Detoxification of ivermectin by ATP binding cassette transporter C4 and cytochrome P450 monooxygenase 6CJ1 in the human body louse, Pediculus humanus humanus

We previously observed that ivermectin-induced detoxification genes, including ATP binding cassette transporter C4 (PhABCC4) and cytochrome P450 6CJ1 (CYP6CJ1) were identified from body lice following a brief exposure to a sublethal dose of ivermectin using a non-invasive induction assay. In this current study, the functional properties of PhABCC4 and CYP6CJ1 were investigated after expression in either X. laevis oocytes or using a baculovirus expression system, respectively. Efflux of [H-3]-9-(2-phosphonomethoxyethyl) adenine ([H-3]-PMEA), a known ABCC4 substrate in humans, was detected from PhABCC4 cRNA-injected oocytes by liquid scintillation spectrophotometric analysis and PhABCC4 expression in oocytes was confirmed using ABC transporter inhibitors. Efflux was also determined to be ATP-dependent. Using a variety of insecticides in a competition assay, only co-injection of ivermectin and dichlorodiphenyltrichloroethane led to decreased efflux of [H-3]-PMEA. PhABCC4-expressing oocytes also directly effluxed [H-3]-ivermectin, which increased over time. In addition, ivermectin appeared to be oxidatively metabolized and/or sequestered, although at low levels, following functional expression of CYP6CJ1 along with cytochrome P450 reductase in Sf9 cells. Our study suggests that PhABCC4 and perhaps CYP6CJ1 are involved in the Phase III and Phase I xenobiotic metabolism of ivermectin, respectively, and may play an important role in the evolution of ivermectin resistance in lice and other insects as field selection occurs.