Pembrolizumab vs Nivolumab/Ipilimumab vs Dostarlimab for MSI-H/dMMR Colorectal Cancer

Updated September 2026

For patients with microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) colorectal cancer, immunotherapy has fundamentally changed treatment. But three important checkpoint strategies are not interchangeable: dostarlimab, pembrolizumab, and nivolumab plus ipilimumab.

The most useful way to compare them is not simply to ask which drug is "strongest." The clinically relevant question is: Which immunotherapy strategy has the best evidence for this disease stage, tumor location, treatment line, and biomarker context?

Bottom line: Pembrolizumab remains the most mature single-agent evidence base for first-line metastatic MSI-H/dMMR colorectal cancer. Nivolumab plus ipilimumab now has randomized phase III evidence supporting an intensified first-line strategy in metastatic disease and exceptionally strong phase II neoadjuvant data in locally advanced dMMR colon cancer. Dostarlimab has become particularly important in dMMR locally advanced rectal cancer because of the remarkable organ-preservation results seen with neoadjuvant treatment.

Table of Contents

• 1. Why MSI-H and dMMR Matter
• 2. The Three Immunotherapy Strategies
• 3. Localized Rectal Cancer
• 4. Localized Colon Cancer
• 5. Metastatic MSI-H/dMMR Colorectal Cancer: First-Line
• 6. Head-to-Head Clinical Comparison
• 7. What Happens After Immunotherapy Resistance?
• 8. Why MSI-H/dMMR Tumors Respond — and Why Some Resist
• 9. Toxicity and Treatment Intensity
• 10. Practical Treatment-Selection Framework
• 11. Common Misconceptions
• 12. Frequently Asked Questions
• 13. Key Evidence and References

1. Why MSI-H and dMMR Matter in Colorectal Cancer

MSI-H and dMMR identify a biologically distinct subgroup of colorectal cancers. The mismatch-repair system normally corrects certain DNA replication errors. When this machinery is defective, mutations accumulate and microsatellite regions become unstable.

The resulting tumors can contain a large number of abnormal peptides, or neoantigens, that are recognizable to the immune system. This helps explain why immune checkpoint blockade can be dramatically more effective in MSI-H/dMMR colorectal cancer than in the much larger population of microsatellite-stable (MSS) or mismatch-repair-proficient (pMMR) tumors.

MSI-H is not the same thing as "all colorectal cancer." An immunotherapy strategy that is highly effective in MSI-H/dMMR disease should not automatically be generalized to MSS/pMMR colorectal cancer.

MSI testing versus MMR testing

MSI status is generally assessed using molecular approaches such as PCR or next-generation sequencing, whereas MMR deficiency is usually assessed by immunohistochemistry for MMR proteins such as MLH1, PMS2, MSH2 and MSH6.

Although MSI-H and dMMR are closely related and often overlap, they are not literally the same test. For treatment selection, the pathology report and the method used should be reviewed carefully.

2. The Three Immunotherapy Strategies

Strategy	Primary checkpoint target	Key colorectal evidence	Most important current role
Pembrolizumab	PD-1	KEYNOTE-177; multiple MSI-H/dMMR studies	Established first-line option for unresectable/metastatic MSI-H/dMMR CRC
Dostarlimab	PD-1	Neoadjuvant dMMR rectal-cancer studies; GARNET tumor-agnostic data	Particularly important for dMMR rectal cancer and organ-preservation strategies
Nivolumab + ipilimumab	PD-1 + CTLA-4	CheckMate 8HW; NICHE-2 and related neoadjuvant studies	Intensified immunotherapy for MSI-H/dMMR metastatic disease and a major neoadjuvant strategy under active development

The key biological distinction is that pembrolizumab and dostarlimab are PD-1 monotherapies, whereas nivolumab plus ipilimumab combines PD-1 blockade with CTLA-4 blockade.

That dual blockade may broaden or intensify T-cell activation, but increased immune activation also creates greater potential for immune-related toxicity.

3. Localized Rectal Cancer: Dostarlimab Has Become the Defining Story

The most striking clinical development involving dostarlimab has occurred in locally advanced dMMR rectal adenocarcinoma.

