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

The in vitro and in vivo effects of the novel acetylcholinesterase inhibito rs donepezil and NXX-066 have been compared to tacrine. Using purified acet ylcholinesterase from electric eel both tacrine and donepezil were shown to be reversible mixed type inhibitors, binding to a similar site on the enzy me. In contrast, NXX-066 was an irreversible non-competitive inhibitor. All three compounds were potent inhibitors of rat brain acetylcholinesterase ( IC50 [nM]; tacrine: 125 +/- 23; NXX-066: 148 +/- 15; donepezil: 33 +/- 12). Tacrine was also a potent butyrylcholinesterase inhibitor. Donepezil and t acrine displaced [H-3]pirenzepine binding in rat brain homogenates (IC50 va lues [mu M]; 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 in creases in [Ca2+](i) in neuroblastoma cells demonstrated that both donepezi l and tacrine were M-1 antagonists. Ligand binding suggested little activit y of likely pharmacological significance with any of the drugs at other neu rotransmitter sites. Intraperitoneal administration of the compounds to rat s produced dose dependent increases in salivation and tremor (ED50 [mu mol/ kg]; tacrine: 15, NXX-066: 35, donepezil: 6) with NXX-066 having the most s ustained effect on tremor. Following oral administration, NXX-066 had the s lowest onset but the greatest duration of action. The relative potency also changed, tacrine having low potency (ED50 [mu mol/kg]; tacrine: 200, NXX-0 66: 30, donepezil: 50). Salivation was severe only in tacrine treated anima ls. Using in vivo microdialysis in cerebral cortex, both NXX-066 and tacrin e were found to produce a marked (at least 30-fold) increase in extracellul ar acetylcholine which remained elevated for more than 2 h after tacrine an d 4 h after NXX-066. (C) 1999 Elsevier Science Ltd. All rights reserved.