Application of quantitative (19) F nuclear magnetic resonance spectroscopy in tape-stripping experiments with natural microemulsions

The skin penetration of flufenamic acid (Fluf) and fluconazole (Fluc) from innovative natural microemulsions was investigated in tape-stripping experiments on pig ears. The formulations were based on the eudermic surfactants lecithin, sucrose laurate, alkylpolyglycoside or a mixture thereof. The quantification of the penetrated drug amounts was executed by (19) F nuclear magnetic resonance (NMR) in comparison with high-performance liquid chromatography (HPLC). The data obtained by the (19) F NMR method were confirmed by additional quantitative studies using HPLC. An excellent linear correlation was found for Fluf as well as for Fluc between (19) F NMR and HPLC data. This work presents a strategy outlining the use of (19) F NMR to selectively monitor the skin penetration routes of fluorinated compounds. Fluc penetrated generally well into the stratum corneum with the significantly highest amounts from the sucrose laurate microemulsion on the tape strips 1-5. Similarly, the highest amounts of penetrated Fluf could be observed from the formulation based on sucrose laurate. In addition, NMR self-diffusion studies were conducted and revealed a bicontinuous microstructure of the investigated microemulsions. The skin penetration results are in good agreement with the obtained (19) F NMR self-diffusion coefficients of the active compounds in the microemulsion systems.