The role of CYP3A4/5 in alprazolam metabolism

Abstract

Cytochrome P450 3A (CYP3A) enzyme family is involved in the metabolism of about 50 % of all drugs in clinical use. Among CYP3A, CYP3A4 and CYP3A5 are the major enzymes in adults; however CYP3A5 is polymorphic and primarily expressed in black populations. CYP3A5 may therefore contribute significantly to the metabolism of CYP3A substrates in African populations.

The impact of CYP3A5 expression on drug metabolism by CYP3A is fairly unknown. Therefore, a tool for assessment of both CYP3A4 and CYP3A5 using a single probe drug is of interest. Overlapping substrate specificities between CYP3A4 and CYP3A5 have made it difficult to differentiate between the activities of the two enzymes using probe drugs. However, experimental in vitro data have shown a preference for formation of 4-hydroxyalprazolam by CYP3A4 and that of α-hydroxyalprazolam by CYP3A5.

The aim of the present thesis was to investigate the role of CYP3A4 and CYP3A5 in alprazolam metabolism as well as to evaluate the use of alprazolam as a probe drug for these two enzymes.

A liquid chromatograpic-mass spectrometric method for determination of alprazolam and the two metabolites 4- and α-hydroxyalprazolam was developed. When using this method we were able to analyze samples for pharmacokinetic and metabolism studies. The AUC ratio of parent drug/metabolite has been shown to reflect the enzymatic activity. Furthermore the metabolic ratio calculated from a single plasma sample was found to be useful in phenotyping studies.

Alprazolam metabolism was shown to be catalyzed by both CYP3A4 and CYP3A5 in vitro. Formation rates of 4-hydroxyalprazolam in human liver microsomes and recombinant CYP3A4 and CYP3A5 was higher than that of α-hydroxyalprazolam, confirming that 4-hydroxylation is the major metabolic pathway of alprazolam. The relative formation of α-hydroxyalprazolam was 3-fold higher for recombinant CYP3A5 compared to CYP3A4. We have thus confirmed that CYP3A5 catalyses alprazolam metabolism, in vitro. We then evaluated the role of CYP3A4 and CYP3A5 in alprazolam metabolism in vivo in Tanzanian and Swedish healthy subjects who express CYP3A5 and those who do not. As there were no significant differences in MR between subjects with different CYP3A5 genotypes we concluded that CYP3A5 plays no significant role in alprazolam metabolism and that CYP3A4 is the major enzyme involved in the metabolism of alprazolam in vivo.