Influence of drug content, type of semi-solid vehicle and rheological properties on the skin penetration of the model drug fludrocortisone acetate

Throughout Europe, topical creams containing corticosteroids are diluted with various neutral cream bases to meet the specific needs of patients. Even though this practice has been common for years, its effect has not been thoroughly investigated and so the effectiveness of the diluted topical steroidal creams is difficult to predict. In the present study, the model drug fludrocortisone acetate was incorporated into three cream bases of different hydrophilicity that are commonly used in Austria. Different final drug concentrations were chosen for comparative studies. Additionally, a semi-solid preparation developed by our group was investigated for comparison. These formulations were tested in diffusion and tape stripping experiments. Diffusion cell studies showed that changes in drug concentration do not necessarily change the skin permeation behaviour in vitro. The tape stripping protocol was successfully optimised for investigation of semi-solid preparations to provide reproducible and accurate results despite the challenges of investigating semi-solid formulations. The results showed that tape stripping experiments are more suitable to elucidate subtle differences between formulations. The composition of the cream bases exhibited stronger effects on the skin penetration of the steroidal drug irrespective of its concentration than the rheological properties. No correlation between formulation viscosity and skin penetration was found.