Evidence that aspartic acid 301 is a critical substrate-contact residue in the active site of cytochrome P450 2D6

Model building studies have intimated a role for aspartic acid 301 in the substrate binding of cytochrome P450 2D6 (CYP2D6). We have tested this hypothesis by generating a range of CYP2D6 mutants substituting a variety of amino acids at this site. The mutant proteins, which included substitution with a negatively charged glutamic acid residue or neutral asparagine, alanine, or glycine residues, were expressed in Saccharomyces cerevisiae. In addition, a mutant where aspartic acid 301 was deleted was also tested. All the mutants expressed approximately equivalent amounts of recombinant apoprotein and, apart from the alanine 301 and the aspartic acid 301 deletion mutants, gave carbon monoxide difference spectra of similar magnitude to the wild type. In the cases of the alanine and deletion mutants, the amount of holoprotein was significantly reduced or absent relative to the amount of apoprotein, indicating restricted heme incorporation. The glutamic acid mutant was shown to have similar catalytic properties to the wild type enzyme toward the substrates debrisoquine and metoprolol; however, some differences in regioselectivity and ligand binding were observed. The mutants containing neutral amino acids at position 301 exhibited marked reductions in catalytic activity. At low substrate concentrations little, if any, activity toward debrisoquine and metoprolol was measured. However, at a higher substrate concentration (2 mM) some activity was observed (about 10-20% of wild type levels). Consistent with the above findings, the debrisoquine-induced spin changes in the mutant proteins were markedly reduced. These data collectively demonstrate that aspartic acid 301 plays an important role in determining the substrate specificity and activity of CYP2D6 and provide experimental evidence supporting the role of this amino acid in forming an electrostatic interaction between the basic nitrogen atom in CYP2D6 substrates and the carboxylate group of aspartic acid 301.

Potent inhibition of yeast-expressed CYP2D6 by dihydroquinidine, quinidine, and its metabolites

The inhibitory effects of dihydroquinidine, quinidine and several quinidine metabolites on cytochrome P450 2D6 (CYP2D6) activity were examined. CYP2D6 heterologously expressed in yeast cells O-demethylated dextromethorphan with a mean Km of 5.4 microM and a Vmax of 0.47 nmol/min/nmol. Quinidine and dihydroquinidine both potently inhibited CYP2D6 metabolic activity (mean Ki = 0.027 and 0.013 microM, respectively) in yeast microsomes and in human liver microsomes. The metabolites, 3-hydroxyquinidine, O-desmethylquinidine and quinidine N-oxide also inhibited CYP2D6, but their Ki values (0.43 to 2.3 microM) were one to two orders of magnitude weaker than the values for quinidine and dihydroquinidine. There was a trend towards an inverse relationship between Ki and lipophilicity (r = -0.90, N = 5, P = 0.07), as determined by the retention-time parameter k' using reverse-phase HPLC. Thus, although the metabolites of quinidine have the capacity to inhibit CYP2D6 activity, quinidine and the impurity dihydroquinidine are the important inhibitors of CYP2D6.

Pharmacogenetics in clinical pharmacology and toxicology

This subject was particularly important to discuss in the presence of Werner Kalow, 77 years young, who is considered as one of the grandfathers of this unique combination of medical research fields. It has become increasingly appreciated that dozens of human drug metabolism polymorphisms exist. The interindividual variabilities in drug metabolism discussed at this symposium do not represent small differences such as 50% or 3-fold but, rather, represent 10- to greater than 1000-fold differences. When attributed to a single gene, dramatic differences can be seen among family members, just as blue and brown eyes can occur in siblings. These differences can result in acute drug toxicity. In addition, there are chronic effects: over one's lifetime, striking differences in the metabolism of drugs, occupationally hazardous chemicals, and other environmental pollutants can lead to interindividual differences in the buildup of DNA damage (e.g., mutations, chromosomal breaks, rearrangements) leading to toxicity and tumor initiation, as well as leading to a buildup in nongenotoxic signals (signal transduction pathways without DNA damage) important for toxicity, tumor promotion, and tumor progression. The human UDP glucuronosyltransferase (UGT superfamily is known to comprise more than 10 genes in humans, and probably in other mammalian species. Breakthroughs in UGT gene mutations responsible for the Crigler-Najjar syndrome and Gilbert's disease have recently been reported. The human cytochrome P450 termed CYP3A4 is a major P450 enzyme in the liver and gastrointestinal tract, and the full impact of the CYP3A4 polymorphism has yet to be fully appreciated.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the disposition of two novel combined thromboxane synthase inhibitors/thromboxane A2 receptor antagonists in the isolated perfused rat liver

