Nonhypercalcemic Vitamin D Analogs: Interactions with the Vitamin D-Binding Protein

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Abstract
The natural vitamin D hormone, 1α,25-dihydroxyvitamin D3 (lα,25-(OH)2D3), not only regulates serum and bone calcium homeostasis but is probably also a paracrine factor in several cells and tissues including skin, immune system, placenta and brain, where it stimulates cell differentiation and inhibits cell proliferation. Several structural analogs of 1α,25-(OH)2D3 not only have superagonist activity but also display a selective action profile: indeed they maintain or have increased activity on cell differentiation/proliferation but also have substantially decreased calcemic activity when compared to 1α,25-(OH)2D3. This decreased calcemic activity is partially due to mere pharmacological reasons: because of low binding affinity for the plasma vitamin D-binding protein, a more rapid extracellular metabolism and increased cellular uptake is possible when compared to 1α,25-(OH)2D3. Their short extracellular half-life combined with comparable or enhanced transactivation potency together with analog- and cell-type-specific intracellular metabolism can probably explain why some analogs have a unique combination of superagonist activity and specific action profile with favorable dissociation of differentiation versus calcemic potency.

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