Thirty-two people walked into a London hospital with operable bowel cancer. They received nine weeks of a single immunotherapy drug, pembrolizumab, before undergoing surgery. Nearly three years later, not one of them has seen the cancer return.
The trial behind those numbers, called NEOPRISM-CRC, was led by University College London and University College London Hospitals. It targeted a specific biological subset of stage II and III colorectal cancers defined by a biomarker known as mismatch repair deficiency (dMMR), sometimes written as MSI-H (microsatellite instability-high). Follow-up data released by UCL in April 2026 confirmed a zero-relapse rate at a median of 33 months after treatment. In a disease where recurrence within two to three years is a genuine and common fear, that number stopped oncologists in their tracks.
What the trial actually did
NEOPRISM-CRC, formally registered on ClinicalTrials.gov as NEOadjuvant PembRolizumab In Stratified Medicine – ColoRectal Cancer, gave patients up to three cycles of intravenous pembrolizumab, each spaced three weeks apart, for a maximum treatment window of nine weeks. All of this happened before surgery, in what oncologists call the neoadjuvant setting. The idea was to unleash the immune system against the tumor while it was still in the body, then let the surgeon remove whatever remained.
The patients enrolled had operable stage II or III dMMR/MSI-H colorectal cancer. This subtype accounts for roughly 10 to 15 percent of bowel cancer diagnoses overall, though the proportion varies by stage. These tumors carry unusually high numbers of mutations, which makes them more visible to immune cells and, crucially, more vulnerable to drugs like pembrolizumab that release the brakes on the immune response.
After completing immunotherapy and going through surgery, 59 percent of the 32 patients had no detectable cancer left in the surgical specimen, a benchmark known as pathologic complete response. The remaining patients still had their tumors shrink or respond, and at 33 months of follow-up, every single participant remained cancer-free.
Why the result matters
Standard treatment for stage II and III bowel cancer typically means surgery followed by months of adjuvant chemotherapy to mop up any microscopic disease left behind. For rectal cancers, preoperative chemoradiation or short-course radiotherapy is often added to shrink the tumor before the operation. The U.S. National Cancer Institute’s professional overview of rectal cancer treatment outlines how these multimodal regimens remain the backbone of care, particularly for higher-risk tumors.
Chemotherapy works, but it comes at a cost. Months of treatment bring fatigue, neuropathy, nausea, infection risk, and disruption to daily life. NEOPRISM-CRC raises the possibility that for patients whose tumors carry the dMMR/MSI-H biomarker, a short burst of immunotherapy before surgery could replace that entire chemotherapy burden. Nine weeks of pembrolizumab versus months of toxic drugs is a trade-off most patients would take in a heartbeat, if the evidence holds up.
The scientific logic did not come from nowhere. The KEYNOTE-177 trial, a randomized phase 3 study whose final analysis is published and peer-reviewed, had already shown that pembrolizumab outperformed standard chemotherapy as first-line treatment for patients with MSI-H or dMMR metastatic colorectal cancer. That trial established pembrolizumab as a standard of care in advanced disease. NEOPRISM-CRC pushed the question upstream: if the drug controls metastatic cancer, can it eradicate earlier-stage tumors entirely when given before the surgeon operates?
Based on 33 months of data, the answer for this small group of patients appears to be yes. The zero-relapse rate is particularly striking because even after successful surgery, a meaningful fraction of stage II and III bowel cancer patients typically see their disease return within two to three years. By priming the immune system before resection, the pembrolizumab course may have cleared microscopic disease that a scalpel alone would miss.
What we still do not know
Compelling as the results are, several gaps prevent them from immediately changing how doctors treat bowel cancer.
First, 32 patients is a small cohort. It provides a powerful signal, but a single relapse would shift the rate from zero to over three percent. Regulators and guideline committees need larger numbers before rewriting treatment standards.
Second, the trial had no comparator arm. Without a randomized control group receiving standard neoadjuvant chemotherapy or chemoradiation, it is impossible to say definitively how much of the benefit came from pembrolizumab versus how much reflects the naturally favorable biology of dMMR/MSI-H tumors. These cancers tend to respond better to immune-based treatment and sometimes carry a better prognosis even with conventional therapy. Only a randomized phase 3 trial pitting this nine-week immunotherapy approach against current standard care can untangle that question.
Third, the full results have not yet been published in a peer-reviewed medical journal. The data reported so far come from UCL’s Cancer Trials Centre and conference presentations. Until independent peer review examines the complete dataset, including individual patient outcomes, immune-related adverse events, surgical complication rates, and any protocol deviations, the findings carry less formal weight than they would in a published manuscript.
Fourth, oncologists typically track colorectal cancer recurrence for at least five years before drawing firm conclusions about cure. Thirty-three months is encouraging, but late relapses, while less common in MSI-H disease, are not unheard of. Several more years of surveillance are needed before anyone can confidently use the word “cured.”
It is also worth noting that NEOPRISM-CRC is not the only trial exploring immunotherapy before surgery in this space. Researchers at Memorial Sloan Kettering published results in the New England Journal of Medicine showing that another checkpoint inhibitor, dostarlimab, produced clinical complete responses in every patient in a small cohort of dMMR rectal cancers, some of whom avoided surgery altogether. That study, while focused on rectal cancer specifically, reinforces the broader principle that dMMR tumors are exquisitely sensitive to immune checkpoint blockade. Together, these trials are building a case, but neither alone is large enough to rewrite guidelines.
What patients should take from this
For the roughly one in eight bowel cancer patients whose tumors test positive for mismatch repair deficiency, NEOPRISM-CRC is genuinely exciting news. It adds to a growing body of evidence that immunotherapy could transform treatment for this biological subgroup, potentially sparing patients months of chemotherapy and its side effects.
But excitement is not the same as a treatment change. Outside of clinical trials, standard surgery and chemotherapy remain the default approach for operable bowel cancer. Patients who know or suspect they have dMMR/MSI-H disease should ask their oncology teams whether immunotherapy-based trials are available to them. In the UK, where this trial was run, questions about future NHS access and any role for NICE evaluation will inevitably follow as the data mature.
The next steps are clear: larger, randomized trials with longer follow-up and published peer-reviewed results. If those confirm what 32 patients in London have shown so far, a nine-week course of pembrolizumab before surgery could become one of the most significant shifts in bowel cancer treatment in a generation. For now, the data are early, the sample is small, and the peer review is pending. But zero relapses in 33 months is not a number the oncology world will forget easily.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
