Andrew Lees, PhD and Libo Wang, PhD
Conjugate vaccines are developed by linking molecules that are poorly immunogenic to a carrier protein to improve immunogenicity. They can be highly effective, as exemplified by the Haemophilus Influenzae b (Hib) and Prevnar 13® (which prevents infections from pneumococcal bacteria) vaccines, both of which are conjugates. However, conjugate vaccines are complex to formulate and costly, with very few carrier proteins licensed for use.
FinaBio (Fina Biosolutions LLC, Rockville, MD), a developer of conjugate vaccines, has developed an E. coli expression system to produce CRM197, one of the most commonly used carrier proteins, to improve the accessibility of this important class of vaccines.1 In this article we consider the analytical strategies applied to demonstrate the biophysical and immunological comparability of the new carrier protein, and to optimize its formulation.
Introducing a New Carrier Protein for Conjugate Vaccines
CRM197 is a genetically detoxified diphtheria toxin with several characteristics that can prove challenging for formulators and manufacturers. A relatively labile protein, it is prone to nicking and degradation by multiple routes including dimerization, aggregation, and precipitation. Notably it is pH sensitive with a tendency to turn inside out under low (acidic) pH conditions and can dimerize via domain exchange. Formulation conditions are controlled to keep conformational change and degradation to a minimum with comprehensive biophysical characterization applied to ensure that the CRM197 remains structurally intact under the conditions of use, up to the point of patient delivery.