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Seres says SER-155 is Phase 2 ready after FDA agrees transplant infection study design

🇺🇸 Seres Therapeutics, Inc. (MCRB) · Wed 04 Nov 2026

Seres Therapeutics' corporate presentation cites FDA agreement on study design for SER-155 in allogeneic transplant recipients, alongside Phase 1b data and an earlier-stage checkpoint inhibitor programme.

Seres Therapeutics has described its oral live biotherapeutic SER-155 as ready for Phase 2 development in a corporate presentation filed with the US Securities and Exchange Commission. The company points to an FDA agreement on the design of a study aimed at preventing bloodstream infections in people receiving allogeneic stem cell transplants (allo-HCT).

SER-155 holds FDA Breakthrough Therapy designation. The deck cites Phase 1b results showing a 77% relative risk reduction in bloodstream infections, with no drug-related serious adverse events reported. The summary available does not give trial size, comparator details or the endpoints agreed with the agency.

The presentation also covers an investigator-sponsored Phase 1b study of SER-155 in enterocolitis associated with immune checkpoint inhibitors. Seres reports an 80% immunosuppressive-free response in that study. Again, the available material gives no further detail on patient numbers or definitions.

Separately, the company says IND-enabling studies for SER-603 are under way. No timeline for an IND submission is given in the information available.

For context, Breakthrough Therapy designation is an FDA pathway intended to speed development and review of drugs for serious conditions where early evidence suggests substantial improvement over existing options. It does not guarantee approval.

What it means for your team

  • The reported FDA agreement on study design is a company statement in an investor-facing deck; the agency's own documentation was not part of the material reviewed.
  • The 77% and 80% figures come from early-phase studies, so regulatory and market-access teams should treat them as preliminary until fuller data are published.
  • Safety reporting for live biotherapeutics in immunocompromised transplant patients will be a point to watch as the programme moves into Phase 2.
  • Details of the Phase 2 design, including endpoints and size, would be needed to assess likely evidence packages for payers.

Company's press release (SEC filing): Seres Therapeutics, Inc. ↗

Article written by VigiEye from the official source (Seres Therapeutics, Inc.). Claims are the company's own. AI-assisted; check the source before acting.

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