Relation between chloroguanide bioactivation to cycloguanil and the genetically determined metabolism of mephenytoin in humans

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Clinical relevance of genetic polymorphisms in the human CYP2C subfamily

The human CYP2Cs are an important subfamily of P450 enzymes that metabolize approximately 20% of clinically used drugs. There are four members of the subfamily, CYP2C8, CYP2C9, CYP2C19, and CYP2C18. Of these CYP2C8, CYP2C9, and CYP2C19 are of clinical importance. The CYP2Cs also metabolize some endogenous compounds such as arachidonic acid. Each member of this subfamily has been found to be genetically polymorphic. The most well-known of these polymorphisms is in CYP2C19. Poor metabolizers (PMs) of CYP2C19 represent approximately 3-5% of Caucasians, a similar percentage of African-Americans and 12-100% of Asian groups. The polymorphism affects metabolism of the anticonvulsant agent mephenytoin, proton pump inhibitors such as omeprazole, the anxiolytic agent diazepam, certain antidepressants, and the antimalarial drug proguanil. Toxic effects can occur in PMs exposed to diazepam, and the efficacy of some proton pump inhibitors may be greater in PMs than EMs at low doses of these drugs. A number of mutant alleles exist that can be detected by genetic testing. CYP2C9 metabolizes a wide variety of drugs including the anticoagulant warfarin, antidiabetic agents such as tolbutamide, anticonvulsants such as phenytoin, and nonsteroidal anti-inflammatory drugs. The incidence of functional polymorphisms is much lower, estimated to be 1/250 in Caucasians and lower in Asians. However, the clinical consequences of these rarer polymorphisms can be severe. Severe and life-threatening bleeding episodes have been reported in CYP2C9 PMs exposed to warfarin. Phenytoin has been reported to cause severe toxicity in PMs. New polymorphisms have been discovered in CYP2C8, which metabolizes taxol (paclitaxel). Genetic testing is available for all of the known CYP2C variant alleles.

Relationship between proguanil metabolic ratio and CYP2C19 genotype in a Caucasian population

AIMS

To investigate the relationship between proguanil metabolic ratio (MR, proguanil/cycloguanil) and CYP2C19 genotype in a Caucasian population.

METHODS

Ninety-nine Caucasians (age range: 18-55 years, 54 female, 45 male) were genotyped for CYP2C19 and phenotyped for proguanil oxidation by collecting urine for 8 h after taking 100 mg proguanil hydrochloride. Proguanil and cycloguanil concentrations were measured by h.p.l.c. PCR was employed for CYP2C19 genotyping.

RESULTS

The three (3%) individuals who were homozygous for CYP2C19*2 (*2/*2) had the highest proguanil MRs (range: 8.0-134.6). Seventy-three (74%) individuals were homozygous for the wild-type allele (*1/*1) and 23 (23%) were heterozygous (*1/*2). The *1/*1 individuals had lower MRs (median=1.4, range: 0.23-5.9, P=0.003, Mann-Whitney U-test) than the *1/*2 subjects (median=2.5, range: 0.88-7.3).

CONCLUSIONS

A CYP2C19 gene-dose effect for proguanil oxidation to cycloguanil was observed, confirming a role for CYP2C19 in cycloguanil formation in vivo. However, there was substantial overlap of proguanil MRs in subjects of different CYP2C19 genotypes, due possibly to variability in the activity of other enzymes contributing to the formation of cycloguanil.

Molecular modelling of human CYP2C subfamily enzymes CYP2C9 and CYP2C19: rationalization of substrate specificity and site-directed mutagenesis experiments in the CYP2C subfamily

1. The results of molecular modelling of human CYP2C isozymes, CYP2C9 and CYP2C19, are reported based on an alignment with a bacterial form of the enzyme, CYP102. 2. The three-dimensional structures of the CYP2C enzymes are consistent with known experimental evidence from site-directed mutagenesis, antibody recognition and regiospecificity of substrate metabolism. 3. The variations in substrate specificity between CYP2C9 and CYP2C19 can be rationalized in terms of single amino acid residue changes within the putative active site region, of which I99H appears to be the most significant.

Fluvoxamine inhibits the CYP2C19-catalyzed bioactivation of chloroguanide

OBJECTIVE

To investigate the interaction between fluvoxamine and chloroguanide (INN, proguanil) to confirm that fluvoxamine inhibits CYP2C19.

