Increased biological activity of 20-epi-1,25-dihydroxyvitamin D3 is due to reduced catabolism and altered protein binding.

The 20-epi series of vitamin D3 analogs has been shown to be made up of more potent inducers of cell differentiation than calcitriol in vitro. Using 20-epi-1 alpha,25-dihydroxyvitamin D3 (MC 1288), we attempted to rationalize this increased biological activity by examining several parameters including the binding affinity of the analog for the plasma binding globulin (DBP) and the target cell vitamin D receptor (VDR), as well as attempting to measure the rate at which MC 1288 is metabolized. MC 1288 was found to be metabolized 36 times more slowly than its epimer 1,25-dihydroxy vitamin D3 (1,25-(OH)2D3), forming several metabolites which were analogous to metabolites of 1,25-(OH)2D3 formed in the side chain oxidation pathway. Bovine thymus VDR bound MC 1288 with five times greater affinity than calcitriol, while rat plasma DBP did not bind MC 1288 even at a concentration of 50 microM, 5000 times the B50 of 25-OH-D3, the ligand used in the assay. Using a vitamin D-inducible growth hormone gene reporter system we were able to demonstrate that MC 1288 induces human growth hormone (hGH) activity 30-fold more efficiently than 1,25-(OH)2D3 in the presence of fetal calf serum (FCS), while the analog is only 10 times more efficient than 1,25-(OH)2D3 in the absence of FCS. We therefore conclude that MC 1288 is more biologically active than calcitriol in vitro due to a combination of factors: the increased VDR binding affinity, the decreased DBP binding affinity, and the decreased rate of metabolism. As with other analogs of vitamin D, the altered protein binding and decreased catabolism of MC 1288 may be important in pharmaceutical applications such as a topical treatment for psoriasis or skin cancer.