A new rapidly absorbed paracetamol tablet containing sodium bicarbonate. I. A four-way crossover study to compare the concentration-time profile of paracetamol from the new paracetamol/sodium bicarbonate tablet and a conventional paracetamol tablet in fed and fasted volunteers

The primary objective of this four-way crossover study was to compare the concentration-time profile of paracetamol from a new rapidly absorbed paracetamol tablet containing sodium bicarbonate (PS) with a conventional paracetamol tablet (P), in a panel of 28 fed and fasted healthy volunteers. The results demonstrated that paracetamol was absorbed more rapidly from tablets containing sodium bicarbonate compared to conventional tablets, as indicated by a shorter tmax in both the fed and fasted state and a higher Cmax in the fasted state. The two formulations were bioequivalent with respect to area under curve (AUC). Food did not affect the extent of absorption from either formulation, as indicated by AUC, however, food did reduce the rate of absorption from both formulations, as indicated by a longer tmax and a lower Cmax. Metabolic activation of paracetamol to its oxidation metabolites, as assessed by combined partial clearances to subsequent secondary metabolites cysteine and mercapturic acid conjugates, indicated that the two formulations were bioequivalent in this respect.

Analysis of solvent central nervous system toxicity and ethanol interactions using a human population physiologically based kinetic and dynamic model

The effect of acute ethanol-mediated inhibition of m-xylene metabolism on central nervous system (CNS) depression in the human worker population was investigated using physiologically based pharmacokinetic (PBPK) models and probabilistic random (Monte Carlo) sampling. PBPK models of inhaled m-xylene and orally ingested ethanol were developed and combined by a competitive enzyme (CYP2E1) inhibition model. Human interindividual variability was modeled by combining estimated statistical distributions of model parameters with the deterministic PBPK models and multiple random or Monte Carlo simulations. A simple threshold pharmacodynamic model was obtained by simulating m-xylene kinetics in human studies where CNS effects were observed and assigning the peak venous blood m-xylene concentration (C(V,max)) as the dose surrogate of toxicity. Probabilistic estimates of an individual experiencing CNS disturbances given exposure to the current UK occupational exposure standard (100 ppm time-weighted average over 8 h), with and without ethanol ingestion, were obtained. The probability of experiencing CNS effects given this scenario increases markedly and nonlinearly with ethanol dose. As CYP2E1-mediated metabolism of other occupationally relevant organic compounds may be inhibited by ethanol, simulation studies of this type should have an increasingly significant role in the chemical toxicity risk assessment.

In-vitro analysis of the contribution of CYP2D6.35 to ultra-rapid metabolism

From 10 to 30% of CYP2D6 ultra-rapid metabolizers of Caucasian origin harbor alleles with duplicated or amplified functional CYP2D6 genes. Recently, the CYP2D6*35 allele has been reported to be more frequent in ultra-rapid metabolizing subjects than in extensive metabolizers, suggesting a possible role of this variant in CYP2D6 duplication-negative ultra-rapid metabolizing subjects. In this study, we examined the functional consequences of the Val11Met, Arg296Cys and Ser486Thr amino acid substitutions associated with the CYP2D6*35 on the expression and catalytic activity of the variant enzyme, heterologously expressed in yeast. Our results indicate that the functional activity and level of expression of recombinant CYP2D6.35 are comparable with those of the wild-type enzyme, thus precluding the hypothesis that the high level of enzyme activity in CYP2D6 duplication-negative ultra-rapid metabolizing subjects is a consequence of the expression of a more catalytically effective CYP2D6.35 enzyme.