The original 12-patient study

In the landmark phase 2 study published in the New England Journal of Medicine, 12 patients who completed six months of dostarlimab achieved a 100% clinical complete response rate. No patient had required chemoradiotherapy or surgery at the reported follow-up.

The result attracted enormous attention because standard management of locally advanced rectal cancer has traditionally involved some combination of chemotherapy, radiation and surgery, depending on stage, tumor location and treatment strategy.

However, the initial result came from a very small prospective cohort and therefore could not by itself establish dostarlimab as a universal replacement for surgery or chemoradiation.

The larger 2025 nonoperative-management study

The evidence subsequently expanded substantially. In the 2025 New England Journal of Medicine phase 2 study of neoadjuvant dostarlimab for dMMR solid tumors, 49 patients with locally advanced dMMR rectal cancer who completed treatment all achieved a clinical complete response and elected nonoperative management.

Among these patients, 37 had a sustained clinical complete response at 12 months, meeting the prespecified criterion for efficacy. Across the overall study population, the investigators reported a 2-year recurrence-free survival of 92%.

Evidence interpretation: The rectal-cancer dostarlimab data are no longer based only on the original 12-patient report. The 2025 prospective study substantially strengthened the evidence for nonoperative management of carefully selected dMMR rectal-cancer patients who achieve a rigorous clinical complete response.

But watch-and-wait is not simply "skip surgery"

A complete clinical response must be established through appropriate multidisciplinary assessment. The concept of organ preservation depends on intensive surveillance using clinical examination, endoscopy, imaging and other disease-appropriate methods.

A patient with residual tumor, suspicious lymph nodes, metastatic disease, incomplete response or subsequent regrowth may still require surgery or other cancer-directed treatment.

How does pembrolizumab compare in localized rectal cancer?

Pembrolizumab is highly effective in MSI-H/dMMR metastatic colorectal cancer, but the specific nonoperative rectal-cancer evidence base that has made dostarlimab famous is centered on dostarlimab-based prospective studies.

This is an important example of why drugs that act on the same checkpoint cannot simply be assumed to have identical evidence in every clinical setting.

How does nivolumab/ipilimumab compare?

Nivolumab plus ipilimumab is a highly active strategy in MSI-H/dMMR colorectal cancer and has generated important neoadjuvant data, but the most mature disease-specific organ-preservation data for dMMR rectal cancer have been generated with dostarlimab.

Localized dMMR rectal cancer	Evidence position	Key point
Dostarlimab	Very strong emerging organ-preservation evidence	All 49 patients completing therapy in the 2025 study achieved cCR and elected nonoperative management.
Pembrolizumab	Strong biological rationale; less defining rectal-specific organ-preservation evidence	Excellent metastatic evidence does not automatically equal equivalent neoadjuvant rectal evidence.
Nivolumab + ipilimumab	Strong immunologic and colorectal evidence; active neoadjuvant strategy	Dual checkpoint blockade is highly active, but organ-preservation evidence is not the same as the dostarlimab rectal dataset.

4. Localized Colon Cancer: Nivolumab + Ipilimumab Has Particularly Strong Neoadjuvant Data

The localized colon-cancer evidence tells a different story. One of the most important studies is NICHE-2, a phase 2 study of neoadjuvant nivolumab plus ipilimumab in patients with locally advanced dMMR colon cancer.

Among 111 patients evaluated for efficacy, 98% had a pathological response, 95% had a major pathological response, and 68% achieved a pathological complete response. At a median follow-up of 26 months, no disease recurrences had been reported in the study population.

Why this matters: In localized dMMR colon cancer, nivolumab plus ipilimumab currently has some of the most striking prospective neoadjuvant pathological-response data among checkpoint strategies. However, these phase 2 results should not be confused with a universal mandate to replace surgery or standard perioperative treatment outside appropriate specialist protocols and trials.

Where does dostarlimab fit in localized colon cancer?

The 2025 dostarlimab study included a nonrectal dMMR solid-tumor cohort containing several tumor types. Among 54 patients completing therapy, 35 achieved a clinical complete response and 33 chose nonoperative management.

