Current Topics in Medicinal Chemistry - Volume 12, Issue 20, 2012

Parkinson's disease (PD) is a neurodegenerative disorder mainly characterized by a progressive neurodegeneration of the dopaminergic neurons. The available pharmacological therapy for PD aims to stop the progress of symptoms, reduce disability, slowing the neurodegenerative process and/or preventing long-term complications along the therapy. The main strategic developments that have led to progress in the medical management of PD have focused on improvements in dopaminergic therapies. Despite all the recent research, there are only a few classes of drugs approved for the treatment of motor related symptoms of PD which primarily act on the dopaminergic neurons system: L-dopa, dopamine agonists, monoamine oxidase-B (MAO-B) and catechol-O-methyl transferase (COMT) inhibitors. Anticholinergic drugs and glutamate antagonists are also available but are not commonly used in routine practice. As no effective therapeutic strategy has yet been attended, other solutions must be investigated. Privileged structures, such as indoles, arylpiperazines, biphenyls and benzopyranes are currently ascribed as helpful approaches. Different families of nitrogen and oxygen heterocycles, such as pyrazoles, hydrazinylthiazoles, xanthones, coumarins or chromones have also been extensively used as scaffolds in medicinal chemistry programs for searching novel MAO-B inhibitors. Nitrogen derivatives play a key role in this subject with several studies pointing out hydrazines, thiazoles or indoles as important scaffolds for the development of novel MAO-B inhibitors. This review comprises an overview of the state of the art on the actual pharmacological therapy for PD followed by a specific focus on the discovery and development of nitrogen-based heterocyclic compounds analogues as promising MAO-B inhibitors.

Drugs of natural origin still play a major role in the treatment of many diseases and as lead structures for the development of new synthetic drug substances. This review article deals the pharmacological effects on the Central Nervous System (CNS) of some plant extracts and their isolated chemical components due to their monoamine oxidase (MAO) activity. Herbs and herbal preparations containing MAO-A inhibitors have been widely used as an effective alternative in the treatment of neuropsychiatric diseases such as depression. Inhibitors of MAO-B not only enhance dopaminergic neurotransmission but also prevent activation of toxin and free radical formation, alleviating the process of neuron denaturalization, on account of which they are used in Parkinson disease (PD). Several methods have been developed for monitoring MAO activity and its inhibitor screening of bioactive natural products.

The number of papers dealing with the structure-based drug design is continuously growing, which demonstrates the importance of such tools in medicinal chemistry. In the current paper, the published literature concerning the use of the ligand-protein docking methodologies in the study of the monoamine oxidase (MAO) enzymes was reviewed. Ten years of studies aimed at developing new compounds active as MAO inhibitors (MAOIs) were covered. The literature regarding thiazole, caffeine, pyrazole, chromone, indeno-pyridazin, β-carboline, indole, coumarin, anilide and amphetamine derivatives, was discussed in some detail. It is apparent that, through this computational approach, more selective and potent molecules can be proposed as inhibitors by applying precise modifications on the basic scaffold.

Historically, much of the focus on monoamine oxidases and their substrates has been in the area of depression and the monoamine neurotransmitters serotonin (5-hydroxytryptamine), noradrenaline, and to a lesser extent, dopamine. With both forms of monoamine oxidase (A and B), the production of hydrogen peroxide as a byproduct of the reaction between the monoamine oxidases and their monoamine substrates has also implicated monoamine oxidase-sensitive events in intrinsic cell death pathways, particularly those centered on oxidative stress and peroxyradical-mediated mechanisms. Consequently, and perhaps not unexpectedly, the inhibition of monoamine oxidase has been considered as adjunctive therapy in neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, both of which involve a significant oxidative stress component. Yet the literature also provides ambiguities; indeed, not all of the functions of monoamine oxidases are dependent on catalytic activity nor can they all be ascribed to expression levels of the monoamine oxidase protein per se. Recent reports strongly suggest that the functions of monoamine oxidases also rely on posttranslational modifications, epigenetic influences, interactions with other proteins, the cell phenotype and its localization to specific subcellular compartments. These recent developments certainly complicate the issue, yet they need to be duly considered when implicating monoamine oxidases and their inhibitors in both in vitro and in vivo pathological contexts.

In neurodegenerative disorders, including Parkinson's and Alzheimer's diseases, type B monoamine oxidase (MAO-B) has been proposed to play a primary role though generating reactive oxygen species in oxidation of monoamine substrates. MAO-B oxidizes MPTP into MPP+, and an MAO-B inhibitor, deprenyl, prevents the MPTP oxidation and also MPP+neutotoxicity. These results suggest the association of MAO-B with neuronal death in neurodegenerative disorders. On the other hand, deprenyl and rasagiline, selective MAO-B inhibitors, have been proved to protect neuronal cells in cellular and animal models of neurodegeneration. These inhibitors decrease oxidation of the substrates, scavenge oxygen radicals, intervene apoptosis signal pathway in mitochondria and induce pro-survival genes coding anti-apoptotic Bcl-2 and neurotrophic factors. However, the association of MAO-B itself with the neuroprotective function of MAO-B inhibitors remains enigmatic. Recently, the involvement of type A MAO (MAO-A) in neuronal death has been shown by upregulation MAO-A expression in cellular models. MAO-A is a target of an endogenous neurotoxin, Nmethyl( R)salsolinol, and MAO-A knockdown (KO) with short interfering (si)RNA protects neuronal death from apoptosis. In addition, MAO-A mediates the increased expression of genes for anti-apoptotic, pro-survival Bcl-2 and neurotrophic factors by MAO-B inhibitors, whereas MAO-B doe not. In this review, we present our recent results on the novel role of MAO-A and MAO-B in neuronal death and also in the neuroprotective gene induction by MAO inhibitors. The future development of new series of neuroprotective drugs is discussed among compounds, which have high affinity to MAO-A and can induce pro-survival genes. MAO-A is expected to play a role in disease-modifying therapy for neurodegenerative disorders.

