Avelumab

80% of SER-155 recipients achieved immunosuppressive-free clinical response of diarrhea (a primary symptom of irEC) at Day 15 in the Investigator-Sponsored Trial (IST) conducted by Memorial Sloan Kettering Cancer Center (MSK) SER-155 has potential to treat irEC, a frequent side effect of immune checkpoint inhibitors (ICIs) that often causes halting of cancer treatment, without requiring treatment with systemic immunosuppressive drugs SER-155 was well tolerated, with no drug-related serious adverse events observed Drug pharmacology data provide confirmatory evidence of Seres’ live biotherapeutics operating as designed to repair the mucosal epithelial barrier and reduce gastrointestinal inflammation, which supports possible expansion into multiple other inflammatory and immune diseases Company to host webcast today, July 8, at 8:30 am ET CAMBRIDGE, Mass., July 08, 2026 (GLOBE NEWSWIRE) -- Seres Therapeutics, Inc. (Nasdaq: MCRB), (Seres or the Company), a leading live biotherapeutics company, today announced encouraging top-line results from an IST evaluating SER-155 in patients with irEC. The study (NCT06801067), conducted at MSK, by Principal Investigator David Faleck, M.D., Director of Inflammatory and Immune-Related Bowel Diseases, evaluated SER-155 in 15 patients with moderate-to-severe (Grade 2- 3) irEC, which affects approximately 25% of all immune checkpoint inhibitor (ICI) therapy recipients in the US. Clinical guidelines for these patients stipulate halting of ICI cancer treatment and initiation of systemic immunosuppressive corticosteroids, which are associated with clinically meaningful toxicities and other negative consequences, including: systemic immunosuppression, increased infection risk, metabolic complications, and potential interference with cancer treatment. There remains a significant unmet need for systemic immunosuppressive-free therapeutic approaches that reduce gastrointestinal inflammation and diarrhea and may allow patients who develop irEC to avoid disruption of their life-saving ICI cancer treatment. Participants in the study were on a wide range of ICI types, including PD-1 inhibitors (Keytruda ® , Opdivo ® , Zynz ® ), PD-L1 inhibitors (Imfinzi ® , Bavencio ® ), CTLA-4 inhibitors (Yervoy ® , Imjudo ® ), LAG-3 inhibitor (Opdualag ® ), and combinations thereof. The promising study results support continued development of SER-155 to treat irEC. Topline Clinical Results Day 15 Data Following SER-155 dosing, 12 of 15 participants (80%) achieved an immunosuppressive-free clinical response at Day 15, the primary endpoint, defined as at least a 1-grade improvement in diarrhea symptoms without immunosuppressive therapy (i.e., a corticosteroid and/or biologic immunomodulator drug). Notably, 8 of these 12 responders (67%) achieved greater than or equal to a 2-grade improvement in diarrhea symptoms without immunosuppressive therapy at Day 15. At Day 15, 5 of 15 participants (33%) achieved immunosuppressive-free complete clinical remission, defined as total resolution of diarrhea symptoms to Grade 0, without immunosuppressive therapy. Day 43 Data At Day 43, 5 of 15 participants (33%) maintained immunosuppressive-free clinical response, and, notably, 2 of 15 participants (13%) maintained immunosuppressive-free complete clinical remission. All 12 participants with immunosuppressive-free clinical response at Day 15 had the same, or better, diarrhea grade at Day 43. As noted, 5 maintained immunosuppressive-free clinical response at Day 43. The other 7 were treated with non-systemically acting, gastrointestinal-targeted immunosuppressives after Day 15. Safety SER-155 was generally well tolerated through Day 43, with no safety concerns identified, consistent with the outcome in Seres’ Phase 1b study of SER-155 in participants undergoing allogeneic hematopoietic cell transplantation (allo-HCT). There were no serious adverse events assessed as related to SER-155 and no bloodstream infections reported in the study through Day 43. Two participants experienced 2 non-serious adverse events each, assessed as possibly related to vancomycin and SER-155. All 4 adverse events were moderate in severity and were resolved. Pharmacology SER-155 bacterial strain engraftment (a measure of drug pharmacokinetics and strain growth in the gastrointestinal tract) was robust, with the majority of SER-155 strains observed across the study participants, and with kinetics, magnitude, and durability comparable to that observed in prior Seres clinical studies. Translational biomarkers of gastrointestinal inflammation (fecal calprotectin) and of mucosal epithelial barrier integrity (fecal albumin), which were elevated in participants at baseline, prior to SER-155 dosing, both showed decreases post SER-155 treatment, with statistically significant reductions achieved