1. The pharmacokinetics of two combined thromboxane synthase inhibitor/thromboxane A2 receptor antagonists were investigated in an isolated perfused rat liver preparation. 2. The clearance of both UK 101613 (15.4 +/- 1.9 ml/min) and UK 102333 (14.9 +/- 1.6 ml/min) was limited by hepatic bloodflow. Both hepatic uptake and biliary excretion of UK 101613 appeared to be active processes. The apparent biliary clearance of UK 101613 (10.3 +/- 2.3 ml/min) was less than perfusate clearance, indicating accumulation within the hepatocytes. 3. The efficiency of the active hepatic uptake of UK 101613 and UK 102333 explains the rapid removal of these compounds from the systemic circulation. The processes of biliary and/or metabolic clearance of UK 101613 and UK 102333 appear to be subsequent to and independent of perfusate clearance. 4. Although the compounds are of similar structure and have a similar hepatic extraction (> 0.9), UK 101613 had an apparent biliary clearance approximately twice that of UK 102333. This is due to the formation of a metabolite of UK 102333 which reduces the amount of parent compound available for biliary excretion. 5. Inhibition of this latter metabolism by ketoconazole (10 microM) resulted in the two compounds having comparable apparent biliary clearances.

Interindividual variation in the isomerization of 4-hydroxytamoxifen by human liver microsomes: involvement of cytochromes P450

Tamoxifen and its metabolite 4-hydroxytamoxifen can both exist as geometrical isomers. Trans-tamoxifen is an oestrogen receptor antagonist and is used for the treatment of breast cancer. Trans-4-hydroxytamoxifen is 100 times more anti-oestrogenic than trans-tamoxifen. The cis isomers of tamoxifen and 4-hydroxytamoxifen are oestrogenic and weakly anti-oestrogenic or oestrogenic respectively. Both isomers of 4-hydroxytamoxifen have been detected in breast tumours of patients treated with trans-tamoxifen and it has been proposed that enzymatic isomerization of 4-hydroxytamoxifen occurs in vivo, resulting in resistance to tamoxifen therapy. We have investigated the isomerization of 4-hydroxytamoxifen by human liver microsomes and whether it is mediated by cytochromes P450. Microsomes from five of the 12 livers examined catalysed the interconversion of trans- and cis-4-hydroxytamoxifen (0.52 microM) when incubated for 40 min with an NADPH-generating system. Between 51 and 64% conversion of trans-4-hydroxytamoxifen was observed. Cis-4-hydroxytamoxifen was also converted to trans-4-hydroxytamoxifen (range 22-27%). Incubations with control, heat-treated microsomes resulted in approximately 1% isomerization of trans-4-hydroxytamoxifen. The extent of isomerization of trans- to cis-4-hydroxytamoxifen observed in microsomes from the other seven livers (range 2-8%) did not greatly exceed that seen in heat-inactivated microsomes. Enzymatic isomerization required NADPH and was inhibited by SKF 525A and ketoconazole, indicating the involvement of cytochromes P450. Enzymatic isomerization of trans-tamoxifen and trans-droloxifene (the 3-hydroxy synthetic analogue of tamoxifen) was not observed. These findings may have implications for the safe and effective use of tamoxifen.

