A new method of constructing drug-polymer temperature-composition phase diagram relevant to the hot-melt extrusion platform

Current experimental methodologies used to determine the thermodynamic solubility of an API within a polymer typically involves establishing the dissolution/melting endpoint of the crystalline API within a physical mixture, or through the use of the glass transition temperature measurement of a de-mixed amorphous solid dispersion. The measurable "equilibrium" points for solubility are normally well above the glass transition temperature of the system meaning extrapolation is required in order to predict the drug solubility at pharmaceutical relevant temperatures. In this manuscript we argue that, the presence of highly viscous polymers in these systems results in experimental data that exhibits an under or over estimated value relative to the true thermodynamic solubility. In previous work we demonstrated the effects of experimental conditions and their impact on measured and predicted thermodynamic solubility points. In the light of current understanding, we have developed a new method to limit error associated with viscosity effects for the application in small-scale hot-melt extrusion (HME). In this study HME was used to generate an intermediate (multi-phase) system containing crystalline drug, amorphous drug/polymer rich regions as well as drug that was molecularly dispersed in polymer. An extended annealing method was used together with high-speed differential scanning calorimetry to accurately determine the upper and lower boundary of the thermodynamic solubility of a model drug -polymer system (felodipine and Soluplus®). Compaed to our previously published data, the current results confirmed our hypothesis that the prediction of the liquid-solid curve using dynamic determination of dissolution/melting endpoint of the crystalline API physical mixture presents an underestimation relative to the thermodynamic solubility point. With this proposed method, we were able to experimentally measure the upper and lower boundary of liquid-solid curve for the model system. The relationship between inverse temperature and drug-polymer solubility parameter (χ) remained linear at lower drug loadings. Significant higher solubility and miscibility between felodipine-Soluplus® system were derived from the new χ values.