Brii Bio Unveils New Data from Its Ongoing Phase 2 ENSURE Study at APASL 2025, Showcasing BRII-179’s Unique Potential to Prime and Boost Higher HBsAg Loss Through Target Patient Identification

Preliminary d ata from Cohort 4 of the ENSURE study supports a novel enrichment strategy to utilize BRII-179 to identify patients who are immune responders and have the potential to achieve higher HBsAg loss at EOT 48-week EOT data from Cohort 1-3 of the ENSURE study clearly suggests the added benefits of elebsiran towards achieving a higher rate of HBsAg loss in combination with PEG-IFNα DURHAM, N.C., and BEIJING, China – Mar ch 30 , 202 5 – 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 new data from its ongoing Phase 2 ENSURE study as a late-breaking oral presentation at the 34 t h Annual Meeting of Asian Pacific Association for the Study of the Liver (APASL 2025) in Beijing, China. ENSURE (NCT05970289) is a multicenter, open-label Phase 2 study. Cohorts 1-3 were designed to evaluate the contribution of elebsiran, an investigational small interfering ribonucleic acid (siRNA), in combination with pegylated interferon alpha (PEG-IFNα) in participants with chronic HBV infection with baseline hepatitis B surface antigen (HBsAg) of 100-3,000 IU/mL. Participants who completed 9 doses of BRII-179, a recombinant protein-based therapeutic vaccine, in combination with elebsiran (BRII-835) in a previous APAC study BRII-179-835-001 (NCT04749368) were enrolled to Cohort 4 of this study and received elebsiran and PEG-IFNα combination treatment. The design of Cohort 4 as part of this study was based on insight from previous studies that a significant proportion of the chronic HBV patients fail to generate a sufficient immune response after receiving multiple doses of BRII-179, and therefore unlikely to have the immune support to achieve sustainable functional cure. Emerging data from Cohort 4 showed that participants who previously had BRII-179 induced anti-HBs response achieved a substantially higher rate of HBsAg seroclearance than those who did not. At Week 24, more than half of the BRII-179 responders (55.6% [10/18]) achieved HBsAg seroclearance, compared to only 10.0% (1/10) in non-responders. These latest data suggest that BRII-179 can serve as a predictive tool for enriching patients more likely to respond to curative therapies. Additional data from Cohorts 1-3 of the ENSURE study showed that higher end of treatment (EOT) HBsAg loss and seroconversion rates in participants receiving elebsiran in combination with PEG-IFNα than those receiving PEG-IFNα alone. “The positive Cohort 4 data from the ENSURE study opens new doors for HBV functional cure,” said Dr. Grace Lai-Hung Wong, Professor of Gastroenterology and Hepatology at CUHK Medical Data Analytics Centre (MDAC) and Department of Medicine and Therapeutics in Hong Kong SAR, China. “Previous studies have suggested that BRII-179 may offer a unique opportunity to identify CHB patients who are able to elicit the necessary HBsAg antibody response. I believe these new findings provide clear evidence supporting such value proposition and further substantiate the role of BRII-179 in shaping future combination therapies.” “We are encouraged that the Cohort 4 from the ENSURE study continue to support our enrichment strategy in developing a functional cure for chronic HBV in target populations,” said David Margolis, MD, Chief Medical Officer of Brii Bio. “The results underscore the potential of BRII-179 in identifying patients who are more likely to respond to regimens aimed at functional cure, thereby enhancing functional cure rates in the target population while reducing exposure to costly therapies for those with a lower probability of cure. We are committed to advancing BRII-179 in combination with various modalities through our ongoing studies and collaborations with strategic partners, aiming to deliver higher functional cure rates to 254 million patients worldwide living with chronic HBV infection.” Abstract Number: OP0335 Presentation Title: Responders to Prior BRII-179 Treatment