Optimizing bacterial expression of catalytically active human cytochromes P450: comparison of CYP2C8 and CYP2C9

1. Methods for the co-expression in Escherichia coli of human cytochrome P450 (CYP) 2C8 and CYP2C9 with NADPH-cytochrome P450 reductase (OxR) to produce a catalytically active system were compared. 2. Approaches assessed were expression of a CYP:OxR fusion construct, bicistronic plasmids, simultaneous transformation with CYP and OxR plasmids, and separate expression of CYP and OxR with reconstitution of activity by mixing the bacterial membranes. Two N-terminal modifications (Delta3-20 and 17alpha-leader) of the individual P450s were additionally investigated. 3. Each approach gave efficient expression of CYP2C8 and CYP2C9, but the bicistronic constructs under the expression conditions used gave low OxR expression and low catalytic activity. CYP expression was higher with the Delta3-20 construct for CYP2C9 and with the 17alpha-presequence construct for CYP2C8. 4. Using torsemide as substrate, all methods gave catalytically active systems with K(m) values similar to human liver microsomes. Mixing bacterial membranes containing separately expressed CYP and OxR reconstituted a catalytically active system with the Delta3-20 construct for CYP2C9 but not for CYP2C8, and with neither of the 17alpha- presequence constructs. OxR co-expressed with CYP in the same membrane interacted with CYP to reconstitute activity more effectively than addition of exogenous OxR membranes. 5. Expression construct and OxR co-expression strategy should be individualized for CYP isoforms.

Polymorphic hydroxylation of perhexiline in vitro

AIMS

The aims of this study were to examine the in vitro enzyme kinetics and CYP isoform selectivity of perhexiline monohydroxylation using human liver microsomes.

METHODS

Conversion of rac-perhexiline to monohydroxyperhexiline by human liver microsomes was assessed using a high-performance liquid chromatography assay with precolumn derivatization to measure the formation rate of the product. Isoform selective inhibitors were used to define the CYP isoform profile of perhexiline monohydroxylation.

RESULTS

The rate of perhexiline monohydroxylation with microsomes from 20 livers varied 50-fold. The activity in 18 phenotypic perhexiline extensive metabolizer (PEM) livers varied about five-fold. The apparent Km was 3.3 +/- 1.5 micro m, the Vmax was 9.1 +/- 3.1 pmol min-1 mg-1 microsomal protein and the in vitro intrinsic clearance (Vmax/Km) was 2.9 +/- 0.5 micro l min-1 mg-1 microsomal protein in the extensive metabolizer livers. The corresponding values in the poor metabolizer livers were: apparent Km 124 +/- 141 micro m; Vmax 1.4 +/- 0.6 pmol min-1 mg-1 microsomal protein; and intrinsic clearance 0.026 micro l min-1 mg-1 microsomal protein. Quinidine almost completely inhibited perhexiline monohydroxylation activity, but inhibitors selective for other CYP isoforms had little effect.

CONCLUSIONS

Perhexiline monohydroxylation is almost exclusively catalysed by CYP2D6 with activities being about 100-fold lower in CYP2D6 poor metabolizers than in extensive metabolizers. The in vitro data predict the in vivo saturable metabolism and pharmacogenetics of perhexiline.

Impact of the Minimum Pricing Policy and introduction of brand (generic) substitution into the Pharmaceutical Benefits Scheme in Australia

PURPOSE

To describe the effects of introducing the Minimum Pricing Policy (MPP) and generic (brand) substitution in 1990 and 1994 respectively on the dispensing of Pharmaceutical Benefits Scheme (PBS) prescriptions both at the aggregate and individual patient level.

METHODS

The relative proportion of prescriptions with a brand premium and those at benchmark was examined 4 years after introduction of the MPP and again 5 years later after generic substitution by pharmacists was permitted. To determine the impact of a price signal at the individual level, case studies involving a patient tracking methodology were conducted on two drugs (fluoxetine and ranitidine) that received a brand premium.

RESULTS

From a zero base when the MPP was introduced in 1990, there were 5.4 million prescriptions (17%) dispensed for benchmark products 4 years later in 1994. At this stage generic (brand) substitution by pharmacists was then permitted and the market share of benchmark brands increased to 45% (25.2 million) by 1999. In the patient tracking studies, a significantly lower proportion of patients was still taking the premium brand of fluoxetine 3 months after the introduction of a price signal compared with patients taking paroxetine which did not have a generic competitor. This was also the case for the premium brand of ranitidine when compared to famotidine. The size of the price signal also had a marked effect on dispensing behaviour with the drug with the larger premium (fluoxetine) showing a significantly greater switch away from the premium brand to the benchmark product.

CONCLUSIONS

The introduction in 1990 of the Minimum Pricing Policy without allowing generic substitution had a relatively small impact on the selection of medicines within the Pharmaceutical Benefits Scheme. However the effect of generic substitution at the pharmacist level, which was introduced in December 1994, resulted in a marked increase in the percentage of eligible PBS items dispensed at benchmark. Case studies showed a larger premium resulted in a greater shift of patients from drugs with a brand premium to the benchmark alternative.

A cost-effectiveness approach to drug subsidy and pricing in Australia

The Australian government offers its citizens subsidies on a select list of pharmaceuticals. For a drug to qualify for inclusion on this list, its manufacturer must demonstrate that the drug is both clinically effective and cost-effective. In part, this measure, along with others, was introduced to improve clinical and economic outcomes. Although this evidence-based system has provided transparency and consistency in decision making about which drugs will be covered, it may not have contained the rate of increase in drug costs.

The xenobiotic inhibitor profile of cytochrome P4502C8

AIMS

To investigate inhibition of recombinant CYP2C8 by: (i) prototypic CYP isoform selective inhibitors (ii) imidazole/triazole antifungal agents (known inhibitors of CYP), and (iii) certain CYP3A substrates (given the apparent overlapping substrate specificity of CYP2C8 and CYP3A).

METHODS

CYP2C8 and NADPH-cytochrome P450 oxidoreductase were coexpressed in Spodoptera frugiperda (Sf21) cells using the baculovirus expression system. CYP isoform selective inhibitors, imidazole/triazole antifungal agents and CYP3A substrates were screened for their inhibitory effects on CYP2C8-catalysed torsemide tolylmethylhydroxylation and, where appropriate, the kinetics of inhibition were characterized. The conversion of torsemide to its tolylmethylhydroxy metabolite was measured using an h.p.l.c. procedure.

RESULTS

At concentrations of the CYP inhibitor 'probes' employed for isoform selectivity, only diethyldithiocarbamate and ketoconazole inhibited CYP2C8 by > 10%. Ketoconazole, at an added concentration of 10 microM, inhibited CYP2C8 by 89%. Another imidazole, clotrimazole, also potently inhibited CYP2C8. Ketoconazole and clotrimazole were both noncompetitive inhibitors of CYP2C8 with apparent Ki values of 2.5 microM. The CYP3A substrates amitriptyline, quinine, terfenadine and triazolam caused near complete inhibition (82-91% of control activity) of CYP2C8 at concentrations five-fold higher than the known CYP3A Km. Kinetic studies with selected CYP3A substrates demonstrated that most inhibited CYP2C8 noncompetitively. Apparent Ki values for midazolam, quinine, terfenadine and triazolam ranged from 5 to 25 microM.

