Biological potency of a fluorinated vitamin D analogue in hypoparathyroidism

Several in vivo experiments have revealed that a synthesized fluorinated analogue of vitamin D, 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 (26,27-F6-1,25(OH)2D3) has a higher and longer-lasting biological activity than 1,25(OH)2D3 in calcium regulating actions. We evaluated the biological potency and the availability for clinical use of this compound in hypocalcemia associated with hypoparathyroidism. In an experimental setting, daily administration of 650 pmol/kg of 26,27-F6-1,25(OH)2D3 showed an equivalent effect to that of 3250 pmol/kg of 1,25(OH)2D3 in elevating whole blood ionized calcium levels in parathyroidectomized rats. Furthermore, an additional clinical study demonstrated that 0.5-1.5 micrograms/day of 26,27-F6-1,25(OH)2D3 were considered adequate maintenance doses in various types of hypoparathyroidism and that changes of medication to the same doses of 1,25(OH)2D3 resulted in prompt decline of urinary calcium excretion and of whole blood ionized calcium levels, and in recurrence of symptoms related to hypocalcemia. Although the mechanism responsible for the high potency of this analogue remains unclear, our experience confirms that 26,27-F6-1,25(OH)2D3 has higher biological activities in bone calcium mobilization and is more potent than 1,25(OH)2D3 in correcting hypocalcemia of hypoparathyroidism in a hospital setting.