Parkinson’s disease is the second most common age-related neurodegenerative disease after Alzheimer’s disease. The characteristic pathological feature of Parkinson’s disease is the loss of neurons in the substantia nigra pars compacta (SNpc), which leads to a striatal dopamine deficiency responsible for the major symptoms of Parkinson’s disease. These symptoms include tremor at rest, postural instability, bradykinesia and in the later stages of Parkinson’s disease, even psychosis. Presently, there is still no cure for Parkinson’s disease and all treatments are only symptomatic. Current research is therefore directed towards the prevention of further dopaminergic neurodegeneration, while the ultimate aim is the reversal of neurodegeneration. Monoamine oxidase (MAO) enzymes are responsible for the regulation and metabolism of monoamine neurotransmitters, such as dopamine. There are two MAO isoforms, MAO-A and MAO-B. Since MAO-B has greater activity in the basal ganglia, it is of particular importance in movement disorders, which include Parkinson’s disease. The selective inhibition of MAO-B, increases dopamine available for binding, and thus reduces Parkinson’s disease symptoms. MAO inhibitors also have neuroprotective potential and thus may slow down, halt and even reverse neurodegeneration in Parkinson’s disease. It is still unclear exactly how MAO inhibitors protect neurons, but one theory suggests that MAO inhibition decreases oxidative stress by reducing the formation of hydrogen peroxide, a metabolic by-product of MAO oxidation of monoamines. Normally, hydrogen peroxide is inactivated by glutathione (GSH), however, in Parkinson’s disease, GSH levels are low, resulting in the accumulation of hydrogen peroxide, which then becomes available for the Fenton reaction. In the Fenton reaction, Fe2+ reacts with hydrogen peroxide and generates an active free radical, the hydroxyl radical. This radical depletes cellular anti-oxidants, damage lipids, proteins and DNA. MAO inhibitors reduce the formation of hydrogen peroxide thus decreasing the formation of hydroxyl radicals and oxidative stress. The MAO inhibitory potential of natural and synthetic chalcones have been illustrated. For example, in 1987, Tanaka and co-workers determined that natural chalcones, such as isoliquiritigenin, have MAO inhibitory activity in rat mitochondria. In 2009, Chimenti and co-workers synthesized a series of 1,3-diphenyl-2-propen-1-ones which exhibited human MAO-B (hMAO-B) selective inhibitory activity. On the other hand, Robinson and co-workers (2013), synthesized novel furanochalcones which also had hMAO-B selective inhibitory activity. A reversible, competitive mode of binding was demonstrated by these compounds. Since the effect of heterocyclic substitution, other than furan on the MAO inhibitory properties of the chalcone scaffold remains unexplored, the aim of this study was to synthesize and evaluate further heterocyclic chalcone analogues as inhibitors of hMAO. RESULTS Design and synthesis: Heterocyclic chalcone analogues that incorporated pyrrole, 5- methylthiophene, 5-chlorothiophene and 2-methoxypyridine substitution were synthesized using the Claisen-Schmidt condensation reaction. All compounds were characterized with 1H-NMR, 13CNMR, IR, MS, and melting points were recorded. Purity was determined with HPLC analysis. MAO inhibition studies: The 50% inhibitory concentration (IC50) values and selectivity index (SI) of all compounds were determined using a fluorometric assay and kynuramine as substrate. Eight out of the ten synthesized compounds exhibited IC50 values < 1 μM, and can thus be considered as potent MAO-B inhibitors, while all compounds showed selectivity for the MAO-B isoform. Compound 10i was the most potent MAO-B inhibitor with an IC50 value of 0.067 μM and the highest SI of 240.7. The most potent MAO-A inhibitor, compound 10f, had an IC50 value of 3.805 μM. Some structure-activity relationships were derived, for example; it was concluded that heterocyclic substitution with 5-methyl-thiophene ring resulted in optimal hMAO-B inhibition, while pyrrole substitution was less favourable. Further investigation is however required as this is only a preliminary study. Reversibility studies: To determine the reversibility of binding, the recovery of enzymatic activity after dilution of the enzyme inhibitor complexes were determined for selected compounds. Results indicated that the most potent MAO-A inhibitor, the pyrrole derivative 10f, had a reversible mode of binding to both the hMAO-B and hMAO-A isoforms, since the enzyme activities were completely recovered by dilution of the inhibitor concentration. In contrast, enzyme activity was only partially recovered after dilution of the most potent MAO-B inhibitor 10i, indicating that this methylthiophene derivative possibly exhibited tight binding to the hMAO-B isoform, and the inhibition caused by this compound was not readily reversed by dilution. In order to determine whether the tight binding as exhibited by compound 10i was due to the thiophene or phenyl moieties, reversibility of binding was also determined for the pyrrole derivative 10e. The results showed that 10e had a reversible mode of binding to the hMAO-B isoform, and enzyme activity was completely recovered by dilution of the inhibitor. Based on these results, it was concluded that the tight binding as exhibited by compound 10i was due to the presence of the thiophene moiety. To confirm that compound 10i exhibited tight, and not irreversible binding, reversibility of binding was also determined by dialysis of enzyme-inhibitor mixtures. For this purpose hMAO-B and 10i, at a concentration of 4 × IC50, were preincubated for a period of 15 min and subsequently dialyzed for 24 h. The results of this study showed that 10i had a reversible mode of binding for MAO-B, since enzyme activity was recovered to a level of 83% after dialysis. Mode of inhibition: To determine the mode of inhibition of compound 10f, Lineweaver-Burk plots were constructed for the inhibition of hMAO-A and hMAO-B. The lines of the Lineweaver-Burk plots intersected at a single point at the y-axis, indicating that 10f had a competitive mode of binding to both hMAO-B and hMAO-A isoforms. MTT viability assay: To determine the toxicity of the chalcones for cultured cells, selected compounds were evaluated with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) viability assay. The cytotoxicity of the test compounds were evaluated at concentrations of 1 and 10 μM, in HeLa cells. The results indicated that compound 10i was non-toxic at 1 and 10 μM, with 100% and 96% cell viability remaining after 24 h exposure of the compound to the cultured cells. Compound 10f, however, exhibited significant toxicity at 10 μM, with only 5% viable cells remaining. In contrast, compound 10e, with the same pyrrole moiety as 10f, was non-toxic at 1 μM and 10 μM, with 99% and 98%, cell viability remaining. It was concluded that the pyrrole moiety of 10f was not responsible for its higher degree of cytotoxicity, which suggests that the CF3 substituent may play a role in the higher degree of cytotoxicity observed for 10f. Further investigation is required to determine the mode of cytotoxicity, when cultured cells are exposed to 10f. Docking Studies: To complete this study and rationalise the results of the MAO inhibition studies, molecular modelling was carried out and all compounds were docked into the crystal structure of hMAO-B, by using the CDOCKER module of Discovery Studio. Some insights were obtained regarding the binding of compound 10i. This compound bound to MAO-B with the phenyl ring facing the FAD cofactor. This orientation allowed for the formation of pi-pi interaction with Tyr 398 as well as a pi-sigma interaction between the thiophene ring and Ile 199 (which is part of the gating switch of MAO-B). It is speculated that the tight binding component of hMAO-B inhibition by 10i may, at least in part, be attributed to the interaction of this compound with the gating switch amino acid, Ile 199. The docking results also showed that most compounds interacted with Tyr 326 or Tyr 398, while interactions with Cys 172, Gln 206, Ile 199 and Tyr 435 also occurred. In conclusion, novel heterocyclic chalcone analogues with promising MAO-B inhibitory activities were successfully synthesized and evaluated. / MSc (Pharmaceutical Chemistry) North-West University, Potchefstroom Campus, 2014

