Stabilization of the hydrolysis of Haemophilus influenzae type b antigen by aluminum hydroxide adjuvant
Abstract
Glucose-1-phosphate, methyl paraben and bis (p-nitrophenyl) phosphate, were selected as models to study the effect of the microenvironment pH of aluminum hydroxide adjuvant on the hydrolysis of ester type molecules. The surface charge of the aluminum hydroxide adjuvant was modified by pretreatment with phosphate anion to produce isoelectric points ranging from 11.8 to 4.9. The observed hydrolysis rate constant (kobs) of an ester that exhibited acid catalyzed hydrolysis and was completely adsorbed to the aluminum hydroxide adjuvant was lower than kobs of this molecule in aqueous solution. The completely adsorbed molecule hydrolyzed at a rate associated with the surface pH, which was higher than the bulk pH. This higher surface pH resulted from anions that were concentrated at the positively charged surface. By matching this kobs to pH in pH-stability profile, the surface pH was approximately 2.0 pH units higher than the bulk pH. The stability study of an ester molecule that was partially adsorbed and had acid catalyzed hydrolysis showed that kobs decreased as the positive surface charge of the aluminum hydroxide adjuvants increased. On the other hand, kobs increased as the negative surface charge surface of the aluminum hydroxide adjuvants increased. It is hypothesized that the unabsorbed molecules diffused in both the diffuse layer and the bulk area, while the adsorbed molecules were adsorbed within the Stern layer. Thus, the pH conditions that affected the hydrolysis of the ester molecules are not only the bulk pH but also the Stern layer pH and the diffuse layer pH. The stability of the ester molecules that were not adsorbed to the aluminum hydroxide adjuvants was also studied. In this situation, the pH conditions that affected the rate of hydrolysis were the diffuse layer pH and the bulk pH. For molecules that have base catalyzed hydrolysis, the results showed that positively charged adjuvants accelerated the rate of hydrolysis, while negatively charged adjuvants reduced the rate of hydrolysis. These results are opposite to the molecules that undergo acid catalyzed hydrolysis. Therefore, it was concluded that the microenvironment pH affected the stability of the ester type molecules that have pH dependent degradation mechanism.
Degree
Ph.D.
Advisors
Hem, Purdue University.
Subject Area
Pharmaceuticals
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