Clinical importance of non-genetic and genetic cytochrome P450 function tests in liver disease

Metabolomics analysis of urine from patients with alcohol-associated liver disease reveals dysregulated caffeine metabolism

Excess alcohol intake causes millions of deaths annually worldwide. Asymptomatic early-stage, alcohol-associated liver disease (ALD) is easily overlooked, and ALD is usually only diagnosed in more advanced stages. We explored the possibility of using polar urine metabolites as biomarkers of ALD for early-stage diagnosis and functional assessment of disease severity by quantifying the abundance of polar metabolites in the urine samples of healthy controls (n = 18), patients with mild or moderate liver injury (n = 21), and patients with severe alcohol-associated hepatitis (n = 25). The polar metabolites in human urine were first analyzed by untargeted metabolomics, showing that 209 urine metabolites are significantly changed in patients, and 17 of these are highly correlated with patients' model for end-stage liver disease (MELD) score. Pathway enrichment analysis reveals that the caffeine metabolic pathway is the most affected in ALD. We then developed a targeted metabolomics method and measured the concentration of caffeine and its metabolites in urine using internal and external standard calibration, respectively. The described method can quantify caffeine and its 14 metabolites in 35 min. The results of targeted metabolomics analysis agree with the results of untargeted metabolomics, showing that 13 caffeine metabolites are significantly decreased in patients. In particular, the concentrations of 1-methylxanthine, paraxanthine, and 5-acetylamino-6-amino-3-methyluracil are markedly decreased with increased disease severity. We suggest that these three metabolites could serve as functional biomarkers for differentiating early-stage ALD from more advanced liver injury.NEW & NOTEWORTHY Our study using both untargeted and targeted metabolomics reveals the caffeine metabolic pathway is dysregulated in ALD. Three caffeine metabolites, 1-methylxanthine, paraxanthine, and 5-acetylamino-6-amino-3-methyluracil, can differentiate the severity of early-stage ALD.

Digital Microfluidics-Enabled Analysis of Individual Variation in Liver Cytochrome P450 Activity

The superfamily of hepatic cytochrome P450 (CYP) enzymes is responsible for the intrinsic clearance of the majority of therapeutic drugs in humans. However, the kinetics of drug clearance via CYPs varies significantly among individuals due to both genetic and external factors, and the enzyme amount and function are also largely impacted by many liver diseases. In this study, we developed a new methodology, based on digital microfluidics (DMF), for rapid determination of individual alterations in CYP activity from human-derived liver samples in biopsy-scale. The assay employs human liver microsomes (HLMs), immobilized on magnetic beads to facilitate determination of the activity of microsomal CYP enzymes in a parallelized system at physiological temperature. The thermal control is achieved with the help of a custom-designed, inkjet-printed microheater array modularly integrated with the DMF platform. The CYP activities are determined with the help of prefluorescent, enzyme-selective model compounds by quantifying the respective fluorescent metabolites based on optical readout in situ. The selectivity and sensitivity of the assay was established for four different CYP model reactions, and the diagnostic concept was validated by determining the interindividual variation in one of the four model reaction activities, that is, ethoxyresorufin O-deethylation (CYP1A1/2), between five donors. Overall, the developed protocol consumes only about 15 μg microsomal protein per assay. It is thus technically adaptable to screening of individual differences in CYP enzyme function from biopsy-scale liver samples in an automated fashion, so as to support tailoring of medical therapies, for example, in the context of liver disease diagnosis.

Knock-in mouse lines expressing either mitochondrial or microsomal CYP1A1: differing responses to dietary benzo[a]pyrene as proof of principle