Debrisoquine and mephenytoin oxidation in Sinhalese: a population study

The frequency distributions of the 0-8 h urinary metabolic ratios of debrisoquine and mephenytoin were measured in 111 healthy, unrelated Sinhalese resident in Sri Lanka. Blood samples were taken from 77 of these subjects for CYP2D6 genotyping. Bimodality in the distribution of the log10 debrisoquine/4-hydroxydebrisoquine ratio was not evident from visual inspection and by kernel density analysis. The results of genotyping indicated that 82% of the population were either homozygous for the wild-type CYP2D6 gene or heterozygous for the wild type allele and the whole gene deletion. Eighteen per cent of the Sinhalese population were heterozygous for the CYP2D6B mutation and the wild-type allele. All of these genotypes give rise to the extensive metaboliser phenotype in white Caucasians. No CYP2D6A mutations were identified and no individuals who were homozygous for the mutant alleles were detected, which is in accord with an absence of phenotypic poor metabolisers of debrisoquine. The mutant CYP2D6 allele frequency in Sinhalese (9%) is only half that observed in white Caucasians. The S/R-mephenytoin ratio ranged from 0.09 to 2.27 (median 0.38). By visual inspection and kernel density analysis the distribution of the S/R-mephenytoin ratio was bimodal and, using a value of 0.9 for the antimode, 16 (14%) subjects were poor metabolisers. In conclusion, the prevalence of the poor metaboliser phenotype in Sinhalese appears much lower for debrisoquine and higher for mephenytoin than in white Caucasians. These findings are similar to those observed in Indians living in Bombay and in Oriental populations.

Sensitivity of indirect metrics for assessing "rate" in bioequivalence studies--moving the "goalposts" or changing the "game"

The requirement to assess "rate" in bioequivalence tests using indirect metrics reinforces ambiguities as to whether such testing is intended to assure both pharmaceutical quality with respect to drug release characteristics as well as clinical safety and efficacy. Using a one-compartment open pharmacokinetic model with first-order absorption and error-free data, the effects of systematically changing the ratio of the absorption rate constants of test and reference formulations on various indirect metrics of rate of drug absorption [maximum plasma concentration (Cmax), time to reach Cmax (tmax), mean residence time (MRT), partial area under the plasma concentration-time curve (AUC(t))] were evaluated as a function of the ratio of absorption rate constant to elimination rate constant. This simple simulation illustrates the pitfall of judging the performance of different indirect rate metrics on the basis of a fixed universal acceptance interval for bioequivalence. However, turning the issue on its head, since rate, as indicated by a rate constant, cannot be assessed accurately using indirect metrics and may have little clinical relevance, regulatory guidelines should emphasize the use of Cmax and other measures taken from the plasma drug concentration-time curve as empirical indices of safety and efficacy. The acceptance limits should then depend on clinical criteria and the variability of the reference formulation.

Inhibition of CYP2D6 activity by treatment with propranolol and the role of 4-hydroxy propranolol

1. The 4-hydroxylation of propranolol by rat and human liver microsomes is associated with formation of a chemically reactive species which binds irreversibly to cytochrome P4502D6 (CYP2D6) destroying its catalytic function. Therefore, the effect of propranolol treatment (80 mg twice daily) on debrisoquine phenotype was examined, to see if it resulted in phenocopying in vivo. The role of 4-hydroxypropranolol (4OHP) in the inhibition of CYP2D6 activity was also studied using microsomes from yeast expressing CYP2D6 and from human livers; metoprolol was used as the CYP2D6 substrate. 2. Although a significant effect on apparent oxidation phenotype was demonstrated, the absolute change in the urinary debrisoquine/4-hydroxydebrisoquine ratio (D/4HD) was small, such that no extensive metaboliser who received propranolol treatment was reclassified as a poor metaboliser. The in vitro studies indicated that 4OHP is a potent inhibitor of metoprolol metabolism (Ki approximately 1 microM). This inhibitory effect was enhanced when 4OHP was pre-incubated in the presence of a NADPH generating system and human liver microsomes. The effect was decreased significantly when reduced glutathione was added to the pre-incubation mixture. Metabolism of 4OHP occurred when incubated with human liver microsomes in the presence of a NADPH generating system and irrespective of CYP2D6 phenotype; yeast expressing CYP2D6 did not metabolise 4OHP. 3. We conclude that, although treatment with propranolol 80 mg twice daily significantly decreases the catalytic function of CYP2D6, the inhibition is insufficient to result in phenocopying. The reactive intermediate produced by further metabolism of 4OHP is probably scavenged effectively in vivo by glutathione and other nucleophiles.

