The metabolism of the pyrethroid insecticide cypermethrin in rats; excreted metabolites

The metabolism of the pyrethroid insecticide cypermethrin has been studied in rats using three forms of 14C-labelling (benzyl-, cyclopropyl- and cyano-) and separate cis- and trans- isomers. The proportion of the dose absorbed from the intestines (50–70% at 2–3 mg kg−1) is rapidly metabolised and eliminated. The major reaction is cleavage of the ester bond to afford the constituent cis- and trans- acids which are conjugated with glucuronic acid and eliminated in the urine. The 3-phenoxybenzyl portion of the molecule is probably released as the α-hydroxynitrile, which is converted via the aldehyde into 3-phenoxybenzoic acid. This compound is then largely hydroxylated and eliminated as a sulphate conjugate. The cyanide ion is metabolised via predictable routes, for instance, as thiocyanate. Cypermethrin is hydroxylated to some extent before hydrolysis. Most of this hydroxylation occurs at the methyl group trans to the cyclopropane carboxyl group, and at the 4-position of the phenoxy group. cis- Cypermethrin is slightly more stable than the trans-isomer.