Evaluation of rivaroxaban amorphous solid dispersions physical stability via molecular mobility studies and molecular simulations.

The present study evaluates the effect of molecular mobility and molecular interactions in the physical stability of rivaroxaban (RIV) - soluplus® (SOL) amorphous solid dispersions (ASDs). Initially, the use of Adam-Gibbs approach revealed that RIV's molecular mobility (below its glass transition temperature) is significantly reduced in the presence of SOL, while the use of ATR-FTIR spectroscopy showed the formation of hydrogen bonds (HBs) between the two ASD components, indicating that these two mechanisms can be considered as responsible for system's physical stability. Contrary to previously published reports, the utilization of ATR-FTIR spectroscopy in the present study was able to clarify, for the first time, the type of intermolecular interactions formed within the examined ASD system, while the presence of a separate drug-rich amorphous phase (significantly increasing as the content of the drug increases) was also identified. Furthermore, in order to gain an insight into the intermolecular interactions responsible for drug's amorphous phase separation, molecular dynamics (MD) simulation models were utilized as realistic representations of the actual systems. Analysis of the obtained trajectories showed that the formation of strong intermolecular HBs between RIV's secondary amide proton and its three carbonyl oxygens (originating from the οxazolidone, oxomorpholin and carboxamide part of the drug molecule) as well as the significant reduction of the available HB acceptors in SOL due to copolymer's chain shrinkage, were responsible for the formation of a separate drug-rich amorphous phase within the ASD.