Effects of aging on crystallization, dissolution and absorption characteristics of amorphous tolbutamide-2-hydroxypropyl-beta-cyclodextrin complex

The effects of storage on the crystallization, dissolution and absorption of tolbutamide from amorphous tolbutamide-2-hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) complex were investigated, in comparison with those of polyvinylpyrrolidone (PVP) solid dispersion. The amorphous solid complex of tolbutamide with HP-beta-CyD and the solid dispersion of tolbutamide with PVP were prepared by a spray-drying method. During storage, a stable form of tolbutamide (form I) was crystallized from the amorphous PVP dispersion, whereas a metastable form of tolbutamide (form II) was crystallized from the HP-beta-CyD complex. The dissolution rate of tolbutamide from both HP-beta-CyD complex and PVP dispersion was significantly faster than that of tolbutamide alone. However, the dissolution rate from the PVP dispersion markedly decreased with storage, because of the formation of slow dissolving form I crystals. On the other hand, the dissolution rate from the HP-beta-CyD complex was only slightly decreased due to the formation of fast dissolving formII crystals. These in vitro dissolution characteristics were clearly reflected in the in vivo absorption of tolbutamide and the glucose plasma level after oral administration in dogs. The results suggested that HP-beta-CyD is useful not only for converting crystalline tolbutamide to an amorphous substance, but also for maintaining the fast dissolution rate of the drug over a long period. Furthermore, the crystallization of drugs from CyD complexes, with storage, seemed to be different from that involving polymer excipients such as PVP.