Leflutrozole

Neither study achieved its primary endpoint of reduction in annualized clinical fracture rate compared to placebo (ORBIT) or bisphosphonates (COSMIC) Both studies achieved secondary endpoint of improvements in bone mineral density with strong statistical significance LONDON, Dec. 29, 2025 (GLOBE NEWSWIRE) -- Mereo BioPharma Group plc (NASDAQ: MREO) (“Mereo” or the “Company”), a clinical stage biopharmaceutical company focused on rare diseases, today announced results from the Phase 3 ORBIT and COSMIC studies for setrusumab (UX143) in Osteogenesis Imperfecta (OI). Neither study achieved statistical significance against the primary endpoints of reduction in annualized clinical fracture rate compared to placebo or bisphosphonates, respectively. Both studies achieved their secondary endpoints of improvements in bone mineral density (BMD) against comparators (placebo and bisphosphonates) with strong statistical significance. There was no change in the safety profile observed. “Whilst we are disappointed by these results, we will be conducting additional analyses on the data, to assess next steps and the best path forward for the program, especially in pediatrics given the totality of the data and lack of other treatment options for individuals with OI. In the meantime, we are carefully managing our cash resources with immediate reductions in our pre-commercial and manufacturing activities, and we are continuing to advance partnering discussions for alvelestat,” said Dr. Denise Scots-Knight, Chief Executive Officer of Mereo. ORBIT and COSMIC bone mineral density (BMD) improvements In the ORBIT study, participants experienced statistically significant and substantial improvements in BMD compared to placebo, at levels consistent with the treatment effect observed in Phase 2 studies. These BMD changes were not accompanied by a corresponding reduction in annualized fracture rates and there was a low fracture rate in the placebo group. In the pediatric COSMIC study, patients had a substantially higher baseline fracture rate compared to the patients enrolled in ORBIT. In this younger patient population, meaningful improvements in BMD were associated with a reduction in annualized fracture rate for setrusumab treated patients over bisphosphonate treated patients, though the reduction did not meet statistical significance. Additional analyses on the data across both studies are being conducted, including in other bone health and clinical endpoints beyond fractures. Mereo’s cash balance was $48.7 million at the end of the third quarter of 2025. The Company will tightly control costs in parallel with conducting further analysis of the setrusumab data to determine the best path forward. Meanwhile the Company will continue to seek to maximize value in its owned and partnered programs, including: Alvelestat for AATD-lung disease – in partnering discussions Vantictumab for osteopetrosis – partnered with āshibioLeflutrozole for male infertility – partnered with ReproNovo. About the Setrusumab Phase 3 ProgramUltragenyx is developing setrusumab in pediatric and young adult patients across OI sub-types I, III and IV with two late-stage studies: the pivotal Phase 2/3 Orbit study and Phase 3 Cosmic study. The global, seamless Phase 2/3 Orbit study is evaluating the effect of setrusumab on clinical fracture rate in patients aged 5 to 25 years. The pivotal Phase 3 portion of the study enrolled 159 patients at 45 sites across 11 countries, with subjects randomized 2:1 to receive setrusumab or placebo, and a primary efficacy endpoint of annualized clinical fracture rate. The global Phase 3 Cosmic study evaluated the effect of setrusumab on reduction in annualized fracture rate in patients aged 2 to <7 years compared to bisphosphonates. The Cosmic study has enrolled 69 patients at 21 sites across 7 countries with patients randomized 1:1 to receive setrusumab or intravenous bisphosphonates (IV-BP) therapy. About Setrusumab (UX143)Setrusumab is a fully human monoclonal antibody that inhibits sclerostin, a negative regulator of bone formation. Blocking sclerostin is expected to increase new bone formation, bone mineral density and bone strength in OI. In mouse models of OI, the use of anti-sclerostin antibodies was shown to increase bone formation, improve bone mass to normal levels, and increase bone strength against fracture force testing to normal levels. About Osteogenesis Imperfecta (OI)Osteogenesis Imperfecta (OI) includes a group of genetic disorders impacting bone metabolism. Approximately 85% to 90% of OI cases are caused by genetic variants in the COL1A1 or COL1A2 genes, leading to either reduced or abnormal collagen and changes in bone metabolism. The collagen mutations in OI can result in increased bone brittleness, which contributes to a high rate of fractures. Patients with OI also exhibit inadequate production of new bone and excess bone resorption, resulting in decreased bone mineral density, bone fragility and weakness. OI can also lead to bone deformities, abnormal spine curvature, pain, decreased mobility, and short stature. No treatments are globally approved for OI, which affects approximately 60,000 people in commercially accessible geographies. About Mereo BioPharma Mereo BioPharma is a biopharmaceutical company focused on the development of innovative therapeutics for rare diseases. The Company has three rare disease product candidates: setrusumab for the treatment of osteogenesis imperfecta (OI); alvelestat for the treatment