Usefulness and pharmacokinetics of subcutaneous calcitriol in the treatment of secondary hyperparathyroidism

Noteworthy idiosyncrasies of 1α,25-dihydroxyvitamin D3 kinetics for extrapolation from mouse to man: Commentary

Calcitriol or 1,25-dihydroxyvitamin D₃ [1,25(OH)₂ D₃ ] is the active ligand of the vitamin D receptor (VDR) that plays a vital role in health and disease. Vitamin D is converted to the relatively inactive metabolite, 25-hydroxyvitamin D₃ [25(OH)D₃ ], by CYP27A1 and CYP2R1 in the liver, then to 1,25(OH)₂ D₃ by a specific, mitochondrial enzyme, CYP27B1 (1α-hydroxylase) that is present primarily in the kidney. The degradation of both metabolites is mostly carried out by the more ubiquitous mitochondrial enzyme, CYP24A1. Despite the fact that calcitriol inhibits its formation and degradation, allometric scaling revealed strong interspecies correlation of the net calcitriol clearance (CL estimated from dose/AUC_∞ ), production rate (PR), and basal, plasma calcitriol concentration with body weight (BW). PBPK-PD (physiologically based pharmacokinetic-pharmacodynamic) modeling confirmed the dynamic interactions between calcitriol and Cyp27b1/Cyp24a1 on the decrease in the PR and increase in CL in mice. Close scrutiny of the literature revealed that basal levels of calcitriol had not been taken into consideration for estimating the correct AUC_∞ and CL after exogenous calcitriol dosing in both animals and humans, leading to an overestimation of AUC_∞ and underestimation of the plasma CL. In humans, CL was decreased in chronic kidney disease but increased in cancer. Collectively, careful pharmacokinetic data analysis and improved definition are achieved with PBPK-PD modeling, which embellishes the complexity of dose, enzyme regulation, and disease conditions. Allometric scaling and PBPK-PD modeling were applied successfully to extend the PBPK model to predict calcitriol kinetics in cancer patients.

Comparative review of the pharmacokinetics of vitamin D analogues

This article reviews the pharmacokinetic characteristics of calcitriol, paricalcitol, and doxercalciferol, and provides an overview of the metabolism of vitamin D. Calcitriol and paricalcitol have similar pharmacokinetic profiles, with terminal half-lives ranging from 5 to 10 hours in healthy subjects to 15-30 hours in patients undergoing dialysis. Both are active on intravenous administration and little of the active agent remains in the circulation after 24 hours, although they are normally given every 48-72 hours. Doxercalciferol is a prohormone, requiring hepatic metabolism to the active metabolite 1alpha,25-(OH)2D2. The half-life of 1alpha,25-(OH)2D2 is about 34 hours in healthy subjects and about 45 hours in dialysis patients, resembling physiologic blood concentrations of endogenous vitamin D. More studies are warranted to determine the disposition of the vitamin D analogues, especially in selected populations such as pediatric and geriatric dialysis patients.

Effect of age and menopause on serum concentrations of pentosidine, an advanced glycation end product

BACKGROUND

Pentosidine is an advanced glycation end product. Our aim is to investigate (a) the age-related change of serum pentosidine and (b) the effect of menopause on serum pentosidine.

METHODS

Using the high-performance liquid-chromatography method with column switching, we measured serum pentosidine in 140 healthy women aged 20-93 years. Serum creatinine was also measured. The samples of 13 young and 13 old subjects were used for the measurements of free pentosidine and fractions of pentosidine. Free pentosidine was measured without hydrolysis, and the fractions were measured with a 10,000 mol wt cutoff filter. To investigate the effect of menopause on pentosidine, two biochemical markers for bone turnover (CTx and osteocalcin) were measured in age-matched premenopausal and postmenopausal women (16 in each group).

RESULTS

Serum pentosidine significantly increased with age (r = .702, p < .0001 ). The values of serum pentosidine for the groups beyond the age of 50 were significantly higher than those for the younger groups. The value for the group aged 80-93 years was three times higher than that for the group aged 20-29 years. Serum pentosidine moderately and significantly correlated to serum creatinine (r = .483, p = .0001). Free pentosidine was detected in only 3 of 13 young subjects and 2 of 13 old subjects. The ratio of free to total pentosidine was 2.9% and 1.2% in young and old subjects, respectively. Pentosidine <10,000 mol wt was not detected in all subjects. Pentosidine >10,000 mol wt was detected in all subjects. Serum CTx and osteocalcin significantly increased in postmenopausal women compared with those of pre-menopausal women. There was no significant change in serum pentosidine between the premenopause group and the postmenopause group.

CONCLUSION

Serum pentosidine significantly increased with age in healthy subjects aged 20-93 years and correlated to serum creatinine. The changes of fractions of pentosidine with aging were not observed. There was no effect of menopause on pentosidine.