Purpose To assess the long-term efficacy of burosumab for paediatric patients with X-linked hypophosphatemia, focusing on linear growth. Methods This multi-center retrospective study included 35 paediatric patients who began treatment with burosumab between January 2018 and January 2021. We collected clinical data, anthropometric measurements, laboratory results and rickets severity score (RSS), from two years prior to treatment initiation and up to four years after. Results Burosumab was initiated at a mean age of 7.5 ± 4.4 years (range 0.6–15.9), with a mean initial dose of 0.8 ± 0.3 mg/kg, which was subsequently increased to 1.1 ± 0.4 mg/kg. The patients were followed for 2.9 ± 1.4 years (range 1–4) after initiating burosumab. Serum phosphorus levels increased from 2.7 ± 0.8 mg/dl at burosumab initiation to 3.4 ± 0.6 mg/dl after three months, and remained stable (p < 0.001). Total reabsorption of phosphorus increased from 82.0 ± 6.8% to 90.1 ± 5.3% after 12 months of treatment (p = 0.041). The RSS improved from 1.7 ± 1.0 at burosumab initiation to 0.5 ± 0.6 and 0.3 ± 0.6 after 12 and 24 months, respectively (p < 0.001). Both height z-score and weight z-score improved from burosumab initiation to the end of the study: from − 2.07 ± 1.05 to -1.72 ± 1.04 (p < 0.001), and from − 0.51 ± 1.12 to -0.11 ± 1.29 (p < 0.001), respectively. Eight children received growth hormone combined with burosumab treatment. Height z-score improved among those who received growth hormone (from − 2.33 ± 1.12 to -1.94 ± 1.24, p = 0.042) and among those who did not (from − 2.01 ± 1.01 to -1.66 ± 1.01, p = 0.001). Conclusion Burosumab treatment in a real-life setting improved phosphate homeostasis and rickets severity, and enhanced linear growth.
Background:MCTO is a rare disorder, caused by mutations in the MABF gene, a negative regulator of RANKL. Manifestations include carpal tarsal osteolysis and subsequent renal failure in some. Pathophysiology is poorly understood, and no effective treatment is available. Clinical case:A 5y old boy presented (2011) with R wrist pain and diffuse swelling. MRI showed pan-carpal synovitis with joint effusion. He did not respond to different anti-inflammatory medications. Plain films showed central loss of the proximal row of carpal bones. His mother was followed as an adolescent with presumed juvenile rheumatoid arthritis. Genetic testing confirmed MAFB gene mutation (c.206C>T,p.Ser69Leu) in both patient and mother.At 7y, skeletal survey showed diffuse osteopenia and mild height loss in T1. DXA (L1-4) Z-score was -0.7. Calcium phosphate metabolism indices were within reference ranges. Bone Specific Alk Pi was modestly increased and C-telopeptide markedly increased.He received Denosumab (0.5-0.75 mg/Kg) 4-monthly for two years and experienced less pain and increased daily activities with improved R wrist function. Osteolysis stabilized and none was noted in the L wrist or ankles. BMD Z-score was -0.2. A year following treatment (2016) he received two more injections of Denosumab following pain and movement restriction of R elbow, R knee and L ankle. In 2019 (13y) he fell and radiology showed, R knee osteopenia, R wrist almost complete destruction of the carpal bones. Neither ankle nor L wrist showed osteolysis. R upper limb musculature was wasted when compared with the left. Shoulder and elbow strength were preserved. BMD Z-score was -1.2. Serum calcium, 25(OH)Vitamin D and PTH were normal. Bone specific alkaline phosphatase and C-Telopeptide were elevated. Serum creatinine was normal, eGFR 150 ml/min/1.73m2, ACR (6.6 [normal< 3.5] mg/mmol)) with no hypercalciuria or nephrocalcinosis. He was normotensive.High resolution peripheral quantitative computerized tomography (HRpQCT) of the L distal radius and distal tibia compared with 7 age-matched healthy males showed reduced total volumetric BMD (186.4;198.2-306.4), normal trabecular volumetric BMD and markedly reduced cortical volumetric BMD (320.4;636.5-792.5). Cortical thickness was below the expected range. HRpQCT measurements of the R wrist and tibia were similar. sRANKL, 6 weeks after Denosumab were markedly increased in both undiluted (35.4 pmol/L) and averaged diluted samples (83.73 pmol/L) when compared with healthy age-matched children (0.21-0.41pmol/L). Conclusions/Clinical Lessons:MCTO (MAFB, mutation c.206C>T,p.Ser69Leu), has a generalized high turnover skeletal phenotype (osteoporosis), likely driven by very high levels of sRANKL. Denosumab is a targeted treatment for the osteoporosis, which may help stabilize the osteolysis.
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