Polymorphism in drug metabolism--implications for drug toxicity

Cytochrome P450: what have we learned and what are the future issues?

The cytochrome P450 (P450) field came out of interest in the metabolism of drugs, carcinogens, and steroids, which remain major focal points. Over the years we have come to understand the P450 system components, the multiplicity of P450s, and many aspects of the regulation of the genes and also the catalytic mechanism. Many crystal structures are now becoming available. The significance of P450 in in vivo metabolism is appreciated, particularly in the context of pharmacogenetics. Current scientific issues involve posttranslational modification, gene regulation, component interactions, structures of P450 complexed with ligands, details of high-valent oxygen chemistry, the nature and influence of rate-limiting steps, greater details about some reaction steps, cooperativity, and the relevance of P450 variations to cancer risk. Some emerging research areas involve new methods of analysis of ligand interactions, roles of conformational changes linked to individual reaction steps, functions of orphan P450s, "molecular breeding" of new P450 functions and enhanced activity, and the utilization of P450s in chemical synthesis.

Discovery in toxicology: mediation by gene expression array technology

Toxicogenomics is a term that represents the merging of toxicology with novel genomics techniques. Data generated in the new-age era of toxicology is relatively complex, requires new bioinformatics tools for adequate interpretation, and allows for the rapid generation of testable hypotheses. Hazard identification and risk assessment processes will advance from the use of genomics techniques, which will lead to greater understanding of mechanism(s) of action of toxicants, development of novel biomarkers of exposure and effect, and better identification of sensitive subpopulations.

Protein variability in male and female Wistar rat liver proteins

In the emerging field of mechanistic toxicology, growing attention is being paid to the interpretation of sex-related toxic responses. The Wistar rat outbred stock is a frequently used rodent for toxicity testing. Outbred strains, which display a relatively high degree of genetic variability between the individual animals are often used in risk assessment, as they are considered to best approximate the variability present in a human population. In this study two-dimensional (2-D) protein gel electrophoresis was applied to investigate the liver protein patterns of male and female Wistar rats and to search for (i) sex-related and (ii) interindividual qualitative and quantitative differences in protein expression. Among the sexes, six proteins were detected that were shown to be exclusively present in male rats and one that was present only in females. A male-specific protein was tentatively assigned to alpha 2u globulin. Seven protein spots showed statistically significant abundance changes (p < 0.001) between males and females, one was tentatively assigned to heme oxygenase 1 and another to the 23 kDa morphin-binding protein. Four sets of protein spots were detected that showed positional shifts in the individual patterns and are likely to represent polymorphic proteins inherent in the Wistar rat. These results form a valuable basis for future investigations of drug-induced changes in the male and female Wistar rat liver pattern.

Dextromethorphan O-demethylation polymorphism in Jordanians

The O-demethylation of dextromethorphan (DMT) to dextrorphan (DRP) was studied in 241 unrelated, healthy Jordanian volunteers (171 males, 70 females). Urine was collected for 8 h following a single oral dose of DMT bromhydrate 30 mg. A thin-layer chromatographic (TLC) technique was used to identify the metaboliser phenotype. The frequency of the poor metaboliser phenotype was found to be 2.9% (approximate 95% confidence interval 0.8-5.0%). Applying the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling poor metabolism was 0.17 (95% confidence interval 0.108-0.232).

Metronidazole and antipyrine as probes for the study of foreign compound metabolism