Achieved Faster and Higher Rate of HBsAg Seroclearance Following Treatment of Elebsiran and PEG-IFNα in Participants with Chronic Hepatitis B Virus Infection: Preliminary 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 Among the 28 of the 31 participants enrolled in Cohort 4 with baseline HBsAg ≥ 100 IU/mL being analyzed, 18 participants with peak anti-HBs ≥ 10 IU/L induced by prior BRII-179 treatment were defined as BRII-179 responders and 10 participants with peak anti-HBs < 10 IU/L were defined as non-responders. At Week 24 of treatment with elebsiran + PEG-IFNα, 39.3% (11/28) of the Cohort 4 participants achieved HBsAg seroclearance. The rate of HBsAg seroclearance at Week 24 in the BRII-179 responders was 55.6% (10/18), notably higher compared to the non-responders at 10% (1/10). Responders to prior BRII-179 treatment appeared to achieve a faster HBsAg seroclearance compared to BRII-179 naïve participants receiving elebsiran + PEG-IFNα as previously reported. Elebsiran and PEG-IFNα combination therapy was generally safe and well tolerated in participants with virally suppressed chronic HBV infection. Treatment with elebsiran + PEG-IFNα is ongoing for 48 weeks. Abstract Number: LB0009 Presentation Title: Higher end of treatment (EOT) HBsAg loss and seroconversion rates in participants with chronic HBV infection receiving elebsiran (BRII-835) and pegylated interferon alfa-2a (PEG-IFNα) compared to PEG-IFNα alone: Week 48 results from ongoing ENSURE study Presenter: Jidong Jia, M.D., Ph.D., Professor of Medicine at the Liver Research Centre, Beijing Friendship Hospital, Capital Medical University in Beijing, China The rates of HBsAg seroclearance at EOT in elebsiran 200 mg + PEG-IFNα and elebsiran 100 mg + PEG-IFNα cohorts were 26.3% (5/19) and 33.3% (6/18), respectively, notably higher compared to PEG-IFNα alone cohort (5.6%) in participants with baseline HBsAg levels 100-3,000 IU/mL. Greater HBsAg reductions at EOT were observed in elebsiran + PEG-IFNα combination cohorts (mean [SE]: -2.47 [0.28] or -3.01 [0.28] log 10 IU/mL in elebsiran 200 mg or 100 mg, respectively) than in PEG-IFNα cohort (-1.02 [0.30] log 10 IU/mL). Elebsiran in combination with PEG-IFNα at both 200 mg and 100 mg doses achieved similar HBsAg reductions and seroclearance rates. Elebsiran and PEG-IFNα combination therapy was generally safe and well tolerated in participants with virally suppressed chronic HBV infection. Post-treatment follow-up is ongoing and will continue for 24 weeks after discontinuation of treatment. As part of Brii Bio’s unique approach to developing a functional cure for HBV, the Company and its partners are actively progressing multiple combination studies with our differentiated portfolio, including BRII-179, a recombinant protein-based HBV immunotherapeutic being evaluated in multiple combination studies with elebsiran led by Brii Bio; elebsiran being evaluated in combination with PEG-IFNα in studies led by Brii Bio and tobevibart, an investigational broadly neutralizing monoclonal antibody targeting HBV, being evaluated in multiple Phase 2 and 3 tobevibart and elebsiran combination studies led by Vir Biotechnology. Key data readouts will be shared in the coming months at the scientific conferences throughout 2025. 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 (BRII-835, VIR-2218) 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. 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 . Media Inquiry: [email protected] Investor Inquiry: [email protected] Partnering Inquiry: [email protected] --------------------------------- 1 World Health Organization. (April 2024). Global hepatitis report 2024: action for access in low- and middle-income countries . World Health Organization. Retrieved from https://www.who.int/publications/i/item/9789240091672 2 World Health Organization. Hepatitis. World Health Organization. Retrieved from https://www.who.int/china/health-topics/hepatitis#:~:text=There%20are%2087%20million%20people,living%20with%20chronic%20hepatitis%20C. 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