Insights into the Dissolution Behavior of Ledipasvir–Copovidone Amorphous Solid Dispersions: Role of Drug Loading and Intermolecular Interactions

The generation of a colloidal drug-rich phase by dissolving an amorphous solid dispersion (ASD) is thought to have a positive impact on oral absorption and bioavailability. Thus, understanding which formulations generate these species is important. In this study, ledipasvir–copovidone ASDs, with and without surfactants, were prepared, and their release performance was examined at different drug loadings. An intrinsic dissolution rate assembly was used to limit potential surface area variations among formulations, and the release of both polymer and drug was monitored as a function of time. Drug-rich colloids only formed when the drug loading (DL) was at or below 5%; at a DL of 7.5% or above, drug release became negligible. The drug and polymer released congruently at and below 5% DL and incongruently at higher DLs. Thus, the limit of congruency (LoC) is between 5 and 7.5% DL. X-ray photoelectron spectroscopy (XPS) of partially dissolved tablet surfaces revealed that a drug-rich layer formed on the surface of the tablet. This was most evident for the higher DL ASDs and led to amorphous drug-controlled dissolution. Consequently, the surface drug-enriched layer physically hindered the polymer from further release. Evidence is provided that the extent of drug–polymer interactions as a function of DL plays a central role in dictating the observed release behavior. Some surfactants were found to promote the formation of drug-rich colloids at considerably higher DLs, providing a formulation strategy to increase the LoC.