Abstract
Concentrated monoclonal antibody (mAb) solutions can allow subcutaneous administration of effective doses of the therapeutic, but for some mAbs this leads to anomalously high viscosities; mAb-mAb association, which leads to the formation of clusters or gels, is often the driver of such behavior. Statistical mechanical considerations suggest that such an association is likely to be dominated by a single binding configuration. In this work we probe the possible molecular origins of this behavior using atomistic molecular simulations of a mAb known to display high viscosity at low salt concentrations. Orientational exploration identified a small number of high-affinity mAb-mAb configurations based on nonelectrostatic contributions to the protein interactions, which reflect the geometric complementarity characteristic of biomolecular recognition. Consideration of electrostatic interactions, which account for most salt effects, adds several tens of kT of attraction to select configurations, although there are uncertainties in the electrostatic interactions calculated using the Poisson-Boltzmann approach. The resulting overall attractive energies, which are greater than 40 kT, support the existence of a single attractive configuration strong enough to form a mAb network at high concentrations. Simulations with increased ionic strength or for small numbers of point mutations predict some but not all observed experimental trends. Independent molecular dynamics simulations for two select configurations showed partial agreement with the previous results. Overall, although the molecular origin of high viscosity is plausibly due to a single strongly bound configuration, unambiguous identification of this configuration is limited by the accuracy of the underlying calculations.