Adsorption equilibrium and kinetics of mAB charge variants on process-scale cation exchangers

The adsorption equilibrium and kinetics of the deamidated variants of a monoclonal antibody (mAb) were determined for two process scale cation exchangers---UNOsphere S and Capto S. The former consists of macroporous rigid particles, while the latter is a crosslinked agarose matrix with dextran grafted inside the pores. The mAb binding capacity for Capto S was generally much higher than for UNOsphere S but decreased more substantially with increasing pH and salt concentration. The steric mass action model (SMA) provided an accurate description of single-component isotherm data and was extended to successfully predict multicomponent adsorption equilibrium. While the effective charge was the same for all isoforms, the binding affinity constant was higher for the less deamidated variants, suggesting that differences in the overall charge, rather than in the number of surface-interacting charged residues, determine the selectivity. Adsorption kinetics was much faster for Capto S than for UNOsphere S for both single-component and multicomponent co-adsorption cases. A pore diffusion model was consistent with the UNOsphere S results for both single and multicomponent cases. Conversely, a model accounting for adsorbed phase diffusion was consistent with the Capto S results for single component and co-adsorption cases, but failed to predict the rates obtained for sequential adsorption. These results suggest that while co-diffusion of multiple variants occurs rapidly in Capto S, counter-diffusion within the dextran grafts is severely hindered. The results have important practical implications suggesting that the macroporous UNOsphere S is a more effective matrix for frontal analysis and other displacement based separations, while the dextran-grafted Capto S is more effective for capture because of its greater capacity and faster adsorption kinetics.