Computational fluid dynamics based digital twins of fixed bed bioreactors validate scaling principles for recombinant adeno-associated virus gene therapy manufacturing.

Gene therapy using recombinant adeno-associated virus (rAAV) as delivery vehicles has garnered much interest in recent years. There are still significant gaps in our fundamental understanding of the manufacturing processes to deliver sufficient products. Manufacturing efforts of rAAV using HEK293 cells have commonly relied on fixed bed falling film bioreactors like the iCELLis®. We used computational fluid dynamics (CFD) to validate the operating conditions required for a predictive iCELLis® 500 scale-down model. The small-scale and at-scale systems have different flow paths causing validation of the corresponding agitation rates required to achieve the same linear flow through the fixed bed across scales to be non-trivial. Therefore, we used CFD to predict the theoretical scaling relationship. In addition, CFD could predict kLa differences between the two systems and the operating conditions required to match kLa between scales. We also confirmed that the location of DO control must be the same in both systems to achieve proper scaling. Experimental runs confirming the validity of the novel scale-down model showed that based on the modifications to the iCELLis® Nano system, we achieved similar DO, key metabolite, pH, and GC titer trends in both systems.

Stereospecific interactions between histidine and monoclonal antibodies.

Histidine is a frequently used buffer in the final formulation of many commercialized monoclonal antibodies (mAbs), with histidine helping to stabilize the antibody during storage in addition to its buffering function. The objective of this study was to examine the stereospecificity of any histidine-antibody interactions using a combination of experimental studies and molecular dynamics simulations. Isothermal titration calorimetry provided evidence of weak stereospecific interactions, with the antibody showing approximately two to four additional interaction sites for d- versus l-histidine. The greater interactions with d-histidine were confirmed by measurements of the net protein charge using electrophoretic light scattering. The reduction in the net negative charge of the antibody in d-histidine led to significantly different behavior during diafiltration due to Donnan exclusion effects. Molecular dynamics simulations corroborated the presence of additional d-histidine interaction sites. These results provide the first demonstration of weak stereospecific interactions between l- and d-histidine and a mAb and the implications of these interactions for antibody formulation.

The effects of buffer condition on the fouling behavior of MVM virus filtration of an Fc-fusion protein.

A combined pore blockage and cake filtration model was applied to the virus filtration of an Fc-fusion protein using the three commercially available filters, F-1, F-2, and F-3 in a range of buffer conditions including sodium-phosphate and tris-acetate buffers with and without 200 mM NaCl at pH 7.5. The fouling behaviors of the three filters for the feed solutions spiked with minute virus of mice were described well by this combined model for all the solution conditions. This suggests that fouling of the virus filters is dominated by the pore blockage mechanism during the initial stage of the filtration and transformed to the cake filtration mechanism during the later stage of the filtration. Both flux and transmembrane resistance can be described well by this model. The pore blockage rate and the rate of increase of protein layer resistance over blocked pores are found to be affected by membrane properties as well as the solution conditions resulting from the modulation of interactions between virus, protein, and membrane by the solution conditions.

DNA RETENTION ON DEPTH FILTERS.

Depth filtration is a commonly-used bioprocessing unit operation for harvest clarification that reduces the levels of process- and product-related impurities such as cell debris, host-cell proteins, nucleic acids and protein aggregates. Since depth filters comprise multiple components, different functionalities may contribute to such retention, making the mechanisms by which different impurities are removed difficult to decouple. Here we probe the mechanisms by which double-stranded DNA (dsDNA) is retained on depth filter media by visualizing the distribution of fluorescently-labeled retained DNA on spent depth filter discs using confocal fluorescence microscopy. The extent of DNA displacement into the depth filter was found to increase with decreasing DNA length with increasing operational parameters such as wash volume and buffer ionic strength. Finally, using 5ethynyl-2'-deoxyuridine (EdU) to label DNA in dividing CHO cells, we showed that Chinese hamster ovary (CHO) cellular DNA in the lysate supernatant migrates deeper into the depth filter than the lysate re-suspended pellet, elucidating the role of the size of the DNA in its form as an impurity. Apart from aiding DNA purification and removal, our experimental approaches and findings can be leveraged in studying the transport and retention of nucleic acids and other impurities on depth filters at a small scale.

