Percutaneous Penetration Enhancement in Vivo Measured by Attenuated Total Reflectance Infrared Spectroscopy

A novel application of attenuated total reflectance IR Spectroscopy (ATR-IR) was used to monitor the outer several microns of the stratum corneum (SC) and, thereby, demonstrate enhanced percutaneous absorption in vivo in man. 4-Cyanophenol (CP) as a model permeant yielded a unique IR signal, distinct from those of the stratum corneum and the vehicle components. CP was administered for 1, 2, or 3 hr as a 10% (w/v) solution either in propylene glycol or in propylene glycol containing 5% (v/v) oleic acid. The absorbance at 2230 cm⁻¹, which corresponded to C≡N bond stretching, diminished significantly faster when CP was codelivered with oleic acid. An IR absorbance due primarily to propylene glycol at 1040 cm⁻¹ (C–O stretching) also disappeared more quickly following application of the enhancer-containing solution. In addition, only the formulations with oleic acid induced a higher wavenumber shift in the frequency of the asymmetric C–H bond stretching absorbance. This change indicates increased lipid-chain disorder, the mechanism by which oleic acid is believed to cause enhanced drug transport across the stratum corneum. Therefore, ATR-IR permits one to examine noninvasively the kinetics, extent, and mechanism of percutaneous penetration enhancement in vivo in human subjects.