The expression of UDP-glucuronosyltransferases of the UGT1 family in human liver and kidney and in response to drugs

Overview of the Components of Cardiac Metabolism

Metabolism in organs other than the liver and kidneys may play a significant role in how a specific organ responds to chemicals. The heart has metabolic capability for energy production and homeostasis. This homeostatic machinery can also process xenobiotics. Cardiac metabolism includes the expression of numerous organic anion transporters, organic cation transporters, organic carnitine (zwitterion) transporters, and ATP-binding cassette transporters. Expression and distribution of the transporters within the heart may vary, depending on the patient’s age, disease, endocrine status, and various other factors. Several cytochrome P450 (P450) enzyme classes have been identified within the heart. The P450 hydroxylases and epoxygenases within the heart produce hydroxyeicosatetraneoic acids and epoxyeicosatrienoic acids, metabolites of arachidonic acid, which are critical in regulating homeostatic processes of the heart. The susceptibility of the cardiac P450 system to induction and inhibition from exogenous materials is an area of expanding knowledge, as are the metabolic processes of glucuronidation and sulfation in the heart. The susceptibility of various transcription factors and signaling pathways of the heart to disruption by xenobiotics is not fully characterized but is an area with implications for disruption of normal postnatal development, as well as modulation of adult cardiac health. There are knowledge gaps in the timelines of physiologic maturation and deterioration of cardiac metabolism. Cross-species characterization of cardiac-specific metabolism is needed for nonclinical work of optimum translational value to predict possible adverse effects, identify sensitive developmental windows for the design and conduct of informative nonclinical and clinical studies, and explore the possibilities of organ-specific therapeutics.

Comparison of Propofol Consumption and Recovery Time in Caucasians from Italy, with Chinese, Malays and Indians from Malaysia

Differences in sensitivity to anaesthetic drugs may exist among different ethnic groups. Allelic variants for drug metabolizing isoenzymes and pharmacokinetic differences may account for a variable response to some anaesthetic drugs. This study was designed to compare propofol consumption and recovery characteristics in four ethnic groups: Chinese, Malays, and Indians in Malaysia and Caucasians in Italy.

Patients undergoing total intravenous anaesthesia with propofol and fentanyl were evaluated for propofol consumption and recovery time. The Bispectral Index (BIS) was used to maintain the same anaesthesia depth in all patients. The BIS value, the response to verbal stimuli and eye-opening time were used to assess recovery. After propofol discontinuation the BIS values returned to baseline in 11±4.2 min for Caucasians, in 12.5±5.1 min for Chinese, 15.9±6.3 min for Malays and 22.1±8.1 for Indians. Time to eye-opening was 11.63±4.2 min in Caucasians, 13.23±4.9 min in Chinese, 16.97±5.2 min in Malays and 22.3±6.6 min in Indians. The propofol consumption was significantly lower in Indians compared to the other three groups (P<0.01).

The recovery of Indians was much slower compared to Chinese, Malays and Caucasians. The recovery time of Malays is significantly slower compared to Chinese and Caucasians. Differences in propofol consumption and recovery time were not significant between Chinese and Caucasians, but the ratio recovery time/propofol consumption was significantly lower in Caucasians compared to all the other groups.

Quantitative profiling of human renal UDP-glucuronosyltransferases and glucuronidation activity: a comparison of normal and tumoral kidney tissues

Renal metabolism by UDP-glucuronosyltransferase (UGT) enzymes is central to the clearance of many drugs. However, significant discrepancies about the relative abundance and activity of individual UGT enzymes in the normal kidney prevail among reports, whereas glucuronidation in tumoral kidney has not been examined. In this study, we performed an extensive profiling of glucuronidation metabolism in normal (n = 12) and tumor (n = 14) kidneys using targeted mass spectrometry quantification of human UGTs. We then correlated UGT protein concentrations with mRNA levels assessed by quantitative polymerase chain reaction and with conjugation activity for the major renal UGTs. Beyond the wide interindividual variability in expression levels observed among kidney samples, UGT1A9, UGT2B7, and UGT1A6 are the most abundant renal UGTs in both normal and tumoral tissues based on protein quantification. In normal kidney tissues, only UGT1A9 protein levels correlated with mRNA levels, whereas UGT1A6, UGT1A9, and UGT2B7 quantification correlated significantly with their mRNA levels in tumor kidneys. Data support that posttranscriptional regulation of UGT2B7 and UGT1A6 expression is modulating glucuronidation in the kidney. Importantly, our study reveals a significant decreased glucuronidation capacity of neoplastic kidneys versus normal kidneys that is paralleled by drastically reduced UGT1A9 and UGT2B7 mRNA and protein expression. UGT2B7 activity is the most repressed in tumors relative to normal tissues, with a 96-fold decrease in zidovudine metabolism, whereas propofol and sorafenib glucuronidation is decreased by 7.6- and 5.2-fold, respectively. Findings demonstrate that renal drug metabolism is predominantly mediated by UGT1A9 and UGT2B7 and is greatly reduced in kidney tumors.

Effects of nifedipine on the pharmacokinetics of repaglinide in rats: possible role of CYP3A4 and P-glycoprotein inhibition by nifedipine

BACKGROUND

The aim of this study was to investigate the effects of nifedipine on the bioavailability and pharmacokinetics of repaglinide in rats.

METHODS

The effect of nifedipine on P-glycoprotein (P-gp) and cytochrome P450 (CYP) 3A4 activity was evaluated. The pharmacokinetic parameters of repaglinide and blood glucose concentrations were also determined in rats after oral (0.5 mg/kg) and intravenous (0.2 mg/kg) administration of repaglinide to rats in the presence and absence of nifedipine (1 and 3 mg/kg).

RESULTS

Administration of nifedipine resulted in inhibition CYP3A4 activity with an IC50 value of 7.8 μM, and nifedipine significantly inhibited P-gp activity in a concentration-dependent manner. Compared to the oral control group, nifedipine significantly increased the area under the plasma concentration-time curve (AUC0-∞) and the peak plasma concentration (Cmax) of repaglinide by 49.3 and 25.5%, respectively. Nifedipine significantly decreased the total body clearance (CL/F) of repaglinide by 22.0% compared to the oral control group. Nifedipine also increased the absolute bioavailability (AB) of repaglinide by 50.0% compared to the oral control group (33.6%). In addition, the relative bioavailability (RB) of repaglinide was 1.16- to 1.49-fold greater than that of the control group. Compared to the intravenous control, nifedipine significantly increased AUC0-∞ of repaglinide. Blood glucose concentrations had significant differences compared to the oral control groups.

CONCLUSION

Nifedipine enhanced the oral bioavailability of repaglinide, which may be mainly attributable to inhibition of CYP3A4-mediated metabolism of repaglinide in the small intestine and/or in the liver and to inhibition of the P-gp efflux transporter in the small intestine and/or reduction of total body clearance by nifedipine. The current study has raised awareness of potential drug interactions by concomitant use of repaglinide with nifedipine.

