Currently, more than 20% of clinically used drugs contain at least one fluorine atom in their chemical structures. In drug design, fluorine is frequently used as a substitute for hydrogen. The bond energy between carbon and fluorine is notably higher than that of carbon and hydrogen so that replacement of a hydrogen with a fluorine can result in significantly improved stability against CYP enzyme oxidation. In addition, fluorine’s ability to form strong hydrogen bond can be utilized to attain higher binding affinity with the target protein. Similarly, introduction of fluorine atoms can be used to obtain desired physical properties such as hydrophobicity of the molecule and the increased acidity of nearby hydroxy and carboxy groups. Studies on introducing fluorine into vitamin D3, 25-hydroxyvitamin D3 (25(OH)D3), and active vitamin D3 were started in the 1970s. In 2001, falecalcitriol in which the two methyl groups at the end of the side chain were replaced by a trifluoromethyl group, respectively, was marketed as a therapeutic agent for secondary hyperparathyroidism. We performed systematic and comprehensive fluorination on the vitamin D3 side chain and synthesized all possible12 derivatives of fluorinated-25(OH)D3. It was found that the position, number, and stereochemistry of the introduced fluorine caused differences in binding affinity for vitamin D receptor (VDR), transcriptional activity through VDR, and metabolic resistance to CYP24A1.
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