In vitro metabolism of theophylline by rat and human liver tissue

Considering CYP1A2 phenotype and genotype for optimizing the dose of olanzapine in the management of schizophrenia

INTRODUCTION

Schizophrenia, a mental disorder, is a debilitating condition which typically strikes young people in their early 20's. Antipsychotic medications are widely prescribed for the treatment of schizophrenia however a balancing act is necessary to provide the correct dose to each patient. It is suggested that a large number of patients discontinue antipsychotic pharmacotherapy because the treatments provided do not always reduce the positive symptoms of the disease, while many have adverse effects on the patients. This implies that neither the incorrect drug nor the optimal dosage for that patient is achieved.

AREAS COVERED

The current review investigates variability in response to olanzapine with a specific focus on the common intrinsic and extrinsic factors that influence both olanzapine and CYP1A2 activity. Furthermore, the authors discuss the utilization of phenotyping and genotyping of CYP1A2 and their potential utility in clinical practice for olanzapine dosing regimens. The authors also consider the potential of pharmacometrics compared to pharmacogenomics as a tool to personalize medicine.

EXPERT OPINION

Careful consideration must be given to the impact of a genetic variant on the disposition of a drug prior to implementing genetic 'tests' to determine response. CYP1A2 phenotypic assessment can yield important information regarding the disposition of olanzapine; however, it relies on the accuracy of the metric and the minimal impact of other metabolic pathways. The application of pharmacometrics provides an effective method to establish covariates that significantly influence olanzapine disposition which can incorporate phenotype and/or genotype.

Liver slices in in vitro pharmacotoxicology with special reference to the use of human liver tissue

In the early years of research in in vitro pharmacotoxicology liver slices have been used. After a decline in the application of slices in favour of the use of isolated hepatocytes and the isolated perfused liver preparation, the development of the Krumdieck slicer in the 1980s led to a ;comeback' of the technique. This review will focus on the use of human liver, with special reference to the comparison of slices with isolated hepatocytes in in vitro pharmacotoxicology. In addition, an overview on the predictive value of these in vitro systems for drug disposition and toxicity in vivo will be given. Preservation techniques for liver slices and hepatocytes will also be discussed. These techniques ensure an efficient utilization of the scarce human material. For long-term storage of liver slices and hepatocytes, cryopreservation seems most promising. However, cryopreservation is still in its infancy, and reports mainly deal with drug metabolism studies after cryopreservation. Drug toxicity, metabolism and transport data determined in slices and isolated hepatocytes, from both human and animal liver showed good correlation with the corresponding parameters measured in vivo. Therefore, the results obtained in such studies may give rise to more in-depth research on the mechanisms of pharmactoxicology in the human liver.

Comparison of the metabolism of ethylene glycol and glycolic acidin vitroby precision-cut tissue slices from female rat, rabbit and human liver

1. The metabolism of [1,2-(14)C]-ethylene glycol and [1,2-(14)C]-glycolic acid was studied in vitro using precision-cut tissue slices prepared from the livers of female Sprague-Dawley rats, New Zealand white rabbits and humans. The time-course for production of metabolites formed from ethylene glycol at concentrations from 3 to 40 mM was determined to compare quantitatively the differences between species in the rates and amounts of formation of glycolic acid, the presumed developmental toxicant of ethylene glycol. The rates of metabolism of glycolic acid to glyoxylic acid at concentrations from 0.05 to 16 mM by liver tissue from the different species were also determined. The apparent V(max)/K(m) for the metabolic conversions of ethylene glycol to glycolic acid and for glycolic acid to glyoxylic acid in liver tissue from the different species were obtained. 2. There were qualitative differences in the metabolic profiles and quantitative differences in the formation of glycolic acid between the mammalian liver systems. There was an average of 10-fold less glycolic acid produced by liver slices from rabbits compared with rats. With the human liver, the formation of glycolic acid was not detectable using tissue from three of four human donors. A low level of glycolic acid was detected in one liver slice incubation from one of the four subjects, but only at one extended time point; glyoxylate was detected with liver slices from all four humans. 3. Liver slices prepared from female Sprague-Dawley rats, female New Zealand White rabbits and three female human subjects all metabolized glycolic acid to glyoxylic acid. Human liver tissue was the most effective at further metabolizing glycolic acid to glyoxylic acid. The ratios of V(max)/K(m), representing the relative clearance of glycolic acid from liver tissue, were approximately 14:9:1 for human, rat and rabbit liver, respectively. 4. Precision-cut liver slices maintained in dynamic organ culture are good predictors of metabolism by liver tissue in vivo. The results of the present study therefore indicate that levels of glycolic acid, if formed in vivo, following exposures to similar concentrations of ethylene glycol, would be lower in humans than in rabbits and rats.

