Exploring the chemodiversity from the Strychnos genus using molecular networking to unveil and identify novel antiplasmodial compounds

This thesis builds upon previous research into the antiplasmodial properties of plants from the Strychnos genus (Loganiaceae family), focusing on several species that have shown promising potential, including Strychnos icaja Baill., Strychnos usambarensis Gilg ex Engl., and Strychnos variabilis De Wild. The initial phase of this study involved an in vitro screening of 43 methanolic crude extracts from 28 Strychnos species for their antiplasmodial activity against the 3D7 strain of Plasmodium falciparum. This screening was complemented by an exploration of the metabolites from these extracts, as well as an alkaloidic extract, using molecular networking, a chemometric method that maps metabolites and visualizes their relationships based on MS/MS spectra. The application of spectral databases allowed for the rapid differentiation of known and unknown compounds, thereby facilitating the bio- and mass-guided fractionation process. This study initially highlighted the importance of further exploring the Strychnos genus, revealing clusters of previously unidentified metabolites that may exhibit antiplasmodial properties. Consequently, several species with promising (IC50 ≤ 5 μg/mL) or good (IC50 between 5 and 15 μg/mL) antiplasmodial activity were selected, notably the leaves of S. usambarensis Gilg ex Engl. and S. phaeotricha Gilg, as well as the trunk barks of S. longicaudata Gilg.
The second phase of the study focused on validating one of the database identifications, strychnine, which was detected in seven species not previously recognized as producers of this compound. Various analyses confirmed its presence, providing proof of concept for the molecular networking methodology.
The final part of the study centered on isolating and identifying novel antiplasmodial metabolites through bio- and mass-guided fractionation. Analyses of S. usambarensis leaves revealed unknown compounds with masses above m/z 900. One metabolite, with a mass of m/z 944, was isolated. It could be a dimeric or trimeric alkaloid containing a glycosidic moiety. However, its antiplasmodial activity could not be evaluated due to its limited quantity and structural fragility. In the case of S. longicaudata trunk barks, fractionation led to the isolation of alstonine and seven unknown metabolites, five of which demonstrated promising antiplasmodial activities and three of which were subjected to structural elucidation. Lastly, a preliminary study on S. phaeotricha leaves revealed good antiplasmodial activity despite the low quantity of alkaloids it contains. The results suggest that the activity may be attributed to compounds from either the alkaloid or terpene classes. Bio- and mass-guided fractionation has been initiated, and further enrichment of the fractions is necessary to progress this research.