by Day 43, reflecting improvement in two mechanistically linked dimensions of irEC disease. Data suggest that administration of SER-155 led to improvement in mucosal epithelial barrier integrity, providing support for potential further development in multiple other inflammatory and immune diseases. “We are extremely pleased with the study data from our long-time collaborator, MSK, which highlight SER-155’s potential as a non-immunosuppressive treatment for irEC, which could enable patients to continue their ICI cancer treatment,” said Richard Kender, Executive Chairman and Interim CEO of Seres. “We are engaging with potential partners, including companies with ICI franchises, as we evaluate next steps for the development of SER-155 in irEC. We also continue to engage with potential collaborators and financing partners as we seek funding to advance the Phase 2 study of SER-155 in allo-HCT, and development in our broader pipeline.” Matthew Henn, Ph.D., President and Chief Scientific Officer of Seres added, “These clinical and pharmacological results underscore the potential of SER-155 to address mucosal epithelial barrier dysfunction and gastrointestinal inflammation through disease pathways not targeted by existing therapies. Beyond irEC, we believe these encouraging data support continued development of Seres’ live biotherapeutic platform to target microbe-specific functions linked to multiple inflammatory and immune diseases in addition to irEC, such as ulcerative colitis and Crohn’s disease.” “Patients receiving ICI treatment who develop irEC face significant challenges, as current management often necessitates interruption or discontinuation of checkpoint inhibitor therapy and treatment with systemic immunosuppressives, which are associated with clinically significant side effects,” commented Dr. Faleck. “These results suggest SER-155 may offer a promising, novel approach to managing irEC, with the potential to reduce reliance on systemic immunosuppressive therapies, while also helping patients continue ICI treatment. We look forward to continued development to further understand the role microbiome therapeutics could play in this setting and the broader opportunity to support the growing number of patients receiving checkpoint inhibitors across multiple cancer types. We intend to present the IST results at a future medical conference.” Jonathan Peled, M.D., Ph.D., Bone Marrow Transplant Specialist and Cellular Therapist, MSK, noted, “MSK has partnered with Seres for over a decade, advancing research aimed at improving outcomes for cancer patients. As an investigator in Seres’ Phase 1b study of SER-155 in allo-HCT, I observed the meaningful reduction in bloodstream infections in patients receiving SER-155. I am now further encouraged by the results of SER-155 in patients with irEC.” Webcast Information Seres’ management will host a webcast today, July 8, 2026, at 8:30 a.m. ET. To listen to the audio portion of the webcast, please dial 800-715-9871 (domestic) or 646-307-1963 (international) and reference the conference ID number 3327556. To join the live webcast, please visit the “Events and Presentations” tab within the “Investors and News” section of the Seres website at . A replay will be available on the Seres website after the event and will be archived for at least 21 days. About the Investigator-Sponsored Trial (IST) (NCT06801067) This investigator-sponsored, open-label trial evaluated SER-155 in 15 participants with Grade 2-3 (moderate to severe) irEC who were naïve to immunosuppressive therapy. Participants received microbiome conditioning with oral vancomycin on Days 1 and 2, followed by oral administration of SER-155 (2 capsules per day) for 12 consecutive days. The primary efficacy endpoint was the proportion of participants achieving an immunosuppressive-free clinical response on Day 15, defined as at least a 1-grade improvement in diarrhea symptoms, without the use of immunosuppressive therapy. The study is also evaluating the safety, tolerability, and drug pharmacology of SER-155. The IST is designed to inform if SER-155 may provide an alternate treatment for irEC with the potential to prevent interruption of the patient’s ICI cancer treatment. Participants in the study were on a wide range of ICI types, including PD-1 inhibitors (Keytruda ® , Opdivo ® , Zynyz ® ), PD-L1 inhibitors (Imfinzi ® , Bavencio ® ), CTLA-4 inhibitors (Yervoy ® , Imjudo ® ), LAG-3 inhibitor (Opdualag ® ), and combinations thereof. All ICIs administered include irEC as adverse reactions in their labels. About Immune Checkpoint Inhibitors (ICIs) ICIs have revolutionized cancer treatment across multiple tumor types. ICIs represent one of the largest and fastest-growing segments of the oncology market, with the worldwide market valued at well over $50 billion in 