PERCUTANEOUS ABSORPTION OF OMC 395 hydrophobic interior of the liposome bilayer due to OMC's hydrophobic characteristics. This could be yet another reason for the high encapsulation efficiency of OMC in liposomes, since hydrophobic material in the right liposomal formulation could poten­ tially have encapsulation efficiency nearing 100 percent. Soybean PC was used for liposome preparations. Soybean PC contains polyunsaturated fatty acids like linoleic acid, which are beneficial for healthy skin. Furthermore, formu­ lations prepared by soybean PC increase the skin's humidity (40). In the liposome formulation cholesterol was included to stabilize the lipid bilayer and decrease the leakage of encapsulated drugs and vesicle aggregation (21,29). In this study, the SPF of the homosalate reference, the CO LIP A standard, ML Vs con­ taining OMC, and the o/w emulsion were analyzed. The results indicated that the SPF of the liposomes containing OMC was a little bit greater than that of OMC lotion at the same concentration. Ramon et al. (41) have also shown that liposomes could be regarded as alternatives to conventional o/w emulsions in the formulation of lipidic sun filters. They showed that when liposomes with a composition and structural organization similar to that of the stratum corneum lipids are used, skin penetration is retarded. There are different in vitro and in vivo techniques for the evaluation of skin permeation (16,17,42). In this study, the amount of OMC was investigated by a tape stripping method that is frequently used to investigate the penetration of topically applied sub­ stances into the skin (6,7 ,13,36). This method is a practical technique to selectively remove the stratum corneum of the skin and allows direct quantification of UV filters in the skin surface (15). Rougier et al. (43) established a linear relationship between stratum corneum reservoir content and percutaneous absorption using the standard urinary excretion method. The data obtained from this in vivo penetration study show that the ML V formulations delivered significantly higher amounts of OMC to the stratum corneum (total number of tape strips removed) than conventional lotion and SUVs. Since the intercellular lipids are important in controlling the percutaneous absorption, MLVs may mix with the intercellular lipids and cause their swelling without altering the multiple bilayer struc­ ture of the stratum corneum, and can produce an extra lipid barrier in the skin (8). Thus, MLV liposomes are able to provide a sustained release carrier system and act as a reservoir for OMC therefore, the sunscreen remains longer on the outermost layers of the skin. This property is important for sunscreens because the amount of the sunscreen agent inside the stratum corneum has a direct relationship with its sun protection value (7, 12), which has been confirmed by several other studies (10,18,19,23). In these studies, in vivo experiments have shown that liposomal encapsulation of drugs produces several-fold higher concentrations in the epidermis and dermis, with lower systemic concentrations compared to those in the conventional dosage forms (gel, lotion, and ointment) (10,18,19,23). The results support the possibility of developing products utilizing the liposomal dosage form that are superior to products utilizing the existing dosage forms for topical therapy (44). From the results obtained in our study, it was also demonstrated that higher amounts of OMC were recovered from the upper layers of the stratum corneum than from the deeper layers, with all formulations. It may be concluded that cutaneously applied free OMC is able to penetrate into the skin, but that with ML Vs the initial penetration into the upper layers improves OMC's permanence on the skin and the amount remaining in

396 JOURNAL OF COSMETIC SCIENCE the upper surface of the skin. Other studies also confirm our observation that the concentration of the drugs is higher in the external layers and lower in the deeper layers of the stratum corneum when used in liposomal formulations (6,38). It is very interesting to note that the results obtained show that after 3 hours, the amount of OMC in the first strip group (strips 1-2) in the ML V formulation was lower than in the second strip group however, the total amount of OMC in the ML V for­ mulation was greater than in the other formulations. This could be explained by the fact that phospholipids in liposomes may mix with the intercellular lipids in the stratum corneum. Thus the intact liposomes are reduced in the first group of strips. However, in the subsequent groups of strips, the liposome size can affect skin penetration. This can cause liposome penetration to the surface of the skin and the accumulation effect in the stratum corneum, while conventional lotion may remain on the surface without pen­ etration to the skin. Although a higher amount of OMC is available in the first strip group in the o/w emulsion, this is not desirable, as it can easily be washed off from the skin surface. Our findings show that the SUVs penetrated into the skin significantly more than the OMC entrapped in the ML Vs and the conventional o/w emulsion, which could be due to the small sizes of these liposomes (mostly less than 100 nm). There are some studies that demonstrate that liposomes with small sizes can penetrate better into the skin (8,10,26), but conversely, there are some other reports that also claim a lack of this effect (39,43). Verma et al. (8) indicated that liposomes with a 120-nm diameter showed statistically enhanced penetration of carboxyfluoresein into the skin as compared to larger ones. It was observed in the Michel et al. (26) study that for tocopheryl nicotinate, there is a 1.5-fold greater amount of drug penetration with the small SUVs as compared to the larger Ml Vs. In contrast, in the same study they demonstrated that there is no difference in the penetration for SUV and MLV formulations containing an anti­ inflammatory substance (1440) (26). Du Plessis et al. (45) compared the skin penetration of three liposomal sizes of cyclosporine as a lipophilic drug. They observed the highest amount of cyclosporine in the surface of the stratum corneum and the lowest amount in the deeper portion of the stratum corneum of pig skin with 60-nm particle size, as compared to two other liposome sizes with an average of 300 nm and 600 nm. Sentjurc et al. (39) have proven that for some of their vesicular systems some transport into the deeper skin layers is observed. This penetration did not depend on vesicle size signifi­ cantly until the vesicle diameter of approximately 200 nm was reached. However, for small vesicles (with diameter less than 200 nm), the transport was significantly de­ creased. These studies show that the influence of liposome size on skin penetration depends on lipid composition, the nature of the drug, and also on the nature of the skin. Therefore, the penetration of liposomes to and through human skin can vary with composition and particle size, which furthers the need to investigate the effect of particle size for each specific drug incorporated in liposomes. This is an important aspect to optimize the topical formulation and to reduce the absorption into the skin. CONCLUSIONS The results of this study indicate that ML Vs prepared by the fusion method could be a better vehicle for OMC as a sunscreen since the MLV formulation has a slightly better SPF compared to conventional formulation and remains more in the stratum corneum,