[Pharmacokinetics and biotransformation of the analgesic flupirtine in humans].

The pharmacokinetics of the analgesic flupirtine (ethyl-N-[2-amino-6-(4-fluorophenylmethylamino)pyridin-3-yl] carbamate, D 9998) were examined in healthy volunteers after a single intravenous, peroral and rectal dose. Plasma-and urine concentrations were analysed by a photometric procedure specific for flupirtine and its active metabolite D 13223. The bioavailability from the capsule amounted to 90%, from the suppository to 72.5%. Plasma half-life was 8.5-10.7 h. No significant accumulation of the plasma concentrations after multiple peroral administration was observed. After a peroral dose of 14C-flupirtine the radioactivity is predominantly excreted via the urine (72%). Two metabolites could be isolated from urine and their chemical structure determined. Together with the parent drug they explain 54% of the urinary radioactivity. The metabolite D 13223 that still has some analgesic activity is found in plasma, too. The portion of unchanged flupirtine amounts to 56-83% of the total plasma levels.