Expression of cytochrome P-450 enzymes in cultured human hepatocytes

Clinically important pharmacokinetic drug-drug interactions: role of cytochrome P450 enzymes

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues and many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In the future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Metabolic capacities in cultured human hepatocytes obtained by a new isolating procedure from non-wedge small liver biopsies

A new isolating procedure of human hepatocytes has been developed using two-step collagenase digestion by a non-perfusion procedure (NP) of non-wedge liver biopsies. 1. A yield of 2-7 x 10(6) hepatocytes/g liver, 52-95% viability and 13-75% attachment were obtained from liver biopsies weighing 6-60 g, comparable to that obtained when using the classical perfusion procedure (P) to isolate human hepatocytes from wedge liver samples of 50-150 g. 2. In culture, human hepatocytes obtained by NP remained attached to plastic for up to 5 days and displayed the usual morphological characteristics. Their metabolic capacities, assessed by liver-specific albumin and urea synthesis and by CYP-dependent and conjugation pathways, were equivalent to those of human hepatocytes obtained by P. In addition, they responded adequately to specific CYP inducers, demonstrating that they constitute a model in which human drug metabolism and toxicity studies can be performed.

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.

In vitro and in vivo drug interactions involving human CYP3A

Cytochrome P4503A (CYP3A) is importantly involved in the metabolism of many chemically diverse drugs administered to humans. Moreover, its localization in high amounts both in the small intestinal epithelium and liver makes it a major contributor to presystemic elimination following oral drug administration. Drug interactions involving enzyme inhibition or induction are common following the coadministration of two or more CYP3A substrates. Studies using in vitro preparations are useful in identifying such potential interactions and possibly permitting extrapolation of in vitro findings to the likely in vivo situation. Even if accurate quantitative predictions cannot be made, several classes of drugs can be expected to result in a drug interaction based on clinical experience. In many instances, the extent of such drug interactions is sufficiently pronounced to contraindicate the therapeutic use of the involved drugs.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

Prediction of in vivo drug interaction from in vitro systems exemplified by interaction between verapamil and cimetidine using human liver microsomes and primary hepatocytes

Emphasis on drug safety is increasing as newly developed drugs become more potent. Interest in the prediction and description of drug interactions is growing accordingly. The study of potential interactions at a very early stage of drug development requires suitable in vitro models that describe drug interactions both qualitatively and quantitatively. The purpose of the work described here was to help assess the predictive value of in vitro drug interaction tests with liver microsomes and hepatocytes by means of the interaction between verapamil and cimetidine. The in vitro inhibition of verapamil metabolism by cimetidine observed during the studies was quantitatively similar to the results reported in published clinical studies after intravenous application. Studies using liver microsome fractions showed that the intrinsic clearances for the formation of various metabolites could be used to predict drug interactions. In addition, work with hepatocyte cultures revealed that an in vitro system covering both phase I and phase II reactions should be included in such studies to permit quantitative prediction of the various metabolic pathways. Both human hepatocyte cultures and human microsomes offer certain advantages for predicting the degree of drug metabolism and interactions in humans at the biotransformation level. Therefore, it seems likely that the simultaneous application of both systems will yield conclusions that most closely approximate the situation in humans.

Long-term primary cultures of adult human hepatocytes

In this work we have investigated a system of long-term primary cultures of adult human hepatocytes which, in contrast to those previously described, has the advantage of requiring neither the use of additive cells as in co-cultures, nor of matrix component preparations like Matrigel or collagen sandwich. This system has been used previously for long-term cultures of hepatocytes from young baboon, and some modifications have been introduced here to take into account the specificity of adult human hepatocytes. In this system, hepatocytes are plated at confluence on collagen-coated dishes and cultured in a serum-free medium consisting of Williams'E supplemented with hormones and growth factors. Proteins secreted specifically by the liver, including albumin, alpha-1 antitrypsin, plasminogen, fibrinogen, lipoproteins ApoA1 and ApoB100, have been quantified in the extracellular medium as a function of time, either by immunoblot or ELISA. In addition, the expression and inducibility of CYP proteins of the CYP1, CYP2 and CYP3 families in response to their prototypical inducers including 2,3,7,8-tetrachlorodibenzo(p)dioxin and rifampicin, have been evaluated by immunoblot analysis of microsomes or cell lysates. Moreover, the oxidative metabolism of cyclosporin A, a monoxygenase activity depending on CYP3A4, has been monitored directly on the cultured cells by HPLC analysis of extracellular medium. Our results show that, under these culture conditions, adult human hepatocytes retain these phenotypical characteristics for at least 35 days. This system meets the requirements for use as a model for screening CYP protein inducers.

Preclinical evaluation of drug-drug interaction potential: present status of the application of primary human hepatocytes in the evaluation of cytochrome P450 induction