Theophylline pharmacokinetics: comparison of Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, humanized hCYP1A1_1A2 knock-in mice lacking either the mouse Cyp1a1 or Cyp1a2 gene, and Cyp1(+/+) wild-type mice

OBJECTIVES

Pharmacokinetics of theophylline was investigated in Cyp1(+/+) wild-type mice, Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, and humanized hCYP1A1_1A2 mice lacking either the mouse Cyp1a1 or Cyp1a2 gene.

METHODS AND RESULTS

Animals received a single dose of theophylline (8 mg/kg i.p.), either alone or pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10 microg/kg i.p.) 24 h prior to theophylline. We found that mouse or human CYP1A2 is the predominant enzyme for theophylline metabolism, the contribution of mouse or human CYP1A1 to theophylline metabolism is negligible, and another TCDD-inducible enzyme plays a minor role in 1-methyluric acid and 1,3-dimethyluric acid formation as well as enhanced theophylline clearance from the body. The half-life of elimination from plasma was more than four times longer in Cyp1a2(-/-) than Cyp1(+/+) mice and more than 10 times different after TCDD pretreatment. In humanized hCYP1A1_1A2 mice lacking the mouse Cyp1a2 gene, the half-life of elimination from plasma was two to three times longer than that in Cyp1(+/+) mice and four to five times different after TCDD pretreatment.

CONCLUSION

Replacement of mouse Cyp1a2 with a functional human CYP1A2 gene restored the ability to metabolize theophylline, and the metabolism changed to a humanized profile (i.e. 3-methylxanthine formation, not seen in the wild-type mouse). TCDD-pretreated hCYP1A1_1A2 Cyp1a2(-/-) mice exhibited enhanced theophylline metabolism and clearance, due to induction of the human CYP1A2 enzyme. Comparing the hCYP1A1_1A2 Cyp1a2(-/-) and wild-type mice with published clinical studies, we found theophylline clearance to be about 5 times and 12 times, respectively, greater than that reported in humans.

Alteration of the pharmacokinetics of theophylline by rutaecarpine, an alkaloid of the medicinal herb Evodia rutaecarpa, in rats

Rutaecarpine is a main active alkaloid present in the medicinal herb, Evodia rutaecarpa. The cytochrome P450 (CYP) 1A2 substrate, theophylline, is an important therapeutic agent for the treatment of asthma, but has a narrow therapeutic index. To evaluate the pharmacokinetic interaction of theophylline with rutaecarpine, the effects of rutaecarpine on CYP1A2 activity and theophylline pharmacokinetics were investigated. Oral treatment of Sprague-Dawley rats with 50 mg kg(-1) rutaecarpine for three days through a gastrogavage caused a 4- and 3-fold increase in liver microsomal 7-ethoxyresorufin O-deethylation (EROD) and 7-methoxyresorufin O-demethylation activity, respectively. In the kidney, rutaecarpine treatment caused a 3-fold increase in EROD activity. In the lungs, EROD activity was elevated from an undetectable to a detectable level by rutaecarpine. Pharmacokinetic parameters of theophylline were determined using a microdialysis sampling method. Rutaecarpine pre-treatment increased the clearance of theophylline in a dose-dependent manner. Pre-treatment of rats with 50 mg kg(-1) rutaecarpine caused a 3-fold increase in theophylline clearance and a 70%, 68% and 68% decrease in the area under the concentration-time curve (AUC), mean residence time (MRT) and half-life, respectively. These results demonstrated that rutaecarpine treatment elevated CYP1A2 catalytic activity and theophylline excretion in rats. In patients taking theophylline, adverse effects might be noticed when a rutaecarpine-containing herbal preparation is used concomitantly.

