Influence of renal function on the steady-state pharmacokinetics of the antiarrhythmic propafenone and its phase I and phase II metabolites

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Systematic and quantitative assessment of the effect of chronic kidney disease on CYP2D6 and CYP3A4/5

Recent reviews suggest that chronic kidney disease (CKD) can affect the pharmacokinetics of nonrenally eliminated drugs, but the impact of CKD on individual elimination pathways has not been systematically evaluated. In this study we developed a comprehensive dataset of the effect of CKD on the pharmacokinetics of CYP2D6‐ and CYP3A4/5‐metabolized drugs. Drugs for evaluation were selected based on clinical drug–drug interaction (CYP3A4/5 and CYP2D6) and pharmacogenetic (CYP2D6) studies. Information from dedicated CKD studies was available for 13 and 18 of the CYP2D6 and CYP3A4/5 model drugs, respectively. Analysis of these data suggested that CYP2D6‐mediated clearance is generally decreased in parallel with the severity of CKD. There was no apparent relationship between the severity of CKD and CYP3A4/5‐mediated clearance. The observed elimination‐route dependency in CKD effects between CYP2D6 and CYP3A4/5 may inform the need to conduct clinical CKD studies with nonrenally eliminated drugs for optimal use of drugs in patients with CKD.

Characterisation of (R/S)-propafenone and its metabolites as substrates and inhibitors of P-glycoprotein

Digoxin is a drug with a narrow therapeutic index, which is a substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to the inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidney. It is well known that serum concentrations of digoxin increase considerably in humans if propafenone is given simultaneously. However, it has not been investigated in detail whether propafenone and its metabolites are substrates and/or inhibitors of human P-glycoprotein. The aim of this study, therefore, was to investigate the P-glycoprotein-mediated transport and inhibition properties of propafenone and its major metabolites 5-hydroxypropafenone and N-desalkylpropafenone in Caco-2 cell monolayers. Inhibition of P-glycoprotein-mediated transport by propafenone and its metabolites was determined using digoxin as a P-glycoprotein substrate. No polarised transport was observed for propafenone and N-desalkylpropafenone in Caco-2 cell monolayers. However, 5-hydroxypropafenone translocation was significantly greater from basal-to-apical compared with apical-to-basal (P(app) basal-apical vs. P(app) apical-basal, 10.21+/-2.63 x 10(-6) vs. 4.34+/-1.84 x 10(-6) cm/s; P<0.01). Moreover, propafenone, 5-hydroxypropafenone and N-desalkylpropafenone inhibited P-glycoprotein-mediated digoxin transport with IC(50) values of 6.8, 19.9, and 21.3 microM, respectively. In summary, whereas propafenone and N-desalkylpropafenone are not substrates of P-glycoprotein, 5-hydroxypropafenone is translocated by human P-glycoprotein across cell monolayers. In addition, propafenone and its two major metabolites 5-hydroxypropafenone and N-desalkylpropafenone are inhibitors of human P-glycoprotein and therefore contribute to the digoxin-propafenone interaction observed in humans.

In vitro cleavage of paracetamol glucuronide by human liver and kidney beta-glucuronidase: determination of paracetamol by capillary electrophoresis

A capillary electrophoresis (CE) method was developed using paracetamol glucuronide as a novel probe for human beta-glucuronidase activity. Using UV detection without prior sample clean-up procedures, fast and reliable quantitation of the released paracetamol was possible. The method showed good precision, accuracy and sensitivity with a limit of detection of 0.25 microM (38 ng/ml) and a limit of quantitation of 1 microM (151 ng/ml). The suitability of the method has been shown for enzyme kinetic studies using different liver and kidney homogenates, respectively. Our data clearly demonstrate that paracetamol glucuronide is cleaved by human beta-glucuronidase thereby releasing paracetamol. The CE method presented is not only a valuable tool for measuring human beta-glucuronidase activity, but also allows investigation of the contribution of deglucuronidation of paracetamol glucuronide to the disposition of paracetamol.

Gut wall metabolism of verapamil in older people: effects of rifampicin-mediated enzyme induction

AIMS

To investigate prehepatic metabolism of verapamil and its inducibility by rifampicin in older subjects.

METHODS

Eight older subjects (67.1 +/- 1.2 years mean +/- s.d.) received racemic, unlabelled verapamil orally for 16 days (120 mg twice daily). Rifampicin (600 mg daily) was coadministered from day 5 to 16. Using stable isotope technology (i.e. intravenous coadministration of 10 mg deuterated verapamil) during verapamil steady-state without (day 4) and with rifampicin (day 16) bioavailability, prehepatic and hepatic extraction of verapamil were determined. The effects of verapamil on AV-conduction were measured by the maximum PR interval prolongation (%).

RESULTS

Bioavailability of the cardiovascularly more active S-verapamil decreased from 14.2 +/- 4.3% on day 4 to 0.6 +/- 0.5% on day 16 (P < 0.001). As a consequence, effects of orally administered verapamil on the AV-conduction were nearly abolished (14.4 +/- 9.4% vs 2.7 +/- 2.6%, P < 0.01). This could be attributed to a considerable increase of prehepatic extraction during treatment with rifampicin (41.7 +/- 22.1% vs 91.6 +/- 6.6%, P < 0.01) and to a minor extent to induction of hepatic metabolism (73.7 +/- 9.4% vs 91.6 +/- 5.3%, P < 0.01).

CONCLUSIONS

Prehepatic metabolism of verapamil occurred in the group of older people investigated. Induction of gut wall metabolism most likely was the major reason for the loss of verapamil effect during treatment with rifampicin in this group of older subjects.