• Cohort 4 follow-up confirms durable hepatitis B surface antigen (HBsAg) loss at 24 weeks post end of treatment (EOT) in BRII-179 anti-HBs responders treated with elebsiran plus pegylated interferon alfa (PEG-IFNα)
• Data suggest the potential to shorten PEG-IFNα treatment duration for BRII-179 anti-HBs responders showing faster and greater HBsAg reduction

DURHAM, N.C. and BEIJING, Nov. 10, 2025 -- Brii Biosciences Limited ("Brii Bio," or the "Company", stock code: 2137.HK), a biotechnology company developing therapies to improve patient health and choice across diseases with high unmet medical needs, announced 24-week post end of treatment (EOT) follow-up results from Cohort 4 of its ongoing Phase 2 ENSURE study. These late-breaking data were presented at the American Association for the Study of Liver Diseases (AASLD) The Liver Meeting®, held in Washington D.C. from November 7-11, 2025, and were simultaneously published in Nature Medicine (link).

ENSURE (NCT05970289) is a multicenter, open-label Phase 2 study in the Asia-Pacific region. Cohort 4 evaluates a novel sequential combination treatment strategy for hepatitis B virus (HBV), utilizing Brii Bio's therapeutic vaccine, BRII-179, to prime and enrich patients, with a goal to improve responsiveness to therapies potentially offering a functional cure. Participants previously treated with nine doses of elebsiran and BRII-179 treatment in a prior Phase 2 study BRII-179-835-001 (NCT04749368) were enrolled to this cohort to receive 48 weeks of combination treatment with elebsiran and PEG-IFNα. These participants were categorized as BRII-179 anti-HBs responders or non-responders based on their peak hepatitis B surface antibody (anti-HBs) levels during the prior study (≥10 IU/L or <10 IU/L, respectively).

EOT data from Cohort 4 were previously presented at the European Association for the Study of the Liver (EASL) Congress 2025. The newly reported data confirm that the HBsAg loss benefit in the BRII-179 responder group was sustained over time. At EOT, 58% (11/19) HBsAg loss was observed in BRII-179 anti-HBs responders. In anti-HBs non-responders, only 17% (2/12) achieved HBsAg loss. At 24 weeks post EOT, 42% (8/19) of anti-HBs responders maintained HBsAg loss compared to 8% (1/12) of anti-HBs non-responders. 50% (4/8) of anti-HBs responders with sustained HBsAg loss at 24 weeks post EOT had baseline HBsAg levels between 1,514-3,086 IU/mL when enrolled in the previous BRII-179-835-001 study suggesting BRII-179 may elicit anti-HBs responses even in patients with higher baseline surface antigen levels. This supports BRII-179's potential to improve functional cure outcomes and broaden its applicability across diverse patient populations.

To further define BRII-179's role in treatment of HBV infection and optimize combination regimens for pivotal studies, Brii Bio is conducting two additional Phase 2b trials. The ENRICH study evaluates the role of BRII-179 in priming HBV-specific immunity and/or identifying immuno-responsive patients with a higher likelihood of achieving functional cure. The ENHANCE study includes two parts: one to evaluate the triple combination of BRII-179, elebsiran and PEG-IFNα given concurrently for 48 weeks, and another exploring a sequential approach of BRII-179 and elebsiran for 24 weeks followed by triple combination for 24 weeks, and a 48-week PEG-IFNα control arm. Both studies are fully enrolled, and EOT data are expected to be presented in 2026.

"We are encouraged to see that the difference in HBsAg seroclearance rates observed at EOT was maintained through 24 weeks of follow-up." said David Margolis, MD, Chief Medical Officer of Brii Bio. "These results support BRII-179's potential to deliver more rapid and durable HBsAg loss and potentially enable a shorter treatment duration of PEG-IFNα. We look forward to validating these findings in our ongoing confirmatory studies."

Publication Number: 5036

Title: Chronic hepatitis B virus infected participants responding to prior BRII-179 treatment achieved higher rate of sustained hepatitis B virus surface antigen loss on elebsiran plus pegylated interferon alfa: follow-up data from ENSURE study

Presenter: Grace Lai-Hung Wong, MBChB (CUHK), MD (CUHK), FRCP (Lond, Edin), FHKCP, FHKAM (Medicine), Professor of Gastroenterology and Hepatology at CUHK Medical Data Analytics Centre (MDAC) and Department of Medicine and Therapeutics in Hong Kong SAR, China

• A total of 31 participants were analyzed, with 19 BRII-179-induced anti-HBs responders and 12 non-responders.
• Median [range] HBsAg at baseline of this ENSURE study was numerically higher in anti-HBs responders (520 [34-2165] IU/mL) than in non-responders (185 [51-672] IU/mL).
• At 24 weeks post EOT, a notably higher rate of HBsAg loss was achieved in BRII-179-induced anti-HBs responders (8/19, 42.1%) compared to non-responders (1/12, 8.3%).
• Although all participants with sustained HBsAg loss had HBsAg < 1,500 IU/mL at baseline of this ENSURE study, 4/8 (50.0%) from anti-HBs responders had baseline HBsAg ranging from 1,514-3,086 IU/mL when enrolled in the previous study, suggesting that BRII-179 may induce anti-HBs responses regardless of baseline HBsAg level.
• The elebsiran and PEG-IFNα combination was generally safe and well-tolerated.

About Hepatitis B

Hepatitis B virus (HBV) infection is one of the world's most significant infectious disease threats with more than 254 million people infected globally.^[1] Chronic HBV infection is the leading cause of liver disease and an estimated 820,000 people die of complications from chronic HBV infection each year.^[1] HBV is of exceptional concern in China, where 87 million people are chronically infected.^[2]

About BRII-179

BRII-179 is a novel recombinant protein-based HBV immunotherapeutic candidate that expresses the Pre-S1, Pre-S2, and S HBV surface antigens, and is designed to induce enhanced and broad B-cell and T-cell immunity. In November 2023, the Center for Drug Evaluation (the "CDE") of the National Medical Products Administration (the "NMPA") granted BRII-179 Breakthrough Therapy Designation.

About Elebsiran

Elebsiran is an investigational subcutaneously administered HBV-targeting siRNA designed to degrade hepatitis B virus RNA transcripts and limit the production of hepatitis B surface antigen. It has the potential to have direct antiviral activity against HBV and HDV. It is the first siRNA in the clinic to include Enhanced Stabilization Chemistry Plus technology to enhance stability and minimize off-target activity, which potentially can result in an increased therapeutic index. Brii Bio licensed exclusive rights to develop and commercialize elebsiran for the Greater China territory from Vir Biotechnology, Inc. in 2020. In May 2024, the CDE of the NMPA granted elebsiran Breakthrough Therapy Designation.

About Brii Bio

Brii Biosciences Limited ("Brii Bio", stock code: 2137.HK) is a biotechnology company developing therapies to address major public health challenges where patients experience high unmet medical needs, limited choice and significant social stigmas. With a focus on infectious diseases, the Company is advancing a broad pipeline of unique therapeutic candidates with lead programs against hepatitis B virus (HBV) infection. The Company is led by a visionary and experienced leadership team and has operations in key biotech hubs, including Raleigh-Durham, the San Francisco Bay Area, Beijing and Shanghai. For more information, visit www.briibio.com.