Parkinson’s disease (PD) is an age related neurodegenerative disorder that presents with both motor and non-motor symptoms. The most common pathological characteristic of PD is the loss of the pigmented dopaminergic neurons of the substantia nigra pars compacta (SNpc), with the appearance of intracellular inclusions known as Lewy bodies in the affected neurons. The loss of the SNpc neurons results in a deficiency of dopamine in the nigrostriatal pathway of the brain, and it is this deficiency that is responsible for the motor symptoms of PD. Monoamine oxidase B (MAO-B) is predominantly found in the striatum and is responsible for the oxidative metabolism of dopamine. The first-line treatment of PD is dopamine replacement therapy with levodopa, the metabolic precursor of dopamine. Rapid metabolism of levodopa at central and peripheral level, however, hampers its therapeutic potential. MAO-B inhibition enhances striatal dopamine activity by means of inhibiting dopamine metabolism, and MAO-B inhibitors are thus used in the treatment of PD, particularly in combination with levodopa. The aim of this study was to design new potent, reversible MAO inhibitors with selectivity towards MAO-B for the symptomatic treatment of PD. Recent studies have shown that C5-substituted phthalide derivatives are highly potent inhibitors of human MAO-B. Phthalide derivatives were also found to be potent inhibitors of human MAO-A. The structural similarity between phthalide and 3-coumaranone suggests that 3-coumaranone may be a useful scaffold for the design of reversible MAO-B inhibitors. In the present study, 3-coumaranone derivatives were thus synthesised and evaluated as potential MAO-A and MAO-B inhibitors. By reacting 6-hydroxy-3-coumaranone with the appropriate alkylbromide in N,N-dimethylformamide in the presence of potassium carbonate, a series of twenty 3-coumaranone derivatives were synthesised. The structures of the compounds were verified with NMR spectroscopy and mass spectrometry. The purities of the compounds were determined by HPLC analyses. To determine the inhibition potencies, the recombinant human MAO-A and MAO-B enzymes were used, and the inhibition potencies were expressed as IC50 values. The results indicated that the 3-coumaranone derivatives are highly potent MAO-B inhibitors. For example, 9 of the 3-coumaranone derivatives inhibited MAO-B with IC50 values < 0.05 μM, with the most potent inhibitor exhibiting an IC50 value of 0.004 μM. Although the 3-coumaranone derivatives are selective MAO-B inhibitors, some compounds were also potent MAO-A inhibitors with the most potent inhibitor exhibiting an IC50 value of 0.586 μM. The reversibility of MAO-B inhibition by a representative inhibitor was examined by measuring the degree to which the enzyme activity recovers after dialysis of the enzyme-inhibitor complex. Since MAO-B activity was almost completely recovered after dialysis, it may be concluded that the 3-coumaranone derivatives bind reversibly to MAO-B. Lineweaver-Burk plots were constructed to show that the representative 3-coumaranone derivative is a competitive inhibitor of MAO-B. To conclude, the 3-coumaranone derivatives are potent, selective, reversible and competitive inhibitors of MAO-B. These compounds may find application in the treatment of neurodegenerative disorders such as PD. Potent MAO-A inhibitors were also discovered, which suggests that 3-coumaranone derivatives may serve as leads for the design of drugs for the treatment of depression. In addition, 3-coumaranone derivatives which inhibited both MAO-A and MAO-B, may have potential application in the therapy of both PD and depressive illness. / MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2015

Parkinson’s disease is the second most common age-related neurodegenerative disease after Alzheimer’s disease. The characteristic pathological feature of Parkinson’s disease is the loss of neurons in the substantia nigra pars compacta (SNpc), which leads to a striatal dopamine deficiency responsible for the major symptoms of Parkinson’s disease. These symptoms include tremor at rest, postural instability, bradykinesia and in the later stages of Parkinson’s disease, even psychosis. Presently, there is still no cure for Parkinson’s disease and all treatments are only symptomatic. Current research is therefore directed towards the prevention of further dopaminergic neurodegeneration, while the ultimate aim is the reversal of neurodegeneration. Monoamine oxidase (MAO) enzymes are responsible for the regulation and metabolism of monoamine neurotransmitters, such as dopamine. There are two MAO isoforms, MAO-A and MAO-B. Since MAO-B has greater activity in the basal ganglia, it is of particular importance in movement disorders, which include Parkinson’s disease. The selective inhibition of MAO-B, increases dopamine available for binding, and thus reduces Parkinson’s disease symptoms. MAO inhibitors also have neuroprotective potential and thus may slow down, halt and even reverse neurodegeneration in Parkinson’s disease. It is still unclear exactly how MAO inhibitors protect neurons, but one theory suggests that MAO inhibition decreases oxidative stress by reducing the formation of hydrogen peroxide, a metabolic by-product of MAO oxidation of monoamines. Normally, hydrogen peroxide is inactivated by glutathione (GSH), however, in Parkinson’s disease, GSH levels are low, resulting in the accumulation of hydrogen peroxide, which then becomes available for the Fenton reaction. In the Fenton reaction, Fe2+ reacts with hydrogen peroxide and generates an active free radical, the hydroxyl radical. This radical depletes cellular anti-oxidants, damage lipids, proteins and DNA. MAO inhibitors reduce the formation of hydrogen peroxide thus decreasing the formation of hydroxyl radicals and oxidative stress. The MAO inhibitory potential of natural and synthetic chalcones have been illustrated. For example, in 1987, Tanaka and co-workers determined that natural chalcones, such as isoliquiritigenin, have MAO inhibitory activity in rat mitochondria. In 2009, Chimenti and co-workers synthesized a series of 1,3-diphenyl-2-propen-1-ones which exhibited human MAO-B (hMAO-B) selective inhibitory activity. On the other hand, Robinson and co-workers (2013), synthesized novel furanochalcones which also had hMAO-B selective inhibitory activity. A reversible, competitive mode of binding was demonstrated by these compounds. Since the effect of heterocyclic substitution, other than furan on the MAO inhibitory properties of the chalcone scaffold remains unexplored, the aim of this study was to synthesize and evaluate further heterocyclic chalcone analogues as inhibitors of hMAO. RESULTS Design and synthesis: Heterocyclic chalcone analogues that incorporated pyrrole, 5- methylthiophene, 5-chlorothiophene and 2-methoxypyridine substitution were synthesized using the Claisen-Schmidt condensation reaction. All compounds were characterized with 1H-NMR, 13CNMR, IR, MS, and melting points were recorded. Purity was determined with HPLC analysis. MAO inhibition studies: The 50% inhibitory concentration (IC50) values and selectivity index (SI) of all compounds were determined using a fluorometric assay and kynuramine as substrate. Eight out of the ten synthesized compounds exhibited IC50 values < 1 μM, and can thus be considered as potent MAO-B inhibitors, while all compounds showed selectivity for the MAO-B isoform. Compound 10i was the most potent MAO-B inhibitor with an IC50 value of 0.067 μM and the highest SI of 240.7. The most potent MAO-A inhibitor, compound 10f, had an IC50 value of 3.805 μM. Some structure-activity relationships were derived, for example; it was concluded that heterocyclic substitution with 5-methyl-thiophene ring resulted in optimal hMAO-B inhibition, while pyrrole substitution was less favourable. Further investigation is however required as this is only a preliminary study. Reversibility studies: To determine the reversibility of binding, the recovery of enzymatic activity after dilution of the enzyme inhibitor complexes were determined for selected compounds. Results indicated that the most potent MAO-A inhibitor, the pyrrole derivative 10f, had a reversible mode of binding to both the hMAO-B and hMAO-A isoforms, since the enzyme activities were completely recovered by dilution of the inhibitor concentration. In contrast, enzyme activity was only partially recovered after dilution of the most potent MAO-B inhibitor 10i, indicating that this methylthiophene derivative possibly exhibited tight binding to the hMAO-B isoform, and the inhibition caused by this compound was not readily reversed by dilution. In order to determine whether the tight binding as exhibited by compound 10i was due to the thiophene or phenyl moieties, reversibility of binding was also determined for the pyrrole derivative 10e. The results showed that 10e had a reversible mode of binding to the hMAO-B isoform, and enzyme activity was completely recovered by dilution of the inhibitor. Based on these results, it was concluded that the tight binding as exhibited by compound 10i was due to the presence of the thiophene moiety. To confirm that compound 10i exhibited tight, and not irreversible binding, reversibility of binding was also determined by dialysis of enzyme-inhibitor mixtures. For this purpose hMAO-B and 10i, at a concentration of 4 × IC50, were preincubated for a period of 15 min and subsequently dialyzed for 24 h. The results of this study showed that 10i had a reversible