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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/4810 |
| Date | January 2010 |
| Creators | Grobler, Paul Johan |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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