Influence of Temperature on Transdermal Penetration Enhancing Mechanism of Borneol: A Multi-Scale Study
In order to relate barrier function to stratum corneum structure and the thermal transitions of corneum lipids, samples from hairless rat skin were investigated by using ESR and drug penetration techniques. The phase transition of stratum corneum lipids was estimated using a deeper probe (16-doxyl-stearic acid) inserted in the lipid bilayers and measuring the rotational correlation time, tau(c). Results of ESR study showed that stratum corneum lipids underwent thermal transitions at 39.3 +/- 1.6 degrees C and 63.6 +/- 2.6 degrees C roughly similar to the data obtained by differential scanning calorimetry measurements. Cholesterol oxidase treatment decreased the fluidity of the lipids at lower temperatures. The treatment of stratum corneum with laurocapram (1%) and isopropyl myristate (IPM, 2%) little changed both phase transition temperatures, although the treatment highly increased the molecular motion of the lipids. The flux (J(s)) of lipophilic drugs (beta-estradiol, indomethacin and betahistine) through the skin was enhanced with increasing temperatures, with an increase in the diffusion constant within skin and a decrease in the lag time. There was a good relationship between log J(s) or log permeability coefficient (K(p)) and 1/tau(c) in the temperature range of 45 to 64 degrees C. The calculated activation energy (delta E) for diffusion of these drugs across skin was 17-40 kcal/mol. Judging from our data, stratum corneum lipids of rat probably exist as the gel, crystalline state below 39 degrees C, the mesomorphic state between 39 and 64 degrees C and the fluid, liquid-crystalline state at temperatures of 64 degrees C or above. These results are in line with the permeability of these lipophilic drugs through the intercellular lipids disordered is highly increased.