Effects of solution conditions on virus retention by the Viresolve® NFP filter.

Virus filtration can provide a robust method for removal of adventitious parvoviruses in the production of biotherapeutics. Although virus filtration is typically thought to function by a purely size-based removal mechanism, there is limited data in the literature indicating that virus retention is a function of solution conditions. The objective of this work was to examine the effect of solution pH and ionic strength on virus retention by the Viresolve(®) NFP membrane. Data were obtained using the bacteriophage ϕX174 as a model virus, with retention data complemented by the use of confocal microscopy to directly visualize capture of fluorescently labeled ϕX174 within the filter. Virus retention was greatest at low pH and low ionic strength, conditions under which there was an attractive electrostatic interaction between the negatively charged membrane and the positively charged phage. In addition, the transient increase in virus transmission seen in response to a pressure disruption at pH 7.8 and 10 was completely absent at pH 4.9, suggesting that the trapped virus are unable to overcome the electrostatic attraction and diffuse out of the pores when the pressure is released. Further confirmation of this physical picture was provided by confocal microscopy. Images obtained at pH 10 showed the migration of previously captured phage; this phenomenon was absent at pH 4.9. These results provide important new insights into the factors governing virus retention using virus filtration membranes.

Probing effects of pressure release on virus capture during virus filtration using confocal microscopy.

Virus filtration is used to ensure drug safety in the production of biotherapeutics. Several recent studies have shown a dramatic decrease in virus retention as a result of a process disruption, e.g., a transient pressure release. In this work, a novel two-label fluorescence technique was developed to probe virus capture within virus filtration membranes using confocal microscopy. Experiments were performed with Ultipor® DV20, Viresolve® Pro, and Viresolve® NFP membranes using bacteriophage φx174 as a model virus. The filters were challenged with two batches of fluorescently labeled phage: one labeled with red dye (Cy5) and one with green dye (SYBR Gold) to visualize captured phage from before and after the pressure release. The capture patterns seen in the confocal images were a strong function of the underlying membrane morphology and pore structure. The DV20 and Viresolve® NFP showed migration of previously captured phage further into the filter, consistent with the observed loss of virus retention after the pressure release. In contrast, there was no migration of captured virus in the Viresolve® Pro membranes, and these filters were also the only ones to show stable virus retention after a pressure release. The direct visualization of virus capture using the two-label fluorescence technique provides unique insights into the factors controlling the retention characteristics of virus filters with different pore structure.

Mechanistic evaluation of virus clearance by depth filtration.

Virus clearance by depth filtration has not been well-understood mechanistically due to lack of quantitative data on filter charge characteristics and absence of systematic studies. It is generally believed that both electrostatic interactions and sized based mechanical entrapment contribute to virus clearance by depth filtration. In order to establish whether the effectiveness of virus clearance correlates with the charge characteristics of a given depth filter, a counter-ion displacement technique was employed to determine the ionic capacity for several depth filters. Two depth filters (Millipore B1HC and X0HC) with significant differences in ionic capacities were selected and evaluated for their ability to eliminate viruses. The high ionic capacity X0HC filter showed complete porcine parvovirus (PPV) clearance (eliminating the spiked viruses to below the limit of detection) under low conductivity conditions (≤2.5 mS/cm), achieving a log10 reduction factor (LRF) of > 4.8. On the other hand, the low ionic capacity B1HC filter achieved only ∼2.1-3.0 LRF of PPV clearance under the same conditions. These results indicate that parvovirus clearance by these two depth filters are mainly achieved via electrostatic interactions between the filters and PPV. When much larger xenotropic murine leukemia virus (XMuLV) was used as the model virus, complete retrovirus clearance was obtained under all conditions evaluated for both depth filters, suggesting the involvement of mechanisms other than just electrostatic interactions in XMuLV clearance.

A novel approach to achieving modular retrovirus clearance for a parvovirus filter.

