Dissolution, phase behavior and mass transport of amorphous solid dispersions in aspirated human intestinal fluids

Amorphous solid dispersions (ASDs) typically show improved dissolution and generate supersaturated solutions, enhancing the oral bioavailability of poorly soluble drugs. To gain insights into intraluminal ASD behavior, we utilized two poorly soluble drugs with different crystallization tendencies, atazanavir and posaconazole, prepared as ASDs at a 10% drug loading with hydroxypropyl methylcellulose acetyl succinate (HPMCAS). We evaluated their release in aspirated fasted-state human intestinal fluid (FaHIF), and multi-component fasted-state simulated intestinal fluid (composite-FaSSIF), characterizing the supersaturation profiles and drug-rich nanodroplets that formed. Complete release was observed for atazanavir ASDs over a 90 min period. Flux for dissolved atazanavir ASDs remained high over the experimental time period of 3 h. In contrast, posaconazole solution concentrations were initially high and then decreased. Likewise, flux was initially high and then decreased where these changes are attributed to crystallization of the drug. Generation of spherical nano-sized amorphous droplets of ∼100-150 nm was found to occur in ex vivo FaHIF media for both ASDs, maximizing the diffusive flux during the supersaturation window. Moreover, buffer capacity differences were postulated to influence release rates of ASDs in simulated vs aspirated fluids. Importantly, the solution phase phenomena observed during ASD release in simulated fluids, namely amorphous nanodroplet formation and drug crystallization, were also found to occur in aspirated luminal fluids.