A framework for the systematic design of fed-batch strategies in mammalian cell culture

Building blocks needed for mechanistic modeling of bioprocesses: A critical review based on protein production by CHO cells

This paper reviews the key building blocks needed to develop a mechanistic model for use as an operational production tool. The Chinese Hamster Ovary (CHO) cell, one of the most widely used hosts for antibody production in the pharmaceutical industry, is considered as a case study. CHO cell metabolism is characterized by two main phases, exponential growth followed by a stationary phase with strong protein production. This process presents an appropriate degree of complexity to outline the modeling strategy. The paper is organized into four main steps: (1) CHO systems and data collection; (2) metabolic analysis; (3) formulation of the mathematical model; and finally, (4) numerical solution, calibration, and validation. The overall approach can build a predictive model of target variables. According to the literature, one of the main current modeling challenges lies in understanding and predicting the spontaneous metabolic shift. Possible candidates for the trigger of the metabolic shift include the concentration of lactate and carbon dioxide. In our opinion, ammonium, which is also an inhibiting product, should be further investigated. Finally, the expected progress in the emerging field of hybrid modeling, which combines the best of mechanistic modeling and machine learning, is presented as a fascinating breakthrough. Note that the modeling strategy discussed here is a general framework that can be applied to any bioprocess.

CHOmpact: A reduced metabolic model of Chinese hamster ovary cells with enhanced interpretability

Metabolic modeling has emerged as a key tool for the characterization of biopharmaceutical cell culture processes. Metabolic models have also been instrumental in identifying genetic engineering targets and developing feeding strategies that optimize the growth and productivity of Chinese hamster ovary (CHO) cells. Despite their success, metabolic models of CHO cells still present considerable challenges. Genome-scale metabolic models (GeMs) of CHO cells are very large (>6000 reactions) and are difficult to constrain to yield physiologically consistent flux distributions. The large scale of GeMs also makes the interpretation of their outputs difficult. To address these challenges, we have developed CHOmpact, a reduced metabolic network that encompasses 101 metabolites linked through 144 reactions. Our compact reaction network allows us to deploy robust, nonlinear optimization and ensure that the computed flux distributions are physiologically consistent. Furthermore, our CHOmpact model delivers enhanced interpretability of simulation results and has allowed us to identify the mechanisms governing shifts in the anaplerotic consumption of asparagine and glutamate as well as an important mechanism of ammonia detoxification within mitochondria. CHOmpact, thus, addresses key challenges of large-scale metabolic models and will serve as a platform to develop dynamic metabolic models for the control and optimization of biopharmaceutical cell culture processes.

Towards advanced bioprocess optimization: A multiscale modelling approach

Mammalian cells produce up to 80 % of the commercially available therapeutic proteins, with Chinese Hamster Ovary (CHO) cells being the primary production host. Manufacturing involves a train of reactors, the last of which is typically run in fed-batch mode, where cells grow and produce the required protein. The feeding strategy is decided a priori, from either past operations or the design of experiments and rarely considers the current state of the process. This work proposes a Model Predictive Control (MPC) formulation based on a hybrid kinetic-stoichiometric reactor model to provide optimal feeding policies in real-time, which is agnostic to the culture, hence transferable across CHO cell culture systems. The benefits of the proposed controller formulation are demonstrated through a comparison between an open-loop simulation and closed-loop optimization, using a digital twin as an emulator of the process.

Population balance modelling captures host cell protein dynamics in CHO cell cultures

Monoclonal antibodies (mAbs) have been extensively studied for their wide therapeutic and research applications. Increases in mAb titre has been achieved mainly by cell culture media/feed improvement and cell line engineering to increase cell density and specific mAb productivity. However, this improvement has shifted the bottleneck to downstream purification steps. The higher accumulation of the main cell-derived impurities, host cell proteins (HCPs), in the supernatant can negatively affect product integrity and immunogenicity in addition to increasing the cost of capture and polishing steps. Mathematical modelling of bioprocess dynamics is a valuable tool to improve industrial production at fast rate and low cost. Herein, a single stage volume-based population balance model (PBM) has been built to capture Chinese hamster ovary (CHO) cell behaviour in fed-batch bioreactors. Using cell volume as the internal variable, the model captures the dynamics of mAb and HCP accumulation extracellularly under physiological and mild hypothermic culture conditions. Model-based analysis and orthogonal measurements of lactate dehydrogenase activity and double-stranded DNA concentration in the supernatant show that a significant proportion of HCPs found in the extracellular matrix is secreted by viable cells. The PBM then served as a platform for generating operating strategies that optimise antibody titre and increase cost-efficiency while minimising impurity levels.

Culture media optimization for Chinese hamster ovary cell growth and expression of recombinant varicella-zoster virus glycoprotein E

Herpes zoster (HZ) is caused by reactivation of varicella-zoster virus (VZV) latent in the sensory ganglia and causes severe pain, often leading to postherpetic neuralgia (PHN). Two prophylactic vaccines against HZ are currently licensed for human use, a live attenuated vaccine and a subunit vaccine containing recombinant VZV glycoprotein E (gE) as antigen. The latter has superior protective efficacy against HZ and PHN. During HZ subunit vaccine development, we obtained Chinese hamster ovary (CHO) cell clones expressing VZV gE. This study was performed to optimize culture media conditions for CHO cell growth and gE production. Using a high-throughput culture system, three CHO cell clones were cultured in microtiter plates containing 24 different basal media, and three basal media were selected. The clone with the highest gE expression was fed-batch cultured in each of the three basal media in combination with 13 different feed media. A pair of media, BalanCD CHO Growth A and EX-CELL Advanced CHO Feed 1, with the highest productivity was selected for gE production. Scale-up fed-batch cultures of the selected clone cultured in a wave bag bioreactor containing the optimized media yielded 2440 mg gE protein/L culture, a 11.5-fold increase compared to original culture conditions (batch culture in CD OptiCHO medium). The optimized media condition is used to produce VZV gE antigen for an HZ subunit vaccine, which is under phase I clinical trial. This study would provide valuable insights on culture media optimization for CHO cells expressing a recombinant vaccine antigen.

