Effect of phenobarbital and other model inducers on cytochrome P450 isoenzymes in primary culture of dog hepatocytes

Cannabidiol and cannabidiolic acid: Preliminary in vitro evaluation of metabolism and drug-drug interactions involving canine cytochrome P-450, UDP-glucuronosyltransferase, and P-glycoprotein

Phytocannabinoid-rich hemp extracts containing cannabidiol (CBD) and cannabidiolic acid (CBDA) are increasingly being used to treat various disorders in dogs. The objectives of this study were to obtain preliminary information regarding the in vitro metabolism of these compounds and their capacity to inhibit canine cytochrome P450 (CYP)-mediated drug metabolism and canine P-glycoprotein-mediated transport. Pure CBD and CBDA, and hemp extracts enriched for CBD and for CBDA were evaluated. Substrate depletion assays using pooled dog liver microsomes showed CYP cofactor-dependent depletion of CBD (but not CBDA) and UDP-glucuronosytransferase cofactor-dependent depletion of CBDA (but not CBD) indicating major roles for CYP and UDP-glucuronosytransferase in the metabolism of these phytocannabinoids, respectively. Further studies using recombinant canine CYPs demonstrated substantial CBD depletion by the major hepatic P450 enzymes CYP1A2 and CYP2C21. These results were confirmed by showing increased CBD depletion by liver microsomes from dogs treated with a known CYP1A2 inducer (β-naphthoflavone) and with a known CYP2C21 inducer (phenobarbital). Cannabinoid-drug inhibition experiments showed inhibition (IC50  = 4.6-8.1 μM) of tramadol metabolism via CYP2B11-mediated N-demethylation (CBD and CBDA) and CYP2D15-mediated O-demethylation (CBDA only) by dog liver microsomes. CBD and CBDA did not inhibit CYP3A12-mediated midazolam 1'-hydroxylation (IC50  > 10 μM). CBD and CBDA were not substrates or competitive inhibitors of canine P-glycoprotein. Results for cannabinoid-enriched hemp extracts were identical to those for pure cannabinoids. These in vitro studies indicate the potential for cannabinoid-drug interactions involving certain CYPs (but not P-glycoprotein). Confirmatory in vivo studies are warranted.

Cytochrome P450 1A2 and 2C enzymes autoinduced by omeprazole in dog hepatocytes and human HepaRG and HepaSH cells are involved in omeprazole 5-hydroxylation and sulfoxidation

The induction assay for the cytochromes P450 (P450s) is an important tool in drug discovery and development. The inductions of dog P450 1A2 and 3A12 by omeprazole and rifampicin were functionally characterised in dog hepatocytes and were compared with induction in human HepaRG and HepaSH cells.P450 1A2-dependent ethoxyresorufin O-deethylation was induced by R,S-omeprazole and P450 3 A-dependent midazolam 1'-hydroxylation was induced by rifampicin, and both reactions were significantly enhanced in cultured dog hepatocytes and human HepaRG and HepaSH cells.Recombinant dog P450 1A2 exhibited activities towards R- and S-omeprazole 5-hydroxylation with low Km values of 23-28 µM, whereas dog P450 2C21 and 3A12 efficiently mediated S-omeprazole 5-hydroxylation and sulfoxidation, respectively, with high Vmax values of 12-17 min-1.Although omeprazole 5-hydroxylation by human P450 2C19 (and sulfoxidation by P450 3A4) in human HepaSH cells were slightly (∼2-fold) induced by R,S-omeprazole, dog P450 1A2 was autoinduced by omeprazole in dog hepatocytes and showed enhanced R-omeprazole 5-hydroxylation activity (∼5-fold).These results indicate that omeprazole can be an autoinducer of P450 1A2 in hepatocytes, and this enzyme was found to be involved in omeprazole 5-hydroxylation and sulfoxidation in dog hepatocytes and human HepaRG and HepaSH cells.

Induction by Phenobarbital of Phase I and II Xenobiotic-Metabolizing Enzymes in Bovine Liver: An Overall Catalytic and Immunochemical Characterization

In cattle, phenobarbital (PB) upregulates target drug-metabolizing enzyme (DME) mRNA levels. However, few data about PB's post-transcriptional effects are actually available. This work provides the first, and an almost complete, characterization of PB-dependent changes in DME catalytic activities in bovine liver using common probe substrates and confirmatory immunoblotting investigations. As expected, PB increased the total cytochrome P450 (CYP) content and the extent of metyrapone binding; moreover, an augmentation of protein amounts and related enzyme activities was observed for known PB targets such as CYP2B, 2C, and 3A, but also CYP2E1. However, contradictory results were obtained for CYP1A, while a decreased catalytic activity was observed for flavin-containing monooxygenases 1 and 3. The barbiturate had no effect on the chosen hydrolytic and conjugative DMEs. For the first time, we also measured the 26S proteasome activity, and the increase observed in PB-treated cattle would suggest this post-translational event might contribute to cattle DME regulation. Overall, this study increased the knowledge of cattle hepatic drug metabolism, and further confirmed the presence of species differences in DME expression and activity between cattle, humans, and rodents. This reinforced the need for an extensive characterization and understanding of comparative molecular mechanisms involved in expression, regulation, and function of DMEs.