Contributions of depth filter components to protein adsorption in bioprocessing.

Depth filtration is widely used in downstream bioprocessing to remove particulate contaminants via depth straining and is therefore applied to harvest clarification and other processing steps. However, depth filtration also removes proteins via adsorption, which can contribute variously to impurity clearance and to reduction in product yield. The adsorption may occur on the different components of the depth filter, that is, filter aid, binder, and cellulose filter. We measured adsorption of several model proteins and therapeutic proteins onto filter aids, cellulose, and commercial depth filters at pH 5-8 and ionic strengths <50 mM and correlated the adsorption data to bulk measured properties such as surface area, morphology, surface charge density, and composition. We also explored the role of each depth filter component in the adsorption of proteins with different net charges, using confocal microscopy. Our findings show that a complete depth filter's maximum adsorptive capacity for proteins can be estimated by its protein monolayer coverage values, which are of order mg/m2 , depending on the protein size. Furthermore, the extent of adsorption of different proteins appears to depend on the nature of the resin binder and its extent of coating over the depth filter surface, particularly in masking the cation-exchanger-like capacity of the siliceous filter aids. In addition to guiding improved depth filter selection, the findings can be leveraged in inspiring a more intentional selection of components and design of depth filter construction for particular impurity removal targets.

Vitamin B12 association with mAbs: Mechanism and potential mitigation strategies.

Process control for manufacturing biologics is critical for ensuring product quality, safety, and lot to lot consistency of therapeutic proteins. In this study, we investigated the root cause of the pink coloration observed for various in-process pools and drug substances in the antibody manufacturing process. Vitamin B12 is covalently bound to mAbs via a cobalt-sulfur coordinate bond via the cysteine residues. The vitamin B12 was identified to attach to an IgG4 molecule at cysteine residues on light chain (Cys-214), and heavy chain (Cys-134, Cys-321, Cys-367, and Cys-425). Prior to attachment to mAbs, the vitamin B12 needs to be in its active form of hydroxocobalamin. During culture media preparation, storage and cell culture processing, cyanocobalamin, the chemical form of vitamin B12 added to media, is converted to hydroxocobalamin by white fluorescence light (about 50% degradation in 11-14 days at room temperature and with room light intensity about 500-1,000 lux) and by short-wavelength visible light (400-550 nm). However, cyanocobalamin is stable under red light (wavelength >600 nm) exposure and does not convert to hydroxocobalamin. Our findings suggests that the intensity of pink color depends on concentrations of both free sulfhydryl groups on reduced mAb and hydroxocobalamin, the active form of vitamin B12 . Both reactants are necessary and neither one of them is sufficient to generate pink color, therefore process control strategy can consider limiting either one or both factors. A process control strategy to install red light (wavelength >600 nm) in culture media preparation, storage and culture processing areas is proposed to provide safe light for biologics and to prevent light-induced color variations in final products.

Mechanisms of precipitate formation during the purification of an Fc-fusion protein.

Protein precipitates that arise during bioprocessing can cause manufacturing challenges, but they can also aid in clearance of host-cell protein (HCP) and DNA impurities. Such precipitates differ from many protein precipitates that have been studied previously in their heterogeneous composition, particularly in the presence of high concentrations of the product protein. Here, we characterize the precipitates that form after neutralization of protein A purified and viral-inactivated material of an Fc-fusion protein produced in Chinese hamster ovary cells. The physical growth of precipitate particles was observed by optical microscopy, transmission electron microscopy, dynamic light scattering, and small-angle and ultra-small-angle X-ray scattering to characterize the precipitate microstructure and growth mechanism. The precipitate microstructure is well-described as a mass fractal with fractal dimension approximately 2. The growth is governed by a diffusion-limited aggregation mechanism as indicated by a power-law dependence on time of the size of the principal precipitate particles. Optical microscopy shows that these primary particles can further aggregate into larger particles in a manner that appears to be promoted by mixing. Absorbance experiments at varying pH and salt concentrations reveal that the growth is largely driven by attractive electrostatic interactions, as growth is hindered by an increase in ionic strength. The solution conditions that resulted in the most significant particle growth are also correlated with the greatest removal of soluble impurities (DNA and HCPs). Proteomic analysis of the precipitates allows identification of O ( 100 ) unique HCP impurities, depending on the buffer species (acetate or citrate) used for the viral inactivation. Most of these proteins have pI values near the precipitation pH, supporting the likely importance of electrostatic interactions in driving precipitate formation.