Effects of Fluvastatin on the Pharmacokinetics of Repaglinide: Possible Role of CYP3A4 and P-glycoprotein Inhibition by Fluvastatin

The purpose of this study was to investigate the effects of fluvastatin on the pharmacokinetics of repaglinide in rats. The effect of fluvastatin on P-glycoprotein and CYP3A4 activity was evaluated. The pharmacokinetic parameters and blood glucose concentrations were also determined after oral and intravenous administration of repaglinide to rats in the presence and absence of fluvastatin. Fluvastatin inhibited CYP3A4 activity in a concentration-dependent manner with a 50% inhibition concentration(IC₅₀) of 4.1 µM and P-gp activity. Compared to the oral control group, fluvastatin significantly increased the AUC and the peak plasma level of repaglinide by 45.9% and 22.7%, respectively. Fluvastatin significantly decreased the total body clearance (TBC) of repaglinide compared to the control. Fluvastatin also significantly increased the absolute bioavailability (BA) of repaglinide by 46.1% compared to the control group. Moreover, the relative BA of repaglinide was 1.14- to 1.46-fold greater than that of the control. Compared to the i.v. control, fluvastatin significantly increased the AUC_0-∞ of i.v. administered repaglinide. The blood glucose concentrations showed significant differences compared to the oral controls. Fluvastatin enhanced the oral BA of repaglinide, which may be mainly attributable to the inhibition of the CYP3A4-mediated metabolism of repaglinide in the small intestine and/or liver, to the inhibition of the P-gp efflux transporter in the small intestine and/or to the reduction of TBC of repaglinide by fluvastatin. The study has raised the awareness of potential interactions during concomitant use of repaglinide with fluvastatin. Therefore, the concurrent use of repaglinide and fluvastatin may require close monitoring for potential drug interactions.

Effects of the antioxidant baicalein on the pharmacokinetics of nimodipine in rats: a possible role of P-glycoprotein and CYP3A4 inhibition by baicalein

The reduced bioavailability of nimodipine after oral administration might not only be due to the metabolizing enzyme cytochrome P450 3A4(CYP3A4) but also to the P-glycoprotein efflux transporter in the small intestine. The aim of this study was to investigate the effects of baicalein on the pharmacokinetics of nimodipine in rats. The effect of baicalein on P-glycoprotein and CYP3A4 activity was evaluated. A single dose of nimodipine was administered intravenously (3 mg/kg) and orally (12 mg/kg) to rats in the presence and absence of baicalein (0.4, 2 and 8 mg/kg). Baicalein inhibited CYP3A4 enzyme activity in a concentration-dependent manner, with a 50% inhibition concentration (IC(50)) of 9.2 μM. In addition, baicalein significantly enhanced the cellular accumulation of rhodamine-123 in MCF-7/ADR cells overexpressing P-glycoprotein. Baicalein significantly altered the pharmacokinetics of orally administered nimodipine. Compared to the oral control group given nimodipine alone, the area under the plasma concentration-time curve (AUC(0-∞)) and the peak plasma concentration (C(max)) of nimodipine significantly increased (p < 0.05 for 2 mg/kg; p < 0.01 for 8 mg/kg). Consequently, the absolute bioavailability of nimodipine in the presence of baicalein (2 and 8 mg/kg) was 31.0-35.3%, which was significantly enhanced (p < 0.05 for 2 mg/kg; p < 0.01 for 8 mg/kg) compared to the oral control group (22.3%). Moreover, the relative bioavailability of nimodipine was 1.39- to 1.58-fold greater than that of the control group. The pharmacokinetics of intravenous nimodipine were not affected by baicalein in contrast to those of oral nimodipine. Baicalein significantly enhanced the oral bioavailability of nimodipine, which may be mainly due to inhibition of the CYP3A4-mediated metabolism of nimodipine in the small intestine and/or in the liver and the inhibition of the P-glycoprotein efflux pump in the small intestine by baicalein. The increase in oral bioavailability of nimodipine in the presence of baicalein should be taken into consideration as a potential drug interaction between nimodipine and baicalein.

Effects of efonidipine on the pharmacokinetics and pharmacodynamics of repaglinide: possible role of CYP3A4 and P-glycoprotein inhibition by efonidipine

The purpose of this study was to investigate the effects of efonidipine on the pharmacokinetics and pharmacodynamics of repaglinide in rats. The pharmacokinetic parameters of repaglinide and blood glucose concentrations were also determined in rats after oral (0.5 mg/kg) and intravenous (0.2 mg/kg) administration of repaglinide to rats in the presence and absence of efonidipine (1 and 3 mg/kg). Efonidipine inhibited CYP3A4 activity with an IC(50) value of 0.08 μM, and efonidipine significantly inhibited P-gp activity in a concentration-dependent manner. Compared to the oral control group, efonidipine significantly increased the area under the plasma concentration-time curve (AUC(0-∞)) (P < 0.01 for 3 mg/kg) and the peak plasma concentration (C (max)) (P < 0.05 for 3 mg/kg) of repaglinide by 51.3 and 28.6%, respectively. Efonidipine also significantly (P < 0.01 for 3 mg/kg) increased the absolute bioavailability (AB) of repaglinide by 51.5% compared to the oral control group (33.6%). Moreover, efonidipine significantly increased (P < 0.05 for 3 mg/kg) the AUC(0-∞) of intravenously administered repaglinide. Consistent with these kinetic alterations, the hypoglycemic effect in the concurrent administration group was more pronounced than that in the control group (i.e., repaglinide alone) when the drug was given orally. A pharmacokinetic/dynamic model involving 2-compartment open model with inhibition in absorption/elimination and an indirect response model was apparently sufficient in estimating the concentration-time and effect-time profiles of repaglinide with or without efonidipine. Present study has raised the awareness of potential drug interactions by concomitant use of efonidipine with repaglinide, since efonidipine may alter the absorption and/or elimination of repaglinide by the inhibition of CYP3A4 and P-gp efflux pump. Therefore, the concurrent use of efonidipine with repaglinide may require a close monitoring for potential drug interactions.

Phase I and pharmacokinetic study of COL-3 in patients with recurrent high-grade gliomas

COL-3 is a chemically modified tetracycline that targets multiple aspects of matrix metalloproteinase regulation. This phase I clinical trial was conducted to determine the maximum tolerated dose (MTD) of COL-3 in adults with recurrent high-grade glioma, to describe the effects of enzyme-inducing antiseizure drugs (EIADs) on its pharmacokinetics, and to obtain preliminary evidence of activity. Adults with recurrent high-grade glioma were stratified by EIAD use. COL-3 was given orally daily without interruption until disease progression or treatment-related dose-limiting toxicity (DLT). Three patients in each EIAD group were evaluated at each dose level beginning with 25 mg/m(2)/day and escalated by 25 mg/m(2)/day. Toxicity, response, and pharmacokinetics were assessed. Thirty-three patients were evaluated. The MTD was 75 mg/m(2)/day in the -EIAD patients while one was not determined in +EIAD patients. The common toxicities observed were anemia, ataxia, diarrhea, hypokalemia, CNS hemorrhage, and myalgia. One partial response was observed. -EIAD patients tended to have a higher steady-state trough concentration that was apparent only at the 100 mg/m(2)/day dose level (P = 0.01). This study suggests that: (a) EIAD use does affect the pharmacokinetics of COL-3 at higher doses; and (b) there was not enough suggestion of single-agent activity to warrant further study in recurrent high-grade gliomas.