Inducibility of cytochrome P450-mediated 7-methoxycoumarin-O-demethylase activity in zebrafish (Danio rerio) embryos

The zebrafish (Danio rerio) embryo has increasingly been used as an alternative model in human and environmental toxicology. Since the cytochrome P450 (CYP) system is of fundamental importance for the understanding and correct interpretation of the outcome of toxicological studies, constitutive and xenobiotic-induced 7-methoxycoumarin-O-demethylase (MCOD), i.e. 'mammalian CYP2-like', activities were monitored in vivo in zebrafish embryos via confocal laser scanning microscopy. In order to elucidate molecular mechanisms underlying the MCOD induction, dose-dependent effects of the prototypical CYP inducers β-naphthoflavone (aryl hydrocarbon receptor (AhR) agonist), rifampicin (pregnane X receptor (PXR) agonist), carbamazepine and phenobarbital (constitutive androstane receptor (CAR) agonists) were analyzed in zebrafish embryos of varying age. Starting from 36 h of age, all embryonic stages of zebrafish could be shown to have constitutive MCOD activity, albeit with spatial variation and at distinct levels. Whereas carbamazepine, phenobarbital and rifampicin had no effect on in vivo MCOD activity in 96 h old zebrafish embryos, the model aryl hydrocarbon receptor agonist β-naphthoflavone significantly induced MCOD activity in 96 h old zebrafish embryos at 46-734 nM, however, without a clear concentration-effect relationship. Induction of MCOD activity by β-naphthoflavone gradually decreased with progression of embryonic development. By in vivo characterization of constitutive and xenobiotic-induced MCOD activity patterns in 36, 60, 84 and 108 h old zebrafish embryos, this decrease could primarily be attributed to an age-related decline in the induction of MCOD activity in the cardiovascular system. Results of this study provide novel insights into the mechanism and extent, by which specific CYP activities in early life-stages of zebrafish can be influenced by exposure to xenobiotics. The study thus lends further support to the view that zebrafish embryos- at least from an age of 36 h - have an elaborate and inducible biotransformation system.

Metabolism of the Marine Phycotoxin PTX-2 and Its Effects on Hepatic Xenobiotic Metabolism: Activation of Nuclear Receptors and Modulation of the Phase I Cytochrome P450

PTX-2 is a marine biotoxin frequently found in shellfish that can lead to food intoxication in humans. Information regarding PTX-2 metabolism is scarce, and little is known of its effect on xenobiotic-metabolizing enzymes (XME) or its molecular pathways. The aim of this study was consequently to examine PTX-2 Phase I metabolism using rat and human liver S9 fractions, and also to assess the capability of PTX-2: (i) to modulate the gene expression of a panel of Phase I (CYP450) and II (UGT, SULT, NAT, and GST) enzymes, as well as the Phase III or 0 (ABC and SLCO) transporters in the human hepatic HepaRG cell line using qPCR; (ii) to induce specific CYP450 in HepaRG cells measured by immunolabeling detection and the measurement of the cells’ activities; and (iii) to activate nuclear receptors and induce CYP promoter activities in HEK-T and HepG2 transfected cell lines using transactivation and reporter gene assay, respectively. Our results indicate that PTX-2 hydroxylation occurred with both rat and human S9 fractions. Whereas PTX-2 mostly upregulated the gene expression of CYP1A1 and 1A2, no induction of these two CYP activities was observed. Lastly, PTX-2 did not act as an agonist of CAR or PXR. Due to its effects on some key XME, more attention should be paid to possible drug–drug interactions with phycotoxins, especially as shellfish can accumulate several phycotoxins as well as other kinds of contaminants.

Possible drug-drug interaction in dogs and cats resulted from alteration in drug metabolism: A mini review

Pharmacokinetic drug-drug interactions (in particular at metabolism) may result in fatal adverse effects in some cases. This basic information, therefore, is needed for drug therapy even in veterinary medicine, as multidrug therapy is not rare in canines and felines. The aim of this review was focused on possible drug-drug interactions in dogs and cats. The interaction includes enzyme induction by phenobarbital, enzyme inhibition by ketoconazole and fluoroquinolones, and down-regulation of enzymes by dexamethasone. A final conclusion based upon the available literatures and author's experience is given at the end of the review.

Addisonian crisis in a dog treated with phenobarbitone

BACKGROUND

A 2-year-old intact female Irish Setter was presented with a 1-week history of anorexia, lethargy, vomiting and diarrhoea. Previous medical therapy included a 3-week treatment with phenobarbitone for suspected idiopathic epilepsy. In humans, phenobarbitone accelerates metabolism of both exogenous and endogenous steroids.

