Anthelmintic effects of tannin-rich plants and oligomeric proanthocyanidin clusters against parasitic and free-living nematodes

Based on the results of an ethnopharmacological study in Ghana, a leaf extract (EtOH 50%) of Combretum mucronatum was investigated by bioassay-guided fractionation which resulted in condensed tannins as the active compounds. Further components identified were flavonoids, including a biflavanoid with an unusual biphenyl linkage. Therefore, extracts from other tanniferous plants (Paullinia pinnata roots (EtOH 50%; acetone), Rhododendron ferrugineum leaves (H₂O), Rumex acetosa herb (ac. 70%) and Crataegus spp. leaves (EtOH 45%) were included in this follow-up study. Extracts (1 – 1000 µg/mL) and purified oligomeric procyanidin (OPC) clusters (1 – 1000 µM) with a degree of polymerization from 2 to 10 showed a lethal activity against larval stages of Toxocara cati and Trichuris vulpis. The effect was dependent on the molecular size of the compound, with the exception of one tetrameric cluster mainly composed of Procyanidin D1 (LC₅₀ 18 µM). Extracts from P. pinnata (ac.) and C. mucronatum were most active with LC₅₀ 22 µg/mL and LC₅₀ 46 µg/mL. Levamisole (1 mM) served as a positive control. The best selectivity was shown by the Crataegus extract: LC₅₀ 179 µg/mL vs. a low cytotoxicity against Caco-2 cells (IC₅₀ 918 µg/mL). The stability of the active compounds during a simulated gastric passage (pH 2, 37 °C, 2h) was confirmed using C. mucronatum extract followed by an anthelmintic assay against Caenorhabditis elegans. The worms' response to xenobiotic and oxidative stress during the incubation with a flavonoid-rich fraction from C. mucronatum was investigated in transgenic C. elegans expressing a glutathione-S-transferase-4-GFP transcriptional fusion, but no increase in the expression was observed. Microscopic observations of dead larvae of T. cati revealed disruptions of the cuticle. In summary, OPCs exert a strong in vitro anthelminthic activity which is currently being further investigated in controlled animal studies as well as concerning the underlying molecular mechanism.