Use of o-phthalaldehyde assay to determine protein contents of Alhydrogel-based vaccines

Aluminum based adjuvants (alum), including aluminum hydroxide (Alhydrogel®) and aluminum phosphate are the most commonly used adjuvant in the US. In order to ensure quality of vaccines, regulatory authorities require evaluation of antigen content in final vaccine products. Currently, there are no generic methods available for the determination of protein content in alum-based vaccines. Aluminum hydroxide gels exist as particles in solution, which interfere with direct quantitation of protein content in formulations using assays such as Lowry, BCA or Bradford protein assay. The present study adapts a simple fluorescent assay to directly (without the need for antigen extraction) determine antigen content on Alhydrogel® with accuracy and sensitivity using the o-phthalaldehyde (OPA) reagent. Malaria vaccine candidates AMA1-C1/Alhydrogel, AMA1-C2/Alhydrogel, MSP142-3D7/Alhydrogel, MSP142-C1/Alhydrogel or BSAM-2/Alhydrogel were used as model formulations. The results of the present study show that the OPA assay is highly accurate (87–100%), reproducible, and simple with a linear detection range of 25–400 μg/mL for Alhydrogel® vaccines (except for MSP142-C1, which has a linear detection range of 31.25–500 μg/mL). This assay has proven to be highly useful in our laboratory and been used in routine vaccine quality control processes.