NEOPRISM-CRC Trial: Zero Bowel Cancer Relapses at 33 Months With Pre-Surgery Pembrolizumab Alone

The UCL/UCLH-led Phase 1/2 NEOPRISM-CRC trial, presented at AACR 2026, reported 33-month follow-up data showing zero relapses among patients with stage 2/3 MMR-deficient (MSI-high) bowel cancer treated with up to 9 weeks of pembrolizumab before surgery — replacing surgery plus chemotherapy as initial treatment. 59% of the 32 trial participants showed no sign of disease after pembrolizumab treatment alone. At 33-month follow-up, not a single patient had experienced relapse, compared to an expected ~25% relapse rate with standard care. Personalized ctDNA liquid biopsy blood tests accurately predicted treatment response. The results suggest that a substantial fraction of MSI-high colorectal cancer patients — who comprise 15-20% of stage II-III colorectal cancers — may avoid surgery entirely if treated with immunotherapy first. Full results were published and presented at AACR 2026.