Cytochrome P450 (CYP) inhibition and induction are the key mechanisms in drug-drug interactions. Aside from clinical studies, primary human hepatocytes may represent the most appropriate experimental system for the evaluation of CYP induction in humans. A consensus of an international panel on the present status and future research directions in the application of primary human hepatocytes in the evaluation of CYP-induction is presented here. The following observations are concluded to be generally true: (1) Human hepatocytes isolated from both biopsy samples and transplantable livers are suitable for induction studies. (2) Hormonally-defined media can be used for the evaluation of CYP induction. (3) Isozyme-selective induction of CYP1A and 3A by known inducers are observed. (4) Reproducibility of induction could be improved by using hepatocytes plated as confluent cultures. (5) Induction could be observed for hepatocytes treated at 1-3 days after culturing. (6) Treatment duration of 2 days in general leads to near maximal induction. (7) In general, there is a good qualitative correlation between human hepatocyte results in vitro and clinical observations in vivo. (8) When the same inducers were evaluated in independent laboratories, similar data were generally observed. We conclude that primary human hepatocytes represent an appropriate model for mechanistic evaluation of CYP induction and as a screening tool for CYP induction potential of xenobiotics. A set of data acceptance criteria are proposed: (1) Positive response should be observed with concurrent positive control chemicals; (2) reproducible observation should be observed with multiple human donors; (3) for negative response, the doses used should not be cytotoxic; and (4) replicate treatment and/or multiple dose treatment should be performed to allow statistical analysis. Future studies should include the further development of on: (1) The inducibility of CYP isozymes other than CYP1A and 3A, and phase II enzymes; (2) further development of culturing condition to allow optimal gene expression; (3) evaluation of the involvement of nonparenchymal cells on CYP induction of parenchymal cells; (4) the and validation of quantitative approaches to extrapolate in vitro data to in vivo data; (5) evaluation of possible individual variations and potential genetic polymorphism in inducibility; (6) further definition of species differences in CYP induction; (7) development of a 'normal' human hepatocyte cell line for CYP induction studies; (8) improvement of cryopreservation procedure of human hepatocytes; (9) definition of the molecular mechanisms of CYP induction; and (10) evaluation of the induction of phase II metabolic pathways.

Role of cytochrome P450 enzymes in drug-drug interactions

Many adverse drug-drug interactions are attributable to pharmacokinetic problems and can be understood in terms of alterations of P450-catalyzed reactions. Much is now known about the human P450 enzymes and what they do, and it has been possible to apply this information to issues related to practical problems. A relatively small subset of the total number of human P450s appears to be responsible for a large fraction of the oxidation of drugs. The three major reasons for drug-drug interactions involving the P450s are induction, inhibition, and possibly stimulation, with inhibition appearing to be the most important in terms of known clinical problems. With the available knowledge of human P450s and reagents, it is possible to do in vitro experiments with drugs and make useful predictions. The results can be tested in vivo, again using assays based on our knowledge of human P450s. This approach has the capability of not only improving predictions about which drugs might show serious interaction problems, but also decreasing the number of in vivo interaction studies that must be performed. These approaches should improve with further refinement and technical advances.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Time-dependent expression of cytochrome P450 genes in primary cultures of well-differentiated human hepatocytes

We sought to establish an in vitro system to study the regulation of highly differentiated hepatocellular functions, and specifically the time-dependent expression of four cytochrome P450 (P450) genes at the messenger RNA (mRNA) and protein levels. When seeded onto matrigel, hepatocytes could be maintained for 8 days in media that were free of serum and hormones (except for insulin). Cells retained a spherical phenotype; they secreted albumin and not alpha-fetoprotein; and the cellular RNA/DNA ratio rose progressively in culture. The isolation procedure and the duration of culture affected expression of specific P450s differently. CYP1A2, CYP2C9, and CYP2E1 mRNAs were not altered by cell isolation, and levels of CYP1A2 and CYP2C9 mRNA were also maintained for 8 days in culture, whereas CYP2E1 mRNA declined to 9% of values in fresh hepatocytes by day 8. CYP3A4 mRNA content was considerably decreased in freshly isolated hepatocytes compared with normal liver, and expression of this gene during the course of culture was more variable than that of the other P450s. Use of Williams' E medium considerably enhanced accumulation of CYP3A4 mRNA, compared with modified Waymouth 752/1 medium, but had a detrimental effect on levels of the other P450 mRNAs. Despite high levels of expression at the mRNA level, the microsomal protein contents of CYP1A2, CYP2C9, CYP2E1, and CYP3A4 declined progressively during the course of culture; this decline was most rapid for CYP3A4. These results confirm the potential of primary cultures of well-differentiated human hepatocytes for studies of P450 gene regulation in humans, but they also demonstrate that culture conditions are variables that must be carefully controlled when examining liver-specific gene expression in vitro. In particular, time in culture may variably affect expression of P450 enzyme changes at both the mRNA and protein levels.

Extended primary culture of human hepatocytes in a collagen gel sandwich system

To develop a strategy for extended primary culture of human hepatocytes, we placed human hepatocytes between two layers of collagen gel, called a "collagen gel sandwich." Maintenance of hepatocellular functions in this system was compared with that of identical hepatocyte preparations cultured on dry-collagen coated dishes or cocultured with rat liver epithelial cells. Human hepatocytes in a collagen gel sandwich (five separate cultures) survived for more than 4 wk, with the longest period of culture being 78 d. They maintained polygonal morphology with bile canaliculuslike structures and high levels of albumin secretion throughout the period of culture. In contrast, hepatocytes on dry-collagen became feature-less, and albumin secretion could not be detected after 14 d of culture. This loss of albumin secretion was partially recovered by overlaying one layer of collagen gel. Ethoxyresorufin O-deethylase activity, associated with cytochrome P450 1A2, was detected basally up to 29 d in collagen gel sandwich culture. These activities were induced four- to eightfold after induction with dibenz(a,h)anthracene. Cocultures also maintained basal activity up to 29 d. However, their inducibility was lower than that of hepatocytes in collagen gel sandwich. No ethoxyresorufin O-deethylase activity was detected in hepatocytes cultured on dry-collagen at 7 d. Thus, the collagen gel sandwich system preserves differentiated morphology and functions of human hepatocytes in primary culture for a prolonged period of time. This system is a promising model for studying human hepatocellular function, including protein synthesis and drug metabolism in vitro.

Use of hepatocyte cultures for the study of hepatotoxic compounds

Human and animal hepatocytes in primary culture are widely used in pharmacotoxicological research. They represent a unique in vitro model since they retain both phase I and phase II enzyme activities as well as their inducibility by xenobiotics. Hepatocyte cultures are used for drug screening, identification of the lesions induced by toxic compounds and determination of mechanisms by which xenobiotics exert liver injury.

