Modes of action of terpene penetration enhancers in human skin; Differential scanning calorimetry, small-angle X-ray diffraction and enhancer uptake studies

Abstract

The mechanisms through which the terpenes, d-limonene, 1–8-cineole and nerolidol, increase the permeability of human stratum corneum (s.c.) and the mechanisms underlying propylene glycol (PG)/terpene synergy were investigated using differential scanning calorimetry (DSC), small-angle X-ray diffraction (SAXD) and enhancer uptake studies. DSC experiments identified two major lipid transitions at 72° and 83°C. d-Limonene reduced the temperatures of both transitions by approx. 20°C without affecting their enthalpies ( ΔH). 1–8-Cineole also reduced the temperatures of both transitions by approx. 20°C but also reduced ΔH for the first major lipid transition; ΔH for the second was unaffected. d-Limonene increased the combined entropy change ( ΔS) associated with both lipid transitions by 11% whereas 1–8-cineole decreased ΔS by 32%. The decrease in ΔS produced by 1–8-cineole provides evidence that this enhancer is lipid disruptive at normal skin temperature. Nerolidol reduced the transition temperatures of both major lipid transitions by approx. 4°C and also decreased their cooperativity. Reduced bilayer cooperativity indicates that this enhancer also disrupts the intercellular lipids. Lack of a clear baseline prevented accurate measurement of ΔH and ΔS values following nerolidol treatment. SAXD experiments showed that d-limonene and 1–8-cineole act to reduce the intensity of lipid based reflections. Decreases in reflection intensities may be linked to a disruption of lipid packing within the bilayers and/or to a disturbance in the stacking of the bilayers. Treatment with nerolidol did not markedly reduce the intensities of the bilayer based reflections. Uptake studies revealed that large quantities of terpenes can be accommodated by the s.c. (mean uptake of d-limonene, 1–8-cineole and nerolidol was 8.90%, 26.2% and 39.6% w/w dry s.c.). The possibility that terpene enhancers pool in the s.c. is discussed. DSC and SAXD investigations provided fragmented evidence that PG/terpene synergy may produce enhanced lipid bilayer disruption. Enhancer uptake studies showed that PG does not significantly increase terpene delivery to the s.c. above that provided by application of neat terpenes.