CYP2C19*17 is likely to cause therapeutic failures in drug treatment with, for example, proton pump inhibitors and antidepressants.

Cytochrome P450 CYP2D6 is the most extensively characterized polymorphic drugmetabolizing enzyme. A de®ciency of the CYP2D6 enzyme is inherited as an autosomal recessive trait; these subjects (7% of Caucasians, about 1% of Orientals) are classi®ed as poor metabolizers. Among the rest (extensive metabolizers), enzyme activity is highly variable, from extremely high in ultrarapid metabolizers, to markedly reduced in intermediate metabolizers. The CYP2D6 gene is highly polymorphic, with more than 70 allelic variants described so far. Of these, more than 15 encode an inactive or no enzyme at all. Others encode enzyme with reduced,`normal' or increased enzyme activity. The CYP2D6 gene shows marked interethnic variability, with interpopulation differences in allele frequency and existence of`populationspeci®c' allelic variants, for instance among Orientals and Black Africans. The CYP2D6 enzyme catalyses the metabolism of a large number of clinically important drugs including antidepressants, neuroleptics, some antiarrhythmics, lipophilic b-adrenoceptor blockers and opioids. The present-day knowledge on the in¯uence of the genetic variability in CYP2D6 on the clinical pharmacokinetics and therapeutic effects/adverse effects of psychotropic drugs is reviewed.

Deficient hydroxylation of debrlsoquine Is an autosomal recessive trait that affects =7% of the Caucasian population. These individuals (poor metabolizers) carry deficient CYP2D6 gene variants and have an impaired metabolsm of several commonly used drugs. The opposite phenomenon also exists, and certain individuals metabolize the drugs very rapidly, resulting in subtherapeutic plasma concentrations at normal doses. In the present study, we have investigated the molecular genetic basis for ultrarapid metabolism of debrisoquine. Restriction fragment length polymorphism analysis of the CYP2D locus in two families with very rapid metabolism of debrisoquine [metabolic ratio (MR) for debrisoquine = 0.01-0.11 revealed the variant CYP2D6 gene CYP2D6L. EcoRI RFLP and Xba I pulsed-field gel electrophoresis analyses showed that this gene had been amplifled 12-fold in three members (father and his two children) of one ofthe families, and two copies were present among members of the other family. The CYP2D6L gene had an open reading frame and carried two mutations causing amino acid substitutions: one in exon 6, yielding an Arg-296 --Cys exchange and one in exon 9 causing Ser-486 --Thr. The MR of subjects carrying one copy of the CYP2D6L gene did not signicantiy differ from that of those with the wild-ype gene, indicating that the structural alterations were not of importance for the catalytic properties of the gene product. Examination of the MR among subjects carrying wild-type CYP2D6, CYP2D6L, or deficient alleles revealed a relationship between the number of active genes and MR. The data show the principle of inherited amplification of an active gene. Furthermore, the finding of a specific haplotype with two or more active CYP2D6 genes allows genotyping for ultrarapid drug metabolizers. This genotyping could be of predictive value for individualized and more efficient drug therapy.

