BIOASSAY-GUIDED ISOLATION AND CHARACTERIZATION OF ANTIPLASMODIAL COMPOUNDS FROM THE LEAVES AND BARK OF LANDOLPHIA DULCIS AND MORINDA LUCIDA

OLADEJI,, SOLOMON OLUWOLE (2022) BIOASSAY-GUIDED ISOLATION AND CHARACTERIZATION OF ANTIPLASMODIAL COMPOUNDS FROM THE LEAVES AND BARK OF LANDOLPHIA DULCIS AND MORINDA LUCIDA. ["eprint_fieldopt_thesis_type_phd" not defined] thesis, Landmark University, Omu Aran, Kwara State.

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Abstract

Adverse side effects and drug resistance often associated with some of the current antimalarial drugs are partly responsible for the intensive search for safer natural antimalarial remedies from diverse plant genetic resources. Preliminary ethnopharmacological survey revealed that Morinda lucida (L.) Benth. and Landolphia dulcis (Sabine) Pichon require investigations as sources of novel antiplasmodial molecules. Hence, this work isolated and characterized novel antiplasmodial compounds from the leaves and bark of M. lucida and L. dulcis. The leaves and bark were respectively dried, pulverized and successively extracted with solvents of increasing polarity: n-hexane, ethyl acetate and hydroethanol (50%). Qualitative phytochemical methods were used to identify the inherent Secondary metabolites while different standard quantitative procedures were used to quantify the different secondary metabolites present in the different extracts. Furthermore, the extracts were subjected to in vivo antiplasmodial investigations (animal models) using the four-day curative assay. The most active extract from each plant was chromatographically fractionated using solvents of increasing polarity and the resulting fractions were subjected to further antiplasmodial assays (in vivo). Pure and semi-pure compounds were obtained from the most active fraction in L. dulcis (OL-N) via column and preparative thin layer chromatography. The pure compounds were characterized using infrared spectroscopy, 1D and 2D- nuclear magnetic resonance and mass spectrometry. However, the constituents of semi-pure compounds were identified using the gas chromatography-mass spectrometry technique. The identified compounds were subjected to molecular docking against Plasmodium protease of P. falciparum thymidylate kinase (2YOG) and P. falciparum Enoyl ACP Reductase with Triclosan Reductase (2OP0) to understand the mechanism by which the compounds induce antiplasmodial activity. The cold extractions were successful, yielding n-hexane extracts [(L. dulcis leaves (HLDL) and bark (HLDB), M. lucida leaves (HMLL) and bark (HMLB)], ethyl acetate extracts [(L. dulcis leaves (ELDL) and bark (ELDB), M. lucida leaves (EMLL) and bark (EMLB)] and 50% hydroethanol extracts [(L. dulcis leaves (EtLDL) and bark (EtLDB), M. lucida leaves (EtMLL) and bark (EtMLB)]. The preliminary phytochemical screenings revealed varying presence of steroids, terpenoids, tannins, flavonoids, alkaloids, phenols, saponins and anthraquinones in the extracts. The extracts and fractions significantly reduced the level of parasitaemia, and increased packed cell volume as well as body weight of Swiss albino mice infected with P. berghei. Three compounds (OL-1, A1B1 and A3) were successfully isolated from OL-N. Based on spectroscopic data, OL-1 was elucidated as a bianthraquinone, A-3 was characterized as a steroidal glycoside while A1B1 as an ester. OL-1, A-3 and the identified compounds from selected chromatographic fractions inhibited 2OP0 and 2YOG with impressive docking scores, indicating the inhibition of fatty acids in cell wall biosynthesis and enzymatic activity in pyrimidine nucleotide biosynthesis as part of the mechanism by which the compounds effect their antiplasmodial activities. Antimalarial compounds were isolated and identified for the first time from the bark of Landolphia dulcis. The underexplored plant is a sustainable source of therapeutic compounds with high potential for development into novel antiplasmodial drugs.

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