Synthesis and evaluation of small molecules bearing a benzyloxy substituent as novel and potent monoamine oxidase inhibitors

Abstract

A new series of small molecules bearing a benzyloxy substituent have been designed, synthesized and evaluated for hMAO inhibitory activity in vitro. Most of the compounds were potent and selective MAO-B inhibitors, and were weak inhibitors of MAO-A. In particular, compounds 9e (IC₅₀ = 0.35 μM) and 10e (IC₅₀ = 0.19 μM) were the most potent MAO-B inhibitors, and exhibited the highest selectivity for MAO-B (9e, SI > 285.7-fold and 10e, SI = 146.8-fold). In addition, the structure-activity relationships for MAO-B inhibition indicated that electron-withdrawing groups in the open small molecules were more suitable for MAO-B inhibition, and substitutions at the benzyloxy of the open small molecules, particularly with the halogen substituted benzyloxy, were more favorable for MAO-B inhibition. Molecular docking studies have been done to explain the potent MAO-B inhibition of the open small molecules. Furthermore, the representative compounds 9e and 10e showed low neurotoxicity in SH-SY5Y cells in vitro. So the small molecules bearing the benzyloxy substituent could be used to develop promising drug candidates for the therapy of neurodegenerative diseases.

Fig. 1. Structures of known MAO inhibitors bearing a benzyloxy substituent.

Fig. 2. Design strategy for the novel series of heterocycle-opened derivatives as MAO-B inhibitors.

Scheme 1. Syntheses of target compounds 9a–g, 10a–g and 11a–g. Reagents and conditions: (a) K₂CO₃, CH₃CN, reflux, 12 h.

Fig. 3. The time-dependent inhibition of hMAO-B by compounds 9e and 10e. The compounds were preincubated for various periods of time (0–60 min) with hMAO-B at concentrations equal to twofold the IC₅₀ values for the inhibition of the enzyme. After dilution to concentrations equal to the IC₅₀, the inhibitory rates were recorded.

Fig. 4. Kinetic study on the mechanism of hMAO-B inhibition by compound 10e. Overlaid Lineweaver–Burk reciprocal plots of the MAO-B initial velocity at an increasing substrate concentration (50–500 μM) in the absence of the inhibitor and in the presence of 10e are shown. Lines were derived from a weighted least-squares analysis of the data points.

Fig. 5. (A) A 3D docking model of compound 9e with hMAO-B. Atom colors: yellow – carbon atoms of compound 9e, gray – carbon atoms of residues of hMAO-B, dark blue – nitrogen atoms, and red – oxygen atoms. The dashed lines represent the interactions between the protein and the ligand. (B) A 2D schematic diagram of a docking model of compound 9e with hMAO-B. (C) A 3D docking model of compound 10e with hMAO-B. (D) A 2D schematic diagram of a docking model of compound 10e with hMAO-B. (E) A 3D docking model of compound 11e with hMAO-B. (F) A 2D schematic diagram of a docking model of compound 11e with hMAO-B. The figure was prepared using the ligand interactions application in MOE.

Fig. 6. Effects of compounds on cell viability in SH-SY5Y cells. The cell viability was determined by the MTT assay after 48 h of incubation with various concentrations of 9e and 10e. The results were expressed as a percentage of control cells. Values are reported as the mean ± SD of three independent experiments.