METHODS

The study was carried out with a randomized, in vivo, crossover design. Six volunteers were extensive metabolizers of the S-mephenytoin oxidation polymorphism, and six volunteers were poor metabolizers. In period A of the study, each subject took 200 mg chloroguanide orally. In period B, each subject took 100 mg/day fluvoxamine for 8 days and on day 6 ingested 200 mg chloroguanide. In both periods, blood and urine were sampled at regular intervals. Chloroguanide and its two metabolites cycloguanil and 4-chlorphenylbiguanide in plasma and in urine were assayed by means of HPLC.

RESULTS

During fluvoxamine use, the median of the total clearance of chloroguanide decreased in a statistically significant way from 1282 ml/min to 782 ml/min among the extensive metabolizers, whereas there was no change among the poor metabolizers. The partial clearance of chloroguanide by means of cydoguanil and 4-chlorphenylbiguanide formation among the extensive metabolizers decreased from 222 ml/min and 97 ml/min before to 33 ml/min and 11 ml/min during fluvoxamine intake, respectively. Among poor metabolizers the corresponding values were 35 ml/min and 7.6 ml/min before and 38 ml/min and 6.9 ml/min during fluvoxamine intake. For each metabolite clearance the change was statistically significant among the extensive metabolizers but not among the poor metabolizers. Both cycloguanil and 4-chlorphenylbiguanide formation clearances were statistically significantly higher among the extensive metabolizers than the poor metabolizers in period A but not in period B (phenocopy).

CONCLUSION

Fluvoxamine is an effective inhibitor of CYP2C19.

Modified high-performance liquid chromatographic method to measure both dextromethorphan and proguanil for oxidative phenotyping

The activities of the polymorphic enzymes cytochromes P450 2D6 and 2C19 can be assessed by administering the probe drugs, dextromethorphan and proguanil, respectively. An existing high-performance liquid chromatographic technique, which measures dextromethorphan and its metabolites, has been modified to also measure proguanil and its polymorphic metabolite, cycloguanil in urine. Proguanil and cycloguanil are assayed in separate aliquots of urine to that used for dextromethorphan/dextrorphan as pretreatment with beta-glucuronidase is required for the analysis of dextrorphan. To assay all four compounds a common extraction procedure is used and a single reversed-phase column and isocratic mobile phase with UV and fluorescence detectors connected in series are required. This technique is specific and sensitive for each analyte (limits of detection, dextrorphan/dextromethorphan/proguanil: 0.1 microgram/ml, cycloguanil: 0.2 microgram/ml). All assays are linear over the concentration ranges investigated (dextromethorphan/dextrorphan: 0.5-10 micrograms/ml, proguanil/cycloguanil: 1-20 micrograms/ml). The method described therefore uses laboratory resources very efficiently for all the assays required for hydroxylation phenotyping using proguanil and dextromethorphan.

Drug interactions with proton pump inhibitors

Omeprazole, lansoprazole and pantoprazole are all mainly metabolised by the polymorphically expressed cytochrome P450 (CYP) isoform CYP2C19 (S-mephenytoin hydroxylase). All 3 proton pump inhibitors have a very limited potential for drug interactions at the CYP level. Small effects on CYP reported for these compounds are usually of no clinical relevance. No dose related adverse effects have been identified, suggesting that the small proportion of slow metabolisers is at no additional risk for clinically important drug interactions. The absorption of some compounds, e.g. benzylpenicillin (penicillin G), are altered during treatment with proton pump inhibitors as a result of the increased intragastric pH. A synergy has been confirmed between omeprazole and amoxicillin or clarithromycin in the antibacterial effect against Helicobacter pylori.

Pharmacokinetics, metabolism and interactions of acid pump inhibitors. Focus on omeprazole, lansoprazole and pantoprazole

