X-linked hypophosphatemia (XLH), also called X-linked dominant hypophosphatemic rickets, X-linked vitamin d-resistant rickets, is an X-linked dominant form of rickets (or osteomalacia) that differs from most cases of rickets in that ingestion of vitamin D is relatively ineffective. It can cause bone deformity including short stature and genu varum (bow leggedness). It is associated with a mutation in the PHEX gene sequence (Xp.22) and subsequent inactivity of the PHEX protein. The prevalence of the disease is 1 in 20’000 persons.
Burosumab (Crysvita) will be available as a solution for injection (10 mg/ml, 20 mg/ml and 30 mg/ml). The active substance of Crysvita is burosumab, a human monoclonal antibody that binds to and inhibits the activity of fibroblast growth factor 23 (FGF23). Inhibiting the activity of elevated serum fibroblast growth factor 23 in X-linked hypophosphataemia reduces the loss of phosphate from the kidney and other metabolic abnormalities, which are considered to be causative of the disease’s bone changes.
The benefits with Burosumab (Crysvita) are its ability to reduce the loss of phosphate from the kidney, to improve abnormally low serum
phosphate concentrations and to reduce the severity of rickets as shown in x-rays. The most common side effects are injection site reactions, headache, and pain in extremities. The full indication is: “Burosumab (Crysvita) is indicated for the treatment of X-linked hypophosphataemia with radiographic evidence of bone disease in children 1 year of age and older and adolescents with growing skeletons.” It is proposed that Burosumab (Crysvita) be prescribed by physicians experienced in the management of patients with metabolic bone diseases.
Detailed recommendations for the use of this product will be described in the summary of product characteristics (SmPC), which will be published in the European public assessment report (EPAR) and made available in all official European Union languages after the marketing authorisation has been granted by the European Commission.
Note that a conditional marketing authorisation is granted to a medicinal product that fulfils an unmet medical need when the benefit to public health of immediate availability outweighs the risk inherent in the fact that additional data are still required. The marketing authorisation holder is likely to provide comprehensive clinical data at a later stage.