Chalcones, the open-chain precursors of flavonoids, have attracted sustained attention in medicinal chemistry because their two aromatic rings joined by an α,β-unsaturated carbonyl system provide a versatile scaffold for tuning biological activity. Previous studies have shown that the extended conjugation of the chalcone framework can favour binding within the hydrophobic cavity of MAO-B, and derivatives bearing heterocyclic substituents such as morpholine, imidazole, and indole have repeatedly emerged as promising inhibitors. Building on this foundation, the team designed a targeted series in which diverse substituents were introduced at different positions of the phenyl ring, aiming to map precisely how electronic and steric changes govern potency and selectivity.

Synthesis was carried out using a microwave-assisted protocol, an approach that dramatically shortens reaction times and improves yields compared with conventional heating, making it well suited to the rapid generation of combinatorial libraries. All nineteen analogues, designated TC1 through TC19, were characterised by nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and high-resolution mass spectrometry, confirming their structures before biological testing began. This analytical rigour matters: in enzyme-inhibition studies, uncharacterised impurities can masquerade as activity, so full structural confirmation is a prerequisite for credible structure-activity relationships.

The biological evaluation was deliberately broad. Rather than screening against a single target, the researchers tested the library against acetylcholinesterase and butyrylcholinesterase, enzymes relevant to Alzheimer’s disease; against β-site amyloid precursor protein cleaving enzyme 1, or BACE1, a key player in amyloid plaque formation; and against both monoamine oxidase isoforms, MAO-A and MAO-B. The results were strikingly one-sided. MAO-B emerged as the clear target of preference, with the chalcones showing little meaningful activity against the cholinesterases or BACE1. This kind of clean target profile is valuable in early drug discovery because it reduces the risk of pursuing compounds whose apparent effects stem from off-target interactions.

Two compounds stood out. TC12 proved to be the most potent inhibitor of the entire series, with a half-maximal inhibitory concentration of 11.00 ± 1.0 nanomolar against MAO-B and a selectivity index greater than 3,636 relative to MAO-A. Kinetic analysis showed that TC12 inhibits the enzyme competitively, with a Ki of 7.16 ± 1.20 nanomolar, meaning it competes with the natural substrate for access to the active site. Importantly, follow-up experiments demonstrated that the inhibition is reversible and not time-dependent, characteristics that distinguish TC12 from the irreversible, mechanism-based inhibitors of earlier generations such as selegiline. Reversible inhibition offers pharmacological advantages, including more predictable dose-response behaviour and easier management of interactions with tyramine-rich foods, a classic concern with MAO inhibitors.

The second-ranked compound, TC9, was hardly less impressive, inhibiting MAO-B with an IC50 of 50.0 ± 1.0 nanomolar and a selectivity index greater than 800. Together, the two analogues anchor a clear structure-activity relationship within the morpholine-substituted chalcone series, suggesting that the morpholine ring and specific aryl substitution patterns cooperate to position the molecule optimally within the MAO-B cavity. Because MAO-B and MAO-A share a highly conserved catalytic architecture, achieving selectivity ratios in the thousands requires exploiting the subtle differences between their active-site cavities, and the data indicate that this scaffold does exactly that.

To understand the molecular basis of this selectivity, the team turned to computational docking. The models revealed that TC12 establishes favourable interactions within the MAO-B active site, most notably through π-π stacking with the aromatic cage residues TYR398 and TYR435, the tyrosines that flank the flavin cofactor and define the substrate-binding environment of the enzyme. In contrast, TC12 showed only limited engagement with the corresponding key regions of MAO-A, providing a structural rationale for the remarkable isoform selectivity observed in the enzyme assays. Such docking evidence does not prove binding geometry on its own, but when it aligns cleanly with kinetic data, it gives medicinal chemists a working hypothesis for further optimisation.

Potency in an enzyme assay is meaningless for a central nervous system drug unless the compound can actually reach the brain. The researchers therefore evaluated the drug-like properties of the lead compounds using both computational prediction and laboratory experiments. ADME predictions indicated that TC12 complies with Lipinski’s rules, exhibits moderate central nervous system permeability, and displays a pharmacokinetic profile supportive of oral bioavailability. In vitro experiments using the parallel artificial membrane permeability assay, a standard surrogate for blood-brain barrier penetration, showed that TC9 achieved high permeability comparable to carbamazepine, a well-established anticonvulsant, while TC12 showed moderate permeability consistent with its predicted CNS penetration.

Metabolic stability was assessed in rat and human liver microsomes, experimental systems that model the first-pass hepatic metabolism a drug candidate must survive. Both TC9 and TC12 demonstrated favourable stability profiles in these assays, an encouraging sign that the compounds would not be rapidly cleared from circulation. The combination of brain penetration, microsomal stability, and predicted oral bioavailability places these chalcones among the more thoroughly vetted early-stage MAO-B inhibitors reported in recent years, and the authors conclude that TC12 in particular represents a promising candidate for further pharmacological evaluation.

The broader significance of the work lies in the therapeutic context. MAO-B degrades dopamine in the brain, and its activity is elevated in the astrocytes of patients with Parkinson’s disease, making it a validated target for drugs such as rasagiline, selegiline, and safinamide. Inhibitors have also been explored for Alzheimer’s disease, where MAO-B activity contributes to oxidative stress and neuroinflammation. Yet existing drugs have limitations, including side effects and, in some cases, irreversible enzyme binding. A reversible, highly selective, nanomolar-potency inhibitor built on a synthetically accessible chalcone scaffold offers a route toward next-generation agents that could modulate dopamine metabolism with greater precision. The study was supported by Istanbul University and the National Research Foundation of Korea, and while the road from a promising in vitro profile to an approved medicine is long, the convergence of potency, selectivity, reversibility, and drug-like properties in TC12 makes this morpholine-substituted chalcone library a noteworthy addition to the neurodegenerative disease drug-discovery pipeline.

Subject of Research: Discovery of selective and reversible MAO-B inhibitors from a morpholine-substituted chalcone library for neurodegenerative disorders

Article Title: Discovery of potent and selective MAO-B inhibitors from a morpholine-substituted chalcone library for neurodegenerative disorders

Article References: Taghiyeva, A., Lim, J., Biswas, A., Mondal, S., Mukherjee, R. U., Basavanakatti, V. N., Bayrak, N., Jayaprakash, V., Paik, M.-J., Kim, H., & TuYuN, A. F. (2026). Discovery of potent and selective MAO-B inhibitors from a morpholine-substituted chalcone library for neurodegenerative disorders. Molecular Diversity. https://doi.org/10.1007/s11030-026-11733-w

Image Credits: AI Generated

DOI: 10.1007/s11030-026-11733-w

Keywords: MAO-B, chalcones, Parkinson's disease, neurodegenerative disorders, monoamine oxidase, molecular docking, drug discovery, morpholine, reversible inhibition, blood-brain barrier, medicinal chemistry, Alzheimer's disease