With the advancement of the biopharmaceutical industry, an increasing number of high-concentration monoclonal antibody (mAb) formulations have been approved for marketing. Since 1998, approximately one-third of FDA-approved mAb products are high-concentration formulations with a protein concentration exceeding 100 mg/mL. Among the 46 approved high-concentration protein formulations, most are indicated for chronic diseases in the fields of immunology, neurology and cardiovascular disorders, which require long-term medication. The production of high-concentration mAb drug substances is critical for enhancing therapeutic efficacy, reducing administration volume and improving patient compliance.
Tangential flow filtration (TFF) serves as a core unit operation in mAb manufacturing. It enables impurity removal, antibody concentration and buffer composition adjustment via concentration and diafiltration steps. Nevertheless, the inherent properties of high-concentration mAb solutions, such as elevated viscosity and high aggregation propensity, pose substantial challenges to TFF processes. Therefore, establishing robust manufacturing strategies is essential to guarantee the quality and yield of high-concentration mAb products.
I. Key Challenges
1. Elevated Feed Viscosity and Excessive Operating Pressure
At a constant temperature, the viscosity of mAb solutions rises along with protein concentration. When concentrated to 150–200 g/L, the viscosity of antibody solutions can reach 10–50 centipoise (cP), varying with the intrinsic characteristics of individual antibodies. Low temperature further increases solution viscosity: reduced thermal motion of antibody molecules facilitates intermolecular association and restricts fluidity, and this phenomenon is far more pronounced in high-concentration systems.
High feed viscosity tends to cause membrane channel fouling, leading to elevated inlet pressure during ultrafiltration (UF). Particularly in the final concentration stage, continuous pressure build-up may exceed the maximum allowable inlet pressure and force process shutdown.
A 0.2 μm sterile filtration step is generally implemented post-TFF to control microbial bioburden. The high viscosity of concentrated mAb solutions results in excessive filtration pressure, hindering the normal completion of sterile filtration.
2. Flux Decline
High solution viscosity impairs fluid mobility and reduces membrane permeability. At sufficiently high antibody concentrations, a gel layer readily forms on the membrane surface, creating significant resistance to solvent permeation and exacerbating flux decay.
3. Increased Aggregation Risk
In high-concentration mAb solutions, the average distance between antibody molecules decreases and intermolecular interactions intensify, which greatly elevates the risk of protein aggregation. Antibody aggregates compromise product quality by reducing biological activity and increasing immunogenicity.
4. Low Product Recovery
Functional groups on the surface of UF membranes tend to adsorb protein molecules. The high protein load in concentrated solutions increases contact opportunities between antibodies and membranes, resulting in severe protein adsorption.
In addition, dead volumes exist at pipeline bends, diameter transitions, valves, internal gaps of membrane cassettes and flow channel terminals, where residual mAb solution cannot be fully collected, further lowering overall recovery. Protein retention within 0.2 μm filters during post-TFF sterile filtration also contributes to yield loss.
5. Deviation of Excipient Concentration
Donnan effect and size exclusion effect are markedly amplified during the ultrafiltration of high-concentration mAbs.
Size exclusion effect: UF membranes possess defined pore size distributions. Theoretically, small-molecule excipients can permeate through membranes while large-molecule antibodies are retained. In practice, excipients may exist as monomers or bind to antibody aggregates to form larger species, which are then retained by membranes, leading to low excipient concentration in the permeate and high concentration in the retentate. Furthermore, abundant antibody molecules occupy a large portion of membrane pore space, limiting the available pathways for excipient permeation and intensifying the size exclusion effect.
Donnan effect: Antibody molecules carry intrinsic surface charges. They compete with charged excipient molecules for electrostatic adsorption sites on the membrane surface. At high protein concentrations, antibodies preferentially occupy these sites, disrupting the normal distribution of excipients governed by the Donnan equilibrium. This electrostatic exclusion impedes the permeation of certain excipients, causing abnormal excipient distribution between permeate and retentate.
II. Mitigation Strategies
1. Selection of Ultrafiltration Membranes
Hydrophilic membranes with low protein adsorption and stable flux performance are preferred to mitigate fouling, aggregation and improve process efficiency and recovery. Typical options include Pellicon® 3 Cassette equipped with Ultracel® Membrane D Screen and T-Series cassettes with Delta RC membrane. These products feature excellent hydrophilicity and minimal mAb adsorption. Their specialized screen structures reduce pressure drop and prevent excessive inlet pressure. Meanwhile, they enhance mass transfer, alleviate concentration polarization at high protein concentrations and maintain stable permeate flux.
2. Buffer Formulation Optimization
Optimization of buffer type, ionic strength and pH is critical for minimizing deviations caused by Donnan effect and size exclusion effect. The recommended ionic strength of diafiltration buffers ranges from 5–50 mM with a pH of 6.0–7.0. Commonly used buffers include histidine-HCl buffer, citrate buffer, phosphate buffer and acetate buffer. Tuning buffer pH has been proven effective in eliminating excipient concentration deviation between retentate and permeate.
3. Process Temperature Control
Maintain the temperature of feed solution and diafiltration buffer at ambient temperature (20–25 °C) throughout the TFF process to avoid excessive viscosity elevation induced by low temperature.
4. Minimize Dwell Time in Final Concentration Stage
The final concentration stage with extremely high protein concentration carries the highest aggregation risk. Dwell time in this stage should be strictly shortened. Instead of conventional multiple sampling and offline testing to verify target concentration (a method lacking representativeness due to severe concentration polarization), the actual permeate volume can be used to calculate the average retentate concentration. Once the target volume is reached, terminate concentration immediately and proceed to product recovery.
5. Shear Force Regulation
Excessive shear force can trigger antibody aggregation. Low-shear pumps such as diaphragm pumps or rotary lobe pumps are recommended for TFF systems. During final concentration, operate at a low transmembrane pressure (TMP) to reduce flow-induced shear stress.
The addition of non-reducing sugars (e.g., sucrose, trehalose) can improve solution stability, protect antibody molecules and mitigate the adverse impacts of shear force.
6. Selection and Flushing of 0.2 μm Filters
Select low-protein-adsorption filters with high volumetric capacity for sterile filtration to avoid pressure build-up. Oversized filters shall be avoided as they lead to larger dead volume and higher product loss.
Post-filtration buffer flushing is adopted to recover residual antibodies inside filters. The flushing volume must be precisely controlled to prevent undesired dilution of the final mAb solution.
III. Conclusion
Ultrafiltration and concentration of high-concentration mAbs is a sophisticated unit operation confronted with multiple technical challenges. The aforementioned optimization strategies can effectively resolve issues including excessive operating pressure, flux decline, protein aggregation, excipient concentration deviation and low recovery. Given the distinct physicochemical properties of different monoclonal antibodies, relevant strategies should be adjusted and customized according to specific product characteristics in practical manufacturing.