Effects of myricetin, an anticancer compound, on the bioavailability and pharmacokinetics of tamoxifen and its main metabolite, 4-hydroxytamoxifen, in rats

This study examined the effect of myricetin, an anticancer compound, on the bioavailability and pharmacokinetics of tamoxifen and its metabolite, 4-hydroxytamoxifen, in rats. The effect of myricetin on P-glycoprotein (P-gp), cytochrome P450 (CYP)3A4 and 2C9 activity was evaluated. Myricetin inhibited CYP3A4 and 2C9 activity with IC(50) values of 7.81 and 13.5 μM, respectively, and significantly inhibited P-gp activity in a concentration-dependent manner. Pharmacokinetic parameters of tamoxifen and 4-hydroxytamoxifen were determined in rats after oral (10 mg/kg) and intravenous (2 mg/kg) administration of tamoxifen in the presence and absence of myricetin (0.4, 2, and 8 mg/kg). Compared with the oral control group (given tamoxifen alone), the area under the plasma concentration-time curve (AUC(0-∞)) and the peak plasma concentration (C (max)) of tamoxifen were significantly (P < 0.05, 2 mg/kg; P < 0.01, 8 mg/kg) increased by 41.8-74.4 and 48.4-81.7%, respectively. Consequently, the absolute bioavailability (AB) of tamoxifen with myricetin (2 and 8 mg/kg) was 29.0-35.7%, which was significantly enhanced (P < 0.05 for 2 mg/kg, P < 0.01 for 8 mg/kg) compared with the oral control group (20.4%). Moreover, the relative bioavailability (RB) of tamoxifen was 1.14- to 1.74-fold greater than that of the control group. The metabolite-parent AUC ratio (MR) was significantly reduced (P < 0.05, 8 mg/kg), implying that the formation of 4-hydroxytamoxifen was considerably affected by myricetin. The enhanced bioavailability of tamoxifen might be mainly due to inhibition of the CYP3A4- and CYP2C9-mediated metabolism of tamoxifen in the small intestine and/or in the liver, and inhibition of P-gp efflux pump in the small intestine by myricetin.

Phase II drug metabolizing enzymes

BACKGROUND

Phase II biotransformation reactions (also 'conjugation reactions') generally serve as a detoxifying step in drug metabolism. Phase II drug metabolising enzymes are mainly transferases. This review covers the major phase II enzymes: UDP-glucuronosyltransferases, sulfotransferases, N-acetyltransferases, glutathione S-transferases and methyltransferases (mainly thiopurine S-methyl transferase and catechol O-methyl transferase). The focus is on the presence of various forms, on tissue and cellular distribution, on the respective substrates, on genetic polymorphism and finally on the interspecies differences in these enzymes.

METHODS AND RESULTS

A literature search using the following databases PubMed, Science Direct and EBSCO for the years, 1969-2010.

CONCLUSIONS

Phase II drug metabolizing enzymes play an important role in biotransformation of endogenous compounds and xenobiotics to more easily excretable forms as well as in the metabolic inactivation of pharmacologically active compounds. Reduced metabolising capacity of Phase II enzymes can lead to toxic effects of clinically used drugs. Gene polymorphism/ lack of these enzymes may often play a role in several forms of cancer.

Enhanced oral paclitaxel bioavailability after administration of paclitaxel-loaded nanosponges

The aim of this study was to evaluate the pharmacokinetics of paclitaxel-loaded nanosponges (PLN) in rats. The study also evaluates the intrinsic effect of the dosage form on the improvement of paclitaxel oral bioavailability. Paclitaxel-loaded nanosponges were prepared and characterized in terms of size distribution, drug solubilization, and the kinetics of paclitaxel sedimentation. Taxol((R)) and paclitaxel-loaded nanosponges were administered orally to rats. The plasma concentration of paclitaxel was determined using liquid chromatography. The average size of PLN was 350 +/- 25 nm. The drug payload of paclitaxel was 500 +/- 0.27 mg/g of lyophilized powder. The encapsulation efficiency was 99.1 +/- 1.0%, and 1.7 +/- 0.2% of paclitaxel was crystallized after 48 h. The relative oral bioavailability of paclitaxel-loaded nanosponges was 256. After oral administration of paclitaxel-loaded PLN, the area under the plasma concentration time curve was significantly increased ( approximately 3-fold) in comparison to the control group (p < 0.05). The results indicated that PLN provided a promising new formulation to enhance the oral bioavailability of paclitaxel while avoiding the use of cremophore El: Ethanol in Taxol((R)).

Prediction of human pharmacokinetics — renal metabolic and excretion clearance

The kidneys have the capability to both excrete and metabolise drugs. An understanding of mechanisms that determine these processes is required for the prediction of pharmacokinetics, exposures, doses and interactions of candidate drugs. This is particularly important for compounds predicted to have low or negligible non-renal clearance (CL). Clinically significant interactions in drug transport occur mostly in the kidneys. The main objective was to evaluate methods for prediction of excretion and metabolic renal CL (CLR) in humans. CLR is difficult to predict because of the involvement of bi-directional passive and active tubular transport, differences in uptake capacity, pH and residence time on luminal and blood sides of tubular cells, and limited knowledge about regional tubular residence time, permeability (Pe) and metabolic capacity. Allometry provides poor predictions of excretion CLR because of species differences in unbound fraction, urine pH and active transport. The correlation between fraction excreted unchanged in urine (fe) in humans and animals is also poor, except for compounds with high passive Pe (extensive/complete tubular reabsorption; zero/negligible fe) and/or high non-renal CL. Physiologically based in-vitro/in-vivo methods could potentially be useful for predicting CLR. Filtration could easily be predicted. Prediction of tubular secretion CL requires an in-vitro transport model and establishment of an in-vitro/in-vivo relationship, and does not appear to have been attempted. The relationship between passive Pe and tubular fraction reabsorbed (freabs) for compounds with and without apparent secretion has recently been established and useful equations and limits for prediction were developed. The suggestion that reabsorption has a lipophilicity cut-off does not seem to hold. Instead, compounds with passive Pe that is less than or equal to that of atenolol are expected to have negligible passive freabs. Compounds with passive Pe that is equal to or higher than that of carbamazepine are expected to have complete freabs. For compounds with intermediate Pe the relationship is irregular and freabs is difficult to predict. Tubular cells are comparably impermeable (for passive diffusion), and show regional differences in enzymatic and transporter activities. This limits the usefulness of microsome data and makes microsome-based predictions of metabolic CLR questionable. Renal concentrations and activities of CYP450s are comparably low, suggesting that CYP450 substrates have negligible metabolic CLR. The metabolic CLR of high-Pe UDP-glucuronyltransferase substrates could contribute to the total CL.

A Rare Case of Propofol-Induced Acute Liver Failure and Literature Review

The incidence of drug-induced acute liver failure is increasing. A number of drugs can inhibit mitochondrial functions, alter β-oxidation and cause accumulation of free fatty acids within the hepatocytes. This may result in hepatic steatosis, cell death and liver injury. In our case, propofol, an anesthetic drug commonly used in adults and children, is suspected to have induced disturbance of the mitochondrial respiratory chain, which in consequence led to insufficient energy supply and finally liver failure. We report the case of a 35-year-old Caucasian woman with acute liver failure after anesthesia for stripping of varicose veins. Liver histology, imaging and laboratory data indicate drug-induced acute liver failure, presumably due to propofol. Hepatocyte death and microvesicular fatty degeneration of 90% of the liver parenchyma were observed before treatment with steroids. Six months later, a second biopsy was performed, which revealed only minimal steatosis and minimal periportal hepatitis. We suggest that propofol led to impaired fatty acid oxidation possibly due to a genetic susceptibility. This caused free fatty acid accumulation within hepatocytes, which presented as hepatocellular fatty degeneration and cell death. Large scale hepatocyte death was followed by impaired liver function and, consecutively, progressed to acute liver failure.

In-vitro and in-vivo metabolic studies of the candidate chemopreventative pentamethylchromanol using liquid chromatography/tandem mass spectrometry

Objectives

This study focuses on the in-vitro metabolic profiles of pentamethyl-chromanol in human, rat, dog and non-human primates, and characterizes the associated metabolic kinetics and specific human isozymes responsible for metabolism. Additional investigations compare in-vitro data with in-vivo metabolic data from rats and dogs.

