Effect of protein adsorption by aluminum-containing adjuvants on colloidal interactions in combination vaccines

Ragheb Hilmi Al-Shakhshir, Purdue University

Abstract

The colloidal stability of combination vaccines depends on the surface properties the antigen-adjuvant complex. The surface charge characteristics of the adjuvants and model proteins were determined by electrophoretic light scattering. The point of zero charge (PZC) was determined to be 11.1-11.8 and 5.0 for aluminum hydroxide and aluminum phosphate adjuvant, respectively. The isoelectric point (pI) was 9.6 and 5.0 for lysozyme and bovine serum albumin (BSA), respectively. Thus, aluminum hydroxide adjuvants and lysozyme will have a net positive charge at physiological pH, 7.4, and aluminum phosphate adjuvant and BSA will have a net negative charge. At pH 7.4 the adsorption of the negatively charged BSA by the positively charged aluminum hydroxide adjuvant resulted in a decrease in the PZC. On the other hand, the adsorption of positively charged lysozyme by the negatively charged aluminum phosphate adjuvant resulted in an increase in the PZC. The surface charge characteristics of the aluminum-containing adjuvants dominated at low protein coverage. In contrast, the surface charge characteristics of the adsorbed protein dominated at high protein coverage. Therefore, the physiochemical properties of the antigen-adjuvant complex and not the adjuvant alone should be considered during vaccine preparation. The adsorptive capacity at pH 7.4 of BSA by aluminum hydroxide adjuvant and lysozyme by aluminum phosphate adjuvant decreased as the ionic strength increased. Ethylene glycol did not affect the adsorptive capacity of aluminum hydroxide adjuvant for BSA but reduced the adsorption of lysozyme by aluminum phosphate adjuvant. Thus, both electrostatic and hydrophobic forces contribute to the adsorption of proteins by aluminum-containing adjuvants. However, electrostatic repulsive forces are also important as negligible adsorption occurs when electrostatic repulsive forces are present. Aluminum adjuvants have distinct surface characteristics which makes them candidates for adsorbing a wide array of antigens and possibly combining these antigen-adjuvant suspensions to prepare combination vaccines. The adsorption of model proteins significantly enhanced the stability of mixed aluminum adjuvant suspensions. The stability was dependent on the degree of protein surface coverage. The redistribution of ions and proteins upon mixing the protein-adjuvant suspensions affected the surface charge and thus the stability of the system.

Degree

Ph.D.

Advisors

Hem, Purdue University.

Subject Area

Pharmacology|Pharmaceuticals

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