Influence of formulation pH and suspension state on freezing-induced agglomeration of aluminum adjuvants

Freezing and thawing of vaccines containing aluminum adjuvants can lead to formation of aggregates and loss in vaccine potency. We sought to understand whether and to what extent the freezethaw damage to aluminum adjuvants would differ based on suspension state (flocculation and settlement) at the time of freezing. As flocculation and settlement characteristics of aluminum adjuvants are driven largely by the electrostatic charges on the adjuvant particles, which, in turn, are strongly influenced by the pH of the suspension, we conducted freezethaw studies on both Adjuphos and Alhydrogel (TM) samples at three pH levels (4, 6.5, and 7.2) in buffer solutions with 9% sucrose. Significantly less aggregation occurred in the buffered sucrose solutions at the pH furthest from the aluminum adjuvant point of zero charge during slow freezing at -20 degrees C. The freezing-induced aggregation for the samples with 9% sucrose at each pH was minimal during fast freezing at -70 degrees C and -115 degrees C. Suspensions that were flocculated and settled to a greater extent experienced the most freezethaw aggregation, whereas suspensions that were frozen before significant flocculation and settlement occurred showed little or no aggregation. Because pH of formulation can affect flocculation and settling time, it indirectly affects the extent of freezethaw aggregation. (c) 2011 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:10501062, 2012