Use of vitamin D4 analogs to investigate differences in hepatic and target cell metabolism of vitamins D2 and D3

Abstract In this study, we used molecules with either of the structural differences in the side chains of vitamin D 2 and vitamin D 3 to investigate which feature is responsible for the significant differences in their respective metabolism, pharmacokinetics and toxicity. We used two cell model systems—HepG2 and HPK1A-ras—to study hepatic and target cell metabolism, respectively. Studies with HepG2 revealed that the pattern of 24- and 26-hydroxylation of the side chain reported for 1α-hydroxyvitamin D 2 (1α-OH-D 2 ) but not for 1α-OH-D 3 is also observed in both 1α-OH-D 4 and Δ 22 -1α-OH-D 3 metabolism. This suggests that the structural feature responsible for targeting the enzyme to the C24 or C26 site could be either the C24 methyl group or the 22–23 double bond. In HPK1A-ras cells, the pattern of metabolism observed for the 24-methylated derivative, 1α,25-(OH) 2 D 4 , was the same pattern of multiple hydroxylations at C24, C26 and C28 seen for vitamin D 2 compounds without evidence of side chain cleavage observed for vitamin D 3 derivatives, suggesting that the C24 methyl group plays a major role in this difference in target cell metabolism of D 2 and D 3 compounds. Novel vitamin D 4 compounds were tested and found to be active in a variety of in vitro biological assays. We conclude that vitamin D 4 analogs and their metabolites offer valuable insights into vitamin D analog design, metabolic enzymes and maybe useful clinically.