Characterization and relevance of physicochemical interactions among components of a novel multiparticulate formulation for colonic delivery

The primary objective of this study was to investigate potential interactions among a model drug (azathioprine; AZA), polymers and a divalent metal ion, which were utilized in the development of a novel multiparticulate formulation for colonic delivery. The approach involved preparation of beads by ionotropic gelation of deacylated gellan gum (DGG) in the presence of Ca2+ ions, followed by coating with Eudragit®S-100. Various possible physicochemical interactions among formulation components were characterized by DSC, FT-IR, XRPD, 1H-NMR, and an isothermal stress test. Results of isothermal stress testing indicated that there was no significant interaction of AZA with DGG and Eudragit® S-100. However, results of DSC and XRPD studies suggested that there could be interactions between AZA and DGG, and ionotropic gelation can affect the physical state of AZA, which may have implications for drug release characteristics and, physical and chemical stability. The results of FT-IR studies were suggestive of interactions of DGG with AZA and Eudragit® S-100, and provided evidence for interactions of AZA and DGG with Ca2+ ions. The electrostatic interaction of DGG with Ca2+ was also supported by results of DSC studies while that between AZA and Ca2+ was confirmed by 1H-NMR studies. This study, to our knowledge, is first reported investigation in which the unique thermal property of gellan gum gels, and possible interactions between a drug and counter ions of an ionotropic agent have been demonstrated through bead characterization studies. The formation of AZA–Ca2+ complex could have an impact on drug release kinetics, product stability and clinical efficacy for treatment of inflammatory bowel diseases or other diseases, which merit further investigation.