| dc.description.abstract | Monoamienoksidase (MAO)-inhibeerders word gebruik vir die behandeling van toestande wat veroorsaak word deur verlaagde vlakke van neuro-oordragstowwe soos dopamien, noradrenalien en serotonien. MAO-inhibeerders is aanvanklik vir die behandeling van depressie gebruik, maar verdere kliniese gebruik is beperk deur ʼn potensieel-ernstige hipertensiewe krisis wat ontstaan indien dié middels saam met tiramien-bevattende kossoorte gebruik word. Dit staan bekend as die “kaasreaksie”. Met die ontdekking van middels wat die MAO-B-isoform meer spesifiek inhibeer of MAO omkeerbaar inhibeer, is die waarskynlikheid vir die kaasreaksie drasties verlaag en word hierdie inhibeerders dus as veilig beskou.
Inhibeerders wat spesifiek MAO-B inhibeer word vir die behandeling van Parkinson se siekte gebruik, wat gekarakteriseer word deur die afsterwing van die dopaminergiese neurone wat vanuit die substansia nigra pars compacta in die brein na die striatum projekteer. Dopamien word deur MAO-B in die brein gemetaboliseer en dus word MAO-B-inhibeerders gebruik om dopaminergiese neurotransmissie te verbeter en op dié manier verskaf dit simptomatiese verligting van die motoriese simptome van Parkinson se siekte. Die primêre behandeling vir Parkinson se siekte is orale toediening van L-dopa, die metaboliese voorloper vir dopamien. MAO-B-inhibeerders word dikwels in kombinasie met L-dopa gebruik sodat die dosis vir L-dopa verlaag kan word. Dit verlaag ook die newe-effekte soos diskinesie, wat met die langtermyngebruik van L-dopa geassosieer word.
Vir die doel van hierdie studie is nuwe middels gesintetiseer, wat moontlik MAO-B omkeerbeer sal inhibeer, deur gebruik te maak van 1-tetraloon as leidraad. Dit is voorheen bewys dat derivate van 1-tetraloon hoogs potente en baie spesifieke inhibeerders van MAO-B is. Om die 1-tetraloonderivate te sintetiseer is substitusie op die C5, C6 en C7 posisies van 1-tetraloon uitgevoer met bensieloksi, 4-chloorbensieloksi en 2-fenoksietoksi as substituente. Die belangrikste doel van hierdie studie was egter om die 1-tetraloonderivate na die ooreenstemmende 1-tetralolderivate te reduseer, wat nog nie voorheen as potensiële MAO-inhibeerders geëvalueer is nie. Hierdie studie gaan dus die inhibisie-eienskappe van die 1-tetraloonderivate vergelyk met die van die 1-tetralolderivate.
Die 1-tetraloonderivate is gesintetiseer deur 5-, 6- en 7-hidroksie-1-tetraloon te laat reageer met die gepaste gesubstitueerde alkielbromied in die teenwoordigheid van aluminiumchloried en tolueen. Hierdie reaksies het die 1-tetraloonderivate 1a-h gelewer. Die 1-tetraloonderivate is in die teenwoordigheid van etanol en natriumboorhidried gereduseer na die ooreenstemmende 1-tetralol (1,2,3,4-tetrahidro-1-naftol)-derivate 1i-p. Al die strukture is met behulp van
kernmagnetieseresonans (KMR)-spektroskopie en massaspektrometrie (MS) opgeklaar. Die suiwerhede is deur hoë-prestasie vloeistofchromatografie (HPVC) bepaal.
