London - Mereo BioPharma Group plc (NASDAQ: MREO, AIM: MPH), 'Mereo' or the 'Company,' today announced additional endpoint data from the Company's Phase 2b dose-ranging 'ASTEROID' clinical study of setrusumab (BPS-804), an anti-sclerostin antibody, in adults with Type I, III or IV osteogenesis imperfecta ('OI').

These additional prespecified endpoint data build upon the Company's 12-month topline data from the ASTEROID study announced in November 2019, which demonstrated a dose-dependent, statistically significant bone building effect of setrusumab at multiple anatomical sites in adult OI patients (irrespective of OI subtype). OI is a rare disorder characterized by fragile bones and reduced bone mass with no approved treatments.

'Based upon our review of the comprehensive data set from the Phase 2b ASTEROID study, these additional prespecified analyses support our previous conclusions that setrusumab is building bone at the lumbar spine and is increasing bone strength at multiple peripheral sites in adult OI patients,' said Dr. Alastair MacKinnon, Chief Medical Officer of Mereo. 'We believe these additional endpoint data, together with the totality of the 12-month topline data and trend in fracture rate reduction in the high dose cohort, are fully supportive of moving forward with our planned pivotal trial in children with OI. To this end, we have made additional progress on the regulatory front, including a positive recent meeting with the European Medicines Agency ('EMA'), and are currently preparing for a Type B End-of-Phase 2 meeting with the FDA, scheduled for the first quarter of 2020.'

'Finite element analysis ('FEA') is one of the best measures available to monitor bone strength in clinical studies, being sensitive to important changes in trabecular and cortical bone as well as overall bone structure. In OI patients given setrusumab for 12 months, the FEA technique was used to assess forearm (radius) bone strength at the beginning and end of the treatment period,' said Dr. Ken Poole, Reader in Metabolic Bone Disease at the University of Cambridge. 'These new data are consistent with an effect of setrusumab at the high dose on improving radius bone strength as evidenced by a better ability to resist experimental deformation and improved failure load. There are no currently approved therapies for osteogenesis imperfecta, and treatment options are greatly needed.'

ABOUT THE PHASE 2B ASTEROID STUDY

ASTEROID was a 12-month, randomized, double-blind, Phase 2b dose-finding study in 112 adults diagnosed with type I, III or IV Osteogenesis Imperfecta and a confirmed COL1A1/COL1A2 mutation who have fractured over the previous 5 years. ASTEROID was the largest, prospectively-designed, interventional clinical study to be performed in this patient group. The primary endpoint of the study was the change over baseline in Tr vBMD of the wrist at 12 months, followed by bone strength measured by Finite Element Analysts (hierarchical) assessed using HR-pQCT. Change from baseline at 6 and 12 months for areal BMD at the lumbar spine, as measured by DXA, was an important secondary endpoint. Additional secondary endpoints included HR-pQCT parameters (such as total vBMD), bone biomarkers, patient reported outcomes (PRO) and quality of life measures. Fracture data were also collected throughout the duration of the study, although the trial was not statistically powered for fractures.

ABOUT OSTEOGENESIS IMPERFECTA

Osteogenesis Imperfects (OI) is a rare genetic disorder that is characterized by fragile bones and reduced bone mass resulting in bones that break easily, loose joints and weakened teeth. In severe cases patients may experience hundreds of fractures in a lifetime. In addition, people with OI often suffer muscle weakness, early hearing loss, fatigue, curved bones, scoliosis, respiratory problems and short stature, leading to significant impacts on overall health and quality of life. The majority of cases of OI (estimated at approximately 90%) are caused by a dominant mutation in a gene coding for type I collagen, a key component of healthy bone. Current treatment of OI is supportive, focusing on minimizing fractures and maximizing mobility, but to date, there are no EMA or FDA approved treatments.

ABOUT SETRUSUMAB

Setrusumab is a fully humanized monoclonal antibody that inhibits sclerostin, a protein which inhibits the activity of bone-forming cells. The mechanism of action of setrusumab could be particularly well suited for the treatment of OI and has the potential to become the first approved treatment option that could reduce fractures and improve OI patients' quality of life. In addition to evaluating setrusumab in adult OI patients, Mereo's Paediatric Investigation Plan (PIP) has been approved by the European Medicines Agency (EMA) and a study design has been agreed for a pivotal registration trial in children, based on a primary endpoint of fracture rate over a 12-month period. The pivotal study will be conducted in approximately 165 children aged 5 to

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