That finding is important evidence that neoadjuvant PD-1 blockade can eliminate visible disease in some early-stage dMMR tumors beyond rectal cancer. But the nonrectal cohort was not a colon-cancer-only randomized comparison, so the result should not be interpreted as proof that dostarlimab is superior to nivolumab plus ipilimumab for localized colon cancer.

Where does pembrolizumab fit?

Pembrolizumab has one of the strongest clinical evidence bases for advanced MSI-H/dMMR colorectal cancer, but the evidence defining its role in localized colon cancer is not directly interchangeable with KEYNOTE-177, which studied unresectable or metastatic disease.

Localized dMMR colon cancer	Current interpretation
Nivolumab + ipilimumab	Exceptionally strong phase 2 neoadjuvant response data from NICHE-2, including 68% pathological complete response.
Dostarlimab	Promising tumor-agnostic neoadjuvant evidence, but less colon-specific comparative evidence.
Pembrolizumab	Highly established in metastatic disease, but its strongest evidence is not from a localized colon-cancer randomized trial.

5. Metastatic MSI-H/dMMR Colorectal Cancer: First-Line Treatment

Metastatic disease is where the evidence becomes considerably more mature. Two major randomized programs now shape the field: KEYNOTE-177 for pembrolizumab and CheckMate 8HW for nivolumab plus ipilimumab.

Pembrolizumab: KEYNOTE-177

KEYNOTE-177 randomized 307 previously untreated patients with unresectable or metastatic MSI-H/dMMR colorectal cancer to pembrolizumab or chemotherapy.

The initial analysis showed a median progression-free survival of 16.5 months with pembrolizumab versus 8.2 months with chemotherapy, with a hazard ratio of approximately 0.60.

Longer follow-up has strengthened the case for pembrolizumab. The 5-year analysis reported a median overall survival of 77.5 months with pembrolizumab versus 36.7 months with chemotherapy, although interpretation of overall survival requires recognition that patients assigned to chemotherapy could receive pembrolizumab after progression.

The 5-year overall survival rate was approximately 54.8% with pembrolizumab versus 44.2% with chemotherapy.

Evidence tier: Pembrolizumab has a mature randomized phase III evidence base and remains one of the clearest first-line single-agent choices for metastatic MSI-H/dMMR colorectal cancer.

Nivolumab + ipilimumab: CheckMate 8HW

CheckMate 8HW changed the metastatic immunotherapy landscape by directly testing dual checkpoint blockade. The trial enrolled immunotherapy-naive MSI-H/dMMR metastatic colorectal cancer patients and compared nivolumab plus ipilimumab with chemotherapy in the first-line setting.

At the prespecified interim analysis, the 24-month progression-free survival rate was 72% with nivolumab plus ipilimumab versus 14% with chemotherapy. Grade 3 or 4 treatment-related adverse events occurred in approximately 23% versus 48%, respectively.

The subsequent analysis comparing nivolumab plus ipilimumab with nivolumab alone showed a progression-free survival hazard ratio of 0.62, with median PFS not reached for the combination versus 39.3 months with nivolumab alone in the reported analysis. Grade 3-4 treatment-related adverse events were more frequent with the combination.

Clinical meaning: Nivolumab plus ipilimumab is no longer merely an interesting salvage regimen. It has become a major first-line immunotherapy strategy for immunotherapy-naive MSI-H/dMMR metastatic colorectal cancer.

What about dostarlimab in metastatic CRC?

Dostarlimab is also biologically relevant because it is a PD-1 inhibitor and has tumor-agnostic activity in dMMR solid tumors.

However, its most distinctive colorectal-cancer clinical story remains the dMMR rectal-cancer neoadjuvant program rather than a randomized first-line metastatic CRC trial comparable with KEYNOTE-177 or CheckMate 8HW.

In the United States, dostarlimab also has an FDA accelerated tumor-agnostic indication for adults with dMMR recurrent or advanced solid tumors that have progressed on or following prior treatment and have no satisfactory alternative treatment options. Regulatory indications can differ by country.