CONCLUSIONS

Inhibition of CYP2C8 occurred at concentrations of ketoconazole and diethyldithiocarbamate normally employed for selective inhibition of CYP3A and CYP2E1, respectively. Some CYP3A substrates have the capacity to inhibit CYP2C8 activity and this may have implications for inhibitory drug interactions in vivo.

Nonspecific binding of drugs to human liver microsomes

AIMS

To characterize the nonspecific binding to human liver microsomes of drugs with varying physicochemical characteristics, and to develop a model for the effect of nonspecific binding on the in vitro kinetics of drug metabolism enzymes.

METHODS

The extent of nonspecific binding to human liver microsomes of the acidic drugs caffeine, naproxen, tolbutamide and phenytoin, and of the basic drugs amiodarone, amitriptyline and nortriptyline was investigated. These drugs were chosen for study on the basis of their lipophilicity, charge, and extent of ionization at pH 7.4. The fraction of drug unbound in the microsomal mixture, fu(mic), was determined by equilibrium dialysis against 0.1 M phosphate buffer, pH 7.4. The data were fitted to a standard saturable binding model defined by the binding affinity KD, and the maximum binding capacity Bmax. The derived binding parameters, KD and Bmax, were used to simulate the effects of saturable nonspecific binding on in vitro enzyme kinetics.

RESULTS

The acidic drugs caffeine, tolbutamide and naproxen did not bind appreciably to the microsomal membrane. Phenytoin, a lipophilic weak acid which is mainly unionized at pH 7. 4, was bound to a small extent (fu(mic) = 0.88) and the binding did not depend on drug concentration over the range used. The three weak bases amiodarone, amitriptyline and nortriptyline all bound extensively to the microsomal membrane. The binding was saturable for nortriptyline and amitriptyline. Bmax and KD values for nortriptyline at 1 mg ml-1 microsomal protein were 382 +/- 54 microM and 147 +/- 44 microM, respectively, and for amitriptyline were 375 +/- 23 microM and 178 +/- 33 microM, respectively. Bmax, but not KD, varied approximately proportionately with the microsome concentration. When KD is much less than the Km for a reaction, the apparent Km based on total drug can be corrected by multiplying by fu(mic). When the substrate concentration used in a kinetic study is similar to or greater than the KD (Km >/= KD), simulations predict complex effects on the reaction kinetics. When expressed in terms of total drug concentrations, sigmoidal reaction velocity vs substrate concentration plots and curved Eadie Hofstee plots are predicted.

CONCLUSIONS

Nonspecific drug binding in microsomal incubation mixtures can be qualitatively predicted from the physicochemical characteristics of the drug substrate. The binding of lipophilic weak bases is saturable and can be described by a standard binding model. If the substrate concentrations used for in vitro kinetic studies are in the saturable binding range, complex effects are predicted on the reaction kinetics when expressed in terms of total (added) drug concentration. Sigmoidal reaction curves result which are similar to the Hill plots seen with cooperative substrate binding.

Compliance with restrictions on the subsidized use of proton pump inhibitors in Australia

AIMS

To determine among a cohort of patients newly dispensed a prescription for a proton pump inhibitor (PPI) the extent of prior use of other less expensive agents such as antacids and H2-receptor antagonists as evidence of a 'stepped care' approach to peptic ulcer and oesophageal disease.

METHODS

A retrospective drug utilization study was conducted within the Pharmaceutical Benefits Scheme (PBS) claims database in Australia. A cohort of social security recipients, who received approval for PPI supply in the month of October 1996, had no prior PPI approval in the previous 18 months and went on to have the drug dispensed, was assembled. This group of 'new PPI starters' was then examined for supply of less expensive prescription medicines to treat peptic ulcer and oesophageal disease in the 12 months prior to obtaining their PPI approval.

RESULTS

In a cohort of 4554 defined new PPI users, 1205 (26.5%) showed no use of H2-receptor antagonists, antacids, cisapride, cytoprotectants or antiregurgitants in the 12 month period prior to commencing the PPI. The major reason for use given by prescribers for PBS supply was 'severe refractory ulcerating oesophagitis'.

CONCLUSIONS

Subsidized supply is currently restricted on cost-effectiveness grounds to refractory peptic ulcer disease or severe oesophageal disease. Despite this, utilization and epidemiological data suggest that there is widespread leakage of use outside these indications particularly to less severe forms of oesophageal disease. This patient tracking study has shown within the PBS database that around a quarter of the patients are treated directly with a PPI without being prescribed less expensive agents at least in the preceding 12 months.

Cytochrome P4502C9: an enzyme of major importance in human drug metabolism

Accumulating evidence indicates that CYP2C9 ranks amongst the most important drug metabolizing enzymes in humans. Substrates for CYP2C9 include fluoxetine, losartan, phenytoin, tolbutamide, torsemide, S-warfarin, and numerous NSAIDs. CYP2C9 activity in vivo is inducible by rifampicin. Evidence suggests that CYP2C9 substrates may also be induced variably by carbamazepine, ethanol and phenobarbitone. Apart from the mutual competitive inhibition which may occur between alternate substrates, numerous other drugs have been shown to inhibit CYP2C9 activity in vivo and/or in vitro. Clinically significant inhibition may occur with coadministration of amiodarone, fluconazole, phenylbutazone, sulphinpyrazone, sulphaphenazole and certain other sulphonamides. Polymorphisms in the coding region of the CYP2C9 gene produce variants at amino acid residues 144 (Arg144Cys) and 359 (Ile359Leu) of the CYP2C9 protein. Individuals homozygous for Leu359 have markedly diminished metabolic capacities for most CYP2C9 substrates, although the frequency of this allele is relatively low. Consistent with the modulation of enzyme activity by genetic and other factors, wide interindividual variability occurs in the elimination and/or dosage requirements of prototypic CYP2C9 substrates. Individualisation of dose is essential for those CYP2C9 substrates with a narrow therapeutic index.

Inhibition of human hepatic cytochrome P4502E1 by azole antifungals, CNS-active drugs and non-steroidal anti-inflammatory agents

1. The capacity of a number of antifungal azoles, CNS-active drugs (anticonvulsants, antidepressants, antipsychotics and benzodiazepine hypnosedative-anxiolytics) and nonsteroidal anti-inflammatory agents (NSAIDs) to inhibit human liver microsomal 4-nitrophenol (4NP) hydroxylation, a marker of CYP2E1 activity, was investigated. 2. The imidazoles bifonazole, clotrimazole, econazole and miconazole were un- or non-competitive inhibitors of 4NP hydroxylation, with apparent Ki values ranging from 4 to 25 microM. Fluonazole, itraconazole and ketoconazole caused minor or negligible inhibition. 3. Of the CNS-active drugs screened, significant inhibition occurred only with tricyclic antidepressants, phenothiazine antipsychotics and two benzodiazepines (flurazepam and medazepam). Un- or non-competitive inhibition was similarly observed for the tricyclic antidepressants, phenothiazines, flurazepam and medazepam, with apparent Ki values ranging from 175 to 1000 microM. 4. Diclofenac and flufenamic acid were the only NSAIDs found to inhibit 4NP hydroxylation substantially; kinetic analysis was suggestive of activation-inhibition phenomena. 5. These data indicate that, although not substrates for CYP2E1, some clinically used drugs have the capacity to inhibit this enzyme and hence have the potential to modulate the toxicity of non-drug xenobiotics metabolized by CYP2E1.

