Comparison between the 5,6-cis and 5,6-trans isomers of 25-hydroxyvitamin D3 in chronic hypoparathyroidism

Five patients with chronic post-operative hypoparathyroidism were treated with 450 microgram/day 5,6-trans-25-hydroxyvitamin D3 (5,6-trans-25OHD3) for 14 days, and the treatment was continued with 150 microgram/day for one year. At the end of this period the patients received 450 microgram/day 5,6-cis-25-hydroxyvitamin D3 (5,6-cis-25OHD3) for 14 days. Comparison of the effects of both isomers revealed a similar ability to enhance intestinal calcium absorption and to normalize serum calcium; serum phosphate and alkaline phosphatase, however, remained unaffected. Urinary phosphate and hydroxyproline excretion decreased on the cis-isomer and increased on the trans-isomer. During treatment with the lower dose of 5,6-trans-25OHD3 intestinal calcium absorption remained in the normal range for one year, whereas the serum calcium decreased to the levels observed before administration of 450 microgram/day within 6 weeks. The results suggest that in hypoparathyroidism 5,6-cis-25OHD3 and 5,6-trans-25OHD3 are equally effective on serum calcium and on intestinal calcium absorption, but that their mode of action on renal phosphate handling and on calcium release from bone is different.