The placebo response to citric acid-induced cough: pharmacodynamics and gender differences

Characteristics of the response to placebo in a citric acid-induced cough challenge were investigated as part of a randomized, double-blind crossover trial to assess the antitussive effect of dextromethorphan. Baseline cough responses were established on two occasions in 22 healthy subjects. They received 60 ml placebo antitussive syrup and cough frequency following five inhalations of 10% citric acid over 5 min was measured at regular intervals up to 12 h. Response-time models of varying complexity were used to describe the placebo cough suppression data. The cough response to placebo was also compared to that of the untreated state. The placebo cough response was best characterized by a non-linear increase in cough suppression up to a maximum reduction of 1.6 coughs from baseline at 4-4.5 h, followed by a non-linear return to baseline. The cough response in the untreated state was not different from that of placebo (P=0.99). Females coughed more frequently than males (median number of coughs=10.5 vs. 9.0, respectively P<0.001; Mann-Whitney U test), and adaptation to the cough stimulus was significantly more rapid in females (P<0.025). Accordingly, in trials that use citric acid-induced cough, gender should be considered in study design, particularly in relation to the timing of measurements.

Enterocytic CYP3A4 in a paediatric population: developmental changes and the effect of coeliac disease and cystic fibrosis

AIMS

To investigate the effects of age and disease states on the expression and activity of intestinal CYP3A4 in a paediatric population.

METHODS

Duodenal biopsies and surgical sections were collected from 104 paediatric patients (age range 2 weeks to 17 years) and from 11 foetuses. An S9 fraction was prepared in each case. CYP3A4 expression was assessed by Western blotting and by immunohistochemistry; activity was measured by the rate of formation of 6beta-hydroxytestosterone from testosterone. Villin expression was used as a marker of enterocyte harvest to normalize CYP3A4 expression and activity data.

RESULTS

In the 74 histologically normal paediatric biopsies there were statistically significant increases in CYP3A4 expression (r2 = 0.19, P = 0.001) and activity (r2 = 0.17, P = 0.02) with age. CYP3A4 was practically absent in fetal duodenum and was expressed at relatively low levels in neonates (P < 0.05 between neonates and children > 5 years). Active coeliac disease resulted in significant (P < 0.001) decreases in CYP3A4 expression and activity.

CONCLUSIONS

Duodenal CYP3A4 is present at significantly lower levels in neonates and in patients with active coeliac disease. This may have clinical significance with respect to the oral bioavailability of CYP3A4 substrates.

Influence of phenylalanine-481 substitutions on the catalytic activity of cytochrome P450 2D6

Homology models of the active site of cytochrome P450 2D6 (CYP2D6) have identified phenylalanine 481 (Phe(481)) as a putative ligand-binding residue, its aromatic side chain being potentially capable of participating in pi-pi interactions with the benzene ring of ligands. We have tested this hypothesis by replacing Phe(481) with tyrosine (Phe(481)-->Tyr), a conservative substitution, and with leucine (Phe(481)-->Leu) or glycine (Phe(481)-->Gly), two non-aromatic residues, and have compared the properties of the wild-type and mutant enzymes in microsomes prepared from yeast cells expressing the appropriate cDNA-derived protein. The Phe(481)-->Tyr substitution did not alter the kinetics [K(m) (microM) and V(max) (pmol/min per pmol) respectively] of oxidation of S-metoprolol (27; 4.60), debrisoquine (46; 2.46) or dextromethorphan (2; 8.43) relative to the respective wild-type values [S-metoprolol (26; 3.48), debrisoquine (51; 3.20) and dextromethorphan (2; 8.16)]. The binding capacities [K(s) (microM)] of a range of CYP2D6 ligands to the Phe(481)-->Tyr enzyme (S-metoprolol, 22.8; debrisoquine, 12.5; dextromethorphan, 2.3; quinidine, 0.13) were also similar to those for the wild-type enzyme (S-metoprolol, 10.9; debrisoquine, 8.9; dextromethorphan, 3.1; quinidine, 0.10). In contrast, the Phe(481)-->Leu and Phe(481)-->Gly substitutions increased significantly (3-16-fold) the K(m) values of oxidation of the three substrates [S-metoprolol (120-124 microM), debrisoquine (152-184 microM) and dextromethorphan (20-31 microM)]. Similarly, the K(s) values of the ligands to Phe(481)-->Leu and Phe(481)-->Gly mutants were also increased 3 to 10-fold (S-metoprolol, 33.2-41.9 microM; debrisoquine, 85-90 microM; dextromethorphan, 15.7-18.8 microM; quinidine 0.35-0.53 microM). However, contrary to a recent proposal that Phe(481) has the dominant role in the binding of substrates that undergo CYP2D6-mediated N-dealkylation routes of metabolism, the Phe(481)-->Gly substitution did not substantially decrease the capacity of the enzyme to N-deisopropylate metoprolol (wild-type, 1.12 pmol/min per pmol of P450; Phe(481)-->Gly, 0.71), whereas an Asp(301)-->Gly substitution decreased the N-dealkylation reaction by 95% of the wild-type rate. Overall, our results are consistent with the proposal that Phe(481) is a ligand-binding residue in the active site of CYP2D6 and that the residue interacts with ligands via a pi-pi interaction between its phenyl ring and the aromatic moiety of the ligand. However, the relative importance of Phe(481) in binding is ligand-dependent; furthermore, its importance is secondary to that of Asp(301). Finally, contrary to predictions of a recent homology model, Phe(481) does not seem to have a primary role in CYP2D6-mediated N-dealkylation.

