Inhibition of glutathione S-transferases (GSTs) from parasitic nematodes by extracts from traditional Nigerian medicinal plants

Piliostigma thonningii, Ocimum gratissimum, Nauclea latifolia and Alstonia boonei are used in Nigerian traditional medicines against gastrointestinal helminths of animals and man. Proanthocyanidins were detected in Piliostigma and Nauclea, but not Alstonia or Ocimum. Extracts of these plants killed 50% of brine shrimp nauplii at <10 ppm (Nauclea), 100 ppm (Piliostigma) and <1000 ppm (Ocimum and Alstonia), the Nauclea LD50 being similar to the anthelmintic drug piperazine. Extracts were also toxic to the parasitic nematode Haemonchus infective L3 stage. Nematode glutathione-S-transferases (GSTs) are potential drug targets. Apart from Alstonia all the medicinal plants contained heat-stable inhibitory activities against recombinant Ascaris and Onchocerca GSTs in vitro. Piliostigma, Ocimum and Nauclea had IC50s of 2, 10 and 15 µg/mL respectively for Ascaris GST and 4, 8, 28 µg/mL respectively for Onchocerca GST. We suggest that the inhibitory properties of some of these Nigerian plant extracts against GST may contribute to the pharmacological basis of their efficacy against helminths in traditional herbal use.