336 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II The Donor and Receptor Solutions Used in the In Vitro Skin Permeation Study Drug in donor Drug in receptor Donor solution Receptor solution EPC-Na Vitamin C Water 50% glycerin aq. solution Drug (13 mg/ml) Vitamin E 5 mM Tween-80 aq. solution EPC-Na Water were found to degrade according to first-order kinetics. The rate constants for vitamins C and E were found to be 0.31 and 0.015 (hr-•), respectively, under the present experimental conditions. The intrinsic permeation profiles were then computed from the apparent concentration-time courses by the following mathematical treatment: The flux J(t) per unit skin area and the cumulative amount Q(t) of the vitamins ap- pearing in receptor solution are given by: J(t) = + kC (1) Q(t) = J(t)dt = '•tt + kC dt (2) where C is the vitamin concentration in the receptor solution at time t, k is the rate constant of degradation, and V and A are the volume (3 ml) and surface area (0.64 cm 2) of the receptor compartment, respectively. The apparent concentration-time profile was approximated by a polynomial equation by the Fletcher method (18). The right-hand side of Eqs. (1) and (2) was evaluated analytically. RESULTS AND DISCUSSION The cumulative amount of vitamin C, following the bioconversion of the provitamin in the skin and appearing in the receptor solution is plotted in Figure 3. It is interesting to see that the cumulative amount increased linearly immediately after the onset of the permeation/bioconversion experiment virtually no time lag was observed in the appear- ance profile of vitamin C. This is quite different from the appearance profile of vi:amin E. This finding may indicate that the hairless mouse maintains a certain vitamin C level in its skin tissue which causes a bursting release in the early stage of the permeation and bioconversion profile. It was reported previously that all laboratory animals, including mice, synthesize ascorbate (19). Further experiments will be carried out to elucidate the mechanism of vitamin C permeation across the skin. The steady-state rate of vitamin C appearance was found to be 2.6 X 10 -2 !xmole/cm2/hr from the slope of the time course of the cumulative amount. Figure 4 shows the profile for the appearance of vitamin E from the provitamin biocon- version. It was found that the rate of appearance of vitamin E, after the bioconversion, increased gradually during the entire period of the permeation experiment. Since vi- tamin E is a very lipophilic compound, a considerable amount of vitamin E that is bioconverted in the viable skin is expected to diffuse back into the stratum corneum

PROVITAMIN TO C AND E IN SKIN 337 360 0 12 24 36 48 60 72 Time Figure 3. Cumulative amount o• v•i• C Noco•ver•ed {ro• until the concentration in the stratum corneum reaches a steady state. This process may require a substantial period of time. The cumulative amount of the provitamin EPC-Na appearing in the receptor solution is shown in Figure 5. It was found that the concentration of provitamin increased linearly after the lag time of about 24 hours. This finding indicates that the provitamin was not completely bioconverted to vitamins C and E in the hairless mouse skin. The steady- state rate of appearance of intact provitamin was calculated to be 1.4 x 10 -3 p•mole/ cm2/hr from the slope of the linear portion of the permeation profile. Based on the 4O •.. 30 lO 0 24 48 72 Time (hi Figure 4. Cumulative amount of vitamin E bioconverted from EPC-Na and appearing in the receptor solution. The circles and error bars represent the average and standard error of six data points.