Percutaneous absorption of malathion in the guinea-pig: effect of repeated topical application.

The effects of daily repeated topical application and of washing on the percutaneous absorption of malathion have been investigated in guinea-pigs. Skin absorption was determined indirectly by measurement of radioactivity excreted into the urine following topical administration of 14C-labelled malathion, with correction of these values for incomplete renal elimination. Malathion was applied at a concentration of 5 mg/cm2 every 24 hr to the same site on the post-auricular bald area for 15 days. Doses 1, 8 and 15 were radiolabelled. The effect of multiple application and washing was assessed by Newman-Keuls multiple range test for statistical significance. The percutaneous absorption of malathion was 2-3 times higher with washing than without. There was no significant increase (P greater than 0.05) in the percutaneous absorption of malathion with repeated application without washing. These studies suggest that the total penetration of malathion resulting from daily topical dosing without daily washing may be predicted from a single-dose application to the same unwashed site at an equivalent surface concentration, and also that repeated washing with soap and water may significantly decrease the barrier function of guinea-pig skin.