SUSTAINED-RELEASE FROM REVERSE MICELLAR SOLUTIONS BY PHASE-TRANSFORMATIONS INTO LAMELLAR LIQUID-CRYSTALS

Reverse micellar solutions of either fenoprofen acid (FH) or fenoprofen sodium salt (FNa) in isopropylmyristate (IPM) were formed in presence of phospholipids (PL), Phospholipon 100. Solubilization of water within the micelles is also possible. However, the increase of water content causes a change in shape and size and finally a phase transformation from the reverse micellar solution into a lamellar liquid crystal. The colloidal microstructure of the micelles was studied by small angle X-ray scattering (SAXS) and viscosimetry. Solubilization of the drug in its free acid form results in almost spherical micelles, while solubilization of drug in its sodium salt form results in cylindrical micelles. The lamellar liquid crystals which form on contact with aqueous media can be used for sustained release, as the diffusion coefficient of the drug within the liquid crystals is smaller by factor 100 than that within an oily solution. The apparent diffusion coefficient of the drug depends on the thickness of the liquid crystalline interface which is also influenced when either the free acid or the salt is solubilized in the system.