234 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Influence of Skin Thickness for Saturated Aqueous Systems Theophylline Methylparaben Thickness Flux _+ SD Lag time -+ SD Flux + SD Lag time -+ SD (Ixm) (mg/cm2/h) (h) (mg/cm2/h) (h) 320 0.024 + 0.003 1.8 + 0.7 0.11 _+ 0.02 2.9 + 0.7 450 0.024 + 0.003 1.2 +- 0.5 0.096 + 0.02 3.0 -+ 0.3 permeant (approximately 3 ml/h). The mounted skin was allowed to equilibrate for one hour before application of the donor solution. The donor compartment was sealed with parafilm to eliminate evaporative loss. Effluent samples were collected periodically and weighed to determine the actual amount of solution delivered. Samples were filtered through a 0.45 !xm filter, and 20 !xl were injected onto a reversed phase HPLC column (Zorbax ODS-3 column, Dupont, Wilmington, DE) with UV detection-theophylline at 272 nm, parabens at 254 nm. 0 0 20 40 60 80 100 120 Time (h) Figure 2. The cumulative amount of theophylline penetrating the excised skin sample versu• time from infinite aqueous dosing.

SKIN PENETRATION 235 0.25 0.20 0.15 0.10 0.05 0.00 ß ' I i 1 ß ' I ' ' I I 0 20 40 60 80 100 120 Time (h) Figure 3. Flux versus time profile for theophylline from aqueous suspension. The mobile phase consisted of methanol:water:HCl in the proportions 60:40:1 v/v at a flow rate of 1.0 ml/min. Donor solutions were prepared by adding an excess of per- meant to each of the neat solvents employed. The suspensions were allowed to equili- brate for several days in a 37øC water bath with periodic shaking. Several systems were examined as a percent of saturation (w/w) and were not used until completely dissolved. Solubilities were determined in a previous study (5). RESULTS AND DISCUSSION Experiments were performed on excised fuzzy rat skin to determine inter- and intra- subject variability, stability to hydration, and influence of membrane thickness on flux and lag time. The variability in flux and lag time between animals was not significantly different from the corresponding inter-subject data (Table I). Values of steady-state flux and lag time were independent of skin thickness (pooled t-test, ot = 0.05) for samples dermatomed to 320 !.•m and 450 !.•m (Table II). The effect of continuous exposure to water on fuzzy rat skin integrity is shown in Figures 2 and 3. The cumulative plot (Figure 2) shows that a steady state is reached after a few hours. After about 50 hours,