Comparative Analysis of the Impact of Protein on Virus Retention for Different Virus Removal Filters.

The performance of virus filters is often determined by the extent of protein fouling, which can affect both filtrate flux and virus retention. However, the mechanisms governing changes in virus retention in the presence of proteins are still not well understood. The objective of this work was to examine the effect of proteins on virus retention by both asymmetric (Viresolve® NFP and Viresolve® Pro) and relatively homogeneous (Ultipor® DV20 and PegasusTM SV4) virus filtration membranes. Experiments were performed with bacteriophage ϕX174 as a model parvovirus and human serum immunoglobulin G (hIgG) as a model protein. The virus retention in 1 g/L hIgG solutions was consistently less than that in a protein-free buffer solution by between 1 to 3 logs for the different virus filters. The virus retention profiles for the two homogeneous membranes were very similar, with the virus retention being highly correlated with the extent of flux decline. Membranes prefouled with hIgG and then challenged with phages also showed much lower virus retention, demonstrating the importance of membrane fouling; the one exception was the Viresolve® Pro membrane, which showed a similar virus retention for the prefouled and pristine membranes. Experiments in which the protein was filtered after the virus challenge demonstrated that hIgG can displace previously captured viruses from within a filter. The magnitude of these effects significantly varied for the different virus filters, likely due to differences in membrane morphology, pore size distribution, and chemistry, providing important insights into the development/application of virus filtration in bioprocessing.

Buffer effects on protein sieving losses in ultrafiltration and their relationship to biophysical properties.

The design of effective ultrafiltration/diafiltration processes for protein formulation requires the use of membranes with very high protein retention. The objective of this study was to examine the effects of specific buffers on the retention of a model protein (bovine serum albumin) during ultrafiltration. Albumin retention at pH 4.8 was significantly reduced in phosphate buffer compared with that in acetate, citrate, and histidine. This behavior was consistent with a small change in the effective albumin hydrodynamic diameter as determined by dynamic light scattering. The underlying conformational changes leading to this change in diameter were explored using circular dichroism spectroscopy and differential scanning calorimetry. These results provide important insights into the factors controlling protein retention during ultrafiltration and diafiltration.

Protein loss during membrane processes in biopharmaceutical manufacturing.

Maximizing product yield in biopharmaceutical manufacturing processes is a critical factor in determining the overall cost of goods, especially given the high value of these biological products. However, there has been relatively limited research on the quantitative analysis of protein losses due to adsorption and fouling during the different membrane filtration processes employed in typical downstream operations. This study aims to provide a comprehensive analysis of protein loss in the range of membrane systems used in downstream processing including clarification, virus removal filtration, ultrafiltration/diafiltration for formulation, and final sterile filtration, all using commercially available membranes with three model proteins (bovine serum albumin, human serum albumin, and immunoglobulin G). The correlation between protein loss and various parameters (i.e., protein type, protein concentration, throughput, membrane morphology, and protein removal mechanism) was also investigated. This study provides important insights into the nature of protein loss during membrane processes as well as a methodology for quantifying protein yield loss in bioprocesses.

Addition of sodium malonate alters the morphology and increases the critical flux during tangential flow filtration of precipitated immunoglobulins.

Recent studies have demonstrated that one can control the packing density, and in turn the filterability, of protein precipitates by changing the pH and buffer composition of the precipitating solution to increase the structure/order within the precipitate. The objective of this study was to examine the effect of sodium malonate, which is known to enhance protein crystallizability, on the morphology of immunoglobulin precipitates formed using a combination of ZnCl2 and polyethylene glycol. The addition of sodium malonate significantly stabilized the precipitate particles as shown by an increase in melting temperature, as determined by differential scanning calorimetry, and an increase in the enthalpy of interaction, as determined by isothermal titration calorimetry. The sodium malonate also increased the selectivity of the precipitation, significantly reducing the coprecipitation of DNA from a clarified cell culture fluid. The resulting precipitate had a greater packing density and improved filterability, enabling continuous tangential flow filtration with minimal membrane fouling relative to precipitates formed under otherwise identical conditions but in the absence of sodium malonate. These results provide important insights into strategies for controlling precipitate morphology to enhance the performance of precipitation-filtration processes for the purification of therapeutic proteins.

Proteomic analysis of host cell protein fouling during bioreactor harvesting.

