We previously discussed the possibility of transdermal application of morphine hydrochloride (MPH) [1][2][3] and examined a mixed solvent system containing water, ethanol (EtOH) and l-menthol (LM) as a skin penetration-enhancing system. [4][5][6][7][8] The phase condition of the system markedly affected the penetration enhancing effect.6) The saturated LM in the applied aqueous solvent system was necessary to obtain a strong enhancing effect, but an excessive amount of LM, which made an o/w emulsion, decreased the enhancing effect. When a finite amount of the system was applied to skin, the system composition on skin changed over time due to the high skin permeability of EtOH. 9,10) The composition change of the system must be considered to design a useful transdermal delivery system containing water, EtOH and LM as penetration enhancers.In this study, a finite amount of the mixed solvent system containing MPH, water, EtOH and LM were applied to excised hairless rat skin, and MPH, EtOH and LM permeation was examined. 1,3-Butylene glycol (BG) was used as one more component to control hydrophilicity or lipophilicity of the mixed system. 11) Five different formulas were used for the application systems (Table 1). Three contained almost saturated LM, and the initial content of LM in the rest was below the limit of solubility of LM. Polyethyleneoxide E-45 (PEO) was used as a neutral polymer to increase viscosity of the systems.
MATERIALS AND METHODSMaterials MPH and naloxone hydrochloride were purchased from Takeda Pharmaceutical Co. (Osaka, Japan) and Sigma Chemical Industries (St. Louis, MO, U.S.A.), respectively. LM was purchased from Wako Pure Chemical Industries (Osaka). All other chemicals were of reagent grade and were used without further purification.In Vitro Skin Permeation Experiments The abdominal skin of male WBN/ILA-Ht hairless rats (150-200 g, Ishikawa Laboratory Animals, Saitama, Japan) was excised and mounted in a vertical diffusion cell with a receiver volume of 2.8 ml and an effective diffusion area of 0.95 cm 2 . The receiver cell was filled with distilled water and the cell set was kept at 37°C. Two hundred milligrams of each solution shown in Table 1 was applied to the skin. One milliliter of receiver solution was withdrawn at predetermined times for 24 h to determine concentrations of MPH, LM and EtOH. The same volume of water was added to the receiver compartment to keep the volume constant.Analysis The concentration of MPH was assayed by an HPLC system (pump, LC-9A; UV detector, SPD-6A; Shimadzu Seisakusho, Kyoto, Japan). The column used was a LiChrospher 100 RP-18(e) 5 mm (4.0ϫ250 mm, Kanto Chemical Co., Tokyo, Japan). A mixture of 0.1% phosphoric acid-acetonitrile (65 : 35) containing 5 mM sodium dodecylsulfate was used as the mobile phase, and elution was performed at 40°C with a flow rate of 1.2 ml/min. Aliquots of 200 ml of the receiver solution were withdrawn and mixed with the same volume of methanol containing naloxone hydrochloride as an internal standard, and then the mixture was ce...