated approval to pembrolizumab (Keytruda) for adult and pediatric patients with unresectable or metastatic solid tumors that are microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) — regardless of tumor type. This was the first tissue-agnostic cancer approval in FDA history. This lineage traces backward from that regulatory milestone through the science that made it possible.
Endpoint
First tissue-agnostic FDA cancer approval
Pembrolizumab was approved for patients whose tumors carry MSI-H or dMMR status, regardless of where in the body the cancer originated. The approval was based on data from five single-arm clinical trials enrolling 149 patients across 15 different tumor types. Overall response rate was 39.6%, with 78% of responses lasting ≥6 months.
This regulatory decision established a new precedent: a cancer drug could be approved based on a molecular biomarker rather than the organ where the tumor arose. The logic was that MSI-H/dMMR tumors — regardless of site — share a biological feature (high mutational burden and neoantigen load) that predicts response to PD-1 blockade.
At a glance
- The endpoint: FDA accelerated approval of pembrolizumab for MSI-H/dMMR solid tumors, May 2017 — first tissue-agnostic cancer approval
- The drug: Pembrolizumab (Keytruda), anti-PD-1 monoclonal antibody
- The biomarker: Microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) status
- The precedent: Drug approved for a molecular feature, not a tumor location — tissue-agnostic regulatory logic
- Why it took decades: PD-1 had to be discovered, mismatch repair had to be linked to immunotherapy response, and the FDA had to accept biomarker-based (not organ-based) approval
Reverse-lineage scaffold
What had to happen first
1
T-cell activation and regulation understood
Immunologists mapped how T cells recognize foreign antigens and how co-stimulatory and co-inhibitory signals regulate their activity. Without this framework, checkpoint inhibitors could not have been conceived. [NEEDS REVIEW: key labs, 1980s–1990s timeline]
2
PD-1 discovered
Tasuku Honjo's lab (Kyoto University) identified PD-1 in 1992 as a receptor involved in programmed cell death. Subsequent work showed PD-1 functions as an immune checkpoint — a brake that tumors exploit to evade immune destruction. [Verified: Ishida et al., EMBO J 1992]
3
PD-L1 identified on tumors
Researchers demonstrated that tumors express PD-L1, which engages PD-1 on T cells to suppress anti-tumor immunity. Blocking this interaction could re-activate T cells against cancer. [NEEDS REVIEW: Dong et al., 2002; Freeman et al., 2000]
4
Anti-PD-1 antibodies developed
Pharmaceutical companies developed monoclonal antibodies against PD-1 (pembrolizumab by Merck, nivolumab by BMS). These entered clinical testing across multiple tumor types. [Verified: pembrolizumab first approvals 2014]
5
DNA mismatch repair linked to cancer
Researchers identified that defects in DNA mismatch repair genes (MLH1, MSH2, MSH6, PMS2) cause microsatellite instability and dramatically increase mutation rate in tumors — particularly colorectal cancer (Lynch syndrome). [NEEDS REVIEW: Fishel et al., 1993; Papadopoulos et al., 1994]
6
MSI-H linked to immunotherapy response
Luis Diaz and Dung Le (Johns Hopkins) observed that colorectal cancers with MSI-H responded dramatically to PD-1 blockade while microsatellite-stable tumors did not. The hypothesis: high mutation burden creates many neoantigens, making these tumors visible to reinvigorated T cells. [Verified: Le et al., NEJM 2015]
7
Basket trials across tumor types
Instead of testing one tumor type at a time, researchers enrolled patients with MSI-H/dMMR tumors from any anatomic site. Response was consistent across 15+ tumor types — supporting a biomarker-based rather than organ-based approval. [Verified: multiple single-arm trials, 149 patients]
8
FDA tissue-agnostic approval
The FDA accepted the logic: if the biomarker (MSI-H/dMMR) predicts response regardless of tumor site, the drug can be approved across all solid tumors with that feature. This was the first time a cancer drug was approved this way. [Verified: FDA accelerated approval, May 2017]
Discovery timeline
[TO BE BUILT]
Full categorized timeline to be developed after source review. Key events to include: T-cell biology foundations, PD-1 discovery (1992), PD-L1 on tumors, mismatch repair gene identification, Lynch syndrome genetics, Le et al. 2015 NEJM, basket trial enrollment, FDA tissue-agnostic precedent.
Known vs. unknown
What the evidence says and what remains open
Established
- MSI-H/dMMR tumors respond to pembrolizumab across multiple tumor types
- The FDA accepted biomarker-based (tissue-agnostic) approval logic
- PD-1 blockade reactivates T cells suppressed by PD-L1-expressing tumors
- High mutation burden in MSI-H tumors generates neoantigens visible to T cells
- Overall response rate ~40% with durable responses in most responders
Not yet known
- Why ~60% of MSI-H patients do not respond [NEEDS REVIEW]
- Whether MSI-H status alone is sufficient or additional biomarkers improve prediction
- Optimal treatment duration
- Whether tissue-agnostic logic will extend to other biomarker-drug pairs
- Long-term outcomes across rare tumor types with limited data
Sources and confidence
Confidence flag
High for endpoint facts (FDA approval, trial data, tissue-agnostic precedent). Moderate for reverse-lineage details (PD-1 discovery timeline verified; mismatch repair history and early T-cell biology sections need source confirmation for specific labs and dates).
Source links
- FDA accelerated approval announcement, May 23, 2017
- Le et al., NEJM 2015 — PD-1 blockade in mismatch repair-deficient tumors
- Le et al., Science 2017 — MSI-H across tumor types
- Ishida et al., EMBO J 1992 — PD-1 discovery
- [TODO: Fishel et al., Cell 1993 — MSH2 cloning]
- [TODO: Freeman et al., J Exp Med 2000 — PD-L1 identification]