6. Dostarlimab vs Pembrolizumab vs Nivolumab/Ipilimumab

Clinical setting	Dostarlimab	Pembrolizumab	Nivolumab + ipilimumab
Localized dMMR rectal cancer	Leading evidence for organ preservation	Strong PD-1 rationale, but less defining rectal organ-preservation evidence	Highly active, but not the same dostarlimab-specific watch-and-wait evidence base
Localized dMMR colon cancer	Promising tumor-agnostic neoadjuvant data	Strong metastatic evidence; localized role remains less mature	Exceptional neoadjuvant phase 2 data
Metastatic first-line	Less mature CRC-specific randomized evidence	Established first-line PD-1 monotherapy	Major first-line dual-checkpoint option
After prior chemotherapy	Potential tumor-agnostic dMMR option depending on regulatory jurisdiction/label	Established settings for MSI-H/dMMR disease	Established in specified previously treated MSI-H/dMMR metastatic settings
After progression on prior PD-1 therapy	Not automatically expected to overcome class resistance	Not generally a rational "switch" simply because it is another PD-1 antibody	Potentially interesting, but evidence after prior PD-1 therapy is limited and clinical-trial strategies are important
Therapeutic intensity	PD-1 monotherapy	PD-1 monotherapy	PD-1 + CTLA-4 dual checkpoint blockade
Immune-related toxicity	Usually lower than dual checkpoint strategies	Usually lower than dual checkpoint strategies	Generally higher because of combined checkpoint blockade

7. What Happens After Immunotherapy Resistance?

This is one of the most important and least understood questions in MSI-H/dMMR colorectal cancer. A patient can have a dramatic response to a PD-1 inhibitor and later experience disease progression. That phenomenon is generally described as acquired resistance.

Do not assume that another PD-1 antibody will reverse resistance. Dostarlimab and pembrolizumab both target PD-1. Switching from one PD-1 antibody to another does not automatically change the biological resistance mechanism.

Why PD-1-to-PD-1 switching is problematic

Pembrolizumab and dostarlimab bind different molecular epitopes, but both primarily interrupt the PD-1 immune checkpoint pathway. If a tumor has developed a resistance mechanism downstream of PD-1 blockade, simply changing the antibody may not solve the problem.

Potential resistance mechanisms include defects in antigen presentation, alterations in interferon signaling, changes in tumor immune infiltration, oncogenic signaling that excludes T cells, and evolution of the tumor under treatment pressure.

Could CTLA-4 blockade help?

This is biologically more interesting than simply switching PD-1 antibodies because ipilimumab targets CTLA-4, a different immune checkpoint.

Nivolumab plus ipilimumab has demonstrated greater efficacy than nivolumab alone in an immunotherapy-naive metastatic MSI-H/dMMR population in CheckMate 8HW.

However, an essential limitation must be emphasized: this does not prove that nivolumab plus ipilimumab will rescue a patient whose cancer has already progressed on pembrolizumab or another PD-1 inhibitor.

The CheckMate 8HW evidence establishing the combination's major efficacy included immunotherapy-naive populations. Treatment after acquired PD-1 resistance is a different biological and clinical question.

What should be considered after confirmed progression?

The first step is generally to confirm that the apparent progression represents true cancer progression rather than pseudoprogression, measurement variability or another clinical explanation.

A multidisciplinary team may then reassess:

• Repeat imaging and the pattern of progression.
• Whether a biopsy is useful or feasible.
• Current MSI/MMR status and whether additional molecular testing is warranted.
• RAS, BRAF, HER2 and other actionable molecular alterations when clinically appropriate.
• Potential resectability or local treatment of oligometastatic disease.
• Standard chemotherapy or targeted therapy appropriate to the molecular profile and previous exposure.
• Clinical trials testing combinations designed to overcome checkpoint resistance.

Resistance is increasingly viewed as a biological problem

Research in dMMR/MSI-H CRC has identified several potential escape routes. These include loss of tumor-antigen recognition, impaired antigen presentation, JAK/STAT or interferon-pathway alterations, changes in the tumor microenvironment, T-cell exhaustion, and activation of alternative immune checkpoints.