Methods

In-vitro metabolites were generated from commercially available microsomes, S9 fractions and cytochrome P450 isozymes. Reaction mixtures were analysed using liquid chromatography/tandem mass spectrometry for metabolite identification, stability, pheno-typing and kinetic profiles. Plasma samples were collected from 28-day toxicology studies in rats and dogs, and analysed using the same methodology as for the identification of in-vitro metabolites.

Key findings

Samples from in-vitro experiments produced a total of eight identified metabolites while five were observed in the in-vivo samples. Kinetic analysis of metabolites in human microsomes generated Michaelis constants (KM) ranging from 10.9 to 104.9 μM. Pentamethylchromanol metabolic stability varied by species and multiple isozymes were identified for the observed biotransformation pathways. Pentamethylchromanol is susceptible to multiple metabolic pathways and differential metabolic stability, which is species dependent.

Conclusions

In-vitro metabolism was not a strong predictor of in-vivo metabolism for the samples assays but showed glucuronidation and sulfation as common biotransformation pathways.

Effects of epigallocatechin gallate on the oral bioavailability and pharmacokinetics of tamoxifen and its main metabolite, 4-hydroxytamoxifen, in rats

The effects of epigallocatechin gallate (EGCG) on the oral bioavailability and pharmacokinetics of tamoxifen and its metabolite, 4-hydroxytamoxifen, were investigated in rats. A single dose of tamoxifen was administered intravenously (2 mg/kg) and orally (10 mg/kg) with or without epigallocatechin (0.5, 3 and 10 mg/kg) to rats. The presence of EGCG significantly altered the pharmacokinetics of orally administered tamoxifen. Compared with the oral control group (given tamoxifen alone), the area under the plasma concentration-time curve and the peak plasma concentration of tamoxifen significantly (P<0.05 for 3 mg/kg of EGCG, P<0.01 for 10 mg/kg of EGCG) increased 48.4-77.0 and 57.1-89.7%, respectively. Consequently, the absolute bioavailability of tamoxifen in the presence of EGCG (3 and 10 mg/kg) was 48.9-78.1%, which was significantly enhanced (P<0.05 for 3 mg/kg of EGCG, P<0.01 for 10 mg/kg of EGCG) compared with the oral control group (23.7%). Moreover, the relative bioavailability of tamoxifen was 1.48-1.77-fold greater than that of the control group. EGCG at a dose of 10 mg/kg significantly increased the area under the plasma concentration-time curve (P<0.05, 40.3%) of 4-hydroxytamoxifen, but the metabolite-parent ratio of 4-hydroxytamoxifen was also significantly altered (P<0.05 for 10 mg/kg of EGCG), implying that the formation of 4-hydroxytamoxifen was considerably affected by EGCG. The increase in bioavailability of tamoxifen is likely to be due to the decrease in first-pass metabolism in the intestine and liver by inhibition of P-glycoprotein and CYP3A by EGCG. The increase in oral bioavailability of tamoxifen in the presence of EGCG should be taken into consideration of potential drug interactions between tamoxifen and EGCG.

Almokalant glucuronidation in human liver and kidney microsomes: evidence for the involvement of UGT1A9 and 2B7

1. Almokalant, a class III antiarrythmic drug, is metabolized to form isomeric glucuronides identified in human urine. Synthesis of the total glucuronide was studied in human liver and kidney microsomes. Recombinant UDP-glucuronosyltransferases (UGTs) were screened for activity and kinetic analysis was performed to identify the isoform(s) responsible for the formation of almokalant glucuronide in man. 2. From a panel of recombinant isoforms used, both UGT1A9 and 2B7 catalysed the glucuronidation of almokalant. The Km values in both instances were similar with 1.06 mM for the 1A9 and 0.97 mM for the 2B7. Vmax for 1A9 was fourfold higher than that measured for UGT2B7, 92 compared with 21 pmol min(-1) mg(-1), respectively, but UGT1A9 was expressed at approximately twofold higher level than the UGT2B7 in the recombinant cell lines. Therefore, the contribution of UGT2B7 to almokalant glucuronidation could be as significant as that of UGT1A9 in man. 3. Liver and kidney microsomes displayed similar Km values to the cloned expressed UGTs, with the liver and kidney microsomes at 1.68 and 1.06 mM almost identical to the 1A9. 4. The results suggest a significant role for UGT1A9 and 2B7 in the catalysis of almokalant glucuronidation.

Human Renal Cortical and Medullary UDP-Glucuronosyltransferases (UGTs): Immunohistochemical Localization of UGT2B7 and UGT1A Enzymes and Kinetic Characterization ofS-Naproxen Glucuronidation

There is currently little information regarding the localization of UDP-glucuronosyltransferases (UGTs) in human renal cortex and medulla, and the functional contribution of renal UGTs to drug glucuronidation remains poorly defined. Using human kidney sections and human kidney cortical microsomes (HKCM) and human kidney medullary microsomes (HKMM), we combined immunohistochemistry to investigate UGT1A and UGT2B7 expression with in vitro microsomal studies to determine the kinetics of S-naproxen acyl glucuronidation. With the exception of the glomerulus, Bowman's capsule, and renal vasculature, UGT1A proteins and UGT2B7 were expressed throughout the proximal and distal convoluted tubules, the loops of Henle, and the collecting ducts. Additionally, UGT1A and UGT2B7 expression was demonstrated in the macula densa, supporting a potential role of UGTs in regulating aldosterone. Consistent with the immunohistochemical data, S-naproxen acyl glucuronidation was catalyzed by HKCM and HKMM. Kinetic data were well described by the two-enzyme Michaelis-Menten equation. K(m) values for the high-affinity components were 34 +/- 14 microM (HKCM) and 45 +/- 14 microM (HKMM). Fluconazole inhibited the high-affinity component establishing UGT2B7 as the enzyme responsible for S-naproxen glucuronidation in cortex and medulla. The low-affinity component was relatively unaffected by fluconazole (<15% inhibition), supporting the presence of other UGTs with S-naproxen glucuronidation capacity (e.g., UGT1A6 and UGT1A9) in cortex and medulla. We postulate that the ubiquitous distribution of UGTs in mammalian kidney may buffer physiological responses to endogenous mediators, but at the same time competitive xenobiotic-endobiotic interactions may provide an explanation for the adverse renal effects of drugs, including nonsteroidal anti-inflammatory drugs.

Glucuronidation of curcuminoids by human microsomal and recombinant UDP-glucuronosyltransferases

Glucuronidation is an important pathway in the metabolism of curcumin, but the isoforms of uridine-5'-diphosphoglucuronosyltransferase (UGT) involved are not known. Here, we report on the glucuronidation of the three natural curcuminoids and their major phase I metabolites with microsomes from human liver and intestine as well as with human recombinant UGTs. Microsomes from human liver generated predominantly the phenolic and small amounts of the alcoholic glucuronide of each curcuminoid, whereas intestinal microsomes formed only the phenolic conjugates but with higher activities. The phenolic glucuronidation of the curcuminoids was predominantly catalyzed by hepatic UGT1A1 and intestinal UGT1A8 and 1A10, whereas UGT1A9, 2B7, and 1A8 exhibited high activities for hexahydro-curcuminoids. UGT1A9 was able to form the alcoholic glucuronide of each curcuminoid in addition to the phenolic conjugate. These data suggest that the gastrointestinal tract contributes substantially to the glucuronidation of curcuminoids in humans, which may have important implications for their pharmacokinetic fate in vivo.

