Thiocaffeine derivatives as inhibitors of monoamine oxidase

Abstract

Parkinson’s disease (PD) is a neurodegenerative disorder which is characterized by selective loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) of the brain and reduced striatal dopamine (DA). Neuropathologically, PD is characterized by the presence of intraneuronal inclusions called Lewy Bodies (LBs). While the pathogenesis of PD is unknown, it is thought that monoamine oxidase (MAO) may play an important role in the neurodegenerative process. In the basal ganglia DA is oxidized by MAO, a process which is associated with the formation of toxic metabolic by–products. For each mole of DA oxidized by MAO, one mole of hydrogen peroxide and dopaldehyde are formed. Both these products are potentially neurotoxic if not quickly cleared. Inhibitors of MAO reduce the MAO–catalyzed metabolism of DA and as a result, reduce the formation of these toxic by–products. MAO inhibitors are therefore considered useful as a treatment strategy to slow the progression of PD since they may exert a neuroprotective effect in the brain. Since MAO is the principal enzyme for the catabolism of DA in the brain, inhibitors of MAO may conserve the dopamine supply in the brain and therefore exert a symptomatic benefit in PD. MAO inhibitors are frequently combined with L–dopa, the metabolic precursor of DA, in the therapy of PD. MAO inhibitors have been shown to enhance the levels of DA derived from L–dopa, and therefore enhance the therapeutic efficacy of L–dopa. MAO exists as two isoforms, MAO–A and MAO–B. These enzymes are products of distinct genes and exhibit differing substrate and inhibitor specificities. Both isoforms are present in the brain and utilize DA as substrate. In the brain, the MAO–B isoform exhibits higher activity and density than MAO–A and is therefore considered to play a more important role in DA metabolism than MAO–A. Also MAO–B activity in the brain increases with age while MAO–A activity remains unchanged. In the aged PD brain MAO–B is therefore thought to be the main MAO isozyme responsible for DA catabolism and inhibitors of this enzyme are considered to be useful in the treatment of PD. As mentioned above, MAO–B inhibitors may conserve dopamine in the PD brain and offer a symptomatic effect. MAO–B inhibitors may also protect against further degeneration by reducing potential toxic by–products associated with the oxidative metabolism of DA. While irreversible inhibitors of MAO–B have been used clinically in the treatment of PD, irreversible inhibition may be associated with certain disadvantages. For example, after terminating treatment with an irreversible MAO inhibitor, recovery of enzyme activity may require several weeks, since the turnover rate for the biosynthesis of MAO in the human brain may be as much as 40 days. In contrast, for reversible inhibitors, following withdrawal of the drug, enzyme activity is recovered quickly upon elimination of the drug from the tissues. This study focuses on the design of new MAO inhibitors that are selective for the MAO–B isoform and which act reversibly with the enzyme. In this study caffeine served as lead compound for the design of new MAO inhibitors. Although caffeine is a weak MAO–B inhibitor, substitution at the C–8 position with a variety of substituents has been shown to enhance the MAO–B inhibition potency of caffeine to a large degree. In a previous study it was shown that substitution at C–8 of caffeine with alkyloxy substituents yielded particularly potent MAO–B inhibitors with IC50 values in the nM range. Based on these promising results, the present study will investigate the possibility that alkylthio substituents at C–8 of caffeine may similarly enhance the MAO–B inhibition potency of caffeine. For this purpose, a series of twelve aryl– and alkylthiocaffeine analogues (4a–l) were synthesized and evaluated as potential inhibitors of recombinant human MAO–A and -B. This study was therefore an exploratory study to discover new caffeine derived MAO inhibitors. Chemistry: The C–8–substituted alkyl– and arylthiocaffeine analogues (4a–l) were synthesized by reacting 8–chlorocaffeine with the appropriate alkyl– and arylthiol derivatives in the presence of a base. The structures and purities of the target inhibitors were verified by NMR, MS and HPLC analysis. MAO inhibition studies: Among the thiocaffeine inhibitors, 8–[4–romobenzenemethanethiol] caffeine (4e) was the most potent MAO–B inhibitor, with an IC50 value of 0.16 μM. This inhibitor also exhibited a high degree of selectivity towards MAO–B. The results indicated that extending the length of the C–8 chain of the 8–thiocaffeine analogues yielded MAO–B inhibitors with enhanced inhibition potency. It was also shown that substitution on the phenyl ring of the C–8 substituent with halogens (Cl, Br and F) enhances the MAO–B inhibition potencies. Another potent MAO–B inhibitor was a phenoxyethyl substituted homologue, 8–(2– phenoxyethanethiol)caffeine (4h), with an IC50 value of 0.332 μM. Time–dependency and mode of inhibition: This study demonstrates that one selected inhibitor, compound 4e, does not reduce the catalytic rates of MAO–A and -B in a time dependent manner. This result shows that the inhibition of MAO–A and -B is reversible. For the inhibition of MAO–A and -B by compound 4e, sets of Lineweaver-Burke plots were constructed. The results showed that the Lineweaver–Burke plots intersected on the y–axis which indicates that this inhibitor is a competitive inhibitor of both MAO–A and -B and is further proof of the reversible interaction of 4e with the MAO enzymes. Future recommendations: Based on the promising MAO–B inhibition potencies of some of the thiocaffeine derivatives, this study recommends that further studies be carried out to optimize the MAO inhibition activities of these compounds. This study specifically recommends that phenylethyl and phenoxyethyl substituted thiocaffeine derivatives, which contain halogens on the phenyl ring, be synthesized and evaluated as MAO inhibitors. Such structures may be particularly potent MAO–B inhibitors. Conclusions: From the results of this study it may be concluded that thiocaffeine derivatives are promising inhibitors of MAO–A and -B. These compounds are competitive and reversible inhibitors of MAO.