Phenotypes and genotypes for CYP2D6 and CYP2C19 in a black Tanzanian population.

AIMS: CYP2D6 and CYP2C19 are polymorphically expressed enzymes that show marked interindividual and interethnic variation. The aim of this study was to determine the frequency of the defective alleles in CYP2D6 and CYP2C19 in Africans and to test whether the genotype for CYP2C19 is better correlated with the proguanil/cylcoguanil ratio than the mephenytoin S/R ratio. METHODS: Two hundred and sixteen black Tanzanians were phenotyped for CYP2D6 with the use of sparteine, and for CYP2C19 with the use of mephenytoin and proguanil. Of these 196 subjects were also genotyped for CYP2D6 (including the CYP2D6*1, CYP2D6*3 and CYP2D6*4 alleles) and 195 were genotyped for CYP2C19 (including the CYP2C19*1, CYP2C19*2 and the CYP2C19*3 alleles). Furthermore 100 subjects were examined for the allele duplication in CYP2D6, leading to ultrarapid metabolism, with long PCR. RESULTS: The sparteine metabolic ratio (MR) was statistically significantly higher in the Tanzanian group of homozygous, extensive metabolizers compared to a historical control group of white Danish extensive metabolizers. Only one poor metabolizer for CYP2D6 (MR=124 and genotype CYP2D6*1/CYP2D6*4 ) was found. The gene frequencies were 0.96 for the CYP2D6*1 allele and 0.04 for the CYP2D6*4 allele. No CYP2D6*3 alleles were found. Nine subjects had an allele duplication in CYP2D6 (9%). For CYP2C19 there were seven subjects (3. 6%) who were phenotyped as poor metabolizers, but only three subjects (1.5%) had a genotype (CYP2C19*2/CYP2C19*2 ) indicative of poor metabolism. The gene frequencies were 0.90 for the CYP2C19*1 allele and 0.10 for the CYP2C19*2 allele. No CYP2C19*3 alleles were found. The mephenytoin S/R ratios were not bimodally distributed. CONCLUSIONS: Both the genotyping and phenotyping results show that there is a substantial difference between an African black population and a Caucasian population in the capacity to metabolize drugs via CYP2D6 and CYP2C19.

[Interaction between paroxetine and clomipramine as a possible reason for admission to a department of internal medicine]. / Interaktion imellem paroxetin og clomipramin som mulig årsag til indlaeggelse på medicinsk afdeling.

A 79-year-old woman was admitted due to increasing dizziness during the past six days. She had been treated with clomipramine 75 mg daily for years, and in addition had received paroxetine 20 mg daily for the last eight days. Both drugs were terminated and the symptoms disappeared. Thirty-six hours after taking the last drugs, the S-clomipramine plus S-desmethylclomipramine were 790 mmol/l. The symptoms were probably related to this high serum level caused by interaction with paroxetine.

Chloroguanide metabolism in relation to the efficacy in malaria prophylaxis and the S-mephenytoin oxidation in Tanzanians.

S-Mephenytoin and chloroguanide (proguanil) oxidation was studied in 216 tanzanians. The mephenytoin S/R ratio in urine ranged from <0.1 to 1.16. The distribution was skewed to the right, without evidence of a bimodal distribution. Ten subjects (4.6%, 2.2% to 8.3%, 95% CI) with an S/R mephenytoin ratio >0.9, were arbitrarily defined as poor metabolizers of mephenytoin. The chloroguanide/cycloguanil ratio ranged from 0.82 to 249. There was a significant correlation between the mephenytoin S/R ratio and the chloroguanide/cycloguanil ratios (rs = 0.73; p<0.00001). This indicates that cytochrome P4502C19 or CYP2C19 is a major enzyme that catalyzes the bioactivation of chloroguanide to cycloguanil. Chloroguanide is a pro-drug, and hence a low CYP2C19 activity may lead to prophylactic failure caused by inadequate formation of cycloguanil. Fifty-eight women who previously took either 200 mg chloroguanide daily (n = 26) or 200 mg chloroguanide daily plus 300 mg chloroquine weekly (n = 32) in a malaria chemoprophylaxis study showed that there was significant correlation between the number of earlier breakthrough parasitemia episodes and the chloroguanide/cycloguanil ratio (rs = 0.30; p = 0.02). The breakthrough rate did not correlate with the S/R mephenytoin ratio. However, other factors, such as exposure to mosquitoes and sensitivity of the plasmodium to cycloguanil, are probably more important.

