2H NMR evidence for dynamic disorder in human skin induced by the penetration enhancer azone

Abstract

Penetration enhancers mediate trans-dermal drug delivery. The penetration enhancer dodecyl-azacycloheptanone (Azone ®) was characterized in situ with 2H NMR on human stratum corneum treated with either uniformly deuterated Azone ® or penetration enhancer with only the entire chain deuterated, in both cases in propylene glycol. The 2H NMR spectrum at ambient temperature constitutes a single narrow line only 180 Hz wide. This contrasts with the complex lineshapes 1–25 kHz wide commonly observed for methylene deuterons of lipids in a bilayer gel phase. Hence there is no evidence for differences in orientational behaviour between different deuterons at ambient temperature. The narrow line translates into a low overall order parameter S CD < 0.0008, for all deuterons, revealing rapid motion of the entire enhancer with correlation times t c ⪡ 6 × 10 −6 s, in all possible directions. At temperatures of about 150 K a 2H NMR pattern characteristic for a random isotropic powder was observed on samples of stacked stratum corncum sheets at different orientations with respect to the magnetic field. This provides strong evidence for random isotropic freezing of the Azone ® molecules upon cooling. The The 2H NMR data suggest than incubation with Azone ® in propylene glycol provokes dynamic structural disorder of the intercellular lamellar lipid structure throughout the stratum corneum and possibly even the creation of fluid domains involving the intercellular lipids, which may be essential for the penetration enhancing effect.