The fate of the cis and trans isomers of the pyrethroid insecticide cypermethrin (NRDC 149), a-cyano-3-phenoxybenzyl 3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate, has been studied in rats (1-5 mg/kg) by using three forms of radiolabeling (benzyl-14C, cyclopropyl-14C, and cyano-14C). Radioactivity derived from the benzyl-14C and cyclopropyl-14C labeling was rapidly eliminated, mostly in the urine. Tissue residues were generally very low, e.g., 0.01 pg/g in brain, with the exception of fat (~1 pg/g). Residues derived from the cis isomer tended to be higher than those derived from the trans isomer. The rate of depletion of the residues derived from [benzyl-14C] -cis-cypermethrin was rapid (tv2 was less than ~1 day) from all tissues except fat, from which radioactivity was eliminated with a half-life of [11][12] days. This residue was largely due to unchanged cis-cypermethrin. [cyono-14C]Cypermethrin afforded radioelimination and distribution characteristics similar to those reported for the cyanide ion.
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.
The disposition of the pyrethroid insecticide cypermethrin, (RS)‐a‐cyano‐3‐phenoxybenzyl (1RS)‐cis, trans‐3‐(2,2‐dichlorovinly)‐2, 2‐dimethylcyclopropane‐carboxylate, has been studied in male and female rats following a single toxic oral dose (200mg kg−1) of two radiolabelled forms ([14C‐benzyl] and [14C‐cyclopropyl]) of the insecticide. The bioaccumulation and elimination of 14C‐benzyl‐labelled cypermethrin, following repeated administration at a sub‐toxic dose (2mg kg−1), has also been studied in male and female rats. Although, at the toxic dose, radioactivity from the two radiolabelled forms was rapidly eliminated in urine and faeces, the increased excretion in the faeces, over that for low doses, was evidence that absorption was incomplete. The major pathways of metabolism involved cleavage of the ester bond, with subsequent hydroxylation and glucuronidation of the cyclopropyl acid moieties, together with hydroxylation and sulphation of the 3‐phenoxybenzyl moiety. The absence of sex‐ or dose‐dependent changes was reflected by the constant proportions of these metabolites found in the urine. Constant levels of radioactivity in tissues were achieved rapidly, generally within the first week of repeated administration. Elimination was rapid on the cessation of dosing, although less rapid from the fat and skin. The material in the fat was mainly the cis‐isomers of cypermethrin, which were eliminated with a mean half‐life of 18.2 days, compared with 3.4 days for the trans‐isomers.
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