X-ray Investigation of the Aging Process of Aluminum Hydroxide Adjuvant in Protein-Based Vaccine Formulations Over a Short Period.

Nearly a century has passed since Glenny and colleagues introduced aluminum-based adjuvants. Over this extensive period, billions of doses of human and veterinary vaccines incorporating these adjuvants have been produced, ensuring both human health and food security. Aluminum-based adjuvants have played a pivotal role during epidemics, allowing scientists to accelerate vaccine development and save lives. Continuous research conducted by institutions worldwide has substantiated the safety and efficacy of aluminum-based adjuvants, establishing them as the gold standard. Consequently, any new adjuvant must be benchmarked against aluminum-based adjuvants and demonstrate substantial advantages in order to gain regulatory approval. This study aims to investigate the short-term structural and physicochemical changes in aluminum hydroxide in protein-based formulations under thermal treatments at 100°C for 24, 48, and 72 hours. These periods were designed to simulate the aging process that occurs during the storage of adjuvants at room temperature. Specifically, the research examines changes in the physicochemical properties of the adjuvant, including pH fluctuations during these thermal treatments, alterations in the sterilization process, protein adsorption capacity for each sample, particle size distribution, and X-ray diffraction (XRD) patterns. These findings not only enhance our understanding of adjuvant stability in vaccine formulations but also provide valuable insights into determining their optimal shelf life and performance. The study demonstrates that the best storage conditions for the adjuvant, with minimal impact from the aging process, are a low pH (pH=5) and higher ionic strength. It was also confirmed that innovative measures, such as reducing the sterilization cycle, stirring the samples after sterilization, and rapidly cooling them afterward, can prevent crystal growth and even produce smaller particle sizes with higher adjuvanticity. This is significant, as previous studies have reported a decline in adjuvanticity following sterilization.