This is why the future of treatment after immunotherapy may be less about "which PD-1 drug should we try next?" and more about: which resistance mechanism has emerged, and what therapy can attack it?

8. Why MSI-H/dMMR Tumors Respond — and Why Some Resist

High mutation burden and neoantigens

Mismatch-repair deficiency can create numerous mutations during tumor development. Some generate abnormal peptides that can be recognized by immune cells. This contributes to the immunogenic character of MSI-H/dMMR tumors.

PD-1 pathway activation

Tumor-specific T cells may become functionally suppressed through checkpoint signaling. PD-1 inhibitors such as pembrolizumab and dostarlimab attempt to release this brake.

CTLA-4 as an additional checkpoint

CTLA-4 operates at a different stage of immune regulation. Combining CTLA-4 inhibition with PD-1 blockade therefore creates a broader form of checkpoint activation than either drug alone.

But high MSI does not guarantee response

A major clinical misconception is: MSI-H means immunotherapy will always work.

In reality, some MSI-H/dMMR tumors show primary resistance, while others initially respond and later acquire resistance. Current research points to multiple mechanisms, including defects in antigen presentation, interferon signaling, immune exclusion and changes within the tumor microenvironment.

9. Toxicity and Treatment Intensity

The choice between single-agent PD-1 blockade and dual checkpoint blockade is not purely an efficacy question. Toxicity matters.

PD-1 monotherapy

Pembrolizumab and dostarlimab can cause immune-mediated adverse events affecting organs including the skin, gastrointestinal tract, liver, lungs, kidneys and endocrine system. Although many immune-related adverse events are manageable, some can be serious or permanent.

Dual checkpoint blockade

Adding ipilimumab increases immune activation and can also increase the frequency or severity of immune-related adverse events. This trade-off must be considered against the potential benefit of deeper or more durable tumor control.

Consideration	PD-1 monotherapy	PD-1 + CTLA-4
Immune activation	Lower intensity	Higher intensity
Potential efficacy	Very high in MSI-H/dMMR disease	Potentially greater in selected patients
Immune toxicity	Generally lower	Generally higher
Monitoring burden	Important	Especially important
Suitability for frail patients	May be easier to tolerate, depending on circumstances	Requires careful assessment of fitness and immune-toxicity risk

10. Practical Treatment-Selection Framework

A useful framework is to start with the tumor's anatomic location and stage, then layer in the biomarker and prior treatment history.

Scenario A: Locally advanced dMMR rectal cancer

Dostarlimab becomes particularly important. The prospective evidence for clinical complete response and nonoperative management is unusually strong. The treatment decision should be made through a multidisciplinary colorectal cancer program experienced in response assessment and watch-and-wait surveillance.

Scenario B: Locally advanced dMMR colon cancer

Nivolumab plus ipilimumab is especially noteworthy. NICHE-2 demonstrated exceptionally high pathological response rates. Nevertheless, the final treatment pathway should take account of surgical anatomy, obstruction risk, stage, patient fitness and the evolving status of neoadjuvant immunotherapy protocols.

Scenario C: Newly diagnosed metastatic MSI-H/dMMR CRC

Two major evidence-based strategies now dominate the discussion:

• Pembrolizumab monotherapy, supported by mature KEYNOTE-177 phase III evidence.
• Nivolumab plus ipilimumab, supported by randomized CheckMate 8HW evidence for intensified checkpoint blockade.

The optimal choice is not necessarily the same for every patient. Factors include disease burden, symptoms, metastatic pattern, pace of disease, performance status, medical comorbidities, prior immune toxicity, treatment goals and the patient's ability to tolerate additional immune-related adverse events.

Scenario D: Progression after pembrolizumab

Do not assume that dostarlimab is automatically the next step. Both are PD-1 inhibitors. The key question is whether the tumor is demonstrating true acquired resistance and what biological mechanism may be responsible.

A clinical trial can be particularly important in this setting. Depending on the individual case, standard chemotherapy, molecularly targeted therapy or a different immunotherapy strategy may also be considered.

