Viscoelastic Behavior of Supercooled and Glassy ASDs at Humid Conditions Can Be Predicted

Amorphous solid dispersions (ASDs) are commonly used to increase the dissolution rate of poorly soluble active pharmaceutical ingredients (APIs). Unfortunately, most ASDs are thermodynamically unstable and, even though kinetically stabilized, will thus eventually crystallize. The crystallization kinetics is determined by the thermodynamic driving force and by molecular mobility, which in turn depend on the drug load, temperature, and relative humidity (RH) at which the ASDs are stored. This work focuses on viscosity as an indicator for the molecular mobility in ASDs. The viscosity and shear moduli of ASDs consisting of the polymer poly(vinylpyrrolidone-co-vinyl acetate) or hydroxypropyl methylcellulose acetate succinate and the API nifedipine or celecoxib were studied using an oscillatory rheometer. The effects of temperature, drug load, and RH on the viscosity were investigated. With the knowledge of how much water is absorbed by the polymer or ASD and thereby also the knowledge of the glass-transition temperature of the wet polymer or ASD, the viscosity of dry and wet ASDs was predicted to be in very good agreement with experimental data just based on the viscosity of neat polymers and the glass-transition temperatures of wet ASDs.