3.4. Phase Solubility Studies

Phase solubility studies of BDQ in the presence of SBE-β-CD was performed according to the method described by Higuchi and Connors [30] and previously published studies from our group with slight modifications [17,25,28]. An excess amount of BDQ (5 mg) was added to centrifuge tubes, containing different concentrations of SBE-β-CD in milliQ water (0–200 mM). Suspensions were bath sonicated for 30 min, then left at room temperature for 24 h under continuous stirring to achieve equilibrium. After 24 h, un-complexed BDQ was separated by filtration with a 0.2 μm polyvinylidene fluoride (PVDF) syringe filter. Filtered solutions were quantified for BDQ content using UV spectrophotometer at 285 nm. Phase solubility diagram was constructed by plotting the dissolved BDQ against respective concentration of SBE-β-CD. The stability constant K_c and complexation efficiency (CE) were calculated from phase solubility diagram using its slope and intercept values using below Equations (2) and (3), respectively.

where, K_c is stability constant and S_o is the solubility of drug without cyclodextrin.