Journal impact factors: a 'bioequivalence' issue?

AIMS

Journal impact factors (IMFs) are used increasingly by institutions as performance indicators of the quality of 'individual research output'. Although the need for discretion when using the numbers has been emphasized, there has been little formal analysis of the issues. We therefore investigated citation profiles for three clinical pharmacology journals to assess the validity of using IMF as a measure of 'individual research'.

METHODS

We compared the pattern of individual citations for random samples of 120 papers published in Clin Pharmacol Ther (CPT), Br J Clin Pharmacol (BJCP) and Eur J Clin Pharmacol (EJCP) in 1981, 1991, 1995 and 1996. Using an analogy between citation-time profiles of papers and concentration-time profiles of drugs, it was possible to define 'lag-time', Cmax, tmax, t(1/2) and AUC(t), and to investigate 'bioequivalence'.

RESULTS

Citation distributions for individual publications were widely variable and skewed (skewness = 1.47, 2.16 and 1.37 for CPT, BJCP and EJCP, respectively). The 90% CI values for the IMF of a publication in each journal (i.e. 90% CI for an observation as opposed to 90% CI for the mean) were 0.24-16.94, 0.08-10.3 and 0.09-5.68.

CONCLUSIONS

IMF does not represent the impact of an individual paper. Furthermore, if the comparison of journals is treated as a bioequivalence issue, the citation data should be log transformed prior to calculating IMF such that they represent the likelihood of citation for the median article. After such transformation, absolute differences between the IMF of clinical pharmacology journals become much smaller.

A comparison of the ontogeny of enterocytic and hepatic cytochromes P450 3A in the rat

Enzymes of the cytochrome P450 3A (CYP3A) sub-family are abundant in adult liver and gut and contribute significantly to the first-pass metabolism of many orally administered drugs. The development of CYP3A enzymes with regard to their expression and activity in enterocytic and hepatic microsomes from 1-day-old through to adult male and female rats has been studied. Microsomes were prepared by calcium precipitation. Enzyme expression was assessed semi-quantitatively by Western blotting using rat polyclonal CYP3A2 and 2C11 antibodies and peptide antibodies specific to rat CYPs 3A1, 3A2, 2C12, and 2C13. The formation of 6beta-hydroxytestosterone (6OHT), determined by HPLC, was used as a measure of enzyme activity. Formation of 6OHT by enterocytic microsomes was similar for males and females and showed a sharp increase at weaning. This pattern was mirrored by levels of immunoquantifiable CYP3A2 (CYP3A9), but CYP3A1 followed a more gradual development. CYPs 2C11, 2C12, or 2C13 were not detected in gut microsomes. In contrast, CYPs 3A1, 3A2, 2C11, 2C12, and 2C13 were all expressed in hepatic microsomes. There was no surge in hepatic enzyme expression or hepatic 6OHT formation at weaning, and a marked sex difference in 6OHT formation was apparent from day 25. The surge in gut activity at weaning may be a protective mechanism against ingested toxins.

