Background: Systemic immunity can be regulated via transient interactions of non-colonizing bacteria and immune cells in the small intestinal mucosa. The potential to harness these interactions for IO treatment was first described by Sivan in 2015. EDP1503 is a non-colonizing preparation of a single strain of Bifidobacterium animalis lactis that, when administered orally, in preclinical models, invokes an anti-tumor immune response by activation of innate and adaptive immune mechanisms. The pleiotropic effects of EDP1503 are initiated in the small intestine and mediated by interactions with multiple pattern-recognition receptors inducing a proinflammatory signature in human PBMCs, activating CD8 and NK cell IFNγ production and cytolytic activity. Preclinically, EDP1503 inhibits tumor growth as a monotherapy and in combination with anti-PD-1/L1. Single agent PD-1/L1 therapies have showed limited clinical benefit in previously treated triple-negative breast cancer (TNBC) patients. Here, we report the safety, tolerability, and efficacy of EDP1503 in combination with pembrolizumab in patients with TNBC (NCT03775850). Methods: EDP1503-101 is an open label Phase 1/2 study of EDP1503 in combination with pembrolizumab. TNBC subjects who had received ≥ 1 treatment regimen for metastatic disease were enrolled. Prior anti-PD-1/L1 treatment was permitted. Subjects initially receive 14 days of EDP1503 monotherapy orally twice a day (b.i.d) and then EDP1503 b.i.d. in combination with pembrolizumab 200 mg iv every 3 weeks. The initial 3 subjects received 2 capsules of EDP1503 b.i.d. All other subjects received 4 capsules b.i.d. Safety and tolerability were assessed using CTCAE v5.0. Evidence of anti-tumor activity was based on investigator-assessed objective response (OR), by RECIST v1.1 and iRECIST, and disease control rate defined as OR and/or stable disease (SD) after 4 cycles of therapy. Paired biopsies at baseline and day 14 were used to establish PD-L1 status and investigate pharmacodynamic biomarker changes. Results: As of 1 October 2020, 15 TNBC subjects had been treated (median age of 52, median of 2 prior lines of therapy and ECOG of 0-1). Additional subjects are being recruited, up to a maximum of 30. EDP1503 was well-tolerated as monotherapy and in combination with pembrolizumab. 53% of subjects experienced treatment-related adverse events (TRAEs). Most common TRAEs were GI-related: abdominal distension (20%), decreased appetite (20%), diarrhea (13%), flatulence (13%). No Grade 4-5 TRAEs were observed. 1 Grade 3 TRAE (diarrhea) leading to treatment discontinuation was reported. In 12 subjects receiving the 4 capsules b.i.d dose, 2 partial responses (PR) and 1 SD were observed, giving an objective response rate (ORR) of 18% and a disease control rate (DCR) of 27% in evaluable patients (n=11). Both responders had tumor burden reductions of >65% by RECIST. One responder had 4 prior lines of therapy for metastatic disease, including a PD-L1 inhibitor combination, has a PD-L1 combined positive score (CPS) of 30 and has been on study treatment for 192 days with a 66% reduction in target lesions. The only residual lesion is 6 mm which is PET-negative. The other responder had 2 prior lines of therapy for metastatic disease, was checkpoint inhibitor naive, has a PD-L1 CPS of 2 and remained on study treatment for 275 days with a 73% reduction in target lesions before discontinuing due to treatment-related AEs. The patient with SD had also previously relapsed on a PD-L1 inhibitor and was on study treatment for 226 days before progressing. Conclusions: EDP1503 administered with pembrolizumab is safe and well-tolerated with no Grade 4-5 TRAEs or SAEs. Clinical benefit was observed in a subset of TNBC patients treated with the combination of EDP1503 and pembrolizumab. This study is continuing to recruit TNBC patients at the high dose of EDP1503. Citation Format: Loise Francisco-Anderson, Shamira Shariffudin, Humphrey Gardner, Peter Sandy, Michael Goldberg, Shubhra Kashyap, Mary Abdou, Maria Sizova, Valeria Kravitz, Holly Ponichtera, Mark Carlson, Shannon Argueta, Chris Davitt, Pooja Parameswaran, Michael Chisamore, Mark Bodmer, Duncan McHale. A phase I/II clinical trial of EDP1503 with pembrolizumab for triple-negative breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS11-27.