CYP3A5 is the major cytochrome P450 3A expressed in human colon and colonic cell lines

CYP3A is known to be expressed in liver, small intestine and colon. However, its isoform distribution (CYP3A4, 3A5 and 3A7) and inducibility have not been clearly elucidated in the colon. Therefore, we analyzed CYP3A in human colon and compared its expression and inducibility to the human colonic cell lines HT29 and Caco2, which were used as models. Patients suffered either from sigmoiditis or colonic adenocarcinoma. Patients as well as HT29 and Caco2 cells were treated with rifampicin. CYP3A protein expression was analyzed in the colon of patients and in the cells by immunoblot and by isoelectric focusing enabling separation of CYP3A isoforms, while mRNA expression was determined using specific reverse transcription-polymerase chain reaction. In both human colon and cells, CYP3A5 was the main isoform expressed at the protein and mRNA levels. Rifampicin treatment had no effect on CYP3A expression. HT29 and Caco2 cells exhibiting the same CYP3A expression and inducibility might therefore be useful in vitro models for studying xenobiotic metabolism in human colon.

Inhibition of tolbutamide 4-methylhydroxylation by a series of non-steroidal anti-inflammatory drugs in V79-NH cells expressing human cytochrome P4502C10

1. To study the role of cytochrome P4502C10 in the metabolism of the non-steroidal antiinflammatory drugs (NSAIDs) diclofenac, phenylbutazone, fenoprofen, ibuprofen, flurbiprofen, ketoprofen and naproxen, a cell line was developed stably expressing CYP2C10 cDNA. A retroviral vector construct, containing a human CYP2C10 cDNA, was transfected in V79-NH Chinese hamster lung cells by calcium phosphate co-precipitation. Sublines stably expressing human cytochrome P450 cDNA were established by selection with the neomycin analogue G418. 2. Enzymatic activity of CYP2C10 was detected by 4-methylhydroxylation of tolbutamide. This activity was inhibited to background levels by preincubation with the CYP2C9/10 inhibitor sulphaphenazole. 3. Preincubations with the NSAIDs ketoprofen, phenylbutazone, flurbiprofen and diclofenac (all 250 microM) caused a decrease in 4-methylhydroxylation of tolbutamide (500 microM), significantly different from control values (p < 0.05). Inhibition of this activity was not seen in preincubations with the NSAIDs fenoprofen, ibuprofen and naproxen (250 microM). 4. The V79-NH CYP2C10 cell line we have developed has been shown to be a useful tool to predict drug-drug interactions.

trans-Acting factors, detoxication enzymes and hepatitis B virus replication in a novel set of human hepatoma cell lines

A panel of four novel human hepatoma cell lines was isolated from a single tumor from a male individual. BC1, B16 and B16A2 lines were well differentiated, while cells of the B9 line were only poorly differentiated, being essentially negative for the functions analyzed. These cell lines have been surveyed for expression of a large set of plasma proteins, accumulation of liver-specific mRNAs and DNA-binding activity of ubiquitous and liver-enriched transcription factors. BC1 cells expressed the highest levels of albumin mRNA, whereas B16 and B16A2 cells accumulated the largest amounts of haptoglobin mRNA. In addition, B16 and B16A2 cells were unique in that they expressed CYP2E1 mRNA, a species absent from the available human liver cells, including HepG2 hepatoma cells, and 3-methylcholanthrene-inducible CYP1A2 mRNA. The activities of genes encoding transcription factors were evidenced in all four cell lines which expressed mRNAs for nuclear factor interleukin 6 and hepatocyte nuclear factor 1 (HNF) together with the DNA-binding activity of NFY and AP1 nuclear proteins. Strikingly, HNF-1 and HNF-4-like DNA-binding activities were restricted to BC1, B16 and B16A2 cells, supporting the idea of the potential role of these (or closely related) factors in the maintenance and/or in the establishment of the differentiated phenotype. B9 cells contained variant HNF1-like DNA-binding activity, similar to dedifferentiated rat hepatoma cells of the H5 line. CCAAT/enhancer-binding protein and HNF-3-like activities were found in all cell lines, although at a lower level and/or activity in B9 cells. Finally, transfection experiments of plasmids containing the whole hepatitis-B virus genome demonstrated that B16 cells, but not B9 cells, were able to support hepatitis-B virus replication and virion production, in agreement with the notion that HNF-1 activity is necessary for viral replication. We believe that the specific complement of transcription factors expressed in the differentiated BC1, B16 and B16A2 cells, and in the poorly differentiated B9 cells, will allow studies on the regulation of hepatic gene expression in these human lines, and will also aid the analysis of xenobiotic metabolism and the biology of hepatitis-B virus replication.

Inducibility of ethoxyresorufin deethylase and UDP-glucuronosyltransferase activities in two human hepatocarcinoma cell lines KYN-2 and Mz-Hep-1

