Preparation and characterization of stearyl glycyrrhetinate/cyclodextrin complex using co-grinding

Aim: In this study, the physicochemical properties of stearyl glycyrrhetinate/β-cyclodextrin (SG/βCD) and SG/γCD complexes were characterized, and the complexes were prepared using the co-milling method. The molecular interactions within the SG/CD complexes were also investigated using nuclear magnetic resonance (NMR) measurements to determine the mode of interaction. Methods: Here, we evaluated the physicochemical properties of SG complexes with CDs prepared by ground mixtures (GM SG/βCD or γCD = 1/1, 1/2). Results: Powder X-ray diffraction (PXRD) showed that the characteristic SG and CD peaks disappeared after co-grinding with GM (SG/CD = molar ratio of 1/2), indicating a halo pattern. Differential scanning calorimetry (DSC) measurements showed that after co-grinding, the endothermic peak due to SG melting, as well as the dehydration peak and the endothermic peak due to SG melting, disappeared. Near-infrared (NIR) spectroscopy results showed that the peaks of SG-derived –CH moieties and CD-derived –OH and –CH moieties broadened in GM, suggesting their involvement in complex formation through SG/CDs intermolecular interactions. In GM (SG/CDs), NMR measurements showed broadened H-A and H-F peaks of the steroid skeleton derived from SG. In GM (SG/βCD = 1/2), the doublet peak, especially OH-3 at the wide edge of CD, shifted to a singlet peak. In GM (SG/γCD = 1/2), the H-3 peak, which is the wide edge of γCD, and the H-6 peak, which is the narrow edge, shifted to broad peaks, suggesting that γCD is deeply encapsulated in the steroidal structure. Conclusions: These findings suggest that complex formation occurred in SG/CDs and that inclusion behavior is different between GM (SG/βCD = 1/2) and GM (SG/γCD = 1/2).