Dermal penetration of 4″-(epi-methylamino)-4″-deoxyavermectin B1a benzoate in the rhesus monkey

The dermal absorption of the experimental avermectin insecticide emamectin benzoate was studied in the Rhesus monkey. Dermal absorption was calculated by comparing radioactivity levels in excreta following dermal application of the compound with those following administration of an equivalent intravenous dose. After iv administration of 300 μg [3H]MAB1a (prepared as a 1:1 solution of propylene glycol:saline) to three monkeys, plasma levels decreased biphasically with a rapid decline in radioactivity during the first 15 min followed by a slower decline to background. By 7 days post-dose, approximately 90% and 5% of the administered radioactivity was recovered in the faeces and urine, respectively. After a washout period, 300 μg [3H]MAB1a (dissolved in emulsifiable concentrate) was applied topically to the shaved forearm of the same monkeys. Following a 10-hr exposure period, approximately 90% of the radioactivity was recovered in a soap and water wash of the exposed forearms. Although plasma radioactivity levels generally remained below background levels, approximately 1.5% of the applied dose was recovered in the excreta. Dermal absorption of [3H]emamectin benzoate was calculated as 1.6%. The low dermal penetration of emamectin benzoate indicates that minimal actual exposure of agricultural workers to this compound will occur.