Morinda lucida Benth (Rubiaceae): A review of its ethnomedicine, phytochemistry and pharmacology

Ethnomedicinal relevance: Natural products derived from plants have served the primary healthcare needs of millions of indigenous people for centuries, many of which have been documented and scientifically validated. Morinda lucida Benth (Rubiaceae), also referred to as brimstone tree, is an ethnomedicinal plant which has been widely used in traditional medicine for several decades, particularly in the African continent. Various parts of the plant, including stem bark, leaves and root, have been applied in traditional medicine for the management of various pathological conditions such as malaria, diabetes, hypertension, inflammation, typhoid fever, cancer, cognitive disorders, sickle cell disease, trypanosomiasis, onchocerciasis and various fevers. In this review, we critically evaluated the relationship between traditional uses, laboratory pharmacological activities and clinical studies on M. lucida so as to unveil opportunities for the development of relevant therapeutic agents against diseases that threaten mankind. Materials and methods: A search for relevant data on M. lucida was done using scientific databases (Google Scholar, Mendeley, ScienceDirect, PubMed, Asian Science Citation Database, Chinese Biomedical Literature Database, Chinese National Knowledge Infrastructure, Chinese Scientific Journal Database, Chinese Science Citation Database, other web sources (such as The Plant List and PROTA), books and other literature sources. Results: A hundred compounds have been isolated from M. lucida. Many of the reported secondary metabolites include alkaloids, tannins, anthraquinones, sterols, saponins, polyphenols, terpenoids, phenols and cardiac glycosides. The in vitro and in vivo experimental studies on various extracts, fractions and isolated compounds of M. lucida support the acclaimed pharmacological activities of the plant, such as antimalarial, antidiabetic, hypotensive, anti-inflammatory, immunostimulatory, antioxidant, antimicrobial, antiproliferative, cognitiveenhancement, anti-sickling, anti-trypanosomal, anti-onchocercal, muscle relaxant, antifungal and antileishmanial activities. These evidence-based scientific reports lend credence to their traditional uses. However, the safety of extracts of M. lucida is a cause for concern following reported toxicities such as antispermatogenic effect, genotoxicity and in vitro inhibition of human cytochrome P450 3A subfamily. Conclusion: Documented evidence suggests that M. lucida remains a rich source of extracts and chemical compounds with diverse bioactivities that are of therapeutic benefit to man and this justifies its traditional uses for the primary healthcare needs of indigenous populations across tropical Africa. Due to the fact that M. lucida extracts may not be safe at some reported doses, more in-depth studies on their toxicities are required to better understand safer approaches to their traditional uses. In addition, mechanistic studies on the isolated compounds with known pharmacological activities are quite limited, thus necessitating future research efforts to be focused on the mechanisms of action of these active principles in order to facilitate their potential enlistment for rational drug design.