Synthesis of [6- ³ H]-1α-Hydroxyvitamin D ₃ and Its Metabolism in Vivo to [ ³ H]-1α,25-Dihydroxyvitamin D ₃

Abstract

[6-3H]-1alpha-Hydroxyvitamin D3 was chemically synthesized and its full biological activity and radiochemical purity were demonstrated. With the use of this preparation it has been possible to demonstrate in vivo that in rats the [6-3H]-1Alpha-hydroxyvitamin D3 is converted to [6-3H]-1alpha,25-dihydroxyvitamin D3, the natural hormone. In fact, in the intestine and bone of rats given 32 picomoles of [6-3H]-1alpha-hydroxyvitamin D3 each day for 6 days, more than 80 percent of the lipid-soluble radioactivity exists as [6-3H]-1alpha,25-dihydroxyvitamin D3, a finding that suggests that much of the biological effectiveness of 1alpha-hydroxyvitamin D3 is due to its conversion to 1alpha,25-dihydroxyvitamin D3.

1 alpha,25-Dihydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3 prolong survival time of mice inoculated with myeloid leukemia cells.

“…Abe et aL (8) reported that la(OH)D3 is only 1/100th as active as la,25(OH)2D3 in suppressing cell growth and inducing differentiation ofM1 cells in vitro. It also has been reported that la(OH)D3 is converted to la,25(OH)2D3 very rapidly in the liver (33,34). Therefore, it was suggested that the la(OH)D3 administered intraperitoneally was converted to la,25(OH)2D3 before it decreased leukemogenicity in vivo.…”

Differentiation of mouse myeloid leukemia cells induced by 1 alpha,25-dihydroxyvitamin D3.

“…It has been reported that la(OH)D3 has to be converted to la,25(0H)2D3 in the liver before it can be effective in stimulating intestinal calcium transport and bone mineral mobilization (24,25). Preliminary experiments, however, showed that la(OH)[3H]D3 was not converted to la,25(OH)2[3H]D3 by M1 cells during 3 days of cultivation (data not shown), suggesting that la(OH)D3 per se may be effective in inducing the differentiation of MI cells.…”

Bioengineering Anabolic Vitamin D-25-Hydroxylase Activity into the Human Vitamin D Catabolic Enzyme, Cytochrome P450 CYP24A1, by a V391L Mutation

“…In cancer cells that possess increased levels of CYP24A1, the test analog would be locally activated before becoming degraded, resulting in a longer-lived anti-proliferative/pro-differentiation signal, originally evaded by high CYP24A1 levels. This concept parallels that of the extra-renal CYP27B1 identified in breast, prostate, colon, and smooth muscle cells, which draws upon the circulating pool of 25-OH-D 3 via megalin/cubilin-mediated endocytosis to become locally activated in the target cell (10). The 1␣-hydroxylated test analog might possess several advantages, including a slightly lower affinity for the vitamin D binding protein than 1␣,25-(OH) 2 D 3 , suggesting increased transport into target cells, circumventing the requirement of the cell to express megalin/cubilin or CYP27B1.…”

“…The second step in activation, 1␣-hydroxylation, is carried out by CYP27B1 and occurs mainly in the kidney and to a lesser extent in extra-renal tissues (8,9). Synthetic 1␣-hydroxylated prodrug forms of vitamin D are also activated in the liver and bypass the kidney entirely (10). In vitro, hepatic CYP27A1 primarily 25-hydroxylates 1␣-OH-D 3 but also yields lesser amounts of 24-and 26-hydroxylated products (6).…”