Determination of Interaction Parameters for Reversibly Self-Associating Antibodies: A Comparative Analysis. 
Monoclonal antibodies (mAbs) represent a major class of biotherapeutics and are the fastest growing category of biologic drugs on the market.  However, mAb development and formulation are often impeded by reversible self-association (RSA), defined as the dynamic exchange of monomers with native-state oligomers.  Here, we present a comparative analysis of the self-association properties for 5 IgG mAbs, under matched conditions and using orthogonal methods.  Concentration-dependent dynamic light scattering and sedimentation velocity studies revealed that the majority of mAbs examined exhibited weak to moderate RSA.  However, because these studies were carried out at mAb concentrations in the mg/mL range, we also observed significant nonideality.  Noting that nonideality frequently masks RSA and vice versa, we conducted direct boundary fitting of the sedimentation velocity data to determine stoichiometric binding models, interaction affinities, and nonideality terms for each mAb.  These analyses revealed equilibrium constants from micromolar to millimolar and stoichiometric models from monomer-dimer to isodesmic.  Moreover, even for those mAbs described by identical models, we observed distinct kinetics of self-association.  The accuracy of the models and their corresponding equilibrium constants were addressed using sedimentation equilibrium and simulations.  Overall, these results serve as the starting point for the comparative dissection of RSA mechanisms in therapeutic mAbs.