Structure-biodegradability relationships in pyrethroid insecticides.

The metabolism of 20 pyrethroids has been examined to evaluate the contribution of detoxification in their selective action between insects and mammals. The studies utilized living houseflies, mice, or rats, or esterase and oxidase systems derived from these organisms. Pyrethroid-hydrolyzing esterases cleave the primary alcohol trans-substituted-cyclopropanecarboxylates much faster than the corresponding cis-isomers but are ineffective in hydrolyzing secondary alcohol esters. Microsomal enzymes oxidize the (+)-trans-chrysanthemate moiety at the trans-methyl group of the isobutenyl substituent and at one of the gem-dimethyl groups whereas the (+)-cis-isomer is attacked at either of the isobutenyl methyl groups. Products isomerized at C3 of the cyclopropane are also detected but only after ester cleavage and oxidation of an isobutenyl methyl group. Each alcohol moiety has its own unique sites for oxidation involving pentadienyl, allyl, benzylic methylene, and aromatic substituents. An enhancement of insecticidal activity is expected on replacement of the biodegradable groupings with substituents relatively resistant to metabolism but this may also increase the mammalian toxicity.