Involvement of Multiple Transport Systems in the Disposition of an Active Metabolite of a Prodrug­type New Quinolone Antibiotic, Prulifloxacin

Effects of drug transporters on volume of distribution

Recently, drug transporters have emerged as significant modifiers of a patient's pharmacokinetics. In cases where the functioning of drug transporters is altered, such as by drug-drug interactions, by genetic polymorphisms, or as evidenced in knockout animals, the resulting change in volume of distribution can lead to a significant change in drug effect or likelihood of toxicity, as well as a change in half life independent of a change in clearance. Here, we review pharmacokinetic interactions at the transporter level that have been investigated in animals and humans and reported in literature, with a focus on the changes in distribution volume. We pay particular attention to the differing effects of changes in transporter function on the three measures of volume. Further, trends are discussed as they may be used to predict volume changes given the function of a transporter and the primary location of the interaction. Because the liver and kidneys express the greatest level and variety of transporters, we denote these organs as the primary location of transporter-based interactions. We conclude that the liver is a larger contributor to distribution volume than the kidneys, in consideration of both uptake and efflux transporters. Further, while altered distribution due to secondary interactions at tissues other than the liver and kidneys may have a pharmacodynamic effect, these interactions, at least at the blood-brain barrier, do not appear to significantly influence overall distribution volume. The analysis provides a framework for understanding potential pharmacokinetic interactions rooted in drug transporters as they modify drug distribution.

Possible multiple transporters were involved in hepatobiliary excretion of antofloxacin in rats

1. The aim of the current study was to investigate the characteristics of biliary excretion of antofloxacin (ATFX) in rats. Rats received a bolus intravenous injection followed by a constant-rate infusion of ATFX. When plasma concentrations of ATFX reached steady state, cyclosporin A, erythromycin, probenecid, cimetidine and diclofenac were administered intravenously to the rats. Samples of blood and bile were collected and the concentrations of ATFX were measured and the corresponding pharmacokinetic parameters were estimated. 2. Biliary excretion of ATFX was observed in rats subjected to CCl(4)-induced experimental hepatic injury for 24 h (CCl(4)-EHI(24h)). Steady state concentrations of ATFX were attained at 60 min following infusion. 3. A slight increase in concentration of ATFX in plasma was observed after cyclosporin A, erythromycin, probenecid and cimetidine treatment. Significant increases in ATFX plasma levels were found in rats treated with diclofenac. Cyclosporin A, erythromycin, probenecid, cimetidine and diclofenac treatment significantly decreased the steady state biliary clearance of ATFX to 55, 68, 54, 53 and 56% of control values, respectively. 4. Cyclosprin A, probenecid, erythromycin and cimetidine also inhibited the biliary excretion of ATFX glucuronide. Significant decrease in the steady state biliary clearance of ATFX and its glucuronide was observed in CCl(4)-EHI(24h) rats. 5. These results indicate that multiple transporters are possibly involved in the biliary excretion of ATFX.

Involvement of Mrp2 (Abcc2) in biliary excretion of moxifloxacin and its metabolites in the isolated perfused rat liver

Moxifloxacin is a novel antibacterial agent that undergoes extensive metabolism in the liver to the glucuronide M1 and the sulfate M2, which are eliminated via the bile. To investigate the role of the multidrug resistance-associated protein (Mrp2) as the hepatic transport system for moxifloxacin and its conjugates, livers of Wistar and Mrp2-deficient TR- rats were perfused with moxifloxacin (10 microM) in a single-pass system. Values for the hepatic extraction ratio (E) and clearance (Cl) were insignificantly higher in TR- rats than Wistar rats (0.193+/-0.050 vs 0.245+/-0.050 for E; 6.85+/-1.96 vs 8.73+/-1.82 mL min(-1) for Cl), whereas biliary excretion and efflux into perfusate over 60 min were significantly lower in the mutant rat strain. Cumulative biliary excretion of M1, M2 and moxifloxacin was significantly reduced to 0.027%, 19.1%, and 29.6% in the TR- rats compared with Wistar rats, indicating that the biliary elimination of M1 is mediated exclusively by Mrp2, whereas that of M2 and moxifloxacin seems to depend mostly on Mrp2 and, to a smaller extent, a further unidentified canalicular transporter. Moxifloxacin stimulates bile flow by up to 11% in Wistar rats, but not in TR- rats, further supporting an efficient transport of this drug and its glucuronidated and sulfated metabolites by Mrp2. Moxifloxacin (10 microM) also reversibly inhibited the Mrp2-mediated biliary elimination of bromsulphthalein in Wistar rats by 34%, indicating competition with the elimination of Mrp2-specific substrates. In conclusion, we found that Mrp2 mediates the biliary elimination of moxifloxacin and its glucuronidated and sulfated metabolites in rats. MRP2 may therefore play a key role in the transport of moxifloxacin and its conjugates into bile in humans.

