Protecting poor metabolizers, a group at high risk of adverse drug reactions

Pharmacogenetics of debrisoquine and its use as a marker for CYP2D6 hydroxylation capacity

Debrisoquine hydroxylation polymorphism is by far the most thoroughly studied genetic polymorphism of the CYP2D6 drug-metabolizing enzyme. Debrisoquine hydroxylation phenotype has been the most used test in humans to evaluate CYP2D6 activity. Two debrisoquine hydroxylation phenotypes have been described: poor and extensive metabolizers. A group with a very low debrisoquine metabolic ratio within the extensive metabolizers, named ultrarapid metabolizers, has also been distinguished. This CYP2D6 variability can be for a large part alternatively determined by genotyping, which appears to be of clinical importance given CYP2D6 involvement in the metabolism of a large number of commonly prescribed drugs. CYP2D6 pharmacogenetics may then become a useful tool to predict drug-related side effects, interactions or therapeutic failures. However, a number of reasons appear to have made research into this field lag behind. The present review focuses on the relevance of genetics and environmental factors for determining debrisoquine hydroxylation phenotype, as well as the relevance of CYP2D6 genetic polymorphism in psychiatric patients treated with antipsychotic drugs.

Role of cytochrome P4502D6 in the metabolism of brofaromine. A new selective MAO-A inhibitor

The metabolic fate of brofaromine (CGP 11 305 A), a new, reversible, selective MAO-A inhibitor, has been assessed in poor (PM) and extensive (EM) metabolizers of debrisoquine. Compared to EM, PM had significantly longer t1/2 (136%) and larger AUC(0-infinity) (110%) of the parent compound brofaromine and a lower Cmax (69%) and AUC (0-72 h) (40%) of its O-desmethyl metabolite. The mean metabolite/substrate ratio (based on urine excretion) was about 6-times greater in EM than in PM. Treatment with quinidine converted all EM into phenocopies of PM. All pharmacokinetic parameters of brofaromine and O-desmethyl-brofaromine in EM treated with quinidine were similar to those of untreated PM, including the metabolite/substrate ratio. Quinidine treatment of PM did not alter the pharmacokinetics of brofaromine or of its metabolite, nor the metabolite/substrate ratio. The results indicate a role for the debrisoquine type of oxidation polymorphism in the O-demethylation and pharmacokinetics of brofaromine.

Pharmacogenetics and drug metabolism: an Irish perspective

Genetic factors, particularly in relation to control of liver drug metabolism, are a major cause of variability in the response to drugs. In 145 Irish subjects 48% were fast acetylators of sulphadimidine in contrast to 80% in Chinese subjects. Eleven (7.6%) of our Irish population showed an improved ability to oxidise delrisoquine. The therapeutic implications of these findings are discussed.

Genetically-determined interaction between propafenone and low dose quinidine: role of active metabolites in modulating net drug effect

1. Quinidine is a potent inhibitor of the genetically-determined debrisoquine 4-hydroxylation. Oxidation reactions of several other drugs, including the 5-hydroxylation of the new antiarrhythmic drug propafenone, depend on the isozyme responsible for debrisoquine 4-hydroxylation. 2. The effect of quinidine on the debrisoquine phenotype-dependent 5-hydroxylation and the pharmacological activity of propafenone was studied in seven 'extensive' metabolizers and two 'poor' metabolizers of the drug receiving propafenone for the treatment of ventricular arrhythmias. 3. In patients with the extensive metabolizer phenotype, quinidine increased mean steady-state plasma propafenone concentrations more than two fold, from 408 +/- 351 (mean +/- s.d.) to 1096 +/- 644 ng ml-1 (P less than 0.001), decreased 5-hydroxypropafenone concentrations from 242 +/- 196 to 125 +/- 97 ng ml-1 (P less than 0.02) and reduced propafenone oral clearance by 58 +/- 23%. 4. Despite these changes in plasma concentrations, electrocardiographic intervals and arrhythmia frequency were unaltered by quinidine coadministration, indicating that 5-hydroxypropafenone contributes to the pharmacologic effects of propafenone therapy in extensive metabolizers. 5. In contrasts, plasma concentrations of propafenone and 5-hydroxypropafenone remained unchanged in the two patients with the poor metabolizer phenotype. 6. Biotransformation of substrates for the debrisoquine pathway can be markedly perturbed by even low doses of quinidine; interindividual variability in drug interactions may have a genetic component.

The molecular mechanisms of two common polymorphisms of drug oxidation--evidence for functional changes in cytochrome P-450 isozymes catalysing bufuralol and mephenytoin oxidation

