The therapeutic efficacy of saquinavir and darunavir is affected by the presence of xenobiotics (such as garlic compounds) capable of modifying transporter-enzyme interplay. To ascertain the mechanism of interactions between antiretroviral drugs and garlic supplements and to identify the garlic constituents responsible, the hepatic pharmacokinetics of two antiretroviral drugs was investigated in the presence of garlic phytochemicals and aged garlic extract. For this purpose, rat liver slices and isolated rat hepatocytes were used. Aged garlic extract significantly inhibited saquinavir efflux from rat hepatocytes, while the efflux of darunavir significantly increased. Phytochemicals inducing distribution changes of saquinavir and darunavir were most probably flavonoids and lipophilic organosulfur compounds, respectively. All tested phytochemicals (except S-allyl L-cysteine) and aged garlic extract inhibited CYP3A4 metabolism of both drugs and modulated hepatic distribution of the corresponding saquinavir and darunavir metabolites. The competition between saquinavir and garlic constituent(s) for the same binding site on the efflux transporter and the positive cooperative effect between darunavir and garlic phytochemical(s), which bind to separate binding places on transporter, are the most probable mechanisms explaining the plasma profile changes, which could occur in vivo during concomitant consumption of antiretrovirals and garlic supplements.

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  We investigated the influence of genetic polymorphisms of CYP2C9 and VKORC1 genotypes on the pharmacokinetics and pharmacodynamics of warfarin and established an equation for predicting the maintenance dose of warfarin in the Thai population using genetic and non-genetic factors. The CYP2C9*2, CYP2C9*3, VKORC1 C1173T and VKORC1 G-1639A genotypes were detected by realtime PCR using fluorogenic hybridization probes. The associations between genetic and demographic factors with respect to warfarin dosage were analyzed. CYP2C9 polymorphisms affect warfarin metabolism as shown by a significant difference in warfarin clearance, whereas VKORC1 genotypes cause a significant difference in warfarin sensitivity index (INR:Cp). The mean weekly warfarin dose was significantly different among different VKORC1 and CYP2C9 genotypes. Patients with the VKORC1 BB haplotype and CYP2C9*1/*1 required about twice the warfarin dose compared to those with the VKORC1 AA haplotype and CYP2C9*1/*1. Using stepwise multiple linear regression, clinical factors (age and weight) and genetic factors (CYP2C9 and VKORC1) could explain about 53.8% of the variance of the warfarin maintenance dose. CYP2C9 and VKORC1 genotypes played an important role in the inter-individual variation in warfarin maintenance dose in a Thai population.

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  Cryopreserved human (h-) hepatocytes are currently regarded as the best in vitro model for predicting human intrinsic clearance of xenobiotics. Although fresh h-hepatocytes have greater plating efficiency on dishes and greater metabolic activities than cryopreserved cells, performing reproducible studies using fresh hepatocytes from the same donor and having an “on demand” supply of fresh hepatocytes are not possible. In this study, cryopreserved h-hepatocytes were transplanted into albumin enhancer/promoter-driven, urokinase-type plasminogen activator, transgenic/severe combined immunodeficient (uPA/SCID) mice to produce chimeric mice, the livers of which were largely replaced with h-hepatocytes. We determined whether the chimeric mouse could serve as a novel source of fresh h-hepatocytes for in vitro studies. h-Hepatocytes were isolated from chimeric mice (chimeric hepatocytes), and cytochrome P450 (P450) activities were determined. Compared with cryopreserved cells, the P450 (1A2, 2C9, 2C19, 2D6, 2E1, 3A) activities of fresh chimeric hepatocytes were similar or greater. Moreover, ketoprofen was more actively metabolized through glucuronide conjugates by fresh chimeric hepatocytes than by cryopreserved cells. We conclude that chimeric mice may be a useful tool for supplying fresh h-hepatocytes on demand that provide high and stable phase I enzyme and glucuronidation activities.

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  Eplerenone deviates from linear pharmacokinetics at doses above the therapeutic dose range. In addition, saturable protein binding of eplerenone is observed in in vitro plasma protein binding studies. The purpose of the present study was to clarify the factors contributing to the nonlinear pharmacokinetics of eplerenone. Plasma concentration data for eplerenone and its metabolite SC-70303, to which eplerenone is reversibly converted, obtained from four phase I studies were analyzed using NONMEM. A population pharmacokinetic model incorporating protein binding and the reversible relationship between eplerenone and SC-70303 was developed. Models with linear and nonlinear protein binding were fitted to the observed concentration data. The observed concentration data of eplerenone and SC-70303 were best described by a model with nonlinear protein binding. The area under the plasma concentration-time curve of eplerenone simulated by the model increased less than proportionally with increasing dose, whereas that of SC-70303 increased proportionally with increasing dose, consistent with observations from the non-compartmental analysis. In conclusion, the nonlinear pharmacokinetics of eplerenone and the apparently linear pharmacokinetics of SC-70303 were described by applying a model with nonlinear protein binding to observed plasma eplerenone and SC-70303 concentrations, suggesting that nonlinear protein binding plays a role in the nonlinear kinetics.