This review updates and evaluates the currently available information regarding the pharmacokinetics, metabolism and interactions of the acid pump inhibitors omeprazole, lansoprazole and pantoprazole. Differences and similarities between the compounds are discussed. Omeprazole, lansoprazole and pantoprazole are all mainly metabolished by the polymorphically expressed cytochrome P450 (CYP) isoform S-mephenytoin hydroxylase (CYP2C19), which means that within a population a few individuals (3% of Caucasians) metabolise the compounds slowly compared with the majority of the population. For all 3 compounds, the area under the plasma concentration-versus-time curve (AUC) for a slow metaboliser is, in general, approximately 5 times higher than that in an average patient. Since all 3 compounds are considered safe and well tolerated, and no dosage-related adverse drug reactions have been identified, this finding seems to be of no clinical relevance. The acid pump inhibitors seem to be similarly handled in the elderly, where a somewhat slower elimination can be demonstrated compared with young individuals. In patients with renal insufficiency, omeprazole is eliminated as in healthy individuals, whereas the data on lansoprazole and pantoprazole are unresolved. In patients with hepatic insufficiency, as expected, the elimination rates of all 3 compounds are substantially decreased. No clinically relevant effects on specific endogenous glandular functions, such as the adrenal (cortisol), the gonads or the thyroid, were demonstrated for omeprazole and pantoprazole, whereas a few minor concerns have been raised regarding lansoprazole. The absorption of some compounds, e.g. digoxin, might be altered as a result of the increased gastric pH obtained during treatment with acid pump inhibitors, and, accordingly, similar effects are expected irrespective of which acid pump inhibitor is given. The effect of the acid pump inhibitors on enzymes in the liver has been intensely debated, and some authors have claimed that lansoprazole and pantoprazole have less potential than omeprazole to interact with other drugs metabolised by CYP. However, after assessment of available data in this area, the conclusion is that all 3 acid pump inhibitors have a very limited potential for drug interactions at the CYP level. In addition, the small effects on CYP reported for these compounds are rarely of any clinical relevance, considering the normal intra- (and inter-)individual variations in metabolism observed for most drugs. In conclusion, omeprazole, lansoprazole and pantoprazole are structurally very similar, and an evaluation of available data indicates that also with respect to pharmacokinetics, metabolism and interactions in general they demonstrate very similar properties, even though omeprazole has been more thoroughly studied with regard to different effects.

Pharmacokinetic evaluation of proguanil: a probe phenotyping drug for the mephenytoin hydroxylase polymorphism

1. Proguanil (PG) oxidative metabolism to cycloguanil (CG) has been linked to the CYP2C19-mediated genetic polymorphism in S-mephenytoin oxidative metabolism. In many countries, rac-mephenytoin can no longer be administered to humans and hence proguanil may be a more suitable probe for phenotyping purposes. 2. There are limited data on the pharmacokinetics of PG and CG and in particular, whether there is a relationship between the urinary metabolic ratio of PG and its partial intrinsic clearance to CG. 3. The disposition of a 100 mg oral dose of PG was investigated in 10 subjects with widely varying metabolic ratios (pre-study urinary metabolic ratio CG to PG = 0.068 to 1.11). Blood samples and all urine were collected for 96 h and assayed for PG and CG by h.p.l.c. 4. The urinary recovery of PG ranged from 30 to 69% of the dose and for CG from 2.8 to 32% of the dose. The overall urinary recovery of PG plus CG ranged from 54 to 77% of the dose. The AUC for PG ranged from 3.2 to 9.5 mg l-1 h whereas for CG it was from 0.02 to 0.71 mg l-1 h. The partial intrinsic clearance to CG ranged 25-fold from 0.41 to 10.1 l h-1. 5. There was a highly significant (r2 = 0.96, P < 0.001) relationship between the urinary metabolic ratio for PG (as CG/PG) and its partial intrinsic clearance to CG. 6. These data have provided evidence for the justification of the use of the urinary metabolic ratio of proguanil for population phenotyping purposes, provided systematic variation in renal drug clearance between populations is considered.

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.

Comparison of chloroguanide and mephenytoin for the in vivo assessment of genetically determined CYP2C19 activity in humans

OBJECTIVES

The main objective of this study was to examine the relations between chloroguanide (proguanil) and mephenytoin metabolic ratios to determine whether or not chloroguanide could replace mephenytoin as a probe for the indirect in vivo measurement of CYP2C19 activity. An additional objective was to examine the interactions between chloroguanide, omeprazole, and mephenytoin, which are three substrates of CYP2C19.

METHODS

Twenty healthy volunteers received 200 mg chloroguanide orally on three separate occasions in an open, randomized-sequence crossover design: once alone, once 2 hours before the oral administration of 100 mg mephenytoin, and once after oral administration for 7 days of 40 mg/day omeprazole. During one additional period, 100 mg mephenytoin was administered orally. The chloroguanide to cycloguanil ratio was determined in plasma 4 hours after drug administration; it was determined in urine collected over 4, 8, and 24 hours. The mephenytoin hydroxylation index was also measured in urine.

RESULTS

All subjects were extensive metabolizers of chloroguanide and mephenytoin. We found no correlation between the mephenytoin hydroxylation index and the chloroguanide to cycloguanil ratio in any of the urine samples collected or in plasma. In the presence of chloroguanide, mephenytoin hydroxylation index increased from a baseline value of 1.2 +/- 0.2 to 1.7 +/- 1.0 (p < 0.05). In the presence of omeprazole, the chloroguanide to cycloguanil metabolic ratio in 24-hour urine increased from 2.2 +/- 1.0 to 5.6 +/- 3.2 (p < 0.001).

