Capillary electrophoretic analysis of meningococcal polysaccharide-diphtheria toxoid conjugate vaccines

Protein-polysaccharide conjugate vaccines are large, complex molecules that present challenges in terms of characterization. Free solution capillary electrophoresis using alkaline (pH 9-10) borate or glycine/NaOH buffers permitted electrophoresis of diphtheria toxoid (Dt) or meningococcal polysaccharide-Dt conjugates as well-formed, relatively broad peaks. Resolution was observed between the conjugate and the Dt peaks for some serotypes; however, overlap between Dt and one or more conjugate peaks was observed with both buffers. Inclusion of SDS in the separation buffer at a level above the critical micelle concentration allowed separation of Dt and Dt-conjugates of meningococcal serotypes A, C, Y and W135. Using borate/SDS, a linear relationship between peak area and Dt concentration was observed between approximately 2 mg/ml and 20 microg/ml. A linear relationship between Dt peak area and injection times from 10 to 50 seconds was also observed. Capillary electrophoresis may therefore be a useful method for quantifying free protein level in meningococcal-Dt conjugate vaccines.