Two human hepatoma cell lines, KYN-2 and Mz-Hep-1 were characterized in terms of glucuronidation capacity and inducibility of cytochrome P4501A1/1A2 and several UDP-glucuronosyltransferases (UGTs). Cytochrome P4501A1/1A2 activity was measured using 7-ethoxyresorufin and that of UGTs with 16 different substrates. The effects of dimethyl sulfoxide (DMSO), 3-naphthoflavone, alpha-naphthoflavone, and rifampicin on these drug-metabolizing enzyme activities were studied. DMSO treatment increased in a dose-dependent manner the ethoxyresorufin O-deethylase (EROD) activity in KYN-2 cells, while an opposite effect was observed in Mz-Hep-1 cells. In KYN-2 cells, EROD was more responsive toward beta-naphthoflavone treatment in combination with DMSO. This activity was enhanced in Mz-Hep-1 cells more than 83 times by beta-naphthoflavone. The enhancement of EROD activity by DMSO and beta-naphthoflavone treatments of KYN-2 cells was abolished by alpha-naphthoflavone treatment. In Mz-Hep-1, only the inducing effect of beta-naphthoflavone was abolished by alpha-naphthoflavone treatment. Rifampicin treatment of KYN-2 cells reversed both the DMSO and beta-naphthoflavone effects on the EROD activity. Glucuronidation of steroids, bile acids, fatty acids and drugs was effective in KYN-2 and Mz-Hep-1 cells. Both 1-naphthol glucuronidation and the level of UGT1*6 protein detected by immunoblot and supporting this activity were lowered by DMSO treatment and increased by beta-naphthoflavone treatment in KYN-2 cells. In Mz-Hep-1 cells, DMSO and beta-naphthoflavone had no effect on 1-naphthol glucuronidation activity. DMSO, beta-naphthoflavone and rifampicin also affected the glucuronidation of various substrates supported by different UGT isoforms. These results indicate that KYN-2 and Mz-Hep-1 cells can be used as new in vitro models for the studies of drug metabolism and the regulation of the corresponding enzymes.

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.

The emerging role of cytochrome P450 3A in psychopharmacology

Recent advances in molecular pharmacology have allowed the characterization of the specific isoforms that mediate the metabolism of various medications. This information can be integrated with older clinical observations to begin to develop specific mechanistic and predictive models of psychotropic drug interactions. The polymorphic cytochrome P450 2D6 has gained much attention, because competition for this isoform is responsible for serotonin reuptake inhibitor-induced increases in tricyclic antidepressant concentrations in plasma. However, the cytochrome P450 3A subfamily and the 3A3 and 3A4 isoforms (CYP3A3/4) in particular are becoming increasingly important in psychopharmacology as a result of their central involvement in the metabolism of a wide range of steroids and medications, including antidepressants, benzodiazepines, calcium channel blockers, and carbamazepine. The inhibition of CYP3A3/4 by medications such as certain newer antidepressants, calcium channel blockers, and antibiotics can increase the concentrations of CYP3A3/4 substrates, yielding toxicity. The induction of CYP3A3/4 by medications such as carbamazepine can decrease the concentrations of CYP3A3/4 substrates, yielding inefficiency. Thus, knowledge of the substrates, inhibitors, and inducers of CYP3A3/ and other cytochrome P450 isoforms may help clinicians to anticipate and avoid pharmacokinetic drug interactions and improve rational prescribing practices.

Acquisition and use of human in vitro liver preparations

Human in vitro liver preparations-i.e., slices, hepatocyte suspensions, primary hepatocyte cultures and microsomes-are increasingly used in the drug development process. The main applications are prediction of drug metabolite profiles, drug-drug interactions and toxicity. The use of these in vitro models is limited, however, because of their erratic availability, the absence of validated protocols and the difficulties of extrapolation of in vitro data to the in vivo situation.

Review article: omeprazole and the cytochrome P450 system

Enzymes of the cytochrome P450 superfamily play a key role in xenobiotic metabolism. Their properties and significance are discussed with particular reference to interactions with the H+,K(+)-ATPase blocker, omeprazole. Such interactions include both inhibitory (subfamily 2C) and inducing effects (subfamily 1A). Delayed metabolic elimination of diazepam, warfarin, carbamazepin and phenytoin is probably due to omeprazole competition for the concerned isoform of subfamily 2C; however, these effects are modest to negligible in magnitude and, for phenytoin, not consistently reproducible. Also, induction of subfamily 1A is only minor as assessed from the resultant changes in N-3-demethylation of caffeine, a reaction specific to this subfamily. Concerns about a possible activation of procarcinogens that might arise from subfamily 1A induction appear ill-founded given the fact that cruciferous vegetables such as broccoli and Brussels sprouts are potent inducers, but rather seem to lower the incidence of certain types of cancer. Likewise, the idea that the toxicity of acetaminophen might increase upon subfamily 1A induction appears far-fetched, mainly because much stronger inducers of subfamily 1A (cigarette smoke and charcoaled beef) are unable to alter acetaminophen metabolism.

The influence of rifampin treatment on caffeine clearance in healthy man

BACKGROUND/AIMS

Since cytochrome P450IA2 is involved in the metabolism of procarcinogens and carcinogens, there is debate about whether induction of this enzyme system by pharmaceuticals leads to a higher risk of malignancy. We investigated rifampin as a potent inducer of the hepatic mixed function oxygenase system and its effect on caffeine metabolism which can be taken as an in vivo marker of cytochrome P450IA2 activity.

METHODS

Caffeine clearance was measured in ten healthy volunteers before and after a 7-day treatment with 600 mg rifampin. Urinary 6 beta-hydroxycortisol output was used as endogenous marker of microsomal enzyme function.

RESULTS

6 beta-hydroxycortisol increased from 7.6 to 26.0 micrograms/24 h per kg after treatment with rifampin, consistent with the induction of mixed function oxidases in the liver. There were significant changes in caffeine plasma half-life (6.2 vs. 3.5 h; p < 0.004) and caffeine plasma clearance (1.3 vs. 2.2 ml/kg per min; p < 0.004) with rifampin.

CONCLUSIONS

The hypothesis that induction of cytochrome P450IA2 entails accelerated activation of procarcinogens would predict rifampin to be associated with a higher risk of malignancy. As with other inducers of cytochrome P450IA2, such as phenytoin or omeprazole, no such linkage is known. Therefore the role of pharmaceutical induction of cytochrome P450IA2 in tumor promotion remains unclear.

