The chemotherapeutic effects of synthetic and natural compounds
Date
2015
Authors
Motau, Tshegofatso Harold
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Abstract
Plasmodium falciparum remains the most virulent cause of malaria. With increasing drug
resistance to artemisinin and other antimalarial drugs, combined with an absence of an
effective vaccine, there’s a critical need for new agents to complement existing treatment and
prophylaxis. Therefore, the aim of the study was to evaluate the in vitro antimalarial activity
and potential toxicity to mammalian cells of select synthetic and natural colourants,
nucleoside and imidazo[1,2a]pyridine (IP) analogues on the erythrocytic stages of the 3D7
chloroquine-sensitive strain of P. falciparum. The P. falciparum 3D7 strain was maintained in
vitro according to standard methods. Quinine and chloroquine were used as positive controls.
The tritiated hypoxanthine incorporation assay was used for evaluating the ability of test
compounds to inhibit the growth of P. falciparum. Active test compounds were tested in
combination studies with quinine. Uninfected human red blood cell (RBC) toxicity was
analysed spectrophotometrically. The ability of test compounds to inhibit -haematin
formation, a metabolic pathway that sequesters toxic haem within the parasites, was
determined. Cytotoxic activity of active compounds was evaluated on two human cell lines
(HEK293 and K562) using the [3H]-thymidine incorporation assay. Data was analysed using
the one-way ANOVA test and reported as the mean ± standard deviation of at least triplicate
experiments and significant difference when p < 0.05. Of the 56 compounds tested, the
synthetic colourants showed the most potent antimalarial activity. Methylene blue and
safranin O were most potent with IC50 values of 4.19 ± 0.16 nM and 86.50 ± 2.61 nM,
respectively, compared to quinine (IC50: 103.90 ± 8.30 nM), and displayed negligible toxicity
to uninfected human RBCs. Combination studies with methylene blue and quinine
demonstrated a synergistic interaction. Methylene blue also demonstrated the highest
selectivity indices (480 and 968) compared to quine (180). Curcumin (diferuloylmethane), a
natural extract was active (IC50: 2.29 ± 0.18 μg/ml) against P. falciparum, but significantly (p
< 0.05) less potent than quinine. Curcumin was 78-fold more active in inhibiting -haematin
formation than quinine, indicating of a possible mechanism of action. The most active
nucleoside analogue, JLP118.1 (IC50: 1.79 ± 0.12 μM), demonstrated inhibitory activity
against the trophozoite stage of P. falciparum. The imidazopyridine analogue, IP-4, displayed
the least potent antimalarial activity (IC50: 15.3 ± 0.41 μM) of the synthetic compounds
tested, with low selectivity indices < 1. The study has confirmed the potent antimalarial
activity and relative safety of methylene blue as well as its potential as an antimalarial drug.
The nucleoside and imidazopyridine analogues showed promising activity and with structural
modification their potency and selectivity indices may be enhanced.
Description
A dissertation submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg,in fulfillment of the requirements for the degree of Master of Science in Medicine. Johannesburg, South Africa, 2015