Burosumab in Chinese adults with X-linked hypophosphatemia: a phase 4 study.

X-linked hypophosphatemia (XLH) is a rare genetic disorder, for which burosumab treatment may be beneficial. This study evaluated the efficacy, pharmacokinetics, and safety of burosumab in Chinese adults with XLH. In this open-label, multicenter, postmarketing phase 4 study, 18 Chinese adult outpatients with XLH received conventional oral phosphate (30 mg/kg/d in 5 doses) and calcitriol (0.50 μg/d in 2 doses) during a 14-wk run-in period. Patients then received subcutaneous burosumab 1.0 mg/kg every 4 wk for 48 wk (12 doses). The main outcome measures were changes from baseline in mean serum phosphate level at the end of burosumab treatment, along with patient-reported outcomes. Sixteen (88.9%) and four (22.2%) patients achieved mean serum phosphate levels >2.5 mg/dL at the mid-point and the end of the dose cycle, respectively. The mean (SD) serum phosphate level increased from 1.6 (0.25) mg/dL at baseline to 2.4 (0.28) mg/dL at the end of the dose cycle. The mean (SD) change from baseline in serum phosphate level was 50.1% (14.11%) following burosumab treatment. The tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR) increased from 1.3 (0.32) mg/dL at week 0 to 2.0 (0.42) mg/dL at week 48. Over 48 wk, patients reported that their worst pain, severity, interference, and stiffness were relieved and Measurement Information System quality of life assessment scores increased. Motor function assessed by the 6-min walking test improved significantly (72.7% increase from baseline to week 48). All treatment-emergent adverse events were mild or moderate; none were serious or serious treatment-related. Burosumab treatment led to sustained correction of serum phosphate levels, enhanced renal phosphate reabsorption (ie, TmP/GFR improvements), and improved stiffness, pain, and functional exercise capacity. Treatment was well-tolerated without any new safety signals in Chinese patients with XLH.