2025. Well known ICI brands include: Keytruda ® , Opdivo ® and Yervoy ® . ICIs work by blocking "checkpoint" proteins—such as PD-1, PD-L1, and CTLA-4—that tumor cells exploit to hide from and inactivate T-cells, effectively turning the immune system's attack mode back on. The amplified immune response can also attack healthy tissue, causing serious immune-related adverse events, such as immune checkpoint inhibitor-related enterocolitis (irEC). About Immune checkpoint inhibitor-related enterocolitis (irEC) IrEC, also called immune-mediated colitis, is among the most frequent and severe immune-related adverse reactions in ICI recipients. irEC can result in diarrhea, gastrointestinal inflammation, abdominal pain, blood in stool, among other symptoms, and may significantly affect quality of life and can progress to serious and life-threatening consequences. irEC has been linked to hospitalizations with longer lengths of stay, higher ICU use, high rehospitalization rates and mortality. Current standard of care for those who suffer from moderate to severe irEC (in the US this approximates 25% of all ICI recipients) requires halting their ICI cancer therapy and initiating a systemic immunosuppressive corticosteroid and/or biologic, further compromising the patient’s immune system. There are numerous toxicities associated with systemic immunosuppressives, and the immunosuppression may negate the anti-cancer effect of the ICI therapy, putting patient cancer outcomes at risk. A significant unmet need remains for systemic immunosuppressive-free therapeutic approaches that reduce gastrointestinal inflammation and diarrhea and may allow patients who develop irEC to avoid disruption of their ICI therapy. About Seres Therapeutics Seres Therapeutics, Inc. (Nasdaq: MCRB) is a clinical-stage biotechnology company developing novel live biotherapeutics, with a focus on therapies designed to allow patients to maintain their cancer therapy when faced with significant side effects, and that treat inflammatory and immune diseases. The Company led the development and FDA approval of VOWST™, the first orally administered microbiome therapeutic, which was subsequently divested to Nestlé Health Science. SER-155, a cultivated multi-strain biotherapeutic, which has received Breakthrough Therapy and Fast Track designations, is being advanced for the prevention of bloodstream infections in patients undergoing allogeneic hematopoietic stem cell transplant (allo-HCT), and is Phase 2 ready, pending receipt of funding. An investigator-sponsored trial of SER-155 in immune checkpoint inhibitor–related enterocolitis (irEC) to further evaluate the potential breadth of the Company’s live biotherapeutic platform is ongoing. The Company is advancing IND-enabling studies for SER-603, which is in development for inflammatory bowel disease. Mechanistically, Seres’ biotherapeutics target the mucosal epithelial barrier-immune interface and are optimized to modulate host function to increase epithelium integrity, induce immune homeostasis, and prevent the colonization and overgrowth of harmful bacteria in the gastrointestinal tract. For more information, please visit . 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 statements about: SER-155 and its intended uses and benefits in irEC; our clinical development plans for SER-155 and SER-603; potential accessibility for patients; the timing and results of clinical studies and data readouts; current or future product candidates and their potential impacts and outcomes; engagement with potential partners and financing sources; our ability to access capital to advance our programs; our planned strategic focus; the anticipated timing of any of the foregoing; and other statements that are not historical fact. 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: (1) our need for additional funding; (2) our ability to continue as a going concern; (3) we have incurred significant losses, are not currently profitable and may never become profitable; (4) our cost reduction actions may not achieve their intended benefits, including an extended cash runway; (5) our limited operating history; (6) we may not be able to realize the anticipated benefits of the VOWST sale, and may face new challenges as a smaller, less diversified company; (7) we have in the past and may in the future receive notice of the failure to satisfy a continued listing rule from The Nasdaq Stock Market LLC; (8) our novel approach to therapeutic intervention; (9) our reliance on third parties to conduct our clinical trials and manufacture our product candidates; (10) our ability to achieve market acceptance necessary for commercial success; (11) the competition we will face; (12) our ability to protect our intellectual property; (13) impact of our recent management transitions and appointments and our ability to retain key personnel; and (14) disruptions at the FDA or other government agencies. These and other important factors discussed under the caption “Risk Factors” in our Quarterly Report on Form 10-Q for the quarterly period ended May 5, 2026, as well as 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, except as required by law, 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. Investor and Media Contacts: IR@serestherapeutics.com Carlo Tanzi, Ph.D. Kendall Investor Relations ctanzi@kendallir.com