In the past, CYP1A1 protein was known to be located in the endoplasmic reticulum (ER; microsomes). More recently, CYP1A1 was shown also to be targeted to the inner mitochondrial membrane; mitochondrial import is dependent on NH(2)-terminal processing that exposes a cryptic targeting signal. It is interesting that microsomal and mitochondrial CYP1A1 enzymes exhibit different substrate specificities, electron donors, and inducer properties. To understand the physiological functions of microsomal versus mitochondrial CYP1A1, we have generated three knock-in lines by altering the CYP1A1 NH(2) terminus. Cyp1a1(mtt/mtt) mice encode an NH(2)-terminal 31-amino acid-truncated protein, deleting the ER-targeting signal and exposing the cryptic mitochondrial-targeting signal. Cyp1a1(mtp/mtp) mice encode a protein carrying L7N and L17N mutations; this mutant lacks the signal recognition particle (SRP)-binding site and subsequent ER-targeting, but requires proteolysis by a cytosolic peptidase for mitochondrial import. Cyp1a1(mc/mc) mice encode a microsomal protein having R34D and K39I mutations, which abolish the mitochondrial targeting signal. After dioxin or beta-naphthoflavone treatment of these mouse lines, the CYP1A1 protein was shown to be located in the mitochondria of the Cyp1a1(mtp/mtp) and Cyp1a1(mtt/mtt) lines and in microsomes of the Cyp1a1(mc/mc) line. To test for differences in function, we compared the response to dietary benzo[a]pyrene (BaP). After 18 days of daily oral BaP, wild-type and Cyp1a1(mc/mc) mice were completely protected, whereas Cyp1a1(-/-) and Cyp1a1(mtp/mtp) mice showed striking toxicity and compensatory up-regulation of CYP1A2 and CYP1B1 mRNA in several tissues. Our data support the likelihood that it is the microsomal rather than mitochondrial CYP1A1 enzyme that protects against oral BaP toxicity.

Cholestasis and endogenous opioids: liver disease and exogenous opioid pharmacokinetics

A class of endogenous opioids is upregulated in liver disease particular to cholestasis, which contributes to symptoms in liver disease such as pruritus, hypotension and encephalopathy. Symptoms associated with cholestasis are reversed or at least ameliorated by mu opioid receptor antagonists. Palliation of symptoms related to cholestatic liver disease also involves bile acid binding agents. Opioid receptor antagonists, unlike bile acid binding agents, have been reported to relieve multiple symptoms, except for pruritus, and improve liver function as demonstrated in experimental cholestasis. Exogenous opioid pharmacology is altered by liver disease. Dose reduction or prolongation of dose intervals is necessary depending on the severity of liver disease.

Magic-angle spinning solid-state NMR spectroscopy of nanodisc-embedded human CYP3A4

Cytochrome P450 (CYP) 3A4 contributes to the metabolism of approximately 50% of commercial drugs by oxidizing a large number of structurally diverse substrates. Like other endoplasmic reticulum-localized P450s, CYP3A4 contains a membrane-anchoring N-terminal helix and a significant number of hydrophobic domains, important for the interaction between CYP3A4 and the membrane. Although the membrane affects specificity of CYP3A4 ligand binding, the structural details of the interaction have not been revealed so far because X-ray crystallography studies are available only for the soluble domain of CYP3A4. Here we report sample preparation and initial magic-angle spinning (MAS) solid-state NMR (SSNMR) of CYP3A4 (Delta3-12) embedded in a nanoscale membrane bilayer, or Nanodisc. The growth protocol yields approximately 2.5 mg of the enzymatically active, uniformly 13C,15N-enriched CYP3A4 from 1 L of growth medium. Polyethylene glycol 3350-precipitated CYP3A4 in Nanodiscs yields spectra of high resolution and sensitivity, consistent with a folded, homogeneous protein. CYP3A4 in Nanodiscs remains enzymatically active throughout the precipitation protocol as monitored by bromocriptine binding. The 13C line widths measured from 13C-13C 2D chemical shift correlation spectra are approximately 0.5 ppm. The secondary structure distribution within several amino acid types determined from 13C chemical shifts is consistent with the ligand-free X-ray structures. These results demonstrate that MAS SSNMR can be performed on Nanodisc-embedded membrane proteins in a folded, active state. The combination of SSNMR and Nanodisc methodologies opens up new possibilities for obtaining structural information on CYP3A4 and other integral membrane proteins with full retention of functionality.

