Indoxacarb biotransformation in the German cockroach

- Indoxacarb undergoes hydrolase-based bioactivation to DCJW in the German cockroach.
- Indoxacarb and DCJW are metabolized to hydroxy and oxadiazine ring-opened compounds.
- Hydroxylated ring-opened metabolites were also detected for indoxacarb and DCJW.
- Formation of hydroxy and ring-opened metabolites is NADPH-/P450-dependent.
- Resistance in field strains is associated with metabolic and target site factors.

Insecticides that are used for pest control undergo physical and biological (enzymatic) degradation. Indoxacarb is an oxadiazine class sodium channel blocker insecticide used for German cockroach (Blattella germanica L.) control. At present, no information is available on enzymatic biotransformation or metabolism of indoxacarb in this important urban pest. We studied the biotransformation pathways of indoxacarb in one susceptible and three field strains with varying susceptibility levels using liquid chromatography and high-resolution mass spectrometry. As shown in other insect species we found evidence for hydrolase-based bioactivation of indoxacarb to a toxic decarbomethoxylated metabolite, DCJW. In addition, both indoxacarb and DCJW were further metabolized to hydroxy, oxadiazine ring-opened and hydroxylated ring-opened metabolites. In general, higher indoxacarb disappearance, increased formation of DCJW and the above-mentioned metabolites were observed in the three field strains. In vitro biotransformation studies showed that hydroxylated and oxadiazine ring-opened metabolite formation is NADPH/cytochrome P450-dependent. Bioassays and in vivo metabolism experiments using the enzyme-inhibiting insecticide synergists, piperonyl butoxide (PBO) and S,S,S,-tributyl phosphorotrithioate (DEF), provided insights into potential indoxacarb resistance mechanisms that may proliferate in German cockroach field strains following unchecked selection pressures. The information presented here is an essential step toward developing indoxacarb resistance management programs and also reveals mechanisms of secondary/tertiary indoxacarb toxicity.