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  The roots of Sophora flavescens (Sf) have been widely used as a herbal medicine for the treatment of diarrhea, gastrointestinal hemorrhage, and eczema. Cytochrome P450 (P450) forms including CYP1A2, CYP2B, CYP2E1, and CYP3A participate in the oxidative metabolism of theophylline, which is an important bronchodilation agent with a narrow therapeutic index. To assess the interaction of Sf with theophylline, the effects of Sf extract on theophylline-metabolizing P450s and on the pharmacokinetic profile of theophylline were investigated in male Sprague-Dawley rats. Oral treatment of rats with the Sf extract caused dose-dependent increases of liver microsomal oxidation activities toward 7-ethoxyresorufin, 7-pentoxyresorufin, and nifedipine. However, nitrosodimethylamine N-demethylation activity was not affected. The ingestion of Sf extract stimulated theophylline 8-oxidation and N-demethylation activities. The increases of oxidative activities were in consensus with the elevation of the protein levels of CYP1A2, CYP2B1/2, CYP2C11, and CYP3A. Sf-treatment increased the clearance of theophylline and decreased the area under the concentration-time curve (AUC) and the area under the moment curve (AUMC). These results demonstrate that Sf reduces blood theophylline concentration through facilitating the elimination of theophylline. In patients taking Sf, possible P450 induction-induced drug interaction should be noted to decrease the risk of therapeutic failure or adverse effects resulting from the use of additional therapeutic agents.

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  Parabens (alkyl esters of 4-hydroxybenzoic acid) are widely used as preservatives in drugs, cosmetic products, and foodstuffs. Safety concerns have recently increased due to the potential health risks associated to exposure to large amounts of these substances. Biotransformation of parabens mainly includes hydrolysis of the ester bond and glucuronidation reactions. The hydrolysis and glucuronidation of a series of six parabens differing by the nature of the alkyl group were investigated in human liver microsomes and plasma, and the major human UDP-glucuronosyltransferase (UGT) isoforms involved in the reaction were identified. Methyl- and ethylparaben were stable in human plasma, with 95% of the initial concentration remaining after 24 h. On the other hand, propyl-, butyl- and benzylparaben concentrations decreased by 50% under similar conditions. In contrast, rapid hydrolysis was measured with human liver microsomes depending on the alkyl chain length, with t_1/2 varying from 22 min for methylparaben to 87 min for butylparaben. All parabens were actively glucuronidated by liver microsomes, in comparison to 4-hydroxybenzoic acid. They were mainly substrates of human recombinant UGT1A1, UGT1A8, UGT1A9, UGT2B7, UGT2B15 and UGT2B17. In conclusion, the parabens were readily metabolized in human liver through esterase hydrolysis and glucuronidation by several UGT isoforms. These results suggest that these parabens do not accumulate in human tissue.

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  Neonatal Fc receptor (FcRn) plays an important role in regulating IgG homeostasis in the body. Changes in FcRn expression levels or activity caused by genetic polymorphisms of FCGRT, which encodes FcRn, may lead to interindividual differences in pharmacokinetics of therapeutic antibodies. In this study, we sequenced the 5′-flanking region, all exons and their flanking regions of FCGRT from 126 Japanese subjects. Thirty-three genetic variations, including 17 novel ones, were found. Of these, two novel non-synonymous variations, 629G>A (R210Q) and 889T>A (S297T), were found as heterozygous variations. We next assessed the functional significance of the two novel non-synonymous variations by expressing wild-type and variant proteins in HeLa cells. Both variant proteins showed similar intracellular localization as well as antibody recycling efficiencies. These results suggested that at least no common functional polymorphic site with amino acid change was present in the FCGRT of our Japanese population.

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  Recent studies indicate that various members of the organic anion transporting polypeptide (OATP) family are expressed on apical membranes of the small intestine. In the present study, we investigated possible interaction of Oatp with the PDZ protein PDZK1 in mouse small intestine, using [³H]estrone-3-sulfate (E3S) as a typical substrate. After intraduodenal administration, the level of [³H]E3S appearing in the portal vein of pdzk1 gene knockout (pdzk1^−/−) mice was much lower than that in wild-type mice. Lower intestinal absorption of [³H]E3S in pdzk1^−/− mice was confirmed in Ussing-type chamber experiments, which showed smaller uptake of [³H]E3S from the apical side in intestinal tissues of pdzk1^−/− mice compared with wild-type mice. The kinetics and inhibition profile of [³H]E3S uptake in the Ussing-type chamber were similar to those in HEK293 cells stably expressing Oatp1a5, suggesting involvement of Oatp1a5 in [³H]E3S uptake. Immunoreactivity to anti-Oatp1a antibody was colocalized with PDZK1 in the small intestine of wild-type mice, whereas apical localization of Oatp1a protein was reduced in pdzk1^−/− mice. An immunoprecipitation study revealed physical interaction of PDZK1 with Oatp1a. Thus, PDZK1 appears to act as an adaptor for Oatp1a. This is the first demonstration of a regulatory protein directly interacting with small-intestinal OATP.

