ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Endocrine Abstracts (2019) 66 P7 | DOI: 10.1530/endoabs.66.P7

Burosumab experience in UK XLH children under five years old

Poonam Dharmaraj1, Christine Burren2, Moira Cheung3, Raja Padidela4, Zulf Mughal4, Nick Shaw5, Vrinda Saraff5, Ruchi Nadar5, Talat Mushtaq6, Renuka Ramakrishnan1, Senthil Senniappan1, Sophia Sakha3, John Barton2, Ian Tucker2, Lauren Rayner4, Paul Arundel7, Robyn Gilbey-Cross3, Alexander Tothill8, James Philip9, Nadine Sawoky9, Paul Connor9 & Leigh Mathieson9

1Alder Hey Children’s Hospital, Liverpool, UK; 2Bristol Royal Hospital for Children, Bristol, UK; 3Evelina Children’s Hospital, London, UK; 4Royal Manchester Children’s Hospital, Manchester, UK; 5Birmingham Children’s Hospital, Birmingham, UK; 6Leeds Teaching Hospitals, Leeds, UK; 7Sheffield Children’s Foundation Trust, Sheffield, UK; 8MAPBiopharma, Cambridge, UK; 9Kyowa Kirin International, Galashiels, UK

Objectives: X-linked hypophosphatemia (XLH) is a rare inherited form of osteomalacia characterised by low blood phosphate levels which lead to inadequate mineralization of bone and rickets. Burosumab is an anti-FGF23 fully human monoclonal-antibody, and the first treatment to target the underlying pathophysiology of XLH. We report relevant real-world biochemical data on children under five years old for the first 6 months of treatment.

Methods: An early access program (EAP) for burosumab was made available for children in the United Kingdom with XLH in 12 specialist centres. Inclusion criteria for the EAP included radiographic evidence of disease, XLH confirmed by genetic PHEX mutation or familial X-linked inheritance mutation or family history. Patients must have also had an unsatisfactory response to best available care and treatment. EAP enrolment was between January and March 2018. A total of 142 applications were received of which 135 were approved with 132 receiving treatment (dose in accordance with EMA marketing authorisation).

Results: Data are available on 10 children under five years (mean age 2.8 years; 1.6–4 years) who have completed a median of 6 months (20–26 weeks) of burosumab treatment. Mean height and weight at week 0 was 85.5 cm (75–97.3 cm) and 12.8 kg (9.9–18.2 kg) respectively. Mean dose administered was 0.81 mg/kg (0.55–1.01 mg/kg) at week 0 and 1.09 mg/kg (0.57–2.01 mg/kg) at the end of the 20–26 week period. Mean fasting serum phosphorus was 0.73 mmol/l (0.6–0.83 mmol/l) in week 0 rising to 1.02 mmol/l (0.82–1.3 mmol/l) at week 20–26 representing a 40% increase in serum phosphate levels. Mean serum ALP fell from 808.2 IU/l (297–2124 IU/l) at week 0 to 612 IU/l (291–1459 IU/l) at week 20–26, representing a 24% decrease in ALP. No patients discontinued treatment due to adverse events.

Conclusions: Early data from treating young children with XLH with burosumab in a real-world UK setting demonstrate that key biochemical responses are aligned with findings from the clinical study program. This provides reassurance that the improvement in key biochemical parameters is consistent across all ages within its licensed indication.

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