Membrane and Filtration Technology
Membrane and filtration technologies are used extensively in the isolation and purification of mAb and other recombinant DNA products, from the initial clarification of cell culture broth to the final sterile filtration of purified bulk solutions. This section focuses on several of these technologies used for the purification of mAb products or removal of contaminants and impurities, including depth filtration, membrane chromatography, ultrafiltration, high performance tangential flow filtration using neutral or charged membranes and virus filtration.
Membrane chromatography or membrane adsorbers, function similarly to packed chromatography columns, but in the format of conventional filtration modules.77 Membrane chromatography uses microporous membranes, usually in multiple layers that contain functional ligands attached to the internal pore surface throughout the membrane structure.
The benefit of membrane chromatography over conventional bead chromatography is the elimination of diffusive pores. For membrane chromatography, binding sites are located along the through pores rather than nestled within long diffusive pores.78 Accordingly, mass transfer of biomolecules to binding sites depends on convection instead of diffusion and binding capacities of membrane chromatography are largely independent of flow rate.
In membrane chromatography, membranes consist of a polymeric substrate to which a functional ligand is chemically coupled. The polymer substrate is composed of multilayers of polyethersulfone, polyvinylidene fluoride and regenerated cellulose membrane. The most widely used functional ligands are the same as those used in chromatography resins, including ion exchange, e.g., quaternary amine (Q), diethylamine (D), polyethyleneimine (E), sulfonic acid (S) and carboxylic acid (C); affinity; reverse-phase, hydrophobic interaction and anionic mixed mode that is made of cross-linked Poly(Allylamine) (PAA).79,80
Among the adsorptive membranes commercially available, the Q membrane adsorber has attracted a lot of industry interest, especially in mAb purification processes.78,81,82 Commercially available Q membranes include Intercept™ (Millipore), Mustang® (Pall) and Sartobind® (Sartorius). Similar to conventional bed chromatography, Q membranes are normally exploited as polishing steps in flow-through mode to remove trace amounts of impurities. Around neutral to slightly basic pH and at low conductivities, viruses, DNA, endotoxin, a large population of host cell proteins and leached Protein A bind to the Q membrane, whereas the typically basic antibody molecules flow through the membrane matrix without being bound.83 Recently available commercial anionic mixed mode membrane such as ChromaSorb (Millipore) improves impurity binding capacity or impurity removal function at a higher loading conductivity range and broadens anionic membrane operation conditions.80
Membrane chromatography is potentially useful for protein purification at laboratory and pilot scale; however, it has limitations that need to be overcome before it can be successfully employed in process-scale production. Major limitations are uneven flow distribution, non-identical membrane size distribution, uneven membrane thickness, the availability of appropriate scale-down devices and low binding capacities.83 Low binding capacity is attributed to low surface area as well as poor flow distribution. Improved binding capacities and flow distribution have been achieved by optimizing pore size, membrane chemistry, membrane thickness or the number of membrane layers and employment of tentacle ligands.77,79 In addition, other important aspects should also be considered for this technology, including membrane lifetime (if membrane is used more than once), integrity testing, lot-to-lot variability and process economics.
Ultrafiltration is a pressure-driven membrane process that is widely used for protein concentration and buffer exchange. Ultrafiltration is a size-based separation, where species larger than the membrane pores are retained and smaller species pass through freely. Separation in ultrafiltration is achieved through differences in the filtration rates of different components across the membrane under a given pressure driving force.84 Buffer exchange is achieved using a diafiltration mode in which buffer of the final desired composition is added to the retentate system at the same rate in which filtrate is removed, thus maintaining a constant retentate volume.
Ultrafiltration with membrane pores ranging from 1 to 20 nm can provide separation of species ranging in molecular weight from 500 daltons to 1,000 kilodaltons.85 This distinguishes the process from reverse osmosis, which involves membrane pores of less than 1 nm that allows water but not salts to pass, and from microfiltration, which has membrane pores ranging in size from 0.05 to 10 µm that allow proteins to pass through.
Ultrafiltration membranes have a unique “skinned” or anisotropic structure. Membrane selectivity is controlled by a thin skin layer that is approximately 0.5 µm thick, and the mechanical strength is provided by a thicker macroporous substrate.86 This allows retention to occur on the membrane surface rather than within the filter structure.85 Filtration fluxes are also greatly increased since a majority of the flow resistance takes place in the thin skin layer.
Ultrafiltration membranes can be cast from a wide variety of polymers, including polysulfone, polyethersulfone, polyvinylidene fluoride and regenerated cellulose. The synthetic polymers exhibit strong resistance to acids, bases, alcohols and higher temperatures, allowing for effective membrane cleaning. In this manner, ultrafiltration membranes can be reused without deterioration of flow rates and cross contamination.85 Synthetic polymers could provide more attractive chemical and thermal stability but are prone to protein fouling. In contrast, cellulose membranes have low protein binding but can be damaged by harsh cleaning methods. New composite regenerated cellulose membranes have significantly less protein fouling, are more easily cleaned, and have excellent mechanical strength. Due to these favorable properties, cellulose membranes are superior to other membranes in process permeability and retention characteristics for protein ultrafiltration and diafiltration.84
Ultrafiltration is normally carried out in tangential flow filtration (TFF) mode, in which fluid passes across the filter (cross-flow), tangential to the plane of the filter surface. The primary advantage of TFF is that the cross-flow continuously sweeps the filter surface, reducing the extent to which materials accumulate on the filter surface, and increasing filtration throughput.