Proguanil metabolism is determined by the mephenytoin oxidation polymorphism in Vietnamese living in Denmark.

1. A sparteine/mephenytoin phenotyping test was carried out in 37 Vietnamese living in Denmark. By visual inspection the urinary S/R-mephenytoin ratio appeared to show a bimodal frequency distribution. Eight putative poor metabolizers of mephenytoin, PMm (22%), had S/R-mephenytoin ratios from 0.79 to 1.12 and 29 putative extensive metabolizers of mephenytoin, EMm, had S/R-mephenytoin ratios < or = 0.55. All of the subjects were extensive metabolizers of sparteine with urinary metabolic ratios from 0.15 to 2.4. 2. The metabolism of the antimalarial prodrug proguanil was studied in 34 of the subjects after a single oral dose of 100 mg. The median 12 h urinary recoveries of the active metabolite cycloguanil and the minor metabolite 4-chlorphenylbiguanide were 5.8 and 1.9% of the dose, respectively, in 26 EMm compared with 1.6 and 0.4%, respectively, in 8 PMm (P < 0.001, Mann-Whitney U-test). 3. There was no statistically significant correlation (Spearmans rs) between any index of proguanil metabolism and the sparteine metabolic ratio.

The N-demethylation of imipramine correlates with the oxidation of S-mephenytoin (S/R-ratio). A population study.

The metabolism of imipramine was investigated in 106 healthy volunteers, all having a sparteine metabolic ratio (MR) of 0.2-0.5 and hence classified as extensive metabolisers. Each subject was given a single oral dose of 25 mg imipramine hydrochloride and blood for assays of imipramine and metabolites was collected 3 h thereafter. The desipramine/imipramine ratio and the 2-OH-desipramine/2-OH-imipramine ratio in plasma, reflecting the demethylation of imipramine and 2-OH-imipramine, respectively, showed significant negative correlations with the mephenytoin S/R ratio (Spearman rank correlation) (rs): -0.46, P < 0.00002 and -0.41, P < 0.00002). No correlations were found between the 2-hydroxylation of imipramine or desipramine and the mephenytoin S/R. These findings confirm those of an earlier panel study showing that the demethylation of imipramine and 2-OH-imipramine cosegregates in part with the mephenytoin oxidation polymorphism.

Fluvoxamine is a potent inhibitor of cytochrome P4501A2.

Fluvoxamine is a new antidepressant and selectively inhibits serotonin reuptake (SSRI). The present study demonstrates that fluvoxamine is a very potent inhibitor of the high-affinity O-deethylation of phenacetin, which is catalysed by cytochrome P4501A2 (CYP1A2), in microsomes from three human livers. Thus, the apparent inhibitor constant of fluvoxamine, Ki, ranged from 0.12 to 0.24 microM. Seven other SSRIs, citalopram, N-desmethylcitalopram, fluoxetine, norfluoxetine, paroxetine, sertraline and litoxetin either did not inhibit or were weak inhibitors of the O-deethylation of phenacetin. Our findings explain the mechanism of the pharmacokinetic interactions between fluvoxamine and drugs that are metabolized by CYP1A2, e.g. theophylline and imipramine.

Inhibitors of imipramine metabolism by human liver microsomes.

1. The aromatic 2-hydroxylation of imipramine was studied in microsomes from three human livers. The kinetics were best described by a biphasic enzyme model. The estimated values of Vmax and Km for the high affinity site ranged from 3.2 to 5.7 nmol mg-1 h-1 and from 25 to 31 microM, respectively. 2. Quinidine was a potent inhibitor of the high affinity site for the 2-hydroxylation of imipramine in microsomes from all three human livers, with apparent Ki-values ranging from 9 to 92 nM. This finding strongly suggests that the high affinity enzyme is CYP2D6, the source of the sparteine/debrisoquine oxidation polymorphism. 3. The selective serotonin reuptake inhibitors (SSRI), paroxetine, fluoxetine and norfluoxetine were potent inhibitors of the high affinity site having apparent Ki-values of 0.36, 0.92 and 0.33 microM, respectively. Three other SSRIs, citalopram, desmethylcitalopram and fluvoxamine, were less potent inhibitors of CYP2D6, with apparent Ki-values of 19, 1.3 and 3.9 microM, respectively. 4. Among 20 drugs screened, fluvoxamine was the only potent inhibitor of the N-demethylation of imipramine, with a Ki-value of 0.14 microM. 5. Neither mephenytoin, citalopram, diazepam, omeprazole or proguanil showed any inhibition of the N-demethylation of imipramine and the role of the S-mephenytoin hydroxylase for this oxidative pathway could not be confirmed.