CASE REPORT

Based on history, the physical examination showing abnormal mentation and laboratory abnormalities including azotaemia, hyponatraemia and hyperkalaemia, Addisonian crisis was suspected. An adrenocorticotropic hormone stimulation test was performed and confirmed the diagnosis. Treatment with intravenous fluid therapy, glucocorticoids and mineralocorticoids led to a resolution of clinical signs in 3 days.

CONCLUSION

To the authors' current knowledge, this is the first reported case of Addisonian crisis in a dog most probably related to phenobarbitone administration. As Addisonian crisis can be life-threatening, clinicians should be aware of this adverse effect of phenobarbitone and use it cautiously in dogs with borderline hypoadrenocorticism.

Substrate specificity for carnitine and glycine conjugation of branched side-chain and cyclic side-chain carboxylic acids in various experimental animals

Substrate specificities for the carnitine and glycine conjugates of branched side-chain and cyclic side-chain carboxylic acids were examined using dog, rabbit, cynomolgus monkey, and squirrel monkey hepatocytes and kidney slices. For all tested samples, the substrate specificity for carnitine or glycine conjugation showed a similar tendency to those for rat experiments reported previously, that is, the best substrate for the carnitine conjugate was cyclopropanecarboxylic acid (CPCA), while that for the glycine conjugate was benzoic acid (BA), followed by cyclohexanecarboxylic acid (CHCA). With respect to carnitine conjugation for CPCA, rat hepatocytes showed the highest ability followed by dog, rabbit, and monkey hepatocytes. Rat kidney slices showed the highest carnitine conjugation ability, followed by rabbit, dog, and monkey kidney slices, in that order. With respect to glycine conjugation for BA, rabbit hepatocytes showed the highest ability, followed by rat and monkey hepatocytes. Dog hepatocytes have no or little glycine conjugate ability for the carboxylic acids studied here. Rabbit kidney slices showed the highest glycine conjugation ability followed by rat, dog, and monkey kidney slices.

Induction of CYP1A in the beagle dog by an inhibitor of kinase insert domain-containing receptor: differential effects in vitro and in vivo on mRNA and functional activity

Compound I [3-[5-(4-methanesulfonyl-piperazin-1-ylmethyl)-1H-indol-2-yl]-1H-quinolin-2-one] is a potent inhibitor of human kinase insert domain-containing receptor (KDR kinase), which is under investigation for the treatment of cancer. Bile duct-cannulated male beagle dogs were administered 6 mg/kg compound I q.d. for 14 days. There was an approximately 2.5-fold decrease in the mean plasma area under the curve of I on days 7 and 14 (approximately 11.3 microM . h), relative to day 1 (28.2 microM . h). In the dog, compound I was eliminated by metabolism, with a major pathway being aromatic hydroxylation and subsequent sulfation to form the metabolite M3. Metabolic profiling suggested that the pathway leading to the formation of the sulfated conjugate M3 was induced upon multiple dosing of I. Studies conducted in vitro suggested that CYP1A1/2 was responsible for the formation of the hydroxylated metabolite, which is sulfated to yield M3. Additional studies confirmed induction of CYP1A protein and activity in the livers of dogs treated with I. However, studies in a dog hepatocyte model of induction showed a surprising decrease both in CYP1A mRNA and enzymatic activity in the presence of I, emphasizing the need to consider the results from a variety of in vitro and in vivo studies in deriving an understanding of the metabolic fate of a drug candidate. It is concluded that the autoinduction observed after multiple treatments with compound I occurs since compound I is both an inducer and a substrate for dog CYP1A.

A Microtiterplate-Based Screening Assay to Assess Diverse Effects on Cytochrome P450 Enzyme Activities in Primary Rat Hepatocytes by Various Compounds

During the development of potential drugs it is useful to identify pharmacological and/or toxicological side effects of a compound as early as possible in order to exclude them from further development for reasons of time and cost. Activation or inactivation of members of the cytochrome P450-dependent monooxygenase system (CYP450) might indicate potential undesired effects of a given compound. However, results using CYP450 assay systems are often inconsistent because of different experimental settings. Therefore, it was the goal of the present study to optimize the CYP450 assay in primary rat hepatocytes with respect to the time point of addition of and duration of exposure to alpha-naphthoflavone (ANF) and beta-naphthoflavone (BNF) as well as trans-resveratrol (RES), which have well-described stimulatory and inhibitory effects on CYP450 enzymes of the 1A and 2B family, respectively. Hepatocytes were also treated with putative lipoxygenase (LOX)/cyclooxygenase (COX) inhibitors with unknown impact on CYP450 enzyme activity in order to detect potential side effects. Cells were cultured for up to 7 days on 96-well microtiter plates, and enzyme activity was determined by a conventional fluorescence spectroscopy assay. ANF and BNF, given to the cells after 4 days of culture, stimulated CYP1A and 2B activities significantly in a concentration-dependent fashion after long-term exposure for at least 1 day. However, during short-term exposure for 1-6 h, CYP1A activity was inhibited, while CYP2B was increased weakly by ANF but not BNF. RES inhibited CYP1A activity during short- and long-term exposure without affecting CYP2B activity. From the results it was concluded that primary rat hepatocytes should be cultured for at least 3-4 days but no longer prior to the assay. The assay should be performed at two different time points of exposure, i.e., 6 h for short-term and 24 h for long-term exposure. The compounds under investigation should be applied at two different concentrations, e.g., at one time and 10 times higher concentrations, which should be oriented to the ED50, provided it is known for the respective substance. Under these assay conditions the LOX/COX inhibitors tested activated CYP1A enzyme activity in long-term but instead inhibited it in short-term experiments. CYP2B activity was stimulated during short- and long-term exposure. These results indicated drug side effects recommending exclusion of the compounds from the drug developmental process. Hence, in order to assess the pharmacological potential of novel compounds it is adequate to perform both short- and long-term experiments to concisely describe the effect of a compound on the CYP450 system.

