Changes of drug pharmacokinetics mediated by downregulation of kidney organic cation transporters Mate1 and Oct2 in a rat model of hyperuricemia.

The effects of hyperuricemia on the expression of kidney drug transporters and on the pharmacokinetics of several substrate drugs were examined. We first established a rat model of hyperuricemia without marked symptoms of chronic kidney failure by 10-day co-administration of oxonic acid (uricase inhibitor) and adenine (biosynthetic precursor of uric acid). These hyperuricemic rats showed plasma uric acid concentrations of up to 6 mg/dL, which is similar to the serum uric acid level in hyperuricemic humans, with little change of inulin clearance. The mRNA levels of multidrug and toxin extrusion 1 (Mate1, Slc47a1), organic anion transporter 1 (Oat1, Slc22a6), organic cation transporter 2 (Oct2, Slc22a2), urate transporter 1 (Urat1, Slc22a12) and peptide transporter 1 (Pept1, Slc15a1) were significantly decreased in kidney of hyperuricemic rats. Since Oct2, Mate1 and Oat1 are important for renal drug elimination, we next investigated whether the pharmacokinetics of their substrates, metformin, cephalexin and creatinine, were altered. The plasma concentration of metformin was not affected, while its kidney tissue accumulation was significantly increased. The plasma concentration and kidney tissue accumulation of cephalexin and the plasma concentration of creatinine were also increased. Furthermore, the protein expression of kidney Mate1 was decreased in hyperuricemic rats. Accordingly, although multiple factors may influence renal handling of these drugs, these observations can be accounted for, at least in part, by downregulation of Mate1-mediated apical efflux from tubular cells and Oct2-mediated basolateral uptake. Our results suggest that hyperuricemia could alter the disposition of drugs that are substrates of Mate1 and/or Oct2.

Comparative Evaluation of Dehydroepiandrosterone Sulfate Potential to Predict Hepatic Organic Anion Transporting Polypeptide Transporter-Based Drug-Drug Interactions.

Pharmacokinetic drug-drug interactions (DDIs) on hepatic organic anion transporting polypeptides (OATPs) are important clinical issues. Previously, we reported that plasma dehydroepiandrosterone sulfate (DHEAS) could serve as an endogenous probe to predict OATP-based DDIs in monkeys using rifampicin as an OATP inhibitor. Since the contribution of hepatic OATPs to the changes in plasma DHEAS by rifampicin remains unclear, however, we performed an in vivo pharmacokinetic study to explore this issue. Since plasma DHEAS concentrations were low in our rat model, the disposition of externally administered DHEAS was evaluated. Intravenously administered DHEAS was recovered mainly in bile (29.1%) and less in urine (2.95%). The liver tissue-to-plasma concentration ratio (Kpliver) decreased from 41.8 to 5.07 by rifampicin, and this decrement was consistent with the decrease in distribution volume from 247 to 59 ml/rat. Comparison of the in vitro IC50 of rifampicin for DHEAS uptake by isolated rat hepatocytes and in vivo plasma rifampicin concentration suggested that the effect of rifampicin on the plasma DHEAS concentration was explained mostly by the inhibition of hepatic OATPs, demonstrating that DHEAS could be a biomarker of hepatic OATP activity. Next, previously reported rifampicin-induced changes in plasma concentrations evaluated as an AUC ratio (AUCR) of possible probe compounds were compared on the basis of rifampicin dose/body surface area. The AUCR values of endogenous compounds and i.v. administered statins, for which possible DDIs in the intestinal absorption process can be excluded, increased proportionally to the rifampicin dose. Simultaneous measurement of these endogenous compounds could be effective biomarkers for the prediction of OATP-based DDIs.

Enzymatic degradation of p-chlorophenol in a two-phase flow microchannel system.

Enzymatic degradation of p-chlorophenol was carried out in a two-phase flow in a microchannel (100 microm width, 25 microm depth) fabricated on a glass plate (70 mm x 38 mm). This is the first report on the enzymatic reaction in a two-phase flow on a microfluidic device. The surface of the microchannel was partially modified with octadecylsilane groups to be hydrophobic, thus allowing clear phase separation at the end-junction of the microchannel. The enzyme (laccase), which is surface active, was solubilized in a succinic aqueous buffer and the substrate (p-chlorophenol) was in isooctane. The degradation of p-chlorophenol occurred mainly at the aqueous-organic interface in the microchannel. We investigated the effects of flow velocity and microchannel shape on the enzymatic degradation of p-chlorophenol. Assuming that diffusion of the substrate (p-chlorophenol) is the rate-limiting step in the enzymatic degradation of p-chlorophenol in the microchannel, we proposed a simple theoretical model for the degradation in the microchannel. The calculated degradation values agreed well with the experimental data.