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Background: Most current drug candidates are poorly water-soluble. For the purpose of improving solubility of poorly water-soluble drugs, various polymers have been widely used. Aminoalkylmethacrylate copolymer Eudragit^® E (EUD-E), which is a basic polymer, shows strong crystallization inhibition effect of drug and drug solubilization effect. Objective: In this study, we evaluated the effect of solution pH on EUD-E nanostructure and its solubilizing ability of poorly water-soluble drugs. The relationship between the nanostructure and drug solubilizing ability of EUD-E was discussed. Materials and Methods: EUD-E solution (3 mg/mL) was prepared by dissolving EUD-E in 20 mM acetate buffer. The pH of EUD-E was adjusted to pH 4.5 and pH 6.5. EUD-E nanostructure in each solution was evaluated by synchrotron small angle X-ray scattering (SAXS) and atomic force microscopy (AFM) measurements. Naringenin (NAR) was used as a model poorly water-soluble drug and solubilizing ability was evaluated by quantifying the equilibrium solubility of NAR in each EUD-E solution. NAR molecular state in each EUD-E solution was evaluated by NMR measurement. Results: Kratky plot analysis from SAXS profiles revealed that EUD-E formed random coil structure at pH 4.5 and globule structure at pH 6.5. From AFM images, worm-like structures with a height of around 1.2 nm and globule structures with a height of 2.5 – 3.5 nm were observed at pH 4.5 and pH 6.5, respectively. NAR solubility in EUD-E solution improved 6.5 and 15.2 times compared to non-polymer solution at pH 4.5 and pH 6.5, respectively. NMR measurement showed that molecular mobility of NAR in EUD-E solution was more strongly suppressed at pH 6.5 than at pH 4.5. From these results, the formation of hydrophobic region derived from the EUD-E globule structure at pH 6.5 would enhance the solubilizing ability, due to its strong interaction with NAR