Effect of genistein on the pharmacokinetics of paclitaxel administered orally or intravenously in rats

As many anticancer agents paclitaxel is a substrate for ATP-binding cassette (ABC) transporters such as P-glycoprotein-mediated efflux, and its metabolism in humans mainly catalyzed by CYP 3A4 and 2C8. Genistein, an isoflavonoid, is supposed to be an inhibitor of some ABC transporters, and its oxidative metobolism catalyzed by CYP 3A4 and 2C8. The purpose of this study was to investigate the effect of orally administered genistein on the pharmacokinetics of paclitaxel administered through oral and intravenous (i.v.) route in rats. A single dose of paclitaxel administered orally (30 mg/kg) or i.v. (3mg/kg) alone or 30 min after oral administration of genistein (3.3mg/kg or 10mg/kg). The presence of 10mg/kg genistein significantly (p<0.05) increased the area under the plasma concentration-time curve (AUC, 54.7% greater) of orally administered paclitaxel, which was due to the significantly (p<0.05) decreased total plasma clearance (CL/F) of paclitaxel (35.2% lower). Genistein also increased the peak concentration (C(max)) of paclitaxel significantly (p<0.05 by 3.3mg/kg, 66.8% higher; p<0.01 by 10mg/kg, 91.8% higher). Consequently, the absolute bioavailability (F) of paclitaxel in the presence of genistein was 0.020-0.025, which was elevated more than the control group (0.016); and the relative bioavailability (Fr) of orally administered paclitaxel was increased from 1.26- to 1.55-fold. Ten milligrams per kilogram genistein also significantly (p<0.05) increased the AUC (40.5% greater) and reduced the total clearance (CLt, 30% lower) of i.v. administered paclitaxel. The presence of genistein improved the systemic exposure of paclitaxel in this study. The pharmacokinetic interaction between them should be taken into consideration when paclitaxel is used with genistein or the dietary supplements full of genistein.

Hepatocyte nuclear factor-1 alpha is associated with UGT1A1, UGT1A9 and UGT2B7 mRNA expression in human liver

Experimental evidence suggests HNF1alpha regulates UGT expression. This study investigates (1) whether the variability in HNF1alpha expression is associated with the variability in UGT1A1, UGT1A9 and UGT2B7 expression in human livers and (2) the functionality of 12 HNF1alpha variants using mRNA expression as phenotype. Controlling for known UGT variation in cis-acting elements known to affect UGT expression, we demonstrate that a combination of HNF1alpha mRNA levels and UGT genotype predicts variance in UGT expression to a higher extent than UGT genotype alone. None of the HNF1alpha polymorphisms studied, however, seem to have an effect on HNF1alpha, UGT1A1, UGT1A9 and UGT2B7 expression, ruling out their functional role. Our data provide evidence for HNF1alpha being a determinant of UGT1A1, UGT1A9 and UGT2B7 mRNA expression. However, the amount of UGT intergenotype variability explained by HNF1alpha expression appears to be modest, and further studies should investigate the role of multiple transcription factors.

Differential UGT1A1 Induction by Chrysin in Primary Human Hepatocytes and HepG2 Cells

Chrysin, a dietary flavonoid, has been shown to markedly induce UGT1A1 expression and activity in HepG2 and Caco-2 cell lines; thus, it has been suggested to have clinical utility in the treatment of UGT1A1-mediated deficiencies, such as unconjugated hyperbilirubinemia or the prevention of 7-ethyl-10-hydroxycamptothecin (SN-38) toxicity. However, little is known about its induction potential in a more physiologically relevant model system, such as primary hepatocyte culture. In this study, induction of UGT1A1 expression (mRNA, protein, and activity) was investigated in primary human hepatocyte cultures after treatment with chrysin and other prototypical inducers. Endogenous nuclear receptor-mediated UGT1A1 induction was studied using transient transfection reporter assays in primary human hepatocytes and HepG2 cells. Results indicated that induction of UGT1A1 expression was minimal in human hepatocytes treated with chrysin compared with that in HepG2 cells (1.2-versus 11-fold, respectively). Subsequent experiments to determine whether the differential response was due to its metabolic stability revealed strikingly different elimination rate constants between the two cell systems (half-life of 13 min in human hepatocytes versus 122 min in HepG2 cell suspensions). Further study demonstrated that UGT1A1 mRNA expression could be induced in human hepatocyte cultures by either increasing the chrysin dosing frequency or by modulating chrysin metabolism, suggesting that the differential induction observed in hepatocytes and HepG2 cells was due to differences in the metabolic clearance of chrysin. In conclusion, this study suggests that the metabolic stability of chrysin likely would limit its ability to induce UGT1A1 in vivo.

Modulation of UDP-glucuronosyltransferase 1A1 in primary human hepatocytes by prototypical inducers

The primary objective of this study was to evaluate the modulation of UGT1A1 expression in human hepatocytes using prototypical CYP450 inducers. A bank of 16 human livers was utilized to obtain an estimate of the range of UGT1A1 protein expression and catalytic activity. Concentration-dependent changes in UGT1A1 response were evaluated in hepatocyte cultures after treatment with 3-methylchloranthrene, beta-napthoflavone, rifampicin, or phenobarbital. Pharmacodynamic analyses of UGT1A1 expression were conducted and compared to those of CYP450 after treatment with inducers in 2-3 different hepatocyte preparations. Additionally, expression of UGT1A1 mRNA and protein was evaluated in human hepatocytes treated with 14 different compounds known to activate differentially the human pregnane-X-receptor or constitutive androstane receptor. Pharmacodynamic modeling revealed EC50 values statistically significant between UGT1A1 and CYP2B6 after treatment with PB, but not statistically distinguishable between UGT1A1 and CYP's 1A2 or 3A4 after treatment with 3-methylchloranthrene or rifampicin, respectively. UGT1A1 was most responsive to the pregnane-X-receptor-agonists rifampicin, ritonavir, and clotrimazole at the mRNA level and, to a lesser extent, the constitutive androstane receptor-activators, phenobarbital and phenytoin. Pharmacodynamic analyses support a mechanism of coordinate regulation between UGT1A1 and a number of CYP450 enzymes by multiple nuclear receptors.

Effects of?-estradiol and propofol on the 4-methylumbelliferone glucuronidation in recombinant human UGT isozymes 1A1, 1A8 and 1A9

The effects of beta-estradiol and propofol on human UGT1A1, 1A8 and 1A9 activities were investigated using 4-methylumbelliferone (4-MU) as a substrate for glucuronidation. The formation of 4-MU glucuronide (4-MUG) from 4-MU, in recombinant human UGT 1A1, 1A8 and 1A9 was determined using HPLC with fluorescence detection. The glucuronidation activity of 4-MU was the highest in UGT1A9 with an apparent K(m) value of 8.3 microM, while that in UGT1A1 and 1A8 was linear to at least 100 microM. beta-estradiol had potent inhibitory effects on UGT1A9 as well as on UGT1A1 with IC(50) values of 2.1 and 7.2 microM, respectively. Propofol inhibited UGT1A9 activity with an IC(50) of 55 microM, while the IC(50) value was much higher for UGT1A8. In contrast, beta-estradiol and propofol activated 4-MU glucuronidation in UGT1A1 and 1A8, respectively. This study therefore indicates that the use of beta-estradiol as a specific inhibitor for UGT1A1 should be used with care in the identification of UGT isozymes responsible for glucuronidation in human liver microsomes.

