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X-Linked Hypophosphatemia: Effect of Calcitriol on Renal Handling of Phosphate, Serum Phosphate, and Bone Mineralization^*

The De Belle Laboratory for Biochemical Genetics, McGill University-Montreal Children's Hospital Research Institute Quebec, Canada
the Genetics Unit, Montreal Shriners Hospital Quebec, Canada
the Genetics Unit, Montreal Shriners Hospital, the Human Genetics Centre Quebec, Canada
Montreal Shriners Hospital, the Human Genetics Centrethe Departments of Biology, McGill University, Montreal Quebec, Canada
Montreal Shriners Hospital, the Human Genetics Centrethe Departments of Pediatrics, McGill University, Montreal Quebec, Canada
Montreal Shriners Hospital, the Human Genetics Centrethe Departments of Surgery, McGill University, Montreal Quebec, Canada

Address all correspondence and requests for reprints to: Charles R. Scriver, Montreal Children's Hospital, Room A-707, 2300 Tupper Street, Montreal, Quebec, Canada H3H 1P3.

Eleven patients with the Mendelian phenotype of x-linked hypophosphatemia (XLH) were treated with calcitriol [1,25-(OH)₂₃] and phosphate. Ten patients had received prior treatment with ergocalciferol and phosphate. Five subjects were prepubertal and six were postpubertal. Response to calcitriol was measured under nonfasting and overnight fasting protocols. Bone biopsies were obtained before and after treatment. Calcitriol (mean dose, 30 ng/kg-day) 1) raised serum phosphorus uniformly in prepubertal patients but in only two of six postpubertal subjects; 2) did not change the theoretical renal phosphate threshold in the total patient group and thus had no effect on the primary transport defect in XLH; 3) improved trabecular bone mineralization in the total patient group, as determined by bone histomorphometry. The beneficial effect on extracellular phosphorus homeostasis was attributed to improved intestinal absorption of phosphorus; improvement in bone mineralization may reflect an additional effect of 1,25-(OH)₂D₃ on bone itself in XLH. Mild transient hypercalcemia occurred during 0.6% of 3545 treatment days and was readily controlled by adjusting the dosage of 1,25-(OH)₂D₃.

^* Presented in part at the 52nd Annual Meeting of the Society for Pediatric Research, San Antonio, TX, May 1, 1980. This work was supported in part by the Quebec Network of Genetic Medicine, the Medical Research Council of Canada, Le Conseil de la Recherche en Sante du Quebec, The Queen Elizabeth II Research Fund, the Shriners of North America, and Hoffman-LaRoche, Ltd. (Canada).

Received July 21, 1980.

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