Design and Synthesis of 1-phenyl-[1,2,4]triazolo[4,3-a]quinoxalin-4(5H)-one Derivatives as a Selective Inhibitor for Acetylcholinesterase and Butyryl Cholinesterase

Abstract

Quinoxaline-based heterocycles are of interest as potential cholinesterase inhibitors, and this study evaluated a substituted 1-phenyl-[1,2,4]triazolo[4,3-a]quinoxalin-4(5H)-one series against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The objective was to synthesise and characterise derivatives with different aromatic substituents and compare their inhibitory activities against both enzymes. Substituted (E)-3-(2-benzylidenehydrazinyl)quinoxalin-2(1H)-ones were cyclised in thionyl chloride to obtain derivatives 2a–2h, and the products were characterised spectroscopically. Cholinesterase inhibition was assessed using the reported modified spectrophotometric assay, with eserine as the reference compound. The reaction was monitored at 412 nm, and the test compounds were evaluated in triplicate under the assay conditions described in the manuscript. The AChE assay showed IC₅₀ values of 0.20±0.01 µM for 2b, 0.30±0.01 µM for 2f, 0.80±0.01 µM for 2h, and 0.88±0.01 µM for 2c. For BChE, compounds 2e, 2a, 2c, and 2b gave IC₅₀ values of 1.10±0.01, 1.30±0.01, 1.50±0.01, and 1.51±0.10 µM, respectively. Eserine showed IC₅₀ values of 0.37±0.10 µM against AChE and 2.70±0.03 µM against BChE. The results indicate differential dual cholinesterase inhibition across the synthesised series. Compounds 2b and 2f showed lower AChE IC₅₀ values than eserine, while several derivatives showed lower BChE IC₅₀ values than the reference. Overall, the findings demonstrate substituent-dependent inhibitory activity and support further evaluation of this triazoloquinoxalinone series for cholinesterase selectivity and structure–activity relationships.