The metabolism of 1,25(OH)(2)D(3) in clinical and experimental kidney disease

Chronic kidney disease (CKD) results in calcitriol deficiency and altered vitamin D metabolism. The objective of this study was to assess the 24-hydroxylation-mediated metabolism of 25(OH)D(3) and 1,25(OH)(2)D(3) in a cross-sectional analysis of participants with a range of kidney function assessed by precise measured GFR (mGFR) (N = 143) and in rats with the induction and progression of experimental kidney disease. Vitamin D metabolites were assessed with LC–MS/MS. Circulating measures of 24-hydroxylation of 25(OH)D(3) (24,25(OH)(2)D(3):25(OH)D(3)) precisely decreased according to mGFR in humans and progressively in rats with developing CKD. In contrast, the 1,24,25(OH)3D3: 1,25(OH)(2)D(3) vitamin D metabolite ratio increased in humans as the mGFR decreased and in rats with the induction and progression of CKD. Human participants taking cholecalciferol had higher circulating 1,24,25(OH)(3)D(3), despite no increase of 1,25(OH)(2)D(3). This first report of circulating 1,24,25(OH)(3)D(3) in the setting of CKD provides novel insight into the uniquely altered vitamin D metabolism in this setting. A better understanding of the uniquely dysfunctional catabolic vitamin D profile in CKD may guide more effective treatment strategies. The potential that 24-hydroxylated products have biological activity of is an important area of future research.