Seres Therapeutics Shifts Strategy, Pauses Key Trial Investment and Extends Cash Runway

Seres Therapeutics, Inc. (Nasdaq: MCRB), a clinical‑stage company specialising in live biotherapeutics, today announced a significant strategic shift in its development priorities and operations following its latest corporate review.

The move comes amid rising costs in late‑stage clinical development and the company’s efforts to maintain financial flexibility into the second half of 2026.

Pause on SER‑155 Phase 2 Investment

Seres confirmed that, after progressing key start‑up activities for the SER‑155 Phase 2 study — including submitting a final clinical protocol to the U.S. Food and Drug Administration, qualifying study sites with a contract research organisation, and advancing manufacturing of the investigational drug substance — it is pausing further investment in this trial for now.

While the Phase 2 study has been prepared in collaboration with regulatory authorities, the company said it will shift resources away from immediate funding of that study until additional capital or partnerships are secured.

The SER‑155 programme, which previously earned Breakthrough Therapy and Fast Track designations from the FDA for its potential to prevent life‑threatening bloodstream infections in patients undergoing allogeneic haematopoietic stem cell transplant (allo‑HSCT), showed promising signals in early studies but remains to be tested in broader Phase 2 cohorts.

Cost Reductions and Workforce Changes

In tandem with the clinical reprioritisation, Seres said it has implemented cost‑reduction measures, including a workforce reduction of approximately 30%.

The company said these changes — part of broader expense management efforts — are expected to extend its available cash runway through at least the third quarter of 2026, based on current plans and operating assumptions.

This move builds on prior restructuring actions reported in 2025, which included smaller personnel reductions and expense controls planned to stretch liquidity earlier in the year.

Refocus on Earlier‑Stage Live Biotherapeutic Programs

With the pause in SER‑155’s Phase 2 investment, Seres’ strategic focus has shifted toward advancing its earlier‑stage pipeline — particularly programmes targeting inflammatory and immune diseases. Among these is SER‑603, a live biotherapeutic development candidate designed to address conditions such as ulcerative colitis, Crohn’s disease, and immune‑related enterocolitis linked to immune checkpoint inhibitor therapy.

The company said it believes this pivot will allow more efficient use of capital while expanding the application of its proprietary microbiome‑based platform to conditions with significant unmet medical needs.

It also said that discussions are ongoing with potential collaborators to support these programmes.

Near‑Term Clinical Catalyst: irEC Investigator‑Sponsored Trial

While the SER‑155 Phase 2 study is paused, Seres highlighted a separately funded, investigator‑sponsored trial evaluating SER‑155 in immune checkpoint inhibitor‑related enterocolitis (irEC) — a severe inflammatory condition sometimes seen in cancer patients receiving checkpoint inhibitors.

That study, which enrolled approximately 15 participants, is now fully enrolled, and the company expects initial clinical data in early second quarter 2026.

Positive results from this read‑out could serve as an important validation of the underlying mechanism of action in a different disease context and potentially support further development of the SER‑155 platform.