The effect of dilution of fusidic acid cream and betamethasone dipropionate cream in complex extemporaneous mixes on formulation performance.

The Aron regimen is an unconventional therapy which entails frequent applications of an extemporaneously prepared three component system (a topical antibiotic, a corticosteroid and an emollient), with the intention of decolonising the skin of S. aureus whilst treating atopic dermatitis. The impact of heavily diluting these topical medicinal products, to differing extents, on formulation performance is not well understood thus was investigated in this study. Following a single application of a range of compounded Aron mixes (fusidic acid and betamethasone dipropionate diluted to varying extents in an emollient base), significant reductions in the expected drug flux across silicone membrane, ex vivo percutaneous absorption and skin retention of both drugs relative to the marketed products were observed. This was attributed to a number of complex formulation effects making such changes difficult to predict in a clinical setting. Further investigations are required to evaluate the impact of frequent applications of the Aron mix to widespread areas on clinical efficacy, antimicrobial resistance and long term side effects.