33% ORR and 67% disease control observed with imneskibart triplet in metastatic melanoma following confirmed progression on potent doublet checkpoint inhibitor therapy Durable responses, Treg reduction and increased CD8/Treg ratio support imneskibart’s differentiated IL-2 mechanism Well-tolerated outpatient regimen shows no treatment discontinuations due to drug-related adverse events Phase 2 enrollment ongoing in melanoma and PD-L1+ advanced NSCLC LARKSPUR, Calif.--(BUSINESS WIRE)--Aulos Bioscience, a clinical-stage immuno-oncology company developing an immune-activating antibody therapeutic designed by leveraging an AI platform, today announced positive Phase 2 data from its ongoing Phase 1/2 study of imneskibart in patients with doublet checkpoint inhibitor (CPI)-refractory metastatic melanoma. The data will be presented in a poster session at the American Society of Clinical Oncology (ASCO) 2026 Annual Meeting in Chicago, Illinois. The Phase 2 data show that imneskibart-based outpatient regimens are well tolerated and demonstrate durable clinical activity, including ongoing and deepening responses. Patients received either imneskibart plus low-dose, subcutaneous aldesleukin or the triplet regimen of imneskibart plus low-dose, subcutaneous aldesleukin and nivolumab. Across the study, imneskibart treatment was associated with sustained regulatory T cell (Treg) reduction, increased CD8/Treg ratios and evidence of durable anti-tumor activity, supporting its differentiated IL-2 mechanism. “Imneskibart’s triplet regimen produced a 33% objective response rate and 67% disease control rate in metastatic melanoma patients whose disease progressed following earlier treatment with potent checkpoint inhibitor doublets,” said Aron Knickerbocker, Aulos Bioscience’s President and Chief Executive Officer . “These patients urgently need safe, life-extending treatment options, particularly those who are not eligible for or do not receive TIL therapy. While li TIL therapy is the only FDA-approved product specifically indicated in the post-PD-1 setting, a substantial treatment gap remains. In addition to durable efficacy consistent with anti-tumor immune memory formation, imneskibart continues to demonstrate a differentiated, potentially best-in-class safety profile. These findings strongly support imneskibart’s mechanism and its ability to make a meaningful difference for patients.” As of the April 6, 2026 data cutoff, data were available from 83 patients across the Phase 1/2 study, including 12 patients in the Phase 2 melanoma triplet cohort evaluable for response: Clinically meaningful activity observed with imneskibart triplet in doublet CPI-refractory melanoma 33% objective response rate (ORR) in confirmed doublet CPI-refractory cutaneous melanoma patients following prior anti-PD-1/CTLA-4 or anti-PD-1/LAG-3 therapy. Four of 12 patients achieved partial responses, with target tumor reductions of 52%, 65%, 74% and 77%. Four of 12 patients achieved stable disease, resulting in a 67% disease control rate. Responses are ongoing and deepening, consistent with the potential generation of de novo anti-tumor immunity and immune memory formation; progression-free survival (PFS) and overall survival (OS) data continue to mature. Strong signal of anti-tumor activity in imneskibart doublet regimen, with ongoing deep and durable tumor reductions in patients who progressed after receiving doublet CPI therapy (prior anti-PD-1 and anti-CTLA-4 and/or anti-PD-1 and anti-LAG-3) 14 evaluable patients received the imneskibart doublet regimen. Three patients with the deepest target tumor reductions continued treatment beyond one year: one patient with a 48% tumor reduction continued on treatment for 13 months; another patient with a 58% reduction in measurable non-target tumors continued on treatment for 18.5 months; and a third patient with a complete response (100% reduction) in the target lesions continues on treatment for more than 25 months. Imneskibart and low-dose, subcutaneous aldesleukin, with or without nivolumab, exhibits unique pharmacodynamic (PD), biological and pharmacokinetic (PK) effects in the IL-2 class, with a higher peripheral blood CD8/Treg ratio correlating with increased survival Durable increases in CD8+ T cells and CD8/Treg ratios