Parkinson's disease and CYP1A2 activity

AIMS MPTP, a neurotoxin which induces parkinsonism is partially metabolized by the enzyme CYP1A2. Smoking appears to protect against Parkinson's disease (PD) and cigarette smoke induces CYP1A2 activity. Thus, we investigated the hypothesis that idiopathic PD is associated with lower CYP1A2 activity using caffeine as a probe compound. METHODS CYP1A2 activity was assessed using saliva paraxanthine (PX) to caffeine (CA) ratios. Caffeine half-life was also estimated from salivary concentrations of caffeine at 2 and 5 h post dose. 117 treated and 40 untreated patients with PD and 105 healthy control subjects were studied. RESULTS PX/CA ratios were 0. 57, 0.93 and 0.77 in treated patients, untreated patients and healthy control subjects, respectively, with no significant differences between study groups (95% CI: treated patients vs controls -0.24, 0.57; untreated patients vs controls -0.75, 0.35). However, patients with PD (treated or untreated) had caffeine half-lives shorter than that in controls (treated patients: 262 min, untreated patients: 244 min, controls: 345 min; 95% CI: controls vs treated patients 23, 143 (P = 0.003); controls vs untreated patients 19, 184 (P = 0.011)). Amongst the patients with PD, caffeine half-life was also inversely related to the age of onset of disease (P = 0.012); gender and concomitant drugs did not influence this significantly.

CONCLUSIONS

Based on PX/CA ratio, there was no evidence of decreased CYP1A2 activity in patients compared with control subjects. The observed decrease in the elimination half-life of caffeine in PD may be caused by increased CYP2E1 activity, an enzyme that also contributes to the metabolism of caffeine. The latter warrants further investigation.

The 1'-hydroxylation of Rac-bufuralol by rat brain microsomes

The 1'-hydroxylation of rac-bufuralol, which is catalyzed by polymorphic CYP2D6 in humans, was studied in brain microsomes from male and female Wistar rats and from the female Dark Agouti rat, a model of the CYP2D6 poor metabolizer phenotype. The kinetics of the 1'-hydroxylation of bufuralol (1-1500 microM) by brain microsomes were biphasic. The activity of the high-affinity site of metabolism was consistent with Michaelis-Menten kinetics (apparent K(m1) = 0. 61-1.42 microM, V(max1) = 4.3-4.8 fmol/min/mg of protein), whereas the low-affinity activity was better described by a Hill function (K(50%(2)) = 253-258 microM, V(max2) = 817-843 fmol/min/mg of protein, n = 1.2-1.3). Values for kinetic constants were similar in all rat strains. Quinine was only a weak inhibitor of both the high- (apparent K(i) = 90 microM) and low-affinity (210 microM) sites of metabolism. In contrast, the kinetics of 1'-hydroxylation of bufuralol by rat liver microsomes were best described by a two-site Michaelis-Menten function. V(max) values were 3 to 5 orders of magnitude greater compared with those for brain microsomes (male and female Wistar), and liver microsomes from female Dark Agouti rats were significantly less active than those from Wistar rats. These data, together with the known potent inhibitory effect of quinine on bufuralol 1'-hydroxylation by rat liver microsomes, indicate tissue-specific differences in the enzymology of this reaction. The role of brain CYP2D enzymes remains to be clarified.

Chiral switches

Developments in synthetic and analytical chemistry have provided the tools to differentiate between two enantiomers (mirror images) of drugs or between the parent compound and metabolite(s) with respect to desired and undesired pharmacological effects. Several drugs are now marketed or being developed as single enantiomers in place of a previous racemic mixture, a process known as "chiral switching". It is easier to understand "pure" as opposed to "mixture" pharmacology but whether the promise of chiral (and metabolite) switches will translate into real clinical advances remains to be seen.

