Effect of pretreatment of skin with cyclic monoterpenes on permeation of diclofenac in hairless rat.

The promoting effect of d-limonene and l-menthol on the percutaneous absorption of nonionized and ionized diclofenac (DF) was investigated employing a pretreatment method. After the pretreatment of hairless rat skin with an ethanol buffer solution containing terpenes, the permeation study was performed in vitro. The permeability coefficients of nonionized (Pn) and ionized diclofenac (Pi) were calculated under the assumption that the total flux was composed of individual fluxes of nonionized and ionized DF. In the case of pretreatment with d-limonene, both Pn and Pi values were increased dramatically. However, the promoting magnitude was not affected by an extension of the pretreatment period. In contrast, when the skin was pretreated with l-menthol, the Pn and Pi values increased gradually as the pretreatment period increased. Based on the measurement of the solubility of terpenes in the pretreatment solution, the difference in promoting efficiency could arise from the difference in thermodynamic activity of the terpenes.