This section highlights FDA-related milestones and regulatory updates for drugs developed by Seres Therapeutics (MCRB).
Over the past two years, Seres Therapeutics has reported clinical trial outcomes, regulatory submissions, approvals, and other FDA events for drugs and therapies such as
SER-603, SER-155, and VOWST. For definitions of regulatory abbreviations such as NDA, BLA, or PDUFA, see the event status legend.
Select a button below to view the list of FDA events for that drug.
SER-603 FDA Regulatory Events
SER-603 is a drug developed by Seres Therapeutics for the following indication: For Inflammatory Bowel Disease.
This drug is under review by the U.S. Food and Drug Administration (FDA).
Below is a timeline of key regulatory milestones for this therapy.
- Drug:
- SER-603
- Announced Date:
- May 4, 2026
- Indication:
- For Inflammatory Bowel Disease
Announcement
Seres Therapeutics, Inc. announced new preclinical data supporting the design and potential of SER-603, a next-generation cultivated live biotherapeutic candidate for inflammatory bowel disease (IBD), being presented at Digestive Disease Week (DDW) 2026, taking place May 2–5, 2026, in Chicago, IL.
AI Summary
Seres Therapeutics announced new preclinical data supporting the design and potential of SER-603, a next-generation cultivated live biotherapeutic candidate for inflammatory bowel disease (IBD). The company will present the findings at Digestive Disease Week (DDW) 2026, held May 2–5 in Chicago, IL. The poster was selected for DDW’s "Poster of Distinction" recognition, highlighting interest in the work.
Data highlight SER-603’s rational design that targets microbial functions linked to mucosal healing and gastrointestinal inflammation in IBD, suggesting a focused, mechanism-based approach rather than broad microbiome changes. The research also advances microbiome-based biomarkers that could help predict response to biologic therapies and better stratify patients. Presentation details: Poster Su1462, "The Rational Design of SER-603: A Next Generation Cultivated Microbial Consortia to Treat IBD", session 7165, May 3, 12:30–1:30 pm, presenter Nicholas Beauchemin.
Read Announcement
SER-155 FDA Regulatory Timeline and Events
SER-155 is a drug developed by Seres Therapeutics for the following indication: Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD).
This drug is under review by the U.S. Food and Drug Administration (FDA).
Below is a timeline of key regulatory milestones for this therapy.
- Drug:
- SER-155
- Announced Date:
- April 20, 2026
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, Inc. announced three presentations at the 2026 European Society of Clinical Microbiology and Infectious Diseases (ESCMID) global congress, taking place April 17-21 in Munich, Germany.
AI Summary
Seres Therapeutics announced three presentations at the 2026 ESCMID global congress, April 17–21 in Munich. The company said pharmacologic data will show durable GI microbiome changes and improved epithelial barrier integrity after SER-155 administration. Other presentations will underscore the expanding role of live biotherapeutics in inflammatory bowel disease and care of immunocompromised patients.
Dr. Jonathan (Tsoni) Peled of Memorial Sloan Kettering Cancer Center will present in a symposium on how the microbiome shapes clinical outcomes in immunocompromised patients. His talk will highlight emerging strategies to target the gut microbiome during hematopoietic cell transplantation.
SER-155 is an investigational oral live biotherapeutic designed to decolonize GI pathogens, support epithelial barrier function, and induce immune tolerance to help prevent bacterial bloodstream and antimicrobial‑resistant infections in patients undergoing allogeneic hematopoietic stem cell transplantation. A Phase 1b placebo‑controlled study reported fewer bloodstream infections, lower systemic antibiotic exposure, and reduced febrile neutropenia.
Read Announcement- Drug:
- SER-155
- Announced Date:
- February 12, 2026
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, Inc. announced that, following advancement of key startup activities for the SER-155 Phase 2 study in allo-HSCT, including the submission of a final protocol to the FDA, study site evaluation and qualification with its CRO, and manufacturing of Phase 2 study drug substance, Seres is pausing additional investment in that program and will shift its operational focus to high-value earlier-stage pipeline programs while continuing to seek funding for the SER-155 Phase 2 study.
AI Summary
Seres Therapeutics reported that it completed key startup steps for the SER-155 Phase 2 study in patients undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT). Those steps include submitting a final protocol to the FDA, completing study site evaluation and qualification with its CRO, and manufacturing the Phase 2 study drug substance. Despite this progress, Seres said it will pause additional investment in the SER-155 allo-HSCT program for now.
The company will shift its operational focus to higher-value, earlier-stage pipeline programs while continuing efforts to secure funding for the SER-155 Phase 2 study. Seres retains the prepared study assets and initial work, so the program could recommence if financing becomes available. The move reflects a prioritization of resources while keeping the option open to advance SER-155 in the future.
Read Announcement- Drug:
- SER-155
- Announced Date:
- October 14, 2025
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, Inc. announced that new post hoc data from its SER-155 Phase 1b trial will be featured in an oral presentation at IDWeek 2025, taking place October 19–22 in Atlanta, Georgia.
