Trans-pinnal movement of methimazole: an in vitro study showing that methimazole can cross from the inner to outer pinna of cats

ObjectivesThe aim of the study was to determine if methimazole applied in a transdermal formulation to the internal pinna will cross to the external pinna in an in vitro Franz cell model.MethodsThe ears from six cats were harvested soon after death. Whole ears were mounted onto Franz-type diffusion cells with the stratum corneum of the inner pinnae uppermost. A commercial transdermal preparation containing methimazole (0.1 ml/10 mg) was applied to the inner pinnae. At 1, 2, 4, 6, 8, 12, 18, 24 and 30 h, a 200 µl sample of reservoir solution was removed to determine the methimazole concentration by high-performance liquid chromatography. The ears were then dissected, separating the internal pinna from the cartilage and the external pinna, before the methimazole concentration was measured at each site. The thickness of the different regions of the ear was measured on paraffin histology sections.ResultsMean ± SD methimazole concentrations at 30 h for the right and left ear, respectively, were: inner ear, 1.2...