An evaluation of cytochrome P450 isoform activities in the female dark agouti (DA) rat: relevance to its use as a model of the CYP2D6 poor metaboliser phenotype

The female dark agouti (DA) rat lacks CYP2D1, the equivalent enzyme in the rat to human CYP2D6 (debrisoquine hydroxylase), and shows impaired metabolism of a number of CYP2D6 substrates. However, from the data available in the literature it is not entirely clear whether the enzyme deficiency in the DA rat is restricted to CYP2D1, and whether factors such as age and substrate concentration are important determinants of interstrain differences in the activity of this enzyme. Given that the female DA rat is used as a model of the human CYP2D6 poor metaboliser phenotype, there is a need for a systematic evaluation of the P450 activities in the DA rat, and of its suitability as a model of the PM phenotype. In the present study metoprolol was used as a probe substrate to investigate CYP2D1 activity since both the alpha-hydroxylation and O-demethylation of this drug are catalysed by CYP2D6 in man. Formation of alpha-hydroxymetoprolol (AHM) and O-demethylmetoprolol (ODM) was 10- and 2.5-fold lower in liver microsomes from female DA rats compared with microsomes from age-matched female Wistar rats, the latter representing the extensive metaboliser strain. Kinetic analysis suggested that in both strains of rat both the alpha-hydroxylation and O-demethylation of metoprolol were catalysed by more than one enzyme. By using quinine as a specific inhibitor of the enzyme, CYP2D1 was identified as an intermediate affinity site in the Wistar strain and was shown to have impaired activity in the DA strain. The activities of lower and higher affinity sites were similar in the two strains. Thus, the only difference between the two strains with respect to both routes of metoprolol metabolism appeared to be in the activity of CYP2D1. Interstrain differences were found to be highly dependent on the choice of substrate concentration, being more marked at lower concentrations. We have also investigated the metabolism of a number of probe compounds for some of the other P450 isoforms commonly involved in drug metabolism to determine the selectivity of the deficiency in the DA strain. p-Nitrophenol hydroxylation and erythromycin N-demethylation were catalysed at higher rates by DA than by Wistar liver microsomes, indicating higher levels of activity of CYP2E1 and CYP3A in the former strain. Felodipine oxidation, tolbutamide hydroxylation and both the hydroxylation and N-demethylation of S-mephenytoin were catalysed at similar rates by microsomes from the two strains, indicating similar activities of enzymes in the CYP2C and CYP3A families.(ABSTRACT TRUNCATED AT 400 WORDS)

The demethylenation of methylenedioxymethamphetamine ("ecstasy") by debrisoquine hydroxylase (CYP2D6)

The metabolism of methylenedioxymethamphetamine (MDMA, "ecstasy") was examined in a microsomal preparation of the yeast Saccharomyces cerevisiae expressing human debrisoquine hydroxylase, CYP2D6. Only one product, dihydroxymethylamphetamine (DHMA), was detected in the incubation mixture, and this product accounted for all of the substrate consumption at low concentration (10 microM). Mean +/- SD values of apparent Km(microM) and Vmax (nmol/min per nmol P450) for the demethylenation of (+) and (-)-MDMA at low concentrations (1-100 microM) were 1.72, 0.12 and 6.45, 0.10 and 2.90, 0.10 and 7.61, 0.06, respectively. At high concentrations (> 1000 microM) substrate inhibition was noted, with Ki values of 14.2 and 28.2 mM, respectively, for the (+) and (-) enantiomers. Incubation of MDMA isomers with human liver microsomes indicated that their demethylenation is deficient in the poor metabolizer phenotype. Thus, MDMA is converted to the catecholamine DHMA by CYP2D6, and this may give rise to genetically-determined differences in toxicity.

