Inhibition of monoamine oxidase B by substituted coumarin derivatives

Abstract

Interest in the selective inhibition of monoamine oxidase B (MAO-B) has increased in the last years due to their therapeutic potential in age related neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Reversibly inhibiting MAO-B reduces the oxidative deamination of dopamine, increases dopamine levels and, consequently, reduces the production of reactive oxygen species (ROS) which may be the therapeutic intervention that positively affect the tempo and progression of neurodegenerative diseases. Coumarin analogues exhibit a broad spectrum of pharmacological activity, ranging from HIV-1-reverse transcriptase inhibitory properties to competitive, reversible and selective MAO-B and acetylcholinesterase (AChE) inhibition. In this study the stereoelectronic properties of novel coumarin compounds were investigated to optimise a pharmacophore for MAO-B inhibition. This structure was subsequently conjugated to the neuroprotective xanthinyl moieties of caffeine, a specific A2A antagonist and the polycyclic structure, a n-methyl-D-aspartic acid (NMDA) antagonist. Other C-7 substitutents were also considered in this study. These drugs may act via a dual mechanism for the treatment of neurodegenerative disorders. Coumarin analogues were synthesised via etherification, esterification and hydrolysis, and conjugated to the neuroprotective xanthinyl moieties by esterification and amidation using activation chemistry with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC) and N,N-dicyclohexylcarbodiimide (DCC). The final products were obtained as oils or amorphous solids from chromatography and were crystallised from organic solvents. MS and NMR were used to confirm the structures and characterise the compounds. The synthesised compounds were evaluated in vitro as competitive inhibitors of MAO-B, using a spectrophotometric assay that utilised 1-methyl-4-(1-methylpyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP), an analogue of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) as substrate. Baboon liver tissue served as enzyme source. The potency of MAO-B inhibition was expressed as IC50 values and the enzyme-inhibitor dissociation constant (Ki value). Molecular modelling studies were carried out with the published MAO-B crystal structure (PDB: 1oj9) and the proposed inhibitors were prepared in a DS Viewer window using the 'Prepare Ligands' module of Discovery Studios 1.7®. Docking was performed in the 'Dock Ligands' module (Ugandfit) with minimisation set as smart minimiser. The docking poses were scored with Ugandfit 2 and the consensus Dock Score. From the molecular modelling studies, it was clear that the inhibitors occupy the entrance and substrate cavities of MAO-B, presenting the coumarin nucleus inside the substrate cavity. In vitro MAO-B inhibition assays established that the 7-substituted coumarin derivatives display MAO-B inhibitory properties with Ki values in the low micromolar range. The most potent inhibitor among the analogues was the 4-methyl-2-oxo-2H-chromen-7-yJ-(2E)-3-phenylacrylate (BPR 4) while the least potent inhibitor was found to be 3-{4-aza-8- oxoheptacyclo[0.4.1.02,10 .03,14 .04,9 .09,13.012,15] tetradecyl} [( 4'-methyl-2' -oxo-2H-chromen-7'- yl)oxy] acetate (BPR 6). The results of an SAR study established that the potency of MAO-B inhibition by the coumarin derivatives examined is dependent on chain length, lipohilicity, planarity and steric bulk of the substituents on position 7 of the coumarin. These results may be of relevance to diminish or delay the incidence and perhaps the progression of debilitating neurodegenerative diseases.