Inclusion complex of usnic acid with β-cyclodextrin: characterization and nanoencapsulation into liposomes

In this study beta-cyclodextrin (beta-CD) was used to improve usnic acid (UA) solubility and the inclusion complex (UA:beta-CD) was incorporated into liposomes in order to produce a targeted drug delivery system for exploiting the antimycobacterial activity of UA. A phase-solubility assay of UA in beta-CD at pH 7.4 was performed. An apparent stability constant of K-1:1 = 234.5 M-1 and a complexation efficiency of 0.005 was calculated. In the presence of 16 mM of beta-CD the solubility of UA (7.3 mu g/mL) increased more than 5-fold. The UA:beta-CD complex was prepared using the freeze-drying technique and characterized through infrared and (HNMR)-H-1 spectroscopy, X-ray diffraction and thermal analyses. The UA:beta-CD inclusion complex presented IR spectral modifications when compared with UA and beta-CD spectra. (HNMR)-H-1 spectrum of UA:beta-CD inclusion complex showed significant chemical shifts in proton H5 located inside the cavity of beta-CD (Delta delta = 0.127 ppm), suggesting that phenyl ring moiety of UA would be expected to be included within the beta-CD cavity, interacting with the H-5 proton. A change in UA from its crystalline to amorphous form was observed on X-ray, suggesting the formation of a drug inclusion complex. DSC analysis showed the disappearance of the UA fusion peak UA:beta CD complex. No differences between the antimicrobial activity of free UA and UA:beta CD were found, supporting the hypothesis that the complexation with cyclodextrin did not interfere with drug activity. Liposomes containing UA:beta CD were prepared using hydration of a thin lipid film method with subsequent sonication. Formulations of liposomes containing UA:beta CD exhibited a drug encapsulation efficiency of 99.5% and remained stable for four months in a suspension form. Interestingly, the encapsulation of UA:beta CD into the liposomes resulted in a modulation of in vitro kinetics of release of UA. Indeed, liposomes containing UA:beta-CD presented a more prolonged release profile of free usnic acid compared to usnic acid-loaded liposomes.