Protective role of erythropoietin during testicular torsion of the rats

Testicular torsion is an important clinical urgency. Similar mechanisms occurred after detorsion of the affected testis as in the ischemia reperfusion (I/R) damage. This study was designed to investigate the effects of erythropoietin (EPO) treatment after unilateral testicular torsion. Fifty male Sprague-Dawley rats were divided into five groups. Group 1 underwent a sham operation of the right testis under general anesthesia. Group 2 was same as sham, and EPO (3,000 IU/kg) infused i.p., group 3 underwent a similar operation but the right testis was rotated 720 degrees clockwise for 1 h, maintained by fixing the testis to the scrotum, and saline infused during the procedure. Group 4 underwent similar torsion but EPO was infused half an hour before the detorsion procedure, and in group 5, EPO was infused after detorsion procedure. Four hours after detorsion, ipsilateral and contralateral testes were taken out for evaluation. Treatment with EPO improved testicular structures in the ipsilateral testis but improvement was less in the contralateral testis histologically, but EPO treatment decreased germ cell apoptosis in both testes following testicular IR. TNF-alpha, IL-1beta, IL-6 and nitrite levels decreased after EPO treatment especially in the ipsilateral testis. We conclude that testicular I/R causes an increase in germ cell apoptosis both in the ipsilateral and contralateral testes. Erythropoietin has antiapoptotic and anti-inflammatory effects following testicular torsion.

Feasibility study and pharmacokinetics of low-dose paclitaxel in cancer patients with severe hepatic dysfunction

The aim of this study is to investigate the feasibility and determine the pharmacokinetics of low-dose paclitaxel in cancer patients with severe hepatic dysfunction. This was a prospective study. Patients with liver metastases who had either transaminase serum levels higher than 10 times the upper normal limit or bilirubin serum levels higher than 5 times the upper normal limit were eligible. All patients underwent pharmacokinetic evaluation during the first course of treatment. Pharmacokinetics in severe hepatic dysfunction patients were compared with data from a reference group of patients with normal hepatic function who participated in a phase I study. Nine severe hepatic dysfunction patients were treated with paclitaxel 70 mg/m administered as a 1-h infusion every 2 weeks. They received a median three treatment courses (range 1-9) without clinically relevant toxicity. The area under the concentration-time curve of paclitaxel was markedly higher in severe hepatic dysfunction patients when compared with the normal hepatic function control group treated with the same dose (98% increase, P<0.001). Area under the concentration-time curve and the time above 0.1 micromol/l (T>0.1) concentration threshold in the severe hepatic dysfunction patients who received paclitaxel 70 mg/m approximated pharmacokinetics of paclitaxel in patients with normal liver function who received 130 mg/m. Maximum plasma concentration (Cmax) did not differ between the two groups. In conclusion, paclitaxel 70 mg/m was safely delivered every 2 weeks in patients with severe hepatic dysfunction and resulted in adequate plasma concentrations. Paclitaxel at this dosage can be taken as an option for severe hepatic dysfunction patients who are expected to get clinical benefits from taxanes.

What common structural features and variations of mammalian P450s are known to date?

Sufficient structural information on mammalian cytochromes P450 has now been published (including seventeen X-ray structures of these enzymes by June 2006) to allow characteristic features of these enzymes to be identified, including: (i) the presence of a common fold, typical of all P450s, (ii) similarities in the positioning of the heme cofactor, (iii) the spatial arrangement of certain structural elements, and (iv) the access/egress paths for substrates and products, (v) probably common orientation in the membrane, (vi) characteristic properties of the active sites with networks of water molecules, (vii) mode of interaction with redox partners and (viii) a certain degree of flexibility of the structure and active site determining the ease with which the enzyme may bind the substrates. As well as facilitating the identification of common features, comparison of the available structures allows differences among the structures to be identified, including variations in: (i) preferred access/egress paths to/from the active site, (ii) the active site volume and (iii) flexible regions. The availability of crystal structures provides opportunities for molecular dynamic simulations, providing data that are apparently complementary to experimental findings but also allow the dynamic behavior of access/egress paths and other dynamic features of the enzymes to be explored.

Modification of CYP2E1 and CYP3A4 activities in haemoglobin E-beta thalassemia patients

OBJECTIVE

Thalassemia disease is a genetic haemoglobinopathy usually associated with an iron overload and some degree of organ impairment. The impact of the disease on the drug metabolising enzyme cytochrome P450 (CYP) is not known. CYP2E1 and CYP3A4 are responsible for the metabolism of a large number of drugs and changes in their activities may have clinical consequences.