Substrate specificity and kinetic properties of seven heterologously expressed dog cytochromes p450

Seven dog cytochromes p450 (p450s) were heterologously expressed in baculovirus-Sf21 insect cells. Of all enzymes examined, CYP1A1 exhibited high 7-ethoxyresorufin O-deethylase activity (low Km enzyme, 1 microM). CYP2B11 and CYP3A12 effectively catalyzed the N1-demethylation and C3-hydroxylation of diazepam (and its derivatives), whereas CYP3A12 and CYP2D15 catalyzed exclusively the N- and O-demethylation, respectively, of dextromethorphan. However, no saturation velocity curves for the N-demethylation of dextromethorphan (up to 500 microM) were achieved, suggesting a high Km for CYP3A12. In contrast to CYP3A12, the CYP2D15-dependent O-demethylation of dextromethorphan was a low Km process (Km = 0.7 microM), similar to that in dog liver microsomes (Km = 2.3 microM). CYP2D15 was also capable of metabolizing bufuralol (1'-hydroxylation), with a Km of 3.9 microM, consistent with that obtained with dog liver microsomes. CYP3A12 was shown to primarily oxidize testosterone at 16alpha-, 2alpha/2beta-, and 6beta-positions. Selectivity of CYP3A12 was observed toward testosterone 6beta-(Km = 83 microM) and 2alpha/2beta-hydroxylations (Km = 154 microM). However, the 16alpha-hydroxylation of testosterone was catalyzed by CYP2C21 also (Km = 6.4 microM for CYP2C21). Therefore, the 6beta- and 16alpha-hydroxylation of testosterone can potentially be employed as markers of CYP3A12 and CYP2C21 (at low concentration), respectively. CYP2C21 was also capable of catalyzing diclofenac 4'-hydroxylation, although some activity was detected with CYP2B11. Surprisingly, none of the p450s selectively metabolized (S)-mephenytoin 4'-hydroxylation. The results described herein are a first step toward the systematic evaluation of a panel of dog p450s and the development of dog p450 isoenzyme-selective marker substrates, as well as providing useful information on prediction and extrapolation of the results from in vitro to in vivo and from dog to human.

Issues related to the use of canines in toxicologic pathology--issues with pharmacokinetics and metabolism

The dog is a commonly used animal model by virtue of its size, well-characterized physiology, and ease of handling. For these reasons and others, dogs are also useful in pharmacokinetic and metabolism studies during the development of both human and veterinary pharmaceutical products. In comparison with humans, or with other animals, dogs have some unique physiologic attributes that can affect the disposition of drugs. Species differences in gastrointestinal physiology, metabolism, renal function, and protein binding can affect the correlation of the pharmacokinetics and toxicology of dogs with those of other species. With the use of relevant examples, this article will provide an introduction to characteristics of dog physiology and their impact on pharmacokinetics, metabolism, drug disposition, toxicity, and dose selection.

