Development and validation of serological methods for human vaccine potency testing: Case study of an anthrax vaccine

Only one anthrax vaccine (Anthrax Vaccine Adsorbed, AVA) is licensed in the US to date, although new vaccines are under development. Logistic difficulties with the potency testing of AVA, plus humane considerations, have prompted the development of an alternative test method that can be used as a potency test not only for AVA, but also for newer vaccines. A potency test is not limited to measuring the concentration and quality of the antigen in the final formulation at the time of vaccine release, but should also detect changes in these characteristics during the dating period of the product, to ensure that the vaccine has retained its potency. We have developed a mouse immunogenicity test with potential for use as a potency test for anthrax vaccines. This model is based on the measurement of antibodies induced by a fixed dose of antigen. The test consists of two stages: a) the induction of antibodies in mice with one pre-selected test dose of vaccine; and b) the measurement of the response. We have established the test dose for AVA and experimental vaccines based on the use of anthrax Protective Antigen (PA). Two types of assays, an ELISA and a toxin neutralization assay (TNA) have been employed to measure antibodies to PA. TNA may be more useful in predicting vaccine efficacy, since it measures the neutralizing activity of sera against the cytotoxic effect of the toxin formed by PA when associated with Lethal Factor. However, anti-PA ELISA is less demanding from a technical point of view. Therefore, if ELISA were capable of detecting accurately changes in antigen quantity and quality, then it could be selected as the antibody-measuring test. Studies in our laboratory suggest that even though ELISA and TNA results are correlated, they may not be strictly interchangeable for quantification of anti-PA antibodies after a single immunization of mice with a test dose of anthrax vaccine. We have also found that TNA is better suited than ELISA to detect changes in immunogenicity caused by vaccine exposure to high temperature for very short periods (two minutes). These results open the possibility that an immunogenicity test in which TNA is used to quantify the anti-PA antibody responses can be used to measure vaccine potency of anthrax vaccines at the end of the manufacturing process and periodically after the finished product has been placed in storage, instead of an active protection test that requires lethal challenge.