The interaction of proton pump inhibitors with cytochromes P450

An understanding of the role of human cytochromes (CYPs) P450 in the metabolism of proton pump inhibitors (PPIs) is important in assuring the safety of these drugs. Species' differences in the metabolism of PPIs tend to overestimate the risk of some forms of toxicity in humans. Therefore, information on metabolism by human enzymes is essential for the prediction of drug interactions, or the lack of them. Although extrapolation from population mean data to the individual patient should be made with care, available data indicate that the use of PPIs, omeprazole, lansoprazole and particularly pantoprazole, should be relatively free of clinically-significant drug interactions. This finding may reflect the level of therapeutic dosages for PPIs, and their short elimination half-lives with respect to exposure to drug-metabolizing enzymes. Concern about low-grade induction of CYPIA-mediated procarcinogen metabolism suggests a need for vigilance on the long-term use of PPIs.

Beeturia and the biological fate of beetroot pigments

Beeturia, the passage of pink or red urine after the ingestion of beetroot, is said to occur in 10-14% of the population, and is more common in iron deficiency and malabsorption. A specific HPLC assay for betacyanins, the red beetroot pigments, in biological fluids was developed to study the prevalence of this apparent polymorphism in humans, and to investigate its basis in rats. Two major peaks were observed in chromatograms of extracts of unpickled beetroot. They had identical UV absorption spectra (lambda max = 535 nm) by diode array analysis, and mass spectrometry indicated that one (betacyanin 1) was betanin or its epimer and the other (betacyanin 2) a disaccharide of betacyanin 1. In a population of 100 normal subjects the 0-8 h urinary recoveries after an oral dose of 60 mg beetroot extract were 0.06-0.54% for betacyanin 1 and 0.01-0.6% for betacyanin 2. The distributions of these data were skewed but not clearly bimodal by visual inspection or by kernel density analysis. Four subjects produced visibly red urine and had betacyanin recoveries at the upper end of the population range. Studies using in situ isolated perfused rat jejunum and liver preparations indicated a negligible absorption of the pigments after 1 h and no detectable metabolism or biliary secretion. Intact anaesthetized rats given i.v. bolus doses of beetroot extract cleared both betacyanins from plasma at the rate of 3.3 +/- 0.9 (SD) ml min-1 (n = 5). The total urinary recovery of both pigments amounted to 80% of the dose, and their renal clearances approached their plasma clearances. These data suggest that beeturia does not arise from deficiencies in hepatic metabolism or renal excretion of betacyanins. After oral administration of beetroot extract to rats the betacyanin content of the stomach decreased rapidly with time but neither the intestines nor the bile duct were stained visibly red. These findings together with those showing instability of the betacyanins in acid conditions suggest that variability in the biological fate of beetroot pigments may be determined largely by gastric pH and emptying rate.

Debrisoquine and metoprolol oxidation in Zambians: a population study

The 0-8 h urinary distributions of the metabolic ratios of debrisoquine (10 mg) and metoprolol (100 mg) were measured in 102 healthy, unrelated, black Zambian medical students. There was a statistically significant correlation (rs = 0.60, p < 0.001; n = 88) between the debrisoquine/4-hydroxydebrisoquine (D/HD) and metoprolol/alpha-hydroxymetoprolol (M/HM) ratios. Bimodality in the distribution of the log10D/HD ratio was not evident from visual inspection and following kernel density analysis of the data, although two subjects (ratios 20, 22) would be classified as phenotypic poor metabolizers (PMs) based on the antimode used for Caucasian populations. The distribution of the log10M/HM ratio was skewed and on the basis of kernel density analysis, bimodal. It was clear from visual inspection of the data that the very high M/HM value (> or = 302) of one individual had a profound influence on the population M/HM ratio distribution. No HM was detected in the urine of this subject but he was not one of the two PMs of debrisoquine (D/HD ratio 1.54). H117/04, the major metabolite of metoprolol was also not detected in this sample. Since H117/04 was shown to be present in all samples from previous population studies, the possibility that this subject did not comply with the protocol could not be excluded. All other subjects had M/HM ratios < or = 12.5. These findings suggest that there is a dissociation in the control of debrisoquine and metoprolol oxidation in Zambians as has been observed previously in Nigerians. Furthermore, clear evidence that the metabolism of these drugs exhibits genetic polymorphism in Zambians was not obtained.