In vivo and in vitro induction of cytochrome P450 enzymes in beagle dogs

The aim of this study was to determine the in vitro and in vivo effects of several prototypical inducers, namely beta-naphthoflavone, 3-methylcholanthrene, phenobarbital, isoniazid, rifampin, and clofibric acid, on the expression of cytochrome P450 (P450) enzymes in beagle dogs. For the in vitro induction study, primary cultures of dog hepatocytes were treated with enzyme inducers for 3 days, after which microsomes were prepared and analyzed for P450 activities. For the in vivo induction study, male and female beagle dogs were treated with enzyme inducers for 4 days (with the exception of phenobarbital, which was given for 14 days), after which the livers were removed and microsomal P450 activities were determined ex vivo. Treatment of male beagle dog hepatocyte cultures (n = 3) with beta-naphthoflavone or 3-methlychloranthrene resulted in up to a 75-fold increase in microsomal 7-ethoxyresorufin O-dealkylase (CYP1A1/2) activity, whereas in vivo treatment of male and female beagle dogs with beta-naphthoflavone followed by ex vivo analysis resulted in up to a 24-fold increase. Phenobarbital caused a 13-fold increase in 7-benzyloxyresorufin O-dealkylase (CYP2B11) activity in vitro and up to a 9.9-fold increase in vivo. Isoniazid had little or no effect on 4-nitrophenol hydroxylase activity in vitro. Rifampin caused a 13-fold induction of testosterone 6beta-hydroxylase (CYP3A12) activity in vitro and up to a 4.5-fold increase in vivo. Treatment of dogs in vivo or dog hepatocytes in vitro with clofibric acid appeared to have no effect on CYP4A activity as determined by the 12-hydroxylation of lauric acid. In general, the absolute rates (picomoles per minute per milligram of microsomal protein) of P450 reactions catalyzed by microsomes from cultured hepatocytes (i.e., in vitro rates) were considerably lower than those catalyzed by microsomes from dog liver (i.e., ex vivo rates). These results suggest that beagle dogs have CYP1A, CYP2B, CYP2E, and CYP3A enzymes and that the induction profile resembles the profile observed in humans more than in rats.

A novel model of acetaminophen-induced acute hepatic failure in rabbits

BACKGROUND

Few reliable and reproducible animal models of acute hepatic failure exist or conform to the criteria proposed by Terblanche and Hickman (Dig. Dis. Sci. 36: 770, 1991). In this prospective randomized study we describe the selective induction of CYP450 enzymes, depletion of glutathione, and hepatotoxic insult using acetaminophen in the development and characterization of a novel rabbit model of acute hepatic failure.

MATERIALS AND METHODS

Male New Zealand white rabbits weighing 3-5 kg were used. After preliminary dose ranging experiments, two groups of New Zealand white (n = 8 in each group) rabbits had CYP450 induction with phenobarbitone (40 mg/kg ip for 5 days) or with 20-methylcholanthrene (80 mg/kg ip). The glutathione synthetase inhibitor buthionine sulfoxime (2 mmol/kg iv) was then administered prior to acetaminophen administration (500 mg/kg sc). Clinical observations were recorded and arterial blood was sampled over 72 h.

RESULTS

Grade I-III encephalopathy occurred at 5-12, 12-25, and 28-56 h, respectively, in animals pretreated with 20-methylcholanthrene, but not in the phenobarbitone pretreated group. Mortality was 75% in the 20-methylcholanthrene group compared to 0% in the phenobarbitone group. Blood lactate (P < 0.05), prothrombin time (P < 0.005), aspartate transaminase (P < 0.005), and creatinine (P < 0.05) were higher in the 20-methylcholanthrene group compared to the phenobarbitone group. Histological changes were marked in the 20-methylcholanthrene group with massive coagulative hepatic necrosis compared to minimal histological damage in the phenobarbitone group.

CONCLUSION

The induction with 20-methylcholanthrene, glutathione depletion with buthionine sulfoxime, and subcutaneous administration of acetaminophen have led to the development of an animal model that parallels clinical, biochemical, and histological features of human hepatic failure.

Time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes

In the present study, time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes, a species of economic importance in Mediterranean countries, were investigated. Cross-bred rabbits were anesthetised and their livers perfused in situ by a two-step collagenase technique; cells suspensions were filtered, seeded in collagen-coated dishes and cultivated at 37 degrees C in a controlled atmosphere for 24 and 72 h. Cytochrome P450 and b(5) contents as well as the catalytic activity of some P450-dependent monooxygenases were measured in subcellular fractions obtained by differential ultracentrifugation; microsomal proteins were also subjected to immunoblotting, using antibodies to rat P4501A, 2B, 2E1 and 3A isoforms. The activity of some microsomal hydrolytic enzymes was also determined. As regards conjugative enzymes, glutathione content and activities of glutathione S-transferase, uridindiphosphoglucuronosyl-transferase, acetyl-transferase and 1,2-epoxibuthane glutathione transferase were assayed. An overall reduction of the catalytic activity was observed 72 h after plating, reaching in certain instances the level of statistical significance. On the whole, our data confirm those previously reported with hepatocytes obtained from other species; however, the evidence that DMEs were still measurable after 72 h supports the usefulness of this in vitro method for drug metabolism studies in the rabbit as well.