mode of binding for MAO-B, since enzyme activity was recovered to a level of 83% after dialysis. Mode of inhibition: To determine the mode of inhibition of compound 10f, Lineweaver-Burk plots were constructed for the inhibition of hMAO-A and hMAO-B. The lines of the Lineweaver-Burk plots intersected at a single point at the y-axis, indicating that 10f had a competitive mode of binding to both hMAO-B and hMAO-A isoforms. MTT viability assay: To determine the toxicity of the chalcones for cultured cells, selected compounds were evaluated with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) viability assay. The cytotoxicity of the test compounds were evaluated at concentrations of 1 and 10 μM, in HeLa cells. The results indicated that compound 10i was non-toxic at 1 and 10 μM, with 100% and 96% cell viability remaining after 24 h exposure of the compound to the cultured cells. Compound 10f, however, exhibited significant toxicity at 10 μM, with only 5% viable cells remaining. In contrast, compound 10e, with the same pyrrole moiety as 10f, was non-toxic at 1 μM and 10 μM, with 99% and 98%, cell viability remaining. It was concluded that the pyrrole moiety of 10f was not responsible for its higher degree of cytotoxicity, which suggests that the CF3 substituent may play a role in the higher degree of cytotoxicity observed for 10f. Further investigation is required to determine the mode of cytotoxicity, when cultured cells are exposed to 10f. Docking Studies: To complete this study and rationalise the results of the MAO inhibition studies, molecular modelling was carried out and all compounds were docked into the crystal structure of hMAO-B, by using the CDOCKER module of Discovery Studio. Some insights were obtained regarding the binding of compound 10i. This compound bound to MAO-B with the phenyl ring facing the FAD cofactor. This orientation allowed for the formation of pi-pi interaction with Tyr 398 as well as a pi-sigma interaction between the thiophene ring and Ile 199 (which is part of the gating switch of MAO-B). It is speculated that the tight binding component of hMAO-B inhibition by 10i may, at least in part, be attributed to the interaction of this compound with the gating switch amino acid, Ile 199. The docking results also showed that most compounds interacted with Tyr 326 or Tyr 398, while interactions with Cys 172, Gln 206, Ile 199 and Tyr 435 also occurred. In conclusion, novel heterocyclic chalcone analogues with promising MAO-B inhibitory activities were successfully synthesized and evaluated. / MSc (Pharmaceutical Chemistry) North-West University, Potchefstroom Campus, 2014

Parkinson’s disease (PD) is an age related neurodegenerative disorder that presents with both motor and non-motor symptoms. The most common pathological characteristic of PD is the loss of the pigmented dopaminergic neurons of the substantia nigra pars compacta (SNpc), with the appearance of intracellular inclusions known as Lewy bodies in the affected neurons. The loss of the SNpc neurons results in a deficiency of dopamine in the nigrostriatal pathway of the brain, and it is this deficiency that is responsible for the motor symptoms of PD. Monoamine oxidase B (MAO-B) is predominantly found in the striatum and is responsible for the oxidative metabolism of dopamine. The first-line treatment of PD is dopamine replacement therapy with levodopa, the metabolic precursor of dopamine. Rapid metabolism of levodopa at central and peripheral level, however, hampers its therapeutic potential. MAO-B inhibition enhances striatal dopamine activity by means of inhibiting dopamine metabolism, and MAO-B inhibitors are thus used in the treatment of PD, particularly in combination with levodopa. The aim of this study was to design new potent, reversible MAO inhibitors with selectivity towards MAO-B for the symptomatic treatment of PD. Recent studies have shown that C5-substituted phthalide derivatives are highly potent inhibitors of human MAO-B. Phthalide derivatives were also found to be potent inhibitors of human MAO-A. The structural similarity between phthalide and 3-coumaranone suggests that 3-coumaranone may be a useful scaffold for the design of reversible MAO-B inhibitors. In the present study, 3-coumaranone derivatives were thus synthesised and evaluated as potential MAO-A and MAO-B inhibitors. By reacting 6-hydroxy-3-coumaranone with the appropriate alkylbromide in N,N-dimethylformamide in the presence of potassium carbonate, a series of twenty 3-coumaranone derivatives were synthesised. The structures of the compounds were verified with NMR spectroscopy and mass spectrometry. The purities of the compounds were determined by HPLC analyses. To determine the inhibition potencies, the recombinant human MAO-A and MAO-B enzymes were used, and the inhibition potencies were expressed as IC50 values. The results indicated that the 3-coumaranone derivatives are highly potent MAO-B inhibitors. For example, 9 of the 3-coumaranone derivatives inhibited MAO-B with IC50 values < 0.05 μM, with the most potent inhibitor exhibiting an IC50 value of 0.004 μM. Although the 3-coumaranone derivatives are selective MAO-B inhibitors, some compounds were also potent MAO-A inhibitors with the most potent inhibitor exhibiting an IC50 value of 0.586 μM. The reversibility of MAO-B inhibition by a representative inhibitor was examined by measuring the degree to which the enzyme activity recovers after dialysis of the enzyme-inhibitor complex. Since MAO-B activity was almost completely recovered after dialysis, it may be concluded that the 3-coumaranone derivatives bind reversibly to MAO-B. Lineweaver-Burk plots were constructed to show that the representative 3-coumaranone derivative is a competitive inhibitor of MAO-B. To conclude, the 3-coumaranone derivatives are potent, selective, reversible and competitive inhibitors of MAO-B. These compounds may find application in the treatment of neurodegenerative disorders such as PD. Potent MAO-A inhibitors were also discovered, which suggests that 3-coumaranone derivatives may serve as leads for the design of drugs for the treatment of depression. In addition, 3-coumaranone derivatives which inhibited both MAO-A and MAO-B, may have potential application in the therapy of both PD and depressive illness. / MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2015

Aims of this project - The aim of this study was to design and synthesise novel 2-aminopyrimidine derivatives as potential adenosine A1 and A2A receptor antagonists. Background and rationale - Parkinson’s disease is the second most common neurodegenerative disorder (after Alzheimer’s disease) and is characterised by the selective death of the dopaminergic neurons of the nigro-striatal pathway. Distinctive motor symptoms include bradykinesia, muscle rigidity and tremor, while non-motor symptoms, of which cognitive dysfunction is an example, also frequently occur. Current therapy provides symptomatic relief mainly by augmentation of dopaminergic signalling (levodopa, dopamine agonists, MAO and COMT enzyme inhibitors), but disease progression is not adequately addressed. New therapies that can prevent further neurodegeneration in addition to providing symptomatic relief are therefore urgently required. Adenosine has an important function as neuromodulator in the central nervous system. The adenosine A2A receptor in particular plays an essential role in the regulation of movement. This, coupled to the fact that it is uniquely distributed in the basal ganglia, contributes to its attractiveness as non-dopaminergic target in the treatment of movement disorders, such as Parkinson’s disease. The efficacy of adenosine receptor antagonists has been illustrated in animal models of Parkinson’s disease and several adenosine receptor antagonists have also reached clinical trials. The neuroprotective properties of adenosine A2A receptor antagonists are further attributed to their ability to modulate neuro-inflammation and decrease the release of the excitatory neurotransmitter glutamate, which is implicated in neurotoxicity. While adenosine A1 receptor antagonism has a synergistic effect on the motor effects of adenosine A2A receptor antagonism, it has the additional benefit of improving cognitive dysfunction, a cardinal non-motor symptom of Parkinson’s disease. Dual antagonism of adenosine A1 and A2A receptors therefore offers the potential of providing symptomatic relief as well as the neuroprotection so desperately needed in the clinical environment. Amino substituted heterocyclic scaffolds, such as those containing the 2-aminopyrimidine motif, have been shown to exhibit good efficacy as dual adenosine receptor antagonists. Since the structure activity relationships of 2-aminopyrimidines have not been comprehensively explored, it is in this regard that this study aimed to make a contribution. Results - Fourteen 2-aminopyrimidines were synthesised successfully over three steps, (although in low yields) and