For patients with a hard-to-treat subtype of colorectal cancer, nine weeks of an immunotherapy drug before surgery wiped out all detectable tumor cells in nearly 6 out of 10 cases. None of those patients saw their cancer come back over almost three years of follow-up. The results, from a UK clinical trial called NEOPRISM-CRC and announced by University College London researchers in May 2026, suggest that a brief pre-surgical course of the drug pembrolizumab could spare certain patients months of grueling chemotherapy after their operations.
The findings center on a molecular subtype known as microsatellite instability-high, or MSI-H, which accounts for roughly 15% of all colorectal cancers. Tumors with this feature carry a high number of genetic mutations, which makes them more visible to the immune system and more responsive to drugs called immune checkpoint inhibitors. Pembrolizumab, marketed as Keytruda and manufactured by Merck, is one of the most widely used checkpoint inhibitors in oncology.
What the NEOPRISM-CRC trial found
The trial, led by Dr. Kai-Keen Shiu at the UCL Cancer Institute, enrolled patients with MSI-H colorectal tumors and gave them three cycles of pembrolizumab over nine weeks before surgery. That sequence flips the conventional approach, in which patients undergo surgery first and then receive up to six months of chemotherapy to reduce the risk of recurrence.
The central result: 59% of participants achieved what oncologists call a pathological complete response, meaning pathologists found zero viable cancer cells in the tissue removed during surgery. Among that group, not a single patient relapsed over a median follow-up of 33 months, nearly three years of sustained, cancer-free survival without additional systemic treatment.
For patients, the implications are concrete. Standard post-operative chemotherapy regimens like FOLFOX carry well-documented side effects: nausea, nerve damage in the hands and feet, fatigue, and suppressed immunity that raises infection risk. If a short course of immunotherapy before surgery can reliably clear the tumor, many patients could skip that entire ordeal. Researchers involved in the trial have argued that treatment intensity should match individual tumor biology, reserving chemotherapy only for those who do not achieve a strong immunotherapy response.
The concept is not without precedent. In 2022, a small study at Memorial Sloan Kettering Cancer Center made headlines when every rectal cancer patient treated with the checkpoint inhibitor dostarlimab achieved a complete clinical response, with no need for surgery or radiation. That trial, published in the New England Journal of Medicine, involved only MSI-H tumors and used a different drug, but it established a proof of concept that NEOPRISM-CRC now extends to colon cancer in a larger, more structured setting. Separately, the KEYNOTE-177 trial demonstrated that pembrolizumab outperformed chemotherapy as a first-line treatment for metastatic MSI-H colorectal cancer, further supporting the drug’s role in this patient population.
What has been verified
The trial is registered under the identifier NCT05197322 on ClinicalTrials.gov, the U.S. National Library of Medicine’s database. The registry confirms the study’s key design elements: pembrolizumab as neoadjuvant therapy, a focus on MSI-H colorectal tumors, primary endpoints related to pathological response, and sponsorship by University College London. The trial also carries a European regulatory identifier (EudraCT 2020-000040-58) and a UK ethics reference (IRAS 1003601), confirming oversight by the Health Research Authority.
These identifiers matter because they show the study was planned prospectively with predefined endpoints and independent monitoring, not assembled after the fact from patient records. That structure gives the findings more weight than a retrospective chart review would carry.
One important caveat: as of June 2026, no formal outcome data have been posted to the results section of the ClinicalTrials.gov record. The 59% response rate and zero-relapse follow-up originate from UCL’s institutional press release, not from a peer-reviewed journal article. Press releases from major research universities typically reflect real data, but they are written to highlight positive results. Details that might temper enthusiasm, such as adverse events, dropout rates, or subgroups where the drug worked less well, usually surface only in the full publication.
What remains uncertain
Several questions sit beyond the reach of what has been publicly disclosed so far.
Sample size and demographics. The total number of patients enrolled has been described only in general terms. Without breakdowns by age, cancer stage, ethnic background, or other health conditions, it is hard to judge how broadly these results would hold up across the wider population of MSI-H colorectal cancer patients seen in routine clinical practice.
Safety data. Pembrolizumab and other checkpoint inhibitors carry known risks of immune-related side effects, including colitis, hepatitis, thyroid dysfunction, and, less commonly, severe organ inflammation. Whether any NEOPRISM-CRC participants experienced serious complications during the nine-week treatment window has not been detailed publicly. Without that information, weighing the benefits of skipping chemotherapy against the potential harms of immunotherapy is difficult.
No concurrent control group. Public summaries of the trial do not describe a comparison arm receiving surgery alone or surgery followed by standard chemotherapy. That means the 59% complete response rate and zero-relapse follow-up cannot be measured against a concurrent baseline. Historical data from prior studies offer indirect benchmarks, but they introduce confounding variables that a randomized control group would handle more cleanly.
The 41% who did not achieve complete response. Nearly half of participants still had detectable cancer cells after immunotherapy and surgery. What happened to those patients, whether they went on to receive chemotherapy, how their outcomes compared, and what factors predicted a weaker response, are all questions the full publication will need to address.
Long-term durability. While 33 months without recurrence is a strong early signal, colorectal cancer can return years after initial treatment, particularly in higher-stage disease. Oncologists generally track patients for at least five years before drawing firm conclusions about cure rates. The current data represent a promising start, not a finish line.
Who this applies to, and who it does not
It is worth underscoring how specific these results are. MSI-H tumors make up roughly 15% of colorectal cancers, which means the vast majority of the more than 150,000 Americans diagnosed with colorectal cancer each year would not be candidates for this approach based on current evidence. Patients with microsatellite-stable tumors, which account for the remaining 85%, respond differently to immunotherapy and are not addressed by this trial.
Even within the MSI-H population, open questions remain about applicability. The trial’s inclusion and exclusion criteria, listed in the registry documentation, may narrow its relevance to patients with clearly resectable, non-metastatic disease. Whether patients with borderline resectable tumors, limited metastases, or prior chemotherapy exposure would see similar benefits is unknown.
For patients with MSI-H colorectal cancer or their families, the practical takeaway is real but bounded. Early results suggest that neoadjuvant pembrolizumab can produce deep, durable responses in a meaningful share of patients with this tumor subtype, and that some who achieve a complete response may avoid recurrence for nearly three years without further treatment. At the same time, the evidence is preliminary. No peer-reviewed paper has laid out the full data, and oncologists are likely to treat NEOPRISM-CRC as an important proof of concept rather than an immediate new standard of care.
Patients interested in this approach should raise it with their oncology teams, who can interpret the evolving evidence in the context of individual disease stage, overall health, and access to clinical trials. As longer follow-up data and formal publications emerge, the field will be better positioned to determine whether nine weeks of immunotherapy before surgery can safely and reliably replace months of chemotherapy for people with MSI-H colorectal cancer.
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*This article was researched with the help of AI, with human editors creating the final content.
