298 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS to human testing. Therefore, laboratory skin permeation studies, using animal models, provide a convenient means for initial screening of new drug formulations for topical/ transdermal application. In vitro methods are particularly useful for rapidly establishing rank-order permeabilities of a series of topical formulations. These data must be corre- latable with human data. Therefore, suitable animal models must have skin perme- ability mechanisms comparable to those found in humans. In our laboratories we are investigating the use of the nude rat as a possible model system. This non-furry animal has a sparse hair distribution, making it a potentially useful model to assess percu- taneous absorption of dermatological preparations. Furthermore, its size is large enough to allow both in vitro and in situ/in vivo experimentation. The permeabilities of n-al- kanols and water through nude rat skin were determined and are compared with pre- viously reported data obtained using human skin, other animal skins, and Silastic membrane © (1-8). Based on these comparisons, the suitability of the nude rat as a laboratory model is assessed. The effect of short-term freezing on the permeability profile of nude rat skin has also been investigated. The permeabilities of n-alkanols obtained with freshly excised skin are compared to those obtained with skin frozen for two days, one week, and four weeks. Such studies provide information which can be used to make the experimental procedure easier, more efficient, and more economical. Variables such as age are better controlled, resulting in more meaningful data. Animal procurement and housing costs are also reduced. Additionally, results from this type of study can provide useful infor- mation for delineating the various mechanisms of skin permeation, regardless of whether the freezing process alters skin permeability. EXPERIMENTAL ANIMALS Female nude rats, strain RNU, were obtained from Harlan Sprague Dawley, Inc., Indi- anapolis, IN. Animals of 8-9 weeks of age were sacrificed by lethal injection of T-61 euthanasia solution (American Hoechst Corp.). The abdominal skin is practically hair- less and hair removal was unnecessary. Rats of this age do not have excessive fat at- tached to the skin, which, otherwise, needs to be removed. SKIN FREEZING The freshly excised skin was tacked to a Teflon © board, sealed in plastic bags, and stored in a freezer at -20øC. Prior to use, it was thawed at room temperature for one-half hour. DIFFUSION CELLS Horizontal glass diffusion cells were used. The cells were assembled with the skin sandwiched between the half-cells. The exposed skin surface area is 0.7 cm 2, and each half-cell has a volume of 3 mL. The assembled cells were immersed in constant temper- ature water baths at 37øC. The cell contents were stirred by propellers in each half-cell rotating at 150 rpm.

NUDE RAT SKIN PERMEABILITY 299 CHEMICALS Tritiated water and a series of •4C-labeled n-alkanols were obtained from New England Nuclear Corp. Included were "neat" solutions of methanol (50 mCi/mmol), ethanol (24 mCi/mmol), butanol (4 mCi/mmol), hexanol (5 mCi/mmol), octanol (5 mCi/mmol), and 3H-water (25 mCi/g). Stock solutions of each compound were prepared by diluting "neat" compound with distilled water to provide specific activities in the range of 0.2- 1.0 }xCi/mL. PERMEATION PROCEDURE Each half-cell was filled with 3 mL of normal saline. The cell contents were stirred for 15 minutes to assure temperature equilibration. Two-hundred microliter samples were withdrawn from each half-cell and assayed for background counts. The donor compart- ments were charged with radiolabeled compounds at concentrations in the micromolar range. This point was considered time zero for the experiments. A 50 }xL sample was withdrawn from the donor compartment after mixing five minutes and assayed radio- isotopically. At predetermined times, 200 }xL samples were withdrawn from the re- ceiver compartments and assayed. These samples were replaced with 200 }xL of saline to maintain a constant volume in the receiver compartment. ASSAY PROCEDURE The samples withdrawn during the experiment were transferred to 10 mL of scintilla- tion cocktail (Aquasol ©, New England Nuclear, Boston, MA) and were assayed on a Beckman Liquid Scintillation Counter, Model LS-5801. DATA ANALYSIS The data were plotted as dpm collected in the receiver compartment as a function of time. The dpm values were corrected for losses due to sampling. The permeability coefficient from a given run was calculated from: Jt = P'A'AC = V'dc/dt where Jt = the total pseudo-steady state flux in dpm/hour across the skin P = the permeability coefficient (cm/hour) A = the diffusional area (cm 2) AC = the concentration differential across the membrane, which was taken to be equal to the donor phase concentration, (dpm/cm 3) V = the half-cell volume of the receptor compartment (cm 3) dc/dt = the steady-state slope in terms of dpm/cmS/hour RESULTS AND DISCUSSION MECHANISTIC CONSIDERATIONS AND SUITABILITY OF THE NUDE RAT AS AN ANIMAL MODEL The in vitro permeability coefficients (P-values) for n-alkanols and water through nude rat skin are summarized in Table I, along with similar data for other animal models,