Binding constants of inclusion complexes of nitroimidazoles with β-cyclodextrins in the absence and presence of PVP

Thermodynamics of complexation of 5-nitroimidazoles with β-cyclodextrin and its methylated and hydroxypropyl derivatives in water and in 0.25% polyvinylpyrrolidone are determined by solution calorimetry. A 1:1 stoichiometry was established. The equilibrium constant (K) for all the nitroimidazoles fall in the range 1000–1900 M−1 suitable for use of cyclodextrins as drug carriers. The complexation ability is significantly enhanced by methylation of the β-cyclodextrin. The stability constant increased in the order metronidazole < ornidazole < tinidazole < secnidazole. The presence of polyvinylpyrrolidone enhances the stability constants.