characterised by nuclear magnetic resonance and infrared spectroscopy, mass spectrometry, by determination of melting points and high performance liquid chromatography. Structure modifications explored included variation of the aromatic substituent on position 4, as well as variations in the substituents of the phenyl ring, present on position 6 of the pyrimidine ring. Radioligand binding assays were performed to determine the affinities of the synthesised compounds for the adenosine A1 and A2A receptor subtypes. Several high dual affinity derivatives were identified during this study; the compound with the highest affinity was 4-(5- methylthiophen-2-yl)-6-[3-(piperidine-1-carbonyl)phenyl]pyrimidin-2-amine (39f) with Ki values of 0.5 nM and 2.3 nM for the adenosine A2A and adenosine A1 receptors, respectively. A few general structure activity relationships were derived, which included: The effect of the aromatic substituent (position 4) on A2A affinity could be summarised (in order of declining affinity) as follows: 5-methylthiophene > phenyl > furan > pyridine > p-fluorophenyl > benzofuran. On the other hand, the effect of this substituent on A1 receptor affinity could be summarised (in order of declining affinity) as follows: phenyl > 5-methylthiophene > pfluorophenyl > benzofuran > pyridine. The affinities as exhibited by the methylthiophene derivatives 39f, 39h – 39j, further showed that while piperidine substitution (39f) resulted in optimal A2A and A1 affinity, pyrrolidine substitution (39j) was less favourable. Substitution at the 4ʹ position of the phenyl ring, as well as thiazole substitution, generally resulted in poor adenosine A1 and A2A receptor affinity. However, 4-[2-amino-6-(5-methylfuran-2-yl)pyrimidin- 4-yl]-N-(1,3-benzothiazol-2-yl)benzamide (39l) surprisingly demonstrated good affinity and selectivity for the adenosine A1 receptor. The results obtained during radioligand binding assays were rationalised by QSAR and molecular modelling (Discovery Studio 3.1, Accelrys) studies. The inverse relationship seen between log Ki (as indicator of affinity) and polar surface area, illustrated the importance of this physico-chemical property in the design of 2-aminopyrimidine A2A antagonists. The results from the docking study further showed that the orientation adopted by derivatives in the binding cavity (and particular hydrogen bonding to Asn 253 and Glu 169) is of importance. Results from the MTT cell viability assay indicated that none of the high affinity derivatives had a significant effect on cell viability at 1 μM, a concentration much higher than their Ki values. However, incorporation of the furan, benzofuran and p-fluorophenyl groups as aromatic substituent and a pyrrolidine as amine substituent, presented liabilities. Lastly, the haloperidol induced catalepsy assay (in rats) was used to give a preliminary indication of adenosine receptor antagonism or agonism. Compound 39f failed to reverse catalepsy under standard conditions, but showed some reversal after an increased time period. Indications therefore exist that 39f is an adenosine receptor antagonist that suffers from bioavailability issues. Compound (39c), 4-phenyl-6-[3-(piperidine-1- carbonyl)phenyl]pyrimidin-2-amine which also demonstrated promising affinity in the radioligand binding assays however showed a statistically significant reduction in catalepsy, indicating adenosine A2A receptor antagonism, and in vivo efficacy. Highly potent, dual affinity aminopyrimidine derivatives with acceptable toxicity profiles were identified in this study, with compound 39c demonstrating in vivo activity. The aim of designing and synthesising a promising dual adenosine A1/A2A receptor antagonist is therefore realised, with compound 39c as the most favourable example. / MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2014

Parkinson’s disease (PD) is a neurodegenerative disorder, which is progressive in nature and usually associated with the elderly. It is the second most common age-related neurodegenerative disorder after Alzheimer’s disease (AD). PD occurs when there is a dramatic loss of dopamine (DA) in the striatum, a substructure of the basal ganglia, of the brain due to the degeneration of the nigrostriatal pathway that contains the dopaminergic neurons. Motor symptoms of PD include bradykinesia, muscular rigidity and resting tremors. Non-motor symptoms include speech and sleep problems, hallucinations and depression. Diverse treatment options are available to treat the symptoms of PD, including levodopa (L-Dopa), DA agonists and monoamine oxidase B (MAO-B) inhibitors. The MAOs are flavoproteins that are bound to the outer membrane of the mitochondria and catalyze the oxidative deamination of neurotransmitters such as serotonin (5-HT), noradrenaline (NA) and DA. Two isoforms occur, namely MAO-A and –B, which share a 70% sequence identity. MAO-A catalyzes the oxidation of 5-HT and MAO-B has a substrate specificity towards benzylamine and 2-phenylethylamine. DA, NA, adrenaline and tryptamine are oxidized by both forms. MAO-A plays an important role in depression while MAO-B plays an important role in PD. The two isoforms are not evenly distributed in the brain. Of particular relevance to PD is the observation that, in the basal ganglia, MAO-B is the predominant form and the oxidation of DA in this region is largely due to MAO-B activity. Also, with an increase in age, there is an up to fourfold increase in MAO-B activity in the brain. In the aged parkinsonian brain, MAO-B is therefore a major DA metabolizing enzyme and MAO-B inhibitors have an important role in the therapy of PD. MAO-B inhibitors may potentially reduce the metabolic destruction of DA and thereby provide relief from the symptoms of PD. MAO-B inhibitors may also exert a neuroprotective effect in PD. In the catalytic cycle of MAO-B, one mole each of an aldehyde, hydrogen peroxide and ammonia are formed for each mole of primary amine substrate oxidized. Ferrous iron, which is abundant in the basal ganglia, may react with the hydrogen peroxide to form hydroxyl radicals in the Fenton reaction. The hydroxyl radical damages virtually all types of biomolecules including proteins, DNA, lipids, carbohydrates and amino acids. The aldehyde, in turn, may react with amino groups of proteins, and thus lead to cell injury. Inhibitors of MAO-B may reduce the MAO-catalyzed formation of hydrogen peroxide and aldehydes in the basal ganglia, and thus act as neuroprotective agents. MAO-B inhibitors that are currently being used in the treatment of PD are selegiline and rasagiline. Both are irreversible inhibitors of MAO-B. While irreversible inhibitors of MAO have been used extensively as drugs, irreversible inhibition has a number of disadvantages. These include the loss of selectivity as a result of repeated drug administration and a slow and variable rate of enzyme recovery following termination of drug treatment. The turnover rate for the biosynthesis of MAO-B in the human brain may require as much as 40 days while with reversible inhibition, enzyme activity is recovered when the inhibitor is eliminated from the tissues. For these reasons the discovery of novel MAO-B inhibitors, which interact reversibly with the enzymes are of value in the therapy of PD. The goal of this study was to design novel and reversible inhibitors of MAO-B, which may find application in the therapy of PD. In the current study, caffeine was used as scaffold for the design of new MAO inhibitors. Caffeine is reported to be a weak inhibitor of MAO-B, with an IC50 value of 5084 μM. Substitution at C-8 of the caffeine moiety, however, yields compounds with potent MAO-B inhibitory properties. Of particular importance to this study is a recent report that a series of 8-sulfanylcaffeine analogues acts as selective inhibitors of human MAO-B. Among the compounds examined, 8-[(phenylethyl)sulfanyl]caffeine was found to be a particularly potent MAO-B inhibitor with an IC50 value of 0.223 μM. In an attempt to further enhance the MAO-B inhibition potency of 8-[(phenylethyl)sulfanyl]caffeine, and possibly to discover highly potent MAO-B inhibitors, a series of five 8-[(phenylethyl)sulfanyl]caffeine analogues was synthesized and evaluated as inhibitors of human MAO-A and –B. For the purpose of this study 8- [(phenylethyl)sulfanyl]caffeine homologues containing C-3 alkyl (CF3, CH3 and OCH3) and halogen (Cl and Br) substituents on the phenyl ring were considered. Furthermore, a series of two 8-sulfinylcaffeine analogues and one 8-sulfonylcaffeine were synthesized and their MAO inhibitory potencies were measured. The purpose with these compounds was to compare the MAO inhibitory properties of the 8-sulfinylcaffeine analogues and 8-sulfonylcaffeine with those of the 8-sulfanylcaffeine analogues. This study also investigates the MAO inhibition properties of three selected 8-[(phenylpropyl)sulfanyl]caffeine and two 8-(benzylsulfanyl)caffeine analogues. Chemistry: The target 8-sulfanylcaffeine analogues were synthesized according to the literature procedure. 