Markers and regulators of osteoblast and osteoclast activity in children with X-linked hypophosphatemia treated with burosumab.

Burosumab is effective for treatment of rickets in children with X-linked hypophosphatemia (XLH). The effects of burosumab on markers and regulators of osteoblast and osteoclast acitivity are largely unknown. In this cross-sectional observational study, we investigated 7 key markers and regulators of osteoblast and osteoclast activity in 107 burosumab-treated children with XLH. We calculated z-scores for bone-specific alkaline phosphatase (BAP), procollagen-1-N-terminal propeptide (P1NP), carboxy-terminal telopeptide of type 1 collagen (CTX), tartrate-resistant acid phosphatase type 5b (TRAP5b), soluble receptor activator of NF-κB ligand (sRANKL), osteoprotegerin (OPG), and sclerostin. The median age at investigation and duration of burosumab treatment were 10.7 and 2.2 yr, respectively, with 18.7% of patients on primary burosumab treatment and 81.3% with prior treatment with phosphate and active vitamin D. Height and body mass index improved by approx. 0.3 z-score from baseline with burosumab treatment, and serum phosphate, 1,25-dihydroxyvitamin D, and alkaline phosphatase improved by 2 z-scores (each p < .01 versus prior burosunab), but phosphate (-1.8 z-score) and calcium (-0.65 z-score) remained reduced (each p < .001 versus age and sex specific reference values). The z-scores for osteoblast (BAP, P1NP) and osteoclast (CTX, TRAP5b) activity markers and for the osteoblast suppressor sclerostin were increased (each p < .001). Standardized sRANKL was reduced (p < .05), while the RANKL inhibitor OPG was increased resulting in a reduced sRANKL/OPG ratio (each p < .001). Multivariate analysis revealed significant associations between serum sclerostin and serum phosphate (negative), serum calcium and vitamin D sufficiency (positive), and primary burosumab treatment with BAP (negative). In this real-world setting, pediatric XLH patients receiving burosumab treatment showed persisting mild hypophosphatemia and increased markers of osteoblast and osteoclast acitivity despite negative counter-regulation of osteoblasts by sclerostin and osteoclasts by OPG/RANKL. Our data suggest a more pronounced normalization osteoblast activity when children are primarily treated with burosumab and underscore the need for adequate vitamin D supplementation.