Experimental methods for determining permethrin dermal absorption.
The objectives of this study were to (1) determine the percutaneous absorption of radiolabeled permethrin and piperonyl butoxide (PBO) in vivo in rats and in vitro to permit a calculation of the ratio of in vitro to in vivo values, and (2) test a method of estimating in vivo human absorption. Carbon-14 labeled permethrin in ethanol solution was applied to the clipped skin of rats in vivo at doses of 2.25, 20, or 200 ?g/cm2. As a reference compound, 14C-labeled PBO in isopropanol solution was applied to rat skin in vivo at a dose of 100 ?g/cm2. All applications were washed at 24 h postapplication, and rats were sacrificed either at 24 h for permethrin or 5 d for both compounds. The radiolabel recovered from carcass, urine including cage wash, and feces was summed to determine percent absorption. For the 24-h time point, at doses of 2.25, 20, and 200 ?g/cm2 of permethrin, values of 22, 22, and 28%, respectively, were obtained for in vivo rat percutaneous absorption (n=6 per dose). For the 5-d time point, at doses of 2.25, 20, and 200 ?g/cm2 of permethrin, values of 38, 38, and 30%, respectively, were obtained for in vivo rat percutaneous absorption (n=6 per dose). The 5-d percutaneous absorption of 14C-PBO at 100 ?g/cm2 was determined to be 42% (n=6). Dose and test duration did not exert a statistically significant effect on percutaneous absorption of permethrin in the rat in vivo. For in vitro absorption determination, 14C-permethrin in ethanol solution was applied to freshly excised human skin in an in vitro test system predictive of skin absorption in humans. Twenty-four hours after application, the radiolabel recovered from dermis and receptor fluid was summed to determine percent absorption. At doses of approximately 2.25, 20, and 200 ?g/cm2 permethrin, values of 1, 3, and 2%, respectively, were obtained for percutaneous absorption (n=9 per dose). Excised human skin absorption of 14C-PBO at 100 ?g/cm2 was determined to be 7% (n=9). Excised rat skin absorptions of permethrin at 2.25, 20, and 200 ?g/cm2 were found to be 20, 18, and 24%, respectively (n=6 per dose), approximately 10-fold higher than human skin absorption. Excised rat skin absorption of PBO was also higher (35%) than the value obtained for human skin by a factor of about 5.