Localisation of aryl sulfotransferase expression in human tissues using hybridisation histochemistry and immunohistochemistry

To date, the laboratory has cloned seven unique human sulfotransferases; five aryl sulfotransferases (HAST1, HAST2, HAST3, HAST4 and HAST4v), an estrogen sulfotransferase and a dehydroepiandrosterone sulfotransferase. The cellular distribution of human aryl sulfotransferases in human hepatic and extrahepatic tissues has been determined using the techniques of hybridization histochemistry and immunohistochemistry. Human aryl sulfotransferase expression was detected in liver, epithelial cells of the gastrointestinal mucosal layer, epithelial cells lining bronchioles and in mammary duct epithelial cells.

Baculovirus-mediated expression of cytochrome P4502C8 and human NADPH-cytochrome P450 reductase: optimization of protein expression

1. High expression levels of cytochrome P450 (CYP) 2C8 and NADPH-cytochrome P450 oxidoreductase (OxR) in Spodoptera frugiperda (Sf21) cells have been achieved using the baculovirus expression system. 2. The baculovirus dual expression plasmid, pAcUW31, was used to insert CYP2C8 and OxR cDNAs downstream of the polyhedrin (polh) or p10 promoters, either separately or together, generating four recombinant baculoviruses; two expressing single proteins (CYP2C8 driven by the p10 promoter, bVp10.2C8 or OxR driven by the polh promoter, bVpolh.OxR) with another two coexpressing both CYP2C8 and OxR under reciprocal control of the polh and p10 promoters (bVpolh.OxR-p10.2C8 and bVpolh.2C8-p10.OxR). 3. High levels of singly expressed CYP2C8 and OxR were achieved from bVp10.2C8 and bVpolh.OxR, with levels of 0.7-1.2 nmol CYP/mg protein and 400-500 nmol cytochrome c reduced/min/mg protein respectively. 4. The two dual gene clones (bVpolh.OxR-p10.2C8 and bVpolh.2C8-p10.OxR) showed, in general, greater variation in CYP content and OxR activity than single gene clones. Screening was therefore necessary for the selection of dual gene clones expressing both proteins optimally. 5. Sf21 microsomes infected by selected dual gene clones were, on average, 14 times more active in tolbutamide hydroxylase activity than those expressing CYP2C8 alone, with a mean spectral CYP content of 79 pmol/mg cell lysate protein and a mean OxR level of 600 nmol/min/mg cell lysate protein.

Evidence for involvement of human CYP3A in the 3-hydroxylation of quinine

AIMS

Our previous studies using in vitro hepatic microsomal preparations suggested that the hepatic metabolism of quinine to form the major metabolite 3-hydroxyquinine is most likely catalysed by human P450 3A (CYP3A). The present study was carried out to investigate the kinetics and to identify and further characterise the human liver CYP isoforms involved in the metabolism of quinine.

METHODS

In vitro human microsomal techniques were employed.

RESULTS

The mean apparent Km value for 3-hydroxyquinine formation was 83 +/- 19 (s.d.) microM, ranging from 57 microM to 123 microM in microsomes from ten human livers. There was a 6.7-fold variation in Vmax values (mean 547 +/- 416 pmol min-1 mg-1). Quinine 3-hydroxylation was inhibited by the specific CYP3A inhibitors, troleandomycin, midazolam and erythromycin. Inhibitors selective for CYP1A1/2, CYP2D6, CYP2E1, CYP2C9/10 or CYP2C19 had little or no effect on quinine 3-hydroxylation. Using microsomes from a panel of livers, significant correlations were found only between 3-hydroxyquinine activity and other CYP3A activities (caffeine 8-oxidation, omeprazole sulphoxidation, midazolam 1'-hydroxylation and midazolam 4-hydroxylation) and immunoreactive CYP3A content. There were no statistically significant correlation with activities selective for CYP1A2, CYP2C9 and CYP2E1. Competitive inhibition of quinine 3-hydroxylation was observed with a substrate known to be specifically metabolized by human CYP3A, i.e. midazolam, with an apparent Ki value of 11.0 microM.

CONCLUSIONS

The present results strongly indicate that the conversion of quinine to 3-hydroxyquinine is the major metabolic pathway in human liver in vitro and that the reaction is catalysed by CYP3A isoforms.

1-Methylxanthine derived from caffeine as a pharmacodynamic probe of oxypurinol effect

AIMS

In the present study we have investigated the use of caffeine, administered in the form of instant coffee, as a prodrug for 1MX to validate the use of the 1MU:1MX ratio following caffeine administration as a pharmacodynamic measure of oxypurinol effect on xanthine oxidase.

METHODS

Five healthy volunteers took caffeine 75 mg 8 hourly administered as instant coffee over a 7 day period. They were given allopurinol 600 mg on day 4. Urine was collected in 8 h aliquots from day 1-day 7. The ratio of 1-methyluric acid (1MU) to 1-methylxanthuric (1MX) was determined.

RESULTS

The relationship between the plasma oxypurinol (the active metabolite of allopurinol) concentration at the midpoint of each caffeine dosage interval and the decrement in the urinary 1MX to 1MU ratio fitted well by a sigmoid Emax model. Mean (+/-s.d.) values of the oxypurinol EC50(3.9 +/- 1.4 mg l-1), EC90(8.7 +/- 1.8 mgl-1) and the exponent, n (3.0 +/- 1.2) were similar to those obtained previously following either the direct administration of 1MX or the use of theophylline as a prodrug for 1MX.

CONCLUSIONS

These data indicate that the use of caffeine as a source of 1MX could provide a simple and ethically acceptable method for monitoring oxypurinol effect in patients taking allopurinol for the treatment of gout.

Allelic and functional variability of cytochrome P4502C9

Single nucleotide substitutions are known to result in a different amino acid at one of four sites in cytochrome P4502C9 (CYP2C9) namely: residue 144: Arg/Cys; residue 358: Tyr/Cys; residue 359: Ile/Leu and residue 417: Gly/Asp. Polymerase chain reaction (PCR)-based amplification of the nucleotide fragments encompassing the four residues (144, 358-359 and 417) in 18 samples of human genomic DNA from a liver bank and one sample of DNA extracted from the blood of a known poor metabolizer of tolbutamide has been carried out. The products of PCR amplification were analysed by either allele-specific restriction endonucleases or probed with radioactively labelled allele-specific oligonucleotides in dot blot hybridizations. Fourteen individuals were homozygous for Arg144 and four were heterozygous Arg/Cys144. All individuals analysed were homozygous for Tyr358 (n = 17) and for Gly417 (n = 18). With the exception of one heterozygote the other 17 subjects were homozygous for Ile359. The genotype of the known poor metabolizer of tolbutamide was homozygous for Arg144, Leu359 and Gly417. The relative levels of expression of the Cys and Arg144 alleles was studied in the heterozygotes. A relative 5- to 10-fold greater expression of the Cys- over the Arg144 allele was noted in two heterozygotes. There was no apparent correlation of genotype to the hydroxylation of the known CYP2C9 substrates phenytoin, tolbutamide, torasemide and diclofenac. Apparent K(m) values for the cDNA-expressed Arg144/Ile359, Cys144/ Ile359 and Arg144/Leu359 variants towards tolbutamide were 91 microM, 62 microM and 229 microM, respectively. It is likely that functional changes occurring as a result of the Ile359Leu transition are responsible for the tolbutamide poor metabolizer phenotype.