AI Summary
Seres Therapeutics announced that new post hoc data from its SER-155 Phase 1b trial will be shared in an oral presentation at IDWeek 2025, October 19–22 in Atlanta. Dr. Tessa Andermann of the University of North Carolina will present on October 20, detailing clinical and microbiology outcomes of bloodstream infections (BSIs) in adults undergoing allogeneic hematopoietic stem cell transplant. The talk will highlight differences between the SER-155 and placebo groups in the bacterial and fungal organisms causing BSIs, including infection outcomes, antibacterial prophylaxis use, and patterns of antimicrobial resistance.
In the randomized, double-blind study, SER-155 reduced BSIs by 77%, lowered systemic antibiotic use, and decreased febrile neutropenia compared to placebo. These findings support the live biotherapeutic’s mechanism of action and suggest broader applications in immune-mediated diseases. Seres is finalizing plans for a Phase 2 study and seeking resources to advance SER-155 toward improving infection outcomes in vulnerable patients.
Read Announcement- Drug:
- SER-155
- Announced Date:
- September 23, 2025
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, announced receipt of additional constructive feedback from the U.S. Food and Drug Administration (FDA) on the Phase 2 study protocol for the Company's lead program, SER-155, for the prevention of bloodstream infections (BSIs) in adults undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT).
AI Summary
Seres Therapeutics announced it received additional constructive feedback from the FDA on its Phase 2 protocol for SER-155, its lead live biotherapeutic aimed at preventing bloodstream infections in adults undergoing allogeneic hematopoietic stem cell transplants. It expects new guidance on study size, primary endpoints, and interim analyses to support protocol finalization and accelerate study launch.
Seres is meeting with investors to secure funding and resources for the Phase 2 study, planning to launch the trial once capital is in place. It will enroll about 248 participants in a well-powered, placebo-controlled design, with an interim analysis at half enrollment.
Interim clinical data are expected within 12 months of study initiation, enabling Seres to engage with the FDA on Phase 3 design and potentially expand into additional vulnerable patient populations.
Read Announcement- Drug:
- SER-155
- Announced Date:
- May 27, 2025
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, Inc. announced the Company will be presenting new exploratory biomarker data from its SER-155 Phase 1b study in a poster session at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting, taking place May 30 –June 3 in Chicago.
AI Summary
Seres Therapeutics, Inc. announced that it will present new exploratory biomarker data from its SER-155 Phase 1b study at the upcoming 2025 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago. The poster session is scheduled for Sunday, June 1, 2025, from 9:00 AM to 12:00 PM CT. The data focuses on how SER-155 may help promote immune reconstitution following allogeneic hematopoietic stem cell transplantation (allo-HSCT). In the study, increases in the homeostatic cytokine IL-7 and higher frequencies of CD4+ T cells were observed, suggesting that SER-155 plays a role in enhancing T-cell recovery after transplantation. This immune support could be crucial in reducing the risk of bloodstream infections in vulnerable patients. The presentation is expected to provide further insights into the drug’s potential benefits and its broader impact on treating inflammatory and immune-mediated diseases.
Read Announcement- Drug:
- SER-155
- Announced Date:
- April 29, 2025
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, announced that it will present three related posters at the upcoming 2025 Digestive Disease Week (DDW) Conference being held May 3-6, 2025, in San Diego, CA.
AI Summary
Seres Therapeutics, Inc. will present three related posters at the upcoming 2025 Digestive Disease Week (DDW) Conference in San Diego, CA, from May 3-6, 2025. The presentations are set to highlight new research into live biotherapeutics for treating inflammatory and immune diseases. One poster titled “Characterization of IBD Microbiomes in Models of Microbiome Disruption and Inflammation to Evaluate Candidate Biomarkers for Patient Selection” will be presented by Principal Scientist Nicholas Beauchemin on May 3 from 12:30 PM to 1:30 PM. A second session, also by Beauchemin, will focus on candidate biomarkers for patient selection in ulcerative colitis clinical trials during the same time slot. An encore presentation will share pharmacokinetic and pharmacodynamic results from a Phase 1b study on SER-155, presented by Christopher Ford on May 4 from 12:30 PM to 1:30 PM.
Read Announcement- Drug:
- SER-155
- Announced Date:
- March 13, 2025
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics provided business updates.
AI Summary
Seres Therapeutics provided a business update on its SER-155 program, highlighting important progress in its development for patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). The company noted that recent constructive FDA feedback supports using a reduction in bloodstream infections at 30 days post-transplant as the primary efficacy endpoint for the next study. SER-155, which has already shown a 77% relative risk reduction in bloodstream infections during its Phase 1b study, is being considered for a standalone Phase 2 trial or a combined Phase 2/3 design. In addition, the company is advancing strategic partnership discussions to accelerate the study and explore potential expansion into other patient populations, including those with inflammatory and immune diseases. Seres plans to submit a draft protocol to the FDA in the second quarter of 2025.
Read Announcement- Drug:
- SER-155
- Announced Date:
- March 3, 2025
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, announced that it has received constructive feedback from a Type B Breakthrough Therapy designation engagement with the U.S. Food and Drug Administration (FDA or the agency) regarding the Company's development strategy for its lead live biotherapeutic, SER-155 in patients undergoing allo-HSCT.
