Human in vivo percutaneous absorption of pyrethrin and piperonyl butoxide

In order to determine the human in vivo percutaneous absorption of pyrethrin and piperonyl butoxide, a commercial formulation containing either [14C]pyrethrin (3.8 mCi/mmol) or [14C]piperonyl butoxide (3.4 mCi/mmol) was applied to the ventral forearm of six human volunteers. The formulation contained 0.3% pyrethrin and 3.0% piperonyl butoxide. Spreadability studies showed that concentrations of 5.5 micrograms pyrethrin/cm2 and 75.8 micrograms piperonyl butoxide/cm2 (used in this study) would be consistent with levels found in actual use. The forearms were thoroughly cleansed with soap and water 30 min after application (as recommended for actual use). Percutaneous absorption was determined by urinary cumulative excretion following dose application. With a 7-day urinary accumulation, 1.9 +/- 1.2% (SD) of the dose of pyrethrin and 2.1 +/- 0.6% of the dose of piperonyl butoxide applied was absorbed through the forearm skin. 1 hr after application blood samples contained no detectable radioactivity. The percutaneous absorption of pyrethrin and piperonyl butoxide from the scalp was calculated to be 7.5% of the applied dose for pyrethrin and 8.3% for piperonyl butoxide. The calculated half-life of 14C excretion was 50 hr for pyrethrin and 32 hr for piperonyl butoxide. The data should be of relevance to appropriate risk assessment in extrapolating animal data to humans.