Energetic insights into alendronate–β-cyclodextrin complexation

Abstract

Abstract Bisphosphonates are widely used for the treatment of bone-related disorders; however, their low bioavailability limits therapeutic efficacy and increases long-term adverse effects. This study investigated the thermal behavior and binding energetics of the alendronate (ALEN)–β-cyclodextrin (β-CD) inclusion complex (IC) at different protonation states. Molecular dynamics simulations identified β-CD as the most suitable natural host for ALEN, promoting the formation of a stable 1:1 IC. The ALEN–β-CD interaction in aqueous solution was evaluated by isothermal titration calorimetry, revealing the spontaneous formation of a 1:1 complex over the physiologically relevant pH range (4.0–8.0). Binding was predominantly entropy-driven at pH 5.3, 7.4, and 8.0, whereas enthalpic contributions dominated at pH 4.0. To the best of our knowledge, this is the first study describing the microscopic binding constants ( K b ) for the ALEN–β-CD inclusion complex. The successful formation of the IC in the solid state was also confirmed by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. Thermal characterization revealed a distinct degradation profile compared with ALEN, β-CD, and their physical mixture, including the appearance of a single well-defined degradation stage. Kinetic analyses further demonstrated that the IC exhibited the highest activation energy among the evaluated systems. In addition, MTT assays showed that complexation significantly reduces the cytotoxic profile of ALEN. Overall, these findings confirm the formation of the ALEN–β-CD inclusion complex in both solution and solid states and provide valuable insights into the thermodynamic aspects of ALEN–β-CD complexation, supporting future investigations of cyclodextrin-based bisphosphonate formulations.