8-Chlorocaffeine was reacted with an appropriate mercaptan in the presence of NaOH, to yield the target 8-sulfanylcaffeine analogues in yields of 6.4–50.7%. 8-Chlorocaffeine, in turn, was conveniently synthesized in high yield by reacting chlorine with caffeine in chloroform. In certain instances, the mercaptan starting materials were not commercially available and were thus synthesized according to the literature procedure by reacting an appropriate alkylbromide with thiourea. The resulting thiouronium salt was hydrolyzed in the presence of NaOH to yield the target mercaptan. The 8-sulfinylcaffeine analogues and 8- sulfonylcaffeine were synthesized by reacting the 8-sulfanylcaffeines with H2O2 in the presence of glacial acetic acid and acetic anhydride. The structures and the purities of the inhibitors were verified by NMR, MS and HPLC analyses. MAO inhibition studies: The MAO inhibitory properties of the test compounds were examined using the recombinant human enzymes. The mixed MAO-A/B substrate, kynuramine, was employed as substrate for both enzymes and the inhibition potencies were expressed as the IC50 values. The 8-[(phenylethyl)sulfanyl]caffeine analogues were found to be highly potent inhibitors of MAO-B. The IC50 values recorded for these homologues ranged from 0.017–0.125 μM, making them twofold to 13-fold more potent MAO-B inhibitors than the lead compound, 8- [(phenylethyl)sulfanyl]caffeine (IC50 = 0.223 μM). For comparison, the reversible MAO-B selective inhibitor, lazabemide, exhibits an IC50 value of 0.091 μM under the same conditions (unpublished data from our laboratory). Interestingly, both alkyl (CF3, CH3 and OCH3) and halogen (Cl and Br) substitution lead to highly potent MAO-B inhibition. It may therefore be concluded that substitution on C-3 is a general strategy to enhance the MAO-B inhibition potency of 8-[(phenylethyl)sulfanyl]caffeine. The results of the MAO inhibitory studies with the 8- [(phenylpropyl)sulfanyl]caffeine analogues showed that these compounds are also inhibitors of MAO-B with IC50 values of 0.061–0.500 μM. Those homologues substituted with chlorine on the para and meta positions of the phenyl ring were found to be exceptionally potent inhibitors with IC50 values of 0.061 μM and 0.062 μM, respectively. For the series of 8- (benzylsulfanyl)caffeines, meta substitution with chlorine (IC50 = 0.227 μM) and bromine (IC50 = 0.199 μM) was also found to enhance the MAO-B inhibition potency of 8- (benzylsulfanyl)caffeine (IC50 = 1.86 μM). The results document that the 8-sulfinylcaffeines are also inhibitors of MAO-B with IC50 values of 11.8–131 μM. The 8-sulfonylcaffeine was also found to be a MAO-B inhibitor. Compared to the 8-sulfanylcaffeines, these homologues are, however, weaker inhibitors. It may, therefore, be concluded that 8-sulfinylcaffeines and 8-sulfonylcaffeines are comparatively weak MAO-B inhibitors and less suited for the design of high potency MAO-B inhibitors. The results also document that the 8-[(phenylethyl)sulfanyl]caffeines are relatively weak MAO-A inhibitors with IC50 values of 5.66–141 μM, with one homologue exhibiting no inhibition under the experimental conditions. As evident from the selectivity indices (SI values), the 8- [(phenylethyl)sulfanyl]caffeines were all selective inhibitors of the MAO-B isoform. Two compounds exhibited SI values in excess of 1000. Since these compounds are also highly potent MAO-B inhibitors, they represent suitable leads for the design of potent and selective MAO-B inhibitors. The 8-sulfinylcaffeines and 8-sulfonylcaffeine were found to be weak MAO-A inhibitors with IC50 values of 166–250 μM. The SI values demonstrate that these compounds are MAO-B selective inhibitors, although to a lesser degree than the 8- [(phenylethyl)sulfanyl]caffeines. The 8-[(phenylpropyl)sulfanyl]caffeines are also MAO-A inhibitors with IC50 values of 0.708–6.48 μM. It is noteworthy that these homologues are the most potent MAO-A inhibitors among the compounds evaluated in this study. In fact, one of the 8-[(phenylpropyl)sulfanyl]caffeines, 8-{[3-(4-chlorophenyl)propyl]sulfanyl}caffeine (IC50 = 0.708 μM), is the only compound with an IC50 value for the inhibition of MAO-A in the submicromolar range. The 8-[(phenylpropyl)sulfanyl]caffeines display, in general, lower degrees of selectivity for MAO-B than the corresponding 8-[(phenylethyl)sulfanyl]caffeines. Reversibility studies: The reversibility of the interaction of a representative inhibitor, 8-{[2-(3- (trifluoromethyl)phenyl)ethyl]sulfanyl}caffeine, with MAO-B was investigated by evaluating the recovery of the enzymatic activity after dilution of the enzyme-inhibitor complex. For this purpose, MAO-B was preincubated with the test compound at concentrations of 10 × IC50 and 100 × IC50 for 30 min. The reactions were subsequently diluted 100-fold to 0.1 × IC50 and 1 × IC50, respectively. The results show that, after dilution to 0.1 × IC50 and 1 × IC50, the MAO-B catalytic activities are recovered to 35% and 22%, respectively, of the control value. For reversible enzyme inhibition, the enzyme activities are expected to recover to levels of approximately 90% and 50%, respectively, after 100-fold dilution of the preincubations containing inhibitor concentrations of 10 × IC50 and 100 × IC50. After preincubation of MAO-B with the irreversible inhibitor (R)-deprenyl (at 10 × IC50), and dilution of the resulting complex to 0.1 × IC50, MAO-B activity is not recovered (3.0% of control). These data indicate that the test compound does indeed react reversibly with MAO-B but because enzyme activities are not recovered to the expected 90% and 50% respectively, it may suggest that the test compound possess a quasi-reversible or tight-binding component. Hansch-type structure activity relationship studies: A limited Hansch-type QSAR study was performed for the inhibition of MAO by the 8-[(phenylethyl)sulfanyl]caffeines. For this purpose, five parameters were used to describe the physicochemical properties of the C-3 substituents on the phenyl rings of the inhibitors. The Van der Waals volume (Vw) and Taft steric parameter (Es) served as descriptors of the bulkiness of the substituents, while the lipophilicities were described by the Hansch constant (π). The electronic properties were described by the classical Hammett constant (σm) and the Swain-Lupton constant (F). A one-parameter fit with the Taft steric parameter versus the inhibition potency (logIC50) yielded the best correlation with a correlation coefficient (R2) of 0.912 and a statistical F value of 41.27 (Fmax = 35). The positive sign of the Es (+0.47) parameter coefficient indicated that the inhibition potencies of the 8- [(phenylethyl)sulfanyl]caffeines towards MAO-B may be enhanced by substitution with sterically large groups at C-3 of the phenyl rings of the inhibitors. / Thesis (MSc (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2013

Aims of this project - The aim of this study was to design and synthesise novel 2-aminopyrimidine derivatives as potential adenosine A1 and A2A receptor antagonists. Background and rationale - Parkinson’s disease is the second most common neurodegenerative disorder (after Alzheimer’s disease) and is characterised by the selective death of the dopaminergic neurons of the nigro-striatal pathway. Distinctive motor symptoms include bradykinesia, muscle rigidity and tremor, while non-motor symptoms, of which cognitive dysfunction is an example, also frequently occur. Current therapy provides symptomatic relief mainly by augmentation of dopaminergic signalling (levodopa, dopamine agonists, MAO and COMT enzyme inhibitors), but disease progression is not adequately addressed. New therapies that can prevent further neurodegeneration in addition to providing symptomatic relief are therefore urgently required. Adenosine has an important function as neuromodulator in the central nervous system. The adenosine A2A receptor in particular plays an essential role in the regulation of movement. This, coupled to the fact that it is uniquely distributed in the basal ganglia, contributes to its attractiveness as non-dopaminergic target in the treatment of movement disorders, such as Parkinson’s disease. The efficacy of adenosine receptor antagonists has been illustrated in animal models of Parkinson’s disease and several adenosine receptor antagonists have also reached clinical trials. The neuroprotective properties of adenosine A2A receptor antagonists are further attributed to their ability to modulate neuro-inflammation and decrease the release of the excitatory neurotransmitter glutamate, which is implicated in neurotoxicity. While adenosine A1 receptor antagonism has a synergistic effect on the motor effects of adenosine A2A receptor antagonism, it has the additional benefit of improving cognitive dysfunction, a cardinal non-motor symptom of Parkinson’s disease. Dual antagonism of adenosine A1 and A2A