Skin Distribution of Imidacloprid by Microautoradiography After Topical Administration to Beagle Dogs

To investigate the cutaneous distribution, localization, and persistence of imidacloprid in dogs, Advantage Topical Solution labeled with carbon 14 (C-14) was topically applied as a single treatment at label rates and application pattern based on body weight to two adult beagles. One clog (8.5 kg) received 1.0 mL of the test solution at a single spot in the interscapular area (14 mg active ingredient/kg body weight); the second dog (12.3 kg) was treated with 2.5 mL of the test solution at four sites, each site receiving approximately 0.625 mL, along the dorsal thoracic and lumbar spine area (21 mg active ingredient/kg body weight). Samples of hair, skin surface residue, and skin taken from the application sites and/or distal body regions of the dogs at four intervals between 7 and 56 clays after treatment demonstrated the migration of C-14 radioactivity from the application sites to distal areas of the canine haircoat and skin. The C-14 radioactivity concentrations in the skin biopsy and stratum corneum samples diminished steadily over 56 days after treatment. Microautoradiography of the skin showed focal concentrations of radioactivity in the superficial epidermis, hair follicles, and sebaceous glands. The presence of imidacloprid-derived radioactivity within hair follicles and sebaceous glands and on the skin surface is in good agreement with the reported efficacy of imidacloprid against fleas on dogs and cats for up to 1 month despite posttreatment bathing, shampooing, and/or swimming.