Phase 1/2, open-label study of oral bacterial supplementation (EDP1503) plus pembrolizumab in participants with advanced or metastatic microsatellite-stable colorectal cancer, triple-negative breast cancer, and checkpoint inhibitor-relapsed tumors.

We report a phase 1/2 study evaluating EDP1503 (capsule containing Bifidobacterium animalis lactis) ± pembrolizumab in participants with microsatellite-stable colorectal cancer (MSS CRC), triple-negative breast cancer (TNBC), or other tumor types that relapsed after responding to immunotherapy (KEYNOTE-939/EDP1503-101; NCT03775850). Participants (≥ 18 years) had confirmed advanced/metastatic tumors and progressive disease (PD), were intolerant/nonresponsive to recommended treatment, and had measurable disease (RECIST v1.1). Cohorts were: MSS CRC (Cohort A), metastatic/locally advanced TNBC (Cohort B), and PD following partial response/stable disease for ≥ 6 months during anti‒PD-(L)1 therapy (≥ 8 months during anti‒PD-(L)1 therapy plus chemotherapy for non-small-cell lung cancer) (Cohort C). Participants received oral EDP1503 (2 or 4 capsules twice daily [BID]) for 2 weeks, then EDP1503 (2 or 4 capsules BID) plus intravenous pembrolizumab 200 mg every 3 weeks until PD, participant withdrawal, investigator decision, intolerable toxicity, or completion of 35 cycles. Primary endpoints were safety/tolerability, objective response rate (RECIST v1.1), and immune-response rate. Secondary endpoints included duration of clinical benefit, progression-free survival (PFS), and overall survival (OS). Of 69 participants, objective responses were observed in 3 (2 in Cohort B and 1 in Cohort C with partial responses received 4 capsules BID). For participants receiving 4 capsules BID, median duration of clinical benefit (95% CI) was 8.7 months (5.5 months‒not evaluable), median PFS was 1.8 (1.7‒1.9) months, and median OS was 7.8 (2.5‒13.5) months. Grade ≥ 3 adverse events occurred in 28 participants (40.6%), with no new safety signals. EDP1503 plus pembrolizumab had manageable safety but limited clinical activity.Trial registration: KEYNOTE-939, EDP1503-101 trial: ClinicalTrials.gov NCT03775850.