Genetically determined interindividual variation in cytochrome P450 dependent drug metabolism : molecular basis and clinical implications

Abstract

Pharmacogenetics is the study of genetic variation that causes variation in drug levels (pharmacokinetics), drug response (pharmacodynamics) and adverse drug reactions. Pharmacogenetics aims at identifying biomarkers that allows a personalized drug treatment and thereby increased drug efficacy and reduced occurrence of adverse drug reactions. The highest genetic influence on interindividual variability in drug bioavailability is exerted by polymorphism in cytochrome P450 (CYP) genes encoding enzymes of critical importance for drug metabolism. In the current thesis, genetic factors behind the interindividual variation in CYP2C19 and CYP3A expression and activity have been investigated.

A yet unidentified member of the CYP3A subfamily, CYP3A43, was cloned and characterized. It was found that CYP3A43 expression was insignificant in human tissues and that the cDNA-expressed protein did not translate into detectable protein or catalytically active enzyme in a variety of heterologous expression systems. Thus, we concluded that CYP3A43 does not contribute to interindividual variation in CYP3A activity.

CYP3A5 has been claimed to be polymorphically expressed at very high levels in Caucasians and CYP3A5 expression has been suggested to account for a large part of the interindividual differences in CYP3A activity. However, using antibodies raised against the Cterminal peptide of CYP3A5 and a peptide-conjugate as standard, we could show that the average expression in Caucasian liver is only 2 % of total CYP3A, although some livers with higher expression were identified. By contrast, we found that the fetal form of CYP3A, i.e. CYP3A7, unexpectedly accounted for a similar amount of CYP3A protein as that of CYP3A5 in adult human livers. The expression was to some extent associated to the presence of the previously described CYP3A7*1C allele. It is concluded that additional genetic factors remain to be identified in order to explain the interindividual differences in CYP3A activity.

CYP2C19 catalyzes the metabolism of e.g. proton pump inhibitors and antidepressants and its genetic polymorphism significantly affects drug metabolism and pharmacokinetics. There is a high interindividual variability in homozygote carriers of wildtype alleles, the basis of which has not been known. We were able to identify a new common allele (CYP2C19*17) containing a -806C>T mutation that introduces a functional transcription factor element that results in binding of nuclear factors and increased transcriptional activity. Analyses in Ethiopians and Swedes revealed a significant influence of CYP2C19*17 on drug pharmacokinetics and it was calculated that homozygous carriers will have about 40% lower omeprazole area under the plasma concentration-time curve after a single dose as compared to homozygotes for the wildtype allele. Thus, this is the first study to provide with a basis for the variability in CYP2C19 activity among subjects previously defined as homozygous for the wildtype allele. It is suggested that CYP2C19*17 may contribute to a higher frequency of non-responders during antiulcer and possibly also antidepressant therapy.