The effect of selective serotonin re-uptake inhibitors on cytochrome P4502D6 (CYP2D6) activity in human liver microsomes

Inhibition of human cytochrome P4502D6 (CYP2D6)-catalysed metabolism can lead to clinically significant alterations in pharmacokinetics. Since there is evidence that the selective serotonin reuptake inhibitor (SSRI) class of antidepressant drugs might inhibit CYP2D6, the effects of five SSRIs on human liver microsomal CYP2D6 activity were compared with each other and with three tricyclic antidepressant drugs. On a molar basis, paroxetine was the most potent of the SSRIs at inhibiting the CYP2D6-catalysed oxidation of sparteine (Ki = 0.15 microM), although fluoxetine (0.60 microM) and sertaline (0.70 microM) had Ki values in the same range. Fluvoxamine (8.2 microM) and citalopram (5.1 microM) also inhibited CYP2D6 activity. The major circulating metabolites of paroxetine in man produced negligible inhibition. In contrast, norfluoxetine the active metabolite of fluoxetine, was a potent CYP2D6 inhibitor (0.43 microM). CYP2D6 activity was also diminished by the tricyclic antidepressant drugs clomipramine (2.2 microM), desipramine (2.3 microM) and amitriptyline (4.0 microM). These findings suggest that compounds with SSRI activity are likely to interact with human CYP2D6 in vivo with the potential of causing drug interactions.

Catalytic activities of human debrisoquine 4-hydroxylase cytochrome P450 (CYP2D6) expressed in yeast

A 1.57kb BamH1 fragment containing a full-length human debrisoquine 4-hydroxylase cytochrome P450 (CYP2D6) cDNA was inserted into the BglII site of the yeast expression plasmid pMA91 and the resulting recombinant plasmid, PELT1, introduced into Saccharomyces cerevisiae strain AH22. Microsomes prepared from AH22/pELT1 cells gave an absorption maximum at 448 nm and a P450 content of 67 +/- 31 pmol/mg of microsomal protein. No P450 was detectable in microsomes prepared from AH22/pMA91 control cells. A western blot of microsomes prepared from yeast transformed with pELT1 were probed with a monoclonal antibody to CYP2D6 and revealed a strong band with a molecular mass consistent with that of CYP2D6 from human liver microsomes. No corresponding band was observed with microsomes from control yeast transformed with pMA91 alone. Microsomes from AH22/pELT cells showed catalytic activity towards metoprolol (alpha-hydroxylation and O-demethylation, 0.17 and 0.78 nmol/mg protein/h, respectively); and towards sparteine (2- and 5-dehydrogenation, 1.82 and 0.59 nmol/mg protein/h, respectively). The inhibition of metoprolol metabolism by quinidine (Qd) was 200 times more potent than that of quinine (Qn), both for alpha-hydroxylation (Qd IC50 = 0.05 microM; Qn IC50 = 4 microM) and O-demethylation (Qd IC50 = 0.05 microM; Qn IC50 = 4 microM). Negligible metabolism of tolbutamide and S-mephenytoin, substrates of the 2C sub-family, and of p-nitrophenol, a substrate of CYP2E1, was detected, although a trace of the N-deethylated metabolite of lignocaine, thought to be metabolised by CYP3A4, was detected with microsomes from CYP2D6-expressing yeast cells. The results indicate that yeast cells containing human CYP2D6 cDNA express a functionally active form of the enzyme, the immunochemical and catalytic properties of which are consistent with those of human liver.