Scenario E: Progression after nivolumab/ipilimumab

A second checkpoint inhibitor is not automatically expected to work. The treatment team should reassess the molecular landscape and previous treatment exposures and look for clinical trials that specifically address immunotherapy resistance.

11. Common Misconceptions

Myth 1: Dostarlimab is better than pembrolizumab because 100% of rectal patients responded.

Not established. The famous 100% response figure came from a highly selected dMMR rectal-cancer population. It cannot be directly compared with KEYNOTE-177 because the diseases, settings, trial designs and endpoints differ. The larger 2025 dostarlimab rectal study is much stronger evidence for this specific setting, but it still does not constitute a head-to-head pembrolizumab comparison.

Myth 2: Nivolumab/ipilimumab is simply a stronger version of pembrolizumab.

Not exactly. The combination adds CTLA-4 inhibition to PD-1 inhibition and produces a different immunologic treatment profile, including a higher immune-toxicity burden.

Myth 3: MSI-H means immunotherapy will always work.

No. MSI-H/dMMR predicts a substantially greater probability of benefit, but primary resistance and acquired resistance occur.

Myth 4: If pembrolizumab stops working, switch to dostarlimab.

There is no established principle that switching from one PD-1 antibody to another overcomes biological PD-1 resistance. Clinical trials and mechanism-directed treatment are more compelling concepts after confirmed progression.

Myth 5: A complete response means no follow-up is necessary.

Especially in nonoperative rectal-cancer management, the opposite is true. Organ-preservation strategies depend on intensive surveillance because local regrowth or distant recurrence can still occur.

12. The Emerging Treatment Map for MSI-H/dMMR CRC

The field is moving from a simple "immunotherapy versus chemotherapy" framework toward a more personalized treatment map:

Biological/clinical question	Why it matters
Is the tumor MSI-H/dMMR?	Determines whether checkpoint blockade is likely to be particularly relevant.
Is it rectal or colon cancer?	Local anatomy and evidence for organ preservation differ substantially.
Localized or metastatic?	The evidence base and treatment objective change dramatically.
PD-1 monotherapy or dual checkpoint blockade?	Balances efficacy, treatment intensity and immune toxicity.
Has the patient already received an ICI?	Prior checkpoint exposure fundamentally changes the evidence landscape.
Is there a molecularly actionable alteration?	Targeted therapy may be relevant, especially after progression.
Could the disease be locally treated or resected?	Selected patients with oligometastatic or residual disease may have additional options.
What resistance mechanism is suspected?	May guide clinical-trial selection and future combination strategies.

13. Overall Verdict

Setting	Most notable evidence-supported strategy	Why
dMMR locally advanced rectal cancer	Dostarlimab	Exceptional prospective clinical-complete-response and organ-preservation data.
dMMR locally advanced colon cancer	Nivolumab + ipilimumab	NICHE-2 produced 98% pathological response and 68% pathological complete response.
Metastatic MSI-H/dMMR CRC, first-line	Pembrolizumab or nivolumab + ipilimumab	Both have strong randomized evidence; the choice involves efficacy, treatment intensity and toxicity.
After PD-1 resistance	No universally established checkpoint switch	Reassess biology, molecular targets, prior treatment and clinical-trial opportunities.

The most important conclusion is that there is no single winner across all MSI-H/dMMR colorectal cancer. The evidence is increasingly stage-specific and location-specific.

For metastatic disease, pembrolizumab has the most mature single-agent evidence, while nivolumab plus ipilimumab has emerged as a powerful dual-checkpoint option with randomized phase III support. For dMMR rectal cancer, dostarlimab has become exceptionally important because of the possibility of organ preservation in carefully selected complete responders. For localized dMMR colon cancer, nivolumab plus ipilimumab has generated striking neoadjuvant pathological-response data.

The next major frontier is not simply another PD-1 antibody. It is understanding why an MSI-H tumor responds, why another does not, and why a responding tumor eventually escapes immune control.

Important: These therapies should be selected and monitored by an oncology team. Treatment indications, reimbursement and regulatory approvals vary by jurisdiction and can change over time. This article is educational information and is not a substitute for individualized medical advice.

14. Frequently Asked Questions

Is dostarlimab better than pembrolizumab for MSI-H colorectal cancer?