Monoamine oxidase (MAO, EC.1.4.3.4) has been a drug target for 60 years, with the primary rationale of developing drugs to treat neuropsychiatric disorders. The biological importance of MAO is to regulate amine levels is the brain and to metabolize amines and drugs in the periphery. This review of the biochemistry of MAO A and MAO B describes the functional properties of the two enzymes integrated with knowledge of the structures of the many MAOinhibitor complexes published in the last 10 years. The analysis of activity, and the chemical and kinetic mechanisms are discussed. Inhibition studies on human MAO in vitro are now facilitated by assays using readily available materials but the kinetics of MAO involving alternative oxidative pathways and sensitivity to the oxygen concentration mean that careful analysis of the data is required as well as the good practice of determining mechanism of inhibition and kinetic constants. Kinetic constants can then be compared with thermodynamic calculations. Inhibitors bind to both the oxidized and reduced forms of MAO present during turnover so both forms should be considered when using molecular dynamics to facilitate drug design. Interaction of inhibitors with the active site can be detected as changes in the visible spectrum and these changes can provide clues about the flavin adduct formed for irreversible inhibitors or about the proximity to the flavin for reversible inhibitors. Developing areas (knock-out mice for behavior, the imidazoline binding site, and imaging to monitor the activity and the inhibition of MAO in the patient) are mentioned.

It is commonly accepted that monoamine oxidase (MAO) may play a critical role in the regulation of the central nervous system activity and contribute to the pathogenesis of human neurodegenerative and depressive disorders. This has encouraged an active research in the development of new drugs, MAO inhibitors, since they may represent an important advance in the treatment of complex diseases such as Alzheimer and Parkinson, which are becoming prevalent pathologies due to the increase of aging population of developed countries societies. Different research groups are intensively working in this area with the aim of finding new MAO selective inhibitors. Differently substituted coumarins have been synthesized and evaluated as MAO-A and MAO-B inhibitors. The purpose of this review is to summarize the findings reported in this area, particularly focuses on the coumarin scaffold.

Among different heterocyclic chemotypes incorporating two nitrogen atoms, pyrazolines could be considered a valid pharmacophore for the synthesis of selective monoamine oxidase (MAO) inhibitors because they were developed by the cyclization of the early hydrazine derivatives such as isocarboxazid. Substituted pyrazolines, decorated with different functional groups, are important lead compounds endowed with a large amount of biological activities. As a matter of this, most of them were also evaluated as dual inhibitors with a synergistic action towards different classes of enzymes (ciclooxygenase, acetylcholinesterase, butyrylcholinesterase). Moreover due to the direct correlation with the recognized MAO inhibition, this scaffold displayed antidepressant and anticonvulsant properties in animal models.

The evolution of bio- and cheminformatics associated with the development of specialized software and increasing computer power has produced a great interest in theoretical in silico methods applied in drug rational design. These techniques apply the concept that “similar molecules have similar biological properties” that has been exploited in Medicinal Chemistry for years to design new molecules with desirable pharmacological profiles. Ligand-based methods are not dependent on receptor structural data and take into account two and three-dimensional molecular properties to assess similarity of new compounds in regards to the set of molecules with the biological property under study. Depending on the complexity of the calculation, there are different types of ligand-based methods, such as QSAR (Quantitative Structure- Activity Relationship) with 2D and 3D descriptors, CoMFA (Comparative Molecular Field Analysis) or pharmacophoric approaches. This work provides a description of a series of ligand-based models applied in the prediction of the inhibitory activity of monoamine oxidase (MAO) enzymes. The controlled regulation of the enzymes' function through the use of MAO inhibitors is used as a treatment in many psychiatric and neurological disorders, such as depression, anxiety, Alzheimer's and Parkinson's disease. For this reason, multiple scaffolds, such as substituted coumarins, indolylmethylamine or pyridazine derivatives were synthesized and assayed toward MAO-A and MAO-B inhibition. Our intention is to focus on the description of ligand-based models to provide new insights in the relationship between the MAO inhibitory activity and the molecular structure of the different inhibitors, and further study enzyme selectivity and possible mechanisms of action.

Since the first generation of MAO inhibitors was developed, more than fifty years ago, this family of drugs has been ups and downs over the last decades. Actually, interest in MAO inhibitors is reviving and the emergence of new advances in the rational design of molecules and new techniques to predict the in vivo behavior has encouraged the research for new drugs with therapeutic potential in this area. The classic MAOIs have been widely used as antidepressants during the two decades after its introduction in clinic. Based on observations made on MAO inhibition by these drugs, it has been postulated hypothesis that have contributed to a better understanding of the mechanism and management of depressive disorders. However, exaggerated concerns about food and drug interactions relegated these drugs from the pharmaceutical landscape. The correct interpretation and the contextualization of side effects and the recent research findings, in which MAO selective inhibitors appear as promising agents in the treatment of emerging and high prevalence diseases, are placing these drugs again into the scientific and pharmacological focus.