The traditional view of drug design is that a single drug should interact with a single
molecular target. As science progressed, there was an understanding that most drugs
interact with more than one target and that multiple targets may be responsible for either
adverse effects or additional therapeutic effects. The idea of polypharmacology, which
suggests that the focus of drug design should shift from a single drug that interacts with a
single target to a single drug that can have interactions with multiple targets and multiple
therapeutic effects, revolutionized the drug discovery process. Discovering new drugs is a
long and costly process with years of research and development and clinical trials required
before the drugs reach the market for much needed therapeutic applications. By repurposing
drugs that are already on the market for a new therapeutic target, the discovery process is
accelerated significantly.
One such a target disease, for which there is a great need for new effective therapies, is
Parkinson’s disease (PD). PD is a progressive neurodegenerative disease that is caused by
the death of dopaminergic neurons in the substantia nigra with the resulting loss of
dopamine from the striatum. Degeneration in PD leads to varying degrees of motor difficulty
and disability, along with other symptoms. Current therapies are focussed on symptomatic
management and an improvement of the quality of life of patients, rather than on a cure.
There are several therapeutic targets that are currently used in the treatment of PD. One of
those targets is the monoamine oxidase (MAO) enzymes, in particular the MAO-B isoform.
The MAO enzymes are responsible for the metabolism of amine neurotransmitters, such as
dopamine, and inhibition of MAO-B has proven to be an effective strategy to increase the
dopamine levels in the brain. Clinically, selective MAO-B inhibitors are administered
concurrently with levodopa (a precursor of dopamine) to increase the levels of dopamine
derived from levodopa. This approach prolongs the beneficial effects of levodopa.
Because MAO-A is responsible for the breakdown of noradrenalin, adrenalin, serotonin and
tyramine, non-selective and selective MAO-A inhibitors have therapeutic applications in
other neurological and psychiatric disorders such as depression. MAO-A inhibitors,
particularly irreversible inhibitors, are also notable from a toxicological point of view.
Irreversible MAO-A inhibitors may lead to potentially dangerous effects when combined with
serotonergic drugs and certain foods containing tyramine, such as cheeses and processed
meats. Selective MAO-B inhibitors and reversible MAO-A inhibitors appear to be free of
these interactions. Based on the considerations above, this study aimed to identify clinically used drugs which
also inhibit the MAO enzymes as a secondary pharmacological property. Such drugs may, in
theory, be repurposed as MAO inhibitors for therapeutic use in the treatment of PD and
depression. The identification of potential MAO-A inhibitory properties among clinically used
drugs are of further importance since the irreversible inhibition of MAO-A may lead to
dangerous effects when combined with certain drugs and foods.
To screen clinically used drugs for potential MAO-A and MAO-B inhibitory activities, a
pharmacophore approach was followed. A pharmacophore model is a virtual 3D
representation of the common steric and electrostatic features of the interaction between an
enzyme and a ligand. By identifying hydrogen bond acceptor, hydrogen bond donor and
hydrophobic interactions between a reference ligand and an enzyme, a model is created that
can search databases for other molecules that would have similar interactions with the
enzyme and arguably also act as ligands. This enables the screening of a large amount of
molecules in a short amount of time. To assist in the identification of MAO inhibitors,
pharmacophore models of the MAO enzymes were constructed using the known
crystallographic structures of MAO-A co-crystallized with harmine, and MAO-B cocrystallized
with safinamide. The Discovery Studio® software package (Accelrys) was used
for this purpose.
In this study, virtual libraries of United States Food and Drug Administration (FDA) approved
drugs and the United States Environmental Protection Agency (EPA) maximum daily dose
databases were screened with pharmacophore models of MAO-A and MAO-B. Among the
hits, 26 drugs were selected on the basis of availability and cost, and were subjected to in
vitro bio-assays in order to determine their potencies (IC50 values) as inhibitors of
recombinant human MAO-A and/or MAO-B. Among the drugs tested, 6 compounds
exhibited inhibitory activity towards the MAO enzymes. Of the 6 compounds, pentamidine
(IC50 = 0.61 μM for MAO-A and IC50 = 0.22 μM for MAO-B) and phenformin (IC50 = 41 μM for
MAO-A) were selected for further analysis.
An examination of the recoveries of the enzymatic activities after dilution and dialysis of the
enzyme-inhibitor complexes showed that both pentamidine and phenformin interact
reversibly with the MAO enzymes. A kinetic analysis suggests that pentamidine acts as a
competitive inhibitor with estimated Ki values of 0.41 μM and 0.22 μM for the inhibition of
MAO-A and MAO-B, respectively. An analysis of the available pharmacokinetic data and
typical therapeutic doses of phenformin and pentamidine suggests that the MAO inhibitory
potencies (and reversible mode of action) of phenformin are unlikely to be of
pharmacological relevance in humans. Pentamidine, on the other hand, is expected to interact with both MAO-A and MAO-B at typical therapeutic doses. Because of its MAO-A
inhibitory activity, pentamidine may thus, in theory, lead to a tyramine-associated
hypertensive crisis when combined with tyramine-containing foods. However, pentamidine is
unlikely to inhibit central MAO since it does not appear to penetrate the central nervous
system to a large degree.
In an attempt to gain further insight into the mode of binding to MAO, pentamidine and
phenformin were docked into models of the active sites of MAO-A and/or MAO-B. An
analysis of the interactions between the enzyme models and the ligands were carried out
and the results are discussed in the dissertation.
The results of this study show that the pharmacophore model approach may be useful in
identifying existing drugs with potential MAO inhibitory effects. The search for new
therapeutic MAO inhibitors, that can be used in the treatment of certain neurological
disorders, including PD and depression, may be accelerated by employing a virtual
screening approach. Such an approach may also be more cost effective than the de novo
design of MAO inhibitors. / MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2014
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/10700 |
| Date | January 2013 |
| Creators | Barkhuizen, Melinda |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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