Activity of methylgerambullin from Glycosmis species (Rutaceae) against Entamoeba histolytica and Giardia duodenalis in vitro

Abstract

Entamoeba histolytica and Giardia duodenalis are widespread intestinal protozoan parasites which both spread via cysts that have to be ingested to infect a new host. Their environment, the small intestine for G. duodenalis and the colon for E. histolytica, contains only very limited amounts of oxygen, so both parasites generate energy by fermentation and substrate level phosphorylation rather than by oxidative phosphorylation. They both contain reducing agents able to reduce and activate nitroimidazole drugs such as metronidazole which is the gold standard drug to treat Entamoeba or Giardia infections. Although metronidazole works well in the majority of cases, it has a number of drawbacks. In animal models, the drug has carcinogenic activity, and concerns about a possible teratogenic activity remain. In addition, the treatment of G. duodenalis infections is hampered by emerging metronidazole resistance. Plant-derived drugs play a dominant role in human medicine, therefore we tested the activity of 14 isolated plant compounds belonging to seven different classes in vitro against both parasites. The tests were performed in a new setting in microtiter plates under anaerobic conditions. The compound with the highest activity was methylgerambullin, a sulphur-containing amide found in Glycosmis species of the family Rutaceae with an EC₅₀ of 14.5 μM (6.08 μg/ml) after 24 h treatment for E. histolytica and 14.6 μM (6.14 μg/ml) for G. duodenalis. The compound was successfully synthesised in the laboratory which opens the door for the generation of new derivatives with higher activity.

Keywords: Aglafoline; Entamoeba histolytica; Giardia duodenalis; Glycosmis spp.; Methylgerambullin; Sulphur-containing amide.

Graphical abstract

Fig. 1

The 14 compounds tested against E. histolytica and G. duodenalis in this study.

Fig. 2

Chemical synthesis of methylgerambullin from commercially available compounds.

Fig. 3

A.Entamoeba histolytica was cultivated for 24 h with different concentrations of methylgerambullin (MG) in medium with different cysteine concentrations and the number of surviving trophozoites was counted. B. The same experiment was performed for G. duodenalis.

Fig. 4

Predicted structures of the sulphur-containing amides as generated by the Molinspiration Galaxy 3D Structure Generator (www.molinspiration.com).

Fig. 5

Proposed reaction scheme for the reaction of methylgerambullin with cysteine.