Antitubercular and antimalarial activity of metabolites isolated from crude and lead-like enhanced (LLE) extracts from selected Namibian medicinal plants
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Date
2020
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University of Namibia
Abstract
Medicinal plants remain an important source of new lead compounds and drugs, but the re-isolation of known compounds and the loss of bioactivity during purification impedes the discovery of novel bioactive compounds. Of interest to this study, is a protocol developed by Camp and co-workers, which involves enhancing the quality of plant extracts by frontloading them with metabolites with drug-like properties, that is, generating lead like enhanced (LLE) extracts. This approach was successfully applied and yielded novel antiplasmodial and antitrypanosomal compounds but has not been explored to search for antitubercular compounds or drug leads. The aim of this study was (i) to prepare crude organic and aqueous extracts of 25 plant parts obtained from eight indigenous Namibian medicinal plants, (ii) to evaluate their antiplasmodial and antimycobacterial activity, and (iii) to correlate their ethnomedicinal use with the biological results obtained in this study. It was further aimed at evaluating the antiplasmodial and antimycobacterial activity of the LLE extracts and MeOH fractions – obtained from the active crude extracts – as well as to isolate and characterize the bioactive compounds. Eight plant species which are used traditionally for the treatment of tuberculosis, malaria and associated symptoms, were collected at Uis in the Erongo region and Tsumkwe in the Otjozondjupa region. Plants included were: Terminalia sericea, Adansonia digitata, Ozoroa paniculosa, Diospyros lycioides, Albizia anthelmintica, Combretum imberbe, Aloe dichotoma and Sarcocaulon marlothii Engl. The antiplasmodial activity was tested in vitro using the parasite lactate dehydrogenase assay against Plasmodium falciparum (CQS) NF54 and the in vitro antimycobacterial activity testing was done using the standard broth microdilution method against Mycobacterium tuberculosis H37Rv-GFP strains. The preliminary biological activity results showed that 10 crude extracts (8 organic and 2 aqueous) displayed antimycobacterial activity with an MIC90 < 90.0 μg/mL, whereas 4 crude extracts (1 organic and 3 aqueous) displayed antiplasmodial activity with IC50 ≤ 18.0 μg/mL. The antimycobacterial and antiplasmodial activities for the fourteen crude extracts ranged from MIC90 9.94 – 86.8 μg/mL and IC50 5.20 - 17.8 μg/mL, respectively. The African baobab tree, A. digitata, displayed the best antimycobacterial (bark, aq.: MIC90 9.94 μg/mL) and antiplasmodial (stems, org.: IC50 5.20 μg/mL) activity. The stems of S. marlothii, an endemic and phytochemically unexplored medicinal plant, are traditionally used to treat tuberculosis. However, both the organic and aqueous extracts displayed poor antimycobacterial activity with MIC90s of 103 μg/mL and >125 μg/mL, respectively. Instead, both the organic and aqueous extracts displayed in vitro antiplasmodial activity with IC50s of 8.80 μg/mL and 17.8 μg/mL, respectively. The antiplasmodial and antimycobacterial bioactive crudes were then subjected to solid phase extraction using Strata-X reversed phase cartridges prepacked with N-vinylpyrrolidone (NVP). The solvent systems used for elution was 70% MeOH:H2O containing 1% trifluoroacetic acid followed by 100% MeOH, to yield the LLE and MeOH fractions, respectively. Compared to the bioactive antimycobacterial crudes, none of the LLE extracts displayed antimycobacterial activity. The MeOH fraction of the bark of A. digitata however, displayed a more than threefold increase in activity (MIC90 19.5 μg/mL) compared to the organic crude (MIC90 70.7 μg/mL). With regard to antiplasmodial activity, the MeOH fraction of the organic extract of the stems of S.mmarlothii displayed a twofold increase in antiplasmodial activity (IC50 4.30 μg/mL) compared to the crude (IC50 8.80 μg/mL). In accordance with the objective of the study, the crude organic extract of the stems of S. marlothii was subjected to flash chromatography, that is, to isolate and characterize potentially novel antiplasmodial compounds. Subfraction on E9 (IC50 6.464 ± 1.767 μg/mL), originating from the organic extract of the stems of S. marlothii, was obtained in small quantities (29.4 mg) and was impure as revealed by LC-MS data. The aromatic region of the 1H-NMR of subfraction E9 revealed that the major bioactive compound, contain a hydroxylated, trisubstituted aromatic ring with an unsaturated side chain, most likely a caffeic acid derivative. The results obtained in this study supports the traditional use of T. sericea and A. digitata for the treatment of malaria and tuberculosis and their associated symptoms. It also supports the enrichment of extracts to expedite antimalarial drug research and recommends further purification of the crude or LLE extracts of S. marlothii for the unambiguously identification of the active compound/s which could serve as leads in antimalarial drug discovery
Description
Dissertation submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in Science (Chemistry and Biochemistry)
Keywords
Medicinal plants, Malaria, Antiplasmodial, Tuberculosis, Antimycobacterial, Lead-like enhanced extracts, Lead compounds