Abstract
Penetration into the nervous tissues and internal distribution of the pyrethroid insecticide α-racemic fenvalerate (R, S) α-cyano-3-phenoxybenzyl (S)-2-(4-chlorophenyl)-isovalerate, and its inactive optical isomer-β-racemic fenvalerate, (R, S) α-cyano-3-phenoxybenzyl (R)-2-(4-chlorophenyl)-isovalerate, were studied using the German cockroach Blatella germanica. The internal concentrations of 14C-labelled α-racemic fenvalerate and β-racemic fenvalerate at various times after topical application were measured in the head, hemolymph, nerve cord, fat body, and gut. At 3-6hr after application, insects showed considerable intoxication by α-racemic fenvalerate but not by β-racemic fenvalerate. The amounts of α-racemic fenvalerate in the head and hemolymph were larger than those of β-racemic fenvalerate at all time intervals. The internal concentrations of α-racemic fenvalerate and β-racemic fenvalerate also differed in the gut, nerve cord, and fat body at 3-6hr after application. However, in the latter case the amount of α-racemic fenvalerate was smaller than that of β-racemic fenvalerate. A clear difference in the time constant of penetration into the ventral nerve cord in vitro between the two optical isomers was observed, i.e. α-racemic fenvalerate showed more rapid penetration than β-racemic fenvalerate and the amount of α-racemic fenvalerate was larger than that of β-racemic fenvalerate. Results obtained in this experiment showed that there is a clear difference in pharmacokinetic behavior between α-racemic fenvalerate and β-racemic fenvalerate. However this difference seems to be only a minor factor in the overall difference of their respective bioactivities.
