BACKGROUND AND RATIONALE
Parkinson’s disease (PD) is a chronic, progressive neurodegenerative disorder affecting the
central nervous system, primarily, the substantia nigra. It is characterized by loss of
dopaminergic neurons in the nigro-striatal pathway, and ultimately patients with Parkinson’s
disease may lose up to 80% of their dopamine-producing cells in the brain. Symptoms
include bradykinesia, muscle rigidity, resting tremor and impaired postural balance.
Symptomatic relief is obtained by using levodopa and various adjunct therapy including
dopamine agonists, catechol-O-methyltransferase inhibitors and monoamine oxidase B
inhibitors. Levodopa is used as the gold-standard for treatment of this disease. It effectively
controls motor symptoms, however, motor complications that impair the quality of life
develop with continued levodopa use. No treatments currently available can halt disease
progression, therefore novel drugs that can slow down or stop disease progression are
urgently required.
The monoamine oxidase (MAO) A and B enzymes are flavoenzymes that play an important
role in the oxidative degradation of amine neurotransmitters such as dopamine, serotonin
and epinephrine. Early attempts to block dopamine metabolism in the brain using nonselective
MAO inhibitors was effective but led to side effects such as hypertensive crisis,
thus they lost favor. The MAO-B enzyme is of particular importance in Parkinson’s disease
because it is more active than MAO-A in the basal ganglia, and is thus primarily responsible
for the catabolism of dopamine in the brain. Selegiline and rasagiline, both irreversible,
selective MAO-B inhibitors have proven efficacy in symptomatic treatment of Parkinson’s
disease, but due to the irreversible nature of their binding, it can take several weeks after
treatment termination for the enzyme to recover. Use of reversible inhibitors such as
lazabemide and safinamide do not have this disadvantage, and have safer side effect
profiles. Unfortunately, clinical trials for lazabemide use in Parkinson’s disease have been
discontinued. Therefore, due to the lack of disease modifying agents for Parkinson’s
disease, as well as safety concerns of current PD therapy, an urgent need exists for novel,
safe and efficient MAO inhibitors. Current research is thus aimed at designing selective or
non-selective reversible inhibitors that bind competitively to the enzyme.
The MAO inhibitory potential of chromone derivatives has been illustrated previously.
Evaluation of C6- and C7-alkyloxy substituted chromones, for example revealed that these
compounds were potent, selective and reversible MAO-B inhibitors. It has further been
shown that chromone 3-carboxylic acid is a potent selective, irreversible MAO-B inhibitor.
Phenylcarboxamide substitution in position 3 of chromone 3-carboxylic acid also results in
potent, selective MAO-B inhibitory activity. Therefore, further evaluation of the effect of
substitution with flexible side chains in the 3-position to evaluate MAO-B inhibition is of
importance.
The chromone ring system is thus a privileged scaffold for the design of inhibitors that are
selective for MAO-B and has the additional advantages of generally exhibiting low
mammalian toxicity and ease of synthesis.
AIM
The aim of this study was to design, synthesize and evaluate novel chromone derivatives as
inhibitors of monoamine oxidase.
RESULTS
Design and Synthesis
3-Aminomethylene-2,4-chromandiones and ester chromone derivatives were synthesized by
coupling several aromatic and aliphatic amines and alcohols, to chromone 3-carboxylic acid,
in the presence of CDI (carbonyldiimidazole). 15 Compounds were successfully synthesized
and characterized by using NMR and IR spectroscopy, as well as mass spectrometry. X-ray
crystallography was used to obtain a crystal structure for the 3-aminomethylene-2,4-
chromandione derivative, 46, in a bid to verify the structures of the synthesized compounds.
Melting points of all compounds were determined, and the purity determined using HPLC
techniques.
MAO inhibition studies
A fluorometric assay was employed using kynuramine as substrate, to determine the IC50
(50% inhibition concentration) values and SI (selectivity index) of the synthesized
compounds. Generally, the esters exhibited weak MAO-A and MAO-B inhibition, while the 3-
aminomethylene-2,4-chromandione derivatives showed promise as selective MAO-B
inhibitors, with IC50 values in the micromolar range. Compound 38, 3-
[(benzylamino)methylidene]-3,4-dihydro-2H-1-benzopyran-2,4-dione, was the most potent
MAO-B inhibitor with an IC50 value of 0.638 μM and a SI of 122 for MAO-B inhibition.
Interesting trends were revealed through analysis of the structure activity relationships, for
example, for the 3-aminomethylene-2,4-chromandione derivatives, the presence of a
chlorine moiety in the side chains of the compounds resulted in a decrease of MAO-B
inhibition activity. Chain elongation further also resulted in weakening the MAO-B inhibition
activity, while chain elongation in the ester derivatives led to a slight increase in MAO-B
inhibition activity.
Reversibility studies
The reversibility of binding of the most potent compound in the 3-aminomethylene-2,4-
chromandione series, 38, was evaluated. None of the synthesized inhibitors were potent
MAO-A inhibitors, therefore reversibility of MAO-A inhibition was not examined. Recovery of
enzyme activity was determined after dialysis of the enzyme-inhibitor complexes. Analysis of
the kinetic data obtained showed that MAO-B catalytic activity was recovered to 115% of the
control value. This suggests that compound 38 is a reversible inhibitor of MAO-B.
Mode of inhibition
A set of Lineweaver-Burk plots were constructed to determine mode of inhibition of
compound 38. The results show linear lines that intersect at a single point just to the left on
the y-axis. This indicates that compound 38 interacts competitively with the MAO-B enzyme.
In conclusion, chromone derivatives were synthesized and evaluated as inhibitors of MAO.
Compound 38 was the most potent MAO-B inhibitor with an IC50 value of 0.638 μM. The
effect of chain elongation and introduction of flexible substituents in position 3 of the
chromone 3-carboxylic acid nucleus was explored and the results showed that 3-
aminomethylene-2,4-chromandione substitution is preferable over ester substitution. / MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2015
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/15449 |
| Date | January 2014 |
| Creators | Mpitimpiti, Annah Nyasha |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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