Chinese hamster ovary (CHO) cells are among the most common cell lines used for therapeutic protein production. Membrane fouling during bioreactor harvesting is a major limitation for the downstream purification of therapeutic proteins. Host cell proteins (HCP) are the most challenging impurities during downstream purification processes. The present work focuses on identification of HCP foulants during CHO bioreactor harvesting using reverse asymmetrical commercial membrane BioOptimal™ MF-SL. In order to investigate foulants and fouling behavior during cell clarification, for the first time a novel backwash process was developed to effectively elute almost all the HCP and DNA from the fouled membrane filter. The isoelectric points (pIs) and molecular weights (MWs) of major HCP in the bioreactor harvest and fouled on the membrane were successfully characterized using two-dimensional gel electrophoresis (2D SDS-PAGE). In addition, a total of 8 HCP were identified using matrix-assisted laser desorption/ionization-mass spectroscopy (MALDI-MS). The majority of these HCP are enzymes or associated with exosomes, both of which can form submicron-sized particles which could lead to the plugging of the filters.

Filtration of a multi-serotype glycoconjugate vaccine drug product through sterilizing grade 0.2/0.22 µm pore size filters.

Glycoconjugate vaccines containing multiple serotypes of a bacterial capsular polysaccharide can provide strong immune protection against pathogenic infections. Sterile filtration is an important component of the fill and finish operations in the preparation of these vaccines, with the capacity of the sterile filter limited by membrane fouling. The objective of this study was to examine the performance of a range of commercial 0.2/0.22 µm nominal pore size sterilizing grade filters with both single-layer and dual-layer structures during filtration of a glycoconjugate vaccine drug product consisting of four polysaccharide serotypes. The highly asymmetric Millipore Express showed much higher capacity than the more homogeneous filters, with the support structure of the Express acting as a prefilter that was able to remove foulants thereby protecting the small pores in the size-selective skin layer. This behavior was confirmed by performing experiments with different batch prefilters and by examining the location of foulant deposition within the sterile filters using confocal microscopy. These results provide important insights into the factors controlling fouling by these multiserotype vaccines as well as a framework for increasing the capacity of the sterile filter.

Membrane technology for the purification of RNA and DNA therapeutics.

Nucleic acid therapeutics have the potential to revolutionize the biopharmaceutical industry, providing highly effective vaccines and novel treatments for cancers and genetic disorders. The successful commercialization of these therapeutics will require development of manufacturing strategies specifically tailored to the purification of nucleic acids. Membrane technologies already play a critical role in the downstream processing of nucleic acid therapeutics, ranging from clarification to concentration to selective purification. This review provides an overview of how membrane systems are currently used for nucleic acid purification, while highlighting areas of future need and opportunity, including adoption of membranes in continuous bioprocessing.

Co-current filtrate flow in TFF perfusion processes: Decoupling transmembrane pressure from crossflow to improve product sieving.

Hollow fiber-based membrane filtration has emerged as the dominant technology for cell retention in perfusion processes yet significant challenges in alleviating filter fouling remain unsolved. In this work, the benefits of co-current filtrate flow applied to a tangential flow filtration (TFF) module to reduce or even completely remove Starling recirculation caused by the axial pressure drop within the module was studied by pressure characterization experiments and perfusion cell culture runs. Additionally, a novel concept to achieve alternating Starling flow within unidirectional TFF was investigated. Pressure profiles demonstrated that precise flow control can be achieved with both lab-scale and manufacturing-scale filters. TFF systems with co-current flow showed up to 40% higher product sieving compared to standard TFF. The decoupling of transmembrane pressure from crossflow velocity and filter characteristics in co-current TFF alleviates common challenges for hollow fiber-based systems such as limited crossflow rates and relatively short filter module lengths, both of which are currently used to avoid extensive pressure drop along the filtration module. Therefore, co-current filtrate flow in unidirectional TFF systems represents an interesting and scalable alternative to standard TFF or alternating TFF operation with additional possibilities to control Starling recirculation flow.

Impact of proteins and protein fouling on virus retention during virus removal filtration.

