Characterization, activity, and computer modeling of a molecular inclusion complex containing rifaldazine

Qunyou Tan,^1,* Dan He,^2,* Mingjun Wu,^2,* Lin Yang,³ Yong Ren,⁴ Juan Liu,² Jingqing Zhang,²

¹Department of Thoracic Surgery, Institute of Surgery Research, Daping Hospital, Third Military Medical University, Chongqing, ²Medicine Engineering Research Center, Chongqing Medical University, Chongqing, ³Chongqing Institute for Food and Drug Control, Chongqing, ⁴Center of Drug Discovery, Nanjing Normal University, Nanjing, People's Republic of China

*These authors contributed equally to this work

Background: The purpose of this study was to develop, characterize, and investigate a molecular inclusion complex containing rifaldazine with good solubility and antibacterial activity.
Methods: Rifaldazine, a lipophilic molecule, was encapsulated into the hydrophobic cavity of ß-cyclodextrin to form a molecular inclusion complex (RAABCD) with good solubility. RAABCD was prepared in a short time using a solid-state grinding method. The inclusion ratio, binding constant, and change in Gibbs free energy were determined by a phase solubility diagram and/or ultraviolet-visible spectroscopy. Differential scanning calorimetry and Fourier transform infrared spectroscopy of RAABCD were performed. Morphological features of RAABCD were observed by photomicroscopy. The most likely optimal configuration for RAABCD was simulated by computer modeling. Broth macrodilution testing was done to investigate the antibacterial activity of RAABCD.
Results: The inclusion ratio, binding constant, and change in Gibbs free energy, determined by a phase solubility diagram and/or ultraviolet-visible spectroscopy were 1:1, 288.33/261.33 L/mol, and 32.29/31.73 kJ/mol, respectively. Differential scanning calorimetry and Fourier transformed infrared spectra of RAABCD confirmed the molecular interaction between rifaldazine and ß-cyclodextrin. The morphological difference between irregular and amorphous-shaped RAABCD and columnar-shaped rifaldazine further confirmed the molecular encapsulation of rifaldazine. The most likely optimal configuration for RAABCD was confirmed by computer modeling. Broth macrodilution testing indicated that RAABCD had good antibacterial activity.
Conclusion: RAABCD had improved solubility and good activity, and might be a promising alternative for treatment of a range of bacterial infections.

Keywords: rifaldazine, cyclodextrin inclusion complex, stoichiometric relationships, differential scanning calorimetry, Fourier transform infrared spectra, computer modeling