Safety Evaluation and Drug Development based on Biological Fate of Drugs —Efforts Made to Overcome Drug Interaction in Drug Development—

1. Assay methods to detect drug interaction in toxicological samples were established by determining cytochrome P450 content and its activity in liver samples. The O-dealkylation reaction of 7-alkoxycoumarin was indicated to reflect changes in the molecular forms of P450s, and the enzyme induction or inhibition in the toxicological samples was easily detected by using the established methods. 2. During toxicological studies of 450191-S or the sleep inducer rilmazafone, a phenobarbital type-induction of hepatic drug metabolizing enzymes was observed in animals, and the doses required for the induction differed markedly between rats and dogs. Enzyme induction was caused by some specific metabolites of 450191-S, and the plasma concentrations of these metabolites were comparable when the enzyme induction was developed in both animals. 3. A nonsteroidal antiinflammatory compound 480156-S showed a slight or no effect on microsomal drug metabolizing activity in rats. On the other hand, repeated administration of this compound to humans resulted in a marked decrease in the oxidative metabolism of 480156-S, followed by a marked increase in the plasma concentrations of the compound. When volunteers were given 480156-S followed by several drugs, such as tolubutamide, the plasma clearance was delayed remarkably, indicating a severe drug interaction. 4. Cytochrome P450 belonging to the CYP2C family was indicated to participate in the oxidative metabolism of 480156-S in both rat and human liver microsomes. The preincubation of microsomes with 480156-S caused a concentration-dependent inhibition of CYP2C-dependent tolubutamide hydroxylation reaction in both rats and humans. There was a marked species difference in the susceptibility to the inhibitory effect of 480156-S, and the concentration required to inhibit rat CYP2C was almost 10 times higher than that required in humans. 5. The cephem antibiotics having N-methyltetrazolethiol (NMTT) at the 3'-position substituent were demonstrated to inhibit mitochondrial low K(m) aldehyde dehydrogenase (ALDH), and produced disulfiram-like (Antabuse) reaction during alcohol metabolism. Pharmacokinetic studies indicated that NMTT released from the antibiotics in bile duct or intestine cause the inhibitory action followed by the development of disulfiram-like reaction. 6. Attempts had been made to develop new cephem antibiotics lacking the disulfiram-like reaction by changing the chemical structure of 3'-position substituents, and a hydroxyethyltetrazolethiol was found not to inhibit the enzyme. Based on this result, together with the antibacterial activity, we have developed a new oxacephem antibiotic flomoxef (6315-S). Flomoxef showed no disulfiram-like reaction both in rats and human.

Use of human tissue in ADME and safety profiling of development candidates

The clinical success of a compound is often curtailed because of inadequate safety, pharmacokinetics or efficacy. Human tissue can be used to identify the potential shortcomings of new drugs before they undergo testing in man. This review highlights the consent and ethical approval required for the use of human tissues and discusses their use for predicting human ADME and safety profiles of drugs in preclinical development. The ability to retrieve a wide range of viable tissues from human donors provides the opportunity to test drugs for many potential use-limiting side-effects.

Predictive value of in vitro model systems in toxicology

The application of in vitro model systems to evaluate the toxicity of xenobiotics has significantly enhanced our understanding of drug- and chemical-induced target toxicity. From a scientific perspective, there are several reasons for the popularity of in vitro model systems. From the public perspective, in vitro model systems enjoy increasing popularity because their application may allow a reduction in the number of live animals employed in toxicity testing. In this review, we present an overview of the use of in vitro model systems to investigate target organ toxicity of drugs and chemicals, and provide selective examples of these model systems to better understand cutaneous and ocular toxicity and the role of drug metabolism in the hepatotoxicity of selected agents. We conclude by examining the value and use of in vitro model systems in industrial development of new pharmaceutical agents.

Comparison of the stability of some major cytochrome P450 and conjugation reactions in rat, dog and human hepatocyte monolayers

The stability of four major cytochrome P450 isoenzymes (CYPIA, CYP2B, CYP2E1 and CYP3A) and of two phase II conjugation enzymes (glucuronyl- and sulfotransferases) was investigated in primary cultures of rat, dog and human hepatocytes in the same conditions. 7-ethoxyresorufin deethylation (EROD), 7-methoxycoumarin demethylation (MCOD), chlorzoxazone (CLOX) 6-hydroxylation, 1'- and 4-hydroxylation of midazolam (MDZ), and p-nitrophenol glucuronidation and sulfation, were used respectively. The EROD activity was stable over 72 hours in rat and dog and only 48 hours in human hepatocytes. The MCOD activity was also stable in rat but decreased in dog by 30% within 72 hours The CLOX hydroxylase activity was most stable in human whereas in rat and dog it fell down to 30% within 72 and 24 hours, respectively. The MDZ hydroxylase activity showed the same unstability profile in the three species investigated. Both conjugation reactions were either stable or showed an increase by up to 60-70% in all three species over 72 hours. The enzymes tested showed different stabilities in rat, dog and human hepatocytes over 72 hours, thus demonstrating the limitations of hepatocyte monolayers as models for metabolic investigations and emphasising the need for validation/characterization studies before routine use.