Viral filtration is routinely incorporated into the downstream purification processes for the production of biologics produced in mammalian cell cultures (MCC) to remove potential viral contaminants. In recent years, the use of retentive filters designed for retaining parvovirus (~20 nm) has become an industry standard in a conscious effort to further improve product safety. Since retentive filters remove viruses primarily by the size exclusion mechanism, it is expected that filters designed for parvovirus removal can effectively clear larger viruses such as retroviruses (~100 nm). In an attempt to reduce the number of viral clearance studies, we have taken a novel approach to demonstrate the feasibility of claiming modular retrovirus clearance for Asahi Planova 20N filters. Porcine parvovirus (PPV) and xenotropic murine leukemia virus (XMuLV) were co-spiked into six different feedstreams and then subjected to laboratory scale Planova 20N filtration. Our results indicate that Planova 20N filters consistently retain retroviruses and no retrovirus has ever been detected in the filtrates even when significant PPV breakthrough is observed. Based on the data from multiple in-house viral validation studies and the results from the co-spiking experiments, we have successfully claimed a modular retrovirus clearance of greater than 6 log10 reduction factors (LRF) to support clinical trial applications in both USA and Europe.

Impact of virus preparation quality on parvovirus filter performance.

Virus-removal filtration technology is commonly used in the manufacturing process for biologics to remove potential viral contaminants. Virus-removal filters designed for retaining parvovirus, one of the smallest mammalian viruses, are considered an industry standard as they can effectively remove broad ranges of viruses. It has long been observed that the performance of virus filters can be influenced by virus preparations used in the laboratory scale studies (PDA, 2010). However, it remains unclear exactly what quality attributes of virus preparations are critical or indicative of virus filter performance as measured by effectiveness of virus removal and filter capacity consistency. In an attempt to better understand the relationship between virus preparation and virus filter performance, we have systematically prepared and analyzed different grades of parvovirus with different purity levels and compared their performance profiles on Viresolve® Pro parvovirus filters using four different molecules. Virus preparations used in the studies were characterized using various methods to measure DNA and protein content as well as the hydrodynamic diameter of virus particles. Our results indicate that the performance of Viresolve® Pro filters can be significantly impacted depending on the purity of the virus preparations used in the spike and recovery studies. More importantly, we have demonstrated that the purity of virus preparations is directly correlated to the measurable biochemical and biophysical properties of the virus preparations such as DNA and protein content and monodispersal status, thus making it possible to significantly improve the consistency and predictability of the virus filter performance during process step validations.

High throughput discovery of new fouling-resistant surfaces.

A novel high throughput method for synthesis and screening of customized protein-resistant surfaces was developed. This method is an inexpensive, fast, reproducible and scalable approach to synthesize and screen protein-resistant surfaces appropriate for a specific feed. The method is illustrated here by combining a high throughput platform (HTP) approach together with our patented photo-induced graft polymerization (PGP) method developed for facile modification of commercial poly(aryl sulfone) membranes. We demonstrate that the HTP-PGP approach to synthesize and screen fouling-resistant surfaces is general, and thus provides the capability to develop surfaces optimized for specific feeds. Surfaces were prepared via graft polymerization onto poly(ether sulfone) (PES) membranes and were evaluated using a protein adsorption assay followed by pressure-driven filtration. We have employed the HTP-PGP approach to confirm previously reported successful monomers and to develop new antifouling surfaces from a library of 66 monomers for four different challenges of interest to the biotechnology community: hen egg-white lysozyme, supernatant from Chinese Hamster Ovary (CHO) cells in phosphate buffered saline (PBS) solution as a model cell suspension, and immunoglobulin G (IgG) precipitated in the absence and presence of bovine serum albumin (BSA) in high salt solution as a model precipitation process.

Tocilizumab.

