Use of cyclodextrins as solubilizing agents for simvastatin: Effect of hydroxypropyl-?-cyclodextrin on lactone/hydroxyacid aqueous equilibrium

The chemical conversion of simvastatin from the lactone (SVL) to the hydroxyacid (SVA) form is becoming an intriguing issue associated with the pharmacological use of SVL. On this matter, recent findings suggest that SVL complexation with cyclodextrins (CDs) may be a useful strategy to affect its aqueous solubility and chemical stability. In this work, a reverse-phase high-performance liquid chromatography (RP-HPLC) method able to selectively identify and quantify SVL and SVA has been set up, validated and applied to follow SVL hydrolysis in the presence of HP?CD. The combination of stability results with simvastatin/HP?CD stability constants achieved from UV-vis measurements and solubility/dissolution studies allowed to get an insight into SVL/HP?CD, SVA/HP?CD and SVL/SVA equilibria taking place in aqueous solution. Results show that in the presence of HP?CD the aqueous SVL/SVA equilibrium is shifted versus the hydroxyacid form. UV-vis results, showing that the lactone and the open-ring form of simvastatin interact with HP?CD in a similar extent, suggest that hydrolysis occurs also on SVL/HP?CD complex, thus supporting a mode of interaction that does not involve the lactone ring. This hypothesis is strengthened by NMR analysis performed on SVA, HP?CD and their inclusion complex, which indicates that the lactone ring is not included in HP?CD hydrophobic cavity. Finally, results suggest that particular attention must be paid to SVL lactonization in aqueous solution when using CD-based formulations and in demonstrating their effective benefit for a specific therapeutic use. © 2010 Elsevier B.V. All rights reserved.