Pierce, Theodore T.

Abstract Context Preclinical studies demonstrate that treatment with calcitriol attenuates skeletal complications in mice with X-linked hypophosphatemia (XLH). Objective This work aimed to assess serum markers of mineral metabolism, nephrocalcinosis, skeletal microarchitecture, growth, and evidence of rickets following calcitriol monotherapy in children and adults with XLH. Methods A 1-year, prospective, single-arm, open-label study was conducted comparing baseline and 12-month outcomes. Participants were recruited from outpatient endocrinology clinics in the United States. Data were collected in a research unit. Eligible participants were aged 4 years or older, not pregnant, with baseline 25-hydroxyvitamin D of 20 ng/mL or greater and normal serum calcium. Participants were treated with calcitriol, with dose based on serum and urinary calcium concentrations, for 1 year. The primary end points were change in serum phosphate and nephrocalcinosis (all participants) and rickets severity score (children). Secondary end points included changes in additional markers of mineral metabolism, skeletal microarchitecture parameters via high-resolution peripheral quantitative computed tomography (all participants), and height z-scores (children). Results Serum phosphate did not change over 12 months of optimized calcitriol treatment. Rickets improved in 2 of 4 children with open epiphyses; height z-scores were unchanged. Nephrocalcinosis scores remained stable. Adults had increased cortical thickness at the radius diaphysis (P = .012). Serum alkaline phosphatase decreased in children (P = .021) and parathyroid hormone trended lower both in children (P = .065) and adults (P = .063). Four participants developed mild hypercalcemia and 2 developed hypercalciuria that resolved with calcitriol titration. Conclusion Calcitriol monotherapy is safe and well-tolerated in XLH, with modest benefits on laboratory indices of mineral metabolism and rickets.