Regioselective hydroxylation of debrisoquine by cytochrome P4502D6: implications for active site modelling

1. Debrisoquine, a prototypic probe substrate for human cytochrome P4502D6 (CYP2D6), is hydroxylated at the alicyclic C4-position by this enzyme. Phenolic metabolites of debrisoquine (5-, 6-, 7- and 8-hydroxydebrisoquine) have also been reported as in vivo metabolites, but the role of CYP2D6 in their formation is unclear. 2. As part of studies to develop a predictive model of the active site of CYP2D6 using pharmacophore and homology modelling techniques, it became important to determine the precise regioselective hydroxylation of debrisoquine by CYP2D6. 3. Data from studies with human liver microsomes and yeast microsomes containing cDNA-derived CYP2D6 demonstrated unequivocally that debrisoquine was hydroxylated by CYP2D6 at each aromatic site in the molecule, as well as at the alicyclic 4-position. The four phenolic metabolites amounted to > 60% of the total identified products and the pattern of regioselective hydroxylation (4-HD > 7-HD > 6-HD > 8-HD > 5-HD) was similar in both in vitro systems. 4. A pharmacophore model for CYP2D6 indicated that while the hydroxylation of debrisoquine at alternative positions could arise from the substrate adopting multiple binding orientations, the energy constraints for the aromatic hydroxylations were unfavourable. An alternative proposal involving essentially a single binding orientation and a mechanism of hydroxylation based on benzylic radical spin delocalization could satisfactorily rationalize all the hydroxylations of debrisoquine. 5. This latter proposal demonstrates the need to consider the mechanism of oxidation as well as the spatial orientation of the substrate in the development of a predictive model of the active site of CYP2D6.

Evidence that serine 304 is not a key ligand-binding residue in the active site of cytochrome P450 2D6

Homology models of cytochrome P450 2D6 (CYP2D6) have identified serine 304 as an active-site residue and implicated a putative role for this residue in substrate enantioselectivity and the differential inhibition of enzyme activity by the diastereoisomers quinine and quinidine. The role of serine 304 in selectivity is thought to be achieved through a preferential hydrogen-bond interaction between the hydroxyl group of the residue and one of the stereoisomers of each ligand. We have tested this hypothesis by substituting serine 304 with alanine, a non-hydrogen-bonding residue, and compared the properties of the wild-type and mutant enzymes in microsomes prepared from yeast cells expressing the appropriate cDNA-derived enzyme. The Ser(304)Ala substitution did not alter the enantioselective oxidation of metoprolol; the O-demethylation reaction remained R-(+)-enantioselective (wild-type, R/S, 1.7; mutant, R/S, 1.6), whereas alpha-hydroxylation remained S-(-)-enantioselective (wild-type and mutant, R/S, 0.7). Similarly, the selective oxidation of the R-(+) and S-(-) enantiomers of propranolol to the major 4-hydroxy metabolite was identical with both wild-type and mutant forms of the enzyme (R/S 0.9), although the formation of minor metabolites (5-hydroxy and deisopropylpropranolol) did show some slight alteration in enantioselectivity. The differential inhibition of enzyme activity by quinine and quinidine was also identical with both forms of CYP2D6, the IC(50) values for each enzyme being approx. 10 microM and 0.1 microM for quinine and quinidine, respectively. The kinetics of formation of alpha-hydroxymetoprolol and 4-hydroxydebrisoquine by wild-type and the Ser(304)Ala mutant was also very similar. However, modest changes in the regioselective oxidation of metoprolol and debrisoquine were observed with the Ser(304)Ala mutant. The regio- and enantioselective oxidation of an analogue of metoprolol, in which the hydroxyl group attached to the chiral carbon was replaced by a methyl moiety, was again identical with both wild-type and Ser(304)Ala mutant. However, the observed selectivity was the reverse of that observed with metoprolol. Collectively, these data indicate that Ser(304) is unlikely to be a key ligand-binding residue, although the residue may indeed be located in the active-site cavity. The reversal of selectivity with the methyl analogue of metoprolol indicates that the hydroxyl group attached to the chiral centre of ligands, such as metoprolol, is important in defining the enzyme's selective properties, and that a hydrogen-bonding residue, other than Ser(304), may be involved in this interaction. Current homology models of the active site of CYP2D6 that predict a hydrogen-bond interaction between Ser(304) and specific ligands will need to be re-evaluated, and other candidate residues capable of such an interaction nominated and tested by site-directed mutagenesis studies.

