The synthesis and evaluation of phthalimide analogues as inhibitors of monoamine oxidase B / Clarina Ilara Manley-King

• Main Author: Manley-King, Clarina Ilara
• Language: en
• Published: North-West University 2013
• Online Access: http://hdl.handle.net/10394/8506

Parkinson’s disease (PD) is a multifactorial neurodegenerative disease believed to be caused by a number of factors. This has made the successful treatment of the disease very difficult, as the underlying cause of degeneration is still unknown. Monoamine oxidase (MAO-B) inhibitors have been used in the treatment of PD. MAO-B is known to be involved in the catalytic oxidation of biogenic amines, a reaction which produces aldehydes and hydrogen peroxide as byproducts. Both these by-products can be toxic if not rapidly cleared. Inhibitors of MAO-B conserve the depleted supply of dopamine and also stoichiometrically decreases the amount of toxic by-products formed. Thus, MAO-B inhibitors may offer both symptomatic and neuroprotective effects that can aid in the treatment of PD. This study is part of the ongoing investigation into the development of new selective reversible inhibitors of MAO-B. Literature reports that isatin, a small, reversible, endogenous MAO inhibitor, found in the brain, can inhibit both MAO-A and MAO-B enzymes. Previous studies have shown that (E)-5-styrylisatin and (E)-6-styrylisatin are reversible inhibitors of human MAOA and -B. Both homologues are reported to exhibit selective binding to the MAO-B isoform with (E)-5-styrylisatin being the most potent inhibitor. To further investigate these structure–activity relationships (SAR), in the present study, additional C5- and C6-substituted isatin analogues were synthesized and evaluated as inhibitors of recombinant human MAO-A and MAO-B. A series of structurally related corresponding anilines, which are synthetic precursors in the synthesis of isatin derivatives, were also evaluated as MAO inhibitors. This study is part of an attempt to identify new inhibitors with enhanced potencies and specificities for both MAO-A or MAO-B. In general, C5- and C6-substitution of isatin leads to enhanced binding affinity to both MAO isozymes, compared to isatin, and in most instances result in selective binding to the MAO-B isoform. The most potent MAO-B inhibitor 5-(4-phenylbutyl)isatin, exhibited an IC50 value of 0.66 nM and the most potent MAO-A inhibitor was found to be 5-phenylisatin with an IC50 value of 562 nM. Crystallographic and modelling studies suggest that the isatin ring binds to the substrate cavities of MAO-A and -B and is stabilized by hydrogen bond interactions between the NH and the C2 carbonyl oxygen of the dioxoindolyl moiety and water molecules present in the substrate cavities of MAO-A and -B. Based on these observations and the close structural resemblance between isatin and its phthalimide isomer, a series of phthalimide analogues were synthesized and evaluated as MAO inhibitors. The results showed that the C5 substituted phthalimides were very potent competitive inhibitors with IC50 values ranging 0.007 to 2.5 μM for MAO-B and IC50 values ranging 0.22 to 9.0 μM for MAO-A. The 5-(4-benzyloxy)phthalimide was the most potent MAO-B inhibitor in the phthalimide series, with an IC50 value of 0.007 μM. The results of modelling studies showed that hydrogen-bond interactions between the phthalimide carbonyl oxygen and the enzyme amino acid residues and the integral water molecules are important for the binding of phthalimide to the active site of MAO-B. The potent competitive inhibition and activities of the C5 substituted phthalimide analogues towards MAO-B has led us to investigate a structurally similar series of C4-substituted phthalonitriles. A series of C4-substituted phthalonitriles were prepared and evaluated as inhibitors of MAO-B. In general, the phthalonitriles were very potent competitive inhibitors of MAO-B with IC50 values ranging from 0.005–6.02 μM. 5-(4-benzyloxy)phthalonitrile was found to be the most potent inhibitor for human MAO-B with an IC50 value of 0.005 μM. To further investigate the effect of the nitrile group in this class of compounds, C3 and C4 substituted benzonitriles were prepared and evaluated for MAO inhibition. The results showed that similar to the phthalonitriles, the benzonitriles were also potent inhibitors of human MAO-B, with IC50 values ranging from 0.785-1.39 μM. The benzonitriles, however, were not as potent as the corresponding phthalonitriles. These findings suggest that, although two nitrile groups are more optimal for inhibition, the presence of a second nitrile group is not a necessity for potent MAO-B inhibition. Placement of the nitrile group at C3 resulted in more potent MAO-B inhibition compared to placement of the nitrile at C4. === Thesis (Ph.D. (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2012