Inhibition of Theophylline Metabolism by Suplatast and Its Metabolites in Rats

The inhibitory effect of suplatast (ST), an anti-allergic drug, on theophylline (TP) metabolism was investigated in rats in vivo and in vitro. Intravenous injection of aminophylline (AP) at 10 mg/kg of TP equivalent was performed with or without pretreatment by oral administration of 100 mg/kg of ST 2.5 h prior to AP. In the ST-pretreated group, plasma concentration (Cp), the area under Cp-time profile (AUC) and urinary excretion of TP increased significantly, and urinary excretion of TP metabolites, 1,3-dimethyluric acid (DMU) and 1-methyluric acid (1MU) decreased significantly. Metabolic clearance of DMU (CL(DMU)) and that of 1MU (CL(1MU)) were remarkably suppressed by ST pretreatment, however, renal clearance (CLr) of TP did not change. To compare the inhibitory effect of ST on TP metabolism with that of its main metabolite (M1) in vivo, a concomitant intravenous injection of AP (10 mg/kg of TP equivalent) with ST or M1 (40 mg/kg of ST equivalent) was carried out. In the M1 group, Cp and AUC of TP increased significantly, and the total body clearance of TP decreased significantly. In contrast, ST did not induce these changes. Then, the inhibitory effect of ST and M1 on TP metabolism in vitro was evaluated using rat-liver microsomes. ST and M1 suppressed DMU formation in a competitively inhibitory manner, and their equilibrium dissociation constants (Ki) were 822 and 731 microM, respectively. In conclusion, inhibition of TP metabolism by ST was demonstrated in vivo and in vitro, and the involvement of M1 and/or other metabolites in this drug interaction was suggested.

Coincubation of tissue slices, a new way to study metabolic cooperation between organs: Hepatorenal cooperation in the biotransformation of CGP 47 969 A

INTRODUCTION

There is limited information on in vitro/ex vivo tools to be used for studying interorgan metabolic cooperation. We report here the use of the tissue slice technique for this purpose.

METHODS

Rat liver and kidney slices were used to study metabolic cooperation for the metabolism of CGP 47 969, a potential anti-inflammatory compound which in vivo is extensively conjugated with glutathione and subsequently degraded via the mercapturic acid pathway.

RESULTS

Upon incubation with liver slices, CGP 47 969 was extensively conjugated with GSH whilst degradation of the GSH conjugate was moderate. Upon incubation with kidney slices, conjugation of CGP 47 969 with GSH was moderate but degradation of the GSH conjugate was complete. Upon coincubation of CGP 47 969 with liver and kidney slices, both conjugation with GSH and its subsequent degradation were almost complete. Thus, coincubation of liver and kidney slices permitted the efficient in vitro reproduction of the complete biotransformation of CGP 47 969 via its GSH conjugate to the ultimate mercapturic acid metabolite in a one step procedure.

DISCUSSION

This novel slice coincubation culture could serve as an in vitro model for interorgan cooperation in multistep metabolic processing.

Biotransformation of benzydamine by microsomes and precision-cut slices prepared from cattle liver

