In Vitro Human Monoamine Oxidase Inhibition and Human Dopamine D4 Receptor Antagonist Effect of Natural Flavonoids for Neuroprotection

Natural flavone and isoflavone analogs such as 3',4',7-trihydroxyflavone (1), 3',4',7-trihydroxyisoflavone (2), and calycosin (3) possess significant neuroprotective activity in Alzheimer's and Parkinson's disease. This study highlights the in vitro human monoamine oxidase (hMAO) inhibitory potential and functional effect of those natural flavonoids at dopamine and serotonin receptors for their possible role in neuroprotection. In vitro hMAO inhibition and enzyme kinetics studies were performed using a chemiluminescent assay. The functional effect of three natural flavonoids on dopamine and serotonin receptors was tested via cell-based functional assays followed by a molecular docking simulation to predict interactions between a compound and the binding site of the target protein. A forced swimming test was performed in the male C57BL/6 mouse model. Results of in vitro chemiluminescent assays and enzyme kinetics depicted 1 as a competitive inhibitor of hMAO-A with promising potency (IC50 value: 7.57 ± 0.14 μM) and 3 as a competitive inhibitor of hMAO-B with an IC50 value of 7.19 ± 0.32 μM. Likewise, GPCR functional assays in transfected cells showed 1 as a good hD4R antagonist. In docking analysis, these active flavonoids interacted with a determinant-interacting residue via hydrophilic and hydrophobic interactions, with low docking scores comparable to reference ligands. The post-oral administration of 1 to male C57BL/6 mice did not reduce the immobility time in the forced swimming test. The results of this study suggest that 1 and 3 may serve as effective regulators of the aminergic system via hMAO inhibition and the hD4R antagonist effect, respectively, for neuroprotection. The route of administration should be considered.

Monoamine oxidase inhibitory activity of 3,5-biaryl-4,5-dihydro-1H-pyrazole-1-carboxylate derivatives

Ethyl and phenyl carbamate derivatives of pyrazoline (3a-3h) were synthesized and tested for their MAO inhibitory activity. All the compounds were found to be selective towards MAO-A. Phenyl carbamates (3e-3h) were better than ethyl carbamates (3a-3d) and displayed the best selectivity index. Compound 3f (KiMAO-A; 4.96 ± 0.21 nM) was found to be equally potent as that of standard drug, Moclobemide (KiMAO-A; 5.01 ± 0.13 nM) but with best selectivity index (8.86 × 10(-5)). Molecular docking studies with R &S conformer of 3f revealed S-enantiomer is better than R-enantiomer as reported earlier by other groups. It is proposed that VdW's radii of the substitution (bulkiness) in ring B determine the potency of phenyl carbamates.