Burosumab is in clinical development for the treatment of adults with X-linked hypophosphataemia (XLH), and is currently licenced in children. XLH is an inherited disorder characterized by low levels of phosphate in the blood. Phosphate levels are low because phosphate is abnormally processed in the kidneys due to high levels of a protein called FGF23, which causes a loss of phosphate in the urine (phosphate wasting) and leads to soft, weak bones (rickets). For the past 40 years, therapy has primarily consisted of multiple daily doses of oral phosphate with active vitamin D. Due to adverse effects of this conventional therapy, the general practice and guidance has been to stop treatment at the end of growth and only treat adult patients who are symptomatic with XLH, leaving a substantial proportion of adults with no treatment.
Burosumab, administered by subcutaneous injection, is a monoclonal antibody (a type of protein) designed to recognise and attach to the FGF23 protein. By attaching to the FGF23 protein, the medicine blocks its activity, allowing the kidneys to reabsorb phosphate into the bloodstream and restore normal levels of phosphate in the blood. In studies of adults with XLH, burosumab is associated with normalisation of serum phosphorus, restoration of bone quality and improvements in pain, stiffness, fatigue, as well as healing of fractures compared to placebo.