The effect of occlusion on epidermal penetration of parabens from a commercial allergy test ointment, acetone and ethanol vehicles

The efficacy of topical allergy screening systems relies on the ability of test agents to effectively penetrate the stratum corneum from applied vehic les and reach the viable cells involved in the cutaneous immune response sy stem. There is very little evidence in the dermatologic literature to justi fy the choice and suitability of vehicles used in many allergy test systems and the effectiveness of occlusion, reported to have variable effects on s olute penetration, often employed in combination with these systems. In thi s study we evaluated the in vitro human epidermal penetration of a mixture of paraben ester preservatives from a commercially available test ointment and two commonly employed solvent vehicles (acetone and ethanol), together with the effect of occlusion on the rate of delivery from these systems. Pa rabens were applied as finite doses (5 mg per cm(2)) to epidermal membranes mounted in horizontal Franz-type diffusion cells. At intervals of 2 h for a total of 10 h the receptor phase (20% ethanol in distilled water) was com pletely removed and replaced. Occlusion was effected by the placement of a piece of high density polyethylene (20 mum) over the application site immed iately after dosing. Concen- trations of parabens in receptor fluid and rem aining in the epidermis at the end of the study were determined by high-per formance liquid chromatography. There was a significant change in the epide rmal flux of parabens from each of the vehicles following occlusion. Wherea s increases were observed for the acetone and ethanol vehicles a decrease w as seen following occlusion of the ointment formulation. Changes in flux ap peared to result from a significant decrease in the epidermal partitioning of the esters following occlusion of the ointment and primarily by an incre ase in paraben epidermal diffusivity (estimated from changes in flux/retent ion) following occlusion of the solvent vehicles. These studies show that t he effects of occlusion are strongly vehicle dependent, having wide implica tions for optimization of this technique with a range of topically applied solutes.