Patients in a British bowel cancer trial who received nine weeks of the immunotherapy drug pembrolizumab before their operation, instead of the usual chemotherapy afterward, have gone nearly three years without a single relapse.
The extended follow-up from the phase 2 NEOPRISM-CRC study was presented at the American Association for Cancer Research annual meeting in San Diego in April 2026, and researchers say the results strengthen the case for using immunotherapy earlier, in a specific subset of colon cancer.
Pembrolizumab is the active ingredient in Merck's Keytruda, a checkpoint inhibitor that helps the immune system attack tumor cells.
It has been approved in the United States since June 2020 for advanced bowel cancers with mismatch-repair deficiency, but the new UK data test something different: giving it upfront, in earlier-stage disease, before the surgeon operates.
Also read: A new scan could help millions of men avoid an unnecessary prostate biopsy
What the trial actually tested
NEOPRISM-CRC recruited 32 patients with stage 2 or stage 3 bowel cancer whose tumors were mismatch-repair deficient, sometimes called MSI-high.
These tumors carry faults in the cellular machinery that normally corrects DNA errors, which makes them unusually visible to the immune system and unusually responsive to checkpoint drugs.
About 10 to 15 percent of stage 2 and stage 3 bowel cancers fall into this group, according to the trial team at University College London Hospitals.
Instead of going straight to surgery followed by three to six months of chemotherapy, patients received up to nine weeks of pembrolizumab first, then had their operation.
Also read: Following these 8 habits may cut your risk of dying from cancer, large study suggests
What the follow-up showed
At the time of surgery, 59 percent of the patients had no detectable cancer left in the removed tissue, a finding known as a pathologic complete response. At 33 months of follow-up, none of the 32 patients had experienced a return of their cancer. The trial team estimates that around one in four patients treated with the usual surgery-plus-chemotherapy approach would be expected to relapse over a comparable window.
"Seeing that no patients have experienced a cancer recurrence after almost three years of follow-up is extremely encouraging and strengthens our confidence that pembrolizumab is a safe and highly effective treatment to improve outcomes in patients with high-risk bowel cancers," said Dr. Kai-Keen Shiu, chief investigator on the trial and consultant medical oncologist at UCL Cancer Institute and UCLH.
The team also reported that an ultrasensitive blood test for circulating tumor DNA, tiny fragments of cancer DNA shed into the bloodstream, correctly identified which patients were clear of disease. Patients whose ctDNA was undetectable four to five weeks after surgery had, in this small cohort, a 100 percent negative predictive value for relapse.
Why the caveats matter
The results are striking, but the trial is small. Thirty-two patients is enough to signal a strong effect, not enough to change practice on its own, and larger studies will need to confirm the durability of the response and the reliability of the ctDNA test.
Also read: How food poisoning can affect your gut for years
The findings also do not apply to most people diagnosed with bowel cancer. The 10 to 15 percent of stage 2 and stage 3 patients with mismatch-repair-deficient tumors are the only group in whom this benefit has been shown. For the roughly 85 percent whose tumors are mismatch-repair proficient, standard surgery and chemotherapy remain the treatment of record.
Bowel cancer, also called colorectal cancer, is one of the most common cancers in the United States. Anyone with persistent changes in bowel habits, blood in the stool or unexplained weight loss should speak with a doctor, and eligible adults are now advised to begin routine colorectal cancer screening at age 45.
The NEOPRISM-CRC investigators are now planning larger studies, including work to test whether some patients with a complete response to pembrolizumab could ultimately avoid major bowel surgery altogether.
Also read: More than 100,000 people in Japan are over 100 – three traditions may help explain why
Also read: Levetiracetam paired with a blood thinner may nearly double clot risk
This article is made and published by Mie Hermansen, who may have used AI in the preparation.