No description available.

No description available.

No description available.

No description available.

This thesis examines the design, synthesis and biological applications of a series of inhibitors which incorporate an azobenzene photoswitch, a peptidyl backbone and a trifluoromethyl ketone warhead. The photoswitch can be isomerised by irradiation with UV or visible light and has been employed to modulate the reactivity of the enzyme. Chapter one gives a brief outline of some of the important areas related to this work. Examples of previously utilised photoswitches as well as some background on serine protease and the uses of fluorine in medicine has been covered. Chapter two outlines the synthesis of the key trifluoromethyl carbinol 2.6 by two different methods. The condensation of a fluorinated aldehyde with a nitroalkane affords an α-nitro trifluoromethyl carbinol which can be reduced to give the desired amine 2.6. Treatment of oxazolones with trifluoroacetic anhydride via a modified Dakin-West reaction gives trifluoromethyl ketones which can be reduced to give trifluoromethyl carbinols. Chapter three investigate the synthesis of substituted stilbenes and phenanthrenes as alternative molecular switches to azobenzenes. Molecular modelling of phenanthrenes suggests they may be suitable mimics of E-azobenzenes. Chapter four outlines the synthesis of a series of mono and disubstituted azobenzenes by direct sulfonation of azobenzene or by condensation of nitroso arenes with aryl amines. The switches incorporate one or two peptidyl residues designed to improve specificity towards the enzyme. Chapter five examines the photoisomerisation of eight potential inhibitors by irradiating with UV or visible light. Irradiation with UV light enriches the sample to give 78-93 % of the Z-isomer. Irradiation with visible light gave photostationary states with 14-21 % Z-isomer. Ambient photostationary states are ca. 22 % Z-isomer. Chapter six looks at the testing of five trifluoromethyl ketones as potential inhibitors ofα-chymotrypsin. The inhibitors vary in substituents, substitution patterns and chain length. The inhibitors were tested at both ambient and Z-enriched photostationary states and were found to exhibit slow binding kinetics. In all cases the Z-enriched inhibitor solution was at least 3-fold more potent than the ambient solution. Chapter seven is an experimental chapter and outlines the synthesis of the compounds prepared in this thesis.

No description available.

The enzyme 3-deoxy D-arabino-heptulosonate 7-phosphate (DAH7P) synthase catalyses the first step of the shikimate pathway. This pathway lies at the heart of bacterial metabolism, and is responsible for the synthesis of a variety of compounds essential to the chemistry of life; from the aromatic amino acids phenylalanine, tyrosine and tryptophan, to a number of aromatic and non-aromatic natural products. This thesis describes the design, synthesis and evaluation of inhibitors of DAH7P synthase. These inhibitors exploit a variety of strategies to interrupt the activity of DAH7P synthase, ranging from simple substrate mimicry to inhibitors that mimic unstable reaction intermediates; inhibitors that exploit metal coordination and entropic effects, and inhibitors that gain improved potency by interacting with multiple sites. In Chapter Two, the synthesis of a mimic for a proposed unstable reaction intermediate is described, and its interaction with DAH7P synthase characterised. The compound was prepared in twelve steps from D-arabinose, and was found to be a slow-tight binding inhibitor of Escherichia coli DAH7P synthase. In Chapter Three, a number of compounds are prepared that were designed to bind to the phosphoenolpyruvate subsite of the DAH7P synthase active site. The binding of these compounds to the enzyme is investigated in order to gain an understanding of the factors involved in DAH7P synthase inhibition. The enantiomeric phospholactates were prepared, and the extent of inhibition of E. coli DAH7P synthase was shown to be dependent on compound chirality. Several other phosphoenolpyruvate-like molecules were prepared, and were also shown to be effective DAH7P synthase inhibitors. In Chapter Four extended compounds are designed that will bind the enzyme by multiple interactions at both substrate binding sites. Four compounds were prepared, and an increase in inhibitory potency was observed. In Chapter Five computational techniques are explored to aid the interpretation of the inhibition of DAH7P synthase by the compounds prepared in these studies. Several approaches for more potent inhibition of this enzyme are outlined and discussed.

This thesis examines the design, synthesis and biological applications of a series of inhibitors which incorporate an azobenzene photoswitch, a peptidyl backbone and a trifluoromethyl ketone warhead. The photoswitch can be isomerised by irradiation with UV or visible light and has been employed to modulate the reactivity of the enzyme. Chapter one gives a brief outline of some of the important areas related to this work. Examples of previously utilised photoswitches as well as some background on serine protease and the uses of fluorine in medicine has been covered. Chapter two outlines the synthesis of the key trifluoromethyl carbinol 2.6 by two different methods. The condensation of a fluorinated aldehyde with a nitroalkane affords an α-nitro trifluoromethyl carbinol which can be reduced to give the desired amine 2.6. Treatment of oxazolones with trifluoroacetic anhydride via a modified Dakin-West reaction gives trifluoromethyl ketones which can be reduced to give trifluoromethyl carbinols. Chapter three investigate the synthesis of substituted stilbenes and phenanthrenes as alternative molecular switches to azobenzenes. Molecular modelling of phenanthrenes suggests they may be suitable mimics of E-azobenzenes. Chapter four outlines the synthesis of a series of mono and disubstituted azobenzenes by direct sulfonation of azobenzene or by condensation of nitroso arenes with aryl amines. The switches incorporate one or two peptidyl residues designed to improve specificity towards the enzyme. Chapter five examines the photoisomerisation of eight potential inhibitors by irradiating with UV or visible light. Irradiation with UV light enriches the sample to give 78-93 % of the Z-isomer. Irradiation with visible light gave photostationary states with 14-21 % Z-isomer. Ambient photostationary states are ca. 22 % Z-isomer. Chapter six looks at the testing of five trifluoromethyl ketones as potential inhibitors ofα-chymotrypsin. The inhibitors vary in substituents, substitution patterns and chain length. The inhibitors were tested at both ambient and Z-enriched photostationary states and were found to exhibit slow binding kinetics. In all cases the Z-enriched inhibitor solution was at least 3-fold more potent than the ambient solution. Chapter seven is an experimental chapter and outlines the synthesis of the compounds prepared in this thesis.

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

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