Pharmacokinetics of 24,25-dihydroxyvitamin D3 in humans

Pharmacokinetic properties of pharmacological doses of 24,25-dihydroxyvitamin-D3 [24,25(OH)2D3] were determined in healthy volunteers. Four male subjects received 25 micrograms of 24,25(OH)2D3 as an intravenous bolus injection. Plasma concentrations of 24,25(OH)2D3, 25-hydroxyvitamin D and 1,25-dihydroxy-vitamin D were monitored during 14 days. In addition, serum ionized calcium, total calcium, inorganic phosphate, albumin, creatinine and intact hPTH(1-84) were measured during 14 days. The concentration-time curve of 24,25(OH)2D3 could be described by a two-exponential curve with half-lives of 3.0 +/- 0.9 hrs and 8.2 +/- 2.9 days (mean +/- SD). The volume of distribution was 0.19 +/- 0.02 liters/kg. None of the mentioned biochemical parameters, except serum 24,25(OH)2D3, changed markedly. In 18 subjects suffering from primary hyperparathyroidism, taking 25 micrograms of 24,25(OH)2D3 daily during three months, an average plateau level of 39 +/- 12 nmol/l of serum was observed. Bioavailability as estimated from this plateau level was approximately 70%.