Development of Halogenated Pyrazolines as Selective Monoamine Oxidase-B Inhibitors: Deciphering via Molecular Dynamics Approach

Abstract

Halogens have been reported to play a major role in the inhibition of monoamine oxidase (MAO), relating to diverse cognitive functions of the central nervous system. Pyrazoline/halogenated pyrazolines were investigated for their inhibitory activities against human monoamine oxidase-A and -B. Halogen substitutions on the phenyl ring located at the fifth position of pyrazoline showed potent MAO-B inhibition. Compound 3-(4-ethoxyphenyl)-5-(4-fluorophenyl)-4,5-dihydro-1H-pyrazole (EH7) showed the highest potency against MAO-B with an IC₅₀ value of 0.063 µM. The potencies against MAO-B were increased in the order of -F (in EH7) > -Cl (EH6) > -Br (EH8) > -H (EH1). The residual activities of most compounds for MAO-A were > 50% at 10 µM, except for EH7 and EH8 (IC₅₀ = 8.38 and 4.31 µM, respectively). EH7 showed the highest selectivity index (SI) value of 133.0 for MAO-B, followed by EH6 at > 55.8. EH7 was a reversible and competitive inhibitor of MAO-B in kinetic and reversibility experiments with a Ki value of 0.034 ± 0.0067 µM. The molecular dynamics study documented that EH7 had a good binding affinity and motional movement within the active site with high stability. It was observed by MM-PBSA that the chirality had little effect on the overall binding of EH7 to MAO-B. Thus, EH7 can be employed for the development of lead molecules for the treatment of various neurodegenerative disorders.

Keywords: halogenated pyrazolines; kinetics; molecular dynamics; monoamine oxidase inhibitors; reversibility.

Figure 1

General blueprint of the design of MAO-B inhibitors.

Figure 2

Lineweaver–Burk plots for MAO-A and MAO-B inhibitions by EH6 (A,C) and their respective secondary plots (B,D) of slopes vs. inhibitor concentrations.

Figure 2

Lineweaver–Burk plots for MAO-A and MAO-B inhibitions by EH6 (A,C) and their respective secondary plots (B,D) of slopes vs. inhibitor concentrations.

Figure 3

Lineweaver–Burk plots for MAO-A and MAO-B inhibitions by EH7 (A,C) and their respective secondary plots (B,D) of slopes vs. inhibitor concentrations.

Figure 4

Recovery of MAO-B inhibition by EH7, using dialysis experiments. The concentrations used were EH7 at 0.15 µM, lazabemide (a reversible reference inhibitor) at 0.20 µM, and pargyline (an irreversible reference inhibitor) at 0.30 µM.

Figure 5

Comparative root mean square deviations (RMSDs) of the Cα carbon atoms of the compounds in whole systems for MAO-B (A) and MAO-A (B) and in active sites for MAO-B (C) and for MAO-A (D). (A,C) EH1, black; EH6, green; EH7, blue; and EH8 red. (B,D) EH1, black; EH6, red; EH7, green; and EH8, blue.

Figure 6

Stability plot of compounds EH1, EH6, EH7, and EH8.

Figure 7

Principal component analysis of the compounds bound to MAO-B (A) and to MAO-A (B), and root mean square fluctuation (RMSF) of the compounds bound to MAO-B (C) and to MAO-A (D). (A,B) EH1, black; EH6, green; EH7, blue; and EH8, red. (C,D) EH1, black; EH6, green; EH7, blue; and EH8, red.

Figure 8

Per residue decomposition of EH1 bound to MAO-B (A), EH6 bound to MAO-B (B), EH7 bound to MAO-B (C), and EH8 bound to MAO-B (D) systems.

Figure 9

Ligand interaction profile of the interaction of EH1 (A), EH6 (B), EH7 (C), and EH8 (D) with MAO-B.

Figure 10

Per residue decomposition of MAO-B_EH7S enantiomer.

Figure 11

Swiss target/ADME prediction: (A) Biological target prediction of lead molecule EH7; (B) bioavailability radar of EH7; (C) the BOILED-Egg construction of EH7.

Scheme 1

Synthetic route of the compounds under the present study.