METHODS

Haemoglobin E-beta thalassemia paediatric, blood transfusion-dependent patients apparently without complications (n = 35) and healthy controls (n = 42) were recruited in this study. The ratios of plasma 6-hydroxychlorzoxazone to chlorzoxazone, and urinary 6-beta-hydroxycortisol (6beta-OHF) to cortisol were used as indices for CYP2E1 and CYP3A4 activities, respectively. Blood and plasma samples were assayed for parameters of clinical biochemistry, oxidants and antioxidants.

RESULTS

There were significant increases in serum iron, protein carbonyl and lipid peroxidation in thalassemia patients, whereas there was a decrease in blood glutathione, but unchanged plasma nitric oxide metabolites. CYP2E1 activity in the patients was unchanged; however, when the patients were stratified by splenectomy status, CYP2E1 activity was increased in non-splenectomised patients in comparison with the controls and splenectomised subjects. On the other hand, 6beta-OHF/cortisol ratios increased markedly in patients associated with depressed growth hormone levels. There were no correlations between CYP2E1 activity and oxidant stress or antioxidant parameters.

CONCLUSIONS

This report is the first demonstration that thalassemia major is associated with an alteration of CYP2E1 and CYP3A4 activities; this could modify the sensitivity of thalassemia patients to the toxic or therapeutic effects of drugs.

Development of dosage design of hepatic metabolizing drugs using serum albumin level in chronic hepatic failure

We have previously reported good correlations among serum aminotransferase (AST) activity, metabolic enzyme activity of CYPs, and total clearance (CL(tot)) of probe drugs in rats with acute hepatic failure induced by CCl4. In this study, we searched for new biochemical indicators that correlate with hepatic function and tried to simulate appropriate drug dosage in chronic hepatic failure. Model rats were prepared by administration of CCl4 (1 ml/kg, s.c., 3 times/week) and used at 48 h after the last administration. Serum albumin concentration was time-dependently decreased and correlated well with 3 major biologic determinants of drug clearance, hepatic blood flow (HBF), intrinsic clearance (CL(int)), and the unbound fraction of drugs in plasma (fp) after intravenous administration of cyclophosphamide, tolbutamide, zonisamide, and chlorzoxazone (as probe drugs for low hepatic extraction) and propranolol and lidocaine (as high-hepatic extraction drugs). By calculating these parameters based on prediction equations by the level of albumin, CL(tot) was obtained. As a result of having evaluated this model using administration of cyclosporin, there was a statistically significant relationship between predicted CL(tot) and observed CL(tot). In conclusion, the value of serum albumin level is a useful parameter that correlates well with chronic hepatic function. We have shown that this quantitative administering design using serum albumin level can predict appropriate dosages of hepatic metabolizing drugs in chronic hepatic failure.

Identification of differentially expressed genes in hepatic HepG2 cells treated with acetaminophen using suppression subtractive hybridization

Acetaminophen (APAP) is widely used for the treatment of pain and fever. Although it is safe at therapeutic doses, APAP is toxic at higher doses and can cause severe damage to the liver. To clarify the mechanism of APAP-related liver damage, we attempted the identification of the differential gene expression in response to APAP treatment in hepatic HepG2 cells. In the present study, we used the technique of suppression subtractive hybridization (SSH) for the identification of the differentially expressed genes between untreated and treated cells and identified 14 candidate genes showing increased expression in response to APAP treatment. RT-PCR and real-time RT-PCR analysis confirmed that the expression of two genes was increased within 24 h following APAP treatment. Among them, only lysyl hydroxylase 2 expression was increased in a time- and dose-dependent manner. Furthermore, the expression of lysyl hydroxylase 2 was shown to be increased in the livers of APAP-treated mice compared to untreated controls. The increased expression of lysyl hydroxylase 2 was also observed when the cells were exposed to other hepatotoxins, ethanol and isoniazid. Since lysyl hydroxylase 2 is known to be a key enzyme of liver fibrosis, the increased expression of lysyl hydroxylase 2 may be involved in hepatotoxins-related liver fibrosis.

