Calcium effects and systemic exposure of vitamin D3 analogues after topical treatment of active vitamin D3-containing ointments in rats.

Topical agents containing vitamin D3 (VD3) analogues such as calcipotriol, maxacalcitol and tacalcitol and the combination of calcipotriol/betamethasone dipropionate (betamethasone) are prescribed for patients with psoriasis. However, they are known to occasionally cause hypercalcemia, and the frequency of hypercalcemia is suggested to vary according to the VD3 analogue used. In this study, to address the reason for these differences, the calcemic effects of maxacalcitol-, calcipotriol- and calcipotriol/betamethasone-containing ointments in rats were evaluated. The serum calcium levels in rats treated with ointments containing maxacalcitol, but not calcipotriol or calcipotriol/betamethasone, were significantly elevated, which is consistent with clinical observations. The serum concentration of VD3 analogue in rats treated with ointments containing calcipotriol and calcipotriol/betamethasone was lower than that in rats treated with maxacalcitol-containing ointment. Thus, the calcemic effects appear to be associated with the systemic exposure of VD3 analogues in rats. To understand the mechanism underlying the different systemic exposures of VD3 analogues, skin permeation and metabolic stability of VD3 analogues were evaluated. The cumulative amount of calcipotriol permeated through rat skin was significantly lower than that of maxacalcitol. On the other hand, the metabolic clearance of calcipotriol in rat hepatocytes was higher than that of maxacalcitol. Similar results were obtained using human skin and human hepatocytes. The current study demonstrates that the lower calcemic effects of calcipotriol- and calcipotriol/betamethasone-containing ointments are caused by the low systemic exposure of calcipotriol according to low skin permeability and rapid hepatic elimination after topical application.

In-vitro evidence of enhanced breast cancer resistance protein-mediated intestinal urate secretion by uremic toxins in Caco-2 cells.

OBJECTIVES: It has been reported that intestinal urate excretion is increased at chronic kidney disease (CKD) state. In this report, whether uremic toxins are involved in the upregulation of intestinal breast cancer resistance protein (BCRP), an intestinal urate exporter, was examined. METHODS: Uremic toxins that were increased at least 15-fold at CKD state were selected for investigation. Caco-2 cells were exposed to these uremic toxins at clinically relevant concentrations. mRNA was quantified by real-time PCR, and flow cytometry was utilized to measure BCRP protein and function in Caco-2 cells. Transcellular secretory transport of [(14) C]urate was determined utilizing Transwell studies after uremic toxin exposure. KEY FINDINGS: Indoxyl sulfate (IS) treatment alone resulted in ∼ 3-fold increase in BCRP mRNA in Caco-2 cells. Membrane protein expression of BCRP in Caco-2 cells also was increased by 1.8-fold after treatment with IS. Intracellular accumulation of pheophorbide A, a selective BCRP substrate, was decreased by 22% after IS treatment for 3 days. Consistent with these findings, transcellular secretory transport of urate across Caco-2 cell monolayers was increased by 22%. CONCLUSION: Intestinal urate secretion may be increased at CKD state partially by upregulation of intestinal BCRP by uremic toxins such as IS.

Extra-renal elimination of uric acid via intestinal efflux transporter BCRP/ABCG2.

Urinary excretion accounts for two-thirds of total elimination of uric acid and the remainder is excreted in feces. However, the mechanism of extra-renal elimination is poorly understood. In the present study, we aimed to clarify the mechanism and the extent of elimination of uric acid through liver and intestine using oxonate-treated rats and Caco-2 cells as a model of human intestinal epithelium. In oxonate-treated rats, significant amounts of externally administered and endogenous uric acid were recovered in the intestinal lumen, while biliary excretion was minimal. Accordingly, direct intestinal secretion was thought to be a substantial contributor to extra-renal elimination of uric acid. Since human efflux transporter BCRP/ABCG2 accepts uric acid as a substrate and genetic polymorphism causing a decrease of BCRP activity is known to be associated with hyperuricemia and gout, the contribution of rBcrp to intestinal secretion was examined. rBcrp was confirmed to transport uric acid in a membrane vesicle study, and intestinal regional differences of expression of rBcrp mRNA were well correlated with uric acid secretory activity into the intestinal lumen. Bcrp1 knockout mice exhibited significantly decreased intestinal secretion and an increased plasma concentration of uric acid. Furthermore, a Bcrp inhibitor, elacridar, caused a decrease of intestinal secretion of uric acid. In Caco-2 cells, uric acid showed a polarized flux from the basolateral to apical side, and this flux was almost abolished in the presence of elacridar. These results demonstrate that BCRP contributes at least in part to the intestinal excretion of uric acid as extra-renal elimination pathway in humans and rats.