AI Summary
Seres Therapeutics announced that it has received constructive feedback from the FDA during a Type B Breakthrough Therapy designation engagement regarding its lead live biotherapeutic SER‑155 for patients undergoing allo‑HSCT. The feedback focused on key elements of the clinical development strategy. Importantly, the FDA supported the proposed primary endpoint of reducing bloodstream infections within 30 days after transplant. The company has submitted clarification questions to the agency, and it expects to receive further feedback in the coming weeks. This information will help shape the protocol for the next study, which is likely to be a Phase 2 trial or possibly a combined Phase 2/3 study offering operational efficiencies. Seres plans to submit the revised protocol for review in the second quarter, marking a critical step toward a clear regulatory path for advancing SER‑155 in this vulnerable patient population.
Read Announcement- Drug:
- SER-155
- Announced Date:
- December 9, 2024
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, Inc. announced that the US Food and Drug Administration (FDA) has granted Breakthrough Therapy designation to SER-155, the Company's lead investigational program, for the reduction of bloodstream infections (BSIs) in adults undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT) for the treatment of hematological malignancies.
AI Summary
Seres Therapeutics recently announced that the FDA has granted Breakthrough Therapy designation to its lead program, SER-155, which is being investigated to reduce bloodstream infections (BSIs) in adults undergoing allogeneic hematopoietic stem cell transplants for hematological malignancies. This designation comes on the back of promising Phase 1b results, where SER-155 led to a 77% relative risk reduction in BSIs compared to placebo. The encouraging clinical data also showed a decrease in systemic antibiotic use and a lower incidence of febrile neutropenia, supporting the potential of SER-155 to address a serious complication in cancer treatment.
In light of this designation, Seres plans to meet with the FDA in early 2025 to discuss the design of the next clinical study. The company is also exploring strategic partnerships to accelerate the study and further expand the potential use of SER-155 in other high-risk patient populations.
Read Announcement- Drug:
- SER-155
- Announced Date:
- September 12, 2024
- Indication:
- Prevention of Antibiotic-Resistant Bacterial Infections and Graft-versus-Host Disease (GvHD)
Announcement
Seres Therapeutics, Inc. today reports topline clinical data from Cohort 2 of its SER-155 Phase 1b placebo-controlled study in patients undergoing allo-HSCT. In this patient population, infections are frequent, severe and often life-threatening.
AI Summary
Seres Therapeutics, Inc. announced topline clinical data from Cohort 2 of its SER-155 Phase 1b placebo-controlled study in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). In this high-risk group, infections are frequent, severe, and often life-threatening. The study found that SER-155 led to a significant reduction in bacterial bloodstream infections (BSIs) and decreased systemic antibiotic use compared to placebo. Additionally, patients receiving SER-155 experienced a lower incidence of febrile neutropenia, and the treatment was generally well tolerated with successful engraftment of the bacteria strains.
Based on these promising results, Seres plans to engage with the FDA to seek Breakthrough Therapy designation. This designation could speed up the development and approval process for SER-155 as an important treatment option to address the urgent need for better infection prevention in allo-HSCT patients.
Read Announcement
VOWST FDA Regulatory Events
VOWST is a drug developed by Seres Therapeutics for the following indication: To prevent the recurrence of Clostridioides difficile infection (CDI) in individuals 18 years of age and older following antibacterial treatment for recurrent CDI (rCDI).
This drug is under review by the U.S. Food and Drug Administration (FDA).
Below is a timeline of key regulatory milestones for this therapy.
- Drug:
- VOWST
- Announced Date:
- January 6, 2026
- Indication:
- To prevent the recurrence of Clostridioides difficile infection (CDI) in individuals 18 years of age and older following antibacterial treatment for recurrent CDI (rCDI).
Announcement
Seres Therapeutics, Inc. announced the publication of two manuscripts in Nature Medicine and the Journal of Infectious Diseases highlighting new insights into the functional mechanism and clinical impact of VOWST™.
AI Summary
Seres Therapeutics announced two manuscripts in Nature Medicine and the Journal of Infectious Diseases that clarify how VOWST works and its clinical effects. The Nature Medicine study found higher VOWST dosing led to faster, stronger engraftment of therapeutic bacteria in the gut and large shifts in the intestinal microbiome and metabolites. Key changes included lower primary and higher secondary bile acids and increased short- and medium-chain fatty acids—metabolic shifts that block C. difficile spore germination and growth. In vitro tests showed VOWST batches produce these metabolites, supporting the idea that the therapy restores microbe-linked functions that help prevent CDI recurrence.
The Journal of Infectious Diseases paper analyzed ECOSPOR IV data and found similar microbiome and bile acid profiles in patients with a first recurrence and those with multiple recurrences, with VOWST showing comparable pharmacology and effects across both groups. Seres said these results, generated using its MbTx platform, inform development of next-generation live biotherapeutics such as SER-155.
Read Announcement
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