98 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS application of PC/CH/CS DRV or PC/CH/CS LUV of the same lipid concentrations. Figures 1 and 2 show representative plots of the percent cumulative uptake of cholesteryl sulfate from different liposomal formulations with various lamellarities and lipid con- centrations in the deeper stratum corneum and deeper skin strata. These results suggest that the interaction of liposomes with skin, leading to lipid deposition in the deeper skin strata, does not depend on the degree of lamellarity. It should be pointed out, however, that lamellarity may play an important role in the deposition of entrapped hydrophilic drugs. For example, we have shown the pronounced advantage of DRV over LUV for the deposition of entrapped interferon into deeper skin strata (12). These data are not conflicting, since the amount of drug associated with the lipid bilayers transported into the deeper skin strata would depend, to a large extent, on the lamellarity and other factors associated with the method of liposomal preparation (13). We also examined the effect of skin species on deposition of lipids after topical appli- cation of liposomes as a function of method of preparation and lipid concentration. These differences could be related to either species differences in stratum corneum lipid struc- ture or to the virtual absence of the follicular route in the mouse skin as compared to the pig skin. The effect of liposomal lipid concentration on in vitro permeation of liposomal lipid into pig skin was evaluated using lipid concentrations 10 mg/ml and 50 mg/ml for three 50mg/mI-Pig 50mg/mI-Mouse 25mg/mI-Pig 25mg/mI-Mouse 10mg/mI-Pig 10mg/mI-Mouse [] REV [] DRV [] taLV ß o I ' I e I 2 4 6 Percent Uptake Figure 2. Comparison of the 24-hr in vitro uptake of •4[C]-cholesteryl sulfate from PC/CH/CS DRV, MLV, and REV with total lipid concentrations of 10, 25, and 50 mg/ml in the deeper skin strata of hairless mouse and pig skin (n = 4-5).

LIPOSOMAL DEPOSITION 99 liposomal systems (MLV, DRV, and LUV). Table II shows the degree of deposition of cholesteryl sulfate into the various strata of pig skin 24 hr after in vitro topical appli- cation of the various liposomal formulations with lipid concentrations of 10 and 50 mg/ml. The data shown in Table II indicate that the amount of lipid deposited in the deeper skin strata of pig skin was ten times higher than that found with mouse skin. These differences could be related to either species differences in stratum corneum lipid structure or to the virtual absence of the follicular route in the mouse skin as compared to the pig skin. The CH/CS ratios in the different mouse skin strata 24 hr after topical application of the liposomal formulations are shown as a function of liposomal type and lipid concentration in Table III. The molecular ratio of radiolabeled lipids of the liposomal preparations was maintained in the surface stratum corneum and the deeper stratum corneum following topical liposomal MLV, DRV, and LUV systems. The results in Table III suggest that the two lipid components are diffusing into the surface and the deeper stratum corneum at the same rate despite a large difference in their polarities. The ratios of the two lipids in these strata are the same as that in the original liposomal suspension, indicating that the lipid components are transported as bilayer units. The fact that the ratio of the Table II Distribution of Cholesteryl Sulfate (expressed as percent formulation applied -+ standard deviation) in Various Strata of Pig Skin 24 hr After In Vitro Topical Application of Various PC/CH/CS Liposomal Formulations Having Different Lipid Concentrations Onto Full-Thickness Skin (n = 4-5) 10 mg/ml 50 mg/ml Compartment REV DRV MLV REV DRV MLV Total donor 43.5 +-- 9.4 47.0 +-- 6.7 52.9 +-- 2.0 27.5 +-- 1.1 23.7 + 0.3 18.1 + 0.8 Surface stratum corneum 24.1 + 5.5 27.2 + 0.1 21.9 + 4.1 27.8 + 4.9 36.8 + 2.2 32.6 +-- 0.9 Deeper stratum corneum 26.2 + 5.4 23.7 + 5.3 20.1 -+ 1.2 37.5 +-- 8.5 34.3 +-- 3.9 41.4 --- 5,8 Deeper skin strata 4.9 +-- 0.3 3.9 +-- 0.8 4.7 +-- 1.6 6.9 +-- 0.0 4.8 + 2.4 7.4 -+ 0.6 Total skin 55.3 + 9.7 54.8 -+ 4.5 46.6 -+ 6.9 72.2 +-- 3.6 75.9 +-- 0.7 81.4 + 5.4 Receiver 1.3 + 0.2 0.6 + 0.06 0.5 +-- 0.1 0.4 + 0.03 4.1 + 0.5 0.6 + 0.1 All values were corrected to 100%. Table III The Cholesterol:Cholesteryl Sulfate Ratio in Various Strata of Hairless Mouse Skin 24 hr after In Vitro Topical Application of PC/CH/CS MLV, DRV, and REV With Different Lipid Concentrations Onto Full-Thickness Skin (n = 4-5) MLV DRV REV 10 25 50 10 25 50 10 25 50 Compartment mg/ml mg/ml mg/ml mg/ml mg/ml mg/ml mg/ml mg/ml mg/ml Surface stratum corneum 1:0.98 1:1.02 1:1.09 1:0.95 1:0.96 1:0.97 1:0.96 1:0.87 1:0.99 Deeper stratum corneum 1:0.95 1:0.83 1:0.50 1:0.92 1:0.97 1:0.94 1:0.88 1:1.44 1:0.94 Deeper skin strata 1:0.61 1:0.53 1:0.65 1:0.93 1:0.96 1:0.32 1:0.58 1:0.20 1:0.82