Die 1-tetraloonderivate is as moontlike inhibeerders van menslike rekombinante MAO-A en MAO-B geëvalueer en die potensies is as die IC50-waarde uitgedruk. Daar is gevind dat beide die 1-tetraloon- en 1-tetralolderivate potente inhibeerders van MAO-B is, en ook dat sekere verbindings potente inhibeerders van MAO-A is. Tetraloonderivaat 1h het die mees potente MAO-A- en MAO-B-inhibisie getoon met IC50-waardes van 0.036 μM en 0.0011 μM, onderskeidelik. Tussen die 1-tetralolderivate was 1p (IC50 = 0.785 μM) en 1o (IC50 = 0.0075 μM) die mees potente inhibeerders van MAO-A en MAO-B, onderskeidelik. Hierdie derivate is gekies om die omkeerbaarheid van MAO-inhibisie te evalueer omdat die omkeerbaarheid van MAO-inhibisie nog nie voorheen vir 1-tetralolderivate geëvalueer is nie. Die resultate toon dat hierdie verbindings omkeerbare inhibeerders van MAO is omdat die ensiemaktiwiteit herwin kon word nadat die inhibeerder deur dialise verwyder is. Lineweaver-Burk grafieke is opgestel en het getoon dat 1p en 1o kompeterende inhibeerders van MAO-A en MAO-B is met Ki-waardes van 0.0065 μM en 1.0 μM, onderskeidelik.
Sommige 1-tetraloon- en 1-tetralolderivate toon hoë potensies vir inhibisie van MAO-B maar lae potensies vir die inhibisie van MAO-A. ʼn Voorbeeld hiervan is die alkoholderivaat 1m wat ʼn IC50-waarde van 0.068 μM toon vir MAO-B, maar geen merkbare inhibisie van MAO-A by die maksimum getoetste konsentrasie besit nie. Molekulêre modulering is met verbindings 1m en 1e uitgevoer om die moontlike bindingsoriëntasies in MAO te voorspel en sodoende ʼn moontlike verduideliking vir hierdie verskynsel te bied. Die resultate toon dat beide verbindings in die aktiewe setel van MAO-A bind en interaksies ondergaan. Molekulêre modulering bied dus nie ʼn verduideliking vir die afwesigheid van MAO-A-inhibisie deur verbinding 1m nie, maar dit is wel bekend dat groter molekules nie in die aktiewe setel van MAO-A pas nie, terwyl dit wel in die aktiewe setel van MAO-B bind. Die bevinding dat 1e beter MAO-B-inhibisie as verbinding 1m besit kan moontlik toegeskryf word aan die feit dat 1m ʼn rasemiese mengsel van 2 enantiomere is, naamlik (R)-1m en (S)-1m. Dit kan gepostuleer word dat 1e deur beide pi-pi en waterstofbindingsinteraksies gestabiliseer word, terwyl die enantiomere van 1m deur of pi-pi óf waterstofbindingsinteraksies gestabiliseer word.
Hierdie studie het bevind dat beide 1-tetraloon- en 1-tetralolderivate potente inhibeerders van MAO-A en MAO-B is. Sommige verbindings het selfs hoër potensies getoon as geneesmiddels wat tans as MAO-inhibeerders gebruik word. Sulke verbindings mag as voorlopers gebruik word vir die verdere ontwerp van MAO-inhibeerders vir die behandeling van Parkinson se siekte.
ABSTRACT
Monoamine oxidase (MAO) inhibitors are used for the treatment of disorders that are caused by deficient levels of neurotransmitters such as dopamine, noradrenaline and serotonin. MAO inhibitors were first used for the treatment of depression, but their clinical use was limited due to a potentially fatal hypertensive crisis that may occur when these drugs are combined with tyramine containing foods. This adverse effect is known as the cheese reaction. With the discovery of inhibitors that are specific for the MAO-B isoform as well as reversible inhibitors of the MAOs, the liability of the cheese reaction was greatly decreased and such MAO inhibitors are considered to be relatively safe drugs.
MAO-B specific inhibitors are used for the treatment of Parkinson’s disease, which is characterised by the loss of the dopaminergic neurons projecting from the substantia nigra pars compacta of the brain to the striatum. Since MAO-B metabolises dopamine in the brain, MAO inhibitors are used to enhance dopaminergic neurotransmission and thus provide symptomatic relief of the motor symptoms of Parkinson’s disease. The primary treatment for Parkinson’s disease is L-dopa, the metabolic precursor of dopamine. MAO-B inhibitors are frequently used in conjunction with L-dopa and may allow for a lower dose of L-dopa to be administered. This, in turn, reduces adverse effects such as dyskinesia associated with long-term L-dopa treatment.
