TRIAL SUCCESS

Seres' SER-155 hits 80% response rate in checkpoint-inhibitor colitis pilot

A 15-patient investigator-run trial at Memorial Sloan Kettering found most patients cleared diarrhea without steroids, but the benefit faded by Day 43 for more than half.

By AppliedXLJuly 8, 2026Trial NCT06801067

Executive Summary

An investigator-run trial at Memorial Sloan Kettering Cancer Center found that most patients treated with Seres' live biotherapeutic cleared checkpoint-inhibitor colitis without steroids at two weeks, meeting the study's primary goal.
The early response did not fully hold: by six weeks, most of the initial responders had needed additional treatment to keep symptoms controlled, though none needed to restart systemic steroids.
The therapy caused no drug-related serious adverse events through six weeks, consistent with safety data from Seres' separate transplant program for the same drug.
Seres has no disclosed regulatory filing plan for this indication and says it is seeking development partners, positioning the data as a proof-of-concept rather than a registration-ready result.

The trial and the problem

The open-label study (NCT06801067), conducted at Memorial Sloan Kettering Cancer Center under principal investigator David Faleck, M.D., enrolled 15 patients with Grade 2-3 immune checkpoint inhibitor-related enterocolitis (irEC), an inflammatory bowel side effect that Seres says affects roughly 25% of patients on checkpoint inhibitors in the US. Standard care for moderate-to-severe irEC requires halting cancer treatment and starting systemic corticosteroids, which carry infection risk and can blunt the immunotherapy's own effect. The trial's primary efficacy endpoint was the proportion of patients achieving an immunosuppressive-free clinical response, defined as at least a 1-grade improvement in diarrhea without steroids or biologic immunomodulators, by Day 15. Seres

The result

Seres reported that 12 of 15 patients (80%) met that bar at Day 15, and 8 of those 12 responders (67%) achieved at least a 2-grade improvement. Five of 15 patients (33%) reached complete clinical remission, defined as full resolution of diarrhea to Grade 0, without immunosuppressive therapy. Patients in the trial were on a range of checkpoint inhibitor classes, including PD-1, PD-L1, CTLA-4 and LAG-3 inhibitors, so the response was not confined to a single drug class. Seres

The durability question

The response narrowed by Day 43: 5 of 15 patients (33%) maintained an immunosuppressive-free clinical response, and 2 of 15 (13%) maintained complete remission. All 12 Day 15 responders held the same or better diarrhea grade at Day 43, but 7 of the 12 needed non-systemic, gastrointestinal-targeted immunosuppressive treatment after Day 15 to stay there. Seres reported no drug-related serious adverse events and no bloodstream infections through Day 43, a safety readout the company said was consistent with its earlier Phase 1b study of SER-155 in allogeneic hematopoietic cell transplant patients. Seres

What the field looks like

No competing trial specifically targets irEC with a live biotherapeutic or a comparable microbiome-based mechanism; the indication has no dedicated approved treatment beyond the steroid-based standard of care that this trial was designed to avoid. Seres already holds Breakthrough Therapy and Fast Track designations for SER-155 in a separate indication, prevention of bloodstream infections after allogeneic stem cell transplant, where the same drug is described as Phase 2-ready pending funding. That program's designations do not extend to irEC, where Seres has disclosed no regulatory filing plan and says it is engaging with potential partners to advance development further. Seres

This analysis was produced using AI-assisted reporting systems, AppliedXL data, and official public records. These systems undergo editorial review, quality checks, and regular audits by human experts. Errors may still occur, as with any automated system. Always consult the linked primary sources. Read our AI Editorial Policy.