Calcitroic Acid–A Review

Calcitroic acid was isolated and characterized almost four decades ago, but little is known about this important vitamin D metabolite. Four reported synthetic strategies to generate calcitroic acid are presented that highlight the scientific progress in the field of chemistry directed to vitamin D analog synthesis. The most recent synthesis described the generation of calcitroic acid with an overall yield of 12.8% in 13 steps. The endogenous formation of calcitroic acid has been demonstrated in perfused rat kidney using 24,25,26,27-tetranor-1,23­(OH)2D3. Although, the majority of vitamin D metabolism is mediated by 24-hydoxylase (CYP24A1), it is not clear why the formation of calcitroic acid was not observed in the presence of recombinant CYP24A1 enzyme. Furthermore, it is not known if enzyme 1α-hydroxylase (CYP27B1) can convert calcioic acid into calcitroic acid. In addition to the lack of research investigating the endogenous formation of calcitroic acid, the physiological role of calcitroic acid remains unknown. Only a few reports mentioned the biological activity of calcitroic acid in connection with the vitamin D receptor (VDR). When administered subcutaneously, calcitroic acid has anthracitic properties and elevates calcium blood levels when administered intravenously. In vitro, calcitroic acid at higher concentrations has been shown to bind VDR and induce gene transcription. However, these studies were not carried out in cells derived from target organs of calcitroic acid such as kidney, liver, and intestine. We can conclude that our current knowledge of calcitroic acid is limited, and more studies are needed to identify its physiological role.