of alpha-1 antitrypsin deficiency-associated lung disease (AATD-LD) and vantictumab for the treatment of autosomal dominant osteopetrosis type 2 (ADO2). The Company’s partner for setrusumab, Ultragenyx Pharmaceutical, Inc., has reported results from the Phase 3 portion of a pivotal Phase 2/3 study in pediatrics and young adults (5 to 25 years old) for setrusumab in OI and in the Phase 3 study in pediatric patients (2 to <7 years old). The partnership with Ultragenyx includes potential additional milestone payments of up to $245 million and royalties to Mereo on commercial sales in Ultragenyx territories. Mereo has retained EU and UK commercial rights and will pay Ultragenyx royalties on commercial sales in those territories. Setrusumab has received orphan designation for osteogenesis imperfecta from the European Commission (“EC”) and the FDA, PRIME designation from the EMA, and has Breakthrough Therapy designation and rare pediatric disease designation from the FDA. Alvelestat has received Orphan Designation for AATD from the EC and the FDA, and Fast Track designation from the FDA for AATD-LD. Following results from ASTRAEUS and ATALANTa in AATD-lung disease, the Company has aligned with the FDA and the EMA on the primary endpoints for a Phase 3 pivotal study which, if successful, could enable full approval in both the U.S. and Europe. The Company’s partner for vantictumab, āshibio, Inc., is funding the global development program. Mereo has retained EU and UK commercial rights. Mereo has also entered into an exclusive global license agreement with ReproNovo SA, a reproductive medicine company, for the development and commercialization of leflutrozole, a non-steroidal aromatase inhibitor for the treatment of infertility in men with low testosterone. In addition, Mereo has two oncology product candidates, etigilimab, an anti-TIGIT; and navicixizumab for the potential treatment of late-line ovarian cancer. Navicixizumab has been partnered with Feng Biosciences, Inc. in a global licensing agreement that includes milestone payments and royalties. Forward-Looking Statements This press release contains “forward-looking statements” that involve substantial risks and uncertainties. All statements other than statements of historical fact contained herein are forward-looking statements within the meaning of Section 27A of the United States Securities Act of 1933, as amended, and Section 21E of the United States Securities Exchange Act of 1934, as amended. Forward-looking statements usually relate to future events and anticipated revenues, earnings, cash flows or other aspects of our operations or operating results. Forward-looking statements are often identified by the words “believe,” “expect,” “anticipate,” “plan,” “intend,” “foresee,” “should,” “would,” “could,” “may,” “estimate,” “outlook” and similar expressions, including the negative thereof. The absence of these words, however, does not mean that the statements are not forward-looking. These forward-looking statements are based on the Company’s current expectations, beliefs and assumptions concerning future developments and business conditions and their potential effect on the Company. While management believes that these forward-looking statements are reasonable as and when made, there can be no assurance that future developments affecting the Company will be those that it anticipates. All of the Company’s forward-looking statements involve known and unknown risks and uncertainties some of which are significant or beyond its control and assumptions that could cause actual results to differ materially from the Company’s historical experience and its present expectations or projections. Such risks and uncertainties include, among others, the uncertainties inherent in the clinical development process; the Company’s reliance on third parties to conduct and provide funding for its clinical trials; the Company’s dependence on enrollment of patients in its clinical trials; and the Company’s dependence on its key executives. You should carefully consider the foregoing factors and the other risks and uncertainties that affect the Company’s business, including those described in the “Risk Factors” section of its Annual Report on Form 10-K, as well as discussions of potential risks, uncertainties, and other important factors in the Company’s subsequent filings with the Securities and Exchange Commission. The Company wishes to caution you not to place undue reliance on any forward-looking statements, which speak only as of the date hereof. The Company undertakes no obligation to publicly update or revise any of our forward-looking statements after the date they are made, whether as a result of new information, future events or otherwise, except to the extent required by law. Mereo BioPharma Contacts: MereoDenise Scots-Knight, Chief Executive Officer Christine Fox, Chief Financial Officer+44 (0)333 023 7300 Burns McClellan (Investor Relations Adviser to Mereo)Lee Roth +01 646 930 4406 Investorsinvestors@mereobiopharma.com

