j. Soc. Cosmet. Chem., 43, 251-258 (September/October 1992) Investigation of the in vitro interaction of various vehicles with hairless mouse skin MOHAMMAD S. RAHMAN, MICHAEL A. GALLO, THOMAS H. UMBREIT, and JOEL L. ZATZ, College of Pharmacy, Rutgers University, P.O. Box 789, Piscataway, NJ 08854 (M.S.R., J.L.Z.), and UMDNJ-RWJ Medical School, 675 Hoes Lane, Piscataway, NJ 08854 (M.A.G., T.H.U.). Received May 14, 1992. Presented in part at the 1989 Annual Scientific Meeting of the Society of Cosmetic Chemists, New York, December 7-8, 1989. Synopsis The interaction of a series of donor solvents including water, propylene glycol, two alcohols, four hydro- carbons, and light mineral oil with hairless mouse skin was studied in vitro by following the dermal absorption of caffeine as a reference compound. Steady-state flux at saturation (J*) of caffeine was calculated for different donor solvents using solubility, donor concentration, and steady-state flux. The value of J* varied markedly with the donor solvent, indicating that caffeine permeation was significantly affected by vehicle/skin interactions. n-Propanol provided the highest value of J* among the various solvents. In the hydrocarbon series, the value of J* decreased exponentially as the chain length was increased. Propylene glycol yielded a lower value of J* compared to water. Light mineral oil provided the lowest value of flux at saturation, suggesting minimal interaction between this vehicle and mouse skin. INTRODUCTION In skin permeation studies, vehicles (solvents) are frequently employed to apply the test material(s) to the membrane surface. The physical and chemical properties of the vehicle play a major role in determining the rate of uptake and penetration of the medicament through the membrane (1). Vehicles can modify either the thermodynamic activity of the drug (2) or the barrier properties of the skin (3). Thus, a thorough understanding of the interaction of the solvent with the drug or the membrane is essential before its selection in any formulation for transdermal drug delivery as well as for dermal toxi- cological evaluation. Mohammad S. Rahman's present address is ManTech Environmental Technology, Inc., 2 Triangle Drive, Research Triangle Park, NC 27709. Thomas H. Umbreit's present address is ATSDR, Atlanta, GA 30333. 251

252 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Many therapeutic and cosmetic agents are nonpolar in nature with limited aqueous solubility. They readily dissolve in various organic solvents, but use of many of these vehicles is restricted in skin permeation experiments because of their membrane- damaging potential. The objective of the present study was to investigate the interaction of a series of donor solvents with full-thickness hairless mouse skin, using caffeine as a reference compound for skin permeation measurements. The octanol-water partition coefficient of this compound is 1 and it has adequate solubility in various polar and nonpolar solvents, thus making it a suitable candidate for this study. MATERIALS AND METHODS MATERIALS Radiolabeled caffeine was utilized to facilitate skin permeation quantitation. [•4C] caffeine (specific activity, 55.7 mCi/mmol) was obtained from ICN Biomedicals, Costa Mesa, CA, and its radiochemical purity was determined by the manufacturer to be greater than 97%. The radioactivity of experimental samples was measured in a 1217 Rackbeta scintillation counter (LKB Instruments, Gaithersburg, MD) using Liquiscint © (Diagnostic Products Corp., Manville, NJ) as the scintillation fluid. Nonradiolabeled caffeine, chlorobutanol, light mineral oil, n-pentadecane, n-dodecane, n-nonane, n-hep- tane, propylene glycol, n-propanol, and isopropanol were obtained from either Sigma Chemical Co., St. Louis, MO, or Fisher Scientific, Springfield, N J, and used as re- ceived. METHODS Preparation of skin samples. Female hairless mice (Skh 1, Charles River Labs, Wilming- ton, MA), 8-14 weeks old, were used as the source for all skin samples. Animals were sacrificed by cervical dislocation and skin was immediately excised. Subcutaneous fat and underlying tissues were carefully removed from the dermal surface. The skin sam- ples were inspected visually for defects such as tears or holes and used within one half hour after animal sacrifice. Full-thickness skin, from both dorsal and ventral side, was used in initial experiments. The dorsal skin was selected for subsequent use for its ease of handling. Preparation of donor solutions. Nine donor solvents were selected for investigating vehicle- skin interactions. These included: water, propylene glycol, isopropanol, n-propanol, n-heptane, n-nonane, n-dodecane, n-pentadecane, and light mineral oil. Radiolabeled caffeine was mixed with unlabeled compound in an alcoholic solution to obtain a final specific activity of 3.63 mCi/mmol. Different volumes of this solution were placed in glass vials, and following evaporation of alcohol, the residues were dissolved in selected volumes of various solvents to prepare donor solutions. In vitro diffusion studies. A modified flow-through diffusion cell system, similar to that described by Bronaugh et al. (4) was utilized in skin permeation studies. The diffusional area in each cell was 0.64 cm 2. The receptor fluid continuously washed the dermal surface of the skin and was collected into vials placed in the fraction collector. Freshly degassed receptor solution was used in each experiment. The skin surface temperature was maintained at 32øC by adjusting the water bath temperature to 34.5øC. After an