A Comprehensive Breakdown of the Complete Purification Process for Antibody Drugs

In the industrialisation of monoclonal antibody therapeutics, whilst upstream cell culture processes are continuously being upgraded and expression titers are constantly being improved, downstream purification processes have become the key bottleneck constraining mass production and regulatory compliance. Industry data indicates that the cost of antibody purification accounts for 50–80 per cent of total production costs. The purification process is not merely a simple protein extraction procedure; it is a critical step that systematically removes host cell proteins (HCPs), host DNA, Protein A-derived ligands, antibody aggregates and fragment variants, whilst simultaneously ensuring dual viral control and meeting pharmacopoeia compliance standards. For the same cell harvest supernatant, the final product’s yield, purity, aggregate profile and safety for medicinal use will vary significantly depending on the purification protocol employed.

Currently, antibody drugs marketed in China generally employ a standardised platform process comprising ‘harvest and clarification – Protein A capture – low-pH virus inactivation – two-step purification – virus nanofiltration – ultrafiltration exchange’. This article provides a comprehensive breakdown of the key process points, core objectives and common challenges in the entire antibody purification workflow from the perspective of industrial-scale production. Our company offers a comprehensive range of customised purification process equipment and supporting solutions, suitable for all scenarios from pilot-scale to large-scale production, helping enterprises overcome downstream purification challenges and achieve stable, compliant and efficient production.

I. Harvest Supernatant Clarification: The First Line of Defence in the Purification Process

Upon completion of CHO cell fermentation, the harvest supernatant contains a large amount of intact cells, cell debris, lipid impurities and residual culture medium. Feeding this directly onto a column can lead to chromatography resin clogging, abnormal column pressure and a sharp increase in pressure during subsequent purification steps; therefore, efficient clarification is the foundation of stable purification.

The mainstream, mature industrial solution is a two-stage clarification process comprising disc centrifugation followed by depth filtration: centrifugation removes the vast majority of intact cells, whilst depth filtration adsorbs fine debris, colloids and lipid impurities. This effectively reduces the turbidity of the feed solution, providing comprehensive protection for downstream chromatography media and significantly extending their service life.

Key process challenge: Incomplete clarification is the primary cause of instability in downstream processes. Excessive feed solution turbidity leads to rapid contamination of Protein A resin and a sharp reduction in its circulation life, whilst also resulting in persistently high levels of HCP residue. The core value of the clarification process lies in alleviating the burden on the entire purification workflow and laying a solid foundation for stability in mass production.

II. Protein A Affinity Chromatography: The Core Capture Step in Antibody Purification

Protein A affinity chromatography is the gold standard capture process for antibody purification. Its primary objective is to specifically enrich IgG antibodies from complex cell supernatants; a single purification step can raise product purity to over 90 per cent, making it a critical stage that determines the overall yield of the process.

This process relies on the specific binding of the Fc fragment of antibodies to Protein A resin. Loading is carried out under neutral pH conditions, whilst contaminating proteins and culture medium impurities are directly eluted. Following a dedicated washing procedure to remove non-specific adsorbed impurities, the antibodies are eluted using an acidic buffer at pH 3.0–3.8. This efficiently removes key impurities such as human cell proteins (HCPs) and host DNA, thereby achieving antibody concentration and enrichment.

Process risks and optimisation points: Low-pH elution can easily induce conformational changes in antibodies and lead to the formation of aggregates. For pH-sensitive antibodies, the buffer formulation must be optimised and stabilisers added to reduce the risk of aggregation; simultaneously, the issue of Protein A ligand detachment may arise during the process, which must be addressed through subsequent purification steps. Furthermore, after each production batch, CIP (Clean-in-Place) washing and sodium hydroxide regeneration must be carried out to ensure the efficiency of media recycling; industrial-grade alkali-resistant media can achieve 100–200 stable cycles.

It is important to note that the intermediate product following Protein A elution still contains aggregates, protein variants and trace process impurities, and cannot be directly used as a qualified bulk solution; subsequent purification and virus control steps must be completed.

III. Low-pH Virus Inactivation: A Mandatory Hurdle for Drug Compliance and Safety

Low-pH virus inactivation is a mandatory compliance requirement under the regulatory framework for antibody drugs, primarily used to efficiently inactivate enveloped viruses and ensure the biosafety of the medicinal product. The standard process involves adjusting the pH of the Protein A-eluted fraction to 3.4–3.8, incubating at room temperature for 30–60 minutes, and then rapidly neutralising it to neutral pH.

This process presents a key balancing challenge: the lower the pH and the longer the incubation time, the more effective the viral inactivation; however, this is highly likely to damage antibody molecules and exacerbate aggregate formation. Particularly for pH-sensitive molecules such as bispecific antibodies, this step represents a critical process bottleneck, with issues such as protein precipitation and excessive aggregate levels frequently arising. The core of process development lies in adopting mild process parameters to balance inactivation efficacy with product quality, whilst meeting viral inactivation validation standards.

