Inhibition of aggregation of aluminum hydroxide adjuvant during freezing and drying

Aluminum-salt adjuvants are widely used to increase immunogenicity of recombinant protein vaccines. However, when vaccines formulated with these adjuvants are frozen or lyophilized, losses of efficacy are often reported. This loss of potency is usually attributed to the aggregation of adjuvant particles during processing. In this study, we examine the aggregation behavior of Alhydrogel, a commercial aluminum hydroxide adjuvant, during freeze-thawing and freeze-drying. By cooling Alhydrogel formulations at faster rates or by the addition of sufficient amounts of a glass forming excipient such as trehalose, aggregation of Alhydrogel, can be prevented or minimized. We propose that freeze-concentration of buffer salts induces modifications in adjuvant surface chemistry and crystallinity, which in turn favor aggregation. These modifications, and the resulting aggregation of Alhydrogel particles can be minimized through choice of buffer ions, or kinetically inhibited by rapidly forming a glassy state during freezing.