Roche is co-developing tocilizumab (Actemra, RoActemra), a humanized anti-interleukin-6 receptor (IL-6R) monoclonal antibody, with Chugai Pharmaceutical. Tocilizumab is marketed in Japan for Castleman disease and several types of arthritis. The product is approved in the European Union for treatment of moderate-to-severe rheumatoid arthritis, and is currently undergoing review by the US Food and Drug Administration for this condition. Tocilizumab has also been studied for potential use in the treatment of other IL-6 related disorders including Crohn disease.

Selective precipitation-assisted recovery of immunoglobulins from bovine serum using controlled-fouling crossflow membrane microfiltration.

Efficient and economic recovery of immunoglobulins (Igs) from complex biological fluids such as serum, cell culture supernatant or fermentation cell lysate or supernatant, represents a substantial challenge in biotechnology. Methods such as protein A affinity chromatography and anion exchange chromatography are limited by cost and selectivity, respectively, while membrane chromatography is limited by low adsorptive area, flow distribution problems and scale-up difficulties. By combining the traditional salt-assisted precipitation process for selective removal of Igs from serum followed by constant-permeate flux membrane microfiltration for low fouling, we demonstrate an exciting new, efficient and economic hybrid method. The high selectivity of an ammonium sulfate-induced precipitation step was used to precipitate the Igs leaving the major undesirable impurity, the bovine serum albumin (BSA), in solution. Crossflow membrane microfiltration in diafiltration mode was then employed to retain the precipitate, while using axial flow rates to optimize removal of residual soluble BSA to the permeate. The selectivity between immunoglobulin G (IgG) and BSA obtained from the precipitation step was approximately 36, with 97% removal of the BSA with diafiltration in 5 diavolumes with resulting purity of the IgG of approximately 93% after the membrane microfiltration step. Complete resolubilization of the IgG was obtained without any aggregation at the concentrations of ammonium sulfate employed in this work. Further, membrane pore size and axial Reynolds number (recirculation rate) were shown to be important for minimizing fouling and loss of protein precipitate.

Optimized removal of soluble host cell proteins for the recovery of met-human growth hormone inclusion bodies from Escherichia coli cell lysate using crossflow microfiltration.

Cross-flow membrane microfiltration was used under optimal conditions to recover met-growth hormone inclusion bodies (IBs) from Escherichia coli cell lysate by removal of the host-cell (bacterial) proteins (HCP) under minimal fouling conditions. This is the first step of a two-step process in which the goal was to isolate IBs at high yield from the HCP. These undesired soluble HCP were removed by passing them through the membrane while retaining the insolubles, including the aggregated IBs. Experiments were conducted at constant permeate flux with flat-sheet membranes of different pore sizes and chemistry, with feeds of varying pH and ionic strengths to determine the optimum combination for HCP removal. Diafiltration, the washing away of impurities with protein-free buffer, was then employed to ensure removal of the host cell proteins at the optimum conditions. About 90% removal of the HCP was obtained in about 5 diavolumes, maintaining high protein transmission and low membrane fouling.

Global model for optimizing crossflow microfiltration and ultrafiltration processes: a new predictive and design tool.

A global model and algorithm that predicts the performance of crossflow MF and UF process individually or in combination in the laminar flow regime is presented and successfully tested. The model accounts for solute polydispersity, ionic environment, electrostatics, membrane properties and operating conditions. Computer programs were written in Fortran 77 for different versions of the model algorithm that can optimize MF/UF processes rapidly in terms of yield, purity, selectivity, or processing time. The model is validated successfully with three test cases: separation of bovine serum albumin (BSA) from hemoglobin (Hb), capture of immunoglobulin (IgG) from transgenic goat milk by MF, and separation of BSA from IgG by UF. These comparisons demonstrate the capability of the global model to conduct realistic in silico simulations of MF and UF processes. This model and algorithm should prove to be an invaluable technique to rapidly design new or optimize existing MF and UF processes separately or in combination in both pressure-dependent and pressure-independent regimes.