386 JOURNAL OF COSMETIC SCIENCE pathologies such as sunburns, photosensitivity, phototoxicity, actinic elastosis, photo­ aging, immunosuppression, and skin cancer (2,5). In order to combat these detrimental effects of solar radiation, sunscreen has been utilized for decades as a means of protection (6,7). However sunscreen's effectiveness is not optimal due to the numerous factors affecting the delivery of the drugs and cosmetics into the skin from topically applied formulations. These factors include the physico­ chemical properties of the drug, the size of the molecule, the lipophilicity of the components in the vehicle, the type of the formulation and vehicle, the presence of penetration enhancers, and the physical state of the stratum corneum (8-10). For a sunscreen to be effective, the UV absorbers must remain in the outermost region of the skin. An ideal sunscreen product should exhibit high skin accumulation of UV absorbers with minimal permeation to the circulation (5,11). Therefore, to be efficient and to avoid toxicity, sunscreens should stay on the skin surface and penetrate minimally through the skin. However, it has been demonstrated that penetration into the skin, permeation through the skin, and retention of UV filters in the skin from topical products can differ significantly among formulations (11,12). Treffel and Gabard (7) showed that sunscreen agents were better retained in the stratum corneum in an emul­ sion-type formulation rather than in petrolatum jelly. In addition, recent studies have demonstrated that sunscreens are absorbed systemically following topical application to the skin (12-14). Since sunscreen products are generally applied to the skin superficially, their effective­ ness is determined by how the product adheres to the skin as a protective film. Fur­ thermore, sunscreens should also have a high affinity for the stratum corneum (12,15). Thus the sunscreen's vehicle can affect the solubility properties of a solute and therefore influence the percutaneous absorption that may enhance or inhibit the movement of the UV filter through the skin (13,16). Since the degree of penetration depends strongly on the physicochemical properties of the active compounds and the nature of the vehicle (9,17), the development of suitable products that prevent penetration of the sunscreen into the skin is a challenge for manufacturers of cosmetic products (17). Liposomes, small vesicles composed of phospholipids, have been used for years to bring active ingredients into the skin (10,18-20). Several factors, such as the physicochemical properties of the drug and other ingredients present in the liposomal product, lamel­ larity, lipid composition, charge on the liposomal surface, the size of the liposomes, the vehicle, the mode of application, and the total lipid concentration have been proven to influence drug deposition into the skin layers (8, 10,21). It has been demonstrated that liposomes can cross the stratum corneum and act as microreservoirs from which drugs may be slowly released (9, 19,22). Liposomes can provide a drug-delivery system that delivers several-fold higher drug concentrations into the skin with lower systemic ab­ sorption compared to conventional dosage forms (10,18,19,23). Another advantage of a liposome-based controlled-released system is that less drug needs to be administered for comparable efficacy. Thus, the probability of systemic absorption and consequent ad­ verse drug reactions is reduced (9,20). There are also studies that have shown that liposomes can enhance the penetration of drugs into the skin or enhance the transdermal flux of drugs, but at the same time some reports also claim a lack of this effect (8,10,24,25). Even though most of the time reducing the size of liposomes enhances the skin penetration, in some cases liposomes with small sizes have shown less penetration into the skin, depending on the drugs incorporated (26).

PERCUTANEOUS ABSORPTION OF OMC 387 In this study we incorporated OMC, as a UV absorber sunscreen, into liposomes. The aim of our work was to establish a comparison between the skin permeation of one o/w lotion vehicle versus the liposomal formulations and also to determine the effect of liposome size reduction on the skin percutaneous absorption. Meanwhile, the SPF of the formulations was determined by an in vivo method in human volunteers, to evaluate the actual efficacy of OMC formulations as a potentially more efficient sunscreen. MATERIALS AND METHODS REAGENTS AND CHEMICALS OMC, cholesterol, and vitamin E were purchased from Merck (Darmstadt, Germany). Lanolin, white petrolatum, stearic acid, propylparaben, methylparaben, disodium EDTA, propylene glycol, and triethanolamine were purchased from Sigma (USA). Soya phosphatidylcholine (Soya PC) was obtained from Avanti Polar Lipids (Alabaster, Ala­ bama, USA). Phosphate-buffered saline (PBS) ingredients were supplied from Sigma (USA). All solvents used in this study were high-performance liquid chromatography (HPLC) grade. All chemicals were of the purest grade available. PREPARATION OF OIL/WATER EMULSION CONTAINING OMC OMC (insoluble in water, 290.40 g/mol, 7 .5%), lanolin (5%), white petrolatum (2.5%), stearic acid (4%), and propylparaben (0.05%) were mixed and heated at 77°-82°C until all the ingredients were melted and dissolved (oil phase). Methylparaben (0.1 %), diso­ dium EDTA (0.05%), and propylene glycol (5%) were dissolved in PBS (pH= 7.2, 75.8%) using indirect heat (77°-82°C) (aqueous phase). The aqueous phase was added to the oil phase at 77°-82°C, and the mixture was stirred until it cooled down to room temperature (27). PREPARATION OF LIPOSOMES CONTAINING OMC Multilamellar liposomes containing OMC were prepared by the fusion method (28). Briefly, the lipid components consisted of Soya PC (15%), cholesterol (2%), vitamin E (0.3%), and propylparaben (0.05%). They were melted in propylene glycol at 77°-82°C (lipid melt). When the lipid melt cooled down to 50°C, OMC (7 .5%) was added and mixed completely. PBS (up to 100%) and methylparaben (0.1 %) were heated separately at 5 5 ° C and added to the previously heated (50°C) lipid melt and vigorously stirred until it cooled down to room temperature. PREPARATION OF SUV LIPOSOMES CONTAINING OMC SUV liposomes were prepared by probe sonication (Soniprep 150 Ultrasonic, England) of ML Vs containing OMC. The probe sonication was performed at 4°C (on ice), at 20 cycles, with 15 seconds of sonication separated by intervals of 15 seconds (29).