Ultrafiltration behavior of monoclonal antibodies and Fc-fusion proteins: Effects of physical properties.

Ultrafiltration (UF) is used for the final concentration and formulation of essentially all antibody-based therapeutics including both monoclonal antibodies (mAbs) and Fc-fusion proteins. The objective of this study was to quantitatively compare the filtrate flux behavior for two highly purified mAbs and an Fc-fusion protein under identical flow and buffer conditions. Filtrate flux data were obtained using a Pellicon 3 tangential flow filtration cassette over a wide range of transmembrane pressures and bulk protein concentrations. Independent experimental measurements were performed to evaluate the protein osmotic pressure and solution viscosity. The maximum achievable protein concentration was directly correlated with the solution viscosity, which controls the pressure drop and extent of back-filtration in the cassette. The filtrate flux data were analyzed using a recently developed model that accounts for the effects of intermolecular interactions and transmembrane pressure gradients on the extent of concentration polarization. These results provide important insights into the factors controlling the filtrate flux during the UF of concentrated protein solutions and an effective framework for the design/analysis of UF processes for the formulation of antibody-based therapeutics. Biotechnol. Bioeng. 2017;114: 2057-2065. © 2017 Wiley Periodicals, Inc.

Effects of Histidine and Sucrose on the Biophysical Properties of a Monoclonal Antibody.

PURPOSE: Histidine is a commonly used buffer in formulation of monoclonal antibodies (mAb), often with excipients like sucrose. The objective of this study was to examine the effects of both histidine and sucrose on the biophysical characteristics of a mAb. METHODS: The hydrodynamic radius of the mAb was determined by dynamic light scattering and confirmed by size exclusion chromatography. Data were also obtained for the osmotic virial coefficients (from osmotic pressure measurements), the solution viscosity, and the mAb thermal stability (using differential scanning calorimetry) at selected conditions. RESULTS: There were no significant changes in mAb conformation / stability as determined by DSC. The hydrodynamic radius initially increased with increasing histidine concentration, going through a maximum at a histidine concentration of about 20 mM. The addition of sucrose increased the mAb hydrodynamic radius at all histidine concentrations by about 0.5 nm. The observed effects of histidine and sucrose on the hydrodynamic radius were also reflected in changes in the osmotic pressure and solution viscosity. CONCLUSIONS: These results provide important insights into the effects of both histidine and sucrose on the behavior of concentrated mAb solutions, including the potential impact on ultrafiltration / diafiltration processes.

Clarification technologies for monoclonal antibody manufacturing processes: Current state and future perspectives.

Considerable progress has been made increasing productivity of cell cultures to meet the rapidly growing demand for antibody biopharmaceuticals through increased cell densities and longer culture times. This in turn has dramatically increased the burden of process and product related impurities on the purification processes. In addition, current trends in the biopharmaceutical industry point toward both increased productivity and targeting smaller patient populations for new indications. Taken together, these developments are driving the industry to explore alternative separation technologies as a future manufacturing strategy. Clarification technologies well established in other industries, such as flocculation and precipitation are increasingly considered as a viable solution to address this bottleneck in antibody processes. However, several technical issues need to be fully addressed including suitability as a platform application, robustness, process cost, toxicity, and clearance. This review will focus on recent efforts to incorporate new generation clarification technologies for mammalian cell cultures producing monoclonal antibodies as well as challenges to their implementation supported by a case study.

Development of robust antibody purification by optimizing protein-A chromatography in combination with precipitation methodologies.

