Quinine metabolism in man : emphasis on the 3-hydroxylation as a biomarker reaction for the activity of CYP3A4

Abstract

Quinine is one of the cinchona alkaloids and used in the treatment of severe forms of malaria. Formation of 3-hydroxyquinine is catalysed by cytochrome P-450 3A4 (CYP3A4), which is the enzyme involved in the metabolism of about 50% of all drugs in clinical use.

In this thesis I investigated the relative importance of four metabolites of quinine in its metabolism in vitro and in vivo and studied the usefulness of the metabolic ratio (MR) of quinine (quinine/3-hydroxyquinine) as a measure of the activity of CYP3A4 in vivo.

High performance liquid chromatographic methods for determination of quinine and four of its metabolites in plasma and urine samples were developed. In healthy subjects co-administration of quinine with ketoconazole (inhibits CYP3A4) decreased the mean apparent oral clearance of quinine and mean area under the plasma concentration time curve (AUC) of 3-hydroxyquinine significantly. The mean total urinary recovery of quinine plus its 3hydroxy metabolite was 28±8 %.

The role of CYP3A4 in the metabolism of quinine to 2'-quininone, (1OS)- and (1OR)-11-dihydroxydihydroquinine in vitro and the relative importance of these metabolites compared to 3-hydroxyquinine in the metabolism of quinine were studied in human liver microsomes. 3-hydroxyquinine had the highest rate of formation and intrinsic clearance in all five human livers used. Incubations with human recombinant CYP3A4 resulted mainly in the formation of 3-hydroxyquinine and to a minor extent 2'-quininone. Formation of these two metabolites but not that of (10R)-11-dihydroxydihydroquinine was completely inhibited by inhibitors of CYP3A4 (ketoconazole and troleandomycin). Formation of (1 OS)-11-dihydroxydihydroquinine was only partly inhibited.

The effects of ketoconazole and fluvoxamine (inhibitor of CY1P1A2) on the formation of 2'-quininone, (1OR)- and (10S)-11-dihydroxydihydroquinine and the relative importance of these metabolites for the elimination of quinine in vivo were investigated. Co-administration with ketoconazole significantly increased the mean AUC of 2'-quininone and (1OR)-11-dihydroxydihydroquinine, but not that of (10S)-11-dihydroxydihydroquinine. The mean total recovery of quinine plus its four metabolites after hydrolysis of conjugates in urine was 56±16%. We investigated the usefulness of the 3-hydroxylation of quinine as a biomarker reaction for the activity of CYP3A4 in man and the intraindividual variation in the metabolic ratio (MR) of quinine. There was no significant difference (p=0.36) between the mean MRs in plasma or urine collected 16h after a single oral dose of 250 mg of quinine hydrochloride on two different occasions. There was a significant correlation between the MR of quinine and omeprazole sulphone formation (r=0.52, p=0.009), but not to the MRs of probe drugs for the other enzymes.

Our results strongly suggest that 3-hydroxyquinine is the major metabolite of quinine both in vitro and in vivo and that CYP3A4 is the major enzyme involved in the 3-hydroxylation of quinine. The metabolic ratio of quinine may serve as a stable measure of the activity of CYP3A4.