Human hepatocytes express trifluoroacetylated neoantigens after in vitro exposure to halothane

Biotransformation of anaesthetic halothane by cytochrome P450-dependent monooxygenases resulted in the production of reactive intermediate trifluoroacetyl (TFA) halide, capable of covalently binding to hepatocyte proteins. TFA-modified liver proteins can act as antigens and are implicated in the pathogenesis of halothane hepatitis in humans. The aim of this study was to investigate the formation of TFA-neoantigens in halothane-treated primary cultures of adult human hepatocytes and to evaluate the usefulness of this in vitro model for studying immune-mediated halothane hepatotoxicity. Cultured human hepatocytes were incubated with halothane under constant temperature, atmosphere and anaesthetic concentration conditions. The results obtained show that halothane-treated hepatocytes isolated from seven different donors produced TFA-antigens as detected by immunocytochemical and western immunoblot analysis using rabbit anti-TFA antiserum. TFA-adducts were localized mainly in the endoplasmic reticulum and in small amounts on the plasma membrane of parenchymal cells. By immunoblotting, several neoantigens, with molecular masses from 42 to 100 kDa, were detected in halothane-exposed hepatocytes. These observations are consistent with the formation of TFA-adducts through metabolism of the anaesthetic and suggest that primary cultures of human hepatocytes represent a suitable in vitro model to study the pathogenesis of immune-mediated halothane hepatotoxicity.

Metabolism of FK506, a potent immunosuppressive agent, by cytochrome P450 3A enzymes in rat, dog and human liver microsomes

The oxidative metabolism of FK506 by liver microsomes and purified cytochrome P450 (P450) enzymes from rats, dogs and humans was studied. The major metabolite formed by liver microsomes from all species was 13-demethylated FK506, named M-I. In adult rats, liver microsomal metabolic activity toward FK506 was higher in males than in females and was stimulated by treatment with P450 3A inducers such as dexamethasone and phenobarbital. In a reconstituted monooxygenase system containing various forms of purified P450 3A enzymes, rat P450 3A2, dog P450 DPB-1 (a form of the P450 3A family) and human P450 3A4 catalyzed FK506 oxidation efficiently in the presence of cytochrome b5, a mixture of phospholipids (dilauroylphosphatidylcholine, dioleoylphosphatidylcholine and phosphatidylserine), and sodium cholate. Rat P450 2C6 and 2D1 and human P450 2CMP also metabolized FK506, with significant lower activity than the P450 3A enzymes, and other rat P450 1A, 2A, 2B, 2C and 2E families including C11 did not show catalytic activities for FK506. Anti-P450 3A2 and anti-P450 3A4 antibodies strongly inhibited FK506 oxidation catalyzed by rat and human liver microsomes, respectively. The formation rate of M-I correlated well with testosterone 2 beta- and 6 beta-hydroxylase activities in rat liver microsomes and with immunoquantified P450 3A4 content, nifedipine oxidase activity, and testosterone 6 beta-hydroxylase activity in human liver microsomes. These in vitro findings indicate that the P450 3A enzymes in liver microsomes from various species of animals, including human, play a major role in the first step oxidation of FK506.

The expression and environmental regulation of P450 enzymes in human placenta

The human placenta oxidizes several xenobiotics, although the spectrum of substrates and metabolic activities when compared with the liver appears somewhat restricted. Maternal cigarette smoking or PCB exposure increases the expression of CYP1A1. This induced activity is able to catalyze the activation of benzo(a)pyrene (B(a)P) into DNA-bound products, both in vitro and in vivo. Studies on adult human liver bolster the concept that CYP1A1 and -1A2 are differentially expressed in hepatic and extrahepatic tissues. Studies with cDNA probe or enzyme specific antibodies and substrates for CYP2A, -2B, -2C, -2D, and -2E gene products have yielded negative results. There are only minimal activities that can be found in substantial quantities in placentas without maternal smoking; one example is 7-ethoxycoumarin O-deethylase (ECOD). Aromatase and cholesterol side-chain cleaving P450 mRNAs, proteins, and activities are measurable in human placentas and do not seem to be affected by maternal cigarette smoking.

Cytochrome P450 induction and metabolism of alkoxyresorufins, ethylmorphine and testosterone in cultured hepatocytes from goats, sheep and cattle

Very little is known of cytochrome P450 (P450) patterns and enzyme characteristics in food-producing animal species. Oxidative metabolism of alkoxyresorufins, ethylmorphine (EtM) and testosterone (TST) was used to monitor the effects of the P450 inducers phenobarbital (PB), beta-naphthoflavone (BNF), dexamethasone (DEX) and triacetyloleandomycin (TAO) in primary cultured hepatocytes from goats, sheep and cattle. BNF effectively and specifically induced ethoxyresorufin deethylase (> 20-fold), indicating the presence of an inducible P450 1A form, and down-regulated EtM demethylation and most selected TST hydroxylations. In non-induced hepatocyte cultures, TST was metabolized to 6 beta-, 2 beta-, 12 beta-, and 11 alpha-hydroxy-TST (OHT). PB and, to a lesser extent, DEX non-specifically induced all OHT formations, and EtM demethylation. TAO almost completely inhibited OHT formation and EtM demethylation. These results indicate the involvement of principally one P450 form, or a restricted number of related P450 forms, presumably belonging to the P450 3A subfamily. In western blot analysis, cross reactivity was found with rat anti-P450 3A1 and anti-sheep P450 3A. A more specific PB effect was observed for 16 alpha-OHT, which may be formed though a ruminant P450 2B form. None of the inducers influenced pentoxyresorufin depentylase (PROD) or EtM O-deethylation. Metabolite patterns and inducibility of selected activities in ruminant hepatocytes are in accordance with previous findings in goats in vivo. Cytochrome P450 characteristics in ruminants appear to differ from those in rats whereas similarities to the situation in humans appear to exist.

