X-linked hypophosphatemia (XLH) is a type of rickets that causes the bone deformity, and unlike other types of rickets, vitamin D supplementation does not cure it. Burosumab is a human monoclonal antibody against FGF23 and can improve the condition of children with XLH. This study aims to assess the efficacy and safety of burosumab in children aged 1-4 years with XLH. 

This is an open-label, phase-2 trial conducted in three hospitals in the USA. The study included a total of 13 children aged 1-4 years with XLH. The participants were assigned to subcutaneous injection burosumab (0.8 mg/kg) every two weeks for 64 weeks. The dose was increased gradually to 1.2 mg per kg. The primary endpoints were the safety and efficacy of the drug measured by adverse events and the change in fasting serum phosphorus concentrations, respectively.

All the 13 children completed the 64-week treatment. All patients had at least one adverse event out of 14 treatment-related adverse events. The results, as per the Thacher Rickets Severity Score, indicated an improvement in the condition of rickets in participants. 

The research concluded that burosumab was generally safe and resulted in increased serum levels and improvement rickets condition in children aged 1-4 years.

Ref: https://www.thelancet.com/journals/landia/article/PIIS2213-8587(18)30338-3/fulltext