Data from Phase 3 Orbit and Cosmic studies of setrusumab in osteogenesis imperfecta expected around year-end 2025 Cash of $56.1 million as of June 30, 2025, expected to support operations into 2027 London, August 12, 2025 – Mereo BioPharma Group plc (NASDAQ: MREO) (“Mereo” or the “Company”), a clinical-stage biopharmaceutical company focused on rare diseases, today announced its financial results for the second quarter ended June 30, 2025, and provided recent corporate highlights. “We look forward to the final analysis for the two ongoing Phase 3 studies for setrusumab in osteogenesis imperfecta, the Phase 3 Orbit study in pediatric and young adult patients, and the Phase 3 Cosmic study in young pediatric patients, around the end of the year. We continue to be excited about the potential of setrusumab to reduce fractures and improve other functional parameters for individuals living with osteogenesis imperfecta,” said Dr. Denise Scots-Knight, Chief Executive Officer of Mereo. “In parallel with the advancement of setrusumab, we are continuing to advance partnering discussions around alvelestat, our first-in-class oral small molecule for AATD-lung disease, and to ready the program for Phase 3 initiation. Our prudent management of our cash and resources means we are well positioned through these key milestones to support our operations into 2027.” Second Quarter 2025 Highlights, Recent Developments, and Anticipated Milestones Setrusumab (UX143) for osteogenesis imperfecta (OI) The Phase 3 Orbit and Cosmic studies, led by our partner Ultragenyx, evaluating setrusumab in pediatric and young adult patients and young pediatric patients with OI, are progressing towards their final analyses around the end of 2025. The randomized, placebo-controlled Phase 3 portion of the Orbit study was evaluated by the Data Monitoring Committee at an interim analysis in July 2025 and they informed Ultragenyx that setrusumab demonstrated an acceptable safety profile and that the study should continue to the final analysis. Data from the Cosmic study were not analyzed at the interim timepoint, consistent with the statistical analysis plan. Patients will continue dosing in both the Phase 3 Orbit and Cosmic studies, with the final analyses to be conducted after patients have been on therapy for at least 18-months. The threshold for the Phase 3 Orbit final analysis is p<0.04 and for the Phase 3 Cosmic final analysis is p<0.05. Pre-commercial efforts continue in Europe where Mereo holds commercial rights. These include continuation of the SATURN program, working with existing OI datasets to better understand the natural history and the unmet medical need in pediatrics and adults and to generate data to support the health economic model, both important to support the assessment by Regulatory and Health Technology Assessment (HTA) bodies and payors. In addition, the activities to define the treatment landscape, including the number of treatment centers and the patient journey from childhood to adulthood, are being extended beyond the five major countries in Europe. Alvelestat (MPH-966) for alpha-1-anti-trypsin deficiency lung disease (AATD-LD) Activities to support initiation of the planned single, global Phase 3 pivotal study are ongoing. The Company continues to be actively engaged with multiple potential partners regarding development and commercialization of alvelestat. Second Quarter 2025 Financial Results Total research and development (“R&D”) expenses increased by $0.4 million from $4.9 million in the three months ended June 30, 2024, to $5.4 million in the three months ended June 30, 2025. The increase was primarily due to increases of $2.2 million in R&D expenses for setrusumab offset by decreases of $1.5 million and $0.2 million in R&D expenses for alvelestat and etigilimab, respectively. The increase in program expenses for setrusumab was primarily driven by amounts due under the manufacturing and supply agreement with our partner, Ultragenyx, as well as ongoing activities related to real-world evidence programs and medical affairs activities in Europe. This is in addition to costs we incur in relation to our collaboration with Ultragenyx, who fund the global development of the program, including input into development, regulatory and manufacturing plans. The decrease in program expenses for alvelestat was primarily due to the completion of the drug formulation and manufacturing activities undertaken in preparation for the Phase 3 study in the three months ended June 30, 2024. General and administrative (“G&A”) expenses decreased by $2.4 million from $7.9 million in the second quarter of 2024 to $5.5 million in the second quarter of 2025. The decrease was primarily due to the recognition of a $1.9 million reduction in expenses in the three months ended June 30, 2025 for amounts received from our depository to reimburse certain expenses incurred by us in respect of our ADR program as well as lower professional fees. Net loss for the second quarter ended June 30, 2025 was $14.6 million, compared to $12.3 million during the comparable period in 2024, primarily reflecting a foreign currency transaction loss of $5.4 million and increases in R&D expenses, offset by a decrease in G&A expenses and $0.5 million in revenue from a one-time milestone payment for the achievement of a clinical milestone on leflutrozole. As of June 30, 2025, the Company had cash and cash equivalents of $56.1 million, compared to $69.8 million as of December 31, 2024. The Company’s guidance remains unchanged, and it continues to expect, based on current operational plans, that its existing cash and cash equivalents balance will enable it to fund its currently committed clinical trials, operating expenses, and capital expenditure requirements into 2027. This guidance does not include any payments associated with a potential partnership for alvelestat or business development activity around any of the Company’s non-core programs. Total ordinary shares issued as