Effect of dipyridamole on serum and urinary phosphate in X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized clinically by rickets, hypophosphatemia and hyperphosphaturia. Conventional treatment of XLH with oral phosphate and vitamin D is associated with hypercalcuria and nephrocalcinosis. Recently, intravenous and oral dipyridamole has been reported to decrease fractional excretion of phosphate in adults with idiopathic hyperphosphaturia. Our objective was to determine whether oral dipyridamole therapy reduces urinary phosphate excretion and increases serum phosphate concentration in children with XLH. A prospective study was performed in six children with XLH. The average age of the patients at the start of the study was 12.5±1.0 years. The effects of 12 weeks of oral dipyridamole therapy, at 4.4±0.4 mg/kg body weight per day, on serum phosphorous, parathyroid hormone (PTH), 1,25 (OH)₂ vitamin D, osteocalcin, tubular maximum for phosphate reabsorption (TmP/GFR), urinary calcium excretion, and cyclic adenosine 3',5'-monophosphate (cAMP) excretion, were compared to baseline levels. Our results show that there was no change in serum phosphorous concentration or TmP/GFR after 12 weeks of dipyridamole therapy. Dipyridamole therapy also had no effect on serum PTH, serum 1,25 (OH)₂ vitamin D, alkaline phosphatase, osteocalcin levels, urinary calcium or cAMP excretion. We therefore concluded that in children with XLH, a 12-week course of dipyridamole had no effect on serum phosphorous or its urinary excretion. Dipyridamole therapy is unlikely to improve the bone disease in children with XLH.