Promising botanical-derived monoamine oxidase (MAO) inhibitors: pharmacological aspects and structure-activity studies

Monoamine oxidase (MAO) is capable of catalysing the oxidative deamination of amines and neurotransmitters. MAO plays a pivotal role in maintaining neurotransmitters linked to neurological disorders viz. Alzheimer's disease (AD), Parkinson's disease (PD) etc. Therefore, inhibition of MAO can be implicated to the cure of such diseases. Synthetic MAO inhibitors are known to inhibit MAO activity. However, there are safety issues with synthetic MAO inhibitors and many of their effects are non-selective and irreversible. Contrasting synthetic drugs, plant-derived natural products have been popularized globally owing to their extensive acceptability and applicability, therapeutic potency and minimum side effects which potentiated the possibility of developing reversible, promising MAO inhibitors based on natural products. The present review comprehensively elucidates plant -derived natural reversible MAO inhibitors using the literature from the popular databases such as Google Scholar, Scopus, PubMed and Web of Science. This literature review reports approximately 51 plants that have been evaluated for MAO inhibitory activity. In addition, 93 plant-derived natural compounds were retrieved as MAO inhibitors. Majority of these investigations predominantly utilized an in vitro approach to evaluate the MAO inhibitors in relation to the developing treatments of related neurological diseases. However, in vivo studies and clinical trials are still lacking in evaluating the botanical-based MAO inhibitors. The aim of this review is to retrieve the recent literature to explore the in vitro and in vivo studies of plant-based natural products as MAO inhibitors, their structure-activity relationship and relevant molecular docking analyses and their roles in the emerging therapy against disorders like AD, and PD. Further, the review also discusses the shortcomings in the existing research in order to generate more coordinated and focused research in future.