1. Benzydamine (BZ), a non-steroidal anti-inflammatory drug used in human and veterinary medicine, is not licensed for use in food-producing species. Biotransformation of BZ in cattle has not been reported previously and is investigated here using liver microsomes and precision-cut liver slices. 2. BZ was metabolized by cattle liver microsomes to benzydamine N-oxide (BZ-NO) and monodesmethyl-BZ (Nor-BZ). Both reactions followed Michaelis-Menten kinetics (Km = 76.4 +/- 16.0 and 58.9 +/- 0.4 microM Vmax = 6.5 +/- 0.8 and 7.4 +/- 0.5 nmolmg(-1) min(-1) respectively); sensitivity to heat and pH suggested that the N-oxidation is catalysed by the flavin-containing monooxygenases. 3. BZ-NO and Nor-BZ were the most abundant products derived from liver slice incubations, and nine other BZ metabolites were found and tentatively identified by LC-MS. Desbenzylated and hydroxylated BZ-NO analogues and a hydroxylated product of BZ were detected, which have been reported in other species. Product ion mass spectra of other metabolites, which are described here for the first time, indicated the formation of a BZ N- -glucuronide and five hydroxylated and N+-glucuronidated derivatives of BZ, BZ-NO and Nor-BZ. 4. The results indicate that BZ is extensively metabolized in cattle. Clearly, differences in metabolism compared with, for example, rat and human, will need to be considered in the event of submission for marketing authorization for use in food animals.

Uncertainty factors for chemical risk assessment: interspecies differences in the in vivo pharmacokinetics and metabolism of human CYP1A2 substrates

The 100-fold default uncertainty factor is used to convert a no-observed-adverse-effect level (NOAEL) from a animal toxicity study, to a "safe" value for human intake. The composite uncertainty factor (100) has to allow for interspecies (10-fold) and interindividual (10-fold) differences in toxicokinetics and toxicodynamics. The aim of the current study was to assess the validity of the interspecies default for toxicokinetics (4.0) for each of the test species (dog, rabbit, rat and mouse), using published data for compounds eliminated by CYP1A2 in humans (caffeine, theobromine, theophylline and paraxanthine). An analysis of the published literature showed that the absorption, bioavailability and route of excretion were generally similar between humans and the test species, for each probe substrate. However, interspecies differences in the route of metabolism, and the enzymes involved in this process, were identified. The magnitude of difference in the internal dose, between species, showed that values for the mouse (10.6) and rat (5.4) exceed the 4.0-fold default, whereas the rabbit (2.6) and dog (1.6) were below this value. This work supports the need to replace the generic default factors by a compound-related value derived from specific, relevant, quantitative data; this would result in more relevant and reliable non-cancer risk assessments.

Cytochrome P450-dependent metabolic pathways and glucuronidation in trout liver slices

We investigated the capacity of trout precision-cut liver slices to metabolize xenobiotics and steroids. As a first approach, liver slices were compared with freshly isolated trout hepatocytes, using 7-ethoxycoumarin (7-EC) and testosterone as substrates. Trout liver slices and freshly isolated hepatocytes had a similar capacity for conducting cytochrome P450-dependent metabolism, as indicated by the rate of oxidative metabolism of 7-EC and testosterone, and by the metabolic profile of these substrates. A lower rate of glucuronidation in slices compared with hepatocytes was observed with testosterone (50 microM), whereas the opposite situation occurred with 7-EC used at higher concentration (100 microM). In a second step, we investigated the effect of beta-naphthoflavone on 7-EC and testosterone biotransformation, using slices maintained in culture for 24 h, with or without the inducer added. The results were compared with the metabolic rates of these substrates incubated with liver slices originating from trout pretreated in vivo with beta-naphthoflavone. Cytochrome P450-mediated rates of 7-EC dealkylation and testosterone hydroxylation decreased to 38 and 55% of the control value, respectively, when incubations were performed in 24-h cultured slices instead of freshly cut slices. Exposure of the slices to 50 microM beta-naphthoflavone resulted in about 3 times higher deethylation rate of 7-EC. A similar value was obtained when treatment occurred in vivo. As demonstrated in rat by several authors, liver slices seem a useful and simple tool for studying the metabolic pathways of xenobiotics and steroids and for the assessment of inducers of the CYP1A1 family.