Quantitative investigation of the role of breast cancer resistance protein (Bcrp/Abcg2) in limiting brain and testis penetration of xenobiotic compounds

The role of breast cancer resistance protein (BCRP/ABCG2) in limiting the brain and testis penetration of xenobiotic compounds in the blood-brain and -testis barriers was investigated using Bcrp(-/-) mice. Tissue/plasma concentration ratios in the brain (K(p,brain)) and testis (K(p,testis)) obtained under steady-state conditions were significantly increased in Bcrp(-/-) mice for PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine), N-hydroxyl PhIP, MeIQx (2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline), dantrolene, and prazosin. In addition, the K(p,brain) of triamterene and the K(p,testis) of 4'-hydroxyl PhIP were also significantly increased in Bcrp(-/-) mice. The effect of functional impairment of Bcrp on the brain uptake of PhIP, dantrolene, and daidzein in Bcrp(-/-) mice determined using in situ brain perfusion was weaker than that observed on the K(p) values. In vitro transcellular transport experiments using cell lines expressing mouse Bcrp or P-glycoprotein (Mdr1a/Abcb1a) showed that, among the tested Bcrp substrates, PhIP, MeIQx, prazosin, and triamterene are common substrates of Bcrp and P-glycoprotein. The K(p) values of common substrates exhibited a smaller increase both in the brain and testis of Bcrp(-/-) mice than expected from the in vitro Bcrp activities. The Bcrp-specific substrates were weak acids, whereas basic or neutral BCRP substrates were also P-glycoprotein substrates. These results suggest that BCRP limits the tissue penetration of xenobiotic compounds in the blood-brain and -testis barriers, but its in vivo importance is also modulated by P-glycoprotein activity.

Transporter-mediated hepatic uptake of ulifloxacin, an active metabolite of a prodrug-type new quinolone antibiotic prulifloxacin, in rats

Prulifloxacin (PUFX) is a prodrug-type new quinolone antibiotic and immediately converted to an active metabolite, ulifloxacin (UFX). It has been previously reported that UFX is highly excreted into the bile, although the hepatic uptake process of UFX has not been investigated yet. In this study, we attempted to characterize the mechanism of hepatic uptake of UFX in rats. The hepatic uptake in vivo was evaluated by integration plot analysis. Furthermore, the uptake of [(14)C]-UFX by isolated rat hepatocytes was measured, and the effects of several transporter inhibitors and other quinolone antibiotics on the uptake were examined. The hepatic uptake clearance of UFX (1 mg/kg) was calculated to be 37.7 mL/min/kg, which was larger than those at doses of 5 and 25 mg/kg and was decreased by co-administration of cyclosporine A (CysA; 30 mg/kg). The uptake of [(14)C]-UFX by isolated rat hepatocytes linearly increased up to 1 min and also inhibited by CysA. Other quinolone antibiotics inhibited the [(14)C]-UFX uptake in a concentration-dependent manner, whereas taurocholate and estrone-3-sulfate partially inhibited the [(14)C]-UFX uptake. These results demonstrate that a carrier-mediated transport system which is common to the quinolone antibiotics is involved in the uptake of UFX in the rat liver.

Involvement of Breast Cancer Resistance Protein (ABCG2) in the Biliary Excretion Mechanism of Fluoroquinolones

Fluoroquinolones are effective antibiotics for the treatment of bile duct infections. It has been shown that the biliary excretion of grepafloxacin is partly accounted for by multidrug resistance-associated protein 2 (MRP2/ABCC2), whereas neither MRP2 nor P-glycoprotein is involved in the biliary excretion of ulifloxacin. In the present study, we examined the involvement of breast cancer resistance protein (BCRP/ABCG2) in the biliary excretion of fluoroquinolones (grepafloxacin, ulifloxacin, ciprofloxacin, and ofloxacin). In Madin-Darby canine kidney II cells expressing human BCRP or mouse Bcrp, the basal-to-apical transport of grepafloxacin and ulifloxacin was greater than that of the mock control, which was inhibited by a BCRP inhibitor, 3-(6-isobutyl-9-methoxy-1,4-dioxo-1,2,3,4,6,7,12,12a-octahydropyrazino[1',2':1,6]pyrido[3,4-b]indol-3-yl)-propionic acid tert-butyl ester (Ko143). Plasma and bile concentrations of fluoroquinolones were determined in wild-type and Bcrp(-/-) mice after i.v. bolus injection. The cumulative biliary excretion of fluoroquinolones was significantly reduced in Bcrp(-/-) mice, resulting in a reduction of the biliary excretion clearances to 86, 50, 40, and 16 of the control values, for ciprofloxacin, grepafloxacin, ofloxacin, and ulifloxacin, respectively. Preinfusion of sulfobromophthalein significantly inhibited the biliary excretion of grepafloxacin in Bcrp(-/-) mice. There was no change in the tissue/plasma concentration ratios of fluoroquinolones in the liver or brain, whereas those in the kidney were increased 3.6- and 1.5-fold for ciprofloxacin and grepafloxacin, respectively, in Bcrp(-/-) mice but were unchanged for ofloxacin and ulifloxacin. The present study shows that BCRP mediates the biliary excretion of fluoroquinolones and suggests that it is also involved in the tubular secretion of ciprofloxacin and grepafloxacin.