Virus filtration is a crucial step in ensuring the high levels of viral clearance required in the production of biotherapeutics produced in mammalian cells or derived from human plasma. Previous studies have reported that virus retention is often reduced in the presence of therapeutic proteins due to membrane fouling; however, the underlying mechanisms controlling this behavior are still not well understood. Experimental studies were performed with a single layer of the commercially available dual-layer PegasusTM SV4 virus removal filter to more easily interpret the experimental results. Bacteriophage ФX174 was used as a model parvovirus, and human immunoglobulin (hIgG) and Bovine Serum Albumin (BSA) were used as model proteins. Data obtained with 5 g/L solutions of hIgG showed more than a 100-fold reduction in virus retention compared to that in the protein-free solution. Similar effects were seen with membranes that were pre-fouled with hIgG and then challenged with ФX174. The experimental data were well-described using an internal polarization model that accounts for virus capture and accumulation within the virus filter, with the hIgG nearly eliminating the irreversible virus capture while also facilitating the release of previously captured virus. These results provide important insights into the performance and validation of virus removal filters in bioprocessing.

The role of intermolecular interactions on monoclonal antibody filtration through virus removal membranes.

The removal of viruses by filtration is a critical unit operation to ensure the overall safety of monoclonal antibody (mAb) products. Many mAbs show very low filtrate flux during virus removal filtration, although there are still significant uncertainties regarding both the mechanisms and antibody properties that determine the filtration behavior. Experiments were performed with three highly purified mAbs through three different commercial virus filters (Viresolve Pro, Viresolve NFP, and Pegasus SV4) with different pore structures and chemistries. The flux decline observed during mAb filtration was largely reversible, even under conditions where the filtrate flux with the mAb was more than 100-fold smaller than the corresponding buffer flux. The extent of flux decline was highly correlated with the hydrodynamic diameter of the mAb as determined by dynamic light scattering (DLS). The mAb with the lowest filtrate flux for all three membranes showed the largest attractive intermolecular interactions and the greatest hydrophobicity, with the latter determined by binding to a butyl resin in an analytical hydrophobic interaction chromatography (HIC) column. These results strongly suggest that the flux behavior is dominated by reversible self-association of the mAbs, providing important insights into the design of more effective virus filtration processes and in the early identification of problematic mAbs/solution conditions.

High-performance countercurrent membrane purification for host cell protein removal from monoclonal antibody products.

The transition to continuous biomanufacturing has led to renewed interest in alternative approaches for downstream processing of monoclonal antibody (mAb) products. In this study, we examined the potential of using high-performance countercurrent membrane purification (HPCMP) for the removal of host cell proteins (HCPs) derived from Chinese Hamster Ovary cells in the purification of a mAb. Initial studies used several model proteins to identify appropriate operating conditions for the hollow fiber membrane modules. HPCMP was then used for mAb purification, with mAb yield >95% and more than 100-fold reduction in HCP. Stable operation was maintained for 48 h for feeds that were first prefiltered through the 3MTM Harvest RC chromatographic clarifier to remove DNA and other foulants. In addition, the Process Mass Intensity for HPCMP can be much less than that for alternative HCP separation processes. These results highlight the potential of using HPCMP as part of a fully continuous mAb production process.

Continuous precipitation-filtration process for initial capture of a monoclonal antibody product using a four-stage countercurrent hollow fiber membrane washing step.

The significant increase in product titers, coupled with the growing focus on continuous bioprocessing, has renewed interest in using precipitation as a low-cost alternative to Protein A chromatography for the primary capture of monoclonal antibody (mAb) products. In this work, a commercially relevant mAb was purified from clarified cell culture fluid using a tubular flow precipitation reactor with dewatering and washing provided by tangential flow microfiltration. The particle morphology was evaluated using an inline high-resolution optical probe, providing quantitative data on the particle size distribution throughout the precipitation process. Data were obtained in both a lab-built 2-stage countercurrent washing system and a commercial countercurrent contacting skid that provided 4 stages of continuous washing. The processes were operated continuously for 2 h with overall mAb yield of 92 ± 3% and DNA removal of nearly 3 logs in the 4-stage system. The high DNA clearance was achieved by selective redissolution of the mAb using a low pH acetate buffer. Host cell protein clearance was 0.59 ± 0.08 logs, comparable to that based on model predictions. The process mass intensity was slightly better than typical Protein A processes and could be significantly improved by preconcentration of the antibody feed material.

Protein fouling during constant-flux virus filtration: Mechanisms and modeling.