Mechanistic basis of adverse drugreactions: the perils of inappropriate dose schedules

Adverse drug reactions (ADRs) have long been recognised as a significant cause of morbidity and mortality. They account for a substantial number of clinical consultations, hospital admissions and extended duration of in-patient stay as well as mortality. By far the most common ADRs are the concentration-dependent pharmacological reactions, the majority of which ought to be preventable. As a result of high concentrations of the parent drug and/or its metabolite(s), there is an augmentation of primary pharmacological activity and/or appearance of new and undesirable secondary pharmacological activity. Typically, these high concentrations result from administration of high doses in an attempt to maximise efficacy and/or modulation of the pharmacokinetics of a drug by either genetic or non-genetic factors. High plasma concentrations of parent drug may result from inherited impairment or drug-induced inhibition of its pharmacokinetic disposition. Conversely, inherited overcapacity or drug-induced induction of the metabolism of a drug may result in low concentrations of parent drug and frequently, rapid accumulation of its metabolites. Environmental, dietary and phytochemical factors may also influence the activity of drug metabolising enzymes. As with inherited polymorphisms of acetylation and cytochrome P450-based drug metabolising enzymes, polymorphisms of other conjugation reactions, such as glucuronidation, increasingly appear to be associated with drug toxicity. Diseases of organs involved in elimination of a drug also alter its pharmacokinetics, plasma concentration and, therefore, the profile of its concentration-dependent ADRs. Inherited mutations, concurrently administered drugs or presence of certain diseases may also alter the sensitivity of some pharmacological targets, accounting for a substantial number of ADRs and interactions. When there is enhanced pharmacodynamic sensitivity, plasma drug concentrations that are apparently within the normal 'non-toxic' range give rise to ADRs. Recent advances have also provided important insights into the wider scope of drug-drug interactions. Interactions that occur at P-glycoproteins, drug transporters and efflux pumps, at various transmembrane interfaces such as the gastrointestinal wall, renal tubules, hepatobiliary border and blood-brain barrier, are beginning to explain many non-metabolic interactions. These alter the systemic exposure to drugs and have so far, begun to explain unexpected neurotoxicity and hepatotoxicity. The function of these transporters is also genetically modulated. These advances, together with continued increased awareness and education of prescribers and pharmacists, offer great opportunities for substantially minimising concentration-related ADRs.

Pharmacogenetic aspects of drug-induced torsade de pointes: potential tool for improving clinical drug development and prescribing

Drug-induced torsade de pointes (TdP) has proved to be a significant iatro-genic cause of morbidity and mortality and a major reason for the withdrawal of a number of drugs from the market in recent times. Enzymes that metabolise many of these drugs and the potassium channels that are responsible for cardiac repolarisation display genetic polymorphisms. Anecdotal reports have suggested that in many cases of drug-induced TdP, there may be a concealed genetic defect of either these enzymes or the potassium channels, giving rise to either high plasma drug concentrations or diminished cardiac repolarisation reserve, respectively. The presence of either of these genetic defects may predispose a patient to TdP, a potentially fatal adverse reaction, even at therapeutic dosages of QT-prolonging drugs and in the absence of other risk factors. Advances in pharmacogenetics of drug metabolising enzymes and pharmacological targets, together with the prospects of rapid and inexpensive genotyping procedures, promise to individualise and improve the benefit/risk ratio of therapy with drugs that have the potential to cause TdP. The qualitative and the quantitative contributions of these genetic defects in clinical cases of TdP are unclear because not all of the patients with TdP are routinely genotyped and some relevant genetic mutations still remain to be discovered. There are regulatory guidelines that recommend strategies aimed at uncovering the risk of TdP associated with new chemical entities during their development. There are also a number of guidelines that recommend integrating pharmacogenetics in this process. This paper proposes a strategy for integrating pharmacogenetics into drug development programmes to optimise association studies correlating genetic traits and endpoints of clinical interest, namely failure of efficacy or development of repolarisation abnormalities. Until pharmacogenetics is carefully integrated into all phases of development of QT-prolonging drugs and large-scale studies are undertaken during their post-marketing use to determine the genetic components involved in induction of TdP, routine genotyping of patients remains unrealistic. Even without this pharmacogenetic data, the clinical risk of TdP can already be greatly minimised. Clinically, a substantial proportion of cases of TdP are due to the use of either high or usual dosages of drugs with potential to cause TdP in the presence of factors that inhibit drug metabolism. Therefore, choosing the lowest effective dose and identifying patients with these non-genetic risk factors are important means of minimising the risk of TdP. In view of the common secondary pharmacology shared by these drugs, a standard set of contraindications and warnings have evolved over the last decade. These include factors responsible for pharmacokinetic or pharmacodynamic drug interactions. Among the latter, the more important ones are bradycardia, electrolyte imbalance, cardiac disease and co-administration of two or more QT-prolonging drugs. In principle, if large scale prospective studies can demonstrate a substantial genetic component, pharmacogenetically driven prescribing ought to reduce the risk further. However, any potential benefits of pharmacogenetics will be squandered without any reduction in the clinical risk of TdP if physicians do not follow prescribing and monitoring recommendations.