Advances in understanding drug metabolism and its contribution to variability in patient response

Recent advances in the understanding of the molecular biology and genetic variability of human drug metabolizing enzymes, particularly cytochromes P450, have contributed immensely towards clarifying the pharmacokinetics and pharmacodynamics of many existing drugs, and are increasingly important in the development of new chemical entities. However, whereas this knowledge has implications for therapeutics, appreciation and application in clinical practice have either been limited or yet to be realized. For example, the debrisoquine polymorphism was discovered 20 years ago, but controlled prospective studies to evaluate its clinical significance and pharmacoeconomic impact are few. Come the millennium it will be seen to what extent traditional therapeutic drug monitoring, involving the measurement of specific plasma drug concentrations, may be complemented or replaced by more general phenotyping or genotyping screens for human drug metabolizing enzymes.

The antitussive effect of dextromethorphan in relation to CYP2D6 activity

AIMS

To test the hypothesis that inhibition of cytochrome P450 2D6 (CYP2D6) by quinidine increases the antitussive effect of dextromethorphan (DEX) in an induced cough model.

METHODS

Twenty-two healthy extensive metaboliser phenotypes for CYP2D6 were studied according to a double-blind, randomised cross-over design after administration of: (1) Placebo antitussive preceded at 1 h by placebo inhibitor; (2) 30 mg oral DEX preceded at 1 h by placebo inhibitor (DEX30); (3) 60 mg oral DEX preceded at 1 h by placebo inhibitor (DEX60); (4) 30 mg oral DEX preceded at 1 h by 50 mg oral quinidine sulphate (QDEX30). Cough frequency following inhalation of 10% citric acid was measured at baseline and at intervals up to 12 h. Plasma concentrations of DEX and its metabolites were measured up to 96 h by h.p.l.c.

RESULTS

Inhibition of CYP2D6 by quinidine caused a significant increase in the mean ratio of DEX to dextrorphan (DEX:DOR) plasma AUC(96) (0.04 vs 1.81, P<0.001). The mean (+/-s.d.) decrements in cough frequency below baseline over 12 h (AUEC) were: 8% (11), 17% (14.5), 25% (16.2) and 25% (16.9) for placebo, DEX30, DEX60 and QDEX30 treatments, respectively. Statistically significant differences in antitussive effect were detected for the contrasts between DEX60/placebo (P<0.001; 95% CI of difference +80, +327) and QDEX30/placebo (P<0.001, +88, +336), but not for DEX30/placebo, DEX30/DEX60 or DEX30/QDEX30 (P=0.071, -7, +241; P=0.254, -37, +211; P=0.187, -29, +219, respectively).

CONCLUSIONS

A significant antitussive effect was demonstrated after 60 mg dextromethorphan and 30 mg dextromethorphan preceded by 50 mg quinidine using an induced cough model. However, although the study was powered to detect a 10% difference in cough response, the observed differences for other contrasts were less than 10%, such that it was possible only to imply a dose effect (30 vs 60 mg) in the antitussive activity of DEX and enhancement of this effect by CYP2D6 inhibition.

Population pharmacokinetics of methadone in opiate users: characterization of time-dependent changes

AIMS

Although methadone is widely used to treat opiate dependence, guidelines for its dosage are poorly defined. There is increasing evidence to suggest that a strategy based on plasma drug monitoring may be useful to detect non-compliance. Therefore, we have developed a population-based pharmacokinetic (POP-PK) model that characterises adaptive changes in methadone kinetics.

METHODS

Sparse plasma rac-methadone concentrations measured in 35 opiate-users were assessed using the P-Pharm software. The final structural model comprised a biexponential function with first-order input and allowance for time-dependent change in both clearance (CL) and initial volume of distribution (V ). Values of these parameters were allowed to increase or decrease exponentially to an asymptotic value.

RESULTS

Increase in individual values of CL and increase or decrease in individual values of V with time was observed in applying the model to the experimental data.

