Predicting Drug-Polymer Compatibility in Amorphous Solid Dispersions by MD Simulation: On the Trap of Solvation Free Energies

Amorphous solid dispersions (ASDs) are a prevalent method for increasing the bioavailability and apparent solubility of poorly soluble drugs. Consequently, extensive research, encompassing both experimental and computational approaches, has been dedicated to developing methods for assessing the key factors influencing their stability, notably drug-polymer interactions. A common computational approach to rank the compatibility of a drug with a set of solvents or polymers is to compare thermodynamic observables, such as solvation free energies at infinite dilution. However, the impact of the molecular weight of the polymer excipient on these interactions remains underexplored. This study delves into this impact through atomistic simulations of Indomethacin in PVP(-VA) and HPMC, and through simulations using a coarse-grained model, emphasizing its critical importance. First, we demonstrate that the molecular weight of the polymer plays a pivotal role in determining the solvation free energy of the drug, at times exerting a more significant influence than the specific chemical identity of the polymer. Additionally, our simulations suggest that higher molecular weight polymers lead to lower solvation free energies and, thus, suggest better compatibility with the drug. Yet, the lower free energy of solvation of the drug in longer polymers does not translate into a higher solubility. This work highlights the subtle role polymer molecular weight plays when measuring thermodynamic observables in amorphous solid dispersions, a role which must be considered when optimizing pharmaceutical formulations.