The relationship between paroxetine and the sparteine oxidation polymorphism.

The relationship between the selective serotonin reuptake inhibitor paroxetine and the sparteine oxidation polymorphism was investigated in a combined single-dose (30 mg) and steady-state (30 mg/day for 2 weeks) study including a panel of nine extensive metabolizers and eight poor metabolizers of sparteine. The median area under the plasma concentration-time curve (AUC) after the first paroxetine dose was about seven times higher in poor metabolizers than in extensive metabolizers (3910 versus 550 nmol.hr/L), whereas at steady state the median AUCss tau interphenotype difference was only twofold (4410 versus 2550 nmol.hr/L). Plasma half-life and steady-state plasma concentration were significantly longer and higher, respectively, in poor metabolizers than in extensive metabolizers (41 versus 16 hours and 151 versus 81 nmol/L). Paroxetine pharmacokinetics were linear in poor metabolizers and nonlinear only in extensive metabolizers. Sparteine metabolic ratio (MR = 12 hour urinary ratio of sparteine/dehydrosparteine), increased during treatment with paroxetine in subjects who were extensive metabolizers, and after 14 days treatment two extensive metabolizers were phenotyped as poor metabolizers and the remaining extensive metabolizers were changed into extremely slow extensive metabolizers with sparteine MRs of 5.7 to 16.5. The inhibition of sparteine metabolism was rapidly reversed after cessation of paroxetine administration. In the poor metabolizers there were no significant changes in MRs during the study. It is concluded that paroxetine and sparteine metabolism cosegregates, but the interphenotype difference in metabolism was less prominent at steady state than after a single dose, presumably because of saturation of the sparteine oxygenase (CYP2D6) in subjects who were extensive metabolizers. Paroxetine is a potent inhibitor of sparteine oxidation by CYP2D6 in vivo.

The activation of the biguanide antimalarial proguanil co-segregates with the mephenytoin oxidation polymorphism--a panel study.

The activation of the antimalarial drug proguanil (PG) to the active metabolite cycloguanil (CG) has been evaluated in a panel of 18 subjects. These subjects had previously been screened and classified as mephenytoin poor (PMm) or extensive metabolisers (EMm) and sparteine poor (PMs) or extensive metabolisers (EMs). Five subjects had the phenotype PMm/EMs, one was PMm/PMs, six subjects were EMm/PMs and six were EMm/EMs. The PG/CG ratio in urine (8 h) was significantly higher in PMm than in EMm (P = 0.0013). This study shows that the P450-isozyme involved in the polymorphic oxidation of mephenytoin is of critical importance in the activation of PG to CG and this may explain the large intersubject variability in CG concentrations in man. PMm make up about 3% of Caucasians, but up to about 20% of Orientals. From the present study, it may be anticipated that the antimalarial effect of PG is absent or impaired in this phenotype. The sparteine polymorphism appeared not to influence the activation of PG to CG significantly.

The mephenytoin oxidation polymorphism is partially responsible for the N-demethylation of imipramine.

The metabolism of imipramine in six poor metabolizers of mephenytoin was compared with the metabolism of 16 extensive metabolizers of mephenytoin from an earlier study. Each subject was given single doses of 100 mg imipramine hydrochloride and 100 mg desipramine hydrochloride on separate occasions. Imipramine demethylation clearance was 0.74 L.min-1 (mean; range, 0.31-1.24) in poor metabolizers of mephenytoin compared with 1.43 L.min-1 (mean; range, 0.61-3.81) in extensive metabolizers of mephenytoin (p = 0.01, Mann-Whitney U test). It has previously been shown that the imipramine clearance by way of other pathways and desipramine oral clearance, both largely representing 2-hydroxylation, are considerably lower in poor metabolizers of sparteine than in extensive metabolizers of sparteine. In contrast, five subjects who were poor metabolizers of mephenytoin and extensive metabolizers of sparteine and a control group of 11 subjects who were extensive metabolizers of mephenytoin and sparteine showed no statistically significant difference with regard to these parameters. One subject who was a poor metabolizer of mephenytoin and sparteine had the lowest imipramine oral clearance of all 22 subjects studied. In conclusion, this and an earlier study show that the oxidation of imipramine is mediated by means of two different polymorphic P450 isozymes, 2-hydroxylation by way of the sparteine oxygenase (P450IID6) and demethylation by way of the mephenytoin oxygenase (P450IIC8).