receptors therefore offers the potential of providing symptomatic relief as well as the neuroprotection so desperately needed in the clinical environment. Amino substituted heterocyclic scaffolds, such as those containing the 2-aminopyrimidine motif, have been shown to exhibit good efficacy as dual adenosine receptor antagonists. Since the structure activity relationships of 2-aminopyrimidines have not been comprehensively explored, it is in this regard that this study aimed to make a contribution. Results - Fourteen 2-aminopyrimidines were synthesised successfully over three steps, (although in low yields) and characterised by nuclear magnetic resonance and infrared spectroscopy, mass spectrometry, by determination of melting points and high performance liquid chromatography. Structure modifications explored included variation of the aromatic substituent on position 4, as well as variations in the substituents of the phenyl ring, present on position 6 of the pyrimidine ring. Radioligand binding assays were performed to determine the affinities of the synthesised compounds for the adenosine A1 and A2A receptor subtypes. Several high dual affinity derivatives were identified during this study; the compound with the highest affinity was 4-(5- methylthiophen-2-yl)-6-[3-(piperidine-1-carbonyl)phenyl]pyrimidin-2-amine (39f) with Ki values of 0.5 nM and 2.3 nM for the adenosine A2A and adenosine A1 receptors, respectively. A few general structure activity relationships were derived, which included: The effect of the aromatic substituent (position 4) on A2A affinity could be summarised (in order of declining affinity) as follows: 5-methylthiophene > phenyl > furan > pyridine > p-fluorophenyl > benzofuran. On the other hand, the effect of this substituent on A1 receptor affinity could be summarised (in order of declining affinity) as follows: phenyl > 5-methylthiophene > pfluorophenyl > benzofuran > pyridine. The affinities as exhibited by the methylthiophene derivatives 39f, 39h – 39j, further showed that while piperidine substitution (39f) resulted in optimal A2A and A1 affinity, pyrrolidine substitution (39j) was less favourable. Substitution at the 4ʹ position of the phenyl ring, as well as thiazole substitution, generally resulted in poor adenosine A1 and A2A receptor affinity. However, 4-[2-amino-6-(5-methylfuran-2-yl)pyrimidin- 4-yl]-N-(1,3-benzothiazol-2-yl)benzamide (39l) surprisingly demonstrated good affinity and selectivity for the adenosine A1 receptor. The results obtained during radioligand binding assays were rationalised by QSAR and molecular modelling (Discovery Studio 3.1, Accelrys) studies. The inverse relationship seen between log Ki (as indicator of affinity) and polar surface area, illustrated the importance of this physico-chemical property in the design of 2-aminopyrimidine A2A antagonists. The results from the docking study further showed that the orientation adopted by derivatives in the binding cavity (and particular hydrogen bonding to Asn 253 and Glu 169) is of importance. Results from the MTT cell viability assay indicated that none of the high affinity derivatives had a significant effect on cell viability at 1 μM, a concentration much higher than their Ki values. However, incorporation of the furan, benzofuran and p-fluorophenyl groups as aromatic substituent and a pyrrolidine as amine substituent, presented liabilities. Lastly, the haloperidol induced catalepsy assay (in rats) was used to give a preliminary indication of adenosine receptor antagonism or agonism. Compound 39f failed to reverse catalepsy under standard conditions, but showed some reversal after an increased time period. Indications therefore exist that 39f is an adenosine receptor antagonist that suffers from bioavailability issues. Compound (39c), 4-phenyl-6-[3-(piperidine-1- carbonyl)phenyl]pyrimidin-2-amine which also demonstrated promising affinity in the radioligand binding assays however showed a statistically significant reduction in catalepsy, indicating adenosine A2A receptor antagonism, and in vivo efficacy. Highly potent, dual affinity aminopyrimidine derivatives with acceptable toxicity profiles were identified in this study, with compound 39c demonstrating in vivo activity. The aim of designing and synthesising a promising dual adenosine A1/A2A receptor antagonist is therefore realised, with compound 39c as the most favourable example. / MSc (Pharmaceutical Chemistry), North-West University, Potchefstroom Campus, 2014

Parkinson’s disease (PD) is a neurodegenerative disorder, which is progressive in nature and usually associated with the elderly. It is the second most common age-related neurodegenerative disorder after Alzheimer’s disease (AD). PD occurs when there is a dramatic loss of dopamine (DA) in the striatum, a substructure of the basal ganglia, of the brain due to the degeneration of the nigrostriatal pathway that contains the dopaminergic neurons. Motor symptoms of PD include bradykinesia, muscular rigidity and resting tremors. Non-motor symptoms include speech and sleep problems, hallucinations and depression. Diverse treatment options are available to treat the symptoms of PD, including levodopa (L-Dopa), DA agonists and monoamine oxidase B (MAO-B) inhibitors. The MAOs are flavoproteins that are bound to the outer membrane of the mitochondria and catalyze the oxidative deamination of neurotransmitters such as serotonin (5-HT), noradrenaline (NA) and DA. Two isoforms occur, namely MAO-A and –B, which share a 70% sequence identity. MAO-A catalyzes the oxidation of 5-HT and MAO-B has a substrate specificity towards benzylamine and 2-phenylethylamine. DA, NA, adrenaline and tryptamine are oxidized by both forms. MAO-A plays an important role in depression while MAO-B plays an important role in PD. The two isoforms are not evenly distributed in the brain. Of particular relevance to PD is the observation that, in the basal ganglia, MAO-B is the predominant form and the oxidation of DA in this region is largely due to MAO-B activity. Also, with an increase in age, there is an up to fourfold increase in MAO-B activity in the brain. In the aged parkinsonian brain, MAO-B is therefore a major DA metabolizing enzyme and MAO-B inhibitors have an important role in the therapy of PD. MAO-B inhibitors may potentially reduce the metabolic destruction of DA and thereby provide relief from the symptoms of PD. MAO-B inhibitors may also exert a neuroprotective effect in PD. In the catalytic cycle of MAO-B, one mole each of an aldehyde, hydrogen peroxide and ammonia are formed for each mole of primary amine substrate oxidized. Ferrous iron, which is abundant in the basal ganglia, may react with the hydrogen peroxide to form hydroxyl radicals in the Fenton reaction. The hydroxyl radical damages virtually all types of biomolecules including proteins, DNA, lipids, carbohydrates and amino acids. The aldehyde, in turn, may react with amino groups of proteins, and thus lead to cell injury. Inhibitors of MAO-B may reduce the MAO-catalyzed formation of hydrogen peroxide and aldehydes in the basal ganglia, and thus act as neuroprotective agents. MAO-B inhibitors that are currently being used in the treatment of PD are selegiline and rasagiline. Both are irreversible inhibitors of MAO-B. While irreversible inhibitors of MAO have been used extensively as drugs, irreversible inhibition has a number of disadvantages. These include the loss of selectivity as a result of repeated drug administration and a slow and variable rate of enzyme recovery following termination of drug treatment. The turnover rate for the biosynthesis of MAO-B in the human brain may require as much as 40 days while with reversible inhibition, enzyme activity is recovered when the inhibitor is eliminated from the tissues. For these reasons the discovery of novel MAO-B inhibitors, which interact reversibly with the enzymes are of value in the therapy of PD. The goal of this study was to design novel and reversible inhibitors of MAO-B, which may find application in the therapy of PD. In the current study, caffeine was used as scaffold for the design of new MAO inhibitors. Caffeine is reported to be a weak inhibitor of MAO-B, with an IC50 value of 5084 μM. Substitution at C-8 of the caffeine moiety, however, yields compounds with potent MAO-B inhibitory properties. Of particular importance to this study is a recent report that a series of 8-sulfanylcaffeine analogues acts as selective inhibitors of human MAO-B. Among the compounds examined, 8-[(phenylethyl)sulfanyl]caffeine was found to be a particularly potent MAO-B inhibitor with an IC50 value of 0.223 μM. In an attempt to further enhance the MAO-B inhibition potency of 8-[(phenylethyl)sulfanyl]caffeine, and possibly to discover highly potent MAO-B inhibitors, a series of five 8-[(phenylethyl)sulfanyl]caffeine analogues was synthesized and evaluated as inhibitors of human MAO-A and –B. For the purpose of this study 8- [(phenylethyl)sulfanyl]caffeine homologues containing C-3 alkyl (CF3, CH3 and OCH3) and halogen (Cl and Br) substituents on the phenyl ring were considered. Furthermore, a series of two 8-sulfinylcaffeine analogues and one 8-sulfonylcaffeine were synthesized and their MAO inhibitory potencies were measured. The purpose with these compounds was to compare the MAO inhibitory properties of the 8-sulfinylcaffeine analogues and 8-sulfonylcaffeine with those of the 8-sulfanylcaffeine analogues. This study also investigates the MAO inhibition properties of three selected 8-[(phenylpropyl)sulfanyl]caffeine and two 8-(benzylsulfanyl)caffeine analogues. Chemistry: The target 8-sulfanylcaffeine analogues were synthesized according to the literature procedure. 