The role of cytochrome P4502D6 in the metabolism of paroxetine by human liver microsomes

Paroxetine is a selective serotonin reuptake inhibitor possessing anti-depressant activity. Demethylenation of the methylenedioxy phenyl group is the initial step in its metabolism, the liberated carbon appearing in vitro as formate. A radioassay involving [14C-methylenedioxy] paroxetine was developed and used to examine the role of cytochrome P4502D6 in paroxetine metabolism by human liver microsomes. The rate of formate production was much higher in microsomes from an extensive metaboliser of debrisoquine than from a poor metaboliser. Also, demethylenation of paroxetine was inhibited by the quinidine and quinine isomer pair in microsomes from the extensive metaboliser only. These observations strongly suggested that the process was catalysed by the enzyme cytochrome P4502D6. Metabolism could not be completely inhibited by quinidine, the residual activity representing the contribution of at least one other enzyme. The ability of microsomes from a poor metaboliser of debrisoquine to demethylenate paroxetine provided further evidence for the involvement of an enzyme distinct from P4502D6. This was confirmed by kinetic analysis of the process in microsomes from both poor and extensive metabolisers. It is concluded that, in man, the initial step of paroxetine metabolism is performed by at least two enzymes, one of which is cytochrome P4502D6.

Evidence for a dissociation in the control of sparteine, debrisoquine and metoprolol metabolism in Nigerians

The 0-8 hour urinary distributions of the metabolic ratios of sparteine (100 mg), debrisoquine (10 mg) and metoprolol (100 mg) were measured in 165 healthy, unrelated, black Nigerian medical students. There was a weak correlation (rs = 0.51, p < 0.001; n = 82) between the metoprolol/alpha-hydroxymetoprolol (M/HM) and the sparteine/total (2- + 5-) dehydrosparteine (S/DHS) ratios. No significant correlations were found between the debrisoquine/4-hydroxydebrisoquine (D/HD) and M/HM ratios (rs = 0.16, n = 33) and between the D/HD and S/DHS ratios (rs = 0.31, n = 38). Both visual inspection and kernel density analysis of the data suggested the presence of two phenotypic groups for sparteine oxidation, with 4% of the population studied being putative poor metabolizers. In contrast biomodality was not apparent in the distribution of the log10M/HM and log10D/HD ratios. These findings provide evidence for a dissociation in the control of metoprolol, sparteine and debrisoquine oxidation in Nigerians and highlight the difficulties in the interpretation of data from pharmacogenetic studies in different ethnic groups.

The site of inversion of R(-)-ibuprofen: studies using rat in-situ isolated perfused intestine/liver preparations

The site of metabolic inversion of R(-)-ibuprofen to the pharmacologically active S(+)-enantiomer has been investigated using an array of in-situ rat perfused organ preparations allowing vascular perfusion (55-60 min) of the separate or combined intestine and liver. After addition of R(-)-ibuprofen (20 mg kg-1 body weight) to the closed (static) lumen of isolated 25 cm lengths of duodenum, jejunum or ileum, and single-pass vascular perfusion, both isomers were measured in the lumen and in vascular perfusate plasma (mean plasma AUC values (+/- s.d., micrograms mL-1 min, n = 5) R(-)-ibuprofen: 1669 +/- 115 (duodenum), 1687 +/- 203 (jejunum), 2061 +/- 188 (ileum); S(+)-ibuprofen: 23 +/- 6 (duodenum), 14 +/- 5 (jejunum), 26 +/- 1 (ileum]. Addition of the same dose of S(+)-ibuprofen to the jejunum (n = 5) resulted in AUC values of 1864 +/- 238 for S(+)-ibuprofen and 6 +/- 3 for R(-)-ibuprofen. After addition of R(-)-ibuprofen (30 micrograms mL-1) to the recirculating vascular perfusate (100 mL) of the entire small intestine (n = 6) AUC values were 1647 +/- 34 for R(-)-ibuprofen and 13 +/- 3 for S(-)-ibuprofen. The same dose of R(-)-ibuprofen to combined intestine/liver (n = 6) and liver only preparations (n = 6) gave AUC values of 1011 +/- 25 and 1021 +/- 49 for R(-)-ibuprofen and 220 +/- 28 and 238 +/- 22 for S(+)-ibuprofen, respectively. In all experiments, except those involving perfusion of the combined intestine/liver and the liver, the concentrations of the isomer opposite to that administered could be accounted for solely by the level of enantiomeric impurity (1.3% for R(-)-ibuprofen and 0.6% for S(+)-ibuprofen). We conclude that inversion of R(-)-ibuprofen to the S(+) antipode occurs in the liver but does not occur on either mucosal or serosal sides of the small intestine of the rat.