Monooxygenation, cytochrome P4501A1 and P4501A1-mRNA in rat liver slices exposed to beta-naphthoflavone and dexamethasone in vitro

Precision-cut liver slices (0.5 mm) were incubated at 30 degrees C in a modified William's Medium E for up to 48 hrs. During the incubation, K+ and GSH/GSSG concentrations did not decrease. Cytochrome P450-dependent dealkylation rates of 7-ethoxycoumarin (ECOD), 7-allyloxycoumarin (ACOD) and 7-ethoxyresorufin (EROD) decreased to 1/3, 1/2 or did not change at all, respectively, after a 48 hrs incubation period. Exposure of the slices to 25 microM beta-naphthoflavone (beta NF) resulted in about 3 times higher monooxygenation rates. An exposure to a combination beta NF and dexamethasone (10(-6)M) caused a marked induction (6 times higher rates) after 48 hrs. Simultaneously an increase in P4501A1 content was observed. P4501A1-mRNA expression (measured by RT-PCR) was distinctly increased following beta NF exposure for 6 or 24 hrs. DMSO (0.2%) and dexamethasone alone modified monooxygenation rates, but did not have significant effects on P4501A1 content or, in the case of DMSO, P4501A1 gene expression (for dexamethasone not determined). Liver slices are a useful and simple tool for the detection of a beta NF-like induction within a few hours after preparation of the slices.

A comparative study of precision cut liver slices, hepatocytes, and liver microsomes from the Wistar rat using metronidazole as a model substance

1. Metronidazole is metabolized by rat liver in vitro models to form a hydroxy metabolite, an acetic acid metabolite, a glucuronic acid conjugate, and a sulphate conjugate. 2. Four different in vitro systems for investigation of drug metabolism based on liver preparations from the male Wistar rat have been investigated. 3. An incubation system where liver slices are incubated in 12-well culture plates was evaluated with respect to metabolism of metronidazole. Optimal viability was observed for a time period of up to 24 h. The Michaelis-Menten parameters for the metabolism of metronidazole in liver slices were calculated and the intrinsic clearance values compared with the values determined in hepatocytes incubated in suspension. It was found that the intrinsic clearance with respect to formation of oxidative metabolites, the hydroxy metabolite, and the acetic acid metabolite correlated, whereas the intrinsic clearance with respect to formation of the glucuronic acid conjugate was lower in slices compared with hepatocytes. 4. The metabolism of metronidazole in liver slices, in hepatocytes in primary monolayer culture, in hepatocytes incubated in suspension, and in liver microsomes was compared. All the incubations were performed under identical incubation conditions including the same incubation medium. The trend observed was that the initial metabolic rates of the production of the hydroxy metabolite, the glucuronic acid metabolite, and the acetic acid metabolite of metronidazole were higher in microsomes than in the other liver preparations. The metabolic rates in hepatocytes in primary culture and in suspension with respect to the oxidative metabolites were higher than in liver slices. The metabolic turnover observed in liver slices was predicted to correlate with in vivo data earlier obtained for rat.

Ethoxycoumarin O-deethylation (ECOD) activity in rat liver slices exposed to beta-naphthoflavone (BNF) in vitro

Precision-cut rat liver slices (0.5 mm) were incubated at 30 degrees C in William's Medium E up to 24 hrs. Our incubation conditions seem to be suitable for maintaining slice function, indicated by constant contents of tissue protein, potassium and glutathione. Thiobarbituric acid reagible substances (TBARS) released into the incubation medium did not significantly increase. Addition of DMSO (0.2 % v/v) or BNF (50 microM) to the incubation medium had no influence on most parameters described above except for increased TBARS release. If ECOD activity was determined in intact liver slices without addition of any cofactor, but substrate only, the main amount of metabolite was found in the medium, and the amount of metabolite retained within the tissue could be neglected. In slices incubated for 24 hrs, no significant changes of ECOD activity occurred for control and DMSO groups, compared with slices incubated for 2 hrs, but in the BNF group activities were more than 3.5 times as high. If ECOD activity was determined in slice homogenate, i.e. with addition of cofactors, decreased activities were measured in all groups after 24 hrs of incubation. This decrease was highest in the control group, lowest in the BNF group. We conclude that intact liver slices can be used as a simple tool to investigate in vitro enzyme induction of BNF type.