Rapid determination of rat hepatocyte mRNA induction potential using oligonucleotide probes for CYP1A1, 1A2, 3A and 4A1

1. A new assay to quantify mRNA levels in small numbers of rat hepatocytes has been developed for cytochrome P450 (CYP) isoforms 1A1, 1A2, 3A and 4A1. The assay uses sets of oligonucleotide probes end-labelled with [35S]-dATP to hybridize to mRNA in control- or drug-treated rat hepatocytes cultured on Cytostar-T 96-well scintillating microplates. 2. The rat hepatocyte induction potential (RHIP) assays for CYP3A, 1A1, 1A2 and 4A1 are sensitive and selective and have an excellent qualitative relationship with CYP induction data ex vivo. The robustness of the CYP3A assay was determined following a run of > 40 plates. The variation of the dexamethasone (DEX) response on each plate, calculated as %coefficient of variation, showed that there was no significant difference between the variability of the response to DEX. 3. Assay specificity for each CYP isoform was achieved by designing probes (four per isoform) antisense to coding regions of each CYP gene sequence. In the CYP3A RHIP assay, pregnenalone 16alpha-carbonitrile (PCN), DEX, clotrimazole (CLOT) and miconazole (MIC) were all good inducers of CYP3A mRNA; beta-napthoflavone (BNF) and methylclofenapate (MCP), however, did not induce CYP3A mRNA, further defining the specificity of this methodology. Specificity was similarly confirmed for the other CYP isoforms. 4. Ind50, the concentration of inducer required to elicit a 50% induction of CYP-specific mRNA, was derived for prototypical CYP inducers: BNF 0.54 and 0.17 microM (CYP1A1 and 1A2 respectively), 3-methylcholanthrene (3MC) 0.11 and 0.04 microM (CYP1A1 and 1A2 respectively), PCN 0.03 microM, DEX 0.17 microM, CLOT 0.48 microM, MIC 3 microM, TAO 3 microM (CYP3A), MCP 1.8 microM, clofibrate (CLOF) 65 microM and ciprofibrate (CIP) 1.9 microM (CYP4A1). Ind50 for BNF and 3MC at CYP1A2 was 3-fold lower than that at CYP1A1 indicating a subfamily difference in inducer potency. 5. Reducing the numbers of animals and the amount of compound required to study CYP induction is an important advantage of the RHIP assays over conventional evaluations in vivo. Typically four rats are dosed for 4 days using oral doses in the range 50-500 mg kg(-1) day(-1). In comparison, the amount of hepatocytes required to carry out all the studies reported herein may be obtained from a single animal (< 2 x 10(8) viable cells) and CYP induction investigated using microg rather than g quantities of drug substance. 6. With appropriately designed oligonucleotide probes, the RHIP technology can assess CYP induction in human hepatocytes, which together with preclinical data can contribute to improving the quality of compounds progressing into the expensive process of drug development.

Characterization of cytochrome P450 (CYP3A12) induction by rifampicin in dog liver

1. Effects of rifampicin (Rif) on the contents of cytochrome P450 (P450) enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) assessed by enzyme-linked immunosorbent assay and catalytic activities (ethoxyresorufin O-deethylase, and testosterone 6 beta-, 16 alpha- and 16 beta-hydroxylase; 6 beta-, 16 alpha- and 16 beta-OHT) in dog liver microsomes were compared between liver lobes of both the male and female dogs. 2. In the control dogs, the contents of individual P450 enzymes and their activities showed no significant differences between individual liver lobes and between the sexes. 3. Rif treatment (10 mg/kg/day, p.o. for 7 days) induced substantial increases in the content of CYP3A12 and 6 beta- and 16 beta-OHT activities, and slight increases in the content of CYP2B11 and 16 alpha-OHT activity, and their elevated levels were virtually the same between liver lobes. The magnitudes of the elevation of the CYP3A12 level and 6 beta- and 16 beta-OHT activities compared with control levels appeared to be greater in the female dogs. However, the ratios of their magnitudes (CYP3A content/6 beta-OHT activity and CYP3A content/16 beta-OHT activity) showed no differences between the sexes. 4. In both the control and Rif-treated dogs, the activities of 6 beta- and 16 beta-OHT were specifically inhibited by anti-CYP3A12 antiserum, and 16 alpha-OHT activity was specifically inhibited by anti-CYP2B11 and anti-CYP2C21 antiserum. 5. These results indicate that Rif treatment induces the expression of CYP3A12 protein, and correlates well with the elevation of its catalytic activity (6 beta- and 16 beta-OHT), and that the female dog is more responsive to Rif treatment as compared with the male.