As biomanufacturers consider the transition from batch to continuous processing, it will be necessary to re-examine the design and operating conditions for many downstream processes. For example, the integration of virus removal filtration in continuous biomanufacturing will likely require operation at low and constant filtrate flux instead of the high (constant) transmembrane pressures (TMPs) currently employed in traditional batch processing. The objective of this study was to examine the effect of low operating filtrate flux (5-100 L/m2 /h) on protein fouling during normal flow filtration of human serum Immunoglobulin G (hIgG) through the Viresolve® Pro membrane, including a direct comparison of the fouling behavior during constant-flux and constant-pressure operation. The filter capacity, defined as the volumetric throughput of hIgG solution at which the TMP increased to 30 psi, showed a distinct minimum at intermediate filtrate flux (around 20-30 L/m2 /h). The fouling data were well-described using a previously-developed mechanistic model based on sequential pore blockage and cake filtration, suitably modified for operation at constant flux. Simple analytical expressions for the pressure profiles were developed in the limits of very low and high filtrate flux, enabling rapid estimation of the filter performance and capacity. The model calculations highlight the importance of both the pressure-dependent rate of pore blockage and the compressibility of the protein cake to the fouling behavior. These results provide important insights into the overall impact of constant-flux operation on the protein fouling behavior and filter capacity during virus removal filtration using the Viresolve® Pro membrane.

Single-Pass Tangential Flow Filtration (SPTFF) of Nanoparticles: Achieving Sustainable Operation with Dilute Colloidal Suspensions for Gene Therapy Applications.

Recent approval of several viral-vector-based therapeutics has led to renewed interest in the development of more efficient bioprocessing strategies for gene therapy products. Single-Pass Tangential Flow Filtration (SPTFF) can potentially provide inline concentration and final formulation of viral vectors with enhanced product quality due. In this study, SPTFF performance was evaluated using a suspension of 100 nm nanoparticles that mimics a typical lentivirus system. Data were obtained with flat-sheet cassettes having 300 kDa nominal molecular weight cutoff, either in full recirculation or single-pass mode. Flux-stepping experiments identified two critical fluxes, one based on boundary-layer particle accumulation (Jbl) and one based on membrane fouling (Jfoul). The critical fluxes were well-described using a modified concentration polarization model that captures the observed dependence on feed flow rate and feed concentration. Long-duration filtration experiments were conducted under stable SPTFF conditions, with the results suggesting that sustainable performance could potentially be achieved for as much as 6 weeks of continuous operation. These results provide important insights into the potential application of SPTFF for the concentration of viral vectors in the downstream processing of gene therapy agents.

Comparison of host cell protein removal by depth filters with diatomaceous earth and synthetic silica filter aids using model proteins.

A number of studies have demonstrated that depth filtration can provide significant adsorptive removal of host cell proteins (HCP), but there is still considerable uncertainty regarding the underlying factors controlling HCP binding. This study compared the binding characteristics of two fine grade depth filters, the X0SP (polyacrylic fiber with a synthetic silica filter aid) and X0HC (cellulose fibers with diatomaceous earth (DE) as a filter aid), using a series of model proteins with well-defined physical characteristics. Protein binding to the X0SP filter was dominated by electrostatic interactions with greatest capacity for positively-charged proteins. In contrast, the X0HC filter showed greater binding of more hydrophobic proteins although electrostatic interactions also played a role. In addition, ovotransferrin showed unusually high binding capacity to the X0HC, likely due to interactions with metals in the DE. Scanning Electron Microscopy with Energy Dispersive Spectroscopy was used to obtain additional understanding of the binding behavior. These results provide important insights into the physical phenomena governing HCP binding to both fully synthetic and natural (cellulose + DE) depth filters.

Sterile filtration of a multi-serotype glycoconjugate vaccine drug product.

Glycoconjugate vaccines consisting of multiple serotypes of the bacterial capsular polysaccharide can provide strong protection against infection by significant pathogens. Previous studies of the sterile filtration behavior of these glycoconjugates have been limited to experiments with individual serotypes even though the formulated vaccines contain several different serotypes to provide broad immunization. The objective of this study was to explore the fouling behavior of a glycoconjugate vaccine drug product consisting of four different polysaccharide serotypes. Sterile filtration data were obtained with 0.22 µm Durapore® membranes at both constant flux and constant pressure for both the individual serotypes and the drug product containing multiple serotypes. Fouled membranes were examined by confocal microscopy, demonstrating that all four serotypes deposit in a narrow band near the filter inlet. The different ionic composition of the formulation buffer (compared to the buffers used with the drug substance) had a large effect on the fouling behavior. In addition, the fouling resistance associated with the drug product was greater than the sum of the resistances of the individual serotypes. These results provide important insights into the sterile filtration behavior of these multivalent glycoconjugate vaccines.