were observed in patients, with a corresponding decrease in Tregs. PD data support hypothesis of selective effector cell expansion, validating imneskibart’s mechanism of action of redirecting interleukin-2 (IL-2) away from trimeric IL-2 receptors (expressed on Tregs and vasculature) and toward dimeric IL-2 receptors (on CD8+ T effector and natural killer cells). The data show that imneskibart demonstrates durable activity coupled with a persistent reduction in Tregs and a higher CD8/Treg ratio that is associated with longer OS, PFS and time on treatment for patients. PK data demonstrate sustained, selective signaling with a half-life of greater than 19 days, enabling potent immune activation without Treg expansion or high-dose IL-2 toxicities such as vascular leak syndrome or pulmonary edema. Well-tolerated triplet regimen, with no apparent increase in Grade 3/4 adverse events following addition of nivolumab Most drug-related adverse events were Grade 1 or 2; no patient discontinued treatment due to a drug-related adverse event. The emerging safety pro the triplet regimen is consistent with the known safety profiles of nivolumab and imneskibart plus aldesleukin, with no new toxicity signals observed. Enrollment is complete in the Phase 2 doublet cohort evaluating imneskibart plus a single loading dose of low-dose, subcutaneous aldesleukin in patients with unresectable locally advanced or metastatic cutaneous melanoma following confirmed progression on doublet CPI therapy. Enrollment continues in the Phase 2 triplet cohort evaluating imneskibart plus low-dose, subcutaneous aldesleukin and nivolumab in approximately 20 evaluable second-line cutaneous melanoma patients, with plans advancing toward potential registrational development. Two ongoing Phase 2 cohorts are evaluating imneskibart and low-dose, subcutaneous aldesleukin administered without and with avelumab (anti-PD-L1 with an active Fc domain and ADCC effector function) in patients with advanced PD-L1+ non-small cell lung cancer (NSCLC) that progressed on prior CPI therapy (with or without chemotherapy). Aulos anticipates presenting comprehensive clinical data from the NSCLC Phase 2 cohorts by year-end. The poster, “Imneskibart + low-dose subcutaneous IL-2 ± nivolumab in patients with CPI-refractory cutaneous melanoma: Promising results from an ongoing phase 1/2 study,” (Abstract 9526) will be presented live in the poster session “Melanoma/Skin Cancers” in the Exhibit Hall at McCormick Place on Sunday, May 31, 2026, 9:00 a.m. to 12:00 p.m. CDT. The poster will also be accessible to meeting registrants as an electronic poster on the ASCO online meeting platform. To learn more about the imneskibart clinical trial program, please visit ClinicalTrials.gov (identifier: NCT05267626 ). For patients and providers in the U.S., please visit . For patients and health professionals in Australia, please visit . About Imneskibart Imneskibart (AU-007) is a human IgG1 monoclonal antibody designed by leveraging artificial intelligence that is highly selective to the CD25-binding portion of IL-2. With a mechanism of action unlike any other IL-2 therapeutic in development, imneskibart redirects IL-2 to reinforce anti-tumor immune effects. This is achieved by preventing IL-2, either exogenous or secreted by effector T cells, from binding to trimeric receptors on regulatory T cells while still allowing IL-2 to bind and expand effector T cells and NK cells. This prevents the negative feedback loop caused by other IL-2-based treatments and biases the immune system toward activation over suppression. Imneskibart also prevents IL-2 from binding to CD25-containing receptors on eosinophils, as well as vasculature and pulmonary endothelium, which may significantly reduce the vascular leak syndrome and pulmonary edema associated with high-dose IL-2 therapy. About Aulos Aulos Bioscience is an immuno-oncology company working to revolutionize cancer patient care through immune-activating antibody therapeutics that direct patients’ immune systems toward killing tumor cells. Matching world-class machine learning from co-founder Biolojic Design with an in-depth understanding of the immune system, Aulos’ initial clinical candidate, imneskibart (AU-007), is a human antibody designed by leveraging artificial intelligence that harnesses the power of IL-2 to induce tumor killing while limiting the immunosuppression and toxicities typically associated with this validated pathway. The company was founded by Biolojic Design and Apple Tree Partners ( ATP ) and is led by pioneers in the fields of artificial intelligence, antibody development and cancer immunotherapies. For more information, visit , X ( @AulosBioscience ) and LinkedIn . Contacts info@aulosbio.com Media inquiries: Mike Beyer, Sam Brown LLC / 312-961-2502 / mikebeyer@sambrown.com