304 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III In Vitro Permeability Coefficients of n-Alkanols and Water Through Frozen Nude Rat Skin Permeability Coefficients x 1000 (cm/hr) Average Freezing Times Frozen Average + Permeant 0 Days 2 Days 1 Week 4 Weeks Frozen Unfrozen Water 4.2 4.1 4.2 3.2 3.8 4.0 Methanol 4.4 5.4 6.1 3.8 5.1 4.8 Ethanol 4.2 6.2 5.7 4.1 5.3 4.8 Butanol 14.2 17.8 18.6 15.1 17.2 15.7 Hexanol 47.8 49.0 53.3 46.2 49.5 48.7 Octanol 59.0 67.8 60.1 72.7 66.9 63.0 able with human data. A good animal model would have the three permeation pathways in approximate balance to those found in humans. The hairless mouse has been shown to be a comparable model. Its zr-value (0.32) is very close to that found in human skin (0.30). The hairless rat may also prove to be equivalent however, to our knowledge it is not commercially available in this country. While the hairless mouse appears mechanis- tically comparable, it does have some functional drawbacks. Its size is quite small, making it unsuitable for in situ/in vivo experimentation. Even for in vitro work, the amount of skin available from one animal is limited, requiring the use of multiple animals for an experiment, introducing more variability. In our laboratories, it was decided to explore other animal models. The fuzzy rat was previously shown to be acceptable (4,5). In the present study, the nude rat is examined. The female animal of this species has no hair or very sparse hair on its abdominal surface, making hair clipping unnecessary. The skin from this area is sufficient for eight to twelve diffusion experiments. The nude rat alkyl chain-length profile is comparable to that found with human skin and its q'r-value (0.26) is close to that for human skin (0.30). Based on these findings, it is suggested that the nude rat would be a suitable laboratory animal model for skin permeation studies. EFFECTS OF FREEZING ON SKIN PERMEABILITY The effect of freezing on the nude rat skin permeability profile was also investigated in our laboratories. Skin frozen for two days, one week, and four weeks was tested for changes in permeability to n-alkanols and water. These data are presented in Table III. Graphical representations, comparing frozen skin data to unfrozen skin data, are pre- sented in Figure 3 (P-value vs. alkyl chain length). Figure 4 shows the permeation- freezing profile (P-value vs. freezing time). These data indicate that freezing the skin for up to one month had no significant effect on its permeability characteristics. Very little data have been published on the effect of freezing on skin permeability. Harrison et al. (11) report that there was no significant difference between the absorp- tion of water in fresh human skin and skin which had been frozen at - 20øC for up to

NUDE RAT SKIN PERMEABILITY 305 lOO o o o x 10' Legend ß UNFROZEN 0 FROZEN 2 d [] FROZEN 1 wk A FROZEN 1 mo 0 2 4 6 8 ALKYL CHAIN LENGTH Figure 3. In vitro permeability profile of n-alkanols in the nude rat: Unfrozen vs. frozen skin. 466 days. Swarbrick et al. (12), however, found increased absorption of chromone acid after freezing three specimens of human skin for 60 hours at - 17øC. The nude rat data cannot be directly compared with these findings. The published data are for human skin which was further treated after thawing to separate the dermal and epidermal layers. In our studies we used full-thickness skin. These data are presented recognizing that a significant amount of basic work is necessary to clearly define the effects of freezing on the permeability patterns of human skin and laboratory animal model skins.