GLUCURONIDATION OF THE OXIDATIVE CYTOCHROME P450-MEDIATED PHENOLIC METABOLITES OF THE ENDOCRINE DISRUPTOR PESTICIDE: METHOXYCHLOR BY HUMAN HEPATIC UDP-GLUCURONOSYL TRANSFERASES

Methoxychlor, a currently used pesticide, is a proestrogen exhibiting estrogenic activity in mammals in vivo. Methoxychlor undergoes oxidative metabolism by cytochromes P450, yielding 1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane (mono-OH-M) and 1,1,1-trichloro-2,2-bis(4-hydroxyphenyl)ethane (bis-OH-M) as main metabolites. Since humans may be exposed to these estrogenic metabolites, which are potential substrates of UDP-glucuronosyltransferases (UGTs), their glucuronide conjugation was investigated with human liver preparations and individual UGTs. Incubation of both mono-OH-M and bis-OH-M with human liver microsomes formed monoglucuronides. The structures of the glucuronides were identified by liquid chromatography/tandem mass spectometry. Examination of cDNA-expressed recombinant human hepatic UGTs revealed that several catalyze glucuronidation of both compounds. Among the cDNA-expressed UGT1A enzymes, UGT1A9 seemed to be the main catalyst of formation of mono-OH-M-glucuronide, whereas UGT1A3 seemed to be the most active in bis-OH-M-glucuronide formation. Furthermore, the chiral selectivity of mono-OH-M glucuronidation was examined. The results of the incubation of single enantiomers generally agreed with the chiral analyses of mono-OH-M derived from the glucuronidase digestion of the glucuronides of the racemic mono-OH-M. There was a relatively slight but consistent enantioselective preference of individual UGT1A1, UGT1A3, UGT1A9, and UGT2B15 enzymes for glucuronidation of the S- over the R-mono-OH-M, whereas in human liver microsomes differences were observed among donors in generating the respective R/S-mono-OH-M ratio. Since it was previously shown that human liver microsomes demethylate methoxychlor mainly into S-mono-OH-M, the observation that UGT1A isoforms preferentially glucuronidate the S-mono-OH-M suggests a suitable mechanism for eliminating this major enantiomer. This enantiomeric preference, however, is not extended to all samples of human liver microsomes that we tested.

Role of orphan nuclear receptors in the regulation of drug-metabolising enzymes

During the past several years, important advances have been made in our understanding of the mechanisms that regulate the expression of genes that determine drug clearance, including phase I and phase II drug-metabolising enzymes and drug transporters. Orphan nuclear receptors have been recognised as key mediators of drug-induced changes in both metabolism and efflux mechanisms. In this review, we summarise recent findings regarding the function of nuclear receptors in regulating drug-metabolising and transport systems, and the relevance of these receptors to clinical drug-drug interactions and the development of new drugs. Emphasis is given to two newly recognised 'orphan' receptors (the pregnane X receptor [PXR] and the constitutive androstane receptor [CAR]) and their regulation of cytochrome P450 enzymes, such as CYP3A4, CYP2Cs and CYP2B6; and transporters, such as P-glycoprotein (MDR1), multidrug resistance-associated proteins (MRPs) and organic anion transporter peptide 2 (OATP2). Although 'cross-talk' occurs between these two receptors and their target sequences, significant species differences exist between ligand-binding and activation profiles for both receptors, and PXR appears to be the predominant or 'master' regulator of hepatic drug disposition in humans. Several important physiological processes, such as cholesterol synthesis and bile acid metabolism, are also tightly controlled by certain ligand-activated orphan nuclear receptors (farnesoid X receptor [FXR] and liver X receptor [LXR]). In general, their ability to bind a broad range of ligands and regulate an extensive array of genes that are involved in drug clearance and disposition makes these orphan receptors attractive targets for drug development. Drugs have the capacity to alter nuclear receptor expression (modulators) and/or serve as ligands for the receptors (agonists or antagonists), and thus can have synergistic or antagonistic effects on the expression of drug-metabolising enzymes and transporters. Coadministration of drugs that are nuclear receptor agonists or antagonists can lead to severe toxicity, a loss of therapeutic efficacy or an imbalance in physiological substrates, providing a novel molecular mechanism for drug-drug interactions.

Conjugation metabolism of acetaminophen and bilirubin in extrahepatic tissues of rats

An anhepatic rat model was used to explore the extrahepatic conjugating metabolism of acetaminophen and serum bilirubin. The recovery of glucuronide- and sulfate-acetaminophen was 47.5% in normal control and 13.4% in model rats in the urine collected for 6 h after administration of acetaminophen 20 mg kg(-1). Following the increase of acetaminophen dose to 150 mg kg(-1), the recovery of urinary glucuronide-acetaminophen increased by 53.9% in normal control; but it decreased by 36.4% in model rats. In contrast to normal control, the pretreatment with phenobarbital did not affect acetaminophen and its metabolite levels in plasma and urine in model rats. After the establishment of anhepatic model the serum direct bilirubin rose dramatically. Urinary bilirubin test was positive in model rats, but not in normal control. No changes were observed in serum total bilirubin and ratio of direct/total bilirubin after the pretreatment with ranitidine or phenobarbital 50 mg kg (-1), i.p. for 5 days in model rats. The results indicate that the glucuronide- and sulfate-acetaminophen formed in the extrahepatic tissues of model rats is 28.2% of normal control, serum free bilirubin can be transformed into conjugated bilirubin in extrahepatic tissues, and the regulation mechanism of phase II conjugating enzymes is different between the hepatic and extrahepatic tissues.

Phase 1 trial of irinotecan (CPT-11) in patients with recurrent malignant glioma: a North American Brain Tumor Consortium study

This study was conducted to determine the maximum tolerated dose and dose-limiting toxicity of irinotecan (CPT-11) administered every 3 weeks to adults with progressive malignant glioma who were treated with enzyme inducing antiepileptic drug (EIAED) therapy, and to compare the pharmacokinetics with those in patients not on EIAED therapy treated at the recommended phase 2 dose for other cancers. The CPT-11 dose was 350 mg/m(2) i.v. every 3 weeks and remained fixed in patients not on EIAED therapy, but the dose was escalated by 50-mg/m(2) increments in patients on EIAED therapy. CPT-11 and its metabolites SN-38, SN-38 glucuronide (SN-38G), and APC (7-ethyl-10[4-N-(5 aminopentanoic acid)-1-piperidine]-carbonyloxycamptothecin) were characterized in both groups. Patients on EIAEDs received 350 to 800 mg/m(2) of CPT-11. Dose-limiting toxicity was due to grade 3 diarrhea despite maximal doses of loperamide. The systemic levels of CPT-11, APC, SN-38G, and SN-38 were all lower in the EIAED group. There was a moderate-to-fair relationship between CPT-11 dose and the area under the curve (AUC) for CPT-11 and APC over the 2, but no relationship dosage range of 350 to 800 mg/m between CPT-11 dose and the AUC for SN-38 or SN-38G. At the 750-mg/m(2) dose, the AUC for CPT-11 (21.6 microg x h/ml) matched the AUC (21.6 microg x h/ml) in the non-EIAED group treated with 350 mg/m(2) of CPT-11. We conclude that the recommended phase 2 dose of CPT-11 for patients on EIAEDs is 750 mg/m(2) when given every 3 weeks. A phase 2 study of patients with recurrent malignant glioma is ongoing to assess the efficacy of CPT-11 when the dose is stratified according to the use of EIAEDs.

