Bone Abstracts

Objective: Burosumab, a recombinant human monoclonal antibody that inhibits FGF23, was approved by the FDA in April, 2018 for the treatment of X-linked hypophosphatemia (XLH) in children and adults. XLH pediatric patients are typically affected with lower extremity deformities, short stature, bone pain, and limited physical activity. Burosumab is superior to conventional therapy in normalizing blood phosphate levels, thereby healing rickets, decreasing leg bowing, and reducing pain. Data on the impact of burosumab on pediatric patients’ growth are limited and data on their body composition are lacking altogether.

Methods: In May 2018, 6 XLH patients (2 males), age range 4–11.5 years, were started on subcutaneous burosumab therapy in our tertiary medical center. Burosumab was administered every 2 weeks, and dose-adjusted (between 0.4–1.9mg/kg) to achieve a serum phosphorus level at the low end of the normal range. Oral phosphate supplement and calcitriol were discontinued before starting burosumab. Anthropometric and body composition measurements, i.e., body fat percentage and distribution, muscle mass and distribution [bioelectrical impedance scale in children>5 years old] were assessed at treatment initiation, at 3 and 6 months.

Results: Growth velocity of burosumab-treated patients increased from baseline to 3 and 6 months: 5.1±0.7 cm/year, 9.0±1.9 cm/year, and 7.8±1.5 cm/year. Mean height z-scores gradually increased, while mean body mass index z-scores gradually decreased (–1.87±0.74 SDS, –1.74±0.72 SDS, and −1.63±0.64 SDS; 1.17±0.55SDS, 1.06±0.44 SDS, and 1.03±0.48 SDS from baseline to 3 and 6 months, respectively). Body composition of 4 patients (2 males; >5 years) showed gradual reduction in body fat percentage (25.7±3.3, 25.2±2.6, and 23.9±3.0), with increased muscle mass that was greater in the lower limbs (right leg: 3.1±1.3, 3.2±1.3, and 3.4±1.4; left leg:3.0±1.3; 3.2±1.3, and 3.3±1.3). All the patients reported decreased pain and increased ability to engage in physical activity. Adverse events were local and mild.

Discussion: We demonstrated a new and not yet recognized important beneficial effect of burosumab therapy on body composition of treated children: decreased fat percentage with simultaneous increased muscle mass, especially in the lower limbs. Burosumab cures rachitic bone changes, reduces pain, and improves patients’ physical capabilities. Studies with long-term follow-up are needed for further examination of burosumab’s impact on body composition and its metabolic implications.

Disclosure: The authors declared no competing interests.