Rational approaches towards the design of (E)-8-(3-Chlorostyryl)-caffeine derivatives as novel reversible inhibitors of monoamine oxidase B
Reversible and selective inhibitors of monoamine oxidase B (MAO-B) are under investigation for the treatment and prevention of Parkinson's disease (PD) and Alzheimer's disease. The mechanism-based inactivator of MAO-B, (R)-deprenyl is frequently used in combination with L-dopa as dopamine replacement therapy in PD. In contrast with reversible inhibitors, following treatment with inactivators such as (R)- deprenyl, enzyme activity can only be regained via de novo synthesis of the MAO-B protein. For these reasons, several studies are currently under way to develop safer inhibitors of MAO-B. The purpose of this study is to develop a pharmacophore model for reversible MAO-B inhibition by studying the stereoelectronic properties of several (E)-8- (substituted-styryl)caffeine analogues. This class of compounds was selected as model compounds since (E)-8-(3-chlorostyryl) caffeine (CSC) was recently found to be a high potency competitive inhibitor of MAO-B. (4-8-(substituted-styry1)caffeine analogues are also known to be potent antagonists of the adenosine A2A-receptor subtype. Such antagonists are currently being investigated as possible therapeutic agents for the symptomatic treatment of motor deficits such as those encountered in PD. Since MAO-B inhibitors are currently under investigation as preventative agents in PD, the possibility of designing dual acting antiparkinsonian drugs that can act both as A2A antagonists and inhibitors of MAO-B may be of value. In PD such dual acting drugs could potentially enhance motility through antagonism of A2A receptors and protect against further neurodegenerative processes, through inhibition of MAO-B. In this study novel (E)-8-(3-chlorostyry1)caffeine analogues were synthesized and evaluated in vitro as competitive inhibitors of MAO-B. The analogues selected were mono substituted with various functional groups on C-3 and C-4 of the styryl phenyl ring. The compounds were evaluated as competitive inhibitors of baboon liver MAO-B. Two different assays were followed to determine the inhibitory activity of the putative inhibitors. The first was a spectrophotometric assay that utilised MMTP, an analogue of the neurotoxin MPTP as substrate and the second was an HPLC method which utilised benzylamine as substrate. The potency of MAO-B inhibition by the synthesised inhibitors were expressed as the enzyme-inhibitor dissociation constant (Ki value) and ranged from 0.1 uM -1.5 uM. Molecular modelling studies were also carried out in order to establish a hypothesis for MAO-B inhibition by the CSC analogues. Such a pharmacophoric model may assist in future design and prediction of MAO-B inhibitors. The pharmacophore contains a hydrogen bond acceptor, two hydrophobic points as well as area for an aromatic ring. In order to determine specific interactions between the inhibitor and the enzyme the compounds were docked into the crystal structure of the MAO-B enzyme. The 2 compounds with respectively the highest and lowest conformational docking scores were then dynamically minimised and the new conformations were docked into the active site. These computational studies indicated favourable interactions of the inhibitors with the enzyme and the flavin-adenine dinucleotide cofactor (FAD). The amino acids that surrounded the ligand were found to be: TYR60, TRPl19, LEU1 67, PHE168, VAL169, ASN170, LEU171, CYSH172, VAL173, THR174, PHEI 85, TYR188, PHE343, TYR398, TYR435 and FAD. Hydrogen bonds formed between GLN206 and N-1 and N- 7 of the xanthinyl moiety, as well as between TYR435 and N-9. Further analysis of the data supported the idea that the inhibitors should have a linear conformation, since inhibitors that were not linear exhibited weaker interactions with the enzyme. The results of a Hansch-type SAR study showed that the potency of MAO-B inhibition by (a-8-styrylcaffeinyl analogues depends upon the van der Waals volume (Vw), lipophylicity as well as the Swain-Lupton electronic parameter (F) of the substituents attached to C-3 of the styryl ring while the potency depends only upon descriptors of bulkiness (such as V,) and lipophylicity of substituents attached to C-4. In conclusion, the (a-8-(substituted-styry1)caffeine analogues represent a class of dual acting antiparkinsonian drugs that may both provide relief of the symptoms through interaction with adenosine A2A receptors and protect from further neurodegeneration through inhibition of MAO-B.
- ETD@PUK