Not overall. There is no randomized head-to-head trial establishing dostarlimab as superior to pembrolizumab across MSI-H/dMMR colorectal cancer. Dostarlimab's strongest distinctive evidence is in dMMR locally advanced rectal cancer, while pembrolizumab has a mature phase III evidence base in first-line metastatic MSI-H/dMMR CRC.

Is nivolumab plus ipilimumab better than pembrolizumab?

It may provide greater progression-free benefit in some analyses, but there is no direct randomized head-to-head trial between first-line pembrolizumab and nivolumab plus ipilimumab. Therefore, an absolute superiority claim would be premature. The combination also carries greater immune-related toxicity.

Can dostarlimab replace surgery in rectal cancer?

In carefully selected patients with dMMR locally advanced rectal cancer who achieve a clinical complete response, prospective studies have demonstrated successful nonoperative management. This does not mean every patient can safely avoid surgery. A rigorous multidisciplinary assessment and surveillance program is essential.

Can nivolumab/ipilimumab be used after pembrolizumab stops working?

It may be considered in selected circumstances, but the evidence is not equivalent to the evidence supporting the combination in immunotherapy-naive metastatic disease. A confirmed progression after PD-1 therapy represents a different biological situation. Clinical trials and mechanism-directed strategies are particularly important.

Are MSI-H and dMMR interchangeable?

They are closely related but refer to different testing concepts. MSI evaluates microsatellite instability, while dMMR usually describes loss of mismatch-repair protein function. They frequently overlap and are both used to identify tumors likely to benefit from checkpoint blockade.

Should patients with MSS or pMMR colorectal cancer receive these drugs?

These immunotherapies do not have the same established efficacy in conventional MSS/pMMR colorectal cancer. Biomarker testing is therefore central to treatment selection.

What is the most important test before choosing immunotherapy?

For this treatment question, accurate MSI/MMR assessment is fundamental. Depending on stage and clinical circumstances, broader molecular profiling may also be appropriate.

15. Key Evidence and References

1. Pembrolizumab — KEYNOTE-177: FDA approval and trial overview for first-line MSI-H/dMMR metastatic colorectal cancer. FDA
2. Pembrolizumab 5-year follow-up: André T, et al. Pembrolizumab versus chemotherapy in MSI-H/dMMR metastatic colorectal cancer: 5-year follow-up of KEYNOTE-177. PubMed
3. Dostarlimab in dMMR locally advanced rectal cancer: Cercek A, et al. PD-1 Blockade in Mismatch Repair–Deficient, Locally Advanced Rectal Cancer. New England Journal of Medicine. NEJM
4. Dostarlimab and nonoperative management: Cercek A, et al. Nonoperative Management of Mismatch Repair–Deficient Tumors. New England Journal of Medicine. 2025. PubMed
5. FDA information for dostarlimab: FDA accelerated approval for dMMR recurrent or advanced solid tumors. FDA
6. CheckMate 8HW: Nivolumab plus ipilimumab in MSI-H/dMMR metastatic colorectal cancer. New England Journal of Medicine
7. CheckMate 8HW later analysis: Nivolumab plus ipilimumab versus nivolumab in MSI-H metastatic colorectal cancer. PubMed
8. FDA approval of nivolumab plus ipilimumab: FDA approval for unresectable/metastatic MSI-H or dMMR colorectal cancer. FDA
9. NICHE-2: Chalabi M, et al. Neoadjuvant Immunotherapy in Locally Advanced Mismatch Repair–Deficient Colon Cancer. New England Journal of Medicine. 2024. NEJM
10. Review of immunotherapy resistance in MSI-H/dMMR CRC: PubMed

16. Editorial Evidence Note

This article distinguishes randomized phase III evidence from phase II studies, single-arm studies, tumor-agnostic evidence and cross-trial comparisons. Response rates from different trials should not be interpreted as direct head-to-head superiority unless the therapies were compared within the same randomized study population.

Regulatory approvals and treatment standards vary by country. The FDA status described above refers to the United States and should not be assumed to represent approval or reimbursement policy in every jurisdiction.