Complexation of Steroid Hormones with Cyclodextrin Derivatives: Substituent Effects of the Guest Molecule on Solubility and Stability in Aqueous Solution

The inclusion complexation of homologous derivatives of steroid hormones with cyclodextrins and 2-hydroxypropyl-β-cyclodextrin (2-HP-β-CD) was investigated with regard to underlying structure-interaction relationship. The interaction was studied by phase solubility analysis and stabilization effects of complex formation with 2-HP-β-CD. The solubilizing and stabilizing abilities of 2-HP-β-CD were generally more effective for testosterone derivatives than for estradiol esters. Within a homologous series of steroid hormones, the steepest linear solubility isotherms were found for 17-methyl and 3-methyl derivatives. The solubilization of steroid esters by 2-HP-β-CD depended on the structure and length of the ester side chain. The interaction of 2-HP-β-CD with the steroids was hindered by long-chain fatty acid ester groups. With increasing length of the side chain, a decline of the isotherms occurred and the phase solubility behavior changed from linear to exponential. Contrary to expectations, benzoylation of steroids considerably decreased the guest-host interaction. The observed rates of degradation of the steroid esters were significantly reduced by 2-HP-β-CD, depending on the chain length, and correlated well with the order found in phase solubility analysis. The degradation showed no deviations from pseudo-first-order kinetics, and the degradation mechanism was not changed because of complexation. The results suggest that interaction of 2-HP-β-CD with steroid esters involves the ester functions of the prodrugs and is more suitable for unsubstituted guest molecules.