8-Chlorocaffeine was reacted with an appropriate mercaptan in the presence of NaOH, to yield the target 8-sulfanylcaffeine analogues in yields of 6.4–50.7%. 8-Chlorocaffeine, in turn, was conveniently synthesized in high yield by reacting chlorine with caffeine in chloroform. In certain instances, the mercaptan starting materials were not commercially available and were thus synthesized according to the literature procedure by reacting an appropriate alkylbromide with thiourea. The resulting thiouronium salt was hydrolyzed in the presence of NaOH to yield the target mercaptan. The 8-sulfinylcaffeine analogues and 8- sulfonylcaffeine were synthesized by reacting the 8-sulfanylcaffeines with H2O2 in the presence of glacial acetic acid and acetic anhydride. The structures and the purities of the inhibitors were verified by NMR, MS and HPLC analyses. MAO inhibition studies: The MAO inhibitory properties of the test compounds were examined using the recombinant human enzymes. The mixed MAO-A/B substrate, kynuramine, was employed as substrate for both enzymes and the inhibition potencies were expressed as the IC50 values. The 8-[(phenylethyl)sulfanyl]caffeine analogues were found to be highly potent inhibitors of MAO-B. The IC50 values recorded for these homologues ranged from 0.017–0.125 μM, making them twofold to 13-fold more potent MAO-B inhibitors than the lead compound, 8- [(phenylethyl)sulfanyl]caffeine (IC50 = 0.223 μM). For comparison, the reversible MAO-B selective inhibitor, lazabemide, exhibits an IC50 value of 0.091 μM under the same conditions (unpublished data from our laboratory). Interestingly, both alkyl (CF3, CH3 and OCH3) and halogen (Cl and Br) substitution lead to highly potent MAO-B inhibition. It may therefore be concluded that substitution on C-3 is a general strategy to enhance the MAO-B inhibition potency of 8-[(phenylethyl)sulfanyl]caffeine. The results of the MAO inhibitory studies with the 8- [(phenylpropyl)sulfanyl]caffeine analogues showed that these compounds are also inhibitors of MAO-B with IC50 values of 0.061–0.500 μM. Those homologues substituted with chlorine on the para and meta positions of the phenyl ring were found to be exceptionally potent inhibitors with IC50 values of 0.061 μM and 0.062 μM, respectively. For the series of 8- (benzylsulfanyl)caffeines, meta substitution with chlorine (IC50 = 0.227 μM) and bromine (IC50 = 0.199 μM) was also found to enhance the MAO-B inhibition potency of 8- (benzylsulfanyl)caffeine (IC50 = 1.86 μM). The results document that the 8-sulfinylcaffeines are also inhibitors of MAO-B with IC50 values of 11.8–131 μM. The 8-sulfonylcaffeine was also found to be a MAO-B inhibitor. Compared to the 8-sulfanylcaffeines, these homologues are, however, weaker inhibitors. It may, therefore, be concluded that 8-sulfinylcaffeines and 8-sulfonylcaffeines are comparatively weak MAO-B inhibitors and less suited for the design of high potency MAO-B inhibitors. The results also document that the 8-[(phenylethyl)sulfanyl]caffeines are relatively weak MAO-A inhibitors with IC50 values of 5.66–141 μM, with one homologue exhibiting no inhibition under the experimental conditions. As evident from the selectivity indices (SI values), the 8- [(phenylethyl)sulfanyl]caffeines were all selective inhibitors of the MAO-B isoform. Two compounds exhibited SI values in excess of 1000. Since these compounds are also highly potent MAO-B inhibitors, they represent suitable leads for the design of potent and selective MAO-B inhibitors. The 8-sulfinylcaffeines and 8-sulfonylcaffeine were found to be weak MAO-A inhibitors with IC50 values of 166–250 μM. The SI values demonstrate that these compounds are MAO-B selective inhibitors, although to a lesser degree than the 8- [(phenylethyl)sulfanyl]caffeines. The 8-[(phenylpropyl)sulfanyl]caffeines are also MAO-A inhibitors with IC50 values of 0.708–6.48 μM. It is noteworthy that these homologues are the most potent MAO-A inhibitors among the compounds evaluated in this study. In fact, one of the 8-[(phenylpropyl)sulfanyl]caffeines, 8-{[3-(4-chlorophenyl)propyl]sulfanyl}caffeine (IC50 = 0.708 μM), is the only compound with an IC50 value for the inhibition of MAO-A in the submicromolar range. The 8-[(phenylpropyl)sulfanyl]caffeines display, in general, lower degrees of selectivity for MAO-B than the corresponding 8-[(phenylethyl)sulfanyl]caffeines. Reversibility studies: The reversibility of the interaction of a representative inhibitor, 8-{[2-(3- (trifluoromethyl)phenyl)ethyl]sulfanyl}caffeine, with MAO-B was investigated by evaluating the recovery of the enzymatic activity after dilution of the enzyme-inhibitor complex. For this purpose, MAO-B was preincubated with the test compound at concentrations of 10 × IC50 and 100 × IC50 for 30 min. The reactions were subsequently diluted 100-fold to 0.1 × IC50 and 1 × IC50, respectively. The results show that, after dilution to 0.1 × IC50 and 1 × IC50, the MAO-B catalytic activities are recovered to 35% and 22%, respectively, of the control value. For reversible enzyme inhibition, the enzyme activities are expected to recover to levels of approximately 90% and 50%, respectively, after 100-fold dilution of the preincubations containing inhibitor concentrations of 10 × IC50 and 100 × IC50. After preincubation of MAO-B with the irreversible inhibitor (R)-deprenyl (at 10 × IC50), and dilution of the resulting complex to 0.1 × IC50, MAO-B activity is not recovered (3.0% of control). These data indicate that the test compound does indeed react reversibly with MAO-B but because enzyme activities are not recovered to the expected 90% and 50% respectively, it may suggest that the test compound possess a quasi-reversible or tight-binding component. Hansch-type structure activity relationship studies: A limited Hansch-type QSAR study was performed for the inhibition of MAO by the 8-[(phenylethyl)sulfanyl]caffeines. For this purpose, five parameters were used to describe the physicochemical properties of the C-3 substituents on the phenyl rings of the inhibitors. The Van der Waals volume (Vw) and Taft steric parameter (Es) served as descriptors of the bulkiness of the substituents, while the lipophilicities were described by the Hansch constant (π). The electronic properties were described by the classical Hammett constant (σm) and the Swain-Lupton constant (F). A one-parameter fit with the Taft steric parameter versus the inhibition potency (logIC50) yielded the best correlation with a correlation coefficient (R2) of 0.912 and a statistical F value of 41.27 (Fmax = 35). The positive sign of the Es (+0.47) parameter coefficient indicated that the inhibition potencies of the 8- [(phenylethyl)sulfanyl]caffeines towards MAO-B may be enhanced by substitution with sterically large groups at C-3 of the phenyl rings of the inhibitors. / Thesis (MSc (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2013

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, characterized pathologically by a marked loss of dopaminergic nigrostriatal neurons and clinically by disabling movement disorders. PD can be treated by inhibiting monoamine oxidase (MAO), specifically MAO–B, since this is a major enzyme involved in the catabolism of dopamine in the substantia nigra of the brain. Inhibition of MAO–B may conserve the dopamine supply in the brain and may therefore provide symptomatic relief for PD patients. Selegiline is an irreversible MAO–B inhibitor and is currently used for the treatment of PD. Irreversible inhibitors inactivate enzymes by forming stable covalent complexes. The process is not readily reversed either by removing the remainder of the free inhibitor or by increasing the substrate concentration. Even dilution or dialysis does not dissociate the enzyme inhibitor complex and restore enzyme activity. From a safety point of view it may therefore be more desirable to develop reversible inhibitors of MAO–B. In this study, caffeine was used as lead compound to design, synthesize and evaluate new reversible inhibitors of MAO–B. This study is based on the finding that C–8 substituted caffeine analogues are potent MAO inhibitors. For example, (E)–8–(3–chlorostyryl)caffeine (CSC) is an exceptionally potent competitive inhibitor of MAO–B with an enzyme–inhibitor dissociation