of June 30, 2025, were 795,001,444. Total ADS equivalents as of June 30, 2025, were 159,000,288, with each ADS representing five ordinary shares of the Company. About Mereo BioPharma Mereo BioPharma is a biopharmaceutical company focused on the development of innovative therapeutics for rare diseases. The Company has two rare disease product candidates: setrusumab for the treatment of osteogenesis imperfecta (OI); and alvelestat for the treatment of alpha-1 antitrypsin deficiency-associated lung disease (AATD-LD). The Company’s partner, Ultragenyx Pharmaceutical, Inc., has completed enrollment in the Phase 3 portion of a pivotal Phase 2/3 study in pediatrics and young adults (5 to 25 years old) for setrusumab in OI and in the Phase 3 study in pediatric patients (2 to <7 years old). The partnership with Ultragenyx includes potential additional milestone payments of up to $245 million and royalties to Mereo on commercial sales in Ultragenyx territories. Mereo has retained EU and UK commercial rights and will pay Ultragenyx royalties on commercial sales in those territories. Setrusumab has received orphan designation for osteogenesis imperfecta from the European Commission (“EC”) and the FDA, PRIME designation from the EMA, and has Breakthrough Therapy designation and rare pediatric disease designation from the FDA. Alvelestat has received Orphan Designation for AATD from the EC and the FDA, and Fast Track designation from the FDA for AATD-LD. Following results from ASTRAEUS and ATALANTa in AATD-lung disease, the Company has aligned with the FDA and the EMA on the primary endpoints for a Phase 3 pivotal study which, if successful, could enable full approval in both the U.S. and Europe. Mereo has also entered into an exclusive global license agreement with ReproNovo SA, a reproductive medicine company, for the development and commercialization of leflutrozole, a non-steroidal aromatase inhibitor for the treatment of infertility in men with low testosterone. In addition, Mereo has two oncology product candidates, etigilimab, an anti-TIGIT; and navicixizumab for the potential treatment of late-line ovarian cancer. Navicixizumab has been partnered with Feng Biosciences, Inc. in a global licensing agreement that includes milestone payments and royalties. Forward-Looking Statements This press release contains “forward-looking statements” that involve substantial risks and uncertainties. All statements other than statements of historical fact contained herein are forward-looking statements within the meaning of Section 27A of the United States Securities Act of 1933, as amended, and Section 21E of the United States Securities Exchange Act of 1934, as amended. Forward-looking statements usually relate to future events and anticipated revenues, earnings, cash flows or other aspects of our operations or operating results. Forward-looking statements are often identified by the words “believe,” “expect,” “anticipate,” “plan,” “intend,” “foresee,” “should,” “would,” “could,” “may,” “estimate,” “outlook” and similar expressions, including the negative thereof. The absence of these words, however, does not mean that the statements are not forward-looking. These forward-looking statements are based on the Company’s current expectations, beliefs and assumptions concerning future developments and business conditions and their potential effect on the Company. While management believes that these forward-looking statements are reasonable as and when made, there can be no assurance that future developments affecting the Company will be those that it anticipates. All of the Company’s forward-looking statements involve known and unknown risks and uncertainties some of which are significant or beyond its control and assumptions that could cause actual results to differ materially from the Company’s historical experience and its present expectations or projections. Such risks and uncertainties include, among others, the uncertainties inherent in the clinical development process; the Company’s reliance on third parties to conduct and provide funding for its clinical trials; the Company’s dependence on enrollment of patients in its clinical trials; and the Company’s dependence on its key executives. You should carefully consider the foregoing factors and the other risks and uncertainties that affect the Company’s business, including those described in the “Risk Factors” section of its Annual Report on Form 10-K, as well as discussions of potential risks, uncertainties, and other important factors in the Company’s subsequent filings with the Securities and Exchange Commission. The Company wishes to caution you not to place undue reliance on any forward-looking statements, which speak only as of the date hereof. The Company undertakes no obligation to publicly update or revise any of our forward-looking statements after the date they are made, whether as a result of new information, future events or otherwise, except to the extent required by law. Mereo BioPharma Contacts: Mereo +44 (0)333 023 7300 Denise Scots-Knight, Chief Executive Officer Christine Fox, Chief Financial Officer Burns McClellan (Investor Relations Adviser to Mereo) +01 646 930 4406 Lee Roth Investors investors@mereobiopharma.com MEREO BIOPHARMA GROUP PLCCONDENSED CONSOLIDATED BALANCE SHEETS(In thousands, except share and per share amounts)(Unaudited) MEREO BIOPHARMA GROUP PLCCONDENSED CONSOLIDATED STATEMENTSOF OPERATIONS AND COMPREHENSIVE LOSS(In thousands, except per share amounts)(Unaudited)