Modeling of Complex Protein Elution Behavior

Industrial downstream process development – high load densities challenge modeling approaches

Figure: High load conditions lead to the development of a trapezoidal peak shape and a shift of the peak front to lower elution volumes.

Within the past years, model-based approaches emerged as a flexible, fast and cheap tool for industrial process development, optimization and characterization, as well as facility fitting. Hereby, the commonly used model for ion-exchange chromatography is the “Steric Mass Action” theory (SMA). It bases on the assumption that a higher concentration of protein in solution will always lead to a higher amount of adsorbed protein – which in fact is not always the case in industrial applications.

Complex elution behavior challenges the modeling of industrial applications. The predictability of high protein load densities is one key requirement for a successful implementation of model-based process development in industry. The SMA equation for ion exchange chromatography does not consider further thermodynamic effects. To account for these, concentration-dependent activities are necessitated.

Implementing an asymmetric activity coefficient to the steric mass action isotherm. The SMA isotherm was extended with an additional coefficient to generate a generalized ion-exchange isotherm. The asymmetric activity coefficient was thereby approximated by means of two protein-specific parameters. These could be identified through the inverse curve fitting method of the simulation software ChromX. The data needed to calibrate the model was only three different gradient elution experiments with varying load densities and gradient slopes.

The new isotherm opens various possibilities for future applications. Now, processes with high load densities can be modeled and simulated. Further it allows model-based process development for instance in the case of continuous ion-exchange chromatography or the operational mode of “overloaded chromatography”.

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