Critical Role for 24-Hydroxylation in Homeostatic Regulation of Vitamin D Metabolism.

CONTEXT: The body has evolved homeostatic mechanisms to maintain free levels of Ca+2 and 1,25-dihydroxyvitamin D [1,25(OH)2D] within narrow physiological ranges. Clinical guidelines emphasize important contributions of PTH in maintaining this homeostasis. OBJECTIVE: To investigate mechanisms of homeostatic regulation of vitamin D (VitD) metabolism and to apply mechanistic insights to improve clinical assessment of VitD status. DESIGN: Crossover clinical trial studying participants before and after VitD3-supplementation. SETTING: Community. PARTICIPANTS: 11 otherwise healthy individuals with VitD-deficiency (25-hydroxyvitamin D [25(OH)D] ≤20 ng/mL). INTERVENTIONS: VitD3-supplements (50,000 IU once or twice a week depending on BMI, for 4-6 weeks) were administered to achieve 25(OH)D≥30 ng/mL. RESULTS: VitD3-supplementation significantly increased mean 25(OH)D by 2.7-fold and 24,25-dihydroxyvitamin D [24,25(OH)2D] by 4.3-fold. In contrast, mean levels of PTH, FGF23, and 1,25(OH)2D did not change. Mathematical modeling suggested that 24-hydroxylase activity was maximal for 25(OH)D≥50 ng/mL and achieved a minimum (∼90% suppression) with 25(OH)D<10-20 ng/mL. The 1,25(OH)2D/24,25(OH)2D ratio better predicted modeled 24-hydroxylase activity (h) (ρ=-0.85; p=0.001) compared to total plasma 25(OH)D (ρ=0.51; p=0.01) and the 24,25(OH)2D/25(OH)D ratio (ρ=0.37; p=0.3). CONCLUSIONS: Suppression of 24-hydroxylase provides a first line of defense against symptomatic VitD-deficiency by decreasing metabolic clearance of 1,25(OH)2D. The 1,25(OH)2D/24,25(OH)2D ratio provides a useful index of VitD status since it incorporates 24,25(OH)2D levels and therefore, provides insight into 24-hydroxylase activity. When VitD availability is limited, this suppresses 24-hydroxylase activity - thereby decreasing the level of 24,25(OH)2D and increasing the 1,25(OH)2D/24,25(OH)2D ratio. Thus, an increased 1,25(OH)2D/24,25(OH)2D ratio signifies triggering of homeostatic regulation, which occurs at early stages of VitD-deficiency.

Vitamin D Deficiency Increases Vulnerability to Canagliflozin-induced Adverse Effects on 1,25-Dihydroxyvitamin D and PTH.

CONTEXT: Canagliflozin has been reported to increase the risk of bone fracture-possibly mediated by decreasing 1,25-dihydroxyvitamin D (1,25(OH)2D) and increasing parathyroid hormone (PTH). OBJECTIVE: This work investigated whether baseline vitamin D (VitD) deficiency renders individuals vulnerable to this adverse effect and whether VitD3 supplementation is protective. METHODS: This community-based, outpatient study had a paired design comparing individual participants before and after VitD3 supplementation. Eleven VitD-deficient (25-hydroxyvitamin D [25(OH)D] ≤ 20 ng/mL) individuals were recruited from the Amish population in Lancaster, Pennsylvania. Participants underwent 2 canagliflozin challenge protocols (300 mg daily for 5 days): the first before and the second after VitD3 supplementation. In the VitD3 supplementation protocol, participants received VitD3 supplementation (50 000 IU once or twice a week depending on body mass index for 4-6 weeks) to achieve 25(OH)D of 30 ng/mL or greater. Two coprimary end points were identified: effects of VitD3 supplementation on canagliflozin-induced changes in 1,25(OH)2D and PTH. Secondary end points included effects of VitD3 supplementation on baseline levels of VitD metabolites and PTH. RESULTS: VitD3 supplementation increased mean 25(OH)D from 16.5 ± 1.6 to 44.3 ± 5.5 ng/mL (P = .0006) and 24,25-dihydroxyvitamin D (24,25(OH)2D) from 1.0 ± 0.1 to 4.3 ± 0.6 ng/mL (P = .0002). Mean 1,25(OH)2D and PTH were unchanged. VitD3 supplementation decreased the magnitude of canagliflozin-induced changes in 1,25(OH)2D (from -31.3%±4.7% to -9.3%±8.3%; P = .04) and PTH (from +36.2%±6.2% to +9.7%±3.7%; P = .005). CONCLUSION: VitD deficiency rendered individuals more vulnerable to adverse effects of canagliflozin on biomarkers associated with bone health. VitD3 supplementation was protective against canagliflozin's short-term adverse effects on 1,25(OH)2D and PTH.

Pharmacogenetics of sodium-glucose co-transporter-2 inhibitors: Validation of a sex-agnostic pharmacodynamic biomarker.

AIM: To validate pharmacodynamic responses to sodium-glucose co-transporter-2 (SGLT2) inhibitors and test for association with genetic variants in SLC5A4, SLC5A9, and SLC2A9. METHODS: Canagliflozin (300 mg), a SGLT2 inhibitor, was administered to 30 healthy volunteers. Several endpoints were measured to assess clinically relevant responses, including drug-induced increases in urinary excretion of glucose, sodium and uric acid. RESULTS: This pilot study confirmed that canagliflozin (300 mg) triggered acute changes in mean levels of several biomarkers: fasting plasma glucose (-4.1 mg/dL; P = 6 × 10-5 ), serum creatinine (+0.05 mg/dL; P = 8 × 10-4 ) and serum uric acid (-0.90 mg/dL; P = 5 × 10-10 ). The effects of sex on glucosuria depended upon how data were normalized. Whereas males' responses were ~60% greater when data were normalized to body surface area, males and females exhibited similar responses when glucosuria was expressed as grams of urinary glucose per gram-creatinine. The magnitude of glucosuria was not significantly correlated with fasting plasma glucose, estimated glomerular filtration rate or age in those healthy individuals without diabetes with an estimated glomerular filtration rate of more than 60 mL/min/1.73m2 . CONCLUSIONS: Normalizing data relative to creatinine excretion will facilitate including data from males and females in a single analysis. Furthermore, because our ongoing pharmacogenomic study (NCT02891954) is conducted in healthy individuals, this will facilitate detection of genetic associations with limited confounding by other factors such as HbA1c and renal function.