– Preclinical data demonstrate SER-155 reduces infection and Graft-versus-Host Disease –

Seres Therapeutics, Inc. (Nasdaq: MCRB), a leading microbiome therapeutics company, will present preclinical findings tomorrow at the European Bone Marrow Transplantation (EBMT) Annual Meeting that suggest SER-155 works by targeting the host immune response, potentially allowing patients undergoing hematopoietic stem cell transplantation (HSCT) to avoid infection or Graft-versus-Host Disease (GvHD), which currently afflict about half of this patient population. SER-155 is being evaluated in an ongoing Phase 1b clinical trial.

“Microbiome-based therapeutics have the potential to protect medically compromised transplant patients from life-threatening infections and GvHD as they start on their road to recovery,” said Matthew Henn, PhD, Chief Scientific Officer at Seres and co-author. “Building on scientific and clinical data from our SER-109 program, we are encouraged by these preclinical findings that bolster our confidence in combating additional important infectious diseases and the continued threat of antimicrobial resistance with further development of SER-155 as a potentially transformative new medicine.”

These results will appear on an e-poster (p194) and will be presented at 9:50 am CET Saturday, March 19, by Elizabeth Halvorsen, PhD, Director of Translational Sciences at Seres.

Across a variety of preclinical models, SER-155 appears to work by reducing gastrointestinal (GI) inflammation, fortifying the protective GI lining, and promoting immune cell balance in the gut. In cultured human cells designed to mimic the lining of the intestines, SER-155 protected the barrier between the colon and invading pathogens from inflammatory damage. In colon organoids – microscopic 3D collections of human cells that function as a tiny colon – SER-155 blocked several key inflammatory gene expression pathways linked with GvHD. In mice, SER-155 increased the ratio of regulatory T cells to inflammatory Th1 and Th17 effector T cells, which could potentially reduce the risk and severity of GvHD. Together, these platforms allow the researchers to pinpoint the potential immunomodulatory effects of gut microbes introduced by SER-155.

The SER-155 Phase 1b trial (ClinicalTrials.gov identifier: NCT04995653), which is being performed in collaboration with Memorial Sloan Kettering Cancer Center and the University of Chicago, is a randomized, double-blind, placebo-controlled, multiple-dose, multicenter study designed to evaluate the safety and efficacy of SER-155 following vancomycin treatment among adult patients undergoing HSCT from a genetically non-identical donor.

According to published clinical data from Memorial Sloan Kettering, patients undergoing this type of transplant who have less diverse gut flora are significantly more likely than patients with a richer microbiome to experience infection or GvHD. The trial will help inform whether SER-155 can restore a healthy microbiome and support transplant recovery.

About SER-155

SER-155, an oral consortium of cultivated bacteria, is a microbiome therapeutic candidate in clinical development. SER-155 is designed using microbiome biomarker data from human clinical data, human cell-based assays, and in vivo disease models, with the aim to decrease infection and translocation of antibiotic-resistant bacteria in the gastrointestinal tract and modulate host immune responses to decrease GvHD. The rationale for this program is based in part on published clinical evidence from Seres’ collaborators at Memorial Sloan Kettering Cancer Center showing that allogeneic HSCT patients with decreased diversity of commensal microbes are significantly more likely to die due to infection and/or lethal GvHD. SER-155 was developed using Seres’ reverse translational discovery platform to potentially reduce incidences of gastrointestinal infections, bloodstream infections and GvHD in immunocompromised patients, including in patients receiving allogeneic HSCT or solid organ transplants.

About Seres Therapeutics

Seres Therapeutics, Inc. (Nasdaq: MCRB) is a leading microbiome therapeutics company developing a novel class of multifunctional bacterial consortia that are designed to functionally interact with host cells and tissues to treat disease. Seres’ SER-109 program achieved the first-ever positive pivotal clinical results for a targeted microbiome drug candidate and has obtained Breakthrough Therapy and Orphan Drug designations from the FDA. The SER-109 program is being advanced to reduce the recurrence of C. difficile infection and has potential to become a first-in-class FDA-approved microbiome therapeutic. Seres is evaluating SER-301 in a Phase 1b study in patients with ulcerative colitis and SER-155 in a Phase 1b study in patients receiving allogeneic hematopoietic stem cell transplantation to reduce incidences of gastrointestinal infections, bloodstream infections and graft-versus-host disease as well as additional preclinical stage programs targeting Infection Protection in medically compromised patients.

For more information, please visit www.serestherapeutics.com.

Forward-Looking Statements

This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. All statements contained in this press release that do not relate to matters of historical fact should be considered forward-looking statements, including the promise and potential impact of microbiome therapeutics, the ability of SER-155 to modulate host immune response or reduce death, the mechanism of action of SER-155, and the possibility that microbiome therapeutics may change the standard of car for any diseases.

These forward-looking statements are based on management’s current expectations. These statements are neither promises nor guarantees, but involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements, including, but not limited to, the following: we have incurred significant losses, are not currently profitable and may never become profitable; our need for additional funding; our limited operating history; the impact of the COVID-19 pandemic; our unproven approach to therapeutic intervention; the lengthy, expensive and uncertain process of clinical drug development; our reliance on third parties and collaborators to conduct our clinical trials, manufacture our product candidates and develop and commercialize our product candidates, if approved; and our ability to retain key personnel and to manage our growth. These and other important factors discussed under the caption “Risk Factors” in our Annual Report on Form 10-K filed with the Securities and Exchange Commission, or SEC, on March 1, 2022, and our other reports filed with the SEC could cause actual results to differ materially from those indicated by the forward-looking statements made in this press release. Any such forward-looking statements represent management’s estimates as of the date of this press release. While we may elect to update such forward-looking statements at some point in the future, we disclaim any obligation to do so, even if subsequent events cause our views to change. These forward-looking statements should not be relied upon as representing our views as of any date subsequent to the date of this press release.

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