The ABCs of drug transport in intestine and liver: efflux proteins limiting drug absorption and bioavailability

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (Pgp; MDR1) and MRP2, may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into blood. Drugs may also be modified by intracellular phase I and phase II metabolising enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for Pgp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subject to further metabolism and biliary excretion, often by a similar system of ATP-binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolising enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. The effectiveness of this system is optimised through dynamic regulation of transporter and enzyme expression; tissues have a remarkable capacity to regulate the amounts of protein both at transcriptional and post-transcriptional levels in order to maintain homeostasis. This review addresses the progress to date on what is known about the role and regulation of drug efflux mechanisms in the intestine and liver.

Effect of intrapatient dosage escalation of irinotecan on its pharmacokinetics in pediatric patients who have high-grade gliomas and receive enzyme-inducing anticonvulsant therapy

The authors set out to determine the effect of intrapatient dose escalation of irinotecan on its disposition in pediatric patients with high-grade glioma who received concomitant enzyme-inducing anticonvulsants (EIAs). During Course 1, a 60-minute intravenous infusion of irinotecan (20 mg/m(2) per day) was administered once daily for 5 days on each of 2 consecutive weeks. The authors measured the concentrations of the lactone forms of irinotecan and its metabolites 7-ethyl-10-hydroxycamptothecin (SN-38), SN-38 glucuronide, and 7-ethyl-10-[4-N-(5-aminopeptanoic acid)-1-piperidino]-carbonyloxycamptothecin (APC) in serial plasma samples collected on Days 1 and 12 of Course 1. For the 6 patients who received EIAs but whose SN-38 areas under the concentration-time curve (AUCs) on Day 1 were below clinically significant levels, irinotecan dosage was increased, and subsequent pharmacokinetic studies were performed. Thirty-five patients were enrolled. The rate of irinotecan clearance was greater for patients who received EIAs than for those who did not (P = 0.0008), whereas systemic exposure to irinotecan (P = 0.02) and SN-38 (P = 0.0001) was lower for those treated with EIAs than for those who were not. Of the 6 patients whose irinotecan dosages were increased, 3 experienced an increase in the SN-38 AUC between Days 1 and 12. For 1 patient, the SN-38 AUC on Day 12 was lower than on Day 1; this result likely was due to an increased dose of EIAs during the same period. Despite irinotecan dose escalation to 60 and 80 mg/m(2), the SN-38 AUCs for 2 patients did not increase to clinically significant levels. The type and grade of toxicity did not differ between the two patient groups. Increasing the dosage of irinotecan increased the SN-38 AUC in some patients who received concomitant EIA therapy.

Serotonin (5-hydroxytryptamine) glucuronidation in vitro: assay development, human liver microsome activities and species differences

1. The main purpose was to develop a high-performance liquid chromatography (HPLC)-based method to assay serotonin glucuronidation activity using liver microsomal fractions. Application of this method was then demonstrated by determining serotonin UDP-glucuronosyltransferase (UGT) enzyme kinetics using human liver microsomes and recombinant human UGT1A6. Interspecies differences were also evaluated using liver microsomes from 10 different mammalian species. 2. Incubation of liver microsomes with serotonin, UDP-glucuronic acid and magnesium resulted in the formation of a single product peak using HPLC with fluorescence and ultraviolet absorbance detection. This peak was confirmed as serotonin glucuronide based on sensitivity to beta-glucuronidase and by obtaining the expected mass of 352 with positive-ion mass spectrometry. 3. Following a preparative HPLC isolation, the structure of this metabolite was established as serotonin-5-O-glucuronide by (1)H-NMR spectroscopy. 4. Enzyme kinetic studies showed apparent K(m) and V(max) of 8.8 +/- 0.3 mM and 43.4 +/- 0.4 nmoles min(-1) mg(-1) protein, respectively, for human liver microsomes, and 5.9 +/- 0.2 mM and 15.8 +/- 0.2 nmoles min(-1) mg(-1), respectively, for recombinant UGT1A6. 5. The order of serotonin-UGT activities in animal liver microsomes was rat > mouse > human > cow > pig > horse > dog > rabbit > monkey > ferret. Cat livers showed no serotonin-UGT activity. Heterozygous and homozygous mutant Gunn rat livers had 40 and 13%, respectively, of the activity of the normal Wistar rat, indicating a significant contribution by a rat UGT1A isoform to serotonin glucuronidation. 6. This assay provides a novel sensitive and specific technique for the measurement of serotonin-UGT activity in vitro.

In vitro analysis of human drug glucuronidation and prediction of in vivo metabolic clearance

The glucuronidation of a number of commonly used hepatic uridine diphosphate glucuronosyltransferase drug substrates has been studied in human tissue microsomes. Prediction of in vivo hepatic drug glucuronidation from liver microsomal data yielded a consistent 10-fold under-prediction. Consideration of protein binding was observed to be pivotal when predicting in vivo glucuronidation for acid substrates. Studies using human intestinal microsomes demonstrated the majority of drugs to be extensively glucuronidated such that the intrinsic clearance (CL(int)) of ethinylestradiol (CL(int) = 1.3 microl/min/mg) was twice that obtained using human liver microsomes (CL(int) = 0.7 microl/min/mg). The potential extrahepatic in vivo glucuronidation was calculated for a range of drug substrates from human microsomal data. These results indicate the contribution of intestinal drug glucuronidation to systemic drug clearance to be much less than either hepatic or renal glucuronidation. Therefore, data obtained with intestinal microsomes may be misleading in the assessment of the contribution of this organ to systemic glucuronidation. The use of hepatocytes to assess metabolic stability for drugs predominantly metabolized by glucuronidation was also investigated. Metabolic clearances for a range of drugs obtained using fresh preparations of human hepatocytes predicted accurately hepatic clearance reported in vivo. The use of cryopreserved hepatocytes as an in vitro tool to predict in vivo metabolism was also assessed with an excellent correlation obtained for a number of extensively glucuronidated drugs (R(2) = 0.80, p < 0.001).

Evaluation of the marmoset as a model species for drug glucuronidation

1. The in vitro glucuronidation of a wide range of compounds has been studied in microsomes prepared from marmoset liver and kidney. These studies have been undertaken to evaluate the marmoset as a model species for drug glucuronidation and for comparison with conjugation by other species. 2. The compounds studied were glucuronidated by marmoset liver microsomes to varying extents (e.g. naproxen CLint 0.4 microl min(-1) mg(-1), 1-naphthol CLint 43 microl min(-1) mg(-1)) Both marmoset and rat liver microsomes glucuronidated morphine at the 3-position (marmoset CLint 19 microl min(-1) mg(-1), rat CLint 6.3 microl min(-1) mg(-1)) but glucuronidation at the 6-position was below, the level of radiodetection in both species. 3. Interestingly, marmoset liver microsomes were able to catalyse the glucuronidation of the tertiary amine imipramine to a significant extent (0.05 nmol min(-1) mg(-1)). However, no glucuronidation was detected by rat liver microsomes. 4. Conjugation of a range of substrates was detectable by marmoset kidney microsomes in contrast to rat kidney microsomes, which only catalysed the glucurondation of bilirubin and 1-naphthol (CLint 17 microl min(-1) mg(-1) and 18 microl min(-1) mg(-1), respectively). 5. This report and previous work in dog and human tissue microsomes suggest that the marmoset may be an alternative animal model for human drug glucuronidation, especially when the pathway of drug glucuronidation is known to differ between lower laboratory species and man.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.