CONCLUSIONS

A time-dependent increase in the clearance of methadone is consistent with auto-induction of CYP3A4, the enzyme responsible for much of the metabolism of the drug. The changes in V with time might reflect both up- and down-regulation of alpha1-acid glycoprotein, the major plasma binding site for methadone. By accounting for adaptive kinetic changes, the POP-PK model provides an improved basis for forecasting plasma methadone concentrations to predict and adjust dosage of the drug and to monitor compliance in opiate-users on maintenance treatment.

Polymorphic debrisoquine 4-hydroxylase activity in the rat is due to differences in CYP2D2 expression

The female Dark Agouti rat is widely used as an animal model for the CYP2D6 poor metabolizer phenotype, males of other strains such as Sprague Dawley or Wistar serving as models for the extensive metabolizer phenotype. To determine the relative level of expression of CYP2D enzymes in the liver of female and male Dark Agouti, Sprague Dawley and Wistar rats, anti-peptide antibodies were raised in rabbits against short synthetic peptides representing the C-termini of the rat P450 enzymes CYP2D1, CYP2D2, CYP2D3, CYP2D4 and CYP2D5. In immunoblotting studies, it was found that the hepatic expression of CYP2D1 was greater in Dark Agouti rats than Sprague Dawley or Wistar rats. In contrast, hepatic CYP2D2 was 30-40-fold less abundant in female Dark Agouti than female Sprague Dawley or Wistar rats and six- to eightfold less abundant in male Dark Agouti than male Sprague Dawley or Wistar rats. No hepatic CYP2D3 could be detected in either sex of any of the three strains. Hepatic CYP2D4 expression was generally greater in male than female rats, and higher in Dark Agouti compared with Sprague Dawley or Wistar strains. CYP2D5 was expressed in the livers of female and male Dark Agouti rats but not in female Sprague Dawley or Wistar rats. This form was variably expressed in livers of male Sprague Dawley and Wistar rats. Hepatic debrisoquine 4-hydroxylase activity was markedly reduced in female and male Dark Agouti rats as compared to Sprague Dawley or Wistar rats and correlated (r = 0.88; P < 0.001) with the hepatic CYP2D2 content. Recombinant CYP2D2 was 18-fold more active at catalysing the 4-hydroxylation of debrisoquine than CYP2D1. Furthermore, quinine markedly inhibited CYP2D2-mediated debrisoquine and metoprolol oxidation, while quinidine, its diastereoisomer, inhibited the reactions to a lesser extent. In conclusion, these results show that impaired debrisoquine 4-hydroxylase activity in the female Dark Agouti rat is due to low levels of CYP2D2.

In-vivo indices of enzyme activity: the effect of renal impairment on the assessment of CYP2D6 activity

Urinary drug:metabolite ratios and urinary recoveries of metabolites, have been used to assess specific enzyme activity non-invasively in vivo. These indices are potentially confounded by the effect of renal function. A recent study of the effects of renal impairment has found discrepancies between different indices used to mark CYP2D6 activity based on sparteine and dextromethorphan urinary recoveries. We have re-examined these experimental data from a theoretical viewpoint. The results suggest that the dependence of fractional urinary recovery of metabolites on renal function varies with the importance of different elimination routes. Therefore, no consistent behaviour of this index is expected when markers with different pharmacokinetics are used. However, when collecting the urine until full recovery of drug and metabolite, drug:metabolite ratios show the same degree of dependence on renal function regardless of the marker. The application of the analysis to the experimental data indicates that CYP2D6 activity is compromised in parallel with deterioration of renal function.

First principles investigation of singly reduced cytochrome P450

1. The application of novel ab initio quantum mechanical methods to the states in the catalytic cycle of cytochrome P450 following the first reduction step is described. 2. A good correlation was found between the calculated energy of reduction and the experimentally determined redox potential for a range of substrate- and substrate analogue-bound systems. 3. On reduction of the haem system, the ground state of Fe remains Fe3+. On binding of a CO molecule, Fe adopts a low-spin Fe2+ state, in agreement with experiment. However, on binding of an O2 molecule, calculations indicate that the system adopts a ferric superoxide ground state, in which the Fe is in a low-spin Fe3+ state.