Thermodynamic and kinetic evaluation of the impact of polymer excipients on storage stability of amorphous itraconazole
Two pharmaceutical polymers with high glass transition temperatures (Tg>100°C), polyvinylpyrrolidone-vinyl acetate copolymer (PVPVA) and hydroxypropyl methylcellulose acetate succinate (HPMCAS), have been assessed for their impact on the storage stability of itraconazole (ITZ) amorphous solid dispersions (ASDs). The results showed that the inhibitory effect of PVPVA on the recrystallization of amorphous ITZ was highly sensitive to surrounding relative humidity (RH), especially at RH above 60%. In contrast, amorphous ITZ in HPMCAS matrix exhibited much stronger resistance to recrystallization even under high RH conditions, reflecting the superior crystallization-inhibitory effect of HPMCAS. While the ITZ loads in the two polymers far exceeded the respective thermodynamic solubility limits, both ASD systems remained physically stable over an extended storage period at RH ≤ 60%. Kinetics rather than thermodynamics dictate the physical stability of ITZ ASDs. Crystallization of ITZ in ASDs appears to involve a complex interplay of multiple factors, including polymer type, glass transition temperatures (Tgs) of drug and polymer, crystallization tendency of amorphous drug, and storage conditions. Specifically, with respect to the impact of polymer type, HPMCAS is particularly effective for maintaining the storage stability of ITZ ASDs, which can be attributed to its higher Tg and lower hydrophilicity.