Aims To identify the principal human cytochrome P450 (CYP) enzyme(s) responsible for the human in vitro biotransformation of repaglinide. Previous experiments have identified CYP3A4 as being mainly responsible for the in vitro metabolism of repaglinide, but the results of clinical investigations have suggested that more than one enzyme may be involved in repaglinide biotransformation. Methods [ 14 C]-Repaglinide was incubated with recombinant CYP and with human liver microsomes (HLM) from individual donors in the presence of inhibitory antibodies specific for individual CYP enzymes. Metabolites, measured by highperformance liquid chromatography (HPLC) with on-line radiochemical detection, were identified by liquid chromatography-mass spectrophotometry (LC-MS) and LC-MS coupled on-line to a nuclear magnetic resonance spectrometer (LC-MS-NMR). Results CYP3A4 and CYP2C8 were found to be responsible for the conversion of repaglinide into its two primary metabolites, M4 (resulting from hydroxylation on the piperidine ring system) and M1 (an aromatic amine). Specific inhibitory monoclonal antibodies against CYP3A4 and CYP2C8 significantly inhibited ( > 71%) formation of M4 and M1 in HLM. In a panel of HLM from 12 individual donors formation of M4 and M1 varied from approximately 160-880 pmol min -1 mg -1 protein and from 100-1110 pmol min -1 mg -1 protein, respectively. The major metabolite generated by CYP2C8 was found to be M4. The rate of formation of this metabolite in HLM correlated significantly with paclitaxel 6 a -hydroxylation ( r s = 0.80; P = 0.0029). Two other minor metabolites were also detected. One of them was M1 and the other was repaglinide hydroxylated on the isopropyl moiety (M0-OH). The rate of formation of M4 in CYP2C8 Supersomes TM was 2.5 pmol min -1 pmol -1 CYP enzyme and only about 0.1 pmol min -1 pmol -1 CYP enzyme in CYP3A4 Supersomes TM . The major metabolite generated by CYP3A4 was M1. The rate of formation of this metabolite in HLM correlated significantly with testosterone 6 b -hydroxylation ( r s = 0.90; P = 0.0002). Three other metabolites were identified, namely, M0-OH, M2 (a dicarboxylic acid formed by oxidative opening of the piperidine ring) and M5. The rate of M1 formation in CYP3A4 Supersomes TM was 1.6 pmol min -1 pmol -1 CYP enzyme but in CYP2C8 Supersomes TM it was only approximately 0.4 pmol min -1 pmol -1 CYP enzyme. Conclusions The results confirm an important role for both CYP3A4 and CYP2C8 in the human in vitro biotransformation of repaglinide. This dual CYP biotransformation may have consequences for the clinical pharmacokinetics and drug-drug interactions involving repaglinide if one CYP pathway has sufficient capacity to compensate if the other is inhibited. T. B. Bidstrup et al .306

When rifampicin and repaglinide are administered concomitantly, rifampicin seems to act as both an inducer and an inhibitor of the metabolism of repaglinide. After discontinuing rifampicin administration, while the inductive effect on CYP3A4 and probably also CYP2C8 is still present, an even more marked reduction in the plasma concentration of repaglinide was observed. Our results suggest that concomitant administration of rifampicin and repaglinide may cause a clinically relevant decrease in the glucose-lowering effect of repaglinide, in particular when rifampicin treatment is discontinued or if the drugs are not administered simultaneously or within a few hours of each other.

AimsThe primary aim of the study was to investigate the possible effect of the CYP2C8 * 3 allele and of grapefruit juice on the pharmacokinetics of repaglinide. Furthermore, the impact of a single dose of grapefruit juice on the pharmacokinetics of repaglinide in relation to dose. MethodsThirty-six healthy male subjects, genotyped for CYP2C8 * 3 (11 genotyped as CYP2C8 * 1/ * 3 , one as CYP2C8 * 3 /* 3 and 24 as CYP2C8 * 1/ * 1 ), participated in a randomized, cross-over trial. In the two phases, the subjects drank 300 mL water or 300 mL grapefruit juice, in randomized order, 2 h before administration of a single dose of either 0.25 mg or 2 mg repaglinide. ResultsNeither the mean AUC 0 −∞ (geometric mean ratio: 1.01; 95% CI: 0.93-1.1, P = 0.88) nor the mean C max (geometric mean ratio: 1.05; 95% CI: 0.94-1.2, P = 0.35) of repaglinide were statistically significantly different in the group carrying the CYP2C8 * 3 mutant allele compared with wild-types. Grapefruit juice caused a 19% decrease in the geometric mean ratio of the 3-hydroxyquinidine to quinidine ratio (difference: 0.81; 95% CI: 0.75-0.87, P < 0.0001), which was used as an index of CYP3A4 activity, and an increase in the mean AUC 0 −∞ of repaglinide (geometric mean ratio: 1.13; 95% CI: 1.04-1.2, P = 0.0048), but had no statistically significant effect on the t 1/2 . There was no statistically significant difference in blood glucose concentration in subjects who had or had not ingested grapefruit juice. The effect was more pronounced at the low dose of repaglinide (0.25 mg) than at the therapeutic dose of 2 mg. ConclusionsThe pharmacokinetics of repaglinide in subjects carrying the CYP2C8 * 3 mutant allele did not differ significantly from those in the wild-types. Grapefruit juice increased the bioavailability of repaglinide, suggesting significant intestinal elimination of the drug which was assumed to be primarily mediated by CYP3A4 in the gut.

Our study showed that formulation of loratadine as a medicated chewing gum results in an almost threefold increase in relative bioavailability. This is most likely due to a bypass of first-pass metabolism as this study suggests that approximately 40% of the absorbed loratadine was absorbed via the oral mucosa.