Characterization of the molecular distribution of drugs in glassy solid dispersions at the nano-meter scale, using differential scanning calorimetry and gravimetric water vapour sorption techniques

Abstract

The molecular distribution in fully amorphous solid dispersions consisting of poly(vinylpyrrolidone) (PVP)–diazepam and inulin–diazepam was studied. One glass transition temperature ( T g), as determined by temperature modulated differential scanning calorimetry (TMDSC), was observed in PVP–diazepam solid dispersions prepared by fusion for all drug loads tested (10–80 wt.%). The T g of these solid dispersions gradually changed with composition and decreased from 177 °C for pure PVP to 46 °C for diazepam. These observations indicate that diazepam was dispersed in PVP on a molecular level. However, in PVP–diazepam solid dispersions prepared by freeze drying, two T g's were observed for drug loads above 35 wt.% indicating phase separation. One T g indicated the presence of amorphous diazepam clusters, the other T g was attributed to a PVP-rich phase in which diazepam was dispersed on a molecular level. With both the value of the latter T g and the Δ C p of the diazepam glass transition the concentrations of molecular dispersed diazepam could be calculated (27–35 wt.%). Both methods gave similar results. Water vapour sorption (DVS) experiments revealed that the PVP-matrix was hydrophobised by the incorporated diazepam. TMDSC and DVS results were used to estimate the size of diazepam clusters in freeze dried PVP–diazepam solid dispersions, which appeared to be in the nano-meter range. The inulin–diazepam solid dispersions prepared by spray freeze drying showed one T g for drug loads up to 35 wt.% indicating homogeneous distribution on a molecular level. However, this T g was independent of the drug load, which is unexpected because diazepam has a lower T g than inulin (46 and 155 °C, respectively). For higher drug loads, a T g of diazepam as well as a T g of the inulin-rich phase was observed, indicating the formation of amorphous diazepam clusters. From the Δ C p of the diazepam glass transition the amount of molecularly dispersed diazepam was calculated (12–27 wt.%). In contrast to the PVP–diazepam solid dispersions, DVS-experiments revealed that inulin was not hydrophobised by diazepam. Consequently, the size of diazepam clusters could not be estimated. It was concluded that TMDSC enables characterization and quantification of the molecular distribution in amorphous solid dispersions. When the hygroscopicity of the carrier is reduced by the drug, DVS in combination with TMDSC can be used to estimate the size of amorphous drug clusters.