Regulation of human liver cytochromes P-450 in family 3A in primary and continuous culture of human hepatocytes

The cytochrome P-450 3A gene family comprises the dominant forms of cytochrome P-450 found in human liver. We examined as a possible useful system for studying the regulation of cytochrome P-450 3A under controlled conditions in vitro, primary monolayer cultures of human hepatocytes and compared the results with those obtained from the study of cytochrome P-450 3A in the human hepatoblastoma cell line HepG2 or in the human hepatocellular carcinoma cell line TONG/HCC. Using 3A antibodies, 3A cDNAs and 3A3, 3A4, 3A5 and 3A7 isozyme-specific oligonucleotides as probes, we determined that primary human hepatocyte cultures routinely expressed a 3A3/4* immunoreactive protein and 3A mRNA. These gene products were well maintained for many days and were induced by treatment of the cultures with dexamethasone, phenobarbital, macrolide antibiotics, the HMG CoA reductase inhibitor lovastatin or an antifungal agent, clotrimazole. Of six donor livers examined, only two contained mRNA or protein for 3A5, a form found in only a few adult human subjects. In cultures prepared from one of these two livers, 3A5 mRNA was detectable for several days. In cultures of hepatocytes from the remaining four human livers that did not contain 3A5 mRNA or protein, we detected neither spontaneous nor inducible 3A5 proteins or mRNAs. HepG2 cells contained only 3A7 protein, a form found in human fetal liver, even after treatment with inducers. treatment of HepG2 cells with dexamethasone, macrolide antibiotics, phenobarbital and phenobarbital-like inducers or lovastatin produced dose-dependent induction of 3A7 mRNA and 3A7 immunoreactive protein. TONG/HCC cells contained 3A3, 3A4 and 3A5 mRNAs, but only 3A5 immunoreactive protein could be detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of human hepatocyte cultures for drug metabolism studies

Among the in vitro models developed to investigate drug metabolism isolated hepatocytes have become the most powerful model. Human hepatocytes can be prepared from whole livers and surgical wedge biopsies. When placed in culture they retain their specific drug metabolizing activities including inducible cytochrome P450 enzymes for several days. Primary human hepatocyte cultures are now increasingly used for studying drug behavior during preclinical development, e.g. drug interactions, stereoselective drug metabolism and drug metabolic profiles. As a rule there is a good in vivo/in vitro correlation in drug biotransformation activity. The main metabolites found in vivo are recovered in vitro. However, quantitative differences are frequently observed.

The establishment and continuous subculturing of normal human adult hepatocytes: expression of differentiated liver functions

The use of normal adult liver hepatocytes in cell culture for biochemical, toxicological and pharmacological studies has been greatly limited owing to the loss of replicative capacity and differentiated liver function. This is contrary to the ability of the liver to regenerate following injury in vivo. This suggests that liver "stem" or "transitional" hepatocytes exist that upon proper stimulus divide and differentiate into mature hepatocytes. In this study we report the establishment and culture of hepatocytes from normal human adult liver, which: (1) possess replicative capacity sufficient to subpassage 12-15 times (27-37 cumulative population doublings); (2) can be cryopreserved for subsequent use without loss of replicative capacity; and (3) upon differentiation in culture synthesize albumin and keratin 18 and metabolize benzo[a]pyrene. The ability of these cells to divide or express differentiated functions appears to be due to a number of cellular, biochemical and physical characteristics that are present during the primary establishment and subsequent growth phases of the cell cultures. Disassociation of cells from excess liver tissue was best achieved by combining the mechanical action of the Stomacher with very low amounts of proteolytic enzymes and EGTA. The cell lines appeared to grow best when established and subpassaged in an mALPHA medium supplemented with insulin, hydrocortisone, transferrin, epithelial growth factor and fetal bovine serum (prescreened for human hepatocyte cell growth). The seeding density and cell-cell contact in culture appeared to be important for both cell division and expression of liver function. When cells were seeded at a low density and subpassaged before confluency, the cells continued to divide. Albumin and keratin 18 synthesis occurred primarily in tightly packed cell clusters. When cells were seeded at a high density, near confluency, albumin and keratin 18 synthesis occurred uniformly in all of the cells of the culture and the culture metabolized benzo[a]pyrene to water-soluble metabolites, which covalently bound to cellular DNA. This appearance of liver functions was consistent with the "transition" of hepatocytes to a terminally differentiated state. Nonhepatic markers, i.e., alpha-fetoprotein, factor VIII and gamma-glutamyl transpeptidase activity were not expressed in cells cultured at either low or high density. Thus, the data presented here indicate that normal human adult liver hepatocytes, once established in culture, can be subpassaged to a high number of population doublings, cryopreserved for later use, and modulated to express differentiated liver functions.

The effects of inducing agents on cytochrome P450 and UDP-glucuronyltransferase activities in human HEPG2 hepatoma cells

Selective induction in vitro of cytochrome P450-dependent mixed-function oxidase (MFO) and UDP-glucuronyltransferase (GT) activities was observed in the human HepG2 hepatoma cell line. 1,2-Benzanthracene (BA) induced MFO O-dealkylation activities for ethoxyresorufin, methoxyresorufin and benzyloxyresorufin, whereas phenobarbitone (PB) selectively induced pentoxyresorufin O-dealkylation and rifampicin (RIF) selectively induced benzyloxyresorufin O-dealkylation. Antibody inhibition experiments indicated that ethoxyresorufin and methoxyresorufin O-dealkylations were catalysed mainly by the P450 1A subfamily in untreated and BA-induced HepG2 cells, that additional unidentified P450 forms were considerably involved in methoxyresorufin and benzyloxyresorufin O-dealkylations and that the P450 2B subfamily was partially responsible for pentoxyresorufin O-dealkylation in PB-induced cells. Bilirubin GT activity was induced by PB, BA, RIF and dexamethasone, but 1-naphthol, morphine and testosterone GT activities were not induced by any of these treatments.

