In vitro antigen measurement and potency tests: challenges encountered during method development...and lessons...

Despite significant investment and technical efforts, veterinary vaccine manufacturers continue to experience challenges with the transition from historic animal-based potency methods to in vitro potency assays. These challenges have a number of contributing factors, including an inadequate understanding of protective antigens and epitopes, a lack of ruggedness and discriminating capabilities in evolving immunologically-based methods, inconsistencies between methods used for in-process antigen measurement and finished product potency, and a lack of clear methods to characterize the finished formulation (including complex adjuvants). A lack of harmonized guidelines and consistent regulatory expectations further complicates these efforts. There is room for optimism, however. There are numerous examples of successful in vitro potency test implementations. Titrations of modified live viral and bacterial vaccines, immune-based quantitative assays, and the recent application of direct physicochemical methods have allowed the transition from animal testing in many applications globally. Specific challenges for assay development and implementation are discussed in the areas of 1) target antigen selection, 2) complexity of finished product formulation, 3) potency discrimination, and 4) stability-indicating relevance.