Seres Therapeutics Inc. announced positive topline results from a study evaluating SER-155 in patients with immune-related enterocolitis (irEC), with most participants achieving a steroid-free clinical response after treatment. The study demonstrated that 80% of participants achieved an immunosuppressive-free clinical response at Day 15, the primary endpoint. SER-155 was well tolerated with no drug-related serious adverse events observed through Day 43. The findings also showed favorable safety and biomarker data, supporting the continued development of the live biotherapeutic candidate.

The study (NCT06801067) was conducted at MSK under Principal Investigator David Faleck, M.D. It evaluated SER-155 in 15 patients with moderate-to-severe (Grade 2-3) irEC, a condition affecting approximately 25% of all immune checkpoint inhibitor (ICI) therapy recipients in the US. Current clinical guidelines require halting ICI cancer treatment and initiating systemic immunosuppressive corticosteroids, which carry risks such as systemic immunosuppression and increased infection risk. The study aimed to address the unmet need for therapeutic approaches that reduce gastrointestinal inflammation without disrupting life-saving cancer treatment.

Topline Clinical Results

Day 15 Data

Following SER-155 dosing, 12 of 15 participants (80%) achieved an immunosuppressive-free clinical response at Day 15. This endpoint was defined as at least a 1-grade improvement in diarrhea symptoms without immunosuppressive therapy. Notably, 8 of these 12 responders (67%) achieved greater than or equal to a 2-grade improvement in diarrhea symptoms without immunosuppressive therapy at Day 15.

At Day 15, 5 of 15 participants (33%) achieved immunosuppressive-free complete clinical remission, defined as total resolution of diarrhea symptoms to Grade 0, without immunosuppressive therapy.

Day 43 Data

At Day 43, 5 of 15 participants (33%) maintained immunosuppressive-free clinical response, and 2 of 15 participants (13%) maintained immunosuppressive-free complete clinical remission. All 12 participants with immunosuppressive-free clinical response at Day 15 had the same, or better, diarrhea grade at Day 43. The other 7 participants were treated with non-systemically acting, gastrointestinal-targeted immunosuppressives after Day 15.

Safety and Pharmacology

SER-155 was generally well tolerated through Day 43, with no safety concerns identified. There were no serious adverse events assessed as related to SER-155 and no bloodstream infections reported. Two participants experienced 2 non-serious adverse events each, assessed as possibly related to vancomycin and SER-155. All 4 adverse events were moderate in severity and were resolved.

SER-155 bacterial strain engraftment was robust, with the majority of strains observed across participants. The kinetics, magnitude, and durability were comparable to prior Seres clinical studies. Translational biomarkers of gastrointestinal inflammation (fecal calprotectin) and mucosal epithelial barrier integrity (fecal albumin) showed statistically significant reductions by Day 43, reflecting improvement in irEC disease dimensions.

Seres Therapeutics will host a webcast on July 8, 2026, at 8:30 am ET, to discuss top-line results from a trial evaluating SER-155 in patients with immune checkpoint inhibitor-related enterocolitis. The Investigator-Sponsored Trial (IST) was conducted at Memorial Sloan Kettering Cancer Center, led by Principal Investigator David Faleck, M.D., Director of Inflammatory and Immune-Related Bowel Diseases. Seres has a long history of collaboration with the center.

Webcast Details

The live webcast will be accessible via the "Events and Presentations" tab within the "Investors and News" section of the Seres website at www.serestherapeutics.com . A replay of the webcast will be available following the event.

Conference Call Access

Participants can join the conference call using the following details: