A comparative study in rats of the in vitro and in vivo pharmacology of the acetylcholinesterase inhibitors tacrine, donepezil and NXX-066

Abstract

The in vitro and in vivo effects of the novel acetylcholinesterase inhibitors donepezil and NXX-066 have been compared to tacrine. Using purified acetylcholinesterase from electric eel both tacrine and donepezil were shown to be reversible mixed type inhibitors, binding to a similar site on the enzyme. In contrast, NXX-066 was an irreversible non-competitive inhibitor. All three compounds were potent inhibitors of rat brain acetylcholinesterase (IC 50 [nM]; tacrine: 125±23; NXX-066: 148±15; donepezil: 33±12). Tacrine was also a potent butyrylcholinesterase inhibitor. Donepezil and tacrine displaced [ 3H]pirenzepine binding in rat brain homogenates (IC 50 values [μM]; tacrine: 0.7; donepezil: 0.5) but NXX-066 was around 80 times less potent at this M 1-muscarinic site. Studies of carbachol stimulated increases in [Ca 2+]i in neuroblastoma cells demonstrated that both donepezil and tacrine were M 1 antagonists. Ligand binding suggested little activity of likely pharmacological significance with any of the drugs at other neurotransmitter sites. Intraperitoneal administration of the compounds to rats produced dose dependent increases in salivation and tremor (ED 50 [μmol/kg]; tacrine: 15, NXX-066: 35, donepezil: 6) with NXX-066 having the most sustained effect on tremor. Following oral administration, NXX-066 had the slowest onset but the greatest duration of action. The relative potency also changed, tacrine having low potency (ED 50 [μmol/kg]; tacrine: 200, NXX-066: 30, donepezil: 50). Salivation was severe only in tacrine treated animals. Using in vivo microdialysis in cerebral cortex, both NXX-066 and tacrine were found to produce a marked (at least 30-fold) increase in extracellular acetylcholine which remained elevated for more than 2 h after tacrine and 4 h after NXX-066.