Inclusion complexes of estrone and estradiol with b-cyclodextrin: Voltammetric and HPLC studies Claudia Yan ˜ ez, * Julio Basualdo, Paola Jara-Ulloa and J. Arturo Squella Organic and Physical Chemistry Department, Chemical and Pharmaceutical Sciences Faculty, University of Chile, Santiago, Chile ABSTRACT: The interaction of estrone and estradiol with b-cyclodextrins (bCD) was investigated by differential pulse voltammetry (DPV) and high-performance liquid chromatography (HPLC) in mixed media. The co-solvent influence on the tendency of these estrogens to form inclusion complexes with bCD was examined. Thus, acetonitrile (MeCN) and ethanol (EtOH) were used in a mixed aqueous medium containing phosphate buffer. The association constant of the inclusion complexes (K a ) of estrone and estradiol with bCD were determined in two different media by using both voltammetric and chromatographic techniques. Estradiol was found to bind to bCD with higher affinities than estrone, irrespective of the medium. We have also found a clear influence of the co-solvent on the K a value, which means a competition of co-solvent molecules with estrogens for binding to the cavity of bCD. Consequently, interaction between bCD and the steroids is weakened when acetonitrile is used. KEYWORDS: b-cyclodextrin; inclusion complexes; steroids INTRODUCTION Cyclodextrins (CDs) are cyclic compounds that have been widely used in the pharmaceutical industry due to their ability to form inclusion complexes. This property has been used to solubilize, stabilize, decrease the volatility of drug molecules, and improve bioavailability. 1 To predict the effects of CDs on these properties, it is important to know the strength of binding of these agents to drugs. Therefore, determination of the association constant (K a ) of cyclodextrin/guest complexes is very important because it accounts for the magnitude of interaction between host and guest molecules. Several techniques have been used to examine the interaction between CD and different drugs, including spectroscopic, 2 calorimetric, 3 and chromatographic methods. 4–6 Some voltammetric methods have also been used to study the formation of some inclusion complexes of bCD in aqueous 7 and mixed 8 media. Furthermore, a polarographic method has also been used to study a- and bCD interaction with chloronitrobenzenes, 9 and with the supramolecular system, meso-tetrakis (4-N-trimethyl- aminobenzyl) porphyrin, with different CDs. 10 In this work, we evaluate the utilization of a voltammetric technique to determine bCD interaction with some steroids of interest such as estrone and estradiol. Estrone and estradiol are naturally occurring steroid hormones (estrogens) essential for the development and maintenance of female sexual characteristics. Moreover, estrogens are known to be powerful antioxidants independently of their binding to estrogen receptors and hormonal functions. 11 CD-encapsulated steroids are widely used in drug formulation 12,13 resulting in beneficial pharmaceutical effects including increase in water solubility and stability. Studies of steroid/cyclo- dextrin complexes are interesting from the point of view of drug design. 14 In this process, formulation becomes an important aspect based on the need for appropriate dosage form, stability, solubility, and dissolution character- istics. 15 In spite of the number of publications in this area, there are only about 30 different pharmaceutical products on the market. On this way, investigations of interactions and structure of the complexes are required because it is important for a pharmaceutical formulator to know an optimized cyclodextrin/drug relationship in relation to drug delivery from the formulations. Inclusion complexes of estrogen-related steroids with cyclodextrins have been studied principally by high-performance liquid chromatography (HPLC). 1,5 The stoichiometry and bind- ing constants of the complexes under study have been *Correspondence to: C. Yan ˜ez, Organic and Physical Chemistry Department, Chemical and Pharmaceutical Sciences Faculty, Univer- sity of Chile, P.O. Box 233 Santiago, Chile. E-mail: cyanez@ciq.uchile.cl