constant (Ki value) of 128 nM. In this study caffeine was similarly conjugated at C–8 to various side–chains. The effect that these chosen side–chains had on the MAO–B inhibition activity of C–8 substituted caffeine analogues will then be evaluated. The caffeine analogues were also evaluated as human MAO–A inhibitors. For the purpose of this study, saturated C–8 side chains were selected with the goal of discovering new C–8 side chains that enhance the MAO–A and ?B inhibition potency of caffeine. As mentioned above, the styryl side chain is one example of a side chain that enhances the MAO–B inhibition potency of caffeine. Should a side chain with promising MAO inhibition activity be identified in this study, the inhibition potency will be further optimized in a future study by the addition of a variety of substituents to the C–8 side chain ring. For example, halogen substitution of (E)–8– styrylcaffeine enhances the MAO–B inhibition potency by up to 10 fold. The saturated side chains selected for the present study included the phenylethyl (1), phenylpropyl (2), phenylbutyl (3) and phenylpentyl (4) functional groups. Also included are the cyclohexylethyl (8), 3–oxo–3– phenylpropyl (5), 4–oxo–4–phenylbutyl (6) moieties. A test compound containing an unsaturated linker between C–8 of caffeine and the side chain ring, the phenylpropenyl analogue 7, was also included. This study is therefore an exploratory study to discover new C–8 moieties that are favorable for MAO– inhibition. All the target compounds were synthesized by reacting 1,3–dimethyl–5,6–diaminouracil with an appropriate carboxylic acid in the presence of a carbodiimide dehydrating agent. Following ring closure and methylation at C–7, the target inhibitors were obtained. Inhibition potencies were determined using recombinant human MAO–A and MAO–B as enzyme sources. The inhibitor potencies were expressed as IC50 values. The most potent MAO–B inhibitor was 8–(5– phenylpentyl)caffeine (4) with an IC50 value of 0.656 ?M. In contrast, all the other test inhibitors were moderately potent MAO–B inhibitors. In fact the next best MAO–B inhibitor, 8–(4– phenylbutyl)caffeine (3) was approximately 5 fold less potent than 4 with an IC50 value of 3.25 ?M. Since the 5–phenylpentyl moiety is the longest side chain evaluated in this study, this finding demonstrates that longer C–8 side chains are more favorable for MAO–B inhibition. Interestingly, compound 5 containing a cyclohexylethyl side chain (IC50 = 6.59 ?M) was approximately 4 fold more potent than the analogue containing the phenylethyl linker (1) (IC50 = 26.0 ?M). This suggests that a cyclohexyl ring in the C–8 side chain of caffeine may be more optimal for MAO–B inhibition and should be considered in future studies. The caffeine analogues containing the oxophenylalkyl side chains (5 and 6) were weak MAO–B inhibitors with IC50 values of 187 ?M and 46.9 ?M, respectively. This suggests that the presence of a carbonyl group in the C–8 side chain is not favorable for the MAO–B inhibition potency of caffeine. The unsaturated phenylpropenyl analogue 7 was also found to be a relatively weak MAO–B inhibitor with an IC50 value of 33.1 ?M. In contrast to the results obtained with MAO–B, the test caffeine analogues were all weak MAOA inhibitors. With the exception of compound 5, all of the analogues evaluated were selective inhibitors of MAO–B. The most potent MAO–B inhibitor, 8–(5–phenylpentyl)caffeine (4) was the most selective inhibitor, 48 fold more potent towards MAO–B than MAO–A. This study also shows that two selected analogues (5 and 3) bind reversibly to MAO–A and ?B, respectively, and that the mode of MAO–A and –B inhibition is competitive for these representative compounds. / Thesis (M.Sc. (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2011.

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, characterized pathologically by a marked loss of dopaminergic nigrostriatal neurons and clinically by disabling movement disorders. PD can be treated by inhibiting monoamine oxidase (MAO), specifically MAO–B, since this is a major enzyme involved in the catabolism of dopamine in the substantia nigra of the brain. Inhibition of MAO–B may conserve the dopamine supply in the brain and may therefore provide symptomatic relief for PD patients. Selegiline is an irreversible MAO–B inhibitor and is currently used for the treatment of PD. Irreversible inhibitors inactivate enzymes by forming stable covalent complexes. The process is not readily reversed either by removing the remainder of the free inhibitor or by increasing the substrate concentration. Even dilution or dialysis does not dissociate the enzyme inhibitor complex and restore enzyme activity. From a safety point of view it may therefore be more desirable to develop reversible inhibitors of MAO–B. In this study, caffeine was used as lead compound to design, synthesize and evaluate new reversible inhibitors of MAO–B. This study is based on the finding that C–8 substituted caffeine analogues are potent MAO inhibitors. For example, (E)–8–(3–chlorostyryl)caffeine (CSC) is an exceptionally potent competitive inhibitor of MAO–B with an enzyme–inhibitor dissociation constant (Ki value) of 128 nM. In this study caffeine was similarly conjugated at C–8 to various side–chains. The effect that these chosen side–chains had on the MAO–B inhibition activity of C–8 substituted caffeine analogues will then be evaluated. The caffeine analogues were also evaluated as human MAO–A inhibitors. For the purpose of this study, saturated C–8 side chains were selected with the goal of discovering new C–8 side chains that enhance the MAO–A and ?B inhibition potency of caffeine. As mentioned above, the styryl side chain is one example of a side chain that enhances the MAO–B inhibition potency of caffeine. Should a side chain with promising MAO inhibition activity be identified in this study, the inhibition potency will be further optimized in a future study by the addition of a variety of substituents to the C–8 side chain ring. For example, halogen substitution of (E)–8– styrylcaffeine enhances the MAO–B inhibition potency by up to 10 fold. The saturated side chains selected for the present study included the phenylethyl (1), phenylpropyl (2), phenylbutyl (3) and phenylpentyl (4) functional groups. Also included are the cyclohexylethyl (8), 3–oxo–3– phenylpropyl (5), 4–oxo–4–phenylbutyl (6) moieties. A test compound containing an unsaturated linker between C–8 of caffeine and the side chain ring, the phenylpropenyl analogue 7, was also included. This study is therefore an exploratory study to discover new C–8 moieties that are favorable for MAO– inhibition. All the target compounds were synthesized by reacting 1,3–dimethyl–5,6–diaminouracil with an appropriate carboxylic acid in the presence of a carbodiimide dehydrating agent. Following ring closure and methylation at C–7, the target inhibitors were obtained. Inhibition potencies were determined using recombinant human MAO–A and MAO–B as enzyme sources. The inhibitor potencies were expressed as IC50 values. The most potent MAO–B inhibitor was 8–(5– phenylpentyl)caffeine (4) with an IC50 value of 0.656 ?M. In contrast, all the other test inhibitors were moderately potent MAO–B inhibitors. In fact the next best MAO–B inhibitor, 8–(4– phenylbutyl)caffeine (3) was approximately 5 fold less potent than 4 with an IC50 value of 3.25 ?M. Since the 5–phenylpentyl moiety is the longest side chain evaluated in this study, this finding demonstrates that longer C–8 side chains are more favorable for MAO–B inhibition. Interestingly, compound 5 containing a cyclohexylethyl side chain (IC50 = 6.59 ?M) was approximately 4 fold more potent than the analogue containing the phenylethyl linker (1) (IC50 = 26.0 ?M). This suggests that a cyclohexyl ring in the C–8 side chain of caffeine may be more optimal for MAO–B inhibition and should be considered in future studies. The caffeine analogues containing the oxophenylalkyl side chains (5 and 6) were weak MAO–B inhibitors with IC50 values of 187 ?M and 46.9 ?M, respectively. This suggests that the presence of a carbonyl group in the C–8 side chain is not favorable for the MAO–B inhibition potency of caffeine. The unsaturated phenylpropenyl analogue 7 was also found to be a relatively weak MAO–B inhibitor with an IC50 value of 33.1 ?M. In contrast to the results obtained with MAO–B, the test caffeine analogues were all weak MAOA inhibitors. With the exception of compound 5, all of the analogues evaluated were selective inhibitors of MAO–B. The most potent MAO–B inhibitor, 8–(5–phenylpentyl)caffeine (4) was the most selective inhibitor, 48 fold more potent towards MAO–B than MAO–A. This study also shows that two selected analogues (5 and 3) bind reversibly to MAO–A and ?B, respectively, and that the mode of MAO–A and –B inhibition is competitive for these representative compounds. / Thesis (M.Sc. (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2011.