mAB production: Step 8 – Anion Exchange Chromatography: Flow‑Through Polishing for DNA and Endotoxin Removal

How we use a negatively charged resin to let the mAb flow through while binding trace impurities that could compromise patient safety.

Welcome back to our deep‑dive series on monoclonal antibody manufacturing. In Step 7, we used cation exchange chromatography (CEX) in bind‑elute mode to remove aggregates, leached Protein A, and many host cell proteins (HCPs). The CEX pool is now highly pure – typically >99% monomer, with HCP levels below 10 ppm.

But two critical impurity classes remain at trace levels:

• DNA (host cell DNA from CHO cells)

• Endotoxins (lipopolysaccharides from Gram‑negative bacteria, introduced from raw materials or environment)

Additionally, regulatory agencies require orthogonal viral clearance – meaning a second, mechanistically distinct viral reduction step (the first was low pH inactivation, which targets enveloped viruses). AEX in flow‑through mode is exceptionally effective at removing both enveloped and non‑enveloped viruses.

Step 8 – Anion Exchange Chromatography (AEX) – is the final polishing step before viral filtration. Unlike CEX, AEX is almost always operated in flow‑through mode for monoclonal antibody manufacturing. Let’s explore why, and how it works.

Overview: What Is Anion Exchange Chromatography?

Principle: AEX resins contain positively charged functional groups (e.g., quaternary ammonium (Q), diethylaminoethyl (DEAE), or quaternary polyethyleneimine (QPEI)) that bind negatively charged molecules.

At typical operating pH (7.0–8.0):

• The mAb (pI 7.5–9.0) has a net neutral or slightly positive charge – it does not bind strongly to the positively charged resin.

• Impurities – DNA (negatively charged phosphate backbone), endotoxins (negatively charged lipid A), HCPs (many have pI <7.0), and viruses (surface negative charges) – bind to the resin.

Result: The mAb flows through the column while impurities are retained. This is flow‑through mode.

Why flow‑through instead of bind‑elute?

• Simpler operation (no elution step, no gradient)

• Higher throughput (shorter cycle time)

• Avoids additional concentration/dilution steps

• Excellent for removing trace negatively charged impurities

For some basic mAbs (pI >8.5), AEX can also be run in bind‑elute mode, but flow‑through is the industry standard for IgG1.

Step 8.1: Resin Selection and Column Preparation

Common AEX resins for flow‑through polishing:

Column packing: Bed height is typically 10–20 cm (shorter than bind‑elute columns because flow‑through requires less bed height). Packing qualification (HETP, asymmetry) is still performed.

Sanitization: 0.5–1.0 M NaOH, 30–60 min. Store in 0.1 M NaOH or 20% ethanol.

Single‑use vs. stainless steel: AEX is often performed in single‑use columns or capsules (e.g., Sartorius Sartobind Q, Pall Mustang Q) at smaller scales. For commercial 10,000–25,000 L scales, packed bed columns (stainless steel or reusable) are common.

Step 8.2: The AEX Flow‑Through Cycle – Step by Step

The cycle is much simpler than bind‑elute chromatography – typically 3 steps.

Step 8.2.1: Equilibration

Purpose: Set the column to the correct pH and conductivity for the mAb to flow through while impurities bind.

Buffer: 20–50 mM Tris, pH 7.0–8.0 (or 20 mM phosphate, pH 7.0–7.5). Conductivity is usually 2–10 mS/cm (no salt or low salt). Higher salt (>15 mS/cm) can reduce impurity binding.

Volume: 5–10 CV (until pH and conductivity are stable)

Critical: The buffer pH must be at least 0.5–1.0 unit above the pI of the mAb to ensure the mAb is neutral or slightly negative. However, for IgG1 with pI ~8.0, pH 7.0–7.5 gives the mAb a slight positive charge – but still weak enough to not bind significantly, while impurities (DNA, endotoxins) are strongly negative. Some processes use pH 8.0 to make the mAb slightly negative, but this risks product binding.

Step 8.2.2: Load (Flow‑Through)

Purpose: Apply the CEX pool to the column and collect the flow‑through containing the purified mAb.

Load conditions:

• Load pH: Adjust the CEX pool to the equilibration pH (e.g., 7.0–8.0) using dilute Tris base or NaOH. This is often done in‑line with a static mixer.

• Load conductivity: Typically 2–10 mS/cm. If the CEX eluate has high salt (e.g., 200–300 mM NaCl), dilute with WFI or low‑salt buffer before loading.

• Load density: Not limited by binding capacity (since the mAb flows through). However, the resin has a finite capacity for impurities. Typical loading: 500–2,000 g mAb per liter of resin (much higher than bind‑elute). The limit is set by breakthrough of DNA or endotoxin.

• Flow rate: 150–500 cm/h (2–5 minute residence time). Higher flow rates are possible because no binding/elution kinetics are involved.

Monitoring:

• UV (A280) – the mAb peak appears immediately (no binding).

• In‑line conductivity and pH.

• Breakthrough monitoring: periodic sampling for DNA (qPCR) and endotoxin (LAL or rFC).

Collection: The entire flow‑through (from UV rise to return to baseline) is collected as the AEX Pool. Unlike bind‑elute, there is no elution peak – the product is collected during load.

Step 8.2.3: Wash and Clean‑In‑Place (CIP)

After loading, the column retains bound impurities (DNA, endotoxins, HCPs, viruses).

