The novel isoxazoline ectoparasiticide fluralaner: Selective inhibition of arthropod γ-aminobutyric acid- and l-glutamate-gated chloride channels and insecticidal/acaricidal activity

Highlights

• Identification and functional expression of the acaricide target genes rdl and glucl from cattle tick.

• Insecticide/acaricide pharmacology studies on RDL from cat flea and fruit fly as well as RDL/GluCl from cattle tick.

• Characterization of the novel ectoparasiticide fluralaner as highly potent insect and acarid RDL and GluCl inhibitor.

• Superior insecticidal and acaricidal activity of fluralaner compared to fipronil, dieldrin, imidacloprid and deltamethrin.

Abstract

Isoxazolines are a novel class of parasiticides that are potent inhibitors of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls) and l-glutamate-gated chloride channels (GluCls). In this study, the effects of the isoxazoline drug fluralaner on insect and acarid GABACl (RDL) and GluCl and its parasiticidal potency were investigated. We report the identification and cDNA cloning of Rhipicephalus (R.) microplus RDL and GluCl genes, and their functional expression in Xenopus laevis oocytes. The generation of six clonal HEK293 cell lines expressing Rhipicephalus microplus RDL and GluCl, Ctenocephalides felis RDL-A₂₈₅ and RDL-S₂₈₅, as well as Drosophila melanogaster RDLCl-A₃₀₂ and RDL-S₃₀₂, combined with the development of a membrane potential fluorescence dye assay allowed the comparison of ion channel inhibition by fluralaner with that of established insecticides addressing RDL and GluCl as targets. In these assays fluralaner was several orders of magnitude more potent than picrotoxinin and dieldrin, and performed 5-236 fold better than fipronil on the arthropod RDLs, while a rat GABACl remained unaffected. Comparative studies showed that R. microplus RDL is 52-fold more sensitive than R. microplus GluCl to fluralaner inhibition, confirming that the GABA-gated chloride channel is the primary target of this new parasiticide. In agreement with the superior RDL on-target activity, fluralaner outperformed dieldrin and fipronil in insecticidal screens on cat fleas (Ctenocephalides felis), yellow fever mosquito larvae (Aedes aegypti) and sheep blowfly larvae (Lucilia cuprina), as well as in acaricidal screens on cattle tick (R. microplus) adult females, brown dog tick (Rhipicephalus sanguineus) adult females and Ornithodoros moubata nymphs. These findings highlight the potential of fluralaner as a novel ectoparasiticide.