Dose-limiting inhibition of acetylcholinesterase by ladostigil results from the rapid formation and fast hydrolysis of the drug–enzyme complex formed by its major metabolite, R-MCPAI

Ladostigil is a pseudo reversible inhibitor of acetylcholinesterase (AChE) that differs from other carbamates in that the maximal enzyme inhibition obtainable does not exceed 50–55%. This could explain the low incidence of cholinergic adverse effects induced by ladostigil in rats and human subjects. The major metabolite, R-MCPAI is believed to be responsible for AChE inhibition by ladostigil in vivo. Therefore we determined whether the ceiling in AChE inhibition resulted from a limit in the metabolism of ladostigil to R-MCPAI by liver microsomal enzymes, or from the kinetics of enzyme inhibition by R-MCPAI. Ladostigil reduces TNF-α in lipopolysaccharide-activated microglia. In vivo, it may also reduce pro-inflammatory cytokines by inhibiting AChE and increasing the action of ACh on macrophages and splenic lymphocytes. We also assessed the contribution of AChE inhibition in the spleen of LPS-injected mice to the anti-inflammatory effect of ladostigil. As in other species, AChE inhibition by ladostigil in spleen, brain and plasma did not exceed 50–55%. Since levels of R-MCPAI increased with increasing doses of ladostigil we concluded that there was no dose or rate limitation of metabolism. The kinetics of enzyme inhibition by R-MCPAI are characterized by a rapid formation of the drug–enzyme complex and fast hydrolysis which limits the attainable degree of AChE inhibition. Ladostigil and its metabolites (1–100nM) decreased TNF-α in lipopolysaccharide-activated macrophages. Ladostigil (5 and 10mg/kg) also reduced TNF-α in the spleen after injection of lipopolysaccharide in mice. However, AChE inhibition contributed to the anti-inflammatory effect only at a dose of 10mg/kg.