Long-term physical stability of lyophilized carbohydrate matrices assessed by positron annihilation lifetime...

Understanding long-term physical changes in lyophilized matrices is essential for rational formulation design and stability assessment. In this study, positron annihilation lifetime spectroscopy (PALS) was applied to monitor time-dependent structural evolution in simple lyophilized carbohydrate-polymer matrices stored for up to 21 months under low and elevated humidity conditions. Formulations containing saccharides, polyols and starch were selected as model amorphous systems to investigate molecular mobility and phase evolution in the absence of active pharmaceutical ingredients. The free volume and ortho-positronium (o-Ps) intensity were determined as complementary indicators of free volume within the amorphous phase and relative amorphous fraction, respectively. Over prolonged storage, composition- and humidity-dependent changes in these parameters were observed. Under elevated humidity, selected formulations exhibited reductions in o-Ps intensity consistent with partial recrystallization, while variations in free volume reflected concurrent structural relaxation within the remaining amorphous phase. Solid-state ¹H NMR and X-ray diffraction provided complementary support for these evolution trends. The results demonstrate that PALS sensitively detects physical aging and structural changes in lyophilized matrices by probing free-volume evolution during storage. These findings support the utility of PALS as a non-destructive analytical tool for monitoring long-term physical stability of amorphous lyophilized matrices under defined environmental conditions.