Interindividual variation in drug metabolism with focus on polymorphic cytochrome P450 2C9

Abstract

Cytochromes P450 (CYP) are enzymes, mainly catalysing the oxidation of xenobiotics, in order to facilitate their excretion from the body. CYP2C9 is a polymorphic enzyme, which is responsible for the metabolism of about 10 % of all known drugs, e.g sulphonylureas, anticoagulants, angiotensin II blockers and NSAIDs, which show a considerable interindividual variation in their metabolic clearance. The primary aim of this PhD thesis was to investigate possible factors, which could be responsible for, or contribute to, the variation in CYP2C9 metabolic activity. We also wanted to further elucidate the impact of CYP2C9 genotypes on the turnover of both exogenous and endogenous substrates.

In healthy Caucasians a three-fold higher metabolic ratio (MR) of the CYP2C9 probe drug losartan was found in volunteers genotyped as CYP2C9*1/*3 compared to CYP2C9*1/*1 subjects (p<0.05), even though there was considerable variation within each genotype group. The subjects homozygous for the CYP2C9*3 allele had metabolic ratios that were between 22 and 220 times higher than CYP2C9*1/*1 subjects. No novel allelic variants of CYP2C9, explaining the low metabolic activity in some of the CYP2C9*1/*1 subjects, could be identified. A two-fold higher metabolic ratio of losartan was evident in women taking oral contraceptives (OCs) compared to women not taking OCs (p< 0.05), showing that CYP2C9-dependent drug metabolism is reduced during concomitant intake of OC.

When comparing healthy Swedish and Korean subjects, it was discovered that Swedes had a slower losartan metabolism than Koreans, regardless of genotype. No difference between female subjects of the two populations, having the same genotype, was detected. Swedish men, though, had a higher metabolic ratio (MR=0.83) than Korean men (MR=0.54), p< 0.001, but no novel polymorphisms could explain this difference.

The intrinsic clearance of the selective COX-2 inhibitor celecoxib was studied in human liver microsomes of different CYP2C9 genotypes, as well as in yeast microsomes with recombinantly expressed CYP2C9 variants. The formation rate of OH-celecoxib was significantly reduced in CYP 2C9*3 samples, compared to samples with two functional alleles. The CYP2C9*2-samples did not differ from CYP2C9*1 in any of the systems studied. Alcohol dehydrogenase (ADH1 and ADH2) was identified as being responsible for the further oxidation of OH-celecoxib, in vitro.

After a single oral dose of 200 mg celecoxib, the exposure was three times higher in healthy CYP2C9*3/*3 subjects than in CYP2C9*1/*1 and CYP2C9*1/*3 subjects. After one week of repeated celecoxib administration, the celecoxib exposure was more than seven times higher in subjects genotyped as CYP2C9*3/*3, compared to individuals in the other two genotype groups. The heterozygous CYP2C9*3 carriers were almost identical to CYP2C9*1/*1 subjects. Subjects genotyped as CYP2C9*3/*3 may be more likely to experience concentration-dependent side-effects after long-term treatment with celecoxib.

In human liver microsomes there was a significantly lower formation rate of epoxyeicosatrienoic acids (EETs) by 34% in samples with the composite genotype of CYP2C8 *3/*3/CYP2C9*2/*2 compared to CYP2C8 *1/*1/CYP2C9*1/*1. Inhibition experiments confirmed the involvement of both CYP2C8 and CYP2C9 in the oxidation of arachidonic acid to vasoactive metabolites. The results imply that variant alleles of CYP2C8 and CYP2C9 might be involved in the pathophysiology of cardiovascular diseases. The ratio between the parent compound and its metabolite. A high MR reflects a low metabolic activity.