Quantitative reverse transcriptase/PCR assay for the measurement of induction in cultured hepatocytes

Hepatic enzyme induction is often evoked as the cause for a variety of effects in animal studies, e.g. hepatic hypertrophy and secondary thyroid neoplasms in rodents. In clinical practice, enzyme induction often enhances drug clearance and may result in reduced efficacy. For example, carbamazepine or rifampin treatment induces P450 3A in humans, and as a result, dramatically reduces the efficacy of midazolam or cyclosporine. Due to species differences in substrate specificity and the regulation of various drug-metabolizing enzymes, assessing enzyme induction in human tissues is a desirable goal. Since induction often occurs as a result of increased synthesis of mRNA coding for a particular enzyme, induction may be quantified by measuring specific mRNA levels. We describe an approach to quantifying mRNA levels using reverse transcriptase-polymerase chain reaction (RT-PCR). This approach makes use of either radiolabeled PCR primers or fluorimetric quantification of product and does not require the synthesis of a competitor RNA or DNA molecule. Thus, Cyp2B1/2 mRNA can be shown to be induced 17-fold in the H4IIE rat hepatoma cell line. Likewise, Cyp3A and Cyp2A6 mRNAs can be shown to be induced in primary human hepatocytes cultured on collagen-coated plates and treated with rifampin for 72 h. By contrast, mRNA levels for Cyp1A1 and Cyp2E1 were not increased by rifampin treatment. This report demonstrates the potential of this approach for examining a number of mRNAs from drug-treated cultured cells, as a means of assessing metabolic enzyme induction.

Increase of cytochrome P-450 1A and glutathione transferase transcripts in cultured hepatocytes from dogs, monkeys, and humans after cryopreservation

The study was designed to investigate the effects of phenobarbital (PB), 3-methylcholanthrene (3-MC), and oltipraz (OPZ), a synthetic derivative of 1,2-dithiole-3-thione, on the levels of cytochrome P450 1A1/2 and gluthathione transferase (GST) mRNAs in both fresh and cryopreserved human, monkey, and dog hepatocytes in primary culture. GST alpha mRNAs were demonstrated in liver parenchymal cells from the three species: after 4 days of culture, their basal levels were decreased, but were strongly higher in PB- and OPZ-treated cells from the three species. In contrast 3-MC was mostly effective on human hepatocytes. The increased levels of GST alpha mRNAs in the presence of PB or OPZ were not observed in all cell populations. GST mu mRNAs, which were detected in both dog and monkey hepatocytes, were induced only in the presence of OPZ. GST pi mRNAs were expressed in dog hepatocytes but did not respond to any of the inducers. In all cases, similar effects were observed in fresh and thawed hepatocytes. Similarly, CYP1A1/2 transcripts were induced by 3-MC in both fresh and cryopreserved cells from the three species but also after OPZ treatment for monkey hepatocytes. These findings demonstrate that enzymes which play a major role in bioactivation/detoxication of xenobiotics remain expressed and inducible in hepatocytes from various species after cryopreservation and thawing.

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.

Quantitation of cytochrome P450 enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) in dog liver microsomes by enzyme-linked immunosorbent assay

1. An enzyme-linked immunosorbent assay (ELISA) using specific antisera has been developed to quantify individual cytochrome P450 (P450) enzymes (1A1/2, 2B11, 2C21 and 3A12) in dog liver microsomes. 2. The specific contents of CYP1A1/2, 2B11, 2C21 and 3A12 in untreated male dog liver microsomes determined by the ELISA were 17, 48, 160 and 69 pmol/mg protein respectively, corresponding to 4, 10, 34 and 15% of total optically determined P450 respectively. These P450 enzymes in untreated female dog liver microsomes showed almost similar amounts and relative proportions to those observed in male dog liver microsomes. 3. The oral treatment of male dogs with phenobarbital (PB), rifampicin (Rif) or beta-naphthoflavone (beta-NF) induced significant increases in the contents of CYP1A1/2 (12-fold by beta-NF), 2B11 (16-fold by PB), 2C21 (2-fold by PB) and 3A12 (5-fold by PB and Rif), resulting in marked proportional alterations of the P450 enzymes in dog liver microsomes. 4. This ELISA method will be a useful tool for investigating possible influences (induct on/suppression) of xenobiotics on the expression of P450 enzymes in dog liver.

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.

Induction of cytochrome P-450 isoenzymes in cultured monkey hepatocytes

The effect of phenobarbital (PB), beta-naphthoflavone (beta-NF) and rifampicin (Rif) on the drug-metabolizing activity of cultured squirrel monkey hepatocytes was examined. The drug metabolizing activity (e.g. alkoxycoumarin dealkylase or steroid hydroxylase) gradually decreased during the culture period with 40-70% activity remaining at 72 hr. When 0.5 mM PB was added to the culture, the activities of 7-methoxycoumarin O-demethylase (MCOD) and 7-ethoxycoumarin O-deethylase (ECOD) increased to 6-7 fold higher level than those of control at 72 hr. Testosterone 6 beta-hydroxylase (6 beta-OH-T) and testosterone 16 beta-hydroxylase (16 beta-OH-T) activities were approx. 3-fold higher than those of the control. Addition of beta-NF significantly increased the activities of 7-ethoxyresorufin O-deethylase (EROD) and ECOD. Though statistically insignificant, Rif slightly increased 6 beta-OH-T activity. Western blot analysis indicated PB induced production of the CYP 2B and 3A subfamilies, while beta-NF and Rif induced that of the CYP 1A and the CYP 3A subfamily, respectively.