Therapeutic Efficacy of Natural Products Towards Protozoan Diseases

The different infections caused by protozoan parasites, such as malaria, leishmaniasis, toxoplasmosis, balantidiasis, trichomoniasis, giardiasis, Chagas disease, amoebic dysentery, are responsible for considerable morbidity and mortality worldwide with desolating social and economic consequences. These protozoan diseases occur all over the world. For the treatment of these diseases, there is a lack of effective, safe, and affordable therapies. Due to the lack of vaccines in most instances and the development of resistant strains to the available synthetic therapeutics, it is important to search for alternative sources of anti-parasitic drugs. Since ancient times, natural products have been used as sources of potential drugs to cure diseases. It has been reported that 80% of drug molecules are natural products. The diversity of natural products can vary, i.e., it may be found in plants, fungi, algae and marine organisms. The plant-based natural products (secondary metabolites), i.e., alkaloids, phenolics, terpenes, and lipids, are potent anti-protozoal molecules. The natural products (secondary metabolites) obtained from microbial origin showed promising anti-protozoal activity. These bio-active molecules 2-(hept-1-enyl)-3-(hydroxymethyl)- 5-(3-methyl but-2-enyl)benzene-- ,4-diol, flavoglaucin, tetrahydroauroglaucin, auroglaucin, 2-(20,3-epoxy-10- 30-heptadienyl)-6-hydroxy-5-(3-methyl-2-butenyl)benzaldehyde, obtained from the fungus Eurotium repens, showed anti-malarial activities even chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. Some of the flavonoid compounds, i.e., eupatilin, jaceosidin and nepetin, isolated from the plant Eupatorium arnottianum, showed the highest activity against Chagas disease. The three most important flavonoids, namely kaempferol, (–)-epicatechin and tiliroside showed promising activity against Entamoeba histolytica. The isoquinoline alkaloid berberine is found in several medicinal plants. Berberine salts have profound inhibitory activity against Giardia trophozoites. Two flavonoids, i.e., luteolin and quercetin, isolated from Vitex negunsdo and Fagopyrum esculentum, showed anti-leishmanial activity. An aclerodane diterpene isolated from an ethanolic extract of Polyalthia longifolia displayed anti-leishmanial activity against Leishmania donovani. A novel triterpene Astrakurkurone isolated from the wild edible mushroom, Astraeus hygrometricus, has a definitive effect on promastigote and amastigote form both in vitro and in vivo against L. donovani. Natural products have displayed promising activity against different protozoan infections, but most of these studies on natural products have been performed in vitro only. The transitions from in vitro study to in vivo trials and also the clinical trials of the new compounds are urgently required to prove their efficacy and safety.

Keywords: Alkaloids, Amoebic dysentery, Anti-malarial, Anti-amoebic, Antigiardial, Antileishmanial, Flavonoids, Giardiasis, Leishmaniasis, Malaria, Marine organisms, Microbial origin, Natural products, Phenolics, Plant origin, Secondary metabolites, Terpenes.