Limited value of the urinary phenytoin metabolic ratio for the assessment of cytochrome P4502C9 activity in vivo

Relationships between the ratio of p-hydroxyphenytoin (p-HPPH), the major metabolite of phenytoin, to unchanged phenytoin excreted in urine (the urinary metabolic ratio or MR) were compared with a number of indices of the metabolic clearances of phenytoin and tolbutamide published previously for seventeen subjects separately administered these known cytochrome P4502C9 (CYP2C9) substrates. Significant correlations (rs = 0.50-0.60, P < 0.05) were observed between the phenytoin MR, derived from either 0-24 or 24-48 h urine collections, and inverse areas under the plasma unbound concentration-time curves (measured over various time intervals) of phenytoin and with plasma unbound tolbutamide clearance. Significant correlations (rs = 0.59-0.74) were also observed between the phenytoin MRs and metabolic unbound clearances for p-hydroxyphenytoin formation. Despite the significant correlations, variability in tolbutamide and phenytoin metabolic clearance parameters tended to account for < 50% of the variability in phenytoin MR. Correlations between the renal clearance of phenytoin and the phenytoin MRs suggest that variability in the renal clearance of unchanged drug limits the usefulness of the phenytoin MR for the investigation of factors influencing CYP2C9 activity in vivo.

The role of the CYP2C9-Leu359 allelic variant in the tolbutamide polymorphism

Tolbutamide undergoes hydroxylation in humans via a cytochrome P450-mediated pathway. The primary P450 isozyme responsible for this metabolism is thought to be CYP2C9. Population studies have indicated the existence of slow metabolizers of tolbutamide (approximately 1 in 500) suggesting a rare polymorphism associated with 2C9. Several allelic variants of 2C9 have been identified; however, the effect of these allelic variations on metabolism in vivo is not established. In the present study, the coding regions, intron-exon junctions, and upstream region of CYP2C9 were amplified by PCR and sequenced in two slow metabolizers. One individual was homozygous for Leu359/Leu359 and the other individual was heterozygous for Arg144/Cys144 and for Ile359/Leu359. No other genetic variations in 2C9 were detected in these individuals. PCR-RFLP tests showed that Arg144 Tyr358 Ile359 Gly417 is the principle CYP2C9 allele. Frequencies of the rarer Leu359 and Cys144 alleles were 0.06 and 0.08, respectively, in a Caucasian-American population and 0.005 and 0.01 respectively in African-Americans. The frequency of the Leu359 allele was 0.026 in Chinese-Taiwanese, but the Cys144 allele was not detected in this population. Studies in a recombinant yeast expression system showed that the Leu359 variant had the highest Km and the lowest Vmac for hydroxylation of tolbutamide of all the CYP2C9 allelic variants. This allelic variant also had the highest Km for the 7-hydroxylation of S-warfarin. The present data suggest that the incidence of the Leu359 allelic variant of CYP2C9 may account for the occurrence of poor metabolizers of tolbutamide.

Pattern of non-steroidal anti-inflammatory drug use in Australia 1990-1994. A report from the Drug Utilization Sub-Committee of the Pharmaceutical Benefits Advisory Committee

OBJECTIVE

To determine the pattern of use of non-steroidal anti-inflammatory drugs (NSAIDs) in the Australian community, 1990-1994.

DESIGN

Data from the national drug utilisation database were expressed in defined daily doses per 1000 population per day (DDDs/1000 population per day). Temporal trends were assessed and comparisons were made with NSAID use in other countries. Epidemiological data were used to estimate the likely impact of changing NSAID use on peptic ulcer hospitalisation rates.

SETTING

Australian community (excluding hospitals).

MAIN OUTCOME MEASURES

Estimated consumption of prescription NSAIDs, expressed in DDDs/1000 population per day.

RESULTS

NSAID use in the Australian community fell from 50.1 DDDs/1000 population per day in 1990 to 34.6 DDDs/1000 population per day in 1994 (down 31%). From this reduced exposure we estimated that the number of admissions for NSAID-related upper gastrointestinal complications will have fallen by about 400 per year. Market research data for this period show a lower percentage use of NSAIDs for osteoarthritis and a decrease in the proportion of use in age groups over 50 years.

CONCLUSIONS

The level of use of non-steroidal anti-inflammatory drugs in Australia has been high in comparison with other countries, but in recent years has fallen markedly. This fall occurred in conjunction with regulatory interventions, educational campaigns and increased concern in the medical and lay press regarding the risks associated with the use of NSAIDs.

Cytochromes P450, 1A2, and 2C9 are responsible for the human hepatic O-demethylation of R- and S-naproxen

A preliminary report implicated cytochrome P450 (CYP) 2C9 in the human liver microsomal O-demethylation of S-naproxen, suggesting that this pathway may be suitable for investigation of human hepatic CYP2C9 in vitro. Kinetic and inhibitor studies with human liver microsomes and confirmatory investigations with cDNA-expressed enzymes were undertaken here to define the role of CYP2C9 and other isoforms in the O-demethylation of R- and S-naproxen. All studies utilised a newly developed sensitive and specific HPLC assay that measured the respective O-desmethyl metabolites of R- and S-naproxen in incubations of human liver microsomes and in COS cell lysates. Microsomal R- and S-naproxen O-demethylation kinetics followed Michaelis-Menten kinetics, with respective mean apparent Km values of 123 microM and 143 microM. Sulfaphenazole, a specific inhibitor of CYP2C9, reduced the microsomal O-demethylation of R- and S-naproxen by 43% and 47%, respectively, and the CYP1A2 inhibitor furafylline decreased R- and S-naproxen O-demethylation by 38% and 28%, respectively. R,S-Mephenytoin was a weak inhibitor of R- and S-naproxen O-demethylation, but other CYP isoform specific inhibitors (e.g., coumarin, diethyldithiocarbamate, quinidine, troleandomycin) had little or no effect on these reactions. cDNA-expressed CYP2C9 and CYP1A2 were both shown to O-demethylate R- and S-naproxen. Apparent Km values (92-156 microM) for the reactions catalysed by the recombinant enzymes were similar to those observed for human liver microsomal R- and S-naproxen O-demethylation. The data demonstrate that CYP2C9 and CYP1A2 together account for the majority of human liver R- and S-naproxen O-demethylation, precluding the use of either R- or S-naproxen as a CYP isoform-specific substrate in vitro and in vivo.

