Method development of an in vitro vertical Franz diffusion cell system to assess permeation of cosmetic active ingredients

Abstract: For evaluation of potential skincare ingredients, an in-house method using Static Franz diffusion cells and dialysis membranes was developed. Benzoic acid was chosen as a model substance along with L-ascorbic acid and α-Tocopherol. The cell conditions were tailored to encourage transmembrane diffusion. Benzoic acid was tested in acetate buffer (pH 4.6), which yielded a maximum flux of 0.91 ± 0.03 mg ∙ cm-2 ∙ h-1 and absorption of 103 ± 4 % out of the applied dose after 8 h. There were strong indications that benzoic acid ionization must be suppressed by lower pH to increase penetration rates. L-ascorbic acid yielded a flux of 0.29 ± 0.01 mg ∙ cm-2 ∙ h-1 in phosphate buffered saline (PBS, pH 7.4) and absorption of 87 ± 7 % of the applied dose after 8 h. Experiments with α-tocopherol showed no penetration in PBS with added bovine serum albumin (BSA), leading to the hypothesis that more hydrophobic membranes and/or receptor medium are needed for the study of lipophilic compounds. In addition, the release of benzoic acid from the amorphous mesoporous magnesium carbonate Upsalite® was investigated. The results showed significant release and penetration of benzoic acid from the solid matrix in both acetate buffer and PBS. The maximum flux was estimated to 6.61 ± 0.96 mg ∙ cm-2 ∙ h-1 in acetate buffer and 99 ± 9 %  of the applied dose was absorbed after 3h. Tests of Upsalite with benzoic acid on hydrophobic silicone and Strat-M membranes showed no significant penetration, likely due to insufficient wetting of the sample. Pre-wetting of Strat-M membrane lead to penetration of benzoic acid into the membrane. Flux rates achieved on synthetic membranes are generally much higher compared to skin, which results in this thesis show. In conclusion, data for pure benzoic acid and L-ascorbic acid in the developed method using dialysis membranes showed reasonable agreement with literature. Penetration of benzoic acid is pH-dependent and may be either increased or decreased by choice of skin model or by using Upsalite as vehicle. Choosing a buffer pH below the pKa of the substance may enhance penetration. Introducing L-ascorbic acid in Upsalite could potentially increase the permeation, similar to that of benzoic acid.