Wash (optional): Some processes include a low‑salt wash (5 CV of equilibration buffer) to recover residual product from the void volume.

CIP (required after each batch):

• 0.5–1.0 M NaOH, 30–60 min – this degrades DNA and endotoxins and sanitizes the resin.

• Followed by WFI wash until pH <8.0.

• Storage in 0.1 M NaOH or 20% ethanol.

Note: Because AEX is typically run in flow‑through mode with high loading, the resin accumulates significant impurities. CIP after every batch is essential.

Step 8.3: Critical Process Parameters (CPPs)

Design of experiment (DoE): For validation, perform a worst‑case study at highest load pH, highest conductivity, highest flow rate, and highest load density to demonstrate robust impurity clearance.

Step 8.4: Impurity Clearance Performance

A well‑designed AEX flow‑through step achieves:

Regulatory note: AEX is often the most effective step for removing non‑enveloped viruses (e.g., reovirus, minute virus of mice) because low pH inactivation does not affect them. For this reason, AEX is a critical part of the viral clearance strategy.

Step 8.5: Process Integration and Conditioning

The CEX eluate (from Step 7) is typically at pH 5.0–5.5 with 150–300 mM NaCl. To load onto AEX at pH 7.0–8.0 and low conductivity, you need conditioning:

Option 1: In‑line dilution (most common)

• Use a static mixer and a second pump to add dilution buffer (e.g., 50 mM Tris, pH 8.5, no salt) to raise pH and lower conductivity.

• The dilution ratio is typically 1:1 to 1:3 (buffer:product).

Option 2: UF/DF before AEX

• Perform ultrafiltration/diafiltration to exchange buffer into the AEX equilibration buffer. This is less common because it adds time and potential yield loss.

Option 3: Adjust pH with base, then dilute with WFI

• Add 1 M Tris base to the CEX pool to raise pH to 7.5, then dilute with WFI to reduce conductivity. Simple but requires a hold tank and mixing.

For large‑scale commercial manufacturing, in‑line dilution is the most efficient and reduces hold volumes.

Step 8.6: Equipment Considerations

Chromatography skid: Same as for Protein A and CEX, but with an additional pump for in‑line dilution. The skid must have:

• Two inlet pumps (one for product, one for dilution buffer)

• Static mixer

• pH and conductivity probes after the mixer (feedback control)

• UV monitor (A280) after the column to confirm product flow‑through

Hold tanks:

• Before AEX: Conditioned pool (after dilution) – often held in a bag or tank for up to 24 hours at 2–8°C.

• After AEX: The AEX pool – collected in a sterile, chilled vessel.

Single‑use options: For scales up to 2,000 L, single‑use AEX capsules (e.g., Sartobind Q, Pall Mustang Q) or prepacked columns (e.g., Repligen OPUS) are convenient. For 20,000 L, packed bed columns are still standard.

Step 8.7: Viral Clearance Validation for AEX

Because AEX is a key orthogonal viral clearance step, it must be validated with scaled‑down studies.

Typical validation design:

1. Select model viruses:

   • X‑MuLV (enveloped, retrovirus) – should be cleared by AEX

   • Reovirus (non‑enveloped) – AEX is very effective

   • MVM (minute virus of mice, non‑enveloped, parvovirus) – smaller, may be harder to clear

2. Spike the virus into the CEX pool (scaled‑down).

3. Adjust pH and conductivity to worst‑case conditions (highest pH, highest conductivity, highest flow rate).

4. Load onto a small AEX column (1–5 mL) at the target load density.

5. Collect flow‑through fractions and measure infectivity.

6. Calculate LRV = log₁₀(virus input) – log₁₀(virus in flow‑through).

Acceptance criteria: Typically ≥4 log for non‑enveloped viruses, ≥3 log for enveloped. The combination of low pH inactivation (≥4 log for enveloped) and AEX (≥4 log for non‑enveloped) gives overall ≥8 log viral clearance – well above regulatory requirements.

Step 8.8: What Can Go Wrong? (Troubleshooting)

Step 8.9: Alternatives and Emerging Approaches

• Membrane AEX (e.g., Sartobind Q, Pall Mustang Q): Uses a membrane instead of resin beads. Higher flow rates, faster cycling, but lower binding capacity for impurities. Ideal for flow‑through polishing at smaller scales (up to 2,000 L).

• Mixed‑mode AEX/hydrophobic (e.g., Capto Adhere): Combines anion exchange with hydrophobic interaction. Can be used in flow‑through or bind‑elute mode. Some processes replace both AEX and viral filtration with Capto Adhere, but it is less common.

• Continuous AEX: In a continuous manufacturing train, AEX can be operated as a simulated moving bed or with periodic column switching. Still emerging.

For a legacy mAb like Humira, standard flow‑through AEX on a packed bed or membrane is the proven approach.

What Comes Next?

The AEX pool is now highly pure, with DNA and endotoxin reduced to trace levels. However, one more viral clearance step is required by regulators – a size‑based step that acts as a fail‑safe for any potential virus that escaped previous steps. That is Step 9: Viral Filtration (Nanofiltration) – using 20 nm pores to physically retain viruses while allowing the mAb to pass.

Questions for our readers:

• Do you use packed bed AEX or membrane AEX at your facility? What scale?

• Have you validated AEX for non‑enveloped virus clearance? What LRV did you achieve?

Share your experiences below. Next week: Step 9 – Viral Filtration (Nanofiltration).

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