Use of scanning electron microscopy and energy dispersive X-ray spectroscopy to identify key fouling species during alternating tangential filtration.

Alternating tangential flow filtration (ATF) has become one of the primary methods for cell retention and clarification in perfusion bioreactors. However, membrane fouling can cause product sieving losses that limit the performance of these systems. This study used scanning electron microscopy and energy dispersive X-ray spectroscopy to identify the nature and location of foulants on 0.2 µm polyethersulfone hollow fiber membranes after use in industrial Chinese hamster ovary cell perfusion bioreactors for monoclonal antibody production. Membrane fouling was dominated by proteinaceous material, primarily host cell proteins along with some monoclonal antibody. Fouling occurred primarily on the lumen surface with much less protein trapped within the depth of the fiber. Protein deposition was also most pronounced near the inlet/exit of the hollow fibers, which are the regions with the greatest flux (and transmembrane pressure) during the cyclical operation of the ATF. These results provide important insights into the underlying phenomena governing the fouling behavior of ATF systems for continuous bioprocessing.

Pressure-dependent fouling behavior during sterile filtration of mRNA-containing lipid nanoparticles.

The COVID-19 pandemic has generated growing interest in the development of mRNA-based vaccines and therapeutics. However, the size and properties of the lipid nanoparticles (LNPs) used to deliver the nucleic acids can lead to unique phenomena during manufacturing that are not typical of other biologics. The objective of this study was to develop a more fundamental understanding of the factors controlling the performance of sterile filtration of mRNA-LNPs. Experimental filtration studies were performed with a Moderna mRNA-LNP solution using a commercially available dual-layer polyethersulfone sterile filter, the Sartopore 2 XLG. Unexpectedly, increasing the transmembrane pressure (TMP) from 2 to 20 psi provided more than a twofold increase in filter capacity. Also surprisingly, the effective resistance of the fouled filter decreased with increasing TMP, in contrast to the pressure-independent behavior expected for an incompressible media and the increase in resistance typically seen for a compressible fouling deposit. The mRNA-LNPs appear to foul the dual-layer filter by blocking the pores in the downstream sterilizing-grade membrane layer, as demonstrated both by scanning electron microscopy and derivative analysis of filtration data collected for the two layers independently. These results provide important insights into the mechanisms governing the filtration of mRNA-LNP vaccines and therapeutics.

Evaluating Nanoparticle Hydrophobicity Using Analytical Membrane Hydrophobic Interaction Chromatography.

Nanoparticle hydrophobicity is a key factor controlling the stability, adhesion, and transport of nanoparticle suspensions. Although a number of approaches have been presented for evaluating nanoparticle hydrophobicity, these methods are difficult to apply to larger nanoparticles and viruses (>100 nm in size) that are of increasing importance in drug delivery and gene therapy. This study investigated the use of a new analytical hydrophobic interaction chromatography method employing a 5.0 µm pore size polyvinylidene fluoride membrane as the stationary-phase in membrane hydrophobic interaction chromatography (MHIC). Experimental data obtained using a series of model proteins were in good agreement with literature values for the hydrophobicity (both experimental and computational). MHIC was then used to evaluate the hydrophobicity of a variety of nanoparticles, including a live attenuated viral vaccine, both in water and in the presence of different surfactants. This new method can be implemented on any liquid chromatography system, run times are typically <20 min, and the experiments avoid the use of organic solvents that could alter the structure of many biological nanoparticles.

Process- and Product-Related Foulants in Virus Filtration.

Regulatory authorities place stringent guidelines on the removal of contaminants during the manufacture of biopharmaceutical products. Monoclonal antibodies, Fc-fusion proteins, and other mammalian cell-derived biotherapeutics are heterogeneous molecules that are validated based on the production process and not on molecular homogeneity. Validation of clearance of potential contamination by viruses is a major challenge during the downstream purification of these therapeutics. Virus filtration is a single-use, size-based separation process in which the contaminating virus particles are retained while the therapeutic molecules pass through the membrane pores. Virus filtration is routinely used as part of the overall virus clearance strategy. Compromised performance of virus filters due to membrane fouling, low throughput and reduced viral clearance, is of considerable industrial significance and is frequently a major challenge. This review shows how components generated during cell culture, contaminants, and product variants can affect virus filtration of mammalian cell-derived biologics. Cell culture-derived foulants include host cell proteins, proteases, and endotoxins. We also provide mitigation measures for each potential foulant.