Preparation and Application of Decanoic Acid/Arginine Hydrogels to a Transdermal Formulation.

We prepared a supramolecular hydrogel composed of decanoic acid and arginine (C10/Arg gel) and evaluated its application to a transdermal formulation. C10/Arg gel adjusted to pH 7 with 1 M NaOH aq or 1 M HCl aq provided a translucent hydrogel with a lamellar liquid crystal structure in the concentration region of decanoic acid ≥12% and arginine ≤9%. Rheological measurements showed that C10/Arg gel is a viscoelastic material with both solid and liquid properties, with elasticity being dominant over viscosity in the low shear stress region. The skin permeability of hydrocortisone (HC) and indomethacin (IM) from C10/Arg gels was investigated in vitro using hairless mouse skin and compared to control formulation drug suspensions (IM or HC) in water. The cumulative permeation amount of HC and IM from the C10/Arg gel at 10 h after application was approximately 16 and 11 times higher than that of the control, respectively. On the other hand, the flux of IM decreased with increasing arginine concentration, likely due to the acid-base interaction between Arg and IM in C10/Arg gel. Adequate drug skin permeation enhancement by C10/Arg gel requires optimizing the gel composition for each specific drug.

Influence of Surfactant on the Skin Permeation of Methylisothiazolinone and Methylchloroisothiazolinone.

Patch tests are often used in safety evaluations to identify the substance causing skin irritation, but the same substance can sometimes give positive or negative results depending on the test conditions. Here, we investigated differences in the skin penetration of two test compounds under different application conditions. We studied the effects of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant polysorbate 80 (PS) on skin penetration of the preservatives methylisothiazolinone (MT) and methylchloroisothiazolinone (MCT), which are used in cosmetics such as shampoos. The skin permeation of MT was enhanced by SDS but was unchanged by PS. Skin impedance decreased in the presence of SDS whereas PS had the same effect as the control aqueous solution, suggesting that SDS reduction of the barrier function of skin affects the permeation of MT, a hydrophilic drug. Application of a mixture of MCT and MT in the presence of SDS did not affect the skin permeation of MCT whereas the permeation of MT was enhanced by SDS, indicating that the skin permeation of MCT is less affected by SDS than is MT. Thus, attention should be paid to the possible effect of co-solutes, especially hydrophilic drugs.

Rheological Properties and Composition Affecting the Skin Permeation of a Model of a Hydrophilic Drug in Lecithin Reverse Wormlike Micelles.

We investigated the effect of the rheological properties and composition of lecithin reverse wormlike micelles (LRWs) on the skin permeation of a model of a hydrophilic drug to determine whether LRWs support uniform hydrophilic drug/oil-based formulations and good drug penetrate into skin. Here, we prepared LRWs with D (-)-ribose (RI) or glycerol (GL) as polar compounds, liquid paraffin (LP) or isopropyl myristate (IPM) as oils, and 6-carboxyfluorescein (CF) as a model for a hydrophilic drug, and evaluated the rheological properties and skin penetration characteristics of the preparations. The LRWs showed moderate viscosity at 25 °C, a typical storage temperature, but decreasing viscosity at 32 °C, the surface temperature of human skin, suggesting that the LRWs would penetrate the microstructure of skin (e.g., wrinkles and hair follicles). The highest skin permeability of CF was observed when IPM was used as the oil, suggesting that both the stratum corneum and hair follicle routes are involved in drug permeation. The penetration of CF into hair follicles is influenced not only by the rheology of the formulation but also by the interaction between IPM and sebum in the hair follicles.