Using the stereospecific metabolism of (+)- and (-)-bufuralol and (+)- and (-)-metoprolol as model reactions, we have characterized the enzymic deficiency of the debrisoquine/sparteine-type polymorphism by comparing kinetic data of subjects in vivo with their microsomal activities in vitro and with reconstituted activities of cytochrome P-450 isozymes purified from human liver. The metabolism of bufuralol in liver microsomes of in vivo phenotyped 'poor metabolizers' of debrisoquine and/or sparteine is characterized by a marked increase in Km, a decrease in Vmax and a virtual loss of the stereoselectivity of the reaction. These parameters apparently allow the 'phenotyping' of microsomes in vitro. A structural model of the active site of a cytochrome P-450 for stereospecific metabolism of bufuralol and other polymorphically metabolized substrates was constructed. Two cytochrome P-450 isozymes, P-450 buf I and P-450 buf II, both with MW 50,000 Da, were purified from human liver on the basis of their ability to metabolize bufuralol to 1'-hydroxy-bufuralol. However, P-450 buf I metabolized bufuralol in a highly stereoselective fashion ((-)/(+) ratio 0.16) as compared to P-450 buf II (ratio 0.99) and had a markedly lower Km for bufuralol. Moreover, bufuralol 1'-hydroxylation by P-450 buf I was uniquely characterized by its extreme sensitivity to inhibition by quinidine. Antibodies against P-450 buf I and P-450 buf II inhibited bufuralol metabolism in microsomes and with the reconstituted enzymes. Immunochemical studies with these antibodies with microsomes and translations in vitro of RNA from livers of extensive and poor metabolizers showed no evidence for a decrease in the recognized protein or its mRNA. Because the antibodies do not discriminate between P-450 buf I and P-450 buf II, both a decreased content of P-450 buf I or its functional alteration could explain the polymorphic metabolism in microsomes. The genetically defective stereospecific metabolism of mephenytoin was determined in liver microsomes of extensive and poor metabolizers of mephenytoin phenotyped in vivo. Microsomes of poor metabolizers were characterized by an increased Km and a decreased Vmax for S-mephenytoin hydroxylation as compared to extensive metabolizers and a loss of stereospecificity for the hydroxylation of S-versus R-mephenytoin. A cytochrome P-450 with high activity for mephenytoin 4-hydroxylation was purified from human liver. Immunochemical studies with inhibitory antibodies against this isozyme suggest the presence in poor-metabolizer microsomes of a functionally altered enzyme.

Discovery of altered pharmacokinetics of CGP 15 210 G in poor hydroxylators of debrisoquine during early drug development

The pharmacokinetics of CGP 15 210 G, a new 5-HT uptake inhibitor in poor and extensive metabolisers of debrisoquine, give indirect evidence of an association between its metabolism and polymorphic hydroxylation of the debrisoquine type.

Genetically determined variability in acetylation and oxidation. Therapeutic implications

The clinical significance of two separate genetic polymorphisms which alter drug metabolism, acetylation and oxidation is discussed, and methods of phenotyping for both acetylator and polymorphic oxidation status are reviewed. Particular reference is made to the dapsone method, which provides a simple means of distinguishing fast and slow - and possibly intermediate - acetylators, and to the sparteine method which allows a clear separation of oxidation phenotypes. Although acetylation polymorphism has been known for some time, definite indications for phenotyping are few. It is doubtful whether acetylator phenotype makes a significant difference to the outcome in most isoniazid treatment regimens, and peripheral neuropathy from isoniazid in slow acetylators is easily overcome by pyridoxine administration. However, in comparison with rapid acetylators, slow acetylators receiving isoniazid have an increased susceptibility to phenytoin toxicity, and perhaps also to carbamazepine toxicity. It is also possible that rapid acetylators receiving isoniazid attain higher serum fluoride concentrations from enflurane and similar anaesthetics than do similarly treated slow acetylators. Thus, when drug interactions of these types are suspected, phenotyping for acetylator status may be advisable. If routine monitoring of serum procainamide and N-acetylprocainamide concentrations is practised, phenotyping of subjects prior to therapy with these agents should not be necessary. Although acetylator phenotype influences serum concentrations of hydralazine, when this drug is given in combination with other drugs acetylator phenotype has not been shown to influence the therapeutic response. Slow acetylator phenotype along with female gender and the presence of HLA-DR antigens appear to be risk factors in the development of hydralazine-induced systemic lupus erythematosus (SLE). Determination of acetylator phenotype may therefore help determine susceptibility to this adverse reaction. In the case of sulphasalazine, adult slow acetylators require a lower daily dose of the drug than fast acetylators in order to maintain ulcerative colitis in remission without significant side effects. It is therefore advisable to determine acetylator phenotype prior to sulphasalazine therapy. Work on the association of acetylation polymorphism with various disease states is also reviewed. It is possible that a higher incidence of bladder cancer is associated with slow acetylation phenotype - especially in individuals exposed to high levels of arylamines. The question as to whether idiopathic SLE is more common in slow acetylators remains unresolved. There appears to be no difference between fa

Red wine contains a potent inhibitor of phenolsulphotransferase

Many ethanolic drinks, especially red wine, contain potent inhibitors of phenolsulphotransferase. At a dilution of 1/75 from the original beverage, extracts from six types of red wine inhibited human platelet phenolsulphotransferase P by a mean of 99% and human platelet phenolsulphotransferase M by 12%. Such extracts had no significant effect on rat liver monoamine oxidase A or human platelet monoamine oxidase B. The inhibitors, which have not yet been identified, can be extracted into ethyl acetate at acid or neutral pH. Thus, they are not monoamines. Flavonoid phenols are plausible candidates. As phenolsulphotransferase M and P are involved in the metabolism of many phenols, including drugs, the inhibition of these enzymes could result in the enhancement of pharmacological potency and have important clinical consequences.

Urinary bile acid and bile alcohol excretion does not reflect the genetic polymorphism of debrisoquine hydroxylation

Excretion of the major urinary bile alcohol 27-nor-5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 24,25- pentol , and of cholic, chenodeoxycholic, deoxycholic and lithocholic acid was measured in 24 h urine collections of 10 extensive and seven poor metabolizers of debrisoquine. There was no significant difference of the excretion of these cholesterol metabolites between the two groups, indicating that cholesterol hydroxylation to bile alcohols and bile acids is probably not controlled by the same genes responsible for the 'debrisoquine-type' hydroxylation polymorphism.