Pharmacodynamics of oxypurinol after administration of allopurinol to healthy subjects

1. Eight healthy subjects received 50, 100, 300, 600 and 900 mg allopurinol daily for 1 week each, in random order with 1 week separating each treatment period. The pre-dose plasma concentration of oxypurinol, the extent of inhibition of xanthine oxidase, plasma urate concentration and urine urate excretion rate were assessed on the last 2 days of each treatment week. 2. The ratio of 1-methyluric acid (1MU) over 1-methylxanthine (1MX) in the urine, following a dose of 50 mg 1MX infused intravenously over 20 min, was used to measure the inhibition of xanthine oxidase. 3. The steady-state plasma concentration of oxypurinol increased linearly with increasing dose of allopurinol between 50 mg to 600 mg day-1, with a weak indication of saturation at the higher 900 mg day-1 dose rate. 4. The relationships between plasma oxypurinol concentration and xanthine oxidase inhibition (1MU/1MX ratio), plasma urate concentration and urine urate excretion rate were fitted to an inhibition sigmoid Emax model and the C50 values for oxypurinol were 26.38 +/- 4.87, (mean +/- s.d.) 36.58 +/- 8.36 and 24.61 +/- 9.08 microM, respectively. 5. 1MU/1MX ratio appeared to be a reliable index of xanthine oxidase activity in vivo as the C50 for oxypurinol observed for 1MU/1MX ratio, plasma urate concentration and urine urate excretion rate were similar. 6. The concentration of oxypurinol required for inhibition of xanthine oxidase, as indicated by C50, was lower than those often observed in clinical practice.

The use of caffeine as a metabolic probe for human drug metabolizing enzymes

1. Caffeine (CA) is metabolized extensively and at least 17 metabolites arising from primary and secondary biotransformation pathways are found in urine following CA ingestion. The enzymes responsible for the formation of most of the metabolites derived from CA have been identified. 2. Given the near ubiquitous consumption of CA, this compound potentially constitutes a useful substrate probe for assessment of certain xenobiotic metabolizing enzyme activities in vivo. Indeed, various ratios of CA metabolites excreted in urine (urinary metabolic ratios; MRs) are now utilized widely for the population screening of enzyme activities. 3. Excretion of the acetylated secondary metabolite 5-actylamino-6-formylamino-3-methyluracil (AFMU) is dependent on the activity of the polymorphic N-acetyltransferase (NAT2), and certain MRs incorporating AFMU may be used for NAT2 phenotyping. 4. The conversion of 1-methylxanthine (1-MX), another secondary metabolite of CA, to 1-methyluric acid (1-MU) is catalyzed by xanthine oxidase (XO), and the urinary 1-MU to 1MX ratio reflects XO activity. 5. N3-demethylation to form paraxanthine (PX), a reaction mediated by cytochrome P4501A2 (CYP1A2), is the dominant primary metabolic pathway of CA. CA N3-demethylation activity may be used as a measure of human hepatic CYP1A2 in vitro. 6. Plasma CA clearance is considered to reflect CYP1A2 activity in vivo. Although a number of MRs are based on the excretion of PX metabolites (PX derived from CA is employed for the assessment of CYP1A2 activity in vivo), factors other than enzyme activity may affect these ratios.

Human cytochrome P450 isoform specificity in the regioselective metabolism of toluene and o-, m- and p-xylene

The conversion of toluene and o-, m- and p-xylene to their respective side-chain and ring monohydroxylated metabolites by human liver microsomes was investigated. Methyl hydroxylation, to form a benzylalcohol, was the major metabolic pathway for all four methylbenzenes. With the exception of 2,4-dimethylphenol formation from m-xylene, ring hydroxylation accounted for < 5% of total metabolite formation. However, regioselectivity of ring hydroxylation was apparent, with hydroxylation occurring only at positions ortho and/or para to a methyl substituent. Toluene and each xylene isomer exhibited biphasic methylhydroxylation kinetics in human liver microsomes. The high-affinity component of each methylhydroxylation was selectively inhibited by diethyldithiocarbamate and correlated significantly with cytochrome P-4502E1 (CYP2E1) content and activities in a panel of human liver microsomes. cDNA-expressed CYP2E1 was shown to catalyze the formation of each benzylalcohol, with apparent Km values similar to those of the high affinity microsomal reactions. In contrast, the conversion of m-xylene to 2,4-dimethylphenol followed single enzyme Michaelis-Menten kinetics, was inhibited selectively by furafylline, and correlated significantly with known CYP1A2 catalyzed reactions. cDNA-expressed CYP1A2 converted m-xylene to 2,4-dimethylphenol, with an apparent Km similar to that of the microsomal reaction. Although CYP1A2 appears to be responsible for the formation of the minor (phenolic) metabolites of toluene and the xylene isomers, CYP2E1 catalyzed methylhydroxylation will be the major determinant of the clearance of these compounds in humans.

Modelling the market uptake of new drugs following listing for subsidy in Australia. A report from the Drug Utilisation Subcommittee of the Australian Pharmaceutical Benefits Advisory Committee

The market uptake of five drugs following subsidy listing in Australia during the period 1990 to 1992 has been modelled using the sigmoid Emax model for drug-receptor binding. Utilisation trends for simvastatin, omeprazole, budesonide, fluoxetine and moclobemide in defined daily dose (DDD) per 1000 population per day were smoothed by expressing as rolling annual averages. The results indicate good fits of the model to the data except for omeprazole, with good estimates of uptake rate and eventual maximum utilisation. Substantial differences between the drugs occurred in uptake rate which may be related to public education campaigns on asthma and coronary heart disease occurring during the release period. The very slow uptake of omeprazole relative to other drugs is likely to be due to restrictions on subsidised use. Modelling the market uptake rate and eventual utilisation of new drugs is useful as an aid to regulatory, quality use of medicines and financial decisions and allows comparisons between drugs to investigate factors important in market uptake.

Human hepatic cytochrome P450 2C9 catalyzes the rate-limiting pathway of torsemide metabolism

Tolyl methylhydroxylation is the rate-limiting step in the elimination of torsemide, a newly developed diuretic, in humans in vivo. Kinetic and inhibitor studies with human liver microsomes and complementary DNA-expressed enzyme were performed to identify the cytochrome P450 (CYP) isoform responsible for torsemide tolyl methylhydroxylation to predict factors that might alter clearance in patients receiving torsemide. As in vivo, tolyl methylhydroxylation was the major biotransformation pathway in human liver microsomes. Microsomal tolyl methyl-hydroxylation kinetics followed Michaelis-Menten kinetics, with the mean apparent Km for the reaction being 11.2 +/- 1.3 microM. The microsomal reaction was almost completely abolished by the specific CYP2C9 inhibitor sulfaphenazole and was inhibited competitively by the alternative CYP2C9 substrate tolbutamide. Torsemide tolyl methylhydroxylase activity in microsomes from 16 human livers correlated significantly (rs = .81-.88) with tolbutamide and phenytoin hydroxylation, both CYP2C9-mediated reactions. Complementary DNA-expressed CYP2C9 catalyzed torsemide tolyl methylhydroxylation with an apparent Km (23 microM) similar to that observed for human liver microsomes and the IC50 values for sulfaphenazole inhibition of the reaction were essentially identical for the two enzyme sources. Taken together, these data demonstrate that human hepatic torsemide tolyl methylhydroxylation is catalyzed predominantly, if not solely, by CYP2C9. The implications of this finding for the regulation of torsemide metabolism in vivo are discussed.

