The monoamine oxidase inhibition properties of caffeine analogues containing saturated C–8 substituents
Grobler, Paul Johan
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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.
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