Clinical pharmacokinetics of fluvoxamine

Fluvoxamine is a selective inhibitor of serotonin reuptake that is widely used in the management of depression. Following oral administration, the drug is absorbed efficiently from the gastrointestinal tract. Peak plasma concentrations are usually observed within 2 to 8 hours postdose for capsules and film-coated tablets and within 4 to 12 hours for enteric-coated tablets. Despite complete absorption, oral bioavailability may be incomplete probably because of first-pass metabolism. Approximately 77% of fluvoxamine is plasma protein bound. Only negligible amounts of fluvoxamine are excreted unchanged in urine. The drug is extensively biotransformed, mostly by oxidation, and at least 11 different metabolites have been detected in human urine. None of the metabolites is known to possess significant pharmacological activity. Following administration of single doses, fluvoxamine shows a biphasic elimination with a mean terminal elimination half-life of about 15 to 20 hours. Steady-state plasma fluvoxamine concentrations are achieved 5 to 10 days after initiation of therapy and are 30 to 50% higher than those predicted from single-dose data. Preliminary data also suggest that plasma drug concentrations may increase nonlinearly with increasing daily dosage. The relationship between plasma fluvoxamine concentration and clinical response has not been clearly defined. Fluvoxamine pharmacokinetics are substantially unaltered in the elderly, whereas higher plasma drug concentrations (relative to dose) are observed in patients with alcoholic cirrhosis of the liver. Fluvoxamine inhibits oxidative drug metabolising enzymes and, therefore, causes a number of clinically significant drug interactions. Drugs whose metabolic elimination is impaired by fluvoxamine include tricyclic antidepressants, alprazolam, bromazepam, diazepam, theophylline, phenazone (antipyrine), propranolol, warfarin, methadone and carbamazepine.