The major cholesterol oxidation products in the human circulation are 27-hydroxycholesterol, 24-hydroxycholesterol, and 7␣-hydroxycholesterol. These oxysterols are formed from cholesterol by specific cytochrome P450 enzymes, CYP27, CYP46, and CYP7A, respectively. An additional oxysterol present in concentrations comparable with 7␣-and 24-hydroxycholesterol is 4␤-hydroxycholesterol. We now report that patients treated with the antiepileptic drugs phenobarbital, carbamazepine, or phenytoin have highly elevated levels of plasma 4␤-hydroxycholesterol. When patients with uncomplicated cholesterol gallstone disease were treated with ursodeoxycholic acid, plasma 4␤-hydroxycholesterol increased by 45%. Ursodeoxycholic acid, as well as the antiepileptic drugs, are known to induce cytochrome P450 3A. Recombinant CYP3A4 was shown to convert cholesterol to 4␤-hydroxycholesterol, whereas no conversion was observed with CYP1A2, CYP2C9, or CYP2B6. The concentration of 4␣-hydroxycholesterol in plasma was lower than the concentration of 4␤-hydroxycholesterol and not affected by treatment with the antiepileptic drugs or ursodeoxycholic acid. Together, these data suggest that 4␤-hydroxycholesterol in human circulation is formed by a cytochrome P450 enzyme.Cholesterol oxidation products (oxysterols) have recently attracted great interest because of their numerous biological actions. They have been implicated in bile acid biosynthesis, cholesterol transport, and gene regulation (1). In addition, many oxysterols are toxic to cells and induce apoptosis (2-4). These compounds can be formed either by cholesterol autooxidation or by the action of cholesterol-metabolizing enzymes. Several oxysterols can be formed by both mechanisms, i.e. 7␣-hydroxycholesterol. This oxysterol is a predominant cholesterol auto-oxidation product but is also formed by the hepatic enzyme cholesterol 7␣-hydroxylase. Major oxysterols in the human circulation include 27-hydroxycholesterol, 24-hydroxycholesterol, and 7␣-hydroxycholesterol (5). One additional oxysterol present in human plasma at a relatively high concentration is 4␤-hydroxycholesterol (6). Very little is known about its formation or metabolism. We have shown earlier that small amounts of this oxysterol are formed, together with 4␣-hydroxycholesterol, during in vitro oxidation of low density lipoprotein, and low levels of the two oxysterols were also found in human atherosclerotic plaques (7). The ratio between 4␣-and 4␤-hydroxycholesterol was close to one both in oxidized LDL 1 and in plaques, and the amount formed in oxidized LDL was only a small percent of the dominating oxysterol, 7-oxocholesterol. These data suggested that very little 4␤-hydroxycholesterol is formed by cholesterol auto-oxidation. Because relatively high levels were reported in human plasma we hypothesized that this compound is formed in vivo by an enzymatic reaction. 4␣-and 4␤-hydroxycholesterol were determined in plasma from volunteers and patients, and it was found that patients treated with certain antiepileptic drugs, known...

The frequency of poor metabolizers of debrisoquin was low and similar in four different native Chinese nationalities. In a total sample of 695 Chinese subjects, only seven (1.01%) had a urinary ratio between debrisoquin and 4-hydroxydebrisoquin greater than 12.6, which is the antimode between poor metabolizers and extensive metabolizers in white populations. This is significantly lower than the 6.82% found in 1011 white Swedish healthy subjects (p less than 0.0001). Admixture analysis indicated the occurrence of two distributions within extensive metabolizers among both Chinese and white subjects. The mean of the distribution of metabolic ratios among Chinese extensive metabolizers was shifted toward higher values compared with Swedish extensive metabolizers (p less than 0.01). The frequency of poor metabolizers of S-mephenytoin was higher in 137 Chinese (14.6%) than in 488 Swedish (3.3%) subjects (p less than 0.0001). Our findings imply that drugs metabolized by these two polymorphic hydroxylases should be prescribed in different dosages to Chinese and white subjects.

We found significant differences in CYP1A2 enzyme activity between Swedes and Koreans that could not be explained by environmental factors or the CYP1A2 haplotypes examined, despite differences in allele frequencies. None of the investigated CYP1A2 haplotypes are critical in inducing variations in enzyme activity, with the exception of CYP1A2*1F.

We have proposed that 4b-hydroxycholesterol (4b-OHC) may be used as an endogenous marker of CYP3A activity. The cholesterol metabolite 4b-OHC is formed by CYP3A4. Treatment of patients with strong inducers of CYP3A enzymes, e.g. anti-epileptic drugs, resulted in 10-fold increased concentrations of plasma 4b-OHC, while treatment with CYP3A inhibitors such as ritonavir or itraconazole resulted in decreased plasma concentrations. There was a relationship between the 4b-OHC concentration and the number of active CYP3A5*1 alleles showing that 4b-OHC was not only formed by CYP3A4, but also by CYP3A5. The concentration of 4b-OHC was higher in women than in men, confirming previous studies indicating a gender difference in CYP3A4/5-activity. The rate of elimination of 4b-OHC is slow (half-life 17 days) which results in stable plasma concentrations within individuals, but limits its use to study rapid changes in CYP3A activity. In short-term studies exogenous markers such as midazolam or quinine may be superior, but in long-term studies 4b-OHC is a sensitive marker of CYP3A activity, especially to assess induction but also inhibition. Under conditions where the cholesterol concentration is changing, the ratio of 4b-OHC : cholesterol may be used as an alternative to 4b-OHC itself. The use of an endogenous CYP3A marker has obvious advantages and may be of value both during drug development and for monitoring CYP3A activity in patients.

Clear differences in the activity of both CYP3A4 and CYP3A5 were shown in the three major human races. Both 4beta-hydroxycholesterol and quinine/3-hydroxyquinine metabolic ratio showed a higher CYP3A activity in women than in men. The results give strong evidence that the plasma concentration of 4beta-hydroxycholesterol may be used as an endogenous marker of CYP3A activity (CYP3A4+5).