Assessing the anthelmintic activity of pyrazole-5-carboxamide derivatives against Haemonchus contortus

Yaqing Jiao,Yuxiu Liu,Anson V Koehler,Jeremy N Burrows,Qingmin Wang,Andreas Hofmann,Robin B Gasser,Michael J Palmer,Katherine T Andrews,Tao Wang,Sarah Preston,Dana Hutchinson,Benoît Laleu,Timothy N C Wells,Hongjian Song,Abdul Jabbar,Jonathan Baell,Gillian M Fisher

doi:10.1186/s13071-017-2191-8

Abstract

BackgroundIn this study, we tested five series of pyrazole-5-carboxamide compounds (n = 55) for activity against parasitic stages of the nematode Haemonchus contortus (barber’s pole worm), one of the most pathogenic parasites of ruminants.MethodsIn an optimised, whole-organism screening assay, using exsheathed third-stage (xL3) and fourth-stage (L4) larvae, we measured the inhibition of larval motility and development of H. contortus.ResultsAmongst the 55 compounds, we identified two compounds (designated a-15 and a-17) that reproducibly inhibit xL3 motility as well as L4 motility and development, with IC50 values ranging between ~3.4 and 55.6 μM. We studied the effect of these two ‘hit’ compounds on mitochondrial function by measuring oxygen consumption. This assessment showed that xL3s exposed to each of these compounds consumed significantly less oxygen and had less mitochondrial activity than untreated xL3s, which was consistent with specific inhibition of complex I of the respiratory electron transport chain in arthropods.ConclusionsThe present findings provide a sound basis for future work, aimed at identifying the targets of compounds a-15 and a-17 and establishing the modes of action of these chemicals in H. contortus.

Highlights

In this study, we tested five series of pyrazole-5-carboxamide compounds (n = 55) for activity against parasitic stages of the nematode Haemonchus contortus, one of the most pathogenic parasites of ruminants
We demonstrated that exsheathed third-stage larvae (xL3) exposed to tolfenpyrad consumed significantly less oxygen than unexposed xL3s, which was consistent with a specific inhibition of complex I of the respiratory electron transport chain in the mitochondrion
Subsequent assays using xL3s of H. contortus showed that the potency of these two test compounds at 72 h of incubation (IC50 values = 55.63 ± 0.18 μM and 51.60 ± 1.41 μM, respectively) was considerably less than tolfenpyrad (IC50 value = 3.05 ± 0.47 μM) (Fig. 2; Table 1)

Summary

Introduction

We tested five series of pyrazole-5-carboxamide compounds (n = 55) for activity against parasitic stages of the nematode Haemonchus contortus (barber’s pole worm), one of the most pathogenic parasites of ruminants. Synthetic pyrazole-5-carboxamide derivatives, such as tebufenpyrad and tolfenpyrad, are important pesticides which are recognized to inhibit complex I of the mitochondrial electron transport (respiratory) chain [1, 2]. Jiao et al Parasites & Vectors (2017) 10:272 compound has an exquisite in vitro activity against parasitic stages of the barber’s pole worm, Haemonchus contortus (Nematoda: Strongylida) [13]. We demonstrated that xL3s exposed to tolfenpyrad consumed significantly less oxygen than unexposed xL3s, which was consistent with a specific inhibition of complex I of the respiratory electron transport chain in the mitochondrion (cf [1]). In vitro cytotoxicity data indicated that tolfenpyrad is ≥ 18-fold more selective for H. contortus than a mammalian cell line [13], raising the possibility of repurposing this agent against (at least some) parasitic nematodes and/or hit-to-lead optimisation

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Results

Discussion

Conclusion