To be administered to patients, therapeutic monoclonal antibodies must have very high purity, with process related impurities like host-cell proteins (HCPs) and DNA reduced to <100 ppm and <10 ppb, respectively, relative to desired product. Traditionally, Protein-A chromatography as a capture step has been the work horse for clearing a large proportion of these impurities. However, remaining levels of process and product related impurities still present significant challenges on the development of polishing steps further downstream. In this study, we have incorporated high throughput screening to evaluate three areas of separation: (i) Harvest treatment; (ii) Protein-A Chromatography; and (iii) Low pH Viral Inactivation. Precipitation with low pH treatment of cell culture harvest resulted in selective removal of impurities while manipulating the pH of wash buffers used in Protein-A chromatography and incorporating wash additives that disrupt various modes of protein-protein interaction resulted in further and more pronounced reduction in impurity levels. In addition, our study also demonstrate that optimizing the neutralization pH post Protein-A elution can result in selective removal of impurities. When applied over multiple mAbs, this optimization method proved to be very robust and the strategy provides a new and improved purification process that reduces process related impurities like HCPs and DNA to drug substance specifications with just one chromatography column and open avenues for significant decrease in operating costs in monoclonal antibody purification.

Clarification of recombinant proteins from high cell density mammalian cell culture systems using new improved depth filters.

Increasingly high cell density, high product titer cell cultures containing mammalian cells are being used for the production of recombinant proteins. These high productivity cultures are placing a larger burden on traditional downstream clarification and purification operations due to higher product and impurity levels. Controlled flocculation and precipitation of mammalian cell culture suspensions by acidification or using polymeric flocculants have been employed to enhance clarification throughput and downstream filtration operations. While flocculation is quite effective in agglomerating cell debris and process related impurities such as (host cell) proteins and DNA, the resulting suspension is generally not easily separable solely using conventional depth filtration techniques. As a result, centrifugation is often used for clarification of cells and cell debris before filtration, which can limit process configurations and flexibility due to the investment and fixed nature of a centrifuge. To address this challenge, novel depth filter designs were designed which results in improved primary and secondary direct depth filtration of flocculated high cell density mammalian cell cultures systems feeds, thereby providing single-use clarification solution. A framework is presented here for optimizing the particle size distribution of the mammalian cell culture systems with the pore size distribution of the gradient depth filter using various pre-treatment conditions resulting in increased depth filter media utilization and improved clarification capacity. Feed conditions were optimized either by acidification or by polymer flocculation which resulted in the increased average feed particle-size and improvements in throughput with improved depth filters for several mammalian systems.

Development of a novel and efficient cell culture flocculation process using a stimulus responsive polymer to streamline antibody purification processes.

Recent advances in mammalian cell culture processes have significantly increased product titers, but have also resulted in substantial increases in cell density and cellular debris as well as process and product related impurities. As such, with improvements in titer, corresponding improvements in downstream processing are essential. In this study we have developed an alternative antibody harvest process that incorporates flocculation using a novel stimulus responsive polymer, benzylated poly(allylamine), followed by depth filtration. As tested on multiple antibodies, this process demonstrates high process yield, improved clearance of cells and cell debris, and efficient reduction of aggregates, host cell proteins (HCP) and DNA. A wide operating window was established for this novel flocculation process through design of experiments condition screening and optimization. Residual levels of impurities in the Protein A eluate were achieved that potentially meet requirements of drug substance and thus alleviate the burden for further impurities removal in subsequent chromatography steps. In addition, efficient clearance of residual polymer was demonstrated using a fluorescence tagged polymer in the presence of a stimulus reagent. The mechanism of HCP and aggregates removal during flocculation was also explored. This novel and efficient process can be easily integrated into current mAb purification platforms, and may overcome downstream processing challenges.

Considerations for Buffering Agent Selection for Frozen rAAV2 Mediated Gene Therapy Products.

