Discovery of tricyclic benzo[4,5]imidazo[2,1-b]thiazole-3-hydroxyindolin-2-one hybrids as selective MAO-B and cholinesterase inhibitors.

A series of 13 novel 3-hydroxyindolin-2-one-benzo[4,5]imidazo[2,1-b]thiazole hybrids was designed, synthesized, and evaluated as potential anti-Alzheimer agents targeting monoamine oxidase B (MAO-B) and cholinesterases. The target compounds were obtained by condensing a benzo[4,5]imidazo[2,1-b]thiazole-based ketone with substituted and N-alkylated isatins and were structurally confirmed by 1H NMR, 13C NMR, and elemental analysis. All synthesized compounds were screened for their inhibitory activities against MAO-A and MAO-B enzymes. Most derivatives displayed weak MAO-A inhibition while exhibiting a marked preference for MAO-B inhibition. Among them, compounds 6c and 6b emerged as the most potent MAO-B inhibitors, with IC50 values of 0.90 ± 0.005 and 0.97 ± 0.010 µM, respectively. Structure-activity relationship studies revealed that substitution at the C-5 position of the isatin ring, particularly with halogen atoms, significantly enhanced MAO-B inhibitory activity, whereas N-alkylation generally reduced both potency and selectivity. Based on their superior MAO-B inhibitory profiles, compounds 6b and 6c were further evaluated against acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). Notably, compound 6c exhibited potent dual cholinesterase inhibition, showing AChE inhibitory activity comparable to that of donepezil and superior BuChE inhibition. Furthermore, SwissADME analysis indicated favorable drug-likeness properties for the most active compounds. Molecular docking studies demonstrated favorable binding interactions of the lead compounds within the active sites of MAO-B and AChE, while a 200 ns molecular dynamics simulation confirmed the stability of the MAO-B-6c complex. Collectively, these findings identify 6c as a promising multitarget lead compound combining selective MAO-B inhibition with potent cholinesterase inhibitory activity and highlight the 3-hydroxyindolin-2-one-benzo[4,5]imidazo[2,1-b]thiazole scaffold as a valuable platform for the development of novel therapeutic agents for Alzheimer's disease.