Impact of a Single Hydrogen Substitution by Fluorine on the Molecular Interaction and Miscibility between Sorafenib and Polymers

We aim to understand the potential impact of a modest chemical modification of a drug molecule on the downstream design of its amorphous solid dispersion (ASD) formulation. To this end, we used sorafenib (SOR) and its fluorinated form, regorafenib (REG), as model drugs, to assess the impact of a single hydrogen substitution by fluorine on the molecular interaction and miscibility between drug and PVP or PVP-VA, two commonly used polymers for ASDs. In this study, we observed that the T-g values of PVP or PVP-VA based ASDs of SOR deviated positively from the Gordon-Taylor prediction, which assumes ideal mixing, yet the T-g of REG ASDs deviated negatively from or matched well with the ideal mixing model, suggesting much stronger drug-polymer interactions in SOR ASDs compared with the REG ASDs. Using solution NMR and computational methods, we proved that a six-member-ring formed between the carbonyl groups on the polymers and the uramido hydrogen of SOR or REG, through intermolecular hydrogen bonding. However, steric hindrance resulting from fluorination in REG caused weaker interaction between REG-polymer than SOR-polymer. To further confirm this mechanism, we investigated the molecular interactions of other two uramido-containing model compounds, triclocarban (TCC) and gliclazide (GCZ), with PVP. We found that TCC but not GCZ formed a hexatomic ring with PVP. We concluded that PVP based polymers can easily interact with N,N'-disubstituted urea compounds with a trans-trans structure in the form of hexatomic rings, and the interaction strength of the hexatomic ring largely depended on the chemistry of drug molecules. This study illustrated that even a slight chemical modification on drug molecules could result in substantial difference in drug-polymer interactions, thus significantly impacting polymer selection and pharmaceutical performance of their ASD formulations.