Therefore, a high-pressure apparatus will be used to determine them experimentally. From the theoretical point of view, the PC-SAFT equation of state will be utilized to calculate the relating phase diagrams. Additionally, the size-dependency of the API solubility in both water and organic solvents will be investigated, especially how the thermodynamic equilibrium solubility changes if the diameter of the API particles reaches the nanoscale. For that manner, the interfacial tension between the solid and the liquid phase will be determined experimentally. Regarding the modelling of this phenomenon, a new theoretical framework will be developed to incorporate this size-dependency into, namely PC-SAFT. Furthermore, the influence of polymeric excipients on API solubilities will be investigated and best stabilizing excipients for long acting (LA) formulations will be identified.