The buffering component selection is a key criterion for the formulation development process for biopharmaceuticals. This decision for recombinant adeno-associated virus (rAAV) mediated gene therapies is receiving special attention due to their rise in clinical trials which may require high concentration, frozen supply chain, and direct delivery to eye and central nervous system related sites. In the present study, we investigate the impact of rates of freezing and thawing on rAAV2 as a model serotype. It was observed that slow rate of thawing impacts rAAV2 colloidal stability in Phosphate based buffering system. Our pre-formulation workflow suggests that rAAV2 has maximum aggregation propensity between pH of 5.5 to 6.5. Thus, the overlap of maximum aggregation propensity pH range with acidic pH shift in Phosphate based buffering system during freezing and thawing appears to be responsible for 42-75% concentration drop noticed for rAAV2. This impact appears to be fully mitigated upon replacement of Phosphate based buffering system with an alternate buffer system such as Tris. The results reported in this study highlight associated risks and provide preliminary guidance on handling of early stage frozen rAAV mediated gene therapies.

Mass Balance Model with Donnan Equilibrium Accurately Describes Unusual pH and Excipient Profiles during Diafiltration of Monoclonal Antibodies.

There is extensive experimental data showing that the final pH and buffer composition after protein diafiltration (DF), particularly with monoclonal antibodies, can be considerably different than that in the DF buffer due to electrostatic interactions between the charged protein and the charged ions. Previous models for this behavior have focused on the final (equilibrium) partitioning and are unable to explain the complex pH and concentration profiles during the DF process. The objective of this study is to develop a new model for antibody DF based on solution of the transient mass balance equations, with the permeate concentrations of the charged species evaluated assuming Donnan equilibrium across the semipermeable membrane in combination with electroneutrality constraints. Model predictions are in excellent agreement with experimental data obtained during DF of both acidic and basic monoclonal antibodies, with the protein charge determined from independent electrophoretic mobility measurements. The model is able to predict the entire pH/histidine concentration profiles during DF, providing a framework for the development of DF processes that yield the desired antibody formulation.

Downstream Processing Technologies/Capturing and Final Purification : Opportunities for Innovation, Change, and Improvement. A Review of Downstream Processing Developments in Protein Purification.

Increased pressure on upstream processes to maximize productivity has been crowned with great success, although at the cost of shifting the bottleneck to purification. As drivers were economical, focus is on now on debottlenecking downstream processes as the main drivers of high manufacturing cost. Devising a holistically efficient and economical process remains a key challenge. Traditional and emerging protein purification strategies with particular emphasis on methodologies implemented for the production of recombinant proteins of biopharmaceutical importance are reviewed. The breadth of innovation is addressed, as well as the challenges the industry faces today, with an eye to remaining impartial, fair, and balanced. In addition, the scope encompasses both chromatographic and non-chromatographic separations directed at the purification of proteins, with a strong emphasis on antibodies. Complete solutions such as integrated USP/DSP strategies (i.e., continuous processing) are discussed as well as gains in data quantity and quality arising from automation and high-throughput screening (HTS). Best practices and advantages through design of experiments (DOE) to access a complex design space such as multi-modal chromatography are reviewed with an outlook on potential future trends. A discussion of single-use technology, its impact and opportunities for further growth, and the exciting developments in modeling and simulation of DSP rounds out the overview. Lastly, emerging trends such as 3D printing and nanotechnology are covered. Graphical Abstract Workflow of high-throughput screening, design of experiments, and high-throughput analytics to understand design space and design space boundaries quickly. (Reproduced with permission from Gregory Barker, Process Development, Bristol-Myers Squibb).

Ultrafiltration behavior of partially retained proteins and completely retained proteins using equally-staged single pass tangential flow filtration membranes.

This work examines the ultrafiltration behavior of partially retained proteins like lysozyme and completely retained proteins like monoclonal antibodies using single pass tangential flow filtration (SPTFF) modules with different screen channels and molecular weight cut-offs. When the staging of the SPTFF used the same membrane area in each stage, there was no impact of the module screened channel or the buffer matrix on the final concentration achieved for completely retained monoclonal antibodies. A hybrid configuration containing 30 kDa membranes and 50 kDa membranes increased the maximum achievable concentration for both the monoclonal antibodies used in this work, at the same time, allowing a two-fold to four-fold increase in normalized feed flow-rate through the system compared to only the 30 kDa or only the 50 kDa membranes. The sieving coefficient of lysozyme measured and calculated using SPTFF was lower than those measured during conventional recirculation TFF indicating a more complicated concentration polarization effect than conventional recirculation TFF. Moreover, the sieving coefficients of lysozyme were the same for the 10 kDa regenerated cellulose and 50 kDa PES membranes while it was higher for the 30 kDa regenerated cellulose membrane. The difference in TFF and SPTFF behavior is important when the product of interest is desired to be permeated. This work presents the first body of data for partially and completely retained solutes together in the SPTFF mode and provides a strategy to increase protein concentration at higher feed flow rates. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 2018 © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:1137-1148, 2018.