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  Xanthine oxidase (XO) catalyzes the oxidation of endogenous and exogenous purines and pyrimidines. In the present study, we investigated polymorphisms in the promoter region of the XO gene. Sequence variations in the 5′-flanking region were screened using denaturing high-performance liquid chromatography (DHPLC) on DNA samples from 196 unrelated Japanese individuals. Thirteen polymorphisms were identified and 13 haplotypes were classified by haplotype analysis. The promoter activities of these polymorphisms were measured by luciferase assay in the human hepatoma cell lines HepG2 and Huh-7. Transcriptional activity was significantly lower in cell lines transfected with the reporter construct containing 5-kb upstream fragments with −1756T than in those with wild-type −1756C. Our results indicate that genetic variation in the promoter region of XO may determine interindividual differences in XO gene expression.

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  Benzbromarone is a uricosuric drug and has been shown to be metabolized predominantly by cytochrome P450(CYP)2C9 in vitro findings. This study aims to investigate the influence of the CYP2C9 genotype on plasma levels of benzbromarone and 6-hydroxybenzbromarone, as well as uric acid lowering effects. A single oral dose pharmacokinetic and pharmacodynamic trial of benzbromarone (100 mg) was performed in 20 healthy volunteers, which included 15 with CYP2C9*1/*1, 4 with CYP2C9*1/*3, and 1 with CYP2C9*3/*3. The oral clearance of benzbromarone in the CYP2C9*1/*1 genotype and CYP2C9*1/*3 genotype was 58.8±25.2 L/hr/kg (mean±SD) and 51.3±7.9 L/hr/kg, respectively, whereas 8.58 L/hr/kg in the CYP2C9*3/*3 genotype. The metabolic ratio (6-hydroxybenzbromarone/benzbromarone) in urine was 38.6±10.7 in the CYP2C9*1/*1 genotype, 35.4±12.4 in the CYP2C9*1/*3 genotype and 12.9 in the CYP2C9*3/*3 genotype. Although benzbromarone significantly increased the urinary excretion and reduced the plasma concentration of uric acid, there were no significant differences in its effects for different CYP2C9 genotypes. These results suggest a critical role for CYP2C9 in the metabolism of benzbromarone in humans and a possible risk of toxicity in the CYP2C9*3 homozygote by lowering clearance of the drug. Further studies are required to assess the clinical impact of CYP2C9 on the metabolism of benzbromarone.

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  We have reported previously that taurine transporter (TauT) mediates γ-aminobutyric acid (GABA) as a substrate in a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells). This study investigates how TauT-mediated GABA transport is regulated in TR-iBRB2 cells under hypertonic conditions. [³H]GABA uptake by TR-iBRB2 cells exposed to 12 h- to 24 h-hypertonic culture medium was significantly greater than that of isotonic culture medium. [³H]GABA uptake by TR-iBRB2 cells was Na⁺-, Cl⁻-, and concentration-dependent with a Michaelis-Menten (K_m) constant of 3.5 mM under isotonic conditions and K_m of 0.324 and 5.48 mM under hypertonic conditions. Under hypertonic conditions, [³H]GABA uptake by TR-iBRB2 cells was more potently inhibited by substrates of TauT, such as taurine and β-alanine, than those of GABA transporters such as GABA, nipecotic acid, and betaine. These results suggest that an unknown high-affinity GABA transport process and TauT-mediated GABA transport are enhanced under hypertonic conditions. In conclusion, hypertonicity enhances GABA uptake by cultured rat retinal capillary endothelial cells.

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  CYP1A2 play an important role in the metabolism of many carcinogens and clinically important drugs. CYP1A2 activity has been found to be influenced by the presence of polymorphic variants which were reported to display wide interethnic variation. This study investigates the frequency distribution and linkage disequilibrium patterns of CYP1A2 genetic polymorphisms, and characterize their haplotype structures in three healthy Asian populations in Singapore (Chinese, Malay, and Indian).
   The entire CYP1A2 gene was screened in 126 healthy subjects from all three ethnic groups (N=42 each). A total of 25 polymorphisms was identified, of which nine were novel. The polymorphisms, −2467delT and −163C>A were detected at high frequencies in all Asian ethnic groups. Significant interethnic differences were observed in the genotypic frequency distribution of IVS2−99G>A (P<0.01) and 1548C>T (P=0.05) across the three ethnic groups while −163C>A (P=0.02) was found to differ between Chinese and Malays. Haplotype analyses revealed four to six major haplotypes in each ethnic population which accounted for more than 60% of the cumulative haplotype frequencies. Future studies should be done to investigate the functional roles of these haplotypes.

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