Multimodal assessment of sedimentation and flocculation in spray-dried injectable peptide suspensions

Understanding sedimentation and flocculation is essential for optimizing the physical stability (i.e., minimizing particle settling and aggregation) of non-aqueous injectable suspensions. Physical stability can be addressed by different analytical techniques based on various measurement principles. These techniques may capture different aspects of sedimentation, for example, image-based method shows macroscopic changes at the supernatant-sediment interface, while light scattering based method detects particle‑level dynamics. Therefore, when used individually, these techniques may provide only limited insights. To gain more comprehensive insight, this study presents a multimodal assessment using multiblock Principal Component Analysis (PCA), where information from techniques based on imaging and light scattering were combined. First, suspensions were produced from spray dried powders with different density and particle size. Then, the sedimentation behaviour was evaluated by both image-based method and light scattering method. Multiblock PCA was applied on the concatenated data obtained from these two techniques. The results uncovered two distinct sedimentation profiles: fast settling driven by discrete flocculation, and slower, more complex settling governed by bridging flocculation. Further analysis confirmed that discrete flocculation is linked to large, dense particles and bridging flocculation is linked to small, light particles. Regression models showed that spray drying parameters have direct effects on powder properties and those properties in turn determine sedimentation rates. Consequently, spray drying influence sedimentation rates indirectly through its effect on powder properties. Overall, this study revealed the correlation between powder properties with flocculation and sedimentation and showed a practical pathway for tuning suspension physical stability directly through spray drying process optimization.