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J. Cosmet. Sci., 55, 65-80 CTanuary/February 2004) Targeted delivery of salicylic acid from acne treatment products into and through skin: Role of solution and ingredient properties and relationships to irritation LINDA RHEIN, BHASKAR CHAUDHURI, NUR JIVANI, HANI FARES, and ADRIAN DAVIS, GlaxoSmithKline Consumer Healthcare, Parsippany, NJ (L.R., H.F., A.D.), and Bertek Corporation, Foster City, CA (B.C., NJ.). Accepted for publication October 18, 2003. Synopsis Salicylic acid (SA) is a beta hydroxy acid and has multifunctional uses in the treatment of various diseases in skin such as acne, psoriasis, and photoaging. One problem often cited as associated with salicylic acid is that it can be quite irritating at pH 3-4, where it exhibits the highest activity in the treatment of skin diseases. We have identified strategies to control the irritation potential of salicylic acid formulations and have focused on hydroalcoholic solutions used in acne wipes. One strategy is to control the penetration of SA into the skin. Penetration of the drug into various layers of skin, i.e., epidermis, dermis, and receptor fluid, was measured using a modified Franz in vitro diffusion method after various exposure times up to 24 hours. A polyurethane polymer (polyolprepolymer-15) was found to be an effective agent in controlling delivery of SA. In a dose-dependent fashion it targeted delivery of more SA to the epidermis as compared to penetration through the skin into the receptor fluid. It also reduced the rapid rate of permeation of a large dose of SA through the skin in the first few hours of exposure. A second strategy that proved successful was incorporation of known mild nonionic surfactants like isoceteth-20. These surfactants cleanse the skin, yet clue to their inherent mildness (because of their reduced critical micelle concentration and monomer concentration), keep the barrier intact. Also, they reduce the rate of salicylic acid penetration, presumably through micellar entrapment (either in solution or on the skin surface after the alcohol evaporates). Cu­ mulative irritation studies showed that targeting delivery of SA to the epidermis and reducing the rapid early rate of penetration of large amounts of drug through the skin resulted in a reduced irritation potential. In vivo irritation studies also showed that the surfactant system is the most important factor controlling irritancy. SA delivery is secondary, as formulations with less SA content reduced the rate of delivery to the receptor and yet were some of the most irritating formulations tested, presumably clue to the action of the specific anionic surfactant on the barrier. Alcohol content also did not appreciably affect irritation and SA delivery formulations with considerably lower alcohol content but containing anionic versus nonionic surfactant systems exhibited considerably higher irritancy. Thus the surfactant type was again the predomi­ nant factor in those studies, although arguably alcohol plays some role (solubilization of SA). Results showed that both polymers and mild surfactants work in concert to provide the optimal formulation benefits of targeted delivery and reduced irritation. Synergistic relationships among hydroalcoholic formulation com­ ponents will be discussed along with the mechanisms likely involved in controlling delivery of SA to skin. INTRODUCTION Various strategies are being used to control delivery of active substances to the skin. The use of polymers to control the delivery of actives from semisolid preparations has 65