Complex formation of sericoside with hydrophilic cyclodextrins: improvement of solubility and skin penetration in topical emulsion based formulations

The main objective of this study was to devise novel methods for improving the solubility of the anti-inflammatory triterpenoid sericoside, the main component of Terminalia sericea extract, thus enabling its incorporation into topical formulations. Sericoside was stabilized by complex formation with hydrophilic derivatives of beta- and gamma-cyclodextrins in a molar ratio of 1.0:1.1. The complex of extract and cyclodextrin was equilibrated in water at 25 degrees C for approximately 24 h. The dehydrated complexes of T. sericea extract and cyclodextrin were characterized by differential scanning calorimetry, thermogravimetry analysis and X-ray diffraction. Complex formation with beta-cyclodextrin as well as gamma-cyclodextrin derivatives was detectable using these three analytical tools; however, only complexes with gamma-cyclodextrin derivatives showed stability upon storage after incorporation into topical o/w or w/o formulations. Furthermore, a T. sericea extract/gamma-cyclodextrin complex incorporated in an o/w formulation resulted in a 2.6-fold higher percutaneous penetration of sericoside in in vitro excised pig skin as compared to pure T. sericea extract. For the first time, the virtually insoluble anti-inflammatory active sericoside was incorporated into a topical emulsion based formulation in a stable manner, resulting in efficient skin penetration.