Diazepam metabolism by human liver microsomes is mediated by both S-mephenytoin hydroxylase and CYP3A isoforms

1. The primary metabolism of diazepam was studied in human liver microsomes in order to investigate the kinetics and to identify the cytochrome P450 (CYP) isoforms responsible for the formation of the main diazepam metabolites, temazepam and N-desmethyldiazepam. 2. The formation kinetics of both metabolites were atypical and consistent with the occurrence of substrate activation. A sigmoid Vmax model equivalent to the Hill equation was used to fit the data. The degree of sigmoidicity was greater for temazepam formation than for N-desmethyldiazepam formation, so that the ratio of desmethyldiazepam:temazepam formation increased as the substrate (diazepam) concentration decreased. 3. alpha-Naphthoflavone activated both reactions but with a greater effect on temazepam formation than on N-desmethyldiazepam formation. In the presence of 25 microM alpha-naphthoflavone the kinetics for both pathways were approximated by Michaelis-Menten kinetics. 4. Studies with a series of CYP isoform selective inhibitors and with an inhibitory anti-CYP2C antibody indicated that temazepam formation was carried out mainly by CYP3A isoforms, whereas the formation of N-desmethyldiazepam was mediated by both CYP3A isoforms and S-mephenytoin hydroxylase.

Identification of human liver cytochrome P450 isoforms mediating secondary omeprazole metabolism

1. The in vitro metabolism of omeprazole was studied in human liver microsomes in order to define the secondary metabolic pathways and identify the cytochrome P450 (CYP) isoforms responsible for the formation of the secondary metabolites of omeprazole. 2. The major secondary omeprazole metabolite was the hydroxysulphone, which was formed during incubation with both hydroxyomeprazole and omeprazole sulphone. A second metabolite, tentatively identified as pyridine-N-oxide omeprazole sulphone, was also formed during incubation with omeprazole sulphone. The formation kinetics of these two metabolites from omeprazole sulphone were biphasic suggesting the involvement of multiple CYP isoforms in each case. In contrast, the formation kinetics of hydroxysulphone from hydroxyomeprazole were linear. 3. Inhibition studies, performed with omeprazole sulphone as substrate at concentrations at which the high affinity activities predominated, with a series of isoform selective inhibitors as well as with an anti-CYP2C3 antibody suggested a dominant role of S-mephenytoin hydroxylase in the formation of hydroxysulphone from omeprazole sulphone. By contrast, CYP3A activities were predominant in the formation of hydroxysulphone from hydroxyomeprazole as well as in the formation of pyridine-N-oxide omeprazole sulphone from omeprazole sulphone.

Caffeine metabolism by human hepatic cytochromes P450: contributions of 1A2, 2E1 and 3A isoforms

Caffeine (CA) N1-, N3- and N7-demethylase, CA 8-hydroxylase and phenacetin O-deethylase activities were measured in microsomes from 18 separate human livers which had been characterized previously for a range of cytochrome P450 (CYP) isoform-specific activities and immunoreactive CYP protein contents. Correlations between the high affinity components of the three separate CA N-demethylations were highly significant (r = 0.77-0.91, P < 0.001) and each of the three high affinity CA N-demethylations correlated significantly (r = 0.64-0.93, P < 0.05-0.001) with the high affinity phenacetin O-deethylase, 2-acetylaminofluorene N-hydroxylation and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) mutagenicity (all predominantly CYP1A2-mediated reactions). Consistent with these observations, cDNA-expressed human CYP1A2 catalyzed the N1-, N3- and N7-demethylation of CA and apparent Km values were similar (0.24-0.28 mM) for all three reactions and comparable to those observed previously with human liver microsomes. The low affinity components of CA N1- and N7-demethylation correlated significantly (r = 0.55-0.85, P < 0.05-0.001) with immunoreactive CYP2E1 content and the CYP2E1-specific activities 4-nitrophenol and chlorzoxazone hydroxylation. Diethyldithiocarbamate, a selective inhibitor of CYP2E1, inhibited the low affinity CA N1- and N7-demethylation, with IC50 values of 23 microM and 11 microM, respectively. The apparent Km values for CA N1- and N7-demethylation by cDNA-expressed CYP2E1 (namely 28 and 43 mM, respectively) were of a similar order to those calculated for the low affinity microsomal activities. Significant correlations (r = 0.87-0.97, P < 0.001) were observed between CA 8-hydroxylation and immunoreactive CYP3A content and the CYP3A-mediated reactions benzo(a)pyrene hydroxylation, omeprazole sulfoxidation and aflatoxin B1 mutagenesis. Effects of alpha-naphthoflavone, erythromycin, troleandomycin and nifedipine on microsomal CA 8-hydroxylation were generally consistent with CYP3A involvement. Taken together with previous data, the results indicate a major involvement of CYP1A2 in the high affinity component of all three human hepatic CA N-demethylations. In contrast, CYP2E1 appears to be the main enzyme involved in the low affinity components of CA N1- and N7-demethylation while CA 8-hydroxylation is catalysed predominantly by a CYP3A isoform(s).

In vitro proguanil activation to cycloguanil by human liver microsomes is mediated by CYP3A isoforms as well as by S-mephenytoin hydroxylase

1. The activation of proguanil to cycloguanil by human liver microsomes was studied to define the cytochrome P450 (CYP) isoforms involved in this reaction. 2. Apparent Km values for proguanil ranged from 35 microM to 183 microM with microsomes from four human livers. 3. There was a 6.3-fold range of activity with microsomes from seventeen human livers. Rates of proguanil activation correlated significantly with CYP3A activities (benzo[a]pyrene metabolism, caffeine 8-oxidation and omeprazole sulphone formation) and CYP3A immunoreactive content. There was also a highly significant correlation with rates of hydroxyomeprazole formation. Correlations with activities selective for CYP1A2, CYP2C9/10 and CYP2E1, and with immunoreactive CYP1A2 content were not significant. 4. Proguanil activation was inhibited by R,S-mephenytoin, troleandomycin and by inhibitory anti-CYP3A antiserum and anti-CYP2C IgG and was activated by alpha-naphthoflavone. Inhibitors selective for CYP1A2, CYP2E1, CYP2A6 or CYP2C9/10 had little or no effect on proguanil activation. The extents of inhibition by R,S-mephenytoin, troleandomycin and the two antibodies varied with the immunoreactive CYP3A content of the microsomes used. 5. It is concluded that proguanil activation to cycloguanil by human liver microsomes is mediated both by S-mephenytoin hydroxylase and isoforms of the CYP3A subfamily. This has implications for the use of proguanil as an in vivo probe for the S-mephenytoin poor metaboliser phenotype.

