The Skin Permeation Enhancement And Reversibility Of L-Carvyl Esters As Novel Percutaneous Absorption Enhancers

Eight l-carvyl esters were synthesized from l-carveol (CV) and fatty acids (C7-C18) and confirmed by1HNMR and MS in this study. The physicochemical parameters of the l-carvyl esters such as solubility parameter, and n-octanol/water partition coefficient were calculated by the approaches of Hoftyzer/Van Krevelen or by ACD/Labs software. Volatility of l-carvyl esters was evaluated by the live weight-loss experiments and l-carvyl esters exhibited no volatility.The enhancing effects of l-carvyl esters on three drugs, indomethacin (IM, acid), isosorbide dinitrate (ISDN, neutrality) and5-fluorouracil (5-FU, base), which were selected based on their lipophilicity, were investigated by the in vitro permeation experiment using rat skin.l-Carvyl esters with a chain length of C8-C12were proved to be more potential enhancers than CV. An evident parabolic relationship was found between the permeation enhancement and the stratum corneum uptake of the l-carvyl esters.Diclofenac diethylamine (DDE) was selected as a model drug to screen the most promising enhancer among l-carvyl esters which were applied in the dug in adhesive patches. The in vitro permeation studies indicated that l-carvyl dodecanoate (C-DOD) was the most promising enhancer in patch for DDE transdermal delivery. The in vivo studies in rats were conducted in order to compare the in vivo enhancing effect of C-DOD and CV on the skin permeation of DDE from patches. The in vitro/in vivo correlation (IVIVC) study of the patch was evaluated using the IVIVC Wizard in WinNonlin(?) program. The results suggested that the in vivo enhancing effect of l-carvyl esters was in accordance with that in vitro.Histological experiments and cytotoxicity assays were applied to exam the irritation of CV and C-DOD. The results of histological experiments revealed that no histological change in rat skin was examined after pretreatment with CV and C-DOD. The results of cytotoxicity assays showed that CV had a concentration-dependent cytotoxicity, while C-DOD had no significant influence (p>0.05) on cell viability. These results indicated that C-DOD were safer than CV.Confocal laser scanning microscopy (CLSM) and ATR-FTIR were employed to study the mechanism of skin permeation enhancement. The results suggested that l-carvyl esters could disrupt the order of the stratum corneum lipid C-H side chain. And as a result, the fluidity of SC lipid increased and the amount of drug distributed in the SC lipid increased, which was benefit for drug penetration into deeper layer.Post-recovery skin permeability after removal of CV and C-DOD were examined using in vitro skin permeation study and in vivo transepidermal water loss determination. After removal of CV, the skin barrier function recovered in8h. While for C-DOD, the skin barrier function was not restored until C-DOD was removed for24h. In addition, post-recovery procedure of the skin barrier was also recorded using ATR-FTIR. After removal of CV and C-DOD for4h, the blue-shift C-H stretching peaks were restored. Thus, it was demonstrated that the enhancing effect of CV and C-DOD were all reversible.