PURPOSE: Novel combinations are required to overcome resistance to immune checkpoint inhibitors in proficient mismatch repair (pMMR) or microsatellite-stable (MSS) metastatic colorectal cancer (mCRC). We aimed to determine whether vorbipiprant, a prostaglandin E2 receptor EP4 subtype antagonist, can convert immune-resistant mCRC into a tumor responsive to anti-PD-1 inhibition. PATIENTS AND METHODS: This phase Ib/IIa prospective, open-label, single-arm trial followed a 3 + 3 dose-escalation and dose-optimization design. A total of 28 patients with chemorefractory pMMR/MSS mCRC were given dose-escalated oral vorbipiprant (30, 90, or 180 mg twice daily), along with biweekly intravenous balstilimab (3 mg/kg), an anti-PD-1 antibody. The primary endpoints included safety and the disease control rate (DCR). Secondary endpoints were the overall response rate, duration of response, progression-free survival, and overall survival. RESULTS: No dose-limiting toxicities were observed. Of the 28 patients, seven (25%) experienced serious adverse events, but only one was attributed to vorbipiprant and one to balstilimab. The trial achieved a DCR of 50% observed across the entire cohort. In the subgroup of patients with liver metastases (n = 12), the DCR was 25%. The overall response rate was 11%, with three patients showing a partial response (median duration of response, 7.4 months). The median progression-free survival was 2.6 months, and the median overall survival was 14.2 months. Translational exploratory analyses suggested that vorbipiprant may boost response to anti-PD-1 in patients with immunogenic tumors. CONCLUSIONS: The combination of vorbipiprant and a PD-1 inhibitor (balstilimab) yielded sufficient activity in refractory pMMR/MSS mCRC, which is worthy of confirmation in future clinical trials in biomarker-enriched populations.