Validation of 4-nitrophenol as an in vitro substrate probe for human liver CYP2E1 using cDNA expression and microsomal kinetic techniques

The involvement of human cytochrome P450 (CYP) 2E1 in the hydroxylation of 4-nitrophenol (4NP) to 4-nitrocatechol (4NC) has been investigated using cDNA expression and liver microsomal kinetic and inhibitor techniques. 4NP hydroxylation by human liver microsomes and cDNA-expressed human CYP2E1 exhibited Michaelis-Menten kinetics; the respective apparent Km values were 30 +/- 7 and 21 microM. Mutual competitive inhibition was observed for 4NP and chlorzoxazone (CZ) (an alternative human CYP2E1 substrate) in liver microsomes, with close similarities between the calculated apparent Km and Ki values for each individual compound. 4NP and CZ hydroxylase activities in microsomes from 18 liver donors varied to a similar extent (3.3- and 3.0-fold, respectively) and 4NP hydroxylase activity correlated significantly (rs > or = 0.75, P < 0.005) with both CZ hydroxylation and immunoreactive CYP2E1 content. The prototypic CYP2E1 inhibitor, diethyldithiocarbamate, was a potent inhibitor of 4NC formation and decreased 4NP hydroxylation by cDNA-expressed CYP2E1 and human liver microsomes in parallel. Probes for other human CYP isoforms namely (alpha-naphthoflavone, coumarin, sulphaphenazole, quinidine, troleandomycin and mephenytoin) caused < 15% inhibition of liver microsomal 4NP hydroxylation. These data confirm that, as in animal species, 4NP hydroxylation is catalysed largely by CYP2E1 in human liver and 4NP may therefore be used as an in vitro substrate probe for the human enzyme.

Identification of human liver cytochrome P450 isoforms mediating omeprazole metabolism

1 The in vitro metabolism of omeprazole was studied in human liver microsomes in order to define the metabolic pathways and identify the cytochrome P450 (CYP) isoforms responsible for the formation of the major omeprazole metabolites. 2 The four major metabolites identified in vitro, in tentative order of importance, were hydroxyomeprazole, omeprazole sulphone, 5-O-desmethylomeprazole, and an unidentified compound termed metabolite X. Omeprazole pyridone was also detected but could not be quantitated. Incubation of hydroxyomeprazole and omeprazole sulphone with human microsomes resulted in both cases in formation of the hydroxysulphone. The kinetics of formation of the four primary metabolites studied were biphasic suggesting the involvement of multiple CYP isoforms in each case. Further studies used substrate concentrations at which the high affinity activities predominated. 3 Formation of the major metabolite, hydroxyomeprazole, was significantly correlated with S-mephenytoin hydroxylase and with benzo[a]pyrene metabolism and CYP3A content. Inhibition studies with isoform selective inhibitors also indicated a dominant role of S-mephenytoin hydroxylase with some CYP3A contribution in the formation of hydroxyomeprazole. Correlation and inhibition data for the sulphone and metabolite X were consistent with a predominant role of the CYP3A subfamily in formation of these metabolites. Formation of 5-O-desmethylomeprazole was inhibited by both R, S-mephenytoin and quinidine, indicating that both S-mephenytoin hydroxylase and CYP2D6 may mediate this reaction in human liver microsomes and in vivo. 4 The Vmax/Km (indicator of intrinsic clearance in vivo) for hydroxyomeprazole was four times greater than that for omeprazole sulphone. Consistent with findings in vivo, the results predict that omeprazole clearance in vivo would be reduced in poor metabolisers of mephenytoin due to reduction in the dominant partial metabolic clearance to hydroxyomeprazole.

Benzodiazepine utilisation in Australia: report from a new pharmacoepidemiological database

This study surveys the total community prescription use of benzodiazepine agents in Australia for the years 1990 and 1991. Also included is information on the utilisation of these agents on the Pharmaceutical Benefits and Repatriation Pharmaceutical Benefits Schemes (PBS/RPBS) over the period 1987 to 1991. The Australian data are from the Drug Utilization Subcommittee (DUSC) database, which is derived from two sources: the PBS/RPBS (subsidized prescriptions), and a national sample of Pharmacy Guild of Australia pharmacies (private and under-copayment general prescriptions). The data are converted to defined daily doses per 1000 inhabitants per day (DDD/1000/day) in accordance with the unit of measurement for drug utilisation studies approved by the World Health Organization. Benzodiazepine utilisation was 33.96 DDD/1000/day for 1990 and 29.31 DDD/1000/day for 1991. The four drugs listed on the Pharmaceutical Benefits Scheme, namely diazepam, oxazepam, nitrazepam and temazepam, constituted 82 per cent of the Australian market. The availability of government subsidy appears to influence benzodiazepine- prescribing behaviour. Benzodiazepine utilisation has been falling in recent years. The fall may be related to the impact of new guidelines and community awareness campaigns. There are major differences in the composition of the market between Australia and the Nordic countries.

High-performance liquid chromatographic assay for human liver microsomal omeprazole metabolism

Assays for the measurement of omeprazole metabolites in plasma and urine have been reported, but when applied to the determination of omeprazole metabolites formed by human liver microsomal incubations there were obvious limitations in sensitivity. The present high-performance liquid chromatographic (HPLC) assay, which comprises extraction, evaporation and reconstitution, is several-fold more sensitive with a limit of detection of approximately 2 pmol (2 nM in incubate) for omeprazole sulphone and 25 pmol (25 nM in incubate) for hydroxyomeprazole. Extraction efficiency is essentially quantitative and is highly reproducible (coefficient of variation = 2.1% for both metabolites). The assay is linear over a wide range of concentrations and the formation of the metabolites is linear with respect to both time (to 15 min) and protein concentration (to 1.5 mg/ml). Two minor metabolites, one of which was identified tentatively as 5-O-desmethylomeprazole, were also formed by human liver microsomes and could be determined by this method. Preliminary studies of the formation of omeprazole sulphone and hydroxyomeprazole showed that the formation kinetics in human liver microsomes were biphasic for both metabolites, suggesting that at least two different cytochrome P450 isoforms are involved in their formation.

High-performance liquid chromatographic assay for 4-nitrophenol hydroxylation, a putative cytochrome P-4502E1 activity, in human liver microsomes

A high-performance liquid chromatographic method which measures formation of product 4-nitrocatechol (4NC) has been developed and applied to the study of human liver microsomal 4-nitrophenol (4NP) hydroxylation. Following diethyl ether extraction, 4NC and the assay internal standard (salicylamide) were separated by reversed-phase (C18) liquid chromatography. Extraction efficiencies of 4NC and internal standard were both > 90%. The assay, which has a limit of detection of 15 pmol injected (or an incubation 4NC concentration of 0.25 microM), is accurate, reproducible and straightforward. With a chromatographic time of 12 min, 40-50 samples may be analyzed per day. Rates of 4NC formation were linear with time and protein concentration to 50 min and 0.5 mg/ml, respectively. Preliminary studies of 4NP hydroxylation showed that this reaction followed single enzyme Michaelis-Menten kinetics in human liver microsomes.

