From Stock Bottle to Vaccine: Elucidating the Particle Size Distributions of Aluminum Adjuvants Using Dynamic Light Scattering

Abstract

The physicochemical properties of aluminum salts are key determinants of their resultant adjuvanticity in vivo when administered as part of a vaccine. While there are links between particle size and the efficacy of the immune response, the limited literature directly characterizing the PSD of aluminum adjuvants has stymied the elucidation of such a relationship for these materials. Hence, this comparative study was undertaken to monitor the PSD of aluminum adjuvants throughout the process of vaccine formulation using DLS. A significant proportion of the stock suspensions was highly agglomerated (>9 μm) and Alhydrogel® exhibited the smallest median size (2677 ± 120 nm) in comparison to Adju-Phos® or Imject alum® (7152 ± 308 and 7294 ± 146 nm respectively) despite its large polydispersity index (PDI). Dilution of these materials induced some degree of disaggregation within all samples with Adju-Phos® being the most significantly affected. The presence of BSA caused the median size of Alhydrogel® to increase but these trends were not evident when model vaccines were formulated with either Adju-Phos® or Imject alum®. Nevertheless, Alhydrogel® and Adju-Phos® exhibited comparable median sizes in the presence of this protein (4194 ± 466 and 4850 ± 501 nm respectively) with Imject alum® being considerably smaller (2155 ± 485 nm). These results suggest that the PSD of aluminum adjuvants is greatly influenced by dilution and the degree of protein adsorption experienced within the vaccine itself. The size of the resultant antigen-adjuvant complex may be important for its immunological recognition and subsequent clearance from the injection site.

Optimizing the utilization of aluminum adjuvants in vaccines: you might just get what you want

“… 25 , 26 These nanoparticles form loosely connected porous aggregates that vary in size from 1 to about 20 µm depending on the adjuvant, the method used for measurement of particle size, and the experimental conditions. 27 – 30 Exposure to shear forces and ultrasonication decreased the size of the aggregate adjuvant particles, 28 , 29 whereas suspension of adjuvants in saline increased aggregation and the size of the aggregates. 30 , 31 The ability of the aggregates to dissociate and reaggregate upon mixing contributes to the even distribution of adsorbed antigens in vaccine formulations.…”

“… 27 – 30 Exposure to shear forces and ultrasonication decreased the size of the aggregate adjuvant particles, 28 , 29 whereas suspension of adjuvants in saline increased aggregation and the size of the aggregates. 30 , 31 The ability of the aggregates to dissociate and reaggregate upon mixing contributes to the even distribution of adsorbed antigens in vaccine formulations. 32 The primary nanoparticles that make up the aggregates provide a very large surface, estimated at 514 m 2 /g for AH (Rehydragel HPA), when measured by water adsorption using gravimetric infrared spectroscopy.…”

Unraveling the enigma: elucidating the relationship between the physicochemical properties of aluminium-based adjuvants and their immunological mechanisms of action

“…Hydrodynamic analyses of ABA suspensions also have their limitations mainly due to the polydispersity of the preparations themselves; however, such methodologies provide an informative reference PSD under relevant environmental conditions. Our group recently reported that the median particle size of Alhydrogel ® and Adju-Phos ® by dynamic light scattering (DLS) was ca 3 and 7 μm respectively [ 89 ]. The latter was in good agreement with that obtained by Kolade et al using low angle laser light scattering for an equivalent commercial preparation ( ca 6 μm) [ 90 ].…”

Established and advanced adjuvants in vaccines' formulation: Mineral adsorbents, nanoparticulate carriers and microneedle delivery systems

“…The disordered, amorphous state is responsible for the high surface area and high adsorptive capacity of this adjuvant whose plate-like particles have a diameter of approximately 50 nm (11,12). The nanoparticles of this aluminium adjuvant form loosely connected porous aggregates that vary in size from 1 to about 20 μm depending on the adjuvant, the method used for measurement of particle size, and the experimental conditions (13,14). Another commercially available adjuvant is amorphous aluminum hydroxyphosphate sulfate, which contains residual sulfate residues because alum was used instead of aluminum chloride for its synthesis.…”

“…The size of the aggregate adjuvant particles can be decreased (i.e. during exposure to shear forces and ultrasonication (13) or increased when these particles are suspended in saline solution (14). The morphology of aluminum-containing adjuvants contributes to the uniform distribution of antigen in vaccines (11).…”