The current study set out to discover novel drugs that inhibit MAO-B reversibly by using 1-tetralone as lead compound. It was previously shown that 1-tetralone derivatives are high potency and specific inhibitors of MAO-B. To synthesise the series of 1-tetralone derivatives, substitution on C5, C6 and C7 with the benzyloxy, 4-chlorobenzyloxy and 2-phenoxyethoxy substituents was carried out. A key objective of this study was to reduce the 1-tetralone derivatives to the corresponding 1-tetralol derivatives, which have not previously been investigated as MAO inhibitors. This study will therefore compare the MAO inhibition properties of 1-tetralone derivatives to the corresponding 1-tetralol compounds.
The 1-tetralone derivatives were synthesised by reacting 5-, 6- and 7-hydroxy-1-tetralone with an appropriate substituted alkyl bromide in the presence of aluminium chloride and toluene. This yielded 1-tetralone derivatives 1a-h. The 1-tetralone derivatives were subsequently reduced in the presence of ethanol and sodium borohydride to the corresponding 1-tetralol (1,2,3,4-tetrahydro-1-naphthol) derivatives 1i-p. All compounds were characterised by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Purities were assessed by high-performance liquid chromatography (HPLC).
The 1-tetralone derivatives were subsequently evaluated as potential inhibitors of recombinant human MAO-A and MAO-B, and the inhibition potencies were expressed as IC50 values. Both
the 1-tetralone and 1-tetralol derivatives were found to be potent inhibitors of MAO-B, while some compounds also acted as MAO-A inhibitors. 1-Tetralone derivative 1h exhibited the most potent inhibition of MAO-A and MAO-B with IC50 values of 0.036 μM and 0.0011 μM, respectively. Among the 1-tetralol derivatives, 1p (IC50 = 0.785 μM) and 1o (IC50 = 0.0075 μM) were the most potent inhibitors of MAO-A and MAO-B, respectively. These derivatives were selected to evaluate the reversibility of MAO inhibition since the reversibility of MAO inhibition by 1-tetralol derivatives has not yet been evaluated. The results showed that these compounds are reversible MAO inhibitors since most of the enzyme activity could be recovered by removal of the inhibitor by dialysis. Lineweaver-Burk plots were subsequently constructed and showed that 1p and 1o are competitive inhibitors of MAO-A and MAO-B with Ki values of 0.0065 μM and 1.0 μM, respectively.
Some of the 1-tetralone and 1-tetralol derivatives exhibit high potency inhibition towards MAO-B but low potency inhibition towards MAO-A, such as the alcohol derivative 1m which possesses an IC50 value of 0.068 μM for the inhibition of MAO-B but no measureable inhibition towards MAO-A at the maximum tested concentration. Molecular modelling was performed with compounds 1m and 1e to explore their possible binding orientations in MAO and to provide a possible explanation for this phenomenon. The results show that both inhibitors are able to bind and interact with the active site of MAO-A. A molecular explanation for the lack of inhibition towards MAO-A by compound 1m is therefore not apparent, but it is well-known that larger molecules fit poorly in the active site of MAO-A compared to the active site of MAO-B. The slightly higher inhibition towards MAO-B by 1e compared to 1m can possibly be explained by the fact that 1m is the racemic mixture of two enantiomers, (R)-1m and (S)-1m. It may be postulated that 1e is stabilised by both pi-pi and hydrogen bond interactions while either enantiomer of 1m is stabilised only by pi-pi or hydrogen bond interactions.
This study found that both 1-tetralone and 1-tetralol derivatives are potent inhibitors of MAO-A and MAO-B, with some compounds exhibiting higher potencies than clinically used MAO inhibitors. Such compounds may act as leads for the design of future MAO inhibitors for the treatment of Parkinson’s disease. | en_US |
