Low-Molecular-Weight Impurity Removal

Bispecific antibodies can generate product-related impurities during expression due to their complex molecular structures. Low molecular weight (LMW) species and homodimers may exhibit physicochemical properties similar to those of the target molecule, making their removal particularly challenging during downstream purification.

In this bispecific antibody project, the upstream-derived material contained relatively high levels of low molecular weight impurities, including HC + ScFv-Fc, as well as the ScFv-Fc homodimer. Their similarities in charge and hydrophobicity to the target molecule limited the effectiveness of conventional ion-exchange, hydrophobic interaction, and mixed-mode chromatography approaches.

A customized Protein A affinity chromatography strategy was therefore developed to exploit differences in affinity between the target molecule and product-related impurities.

Development Challenge

The bispecific antibody feed material showed relatively low purity and contained significant levels of product-related impurities.

The major impurities included:

HC + ScFv-Fc low molecular weight species
(ScFv-Fc)₂ homodimer
Impurities with physicochemical properties similar to the target molecule
Limited separation capability using conventional polishing approaches

Because these impurities had relatively similar charge and hydrophobicity characteristics to the target molecule, separation using ion-exchange, hydrophobic interaction, or mixed-mode chromatography presented significant challenges.

An alternative purification strategy was required to achieve efficient impurity removal while maintaining target molecule recovery and process scalability.

Purification Strategy

Protein A Resin Screening

The molecular architecture of the bispecific antibody was considered when selecting the purification strategy.

Rather than relying solely on differences in charge or hydrophobicity, Protein A affinity chromatography was investigated based on the differences in affinity between the target molecule and the relevant impurity species.

Suitable Protein A affinity media were screened to identify conditions capable of differentiating the target molecule from the low molecular weight impurities and homodimer.

Elution Condition Optimization

The Protein A chromatography conditions were subsequently optimized to improve the separation between the target antibody and product-related impurities.

An isocratic elution strategy was established to achieve effective impurity removal while maintaining an operationally practical purification process.

Scale-Up Confirmation

The optimized chromatography process was further evaluated at 200 L production scale to confirm process robustness and scalability.

Results

The optimized Protein A affinity chromatography process effectively removed the targeted product-related impurities and substantially improved product purity.

NR-CE Purity Increased from 70% to 90%

The optimized affinity chromatography conditions completely removed the identified HC + ScFv-Fc low molecular weight species and (ScFv-Fc)₂ homodimer.

As a result, NR-CE purity increased from approximately 70% to 90%.

Successful 200 L Scale-Up

The optimized process was successfully confirmed at a 200 L production scale, demonstrating its robustness and scalability for larger-scale manufacturing.

Figure 1. Removal of low molecular weight impurities and homodimer during Protein A affinity chromatography optimization.

Key Results

Parameter	Initial Process	Optimized Process
NR-CE Purity	~70%	90%
HC + ScFv-Fc	Present	Completely removed
(ScFv-Fc)₂ Homodimer	Present	Completely removed
Chromatography Strategy	Conventional polishing	Optimized Protein A affinity chromatography
Scale	Development scale	Confirmed at 200 L

Project Outcome

This case demonstrates how molecule-specific affinity differences can be exploited to address challenging product-related impurities in bispecific antibody purification.

By optimizing Protein A resin selection and isocratic elution conditions, the process completely removed the targeted low molecular weight species and homodimer, while increasing NR-CE purity from approximately 70% to 90%.

Successful confirmation at 200 L scale further demonstrated the robustness and scalability of the optimized process. Integrating impurity removal into the affinity capture step also reduced the burden on subsequent polishing operations, providing a more efficient downstream purification strategy.