skin was removed from the chamber and split into epidermis and dermis all skin layers were extracted overnight in 80 isopropanol and 20 water for the glass dish samples about 20 g ziana u00ae gel was prepared and in separate dishes a mixture of ziana u00ae gel 20 g and benzoyl peroxide 5 gel 20 g was prepared and well mixed at 2 4 6 and 8 hours following re mixing of the formulations three 100 u03bcl aliquots of formulation were collected from each dish mixed with 80 20 isopropanol water and saved for subsequent analysis all samples were processed and analyzed for tretinoin and its active isomer isotretinoin the degradation of tretinoin in ziana u00ae gel was evaluated over 24 hours at 32 0 u2212 1 0 u00b0 c the assays were evaluated at baseline 2 4 6 8 and 24 hours after baseline the tretinoin concentrations were analyzed by high performance liquid chromatography u201c hplc u201d a hewlett packard 1100 series hplc system was used with an agilent 1100 series lc with a diode array detector a solvent system consisting of 5 0 1m ammonium acetate ph 5 0 with acetic acid and 95 5 acetic acid acetonitrile was run through a phenomenex luna c18 2 100 a column 100 u00d7 4 6 3 u03bc at a flow rate of 0 5 ml min ten microliters of sample were injected the data over a 24 hour period shows that tretinoin from ziana u00ae gel does penetrate human skin it starts slow and has a progressive rise in epidermal and dermal concentrations when the skin is exposed to benzoyl peroxide aqueous gel 2 hours after the ziana u00ae gel application there was no appreciable effect on tretinoin penetration under light conditions a slight increase.