Advanced investigation of the suitability of confocal Raman spectroscopy for dermatological bioequivalence assessments using caffeine and 1,2-pentanediol

Since the European Medicines Agency released its "Guideline on quality and equivalence of locally applied, locally acting cutaneous products", the interest in bioequivalence studies especially in relation to the approval of generics has grown. The guideline provides a framework for the methodology of such studies with the specification of tape stripping as the sole method for skin penetration tests and the vague requirements for a control formulation presenting slight shortcomings in this context. Thus, the aim of this study was to broaden the understanding of what constitutes a suitable control formulation as well as showing the suitability of confocal Raman spectroscopy (CRS) as an alternative method to tape stripping. Validation of CRS results was achieved by performing tape stripping and CRS experiments using caffeine as a model API consecutively on the same skin samples. In order to investigate a suitable control formulation, the effect of different concentrations of 1,2-pentanediol as a penetration enhancer on thermodynamic activity and thus penetrated API amount was examined. We established that CRS is appropriate for both quantitative skin penetration testing as well as in depth bioequivalence testing by demonstrating that the findings of tape stripping and CRS were extremely similar in terms of both the total amounts of detected API as well as the outcomes of bioequivalence testing. Furthermore, we could show that high concentrations of 1,2-pentanediol, by improving the solubility of caffeine, reduce the thermodynamic activity sufficiently to obtain a suitable control formulation. Lower concentrations achieved the expected penetration enhancement but not to a sufficient degree that the formulation can be used as a control. Finally, against the background of these findings, the penetration-enhancing effect of 1,2-pentanediol was investigated, whereby effects on the lipid structure of the stratum corneum and a water-binding effect in the skin were observed.