Lack of effect of phenobarbital on ex vivo leukocyte oxidative metabolism in healthy volunteers

Oxidative metabolism in activated human polymorphonuclears catabolizes leukotriene B4 by a cytochrome P450 omega-hydroxylase and procainamide by a myeloperoxidase. The percentage of leukotriene B4 metabolized by activated human polymorphonuclears and the apparent enzymatic parameters of procainamide metabolism were studied ex vivo in six healthy volunteers before and after phenobarbital intake (100 mg/day) for 10 days and in six healthy control volunteers. No differences were found between groups for the difference in percentage of leukotriene B4 metabolized between day 11 and day 1. The apparent enzymatic parameters, Km and Vm, of procainamide oxidation did not differ significantly between the groups both on day 1 and day 11. These results do not show any evidence of inducibility of leukotriene B4 and procainamide metabolism in human polymorphonuclears. However, a positive correlation between 6 beta OH-cortisol excretion and percentage of leukotriene B4 metabolized was observed on day 11. This study suggests that human polymorphonuclears cytochrome P450 leukotriene B4 omega-hydroxylase and procainamide metabolism is not a useful method to study cytochrome P450 induction in clinical pharmacology.

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

1. The effects of phenobarbital (PB), beta-naphthoflavone (beta-NF), omeprazole (Omep) and rifampicin (Rif) on drug-metabolizing activities in dog hepatocytes, cultured with William's medium E, were examined. 2. The drug metabolizing activities of the hepatocytes decreased during culture; 7-ethoxycoumarin O-deethylase (ECOD) activity was nearly 70% of initial value at 72 h, but 7-methoxycoumarin O-demethylase (MCOD), 7-propoxycoumarin O-depropylase (PCOD), progesterone 6 beta-hydroxylase (6 beta-OH-P), progesterone 16 alpha-hydroxylase (16 alpha-OH-P), progesterone 21-hydroxylase (21-OH-P), 7-ethoxyresorufin O-deethylase (EROD) activities and total cytochrome P450 content were approx. 50%. 3. When the hepatocytes were cultured with PB, the enzyme activities increased time- and dose-dependently. MCOD, ECOD and PCOD activities increased 5-8 fold with 2 mM PB in 96 h. Similar results were obtained for 6 beta-OH-P, 16 alpha-OH-P and 21-OH-P activities, and total cytochrome P450. The effect of PB was abolished when 2.5 microM cycloheximide or 0.1 microM actinomycin D was included in the culture. 4. Treatment of hepatocytes with 40 microM beta-NF for 72 h resulted in 25-fold elevation of EROD activity. beta-NF enhanced PCOD activity approx. six-fold, while ECOD increased only slightly, and 7-MCOD negligibly. 5. Omep (100 microM) increased EROD activity nearly 10-fold, and 25 microM Rif increased 6 beta-OH-P activity approx. 8-fold, but ECOD only slightly. 6. Western blot analysis of microsomes from cultured dog hepatocytes with anti-rat CYP 2B1 antibodies indicated that PB increased an immunochemically-reactive protein. The protein showed the same mobility as the major dog P450 isozyme (cytochrome P450 PBD-2 or CYP 2B11) purified from liver microsomes of PB-treated male beagle dog. In a similar manner, induction of cytochrome P450 PBD-1 (CYP 3A12) by PB was confirmed.

Induction of P4502E1 by acetone in isolated rabbit hepatocytes. Role of increased protein and mRNA synthesis

The molecular mechanism(s) underlying induction of the hepatic microsomal cytochrome P4502E1 (2E1) by xenobiotics (e.g. ethanol and acetone) is controversial. Proposed mechanisms include increased rates of enzyme synthesis due to elevated 2E1 mRNA levels, enhanced translation of pre-existing mRNA, or stabilization of 2E1 protein. To further assess which, if any, of these events predominates during the initial stages of 2E1 protein induction, we investigated the effects of acetone treatment on 2E1 content in cultured rabbit hepatocytes, an in vitro system that allows for precise control of the cellular mileau. Hepatocytes harvested from female rabbits and plated on plastic dishes with serum-supplemented medium were 90-100% viable for at least 48 hr in culture. Analysis of immunoreactive 2E1 content and aniline hydroxylase activity in microsomes isolated from hepatocytes cultured for up to 24 hr revealed that 2E1 expression was equal to that of microsomes from unplated cells and by 48 hr of culture, 2E1 levels decreased by only 35%. Moreover, microsomes isolated from cells exposed to 17 mM acetone for 24 hr exhibited a 53 and 62% increase in aniline hydroxylase activity and 2E1 content, respectively, compared to untreated cells. To explain these increases, the rate of 2E1 protein synthesis was determined in untreated cells or in cells treated with 17 mM acetone by first exposing hepatocytes to medium supplemented with 35S-labeled methionine and cysteine ([35S]Met/Cys) and subsequently assessing radiolabel incorporation into 2E1 protein. While no difference was found between untreated and acetone-treated cells in the incorporation of [35S]Met/Cys into trichloracetic acid-precipitable microsomal proteins, immunoaffinity purification of 2E1 revealed that incorporation of 35S-labeled amino acids specifically into 2E1 was elevated by acetone to 200% of control values. Treatment of hepatocytes with the transcriptional inhibitor, alpha-amanitin, markedly inhibited this acetone-mediated increase in [35S]Met/Cys incorporation into 2E1. Analysis of hepatocyte RNA revealed that acetone increased 2E1 mRNA to 130 and 160% of control levels at 6 and 24 hr, respectively, and that these increases were prevented by pretreatment with alpha-amanitin. Our results indicate that acetone increases 2E1 protein levels in cultured rabbit hepatocytes by stimulating its rate of de novo synthesis. Since this increase in 2E1 synthesis stems, at least in part, from the acetone-mediated enhancement of hepatocyte 2E1 mRNA content and is inhibitable by alpha-amanitin, transcriptional activation of the rabbit CYP2E1 gene is apparently involved in the induction of 2E1 protein by acetone.

