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Abstract: TH-OR023

Burosumab Improves Phosphorus Metabolism and Fracture Healing in Adults with X-Linked Hypophosphatemia (XLH)

Session Information

Category: Bone and Mineral Metabolism

  • 402 Bone and Mineral Metabolism: Clinical


  • Portale, Anthony A., University of California San Francisco, San Francisco, California, United States
  • Insogna, Karl, Yale University, New Haven, Connecticut, United States
  • Imel, Erik, Indiana University School of Medicine, Indianapolis, Indiana, United States
  • Kamenicky, Peter, Université Paris-Sud, Le Kremlin-Bicêtre, France
  • Imanishi, Yasuo, Osaka City Univ Graduate School of Med, Abeno-ku, Osaka-Fu, Japan
  • Ito, Nobuaki, University of Tokyo Hospital, Tokyo, Japan
  • Perwad, Farzana, University of California San Francisco, San Francisco, California, United States
  • Zhang, Lin, Ultragenyx Pharmaceutical Inc, Novato, California, United States
  • Theodore-Oklota, Christina, Ultragenyx Pharmaceutical Inc., Novato, California, United States
  • Mealiffe, Matt, Ultragenyx Pharmaceutical Inc., Novato, California, United States
  • San martin, Javier, Ultragenyx Pharmaceutical Inc., Novato, California, United States
  • Carpenter, Thomas, Yale University, New Haven, Connecticut, United States

In an ongoing, Phase 3, double-blind, multicenter study, we examined the efficacy and safety of burosumab, a human monoclonal antibody against FGF23, in 134 adults with XLH.


Subjects were randomized 1:1 to receive burosumab 1 mg/kg (n=68) or placebo (n=66) subcutaneously every 4 weeks (wks). After 24 wks, subjects receiving placebo crossed-over to receive burosumab, and all subjects received burosumab during wks 24-48, while remaining blinded to prior treatment.


From wks 0 to 24, a significantly greater percentage of burosumab subjects than placebo subjects (94% vs 8%; p<0.0001) attained the primary endpoint of a mean serum phosphorus level within the normal range at the midpoint of dosing intervals; from wks 24-48, phosphorus was within the normal range in both groups (crossover 89%, burosumab-continuation 84%). At baseline, 65 active fractures/pseudofractures (Fx/PFx) were present in 47% (32/68) of subjects in the burosumab group and 91 Fx/PFx in 58% (38/66) of subjects in the placebo group. At wk 24, 43% (28/65) of Fx/PFx fully healed with burosumab and 8% (7/91) healed with placebo (odds ratio of full healing 16.8, p<0.0001). By wk 48, the burosumab-continuation group showed additional Fx/PFx healing, and the crossover group showed healing similar to that of the burosumab-continuation group at wk 24. At wk 48, both groups showed significant decreases from baseline (or wk 24 for crossover) in stiffness, physical functioning, and pain scores (all p<0.001). At wk 48, PTH decreased from baseline by 9% in the burosumab-continuation group and by 7% in the crossover group. Serum creatinine and calcium levels and urine calcium excretion were unchanged. Most subjects had no change in nephrocalcinosis severity score (5-point scale), and no subject had a change >±1 point. Serious AEs were reported in 15 subjects; none were assessed as drug-related.


Inhibition of FGF23 with burosumab was associated with improvements in serum phosphorus levels, pain, stiffness, and physical functioning, and healing of Fx/PFx in adults with XLH. The improvements seen with burosumab at wk 24 were replicated in placebo subjects who crossed over to receive burosumab from wks 24-48.


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