IV. Purification and Polishing: Thorough Removal of Impurities and Product Variants

Following the capture and virus inactivation steps, the feed solution still contains residual antibody aggregates, degradation fragments, charge variants, as well as trace impurities such as HCPs, DNA, detached Protein A and endotoxins. Deep impurity removal must be achieved through the purification and polishing process, which is a critical step in ensuring the purity and safety of the medicinal product.

The standard, mature industrial approach involves orthogonal impurity removal using AEX (anion exchange chromatography) followed by CEX (cation exchange chromatography):

1. Anion exchange chromatography (AEX, flow-through mode): Under neutral conditions, antibodies, which carry no negative charge, flow freely through the column; this allows for the efficient adsorption and removal of negatively charged host DNA, endotoxins, residual HCPs and viral particles;

2. Cation-exchange chromatography (CEX, binding-elution mode): utilising charge differences to precisely separate antibody monomers from dimers, multimers and acid-base variants; a salt gradient elution is used to isolate the high-purity monomer main peak. Core industry quality control standards strictly limit aggregates in the crude product to below 0.5 per cent, a target primarily achieved through the CEX process.

For samples that are particularly challenging to purify, hydrophobic interaction chromatography (HIC) and hydroxyapatite chromatography can be employed to further enhance the removal of variants and trace impurities. A common misconception in the industry is to skip the refinement stage and rely solely on Protein A purification to determine product compliance; this is highly likely to result in the residual presence of hidden impurities, leading to the failure of product registration.

V. Viral Nanofiltration: A Physical Barrier for Non-enveloped Viruses

Low-pH inactivation is effective only against enveloped viruses and cannot remove non-enveloped viruses. In accordance with Pharmacopoeia regulatory requirements, antibody production must incorporate two viral removal processes based on different principles; consequently, viral nanofiltration is an indispensable core process.

This process relies on the principle of precise membrane pore size sieving: antibody molecules of approximately 10 nm can freely pass through the membrane pores, whilst viral particles larger than 20 nm are physically retained, thereby achieving highly efficient virus removal without compromising the structure or activity of the antibodies. Key process considerations: Prior to nanofiltration, the feed solution must undergo thorough clarification and impurity removal to prevent membrane module blockage, flux decline and reduced yield caused by particulate matter and protein precipitation; this is a crucial step in ensuring the stability of mass production and controlling production costs.

VI. Ultrafiltration and Dialysis (UF/DF): A Key Process in the Production of Stabilised Antibody Stock Solutions

Following chromatography and nanofiltration, the antibody feed solution has a complex buffer system and does not meet the required protein concentration; it must therefore undergo ultrafiltration-dialysis to achieve concentration and buffer replacement. Based on the principle of tangential flow filtration, this process precisely retains large antibody molecules whilst allowing small-molecule salt ions and excess buffer components to pass through, thereby simultaneously concentrating the protein and replacing the formulation buffer to ultimately achieve the standard concentration for the bulk solution.

The key challenge in this process is that high-concentration proteins are prone to increased viscosity and molecular aggregation. This requires the use of specialised equipment and well-established process parameters, optimising membrane material, operating pressure and temperature to mitigate the risk of aggregation at high concentrations, ultimately yielding a qualified antibody bulk solution which, after sterile filtration, is ready for filling and storage.

Process Summary

Antibody drug purification is a systematic engineering process characterised by multiple layers of safeguards and interlinked stages, with a clear, standardised closed-loop workflow: clarification and impurity removal → Protein A capture and enrichment → low-pH virus inactivation → cationic and anionic purification to remove variants → virus protection via nanofiltration → ultrafiltration for medium exchange and finalisation of the bulk solution.

Upstream processes determine product yield, whilst downstream purification determines whether the drug can be launched in compliance with regulations. Even if the cell culture titre meets the target in the upstream stage, issues such as excessive aggregates, residual HCPs and failure to meet viral clearance standards are highly likely to arise if the purification process design is unreasonable or equipment compatibility is insufficient, directly leading to project stagnation.

Given the differences in the physicochemical properties of various antibody molecules, standardised platform processes cannot be directly applied; core parameters such as buffer systems, pH levels and loading quantities must be optimised on a case-by-case basis. With our deep expertise in the downstream purification sector of biopharmaceuticals, we offer a comprehensive, one-stop range of purification process equipment and bespoke process development services. These are tailored to all stages—from R&D and pilot-scale testing through to large-scale production—helping companies rapidly establish compliant purification production lines characterised by high yields, high purity and low impurity levels. This effectively reduces mass production risks and production costs, whilst accelerating the drug’s path to market.