CONCLUSION

Chloroguanide inhibits the CYP2C19-dependent 4'-hydroxylation of mephenytoin. The bioactivation of chloroguanide to cycloguanil is inhibited by the CYP2C19 substrate omeprazole. However, the chloroguanide to cycloguanil metabolic ratio does not reflect the same array of S-mephenytoin hydroxylase activities found in extensive metabolizers as that show by the mephenytoin hydroxylation index.

Metabolic disposition of proguanil in extensive and poor metabolisers of S-mephenytoin 4'-hydroxylation recruited from an Indonesian population

1. The metabolism of proguanil (PG) was studied by measuring PG, cycloguanil (CG) and 4-chlorophenylbiguanide (CPB) in plasma and urine samples after an oral 200 mg dose of PG hydrochloride administered to 14 extensive (EMs) and 10 poor hydroxylators (PMs) of S-mephenytoin of Indonesian origin. 2. The mean ( +/- s.d.) values of the elimination half-life (t 1/2) and AUC of PG were significantly (P < 0.01) greater in the PM than in the EM group (20.6 +/- 3.1 vs 14.6 +/- 3.5 (95% confidence intervals of difference 3.1 to 8.9) h; and 5.43 +/- 1.89 vs 3.68 +/- 0.83 (0.58 to 2.91) micrograms ml-1 h). 3. Plasma concentrations of CG, an active metabolite, could not be detected in all PMs, and those of CPB were sufficiently high to determine a time-course in only four PMs. Mean AUC(0,24 h) values of CPB were significantly (P < 0.05) lower in the PM (n = 4) than in the EM group (n = 14) (0.47 +/- 0.13 vs 0.88 +/- 0.50 (-0.14 to 0.96) micrograms ml-1 h). 4. Log10 percentage urinary recovery of 4'-hydroxymephenytoin correlated significantly (P < 0.05) with the t 1/2 (rs = -0.661) and AUC (rs = -0.652) of PG. 5. PG, CG and CPB were detectable in urine at 12 h in all subjects. Log10 percentage urinary recovery of 4'-hydroxymephenytoin correlated significantly (P < 0.01) with urinary PG/CG (rs = -0.876), PG/CPB (rs = -0.833) and PG/(CG + CPB) (rs = -0.831) metabolic ratios.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of liver metabolic function. Clinical implications

Inter- and intraindividual variability in pharmacokinetics of most drugs is largely determined by variable liver function as described by parameters of hepatic blood flow and metabolic capacity. These parameters may be altered as a result of disease affecting the liver, genetic differences in metabolising enzymes, and various types of drug interactions, including enzyme induction, enzyme inhibition or down-regulation. With the now known large number of drug metabolising enzymes, their differential substrate specificity, and their differential induction or inhibition, each test substance of liver function should be used as a probe for its specific metabolising enzyme. Thus, the concept of model test-substances providing general information about liver function has severe limitations. To test the metabolic activity of several enzymes, either several test substances may be given (cocktail approach) or several metabolites of a single test substance may be analysed (metabolic fingerprint approach). The enzyme-specific analysis of liver function results in a preference for analysis of the metabolites rather than analysis of the clearance of the parent test substance. There are specific methods to quantify the activity of cytochrome P450 enzymes such as CYP1A2, CYP2C9, CYP2C19MEPH, CYP2D6, CYP2E1, and CYP3A, and phase II enzymes, such as glutathione S-transferases, glucuronyl-transferases or N-acetyltransferases, in vivo. Interactions based on competitive or noncompetitive inhibition should be analysed specifically for the cytochrome P450 enzyme involved. At least 5 different types of cytochrome P450 enzyme induction may result in major variability of hepatic function; this may be quantified by biochemical parameters, clearance methods, or highly enzyme-specific methods such as Western blot analysis or molecular biological techniques such as mRNA quantification in blood and tissues. Therapeutic drug monitoring is already implicitly used for quantification of the enzyme activities relevant for a specific drug. Selective impairment of hepatic enzymes due to gene mutations may have an effect on the pharmacokinetics of certain drugs similar to that caused by cirrhosis. Assessment of this heritable source of variability in liver function is possible by in vivo or ex vivo enzymological methods. For genetically polymorphic enzymes and carrier proteins involved in drug disposition, molecular genetic methods using a patient's blood sample may be used for classification of the individual into: (i) the impaired or poor metaboliser (homozygous deficient); (ii) the extensive (homozygous active) metaboliser group; and (iii) the moderately extensive metaboliser (heterozygous) group. For hepatic blood flow determinations, galactose or sorbitol given at relatively low doses may be much better indicators than the indocyanine green.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.