Influence of additives on the properties of nanodroplets formed in highly supersaturated aqueous solutions of ritonavir.

The formation of colloidal drug aggregates of lipophilic drugs is thought to be of relevance for the oral delivery of poorly water-soluble drugs. In this study, the underlying basis for colloid formation from amorphous solid dispersions and the impact of additives on colloidal stability were evaluated. A relationship was found between the concentration at which colloidal droplets formed upon dissolution of an amorphous solid dispersion and the liquid-liquid phase separation (LLPS) transition concentration, whereby the latter is related to the theoretical amorphous "solubility" value. The composition of the dispersed phase in ritonavir-polymer-water solutions was confirmed to be a noncrystalline, ritonavir-rich droplet phase. Additives were found to impact the size, stability, and crystallization behavior of the colloidal phase. In general, charged additives reduced the kinetics of droplet coalescence, but had a variable effect on crystallization kinetics, either promoting or inhibiting crystallization. Through proper selection of formulation components, it thus appears possible to promote the formation of ∼ 250-350 nm colloidal droplets of ritonavir following dissolution of an amorphous solid dispersion, and to inhibit coalescence and crystallization from these two-phase supersaturated solutions.