Serum aminotransferase activity as a predictor of clearance of drugs metabolized by CYP isoforms in rats with acute hepatic failure induced by carbon tetrachloride

The values of serum aminotransferase activity (AST) in untreated rats and rats with acute hepatic failure at 24h after an oral administration of CCl(4) (0.5 ml/kg) were 85+/-9 IU/l and 4260+/-620 IU/l (mean+/-S.D., n=6), respectively. The values of total clearance (CL(tot)) after intravenous administration of caffeine, tolbutamide, chlorzoxazone or lidocaine (as probe drugs for various CYP isoforms) to CCl(4)-treated rats were decreased to about 1/8, 1/3, 1/3 or 1/2 compared with those in untreated rats. Good correlations were observed between mRNA expression and enzyme activity of CYP2C11, CYP2E1, CYP3A2 and CYP1A2 in livers of rats given various doses of CCl(4). There was also a good negative correlation between serum AST activity and hepatic enzyme activity of each CYP. The serum AST activities corresponding to a 50% decrease of CYP2C 11, CYP2E1, CYP3A2 and CYP1A2 activities were about 710, 780, 1030 and 1300 IU/l, respectively. In conclusion, when the serum AST value in CCl(4)-treated rats reached about 4000 IU/l, the hepatic CYP activities were one-tenth or less of the control, although the degree of decrease of the CL(tot) values varied markedly. Nevertheless, the AST value appears to be a promising candidate for an indicator to predict appropriate dose modification of drugs for patients with acute hepatic failure.

How useful is the "cocktail approach" for evaluating human hepatic drug metabolizing capacity using cytochrome P450 phenotyping probes in vivo?

Relatively selective in vivo substrate probes have been developed for several major CYP isoforms involved in oxidative drug metabolism. There are basically two in vivo methods for identifying the phenotype. One method, the selective (CYP-specific) phenotyping method, involves administering one single probe drug, whereas the other is a mixed phenotyping or "cocktail" method involving the simultaneous administration of multiple probe drugs, specific for the individual P450. At present, caffeine and chlorzoxazone are used most often as probe drugs for CYP1A2 and CYP2E1, respectively, but these are not necessarily the best probe drugs. Of the potential probe drugs for CYP2C9, CYP2C19, CYP2D6 and CYP3A4, none is really useful. Despite current limitations, the cocktail method for obtaining information about multiple CYP activities in a single experimental session is likely to be more widely used as a screening or phenotyping method for humans in the future.

Detection and analysis of intracytoplasmic cytokines in peripheral blood mononuclear cells in patients with drug-induced liver injury

BACKGROUND/AIMS

Idiosyncratic immune response to drugs causes two types of liver injury, cholestasis or hepatitis. However, the underlying immune mechanisms of drug-induced liver injury are presently unclear.

METHODS

We examined the cytokine production of peripheral blood mononuclear cells (PBMCs) from 17 patients with drug-induced liver injury and healthy controls during their incubation with and without the drug by flow cytometry. We also analyzed the cytokine production in PBMCs from eight patients after stimulation with the drug-pulsed HepG2 lysates to examine the possibility that the drug or its metabolites conjugated with a putative molecule derived from HepG2 cells might be more immunogenic.

