Seres Therapeutics Presents New Preclinical Data Supporting the Development of SER-401 for Immuno-Oncology at the 2019 American Association for Cancer Research Annual Meeting

Seres Therapeutics, Inc. MCRB today announced that new preclinical data supporting the development of microbiome therapeutics for immuno-oncology (poster title "Leveraging gut microbiota networks to impact tumor immunotherapy")¹ will be presented by Sceneay et al. at the 2019 American Association for Cancer Research Annual Meeting (AACR) in Atlanta, Georgia. The data presented provide new insights on the potential mechanism by which Seres' microbiome therapies, including the Company's SER-401 program, could improve outcomes of cancer patients treated with immune checkpoint inhibitors.

"The novel data presented at AACR provide mechanistic insights into the biological activity of our microbiome therapeutic candidates to augment immune checkpoint inhibitors," said Matthew Henn, Ph.D., Chief Scientific Officer and Executive Vice President at Seres. "The valuable learnings obtained from this work have informed the development of SER-401, which is being evaluated in combination with an FDA approved immunotherapy in the ongoing MCGRAW Phase 1b study in patients with metastatic melanoma."

Seres presented preclinical results evaluating the impact of various consortia of bacterial species on the anti-tumor immune response in murine models following treatment with an anti-PD-1 checkpoint inhibitor. Results demonstrated that germ-free or antibiotic-treated mice lacking a functional gastrointestinal microbiome failed to mount an effective anti-tumor response when administered anti-PD-1 checkpoint inhibitor treatment. The response to anti-PD-1 treatment was restored by the introduction of a specific consortia of commensal bacteria rationally designed using insights from both in vivo and human microbiome signatures of response and the functional properties of specific bacterial strains in Seres' strain library of gastrointestinal bacteria.

These data provide support for the continued development of SER-401, an oral microbiome therapeutic candidate sourced from screened healthy individuals who have been identified to have a microbiome bacterial signature similar to that observed in responders to cancer immunotherapy. The therapeutic aim of SER-401 is to modify the microbiome of cancer patients to increase the efficacy of immunotherapy. SER-401 is being evaluated in a Phase 1b clinical study (NCT03817125) conducted in collaboration with The University of Texas MD Anderson Cancer Center and the Parker Institute for Cancer Immunotherapy in patients with metastatic melanoma. In addition, Seres has an ongoing collaboration with AstraZeneca to advance the mechanistic understanding of the microbiome in augmenting the efficacy of cancer immunotherapy, including in combination with agents in AstraZeneca's oncology pipeline.

About Seres Therapeutics

Seres Therapeutics, Inc. MCRB is a leading microbiome therapeutics platform company developing a novel class of biological drugs that are designed to treat disease by restoring the function of a dysbiotic microbiome, where the state of bacterial diversity and function is imbalanced. Seres' most advanced program, SER-109, has obtained Breakthrough Therapy and Orphan Drug designations from the U.S. Food and Drug Administration and is in Phase 3 development for recurrent C. difficile infection. SER-287 is being evaluated in a Phase 2b study in patients with active mild-to-moderate ulcerative colitis. Seres is also developing SER-401 in a Phase 1b study in patients with metastatic melanoma. 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 potential impact of SER-401, the timing and results of any collaborations or clinical studies, Seres' plans for pre-clinical development, and other statements that are not historical facts.

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; 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; the success of our leadership transition; our ability to retain key personnel and to manage our growth; and our management and principal stockholders have the ability to control or significantly influence our business. 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 6, 2019 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.

Reference

1. Sceneay, Jaclyn et. al., Leveraging gut microbiota networks to impact tumor immunotherapy. Session title "Inflammation and microbiome"; section 4, abstract 2839. American Association for Cancer Research (AACR) Annual Meeting 2019.

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