Spectroscopic and thermal approaches to investigate the formation mechanism of piroxicam–saccharin co-crystal induced by liquid-assisted grinding or thermal stress

Abstract

The use of co-crystal technology applied to pharmaceutical industry has recently attracted considerable interest. It is important to better understand the mechanism of co-crystal formation via specific intermolecular interactions. The objective of the present study was to evaluate a stepwise mechanism of a co-crystal formation between piroxicam (PIR) and saccharin (SAC) after different grinding and thermal treatments by using spectroscopic and thermal analyses. The physical and ground mixtures of PIR–SAC (molar ratio = 1:1) and their preheated mixtures were analyzed using FTIR, DSC and DSC-FTIR techniques. Typical PIR–SAC co-crystal was prepared by solvent evaporation method. Various PIR–SAC ground mixtures after neat grinding process showed the same FTIR spectra as their physical mixtures, but these ground mixtures might be changed to co-crystals after further thermal treatment. By adding two drops of chloroform into PIR–SAC physical mixture, however, the PIR–SAC co-crystal was gradually formed with the increase in grinding time (>57 min) via inter-/intramolecular N–H···O and C–H···O hydrogen bonding between PIR and SAC. By preheating the PIR–SAC physical mixture over 170 °C, it was also gradually transformed into a co-crystal with temperature. The PIR–SAC co-crystal formation might be possibly attributed to a mobile phase formed between PIR and SAC, leading to a co-crystal formation. This mobile phase could be formed by either solution through a lubricating liquid added during grinding process or eutectic melt via thermal stress. A simultaneous DSC-FTIR technique also directly evidenced the PIR–SAC co-crystal formation via a one-step process. The present study concludes that the chloroform-assisted grinding process or thermal stress easily enhanced a PIR–SAC co-crystal formation via gradual induction of inter-/intramolecular hydrogen bonding between PIR and SAC.