Synergism of rotenone by piperonyl butoxide in Haemonchus contortus and Trichostrongylus colubriformis in vitro: Potential for drug-synergism through inhibition of nematode oxidative detoxification pathways

The anthelmintic properties of rotenone and its activity in combination with the cytochrome P450 inhibitor piperonyl butoxide, were examined in in vitro assays with adults and larvae of Haemonchus contortus and larvae of Trichostrongylus colubriformis. Rotenone was toxic to larvae of both species, with LC50 values in larval development assays of 0.54 and 0.64 μg/ml for H. contortus and T. colubriformis, respectively. The compound also caused complete cessation of movement in adult H. contortus after 72 h at a concentration of 20 μg/ml. Toxicity of rotenone towards the larvae of both species was increased in the presence of piperonyl butoxide (synergism ratios of 3–4-fold at the LC50) and the activity against adult H. contortus was also significantly enhanced following pre-treatment with piperonyl butoxide. This significant synergism suggests that these nematode species are able to utilize a cytochrome P450 enzyme system to detoxify rotenone and indicates that a role may exist for cytochrome P450 inhibitors to act as synergists for other anthelmintics which are susceptible to oxidative metabolism within the nematode.