(191an) Pressure-Driven Sterile Filtration of a Model Vaccine Adjuvant

Sterile filtration is an important downstream processing step in pharmaceutical manufacturing of liquid dosage forms that is necessary to eliminate microbiological contamination. Conventional sterile filtration typically utilizes a simple and efficient dead-end filtration model consisting of filter membranes with a nominal pore size of 0.2 or 0.22 µm. These filters are classically validated using Brevundimonas diminuta, one of the smallest known bacteria. However, this validation process becomes more complex when working with nano-sized emulsions due to their size being similar to membrane pores and their tendency to induce membrane fouling. Thus, existing rules regarding optimal membrane operating conditions are subject to change. This study investigates the filtration behavior of a model vaccination adjuvant under various formulations and pressures to identify the optimal operating conditions for sterile filtration of more complex pharmaceuticals.

Our experimental setup is composed of a pressure-regulated system in which fluid flow over the membrane was driven by compressed gas. We utilized two filters in series in this study: first a Millipore® Filter Membranes 0.45 µm PVDF prefilter and followed by a 0.22 µm PES sterilizing-grade filter. Filters were prewetted in a fixed volume of 450 L/m² by different formulations of model vaccine adjuvant, prior to exposure to a bacterial challenge consisting of 10⁷ CFU/cm² of B. diminuta. Bacterial retention was then assessed via serial dilution and plating onto Tryptic Soy Agar. Pressure was varied between 5 and 35 psi, and mass flux data were systematically collected at each pressure setting. Filter integrity was systematically verified using the bubble point test to ensure the filter membrane was intact and free from defects, confirming reliable filtration performance and sterility assurance. Particle size analysis was conducted to test the droplet stability of the nanoemulsion before and after filtration.

Results obtained thus far indicate distinct trends and insights regarding optimal operational parameters. Prewetting solutions influence filtration efficiency due to chemical interactions with subsequent filtration feed solution. Fouling and a significant flux decrease are associated with increased concentrations of model vaccine adjuvant. Moderate filtration pressure enhances mass flux and shows minimal differences compared to lower pressures which indicates optimal filtration conditions are at moderate pressures. Bacterial breakthrough is to be expected at higher pressures (≥35 psi). The stability of the model vaccination adjuvant nanoemulsion is preserved post-filtration, as shown by no significant changes in size.

The findings from this study provide insights into optimizing filtration conditions for pharmaceutical nanoemulsions, enhancing process efficiency and reliability, and thereby contributing to the development of safer and more effective nanoparticle-based vaccine products.