Ultrafiltration systems can be operated with different control strategies. Ultrafiltration and diafiltration processes are typically developed using constant retentate pressure, constant trans-membrane pressure or constant filtrate flux. As these control methods do not factor in the effects of the protein gel layer at the membrane surface, a method of maintaining constant protein concentration at the membrane surface was introduced.87 The potential benefits of this method include enhanced product yield, minimized membrane area, consistent processing times as the variations in feed and membrane properties are controlled.
Delivery of therapeutic mAbs by subcutaneous administration is convenient for patients, but a high concentration formulation must be used in order to keep the injection volume low.88 A high concentration formulation (183 g/L) was developed for an anti-B. anthracis protective antigen antibody in clinical development.88 The optimal antibody concentration at the diafiltration step was selected to minimize the processing time and required surface area. With high concentration ultrafiltration processes, solutions often become very viscous and this limits the final concentration. Recent work has demonstrated use of elevated temperature as an approach to safely manage the rheological properties of high concentration formulations, as well as to enhance overall mass transfer.88,89 Winter et al. developed a high concentration formulation process for an IgG1.88 The high viscosity of the IgG1 limited the ability to achieve the required high concentration of greater than 200 g/L. The process temperature was increased from 23 to 46°C, which resulted in a 2-fold decrease in viscosity and a 2-fold increase in the filtration flux. Processing at the higher temperature did not impact the product quality.
An important process development aspect of a final UFDF formulation step includes the final sterile filtration or bulk filtration of the product. In general, sterile filtration is an important concern for all intermediate purification pools, but considerably more so at the end of the process where the highest protein concentrations are present and greatest value has been imparted onto the product. Proper scale-up techniques using equipment representative to manufacturing is critical.90 Understanding the need for cost-effective, robust sterile filtration, manufacturers have also developed new classes of composite and asymmetric membranes with higher throughput capacities.91,92
High performance tangential flow filtration.
Ultrafiltration has the inherent nature of high throughput and low resolution. Recent studies have shown that ultrafiltration systems can be used for the separation of proteins of similar and moderately different sizes based on differences in protein charge.93–97 Separation is achieved by exploiting electrostatic interactions between protein molecules and the membrane pores. The emerging technique of high performance tangential flow filtration (HPTFF) is a two-dimensional unit operation in which both size and charge differences are utilized for the purpose of purification and separation.98 In addition, protein concentration and buffer exchange can be accomplished in the same unit operation.
Charged proteins in electrolyte solutions are surrounded by a diffuse ion cloud or electrical double layer due to electrostatic interactions with the counter-ions and co-ions.86 In HPTFF, by optimizing buffer pH and ionic strength, the difference in hydrodynamic size between product and impurities can be increased. The hydrodynamic size increases with charges and decreases with ionic strength since higher conductivities shield charges on proteins.99 High selectivity is achieved by increasing the electrostatic exclusion of the more highly charged protein relative to the neutral protein. In addition, membrane charge could further enhance the resolution between charged and neutral molecules.95 The use of charged membranes provides an additional degree of robustness to the process compared to uncharged membranes. A positively charged membrane could provide much greater retention of a positively charged protein than a negatively charged or neutral membrane.
In HPTFF, membrane pore size distribution affects selectivity by altering solute sieving coefficients and filtrate flow distribution. Eliminating large defects and controlling the pore size distribution can significantly improve the performance of the HPTFF membrane.86
As in conventional ultrafiltration processes, the successful implementation of HPTFF processes relies on optimizing operational fluxes, transmembrane pressure, module and flow path design to enhance selectivity and reduce protein fouling on membrane surfaces to reach an optimal performance.96
HPTFF has been reported to achieve removal of host cell proteins and host cell DNA impurities in mAb purification processes utilizing non-affinity chromatography steps combined with HPTFF.100 HPTFF was used in diafiltration mode in which product was retained while impurities were removed from the retentate. One example resulted in the reduction of Chinese hamster ovary proteins (CHOP) from 530 ppm (parts per million, i.e., nanogram of CHOP per milligram of mAb product) after cation exchange purification, to 15 ppm after anion exchange purification and then to less than 0.6 ppm after HPTFF.100
Mammalian cells used in the manufacture of mAbs and other therapeutic recombinant proteins produce endogenous retroviruses and are occasionally infected with adventitious viruses during processing.101 Due to safety requirements, mammalian cell-derived products may contain less than one virus particle per million doses. This typically translates to approximately 12–18 log10 clearance of endogenous retroviruses and 6 log10 clearance for adventitious viruses.102 The inclusion of virus removal and inactivation steps in designing purification processes is an integral component of product safety assurance strategies.101 It is generally required that orthogonal virus clearance steps are used that employ complementary mechanisms, so that viruses not cleared by one mechanism may be cleared by the other.103