The Magnitude and Time Course of Changes in Mycophenolic Acid 12-Hour Predose Levels During Antibiotic Therapy in Mycophenolate Mofetil-Based Renal Transplantation

There is increasing evidence that monitoring predose plasma levels of mycophenolic acid (MPA) is of benefit in renal transplant recipients treated with mycophenolate mofetil (MMF). Concomitant treatment with oral antibiotics leads to a 10% to 30% reduction in MPA area under the curve (AUC)0-12, probably by reducing enterohepatic recirculation (EHR). Because of the timing of EHR (6 to 12 hours postdose), the magnitude of predose MPA level reduction may be disproportionately larger than that of AUC0-12. However, changes in predose MPA levels and the time course over which these changes occur and resolve during antibiotic treatment have not been studied. The purpose of this study was to define the extent and time course of MPA predose level reduction during antibiotic therapy. A total of 64 MMF-treated renal transplant recipients (with tacrolimus cotherapy) were prospectively studied. Clinically indicated cotherapy with either oral ciprofloxacin or amoxicillin with clavulanic acid resulted in a reduction in 12 hour predose MPA level to 46% of baseline within 3 days of antibiotic commencement. No demographic or biochemical variables were associated with the magnitude of MPA level reduction. No further fall in MPA level was seen by day 7, but MPA levels recovered spontaneously to 79% of baseline after 14 days of antibiotics. Levels normalized within 3 days of antibiotic cessation. No changes in daily MMF dose (normalized for body weight) were made during antibiotic treatment. This data should help the clinician to recognize the extent of MPA predose level reduction during antibiotic therapy, and to avoid inappropriate MMF dose escalation and potential risk of toxicity.

Impact of transporter-mediated drug absorption, distribution, elimination and drug interactions in antimicrobial chemotherapy

A comprehensive list of drug transporters has recently become available as a result of extensive genome analysis. Membrane transporters play important roles in determining the pharmacokinetic aspects of intestinal absorption, tissue distribution, and the urinary and biliary excretions of a wide variety of therapeutic drugs. The identification and characterization of transporters responsible for the transfer of nutrients and xenobiotics, including drugs, is expected to provide a scientific basis for understanding drug disposition, as well as the molecular mechanisms of drug-drug/drug-food/drug-protein interactions and inter-individual/inter-species differences. This review focuses on the influence of transporters on the pharmacokinetics of beta-lactam antibiotics, new quinolones, and other antimicrobial agents, as well as focusing on the drug-drug interactions associated with transporter-mediated uptake from the small intestine and transporter-mediated elimination from the kidney and liver.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

P-GLYCOPROTEIN PLAYS A MAJOR ROLE IN THE EFFLUX OF FEXOFENADINE IN THE SMALL INTESTINE AND BLOOD-BRAIN BARRIER, BUT ONLY A LIMITED ROLE IN ITS BILIARY EXCRETION

Fexofenadine is a selective, nonsedating H(1)-receptor antagonist approved for symptoms of allergic conditions, which is mainly excreted into feces via biliary excretion. The purpose of this study is to investigate its pharmacokinetics in mice and rats to determine the role of P-glycoprotein (P-gp) in its biliary excretion. In mice, biliary excretion clearance (17 ml/min/kg) accounted for almost 60% of the total body clearance (30 ml/min/kg). Comparing the pharmacokinetics after intravenous and oral administration indicated that the bioavailability of fexofenadine was at most 2% in mice. Knockout of Mdr1a/1b P-gp did not affect the biliary excretion clearance with regard to both plasma and liver concentrations, whereas the absence of P-gp caused a 6-fold increase in the plasma concentration after oral administration. In addition, the steady-state brain-to-plasma concentration ratio of fexofenadine was approximately 3-fold higher in Mdr1a/1b P-gp knockout mice than in wild-type mice. Together, these results show that P-glycoprotein plays an important role in efflux transport in the brain and small intestine but only a limited role in biliary excretion in mice. In addition, there was no difference in the biliary excretion between normal and hereditarily multidrug resistance-associated protein 2 (Mrp2)-deficient mutant rats (Eisai hyperbilirubinemic rats) and between wild-type and breast cancer resistance protein (Bcrp) knockout mice. These results suggest that the biliary excretion of fexofenadine is mediated by unknown transporters distinct from P-gp, Mrp2, and Bcrp.