RESULTS

Among several cytokines produced by the drug or the drug-pulsed HepG2 lysates, interferon-gamma production from CD8+ cells was associated with hepatocellular injury, and tumor necrosis factor-alpha production from CD14+ cells was with cholestasis. Especially, the latter was apparent when the drug-pulsed HepG2 lysates were used as stimulants, suggesting that a complex consist of the drug, or its metabolite, and a putative molecule derived from HepG2 cells might be more immunogenic than the drug itself.

CONCLUSIONS

The analysis of intracytoplasmic cytokine in PBMCs after stimulation with the drug or the drug-pulsed HepG2 lysates is useful to analyze the immune mechanism underlying drug-induced liver injury.

Relationships between non-occupational cadmium exposure and expression of nine cytochrome P450 forms in human liver and kidney cortex samples

This study was undertaken to assess associations between age, gender, cigarette smoke and non-workplace cadmium exposure, and liver pathology and inter-individual variation in cytochrome P450 (CYP) expression in human tissues. Autopsy specimens of twenty-eight Queensland residents whose ages ranged from 3 to 89 years were analyzed for the presence of nine CYP protein isoforms by immunoblotting. All subjects were Caucasians and their liver cadmium contents ranged from 0.11 to 3.95 microg/g wet weight, while their kidney cadmium contents were in the range of 2 to 63 microg/g wet weight. CYP1A2, CYP2A6, CYP2D6, CYP3A4, and CYP3A5 were detected in liver but not in kidney, and CYP1A1 and CYP1B1 were not found in liver or kidney. Lowered liver CYP2C8/19 protein contents were found to be associated with liver pathology. Importantly, we show elevated levels of CYP2C9 protein to be associated with cadmium accumulation in liver. No mechanism that explains this association is apparent, but there are two possibilities that require further study. One is that variation in CYP2C9 protein levels may be, in part, attributed to an individual's non-workplace exposure to cadmium, or an individual's CYP2C9 genotype may be a risk factor for cadmium accumulation. A positive correlation was found between liver CYP3A4 protein and subject age. Levels of liver CYP1A2 protein, but not other CYP forms, were increased in people more exposed to cigarette smoke, but there was no association between CYP1A2 protein and cadmium. CYP2A6 protein was found in all liver samples and CYP2A6 gene typing indicated the absence of CYP2A6 null allele (CYP2A6(D)) in this sample group, confirming very low prevalence of homozygous CYP2A6(D) in Caucasians. CYP2A6 gene types W/W, W/C, and C/C were not associated with variations in liver microsomal CYP2A6 protein. CYP2D6 protein was absent in all twenty-five kidney samples tested but was detectable in liver samples of all but two subjects, indicating the prevalence of the CYP2D6 null allele (CYP2D6(D)) in this sample group to be about 7%, typical of Caucasian populations.

The clinical importance of conventional and quantitative liver function tests in liver transplantation

The advantages and disadvantages of using monoethylglycinexylidide (MEGX), the major metabolite of lidocaine, as a probe of hepatic function in liver transplantation are reviewed. A 'real time' test of liver function should give a measure of current hepatocellular capacity rather than reflect past damage. The hepatic metabolism of lidocaine to MEGX is the basis of a flow-dependent dynamic test of liver function. In pre-transplantation patients, data from this MEGX test support its role in assessing the risk of morbidity and mortality. In assessing the liver transplant donor, there are differences concerning its apparent usefulness and these must be resolved. In the liver transplant recipient, this MEGX test is also useful for measuring real-time hepatic metabolizing activity, and low MEGX values reflect the clinical condition of the patient. At present, however, this test has several limitations. Therefore, a comprehensive evaluation, not only by the MEGX test but also by a combination of other conventional liver function tests (biochemical parameters, etc.), or with histological evaluation, is thought to be desirable for deciding whether a liver transplantation should be carried out or not.

