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Phytochemical, antimicrobial and antiplasmodial investigations on Guinean plant species

Book - Dissertation

Medicinal plants have historically proven their value as sources of molecules with therapeutic potential, and nowadays still represent an interesting pool for the discovery of novel drug leads. Current research in drug discovery from medicinal plants involves a multifaceted approach combining several methods and techniques. Despite the considerable progress in terms of research and development of new treatment and prevention procedures over the last decades, infectious diseases still remain the leading cause of death in many developing countries. In Guinea, medicinal plants play an important role in the management of infectious diseases including malaria, urinary disorders, skin diseases and oral diseases. As part of a valorization program of these plant species, ethnopharmacological investigations have been carried out and plants species employed for the treatment of malaria, skin diseases, oral diseases and urinary disorders were inventoried. An extensive bibliographic review, followed by a preliminary biological screening resulted in the selection of some promising plant species including Terminalia albida, Tetracera alnifolia Combretum paniculatum and Pavetta crassipes. In the present research, we propose to deepen the biological and phytochemical investigations on some promising plants extracts, through the evaluation of their potential antimicrobial and antiplasmodial properties, and the corresponding active constituents. The bioassay-guided fractionation of Terminalia albida root resulted in the isolation of 14 compounds (1–14), and their antimicrobial properties were evaluated against Plasmodium falciparum, Candida albicans, Staphylococcus aureus and Escherichia coli. Pantolactone (IC50 0.60 ± 0.03 ?M) demonstrated significant activity against P. falciparum. Other compounds, including 3,4,3’-tri-O-methyl-ellagic acid, the triterpenes arjunolic acid, arjungenin, arjunic acid and arjunglucoside II, and the phenol glycoside calophymembranside-B, were less active and showed IC50 values in the range 5 – 15 ?M. None of the tested compound showed antibacterial or antifungal activity. Although the n-butanolic fraction was not active, the possibility cannot be excluded that this polar fraction contains inactive glycosides, which may release active aglycones after removal of the glycosidic moieties in the gastrointestinal tract, more in particular in the colon. Therefore, the n-butanolic fraction of the total root extract of Terminalia albida has been subjected to extensive dereplication studies followed by the isolation of the target compounds. As a result, 10 oleanane triterpenoids (1-10), among which six new compounds, i.e. albidanoside A, albidic acid A, albidinolic acid, albidienic acid, albidolic acid, albidiolic acid; two triterpene aglycones, i.e. albidic acid B and albidic acid C isolated here for the first time from a natural source; and two known compounds. Isolated compounds were evaluated for their antiplasmodial and antimicrobial activity against the chloroquine-resistant strain Plasmodium falciparum K1, Candida albicans and Staphylococcus aureus. Compounds 1 - 4, 6 ,7 and 8 demonstrated moderate antiplasmodial activity with IC50 values between 5 and 15 ?M. None of the tested compounds was active against C. albicans or S. aureus. These findings emphasize the potential of T. albida as a source for discovery of new antiplasmodial compounds. The bioassay-guided fractionation of Tetracera alnifolia leaves extracts led to the isolation of 19 compounds (1–19). Purification of fractions was performed by flash chromatography, followed by semi-preparative HPLC-DAD-MS and LC-SPE-NMR, while the structural elucidation of the isolated compounds was carried out by 1D and 2D NMR and HR-ESI-MS. Isolated compounds were screened against Plasmodium falciparum, Candida albicans and their cytotoxicity against MRC-5 cells was determined. The highest antiplasmodial activity was obtained for pheophorbide-b methyl ester (1.0 ± 0.7 ?M), (1,2)-bis-nor-phytone (2.0 ?M), isophytol (4.0 ?M) , pheophorbide-a methyl ester (2.8 ± 1.2 ?M), epicatechin-3-galloylester (5.5 ± 2.1), and phytol (6.9 ± 2.4 ?M). Other compounds, including myricetin-3-O-rhamnopyranoside, ?-tocopherol and cycloart-24-en-3?-yl ?-linolenate were less active and showed IC50 values in the range 13.5– 25 ?M. None of the tested compounds was active against Candida albicans. A hight cytotoxicity was found for pheophorbide-b methyl ester, pheophorbide-a methyl ester and phytol. Nowadays, the rapid development of modern analytical techniques and various chemometric approaches provide new perspectives for early metabolite identification in natural products research. These techniques represent a potential strategy to streamline the traditional and laborious process of isolating natural products through targeting of unknown active compounds before purification. These innovative techniques have been applied on extracts of the leaves of C. paniculatum, which have demonstrated promising antiplasmodial activity during our preliminary studies, leading to a quick and effective identification of compounds correlated to this activity. The fractionation of crude extracts was carried out, followed by multivariate data analysis of liquid chromatography–high resolution mass spectrometry (LC–HRMS) profiles of the fractions obtained. In parallel, all fractions were screened against Plasmodium falciparum, and their cytotoxicity against MRC-5 cells was determined. Dereplication studies combining UPLC-MS/MS-based molecular networking, in silico analysis and NMR methods were employed to identify the important metabolites. Several compounds strongly correlated with antiplasmodial activity have been highlighted. Six compounds including rutin (IC50 6.7 ?M) and foliasalacioside F (IC50 10.6 ?M ) have been isolated and their OPLS predicted score values were in agreement with antiplasmodial results found in vitro. These preliminary results provided clear evidence on the effectiveness of using these innovative methods (chemometrics and dereplication analysis) for the rapid identification of active metabolites in plant extracts. Further research aiming for the isolating of additional promising compounds which have shown a strong correlation with antiplasmodial activity is ongoing.
Number of pages: 354
Publication year:2021
Keywords:Doctoral thesis
Accessibility:Open