Multiple Complex Formation of Unstable Compounds with Cyclodextrins: Efficient Determination and Evaluation of the Binding Constant With Improved Kinetic Studies

Equations were derived that allow the determination of cyclodextrin–compound complex primary and secondary binding constants by using kinetic analysis (by monitoring the degradation rate constant of the compound in the presence of cyclodextrins). The degradation rate constant is dependent on the concentration of cyclodextrins in the solution of the compound and also on the stoichiometry of the cyclodextrin–compound complex. It was found that riboflavin (a photosensitive compound) forms a 1:2 inclusion complex with α-cyclodextrin and a 1:1 complex with γ-cyclodextrin. Indomethacin (sensitive to hydrolysis) also forms a 1:1 complex with β-cyclodextrin. Both compounds are unstable and can be stabilized through complexation with cyclodextrins. The binding constants were also calculated fluorimetrically for riboflavin and by 1 H NMR spectrometry for indomethacin in order to compare the values from the different methods. The fact that the same compound (riboflavin) can exhibit different binding behaviors with different cyclodextrins could result in new descriptive studies for each particular case.