Study of the Anti-FGF23 Antibody Burosumab in Adults With XLH

  • End date
    Jan 31, 2022
  • participants needed
  • sponsor
    Kyowa Kirin Pharmaceutical Development Ltd
Updated on 8 September 2021


This is phase 3b open-label, international, multicenter study to continue to monitor the long-term safety and efficacy of burosumab in adult patients with XLH that participated in previous clinical trials with burosumab (UX023-CL303 / UX023-CL304).


XLH is a rare, genetic disorder that is serious, chronically debilitating and represents an unmet medical need. XLH is the most common inherited form of rickets and the most common inherited defect in renal tubular phosphate transport. XLH is transmitted as an X-linked dominant disorder. Mutations resulting in the loss of function of PHEX form the genetic basis for XLH. More than 300 different PHEX gene mutations have been identified in patients with XLH (PHEXdb); however, few definitive correlations have been observed between specific mutations and phenotypic severity.

Patients with XLH have hypophosphatemia due to excessive serum FGF23 levels. FGF23 reduces serum phosphorus levels by two distinct mechanisms of action. The primary mechanism is to inhibit phosphate reabsorption in the proximal tubule of the kidney. The secondary mechanism is to decrease phosphate absorption by the small intestine through the inhibition of 1,25(OH)2D production in the kidney.

Burosumab has the potential to block or reduce FGF23 action and improve phosphate homeostasis in XLH patients. Burosumab binds the amino-terminal domain of FGF23 that interacts with the FGF-binding portion of the combination FGFR1/Klotho receptor, preventing FGF23 from binding to and signaling from its receptor. Both intact and fragmented FGF23 polypeptides are immunoprecipitated with burosumab. By inhibiting FGF23, burosumab restores tubular reabsorption of phosphate (as measured by the ratio of renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate [TmP/GFR]) from the kidney and increases the production of 1,25(OH)2D that also enhances intestinal absorption of phosphate. The dual action on kidney reabsorption and intestinal absorption improves serum phosphorus levels, which is expected to improve bone mineralization and reduce the diverse bone and non-bone manifestations associated with hypophosphatemia in XLH patients.

Condition Familial Hypophosphatemic Rickets
Treatment Burosumab
Clinical Study IdentifierNCT03920072
SponsorKyowa Kirin Pharmaceutical Development Ltd
Last Modified on8 September 2021

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