Induction of cytochrome P-4502E1 in the human liver by ethanol is caused by a corresponding increase in encoding messenger RNA

The propensity of centrilobular liver damage to develop in alcohol abusers after exposure to various hepatotoxins, including ethanol itself, has been linked to the induction by ethanol of P-4502E1, a microsomal P-450 enzyme that bioactivates these agents to reactive metabolites. Whereas long-term ethanol consumption elicits a marked increase in hepatic P-4502E1 content, the molecular mechanism by which ethanol produces this effect is the subject of controversy in animals, and it has not been elucidated in human beings. Possible mechanisms include increased enzyme synthesis stemming from elevated 2E1 messenger RNA levels, enhanced translation of preexisting messenger RNA or stabilization of P-4502E1 protein. To determine which, if any, of these mechanisms underlies P-4502E1 induction in human beings, we examined the effects of ethanol intake on the hepatic intralobular distribution of P-4502E1 messenger RNA and the corresponding protein. Liver sections derived from needle biopsy specimens were obtained from five recently drinking alcoholics (last drink no more than 36 hr before) and eight control subjects (five abstaining alcoholics [last drink 96 hr or more before] and three nondrinkers). In situ hybridization of these liver sections with a human P-4502E1 complementary DNA probe was used to localize P-4502E1 messenger RNA transcripts. Quantitative image analysis of hybridized sections from control subjects revealed that P-4502E1 transcript content in perivenular (zone 3) hepatocytes was significantly higher (p < 0.05) than in midzonal (zone 2) and periportal (zone 1) cells (18.3 +/- 1, 9.5 +/- 2 and 3.1 +/- 2 arbitrary density units, respectively; mean +/- S.E.M.). In recent drinkers, acinar regions containing P-4502E1 transcripts were elevated 2.9-fold compared with those in controls (32.8% +/- 7% vs. 11.2% +/- 2%; p < 0.01), with this messenger RNA increase occurring mainly in perivenular cells (29.6 +/- 3 vs. 18.3 +/- 1 units; p < 0.01). P-4502E1 protein distribution, assessed by the immunohistochemical staining of liver sections with P-4502E1 antibodies, was found to be analogous to that of the messenger RNA in control subjects (the level in perivenular cells was greater than that in midzonal cells, which was greater than that in periportal cells), whereas recent drinkers exhibited marked elevations in enzyme content in both perivenular and midzonal hepatocytes. Moreover, cellular levels of P-4502E1 protein and messenger RNA were significantly correlated (rs = 0.79; p < 0.001) in all patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Acid reaction products of indole-3-carbinol and their effects on cytochrome P450 and phase II enzymes in rat and monkey hepatocytes

The effects of three acid condensation products of indole-3-carbinol (I3C), i.e. 3,3'-diindolylmethane (DIM), 5,6,11,12,17,18-hexahydrocyclonona[1,2-b:4,5-b':7,8-b"]tri-indole (CTI) and 2,3-bis[3-indolylmethyl]indole (BII), on cytochrome P450 and phase II enzymes were studied in primary cultures of rat and cynomolgus monkey liver cells. In rat hepatocytes all three indole derivatives dose-relatedly induced the ethoxyresorufin O-dealkylation (EROD) activity (to 24-fold) and 7 alpha-hydroxylation of testosterone (to 4-fold), whereas all three decreased the 16 alpha- and 2 alpha-testosterone hydroxylation (DIM to 60%, CTI and BII to a mere 5% of the control cells). Treatment of monkey hepatocytes with DIM and BII enhanced the EROD activity to 6- and 9-fold, respectively. Furthermore, BII decreased the 6 beta-hydroxylation of testosterone (to 60% of the untreated cultures) in monkey cells. Phase II enzymes were also affected. In rat hepatocytes DIM, CTI and BII enhanced DT-diaphorase (DTD) (= NAD(P)H-quinone reductase) activity, and DIM and BII the glucuronidation of 1-naphthol. In monkey cells BII only enhanced DTD, and no changes were observed in the glucuronidation of 1-naphthol after treatment with either DIM or BII. The indole derivatives did not affect glutathione S-transferase activity and sulfation of 1-naphthol in either rat or monkey hepatocytes. These results identify two novel acid condensation products of I3C, CTI and BII, as potent compounds in affecting biotransformation in rat as well as in monkey hepatocytes.

Phenobarbital induction of cytochrome P-450 gene expression

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Human cytochrome P-450 enzymes

Cytochrome P-450 (P-450) enzymes have been studied extensively in experimental animal models and much is known regarding their structures, regulation, and mechanisms of catalysis. In recent years investigations have been extended to the human P-450s. There are more than 30 different characterized human P-450s in the superfamily, and collectively they are probably the most significant enzymes involved in the metabolism of drugs, carcinogens, and steroids. The levels of many of the P-450s and their catalytic activities can vary considerably because of polymorphism, induction, and inhibition. The catalytic specificity of the P-450s can range from being very non-discriminatory to very exacting, and clinical consequences of drugs and steroids can be related to variations in P-450 levels. Defects in the rate-limiting P-450 reactions in steroidogenesis (due to genetic deficiencies) have been shown to be debilitating and even fatal.