Enzyme induction and inhibition by new antiepileptic drugs: a review of human studies

The aim of this paper is to review a number of new antiepileptic agents (i.e. felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide) for their inducing and/or inhibitory properties in humans, mainly considering the interactions where they are involved as the cause rather than the object of such interactions. Two aspects have been particularly taken into account: the changes or absence of changes in plasma/serum concentrations of concomitant drugs and the direct or indirect evidence of induction, inhibition or lack of effect on the six major human hepatic CYP isozymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), as well as on other CYP isozymes or enzyme systems. Felbamate clearly affects the pharmacokinetics of a number of drugs, generally increasing but also decreasing their concentrations. It induces enzymes such as CYP3A4 and inhibits enzymes such as CYP2C19 and those of the beta-oxidation pathway. Topiramate is not devoid of potential interaction properties: it decreases the plasma concentrations of ethinylestradiol, induces CYP3A4 and inhibits CYP2C19. For oxcarbazepine, no inhibitory, only inductive effects have been observed thus far. Felbamate. topiramate and oxcarbazepine may induce the metabolism of steroidal oral contraceptives. In this respect, tiagabine has been studied at a rather low dose. Pharmacodynamic or pharmacokinetic interaction seems to exist between lamotrigine and carbamazepine. Lamotrigine appears to be a weak inducer of UGTs, whereas induction of CYP3A4 seems improbable as the compound does not change the concentrations of oral contraceptives or the urinary excretion of 6beta-hydroxycortisol. Zonisamide has very peculiar pharmacokinetics and an extensive metabolism. Additional information on its enzyme inducing or inhibiting properties would be necessary, as data so far collected on its effect on the pharmacokinetics of other drugs are conflicting. Gabapentin, vigabatrin and in particular levetiracetam appear to be devoid of significant enzyme inducing or inhibiting properties.

Distribution of uridine diphosphate-glucuronosyltransferase (UGT) expression and activity in cynomolgus monkey tissues: evidence for differential expression of steroid-conjugating UGT enzymes in steroid target tissues

Based on the similarity of pathways and enzymes involved in steroid metabolism, simians represent a relevant animal model to study steroid elimination by glucuronidation. In this study the tissue distribution of UDP-glucuronosyltransferase (UGT) transcripts, proteins, and enzymatic activities were examined in 24 different cynomolgus monkey tissues. RT-PCR and Western blot analysis on total RNA and microsomal proteins demonstrated the presence of UGT1A and UGT2B transcripts and proteins in a wide range of tissues including steroid target tissues. Glucuronidation activity on eugenol, 5alpha-androstane-3alpha,17beta-diol, androsterone, and 4-hydroxyestradiol was measured using tissue homogenates and radiolabeled [14C]UDP-glucuronic acid. All tissues contained conjugation activity on these substrates, but glucuronidation rates were significantly lower in steroid target tissues than in liver, kidney, or gut. However, the ratio of steroid glucuronidation vs. eugenol glucuronidation was higher in steroid target tissues, suggesting a differential expression of steroid-conjugating enzymes in these tissues. Taken together, these results clearly demonstrate the presence of steroid glucuronidation enzymes in extrahepatic steroid target tissues and support the hypothesis that steroid glucuronidation is an important intracrine pathway involved in termination of steroid signaling.

Induction of drug metabolising enzymes: pharmacokinetic and toxicological consequences in humans

Currently, 5 different main mechanisms of induction are distinguished for drug-metabolising enzymes. The ethanol type of induction is mediated by ligand stabilisation of the enzyme, but the others appear to be mediated by intracellular 'receptors'. These are the aryl hydrocarbon (Ah) receptor, the peroxisome proliferator activated receptor (PPAR), the constitutive androstane receptor (CAR, phenobarbital induction) and the pregnane X receptor [PXR, rifampicin (rifampin) induction]. Enzyme induction has the net effect of increasing protein levels. However, many inducers are also inhibitors of the enzymes they induce, and the inductive effects of a single drug may be mediated by more than one mechanism. Therefore, it appears that every inducer has its own pattern of induction; knowledge of the main mechanism is often not sufficient to predict the extent and time course of induction, but may serve to make the clinician aware of potential dangers. The possible pharmacokinetic consequences of enzyme induction depend on the localisation of the enzyme. They include decreased or absent bioavailability for orally administered drugs, increased hepatic clearance or accelerated formation of reactive metabolites, which is usually related to local toxicity. Although some severe drug-drug interactions are caused by enzyme induction, most of the effects of inducers are not detected in the background of nonspecific variation. For any potent inducer, however, its addition to, or withdrawal from, an existing drug regimen may cause pronounced concentration changes and should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events. The toxicological consequences of enzyme induction in humans are rare, and appear to be mainly limited to hepatoxicity in ethanol-type induction.

Molecular genetic basis for deficient acetaminophen glucuronidation by cats: UGT1A6 is a pseudogene, and evidence for reduced diversity of expressed hepatic UGT1A isoforms

The domestic cat has a significantly lower capacity to glucuronidate planar phenolic xenobiotics compared with most other mammalian species. The aim of this study was to determine the mechanistic basis for this anomaly. Current knowledge of the substrate specificity of UDP-glucuronosyltransferase (UGT) isoforms indicates that the cat may either lack or poorly express UGT1A6. Initially, a novel cloning technique was used to identify UGT1A genes expressed in cat liver. Only two unique UGT1A isoforms could be discriminated. The first (28%, of clones) was most homologous to UGT1A1 (the bilirubin-UGT), while the second (72% of clones) showed homology to several isoforms, but could not be unambiguously identified, and was designated cat UGT1A02. Southern blot analysis confirmed the presence of a single UGT1A6-homologous region in the cat genome. Subsequent cloning and sequencing of the entire UGT1A6 exon 1 coding region revealed five deleterious genetic mutations. Identical mutations were found by sequencing of UGT1A6 exon 1 from five other unrelated cats. Four of these five genetic lesions were also identified in the UGT1A6 exon 1 region of a margay (Leopardus wiedii). Finally, RT-PCR of liver mRNA from four different cats confirmed the presence of UGT1A1 and UGT1A02, but not UGT1A6. In conclusion, UGT1A6 is a pseudogene in the domestic cat and in at least one other phylogenetically related species. Furthermore, cats appear to have a less diverse pattern of UGT1A isoform expression compared with other species. Such differences most likely reflect the highly carnivorous diet of Feliform species and resultant minimal exposure to phytoalexins.

Intermittent jaundice in patients with acute leukaemia: a common mutation of the bilirubin uridine-diphosphate glucuronosyltransferase gene among Asians

The Gly71Arg mutation of the hepatic bilirubin UDP glucuronosyl-transferase (B-UGT) gene associated with Gilbert syndrome prevails among Japanese and its gene frequency is 0.13. Among 20 patients with acute leukaemia, 4 patients showed intermittent unconjugated hyperbilirubinaemia during the course of combined chemotherapy. The Gly71Arg mutation was detected in all 4 patients with hyperbilirubinaemia, but was not found in 16 patients without hyperbilirubinaemia. Two of them were heterozygotes and one was a homozygote for the Gly71Arg mutation, and the other was a compound heterozygote of the Gly71Arg mutation and TA insertion mutation in the TATA box of the B-UGT gene. In addition to the complications leading to hyperbilirubinaemia, including liver damage due to drugs, viral infections or tumour cell infiltrations and alloimmune haemolysis, carrier status for the Gly71Arg mutation should be considered in a patient with leukaemia showing intermittent hyperbilirubinaemia during the course of chemotherapy, especially among Japanese, Koreans and Chinese owing to its prevalence in those populations.