The major fluvoxamine metabolite in urine is formed by CYP2D6

Previous studies have shown that fluvoxamine is metabolized by CYP1A2 and CYP2D6, but there is no information on the impact the various CYP enzymes have on the different metabolic pathways of fluvoxamine biotransformation. The present study was designed to investigate this issue.

The major fluvoxamine metabolite, the 5-demethoxylated carboxylic acid metabolite, was analyzed in urine from 50 healthy volunteers after intake of a single oral dose of 50 mg fluvoxamine, and the formation clearance for the metabolite (CLm) was calculated. Of the subjects, 28 were non-smoking CYP2D6 and CYP2C19 extensive metabolizers (EMs), 12 were smokers and were thus considered to have an induced CYP1A2 activity, 5 were CYP2D6 poor metabolizers (PMs), and 5 were CYP2C19 PMs. In 11 of the non-smoking EMs, 200 mg caffeine was given at another occasion in order to calculate oral caffeine clearance as a measure of CYP1A2 activity. In addition, CLm was calculated in ten other subjects given increasing doses of fluvoxamine for 4 weeks.

Oral clearance of fluvoxamine was significantly higher in smokers, and significantly lower in CYP2D6 PMs than in non-smoking EMs. CLm was 78% lower in CYP2D6 PMs than in the EMs. Smoking and being a CYP2C19 PM did not influence CLm. There was no significant correlation between oral caffeine clearance and CLm. CLm decreased with increasing fluvoxamine dosage, but the decrease in oral clearance was even higher.

These results indicate that CYP2D6 catalyzes the major metabolic pathway of fluvoxamine, whereas CYP1A2 seems to catalyze other less important pathways. Both the CYP2D6 and the CYP1A2 pathways seem to be saturated in parallel with increasing fluvoxamine dosage.