Promising Non-cytotoxic Monosubstituted Chalcones to Target Monoamine Oxidase-B

Abstract

A library of monosubstituted chalcones (1-17) bearing electron-donating and electron-withdrawing groups on both aromatic rings were selected. The cell viability on human tumor cell lines was evaluated first. The compounds unable to induce detectable cytotoxicity (1, 13, and 14) were tested using the monoamine oxidase (MAO) activity assay. Interestingly, they inhibit MAO-B, acting as competitive inhibitors, with 13 and 14 showing the best profiles. In particular, 13 exhibited a potency higher than that of safinamide, taken as a reference. Docking studies and crystallographic analysis showed that in human MAO-B 13 binds with the halogen-substituted aromatic ring in the entrance cavity, similar to safinamide, whereas 14 is accommodated in the opposite way. The main conclusion of this cell biology, biochemistry, and structural study is to highlights 13 as a chalcone derivative that is worth consideration for the development of novel MAO-B-selective inhibitors for the treatment of neurodegenerative diseases.

Figure 1

Competitive inhibition of chalcone derivatives on human MAO-B activity. Lineweaver–Burk double-reciprocal plots of MAO-B initial velocity vs substrate concentration (1/v₀ vs 1/[kynuramine]) in the absence and presence of various concentrations of compounds. Continuous lines are the results of linear regression analysis of plotted data (r > 0.98). (a) Reciprocal plots at different concentrations of 13 (5–40 nM) showing that increasing the concentration of the compound increases the effect on K_m (from the x-intercept), while no effect on V_max (from the y-intercept) is observed. (b) Examples of reciprocal plots in the presence of 1, 14, and chalcone (the lead compound) that demonstrate their competitive behavior (effect on K_m only). Safinamide is shown as a standard competitive inhibitor. As these compounds have different inhibitory potencies, different concentrations were used to show the effect on MAO-B.

Figure 2

Predicted docking poses of 13 in the MAO-A and MAO-B crystal structures: (a) binding mode of 13 in the MAO-A protein (PDB ID 2Z5X); (b) the more energetically favored binding mode (binding mode b) predicted for 13 in the MAO-B crystal structure (PDB ID 2V5Z).

Figure 3

Overall structure of the MAO-B dimer represented as a pink ribbon diagram (chain A is on the left) with the membrane-spanning C-terminal helix pointing to the bottom of the figure. The FAD cofactor is shown in stick representation with carbon, nitrogen, oxygen, and phosphorus atoms colored in yellow, blue, red, and magenta, respectively. The structure in complex with 13 (in green, with fluorine atoms in light blue and oxygen in red) is shown. The overall fold is identical to that of the enzyme in complex with 14.

Figure 4

Architectures of the MAO-B active site in complex with (a) 13 and (b) 14. FAD is depicted as in Figure 3. The residues that line the cavity are represented as sticks with carbon, nitrogen, oxygen, and sulfur atoms in pink, blue, red, and orange, respectively. The refined 2F_o – F_c electron density map for the inhibitor molecule (contoured at 1.2σ) is shown in blue chicken-wire style. The fluorine atoms of both inhibitors are in light blue, with carbon atoms in green (13) and gray (14). Water molecules are depicted as red spheres and hydrogen bonds as dashed lines.

Figure 5

Superposition of MAO-B structures in complex with 13, 14, and safinamide (the latter represented with carbon atoms in magenta; PDB code 2V5Z). For the sake of clarity, water molecules and hydrogen bonds shown in Figure 4 have been omitted.