Biological activities of 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 on human promyelocytic leukemic HL-60 cells: effects of fetal bovine serum and of incubation time.

The hexafluorinated vitamin D3 analog, 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3[F6-1,25-(OH)2D3] is more potent than 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] regarding various physiological effects. When the biological potencies of vitamin D3 analogs were assessed 24 h after the addition by the induction of 24-hydroxylation activity in the human promyelocytic leukemia cell line, HL-60,F6-1,25-(OH)2D3 was 6 times as potent as 1,25-(OH)2D3 in a medium containing 5% fetal bovine serum. When the cells were cultured in a serum-free medium, F6-1,25-(OH)2D3 was only equipotent to 1,25-(OH)2D3. Considering a previous report demonstrating a weaker binding of F6-1,25-(OH)2D3 to serum vitamin D-binding protein (DBP) than 1,25-(OH)2D3, it seems that the resultant greater free fraction of F6-1,25-(OH)2D3 might account for its greater activity in a serum-containing medium. As assessed by the suppression of cell proliferation and the induction of cell differentiation along the monocyte/macrophage pathway which requires as long as 96 h for their assessment, the potency ratio of F6-1,25-(OH)2D3 to 1,25-(OH)2D3 increased as the levels of fetal bovine serum increased. It was of great interest that F6-1,25-(OH)2D3 was still significantly more potent than 1,25-(OH)2D3 even in a serum-free medium. Together with the data indicating the equipotency of F6-1,25-(OH)2D3 and 1,25-(OH)2D3 in the induction of 24-hydroxylation activity, it was suggested that decreased metabolic inactivation might contribute in part to the higher potency of F6-1,25-(OH)2D3 in the long-term effect.