The Role of Glass Transition Temperatures in Coamorphous Drug–Amino Acid Formulations

The improved physical stability associated with coamorphous drug–amino acid (AA) formulations may indicate a decrease in mobility of the amorphous drug molecules, compared to the neat amorphous form of the drug. Since the characteristic glass transition temperatures (Tgα and Tgβ) represent molecular mobility in amorphous systems, we aimed to characterize Tgα and Tgβ and to determine their role in physical stability as well as their potential usefulness to determine the presence of an excess component (either drug or AA) in coamorphous systems. Indomethacin (IND)–tryptophan (TRP) and carvedilol (CAR)–TRP were used as model coamorphous systems. The analytical techniques used were X-ray powder diffractometry (XRPD) to determine the solid-state form, dynamic mechanical analysis (DMA) to probe Tgα and Tgβ, and differential scanning calorimetry (DSC) to probe thermal behavior of the coamorphous systems. Tgα analysis showed a gradual monotonous increase in Tgα values with increasing AA concentration, and this in...