Evaluation for intrinsic skin permeation of unstable compounds.

An evaluation method is proposed for the intrinsic skin permeation rate of unstable compounds. Vitamin C and vitamin E were used as the model compounds. The degradation of vitamin C and E in the solutions followed first-order kinetics with degradation constants of 0.26 h-1 and 0.014 h-1, respectively. The apparent skin permeation profiles of vitamin C and E in vitro, approximated by a nonlinear profile of the polynomial regression method, was corrected for intrinsic permeation rate considering first-order degradation in the receptor solution. The intrinsic profiles evaluated agreed well with the ones determined from radio-labelled compounds, indicating the feasibility of the present analysis.