Vitamin D analogs with low affinity for the vitamin D binding protein: Enhanced in vitro and decreased in vivo activity

Abstract

The affinity of 1α,25‐dihydroxyvitamin D₃ [1α,25‐(OH)₂D₃] and analogs with side‐chain modifications [MC 903 or calcipotriol, MC 1147 or 24,24‐dihomo‐1α,25‐(OH)₂D₃ and 1,25‐(OH)₂‐16ene‐23yne‐D₃] for the vitamin D receptor and the serum vitamin D binding protein (DBP) were compared. The affinity of MC 903 for the receptor from chick and rat duodenum or from human peripheral blood mononuclear cells or HL‐60 cells varied between 60 and 100% relative to the affinity of 1,25‐(OH)₂D₃. The relative affinity of 1,25‐(OH)₂‐16ene‐23yne‐D₃ and MC 1147 varied for the same receptors between 45–70 and 3.5–25%, respectively. The relative affinity of MC 903 for human DBP was 30‐fold decreased, whereas the two other analogs did not bind to DBP at all even in more than 1000‐fold excess. The in vitro biologic activity of 1α,25‐(OH)₂D₃ on phytohemagglutinin‐stimulated normal human lymphocyte proliferation was markedly inhibited by the addition of physiologic amounts of DBP to the cell culture medium. No such inhibition was observed when MC 903 or 1147 was evaluated similarly. DBP therefore reversed the rank order of the in vitro potency of these analogs. Intramuscular injections for 10 consecutive days to vitamin D‐deficient chicks demonstrated a ≥ 100‐fold lower biologic activity of MC 903, MC 1147, and 1,25‐(OH)₂‐16ene‐23yne‐D₃ compared to that of 1α,25‐(OH)₂D₃ as evaluated by serum calcium and osteocalcin concentrations, as well as by duodenal calbindin D_28K and bone calcium content. We conclude that the biologic activity of vitamin D metabolites and analogs depends on their affinity for the vitamin D receptor as well as their affinity for DBP. Analogs with a low DBP but good receptor binding properties display low in vivo biologic activity on calcium and bone homeostasis, at least partly due to altered pharmacokinetics.