Specificity of substrate and inhibitor probes for human cytochromes P450 1A1 and 1A2

Kinetic and inhibitor studies using cDNA-expressed enzymes and human liver microsomes have characterized the specificity of a range of cytochrome P450 (CYP) 1A substrate and inhibitor probes towards the two isoforms comprising this subfamily. Expressed CYP1A1 and CYP1A2 both catalyzed the O-deethylation of phenacetin, although the apparent Km was about 4-fold lower for CYP1A2 (25 vs. 108 microM). Phenacetin O-deethylation exhibited biphasic kinetics in human liver microsomes, and the apparent Km for the high-affinity component (9 +/- 6 microM) was consistent with the involvement of CYP1A2 in this reaction. The prototypic CYP1A xenobiotic inhibitor and substrate probes alpha-naphthoflavone, ellipticine, 7-ethoxycoumarin and 7-ethoxyresorufin all inhibited CYP1A1- and CYP1A2-mediated phenacetin O-deethylation as well as the high-affinity component of human liver phenacetin O-deethylase activity. alpha-Naphthoflavone and 7-ethoxycoumarin were, however, approximately 10-fold more potent as inhibitors of CYP1A2 than CYP1A1. Other putative human CYP1A xenobiotic substrates and inhibitors, including caffeine, 5- and 8-methoxypsoralen, nifedipine, paraxanthine, propranolol and theophylline similarly inhibited CYP1A1- and 1A2-catalyzed phenacetin O-deethylation and the high-affinity human liver phenacetin O-deethylase. In contrast, the monoclonal antibody MAb 1-7-1, raised against 3-methylcholanthrene-inducible rat cytochromes 450, almost abolished CYP1A1-mediated phenacetin O-deethylation, but had no effect on human liver microsomal- or CYP1A2-catalyzed phenacetin dealkylation. Together with previous data, the results indicate that the majority of human CYP1A xenobiotic inhibitor and substrate probes are nonspecific in their recognition of CYP1A1 and CYP1A2, although selectivity is apparent for some compounds.

Tolbutamide hydroxylation in humans: lack of bimodality in 106 healthy subjects

The tolbutamide hydroxylation capacity was studied in 106 healthy unrelated volunteers from the Australian population. Following a 500 mg oral dose of tolbutamide, the ratio of metabolites (hydroxytolbutamide plus carboxytolbutamide) to unchanged tolbutamide excreted in urine from 6 to 12 h post-dose (urinary metabolic ratio, MR) was determined. Metabolic ratio values did not appear bimodally distributed, even following various transformations of the data (i.e. Log10, inverse, Log10 inverse). A poor metabolizer (PM) subject from a previous clinical study, however, could be distinguished (MR value 159) from the above subjects (MR value range 324-3033), particularly from the histogram plot of inverse tolbutamide metabolic ratio. The poor metabolizer's parents had metabolic ratio values (526 and 478) that were at the lower end of the range of metabolic ratios obtained from the population study, and may indicate that they both have a heterozygous genotype and that a recessive form of inheritance is most likely. As the hydroxylations of tolbutamide and phenytoin are closely linked, the incidence of slow tolbutamide metabolizers is likely to be similar to that for phenytoin (about 1:500) and this is consistent with the failure to detect a single poor tolbutamide metabolizer in our random sample of 106 individuals.

Site-directed mutation studies of human liver cytochrome P-450 isoenzymes in the CYP2C subfamily

Evidence from human studies in vivo and in vitro strongly suggests that the methylhydroxylation of tolbutamide and the 4-hydroxylation of phenytoin, the major pathways in the elimination of these two drugs, are catalysed by the same cytochrome P-450 isoenzyme(s). In the present study we used site-directed mutagenesis and cDNA expression in COS cells to characterize in detail the kinetics of tolbutamide and phenytoin hydroxylations by seven CYP2C proteins (2C8, 2C9 and variants, and 2C10) in order to define the effects of small changes in amino acid sequences and the likely proteins responsible in the metabolism of these two drugs in man. Tolbutamide was hydroxylated to varying extents by all expressed cytochrome P-450 isoenzymes, although activity was much lower for the expressed 2C8 protein. While the apparent Km values for the 2C9/10 isoenzymes (71.6-131.7 microM) were comparable with the range of apparent Km values previously observed in human liver microsomes, the apparent Km for 2C8 (650.5 microM) was appreciably higher. The 2C8 enzyme also showed quite different sulphaphenazole inhibition characteristics. The 4-hydroxylation of phenytoin was also more efficiently catalysed by the 2C9/10 enzymes. These enzymes showed similarities in kinetics of phenytoin hydroxylation and sulphaphenazole inhibition compared with human liver phenytoin hydroxylase. Also of interest was the observation that, among the 2C9 variants, small differences in amino acid composition could appreciably affect both tolbutamide and phenytoin hydroxylations. The amino acid substitution Cys-144-->Arg increased both the rates of tolbutamide and phenytoin hydroxylations, while the Leu-359-->Ile change had a greater effect on phenytoin hydroxylation. We conclude that: (1) although 2C8 and 2C9/10 proteins metabolize tolbutamide. only 2C9/10 proteins play a major role in human liver; (2) 2C9/10 proteins also appear to be chiefly responsible for phenytoin hydroxylation; and (3) subtle differences in the amino acid composition of these 2C9/10 proteins can affect the functional specificities towards both tolbutamide and phenytoin.

Co-regulation of phenytoin and tolbutamide metabolism in humans

1. The disposition of phenytoin and tolbutamide was compared in eighteen healthy young adults separately administered single therapeutic doses (sodium phenytoin 300 mg, tolbutamide 500 mg) of the two drugs. 2. Within the group, ratios of ranges of total and unbound areas under the plasma concentration-time curves were similar for both drugs. 3. There were significant (P < 0.001) correlations between total (r = 0.88) and unbound (r = 0.86) areas under the plasma phenytoin and tolbutamide concentration-time curves. 4. The results are consistent with the involvement of the same cytochrome P-450 isoenzyme(s) in the metabolism of tolbutamide and phenytoin.

The formation of proximate carcinogens from three polycyclic aromatic compounds by human liver microsomes

1. The metabolism of 3H-benzo[a]pyrene (BP), 3H-7-methylbenz[c]acridine (7MBAC) and 3H-dibenz[a,j]acridine (DBAJAC) have been studied in human liver microsomes from 13 subjects. 2. When the metabolism of these carcinogens to more polar ethyl acetate-soluble metabolites were compared, the activities towards the nitrogenous carcinogens were twice that determined for BP. 3. The specific rates of formation of the three proximate carcinogens, BP-7,8-dihydrodiol, 7MBAC-3,4-dihydrodiol and DBAJAC-3,4-dihydrodiol per nmol cytochrome P-450 for 12 subjects were positively correlated. 4. These dihydrodiols constituted 5.9 +/- 0.7% (mean +/- SEM), 57.8 +/- 2.6% and 3.0 +/- 0.4% of the total metabolites identified by cochromatography with standards, 7MBAC, DBAJAC and BP respectively.

Perturbation of paracetamol urinary metabolic ratios by urine flow rate

The effects of high and low urine flow rates on the urinary metabolic ratios for paracetamol glucuronidation, sulphation and oxidation were determined at steady-state in seven healthy young adult volunteers. Metabolic partial clearances were unaffected by urine flow rate, but individual paracetamol metabolic ratios varied 2.5- to 3.2-fold over a 7.4-fold range of urine flow rates (0.81-6.00 ml min-1). The change in metabolic ratios was due entirely to a 2.5-fold change in renal clearance of unchanged paracetamol. These data emphasise the limitations of the metabolic ratio as a measure of intrinsic clearance for compounds which undergo some degree of tubular reabsorption.