400 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Percutaneous Absorption Levels of the Test Molecules Amounts found 96 h after topical administration Total penetration after 96 hours Epidermis % of -t- dermis Animal applied Molecules Urine Feces area treated body nmoles ß cm- 2 dose Amounts in stratum corneum of treated area 30 min after application Benzoic acid 24.80 a 1.32 0.50 0.01 26.60 13.30 17.60 (0.50) b (0.30) (0.05) (0.004) (0.70) (0.35) (1.50) Acetylsalicylic 5.40 0.43 0.85 0 6.70 3.35 5.15 acid (0.70) (0.15) (0.20) (0.70) (0.35) (0.64) Dehydroepi- 1.67 3.00 0.19 0.43 5.28 2.65 1.95 androsterone (0.15) (0.30) (0.03) (0.08) (0.20) (0.10) (0.40) Sodium 3.50 0.48 1.00 0 4.98 2.50 3.85 salicylate (0.50) (0.10) (0.17) (0.71) (0.35) (0.29) Testosterone 1.72 2.05 0.24 0.33 4.32 2.16 2.06 (0.43) (0.60) (0.05) (0.15) (0.46) (0.23) (0.23) Caffeine 2.52 0.42 0.15 0.65 3.73 1.86 2.76 (0.25) (0.07) (0.01) (0.06) (0.30) (0.15) (0.34) Thiourea 2.10 0.76 0.20 0.20 3.23 1.61 2.63 (0.40) (0.04) (0.02) (0.04) (0.48) (0.24) (0.33) D-mannitol 1.60 0.42 0.30 0.30 2.63 1.32 2.22 (0.20) (0.15) (0.10) (0.10) (0.17) (0.08) (0.37) Hydrocortisone 0.46 0.33 0.04 0.03 0.86 0.43 0.93 (0.07) (0.07) (0.01) (0.01) (0.08) (0.04) (0.15) Dexamerhasone 0.16 0.30 0. l0 0.04 0.60 0.30 0.41 (0.02) (0.05) (0.02) (0.01) (0.06) (0.03) (0.02) Expressed in nanomoles ß cm -2 application area S.D:(n = 6). When the compounds benzoic acid, acetylsalicylic acid, dehydroepiandrosterone, so- dium salicylate, testosterone, hydrocortisone, and dexamethasone are classified ac- cording to a decreasing order of penetration rate, we observe that this order is similar to that found in the literature concerning studies in man (12, 13). Likewise, it is estab- lished that acetylsalicylic acid and salicylic acid have similar penetrating properties (13), whereas their sodium salts exhibit diminished penetration (8), and, indeed, we observed that sodium salicylate penetrates less than acetylsalicylic acid. Figure 3 shows the amount of material present on each stripping of the dosed area of the animals of group 2 at the end of the application. It is of note that for both in vitro (14) and in vivo systems, the substance concentration decreases inside the stratum corneum, following an exponential relation. Considering the diversity of the compounds tested, it would seem that this observation can be outlined as one of the factors governing percu- taneous absorption. As shown in Table II and Figure 4, independent of the physicochemical nature of the test material there exists a significant linear relationship between the total amount of material penetrating within four days (y) and the amount present in the stratum cor- neum at the end of the application (30 min) (x) (r = 0.98, p 0.001). From a theoretical viewpoint, this correlation sheds some light on a possible explana-

PREDICTING PERCUTANEOUS ABSORPTION 401 o o 30 t 2O 10 BENZOIC ACID LtJ 7' ß -J , --• 0 u• 0 (/) Lu 0 •u • 0 ß • • o 7' "• o n- • 0 • u_ :D -- 0 ',r' o T .• O2 "• • x PRODUCTS Figure 2. Percutaneous absorption levels of the test materials four days after their topical administration in the hairless rat. tion for the stratum corneum barrier effect. For a low reservoir capacity, we find low or minimal penetration and therefore a strong barrier effect. On the other hand, for a high reservoir capacity, we find substantial penetration and therefore a weak barrier effect. As a consequence, it is possible that barrier and reservoir functions of the horny layer may reflect the same physiological reality. From a practical viewpoint, the simple measurement of the amount of a chemical found in the stratum corneum at the end of a 30-rain application provides a good predictive assessment of the total amount penetrating within four days. As previously mentioned, the absorption level has been shown to be dependent on application conditions. It was therefore important to ascertain whether or not the "stripping method" was independent of the main factors capable of modifying the pen- etration level of a chemical, i.e., application time, dose applied, vehicle used, and anatomic site involved. PART II: INFLUENCE OF APPLICATION CONDITIONS ON THE RELATIONSHIP BETWEEN STRATUM CORNEUM CONCENTRATION AND PERCUTANEOUS ABSORPTION INFLUENCE OF APPLICATION TIME The duration of application of a compound may considerably influence the total