Eptastigmine is a long-lasting acetyl-cholinesterase inhibitor, currently being developed for the symptomatic treatment of Alzheimer's disease. In the present study, we investigated the relationship between pharmacokinetics and pharmacodynamics of eptastigmine in young healthy volunteers. Eight male subjects received single oral doses of 10, 20, and 30 mg of eptastigmine and placebo according to a double-blind, randomized, crossover design. Blood was collected before and 0.5, 1, 1.5, 2, 3, 4, 6, and 24 hours after drug administration. Cholinesterase activity was measured using a potentiometric method in both plasma (butyryl-cholinesterase) and in red blood cells (acetyl-cholinesterase). Eptastigmine plasma levels were measured by a very sensitive high-performance liquid chromatography method (limit of quantitation 0.2 ng/mL). Eptastigmine plasma concentrations increased proportionally with the dose (mean +/- SEM AUC0-24 was 0.74 +/- 0.58, 3.61 +/- 1.15, and 6.25 +/- 1.51 ng.h/mL with 10, 20, and 30 mg, respectively) and were undetectable at 24 hours. The inhibition of acetyl-cholinesterase was dose-dependent (peak inhibition was 15 +/- 2%, 30 +/- 4%, and 36 +/- 6% with 10, 20, and 30 mg, respectively) and long-lasting, with a residual inhibition of 8 to 11% at 24 hours. Acetyl-cholinesterase inhibition and drug plasma levels were related over time with a counterclockwise hysteresis curve, suggesting the formation of active metabolites and/or a slow association to and dissociation from the enzyme in red blood cells. Butyryl-cholinesterase inhibition was weak and not dose-dependent (peak inhibition was 12 +/- 4%, 13 +/- 3%, and 12 +/- 2% with 10, 20, and 30 mg, respectively). The drug was well tolerated by all subjects.(ABSTRACT TRUNCATED AT 250 WORDS)