6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Influence of Vehicle on Docosane Absorption and Transdermal Delivery (% Total Recovered Label) Vehicle Layer Petrolatum Polydecene Mineral oil Soya oil Cream (w/o) Surface 94.5 61.1 62.9 69.8 77.1 Horny layer 4.3 36.5 35 28.3 19.2 Epidermis 0.9 1.8 1.5 1.4 2.7 Dermis 0.3 0.6 0.6 0.5 1.0 Receptor fluid 0 0 0 0 0 Total 100.0 100.0 100.0 100.0 100.0 Distribution of absorbed H3-docosane percentage of absorbed H3-activity 100% 75% 50% 25% O% petrolatum polydec. miner. oil soya oil cream horny layer [7• I 78,9 epidermis ß 15,6 dermis I• 5,5 receptor fl. [• 0 93,9 4,6 1,5 0 94,4 4 1,6 0 93,7 4,6 1,7 0 83,8 11,8 4,4 0 Figure 3. Absorption of 3H-docosane across intact, freshly excised porcine skin. In contrast to hexadecane, less label was absorbed from the petrolatum vehicle than from the other vehicles (see text), yet more label reached deeper skin layers from petrolatum than from the other vehicles. As with hexadecane (cf., Figure 2), no label appeared in the receptor fluid. greater proportion of 3H-docosane reached the epidermis and dermis out of petrolatum, again no label reached the receptor fluid, regardless of vehicle. These results show that the longer-chain hydrocarbon, docosane, as with the shorter-chain molecule, hexade- cane, also remains restricted to the outer skin layers, and that it does not traverse the dermis to deeper layers. Moreover, these studies show further that although the nature of the vehicle influences absorption, and to some extent the depth of skin penetration of the longer-chain species, modifications in the vehicle do not result in transdermal delivery to deeper tissues. PENETRATION OF DOCOSANE THROUGH DAMAGED HAIRLESS MOUSE STRATUM CORNEUM We next ascertained whether 3H-docosane in petrolatum could penetrate to deeper skin

TOPICAL HYDROCARBONS 7 layers, following application to an abrogated barrier, and further, whether application of the labeled alkane from "neat" petrolatum or from a suspension of petrolatum in propylene glycol:ethanol would influence absorption across damaged skin. In "neat" petrolatum, about 90% of the applied label remained on the surface of the skin after 2 •/2 hours, while in contrast, over 50% of the label penetrated to deeper layers of murine skin out of the liquid vehicle (Table III). Moreover, slightly higher levels of label reached the dermis and subcutaneous fat layers from the liquid vehicle (Table III). But even in the extreme case of a damaged skin barrier, and application in a liquid vehicle, detectable levels of label did not reach the blood. These data show that docosane displays vehicle-dependent differences in absorption in damaged skin. The results also show that, even in damaged skin and from a liquid vehicle, negligible amounts of label reach the blood under these experimental conditions. VISUALIZATION OF PETROLATUM ACROSS INTACT AND DAMAGED SKIN In parallel studies to the radioisotope studies, we next assessed the depth of penetration of "neat" petrolatum across intact and damaged skin by fluorescence microscopy. As previously described, petrolatum appeared to remain strictly restricted to the skin surface and stratum corneum in intact skin (not shown see references 6, 7). Moreover, both 2 and 4 hours after acetone treatment, petrolatum again appeared largely restricted to the surface and outer epidermis of murine skin (Figure 4). These results confirm the radioisotope studies by an independent, morphological method. DISCUSSION Despite several decades of apparently safe use in topical products, recent feeding studies Table III Penetration of 3H-Docosane Across Acetone-Treated Hairless Mouse Skin (% Total Recovered Label) Vehicle Petrolatum in propylene Sample site 100% Petrolatum a-e glycol:ethanoP -e Surface (cottonball) 90.6 +-- 2.2 46.1 +-- 1.5 Total SC 7.82 45.1 Tape 1 5.97 +-- 1.2 (63) 36.09 + 1.5 (67) Tape2 0.79 + 0.1 (8) 5.96 --- 1.1 (11) Tape 3 1.06 +-- 0.6 (11) 3.04 +-- 0.2 (6) Epidermis 1.38 +-- 0.04 (2) 1.20 -+ 0.7 (2) Dermis 0.19 +-- 0.04 (2) 1.20 +-- 0.7 (2) Fat 0.08 -+ 0.04 (1) 0.39 --- 0.1 (1) Blood Trace Trace Total 100.05 94.01 % Applied label. MEAN +-- SEM n = 4 animals in each group. TEWL levels in both groups 2 mg/cm2/hr. Trace = not significantly different from background. Parentheses indicate % total absorbed label.