Pharmacokinetics of galanthamine in humans and corresponding cholinesterase inhibition.

Measurements were done to determine the plasma concentrations of galanthamine and two of its metabolites, as well as the corresponding inhibition of acetylcholinesterase activity in erythrocytes after applying 5 and 10 mg galanthamine hydrobromide as a constant-rate intravenous infusion for 30 minutes and single oral doses of 10 mg in eight healthy male volunteers. The data obtained revealed first-order pharmacokinetics, complete oral bioavailability, and a mean terminal half-life of 5.68 hours (95% confidence interval, 5.17 to 6.25 hours). Renal clearance accounted for only 25% of the total plasma clearance (CL = 0.34 L.kg-1.hr-1). Only negligible quantities of the putative metabolites, epigalanthamine and galanthaminone, were detected in blood and urine. The inhibition of acetylcholinesterase activity was closely correlated with the pharmacokinetics of galanthamine, a median maximal value of 53% being achieved by applying 10 mg galanthamine intravenously. Analysis of in vitro and ex vivo concentration responses revealed no differences, indicating that no metabolites of galanthamine exert additional inhibition of acetylcholinesterase activity.