The aim of the present work was to develop a tool for the study of the enzyme activities relevant for the biotransformation of foreign compounds, their elimination and/or activation to toxic substances. The activity of an enzyme may be assessed by the rate of metabolism of a preferably specific probe or model compound. The cytochrome P450'ies, the key enzymes for the elimination and/or activation of most foreign compounds, exist in multiple forms with variable substrate specificity and regulation. Some cytochrome P450'ies are under genetic control, whereas the activity of others is mainly regulated by the influence from factors in the environment. Only some of the cytochrome P450'ies are relevant for the formation of harmful metabolites. Thus, the activity of as many cytochrome P450 forms as possible should be assessable, preferably simultaneously. The present work evaluated metronidazole in a cocktail with antipyrine as a tool for the study of the regulation of foreign compound metabolism in the liver. The cytochrome P450 catalyzed metabolism of metronidazole and antipyrine was studied in humans and in isolated rat hepatocytes. In humans the influence of dose, route of administration, enzyme induction and inhibition and liver disease was investigated. Rats of either sex were studied with and without pretreatment with specific enzyme inducers and incubations included specific enzyme inhibitors. Evidence was provided that the oxidative formation of the five major metabolites, two from metronidazole and three from antipyrine, depends on different cytochrome P450'ies. In humans it was demonstrated that the clearance of metronidazole and antipyrine could be determined from the same saliva sample collected 16-24 hours after their oral administration and so could the clearance for formation of each metabolite if urine was collected for 48 hours. Thus, with the cocktail of metronidazole and antipyrine and simple non-invasive sampling the activity of five different cytochrome P450'ies can be assessed in vivo. In addition, metronidazole may also be used for assessment of the glucuronidation capacity although this is a minor pathway in man. Because the variation within subjects is much less than between them, the cocktail test is particularly suited for paired designs with measurements before and after an environmental change and the subjects serving as their own control. The metronidazole/antipyrine cocktail may have many applications in the study of the regulation of foreign compound metabolism in man and in animals, in vivo and in vitro.

Propranolol metabolism by Cunninghamella bainieri

1. Incubations of racemic propranolol alone or in the presence of either quinidine or sparteine were performed with Cunninghamella bainieri. 2. Five mammalian metabolites of propranolol (4-hydroxypropranolol, desisopropyl-propranolol, 1-naphthoxylactic acid, propranolol glycol and 1-naphthoxyacetic acid) were present in unhydrolysed extracts of the incubation medium according to h.p.l.c. and g.l.c. analyses. The relative proportion of 4-hydroxypropranolol increased after enzymic treatment. 3. Propranolol not only had a fungistatic effect, but also caused morphological changes in the organism, which were accompanied by decomposition of 4-hydroxypropranolol and formation of a greenish-brown colour in the incubation medium. 4. Drug interaction experiments yielded results which paralleled those reported in mammals. 5. The findings indicate that C. bainieri may be a useful microbial model for drug disposition and interaction studies.

Effect of sparteine and quinidine on the metabolism of methoxyphenamine by Cunninghamella bainieri

1. The fungus C. bainieri, incubated for 7 days with methoxyphenamine alone or in combination with either sparteine or quinidine, gave N-desmethylmethoxyphenamine and its N-acetyl conjugate as major metabolites, while O-desmethylmethoxyphenamine, 5-hydroxymethoxyphenamine and 2-hydroxyamphetamine were produced in lesser amounts. In addition, 1-(2-hydroxyphenyl)-2-aminopropane, 1-hydroxy-1-(2-methoxyphenyl)-2-propanone, beta-hydroxymethoxyphenamine, and 1-(5-hydroxy-2-methoxyphenyl)-2-aminopropane were tentatively identified as minor components of the fungal biotransformation of methoxyphenamine. 2. As observed in mammalian systems, the addition of either sparteine or quinidine decreased the rate and extent of methoxyphenamine biotransformation. 3. C. bainieri may be a useful model for drug interaction studies.

Genetic polymorphism in drug oxidation

Of the two clearly established drug oxidation polymorphisms, only the one referred to as debrisoquine polymorphism affects many drugs. The only known polymorphic substrates of mephenytoin hydroxylase are mephenytoin and mephobarbital. Relatively recently discovered drug substrates of debrisoquine hydroxylase are propafenone, diltiazem, and codeine. The list of substrates contains 28 items. The fate of slightly less than half of these is clinically affected in poor metabolizers, and several of the latter drugs are no longer marketed. There are many reasons why a failure of metabolism may not alter the fate of a drug sufficiently to affect its clinical use. Of interest and clinical importance is the inhibition of debrisoquine hydroxylase by inhibitors such as quinidine and by some neuroleptics; also the simultaneous use of two substrates has led to serious toxicity by mutual metabolic inhibition. The study of these oxidation polymorphisms has been instructive not only for formal pharmacogenetics but also for the understanding of problems of therapy in patients without genetic defects.