Protection of therapeutic antibodies from visible light induced degradation: Use safe light in manufacturing and storage.

As macromolecules, biologics are susceptible to light exposure, which induces oxidation of multiple amino acid residues including tryptophan, tyrosine, phenylalanine, cysteine and methionine. Pertaining to safety, efficacy and potency, light-induced oxidation of biologics has been widely studied and necessary precautions need to be taken during biologics manufacturing process, drug substance and products handling and storage. Proteins will degrade to varying extents depending on the protein properties, degradation pathways, formulation compositions and type of light source. In addition to UV light, which has been widely known to degrade proteins, visible light from indoor fluorescent lighting also can mediate protein degradation. In this report, we examine and identify wavelengths in the visual spectrum (400-700 nm) that can cause monoclonal antibody and histidine buffer degradation. Installation of safe lights which exclude the identified damaging wavelengths from visible spectra in manufacturing and storage areas can provide a balance between lighting requirement for human operators and their safety and conservation of product quality.

Using hydrogen peroxide to prevent antibody disulfide bond reduction during manufacturing process.

During large-scale monoclonal antibody manufacturing, disulfide bond reduction of antibodies, which results in generation of low molecule weight species, is occasionally observed. When this happens, the drug substance does not meet specifications. Many investigations have been conducted across the biopharmaceutical industry to identify the root causes, and multiple strategies have been proposed to mitigate the problem. The reduction is correlated with the release of cellular reducing components and depletion of dissolved oxygen before, during, and after harvest. Consequently, these factors can lead to disulfide reduction over long-duration storage at room temperature prior to Protein A chromatography. Several strategies have been developed to minimize antibody reduction, including chemical inhibition of reducing components, maintaining aeration before and after harvest, and chilling clarified harvest during holding. Here, we explore the use of hydrogen peroxide in clarified harvest bulk or cell culture fluid as a strategy to prevent disulfide reduction. A lab-scale study was performed to demonstrate the effectiveness of hydrogen peroxide in preventing antibody reduction using multiple IgG molecules. Studies were done to define the optimal concentration of hydrogen peroxide needed to avoid unnecessary oxidization of the antibody products. We show that adding a controlled amount of hydrogen peroxide does not change product quality attributes of the protein. Since hydrogen peroxide is soluble in aqueous solutions and decomposes into water and oxygen, there is no additional burden involved in removing it during the downstream purification steps. Due to its ease of use and minimal product impact, we demonstrate that hydrogen peroxide treatment is a powerful, simple tool to quench reducing potential by simply mixing it with harvested cell culture fluid.

The role of independently variable grafting density and layer thickness of polymer nanolayers on peptide adsorption and cell adhesion.

This contribution demonstrates a simple and reproducible method for fabricating surface-tethered polymer brushes that vary in grafting density and layer thickness for peptide adsorption and cell-adhesion studies. Surface-initiated atom transfer radical polymerization was used together with thiol self-assembly to generate these nanothin polymer brush layers of poly((polyethylene glycol) methacrylate). A kinetic study was done to measure the layer thickness growth rate at room temperature from flat gold substrates presenting different polymerization initiator molecule surface densities. The polymer brush layers transition from mushroom to brush regimes with increasing grafting density. A crossover density of 0.038+/-0.005 chains/nm(2) was determined for the PPEGMA polymer brushes. The results described in this paper show that layer properties such as wettability and dry layer thickness depend strongly on initiator surface density. Ultimately, the adsorbed concentration of an RGD-containing synthetic peptide Gly-Arg-Gly-Asp-Ser and the adhesion and spreading of cells were correlated with surface properties, which continues to be a major research theme in biomedical and biomaterials research.