Multi-target directed ligands: Electrophysiological characterization of an anticholinesterase inhibitor coupled to an agonist of α7 nicotinic acetylcholine receptor

Alzheimer’s disease (AD) is the neurodegenerative disorder provoking memory loss and cognitive dysfunction in elderly patients. It is the fourth cause of death in the developed countries (after heart disease, cancer and stroke). This disease is incurable for the moment, only palliative and long-term treatment is available having acetylcholinesterase and a7 nicotinic acetylcholine receptor as molecular targets. The key event of AD seems to be beta amyloid peptide formation and aggregation, however a reduced synthesis of neurotransmitter – acetylcholine – was also observed in the etiology of this dysfunction. Additionally, amyloid peptide Ab binds to a7 nicotinic acetylcholine receptor to induce either receptor activation or inhibition in an Ab concentration-dependent mode. Ab oligomers induce Tau phosphorylation via a7 nicotinic receptor activation as well. In our synthetic work, we took advantage of ‘‘click-chemistry’’ to couple modified acetylcholinesterase inhibitors (Cl- orNH2-tacrine and Cl-NH2- and acyl-huprine) to azide-alkyne derivatized agonists of a7 nicotinic acetylcholine receptor in order to obtain a series of multi-target directed ligands which could ideally simultaneously interact with acetylcholinesterase and with the a7 nicotinic acetylcholine receptor. The acetylcholinesterase inhibition from the synthesized two-target ligands ranged between 3 and 15 nM. The agonistic behaviour of the two-target compounds was evaluated by manual and automated two-electrode voltage clamp on Xenopus oocytes expressing human a7 nicotinic acetylcholine receptors. Among the double functionalized organic molecules, one compound showed an in vitro inhibitory activity towards human acetylcholinesterase in nanomolar scale and an agonistic activity in the micromolar range. Moreover, compound 1 showed a good ability to cross the blood–brain barrier. It is worth noticing that at high concentrations the referred compound 1 showed a pronounced antagonistic activity towards the nicotinic receptor. The synthetic precursors were as well tested on Xenopus oocytes: (i) the agonistic activity of the agonist moiety towards a7 nicotinic acetylcholine receptors in nanomolar scale was verified and (ii) the presumed nicotinic antagonistic activity of the anticholinesterase precursor group was also demonstrated by this approach explaining why at high concentrations, the partial agonist compound 1 behaves as an antagonist of a7 nicotinic acetylcholine receptors.