DELIVERY AND METABOLISM OF o•-TAc 233 2 exceeded that of the emulsion 1 formulation. The total amount of c•-TAc permeated was affected by all three terms mentioned above. Emulsion 2 had significantly higher amounts of total permeation than the IPM solution and other emulsions used in this study. The hydroalcoholic gel 3 also had higher total permeation than the alcoholic gel 2. Permeation of ot-TAc in viable skin, which is the sum of both the parent molecule and the metabolite in viable skin is also depicted in Figure 2 (light bars). Emulsion 2 had significantly higher amounts of permeation as compared to the IPM solution. The therapeutic benefit from these formulations can be obtained only by bioconversion of the ot-TAc to ot-T. An important measure of such conversion was thought to be the extent of metabolism (EX). Extent of metabolism (EX, %) has been calculated in the viable skin as shown in equation 1. T Atvs) EX(vs) = T AT^c X 100 (1) A(vs) + ' •(vs) A is the amount of o•-T or o•-TAc as micrograms of active. The subscript (vs) refers to the viable skin. The superscript refers to either o•-T or o•-TAc. The numerator refers to the amount of o•-T in micrograms present in viable skin. The extent of metabolism was found to be formulation-dependent. The IPM-solution, emulsion 2, and the hydroalco- holic gel 3 had a significantly higher extent of metabolism compared to the other formulations. The hydroalcoholic gel 3 was superior to the alcoholic gel 2 in terms of the extent of metabolism. This is depicted in Figure 3. Table V shows the statistical summary of the results for the formulations. Check marks indicate those comparisons for which a statistically significant difference at the 95% confidence interval was found. DISCUSSION We have shown the metabolism of ot-TAc to ot-T in micro-Yucatan pig skin for the first 80_ 70 60 50 40 30 20 10 0 IPM- Gel 1 Gel 2 Gel 3 Emul 1 Emul 2 solution Emul 3 Figure 3. Extent of metabolism of t•-TAc in viable skin. Values are percentage of applied dose mean e SEM (n = 4). *Statistically significant difference (Tukey's test, t• = 0.05) from the unmarked formulations.

234 JOURNAL OF COSMETIC SCIENCE Table V Statistical Comparison of Data for the Formulations Total o•-TAc permeated (% of applied dose) Extent of metabolism • [o•-T/(o•-T + o•-TAC)] x 100 Metabolite-o•-T concentration in viable tissue (% of dose) IPM b •' gel 1 x •/ x IPM •, gel 2 x ,/ x IPM +* gel 3 x x x IPM •-• Emul 1 x •/ x IPM *-• Emul 2 • x •/ IPM +* Emul 3 x • x Gel 1 •,gel2 x x x Gel 2 •' gel 3 x •/ x Gel t *-• get 3 •/ x x Emul 1 +* Emul 2 • • x Emul 1 ** Emul 3 x x x Emul 2 •' Emul 3 •/ x x 7: Statistically significant difference (o• = 0.05) exists between the terms connected by the arrow symbol determined using Tukey's test. x: No statistically significant difference (o• = 0.05) exists between the terms connected by the arrow determined using Tukey's test. a In viable skin at 24 hours. b An IPM solution of •-TAc. time. No other metabolites of ot-T were detected in viable skin in the 24-hour study period. Metabolism did not occur in the stratum corneum, but was restricted to the viable skin. The F-test revealed no interday variability, thus validating the efficiency of the randomized complete block design we had chosen as the statistical model for the experiment. We have shown that ot-TAc can be delivered through skin by using a simple topical formulation (an IPM-solution). Wester and Maibach (13) demonstrated delivery of radiolabeled ot-TAc across human skin using a simple cream formulation. Gehring et a/. (14) showed that ot-T leads to an increase in stratum corneum hydration irrespective of whether it was incorporated in an o/w or w/o emulsion. Earlier, Norkus eta/. (3) and Trevithick and Mitton (2) had demonstrated metabolism of ot-TAc to ot-T in mouse skin. Norkus eta/, (3) reported about 10% metabolism of the ot-TAc to o•-T in total skin, inclusive of the stratum corneum. We found, on an average, about 25% metabolism in total skin, about 2.5 times their value. However, Alberts et M. (5) have reported negligible metabolism in human skin. We found o•-TAc to be absorbed into pig skin after topical application. Other authors have observed similar absorption of ot-TAc in mouse and human skin (2,3,15). Kamimura and Matsuzawa have used autoradiographic studies to show the cutaneous transport of ot-TAc in skin (15). Although there are reports on ot-TAc metabolism in rat and mouse skin, we did not come across any report on pig skin. Tojo and Lee (16) studied the bioconversion of a provitamin to vitamins C and E in mouse skin dermis. The provitamin was inherently stable and was expected to develop actions of both vitamins E and C in the body through splitting off the phosphoric acid esters by enzyme phosphatase. They calculated the yield of bioconversion to be about 96% in the hairless mouse skin. Nabi et aL (17) have used skin culture models and explants to demonstrate the bioconversion of ot-TAc to o•-T.