Circulating 1 alpha,25-dihydroxyvitamin D levels after a single dose of 1 alpha,25-dihydroxyvitamin D3 or 1 alpha-hydroxyvitamin D3 in normal men.

In order to clarify the differences between 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 1 alpha-hydroxyvitamin D3 (1(OH)D3) as a drug, we determined the serum levels of 1,25(OH)2D at different times after oral administration of each drug to normal men. In the paired test which was carried out on the same people, the integrated change of serum 1,25(OH)2D levels, defined as the mean area under the concentration curve over 24 h, was significantly higher in the 1,25(OH)2D3 group than in the 1(OH)D3 group. The mean of the peak serum level (Cmax) was also significantly higher in the former group. On the other hand, the mean time to reach the peak level (Tmax) was significantly shorter in the 1,25(OH)2D3 group than in the 1(OH)D3 group. The same results were obtained even when we performed the non-paired administration tests. In the present study, the peak serum level of 1,25(OH)2D after ingestion of a single dose of 1,25(OH)2D3 was reached rapidly, i.e., between 4 and 6 h, and the value had usually returned to the baseline by 24 h. On the contrary, 1(OH)D3 administration resulted in a slow rise in the serum level of 1,25(OH)2D with a relatively low peak level. Therefore, clinical use of these two drugs demands careful consideration of their differences.