Adsorption modeling rethought – the colloidal particle adsorption model CPA

Mechanistic models for ion exchange chromatography were mostly based on stoichiometric equations until this very day. With the release of DSPX, GoSilico has introduced a new adsorption isotherm class that is based on a colloidal description of proteins: The colloidal particle adsorption model (CPA). In comparison to the standard “SMA” model, the new colloidal models achieve a completely new level of accuracy at high column loading. Read more…

Established: Mechanistic chromatography models based on stoichiometric equations

For the past three decades, mechanistic models mainly relied on simple stoichiometric equations to describe protein adsorption in ion exchange (IEX) processes. The most prominent and widely used representative is the steric mass action (SMA) isotherm. The SMA model describes the interaction between proteins and IEX resins by a reversible exchange of adsorber counter-ions by proteins in the mobile phase, while explicitly considering the influence of the ionic strength on protein adsorption. The protein binding capacity of an IEX adsorber is thereby physically constrained by its ionic capacity. The nonlinear adsorption behavior at high protein loads is further described by a steric shielding of adsorber ligands as sketched in the following graphic.

Steric mass action depiction

SMA vs. reality: SMA can not explain observed shoulders in front of the peak

Despite its popularity, in recent years we have more often seen shortcomings of the SMA model in describing industrial process behavior. Discrepancies between the SMA model and experimental data are particularly pronounced at high protein load densities combined with linear gradient elution. As illustrated by the following simulations using increasing loads, we thereby often observe the formation of a characteristic peak shoulder or “shark fins” in the front part of the elution peak that cannot be reflected by the SMA model.

For the last three decades, our understanding of IEX chromatography has been based predominantly on principles of stoichiometry. Given the increasing number of processes that do not follow the behavior proposed by these models, we started to consider a non-stoichiometric description of protein adsorption.

Till Briskot, Research Engineer at GoSilico GmbH

Proudly presenting: The colloidal particle adsorption model CPA

To overcome the shortcomings of the SMA model, we introduced an alternative model for describing protein adsorption in IEX columns that is not based on stoichiometric equations. Using the colloidal nature of proteins, the new model allows a more fundamental description of interactions between proteins and charged ion adsorbers. Non-linear adsorption effects are thereby ascribed to steric hindrance at the adsorber surface and electrostatic interactions between adsorbed proteins. In contrast to the SMA model, the maximum protein binding capacity of the adsorber is physically constrained by the adsorber surface area accessible to proteins and not by its ionic capacity.

Depiction of a single colloid explaining the maximum protein binding capacity

Depiction of antibodies connecting to ligands inside a resin bead

Time-out for stoichiometry – Go colloidal!

In a recent case study together with Cytiva, we demonstrated the ability of the CPA model to describe complex elution behavior of a monoclonal antibody (mAb) on the strong cation exchanger Capto S ImpAct™. The following figure shows three gradient elution experiments (rows) at different load densities, performed and provided by Cytiva. In addition to charge variants of the mAb (second column), the CPA model accounted for different product-related and process-related impurities including low-molecular and high-molecular weight species (third column) as well as host cell proteins and leached Protein A (fourth column). At very high protein load densities close to or beyond the protein breakthrough (second and third row), elution peaks show a distinct shoulder that cannot be explained by the SMA model but can be reproduced by the CPA model.

Multiple graphs of the colloidal particle adsorption model case study with Cytiva
Figure: Linear gradient elution experiments of a monoclonal antibody (mAb) on Capto S ImpAct™ at three different protein load densities (rows). First column: measured UV absorbance (dashed lines) and simulated curves (continuous lines); second column: simulated elution profiles of three mAb charge variants.; third column: simulated elution profiles of low-molecular weight species (purple) and high-molecular weight species (cyan); fourth column: simulated elution profiles of host cell proteins (green) and leached Protein A (red). Experimental fraction data are shown as dots. All experimental data provided by Cytiva.
The improvement in model performance compared to the SMA approach is stunning and addresses a problem we’ve been struggling with. The ability to describe the strange peak shapes at high loading, both with respect to product related and process related impurities, suggests that better processes can be developed with the colloidal model.

Gunnar Malmquist, Principal Scientist at Cytiva

The future of IEX modeling: CPA and DSPX

With the launch of its new downstream simulation suite DSPX, GoSilico has also launched the colloidal particle adsorption model (CPA) and made it commercially available. In DSPX – ChromX it can be activated using a switch between steric mass action and colloidal model in the model selection.

The results achieved so far clearly show the ability of the CPA model to describe complex elution behavior. Based on mechanistic principles, the CPA model enables a causal interpretation which can help to better understand industrial IEX processes and to support the development of these processes in a model-based approach. Stop stoichiometric thinking. Go colloidal.