Cytochrome P450 3A: ontogeny and drug disposition

The maturation of organ systems during fetal life and childhood exerts a profound effect on drug disposition. The maturation of drug-metabolising enzymes is probably the predominant factor accounting for age-associated changes in non-renal drug clearance. The group of drug-metabolising enzymes most studied are the cytochrome P450 (CYP) superfamily. The CYP3A subfamily is the most abundant group of CYP enzymes in the liver and consists of at least 3 isoforms: CYP3A4, 3A5 and 3A7. Many drugs are mainly metabolised by the CYP3A subfamily. Therefore, maturational changes in CYP3A ontogeny may impact on the clinical pharmacokinetics of these drugs. CYP3A4 is the most abundantly expressed CYP and accounts for approximately 30 to 40% of the total CYPcontent in human adult liver and small intestine. CYP3A5 is 83% homologous to CYP3A4, is expressed at a much lower level than CYP3A4 in the liver, but is the main CYP3A isoform in the kidney. CYP3A7 is the major CYP isoform detected in human embryonic, fetal and newborn liver, but is also detected in adult liver, although at a much lower level than CYP3A4. Substrate specificity for the individual isoforms has not been fully elucidated. Because of large interindividual differences in CYP3A4 and 3A5 expression and activity, genetic polymorphisms have been suggested. However, although some gene mutations have been identified, the impact of these mutations on the pharmacokinetics of CYP3A substrates has to be established. Ontogeny of CYP3A activity has been studied in vitro and in vivo. CYP3A7 activity is high during embryonic and fetal life and decreases rapidly during the first week of life. Conversely, CYP3A4 is very low before birth but increases rapidly thereafter, reaching 50% of adult levels between 6 and 12 months of age. During infancy, CYP3A4 activity appears to be slightly higher than that of adults. Large interindividual variations in CYP3A5 expression and activity were observed during all stages of development, but no apparent developmental pattern of CYP3A5 activity has been identified to date. Profound changes occur in the activity of CYP3A isoforms during all stages of development. These changes have, in many instances, proven to be of clinical significance when treatment involves drugs that are substrates, inhibitors or inducers of CYP3A. Investigators and clinicians should consider the impact of ontogeny on CYP3A in both pharmacokinetic study design and data interpretation, as well as when prescribing drugs to children.

Effects of liver disease on pharmacokinetics. An update

Liver disease can modify the kinetics of drugs biotransformed by the liver. This review updates recent developments in this field, with particular emphasis on cytochrome P450 (CYP). CYP is a rapidly expanding area in clinical pharmacology. The information currently available on specific isoforms involved in drug metabolism has increased tremendously over the latest years, but knowledge remains incomplete. Studies on the effects of liver disease on specific isoenzymes of CYP have shown that some isoforms are more susceptible than others to liver disease. A detailed knowledge of the particular isoenzyme involved in the metabolism of a drug and the impact of liver disease on that enzyme can provide a rational basis for dosage adjustment in patients with hepatic impairment. The capacity of the liver to metabolise drugs depends on hepatic blood flow and liver enzyme activity, both of which can be affected by liver disease. In addition, liver failure can influence the binding of a drug to plasma proteins. These changes can occur alone or in combination; when they coexist their effect on drug kinetics is synergistic, not simply additive. The kinetics of drugs with a low hepatic extraction are sensitive to hepatic failure rather than to liver blood flow changes, but drugs having a significant first-pass effect are sensitive to alterations in hepatic blood flow. The drugs examined in this review are: cardiovascular agents (angiotensin converting enzyme inhibitors, angiotensin II receptor antagonists, calcium antagonists, ketanserin, antiarrhythmics and hypolipidaemics), diuretics (torasemide), psychoactive and anticonvulsant agents (benzodiazepines, flumazenil, antidepressants and tiagabine), antiemetics (metoclopramide and serotonin antagonists), antiulcers (acid pump inhibitors), anti-infectives and antiretroviral agents (grepafloxacin, ornidazole, pefloxacin, stavudine and zidovudine), immunosuppressants (cyclosporin and tacrolimus), naltrexone, tolcapone and toremifene. According to the available data, the kinetics of many drugs are altered by liver disease to an extent that requires dosage adjustment; the problem is to quantify the required changes. Obviously, this requires the evaluation of the degree of hepatic impairment. At present there is no satisfactory test that gives a quantitative measure of liver function and its impairment. A critical evaluation of these methods is provided. Guidelines providing a rational basis for dosage adjustment are illustrated. Finally, it is important to consider that liver disease not only affects pharmacokinetics but also pharmacodynamics. This review also examines drugs with altered pharmacodynamics.