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Filtration Process for High-Concentration Biopharmaceuticals

A high-concentration formulation refers to a dosage form with a high content of active pharmaceutical ingredients (API) per unit volume of solution. This formulation is primarily developed to address the issue of frequent drug administration. By elevating drug concentration, the injection frequency can be reduced to streamline administration and improve patient compliance. For macromolecular biopharmaceuticals with large dosage requirements, conventional formulations often fail to meet clinical demands, which further drives the development of high-concentration products.

Generally, such liquid formulations feature a viscosity ranging from 10 to 50 centipoise (cp), with protein concentrations exceeding 100 mg/mL and even reaching up to 250 mg/mL. For biotech drugs, high-concentration formulations are commonly administered via subcutaneous injection instead of intravenous infusion. This enables self-administration by patients, improves their quality of life and cuts down medical costs.

Currently, over 20% of monoclonal antibody (mAb) products approved by the U.S. Food and Drug Administration (FDA) are high-concentration formulations. Nevertheless, the development and manufacturing of high-concentration formulations face substantial technical hurdles. High solution viscosity and the propensity for protein aggregation complicate process development, compromise product stability and raise the risk of immunogenicity.

Technical Challenges and Countermeasures for High-Concentration Formulation Processing

The production of high-concentration formulations mainly involves two core unit operations: ultrafiltration/diafiltration (UF/DF) and sterile filtration.

1. Ultrafiltration/Diafiltration (UF/DF)

UF/DF is a critical downstream process step for high-concentration mAb manufacturing. Positioned at the end of the purification workflow, this process concentrates target proteins to the specified concentration and performs buffer exchange to transfer the product into the final formulation buffer.

Meeting the specifications of excipient concentrations in the final formulation is essential to maintain physiological osmotic balance for safe administration, preserve drug efficacy and extend shelf life. Multiple technical obstacles associated with high-concentration feedstocks must be overcome during UF/DF, including excipient drift, excessive pressure drop, shear-induced protein aggregation, low product recovery and residual volume issues.

Excipient Concentration Drift

The higher the protein concentration of the feedstock, the more significant the drift of pH and excipient concentrations relative to the target values of the diafiltration buffer. This phenomenon is attributed to the Donnan exclusion effect and steric exclusion effect.

Elevated Viscosity and Pressure Drop

High protein concentration drastically increases solution viscosity, which leads to a sharp rise in pressure drop across tangential flow filtration (TFF) systems and higher power consumption for material pumping. High-viscosity feedstock tends to foul membrane channels, resulting in elevated inlet pressure during ultrafiltration. Particularly in the final concentration stage, continuous rising inlet pressure may exceed the equipment limit and force process shutdown.

Moreover, a gel layer readily forms on the membrane surface at extremely high protein concentrations, creating substantial resistance against solvent permeation. At a constant temperature, solution viscosity increases with protein concentration. Low temperature also raises viscosity: reduced thermal motion brings protein molecules into closer proximity and facilitates the formation of stable intermolecular interactions, impairing fluidity. This effect is far more pronounced in high-concentration protein solutions.

Viscosity Modification Requirement

Viscosity-reducing excipients are commonly added to high-concentration formulations to facilitate subcutaneous injection.

Low Product Recovery and Residual Volume

The post-UF/DF product recovery process poses challenges. To avoid concentration dilution, only a minimal volume of flushing buffer can be used. Liquid retention frequently occurs at pipe bends, diameter transitions, valves, internal gaps of membrane cassettes and channel terminals, leaving a considerable amount of protein solution unrecoverable and lowering overall process yield.

Protein Aggregation Risk

In high-concentration protein solutions, shortened intermolecular distance enhances molecular interactions and self-association, leading to protein aggregation. Aggregation degrades product quality, manifested as reduced biological activity and increased immunogenic potential.

Countermeasures for UF/DF Challenges

Optimized Membrane Selection

Adopt ultrafiltration membranes with excellent hydrophilicity, low protein adsorption and stable flux to mitigate fouling and aggregation, so as to improve filtration efficiency and product yield. T-Series cassettes equipped with Delta RC membranes are ideal options. Featuring superior hydrophilicity and minimal protein binding, their specialized screen design reduces pressure loss and restrains excessive inlet pressure. Meanwhile, the cassettes enhance mass transfer, alleviate concentration polarization under high-concentration conditions and maintain stable permeate flux.

Buffer Formulation Optimization

Optimizing buffer type, ionic strength and pH is vital for reliable UF/DF performance. Proper buffer formulations can mitigate excipient deviation caused by the Donnan effect and steric exclusion effect. The diafiltration buffer is typically prepared at an ionic strength of 5–50 mM with a pH range of 6.0–7.0. Commonly used buffer systems include histidine-HCl buffer, citrate buffer, phosphate buffer and acetate buffer. Optimizing buffer pH effectively eliminates deviations of excipient concentrations between retentate and permeate during ultrafiltration.

Process Temperature Control

Maintain the temperature of feedstock and diafiltration buffer in the TFF system at ambient temperature (20–25 °C) to prevent excessive viscosity of high-concentration mAb solutions at low temperatures.

Minimize Residence Time in Final Concentration Stage

The final concentration step with ultra-high protein concentration carries the highest aggregation risk. Shortening the residence time in this stage can effectively suppress protein aggregation.

Instead of the conventional workflow of repeated sampling and offline concentration detection (where the bulk tank concentration cannot accurately reflect the actual concentration due to severe concentration polarization), calculate the average retentate concentration based on the cumulative permeate volume, and terminate the concentration process immediately once the target volume is reached to proceed to product recovery. Alternatively, single-pass tangential flow filtration (SPTFF) can be applied for the final concentration step.

Shear Force Control

Excessive shear force induces mAb aggregation, so shear stress must be strictly controlled throughout the process.

Select low-shear pumps such as diaphragm pumps or rotary lobe pumps, and operate the final concentration step at low flow rates and low transmembrane pressure (TMP).

Add non-reducing sugars including sucrose and trehalose to enhance solution stability. These excipients protect protein molecules and mitigate the adverse impacts of fluid shear.

Implement SPTFF technology to reduce the number of material recirculation cycles across pumps, thereby lowering overall shear exposure.

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Filtration Process for High-Concentration Biopharmaceuticals

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