Carboxamides vs. methanimines: Crystal structures, binding interactions, photophysical studies, and biological evaluation of (indazole-5-yl)methanimines as monoamine oxidase B and acetylcholinesterase inhibitors

A comprehensive study was performed for the first time to compare two structurally related substance classes, namely indazole-5-carboxamides (11–16) and (indazole-5-yl)methanimines (17–22). Both chemical entities are potent, selective and reversible MAO-B inhibitors and, therefore, may serve as promising lead structures for the development of drug candidates against Parkinson's disease (PD) and other neurological disorders. Compounds 15 (Ki = 170 pM, SI = 25907) and 17 (Ki = 270 pM, SI = 16340) were the most potent and selective MAO-B inhibitors in both series. To investigate the multi-target inhibitory activity, all compounds were further screened for their potency against human AChE and BuChE enzymes. Compound 15 was found to be the most potent and selective AChE inhibitor in all series (hAChE IC50 = 78.3 ± 1.7 μM). Moreover, compounds 11 and 17 showed no risk of drug-induced hepatotoxicity and a wider safety window, as determined in preliminary cytotoxicity screening. Molecular modeling studies into the human MAO-B enzyme-binding site supported by a HYDE analysis suggested that the imine linker similarly contributes to the total binding energy in methanimines 17–22 as the amide spacer in their carboxamide analogs 11–16. Amplified photophysical evaluation of compounds 17 and 20, including single X-ray analysis, photochemical experiments, and quantum-chemical calculations, provided insights into their more favourable isomeric forms and structural features, which contribute to their biologically active form and promising drug-like properties.