Mechanism for enhancement effect of lipid disperse system on percutaneous absorption

To clarify the mechanism involved in the enhancement effect of lipid disperse systems (LDS) on percutaneous absorption, the effect of the LDSs of betahistine (BH), prepared using egg phosphatidylcholine (EPC, phase transition temperature, tau m, -15 to -17 degrees C) or hydrogenated soybean phosphatidylcholine (HSPC, tau m, 50 to 60 degrees C), cholesterol, and dicetylphosphate, on the percutaneous absorption of BH, the amount of skin lipids (ceramides, triglycerides, and phospholipids), the fluidity of skin lipids, and the partitioning of LDS-BH into the skin layers were investigated using Wistar and hairless rats. Also examined was whether the LDS penetrated through the stratum corneum (SC) or follicles, using a fluorescent probe (Nile Red). The plasma concentrations of BH were much higher and more sustained after application of a gel formulation containing EPC-LDS and D-limonene (prep. 2) than those after the non-LDS formulation containing D-limonene (prep. 1), whereas the plasma levels after application of a formulation containing HSPC-LDS (prep. 5) were not largely increased compared with those after prep. 1. The content of ceramides (intercellular lipids) and triglycerides (sebaceous gland lipides) in the SC were dramatically decreased by the treatment with prep. 1 and prep. 2, with the more decreased levels of these lipids by the treatment with prep. 2. The phospholipid content of the SC was enhanced by 2-fold following the prep. 2 treatment, indicating the extensive incorporation of LDS lipids into the SC. The histochemical examination of the skin, following application of EPC-LDS with a fluorescent probe, indicated that the LDS lipids penetrated rapidly through the SC and follicles into the viable skins. The fluidity of the SC lipids was dramatically increased following the treatment with the fluid EPC-LDS, whereas the fluidity was significantly decreased by the solid HSPC-LDS. The BH in each skin layer was also significantly increased by the treatment with prep. 2. These results surely demonstrated that the fluid LDS permeated rapidly into the SC and the viable epidermis through the intercellular domains and the follicles in intact vesicles or lipid mixtures, thus ensuring the facilitated transport of LDS-drug through the skin.