Osmolality Effects on CHO Cell Growth, Cell Volume, Antibody Productivity and Glycosylation

The addition of nutrients and accumulation of metabolites in a fed-batch culture of Chinese hamster ovary (CHO) cells leads to an increase in extracellular osmolality in late stage culture. Herein, we explore the effect of osmolality on CHO cell growth, specific monoclonal antibody (mAb) productivity and glycosylation achieved with the addition of NaCl or the supplementation of a commercial feed. Although both methods lead to an increase in specific antibody productivity, they have different effects on cell growth and antibody production. Osmolality modulation using NaCl up to 470 mOsm kg^-1 had a consistently positive effect on specific antibody productivity and titre. The addition of the commercial feed achieved variable results: specific mAb productivity was increased, yet cell growth rate was significantly compromised at high osmolality values. As a result, Feed C addition to 410 mOsm kg^-1 was the only condition that achieved a significantly higher mAb titre compared to the control. Additionally, Feed C supplementation resulted in a significant reduction in galactosylated antibody structures. Cell volume was found to be positively correlated to osmolality; however, osmolality alone could not account for observed changes in average cell diameter without considering cell cycle variations. These results help delineate the overall effect of osmolality on titre and highlight the potentially negative effect of overfeeding on cell growth.

Effect of Sialic Acid on Mammalian Cell Culture and Protein Expression: A Potential Productivity Enhancer for Biopharmaceutical Cell Culture Processes

Improved productivity of the two most commonly used cell lines in the biopharmaceutical industry, such as human embryonic kidney 293 (HEK293) and Chinese hamster ovary (CHO), could reduce production costs and increase manufacturing capacity. One method for increasing protein productivity is the addition of antioxidants during the cell culture process. In this study, we examined the effect of sialic acid (SA) on one HEK293 cell line and two CHO cell lines. The addition of SA to HEK293 cell led to a higher viable cell density (VCD), viability (Via), and a lower lactate content in the later stage of cultures. Further results showed that SA reduced the reactive oxygen species (ROS), improved cell viability, reduced lactate production, and increased antibody expression by more than 20% in the later stage of the two CHO cell lines cultures. Besides, an optimized dose of SA had no significant effect on acidic variants level aggregation level, N-linked glycosylation pattern, and SA content on antibodies. These results suggest that the addition of SA can improve the productivity of biopharmaceutical cell culture processes.

Increased MSX level improves biological productivity and production stability in multiple recombinant GS CHO cell lines

Increasing cell culture productivity of recombinant proteins via process improvements is the primary focus for research groups within biologics manufacturing. Any recommendations to improve a manufacturing process obviously must be effective, but also be robust, scalable, and with product quality comparable to the original process. In this study, we report that three different GS-/- CHO cell lines developed in media containing a standard concentration of the selection agent methionine sulfoximine (MSX), but then exposed to increased MSX concentrations during seed train expansion, achieved titer increases of 10-19%. This result was observed in processes already considerably optimized. Expanding the cells with a higher MSX concentration improved cell line production stability with increased culture age. Production cultures in 500-L and 1000-L bioreactors replicated laboratory results using 5-L bioreactors, demonstrating process robustness and scalability. Furthermore, product quality attributes of the final drug substance using the higher MSX process were comparable with those from cells expanded in media with the standard selection MSX concentration. Subsequent mechanistic investigations confirmed that the cells were not altered at the genetic level in terms of integration profiles or gene copy number, nor transcriptional levels of glutamine synthetase, heavy chain, or light chain genes. This study provides an effective and applicable strategy to improve the productivity of therapeutic proteins for biologics manufacturing.

Early integration of Design of Experiment (DOE) and multivariate statistics identifies feeding regimens suitable for CHO cell line development and screening

In Chinese Hamster Ovary (CHO) cell lines, the establishment of the ideal fed-batch regimen promotes metabolic conditions advantageous for the bioproduction of therapeutic molecules. A tailored, cell line-specific feeding scheme is typically defined during process development (PD) activities, through the incorporation of Design of Experiment (DOE) and late stage cell culture approaches. The feeding during early stage cell line development (CLD) was a simplified "one-fits-all" design, inherited from PD lab, that didn't account for CLD needs of throughput and streamlined workflow. The "one-fits-all" efficiency was not routinely verified when novel technologies were incorporated in CLD and sub-optimal feeding carried the risk of not selecting the most desirable cell lines amenable to late stage PD. In our work we developed the DOE-feed method; a streamlined, three-stages framework for identifying efficient feeding schemes as the CLD technologies evolved. We combined early stage cell culture input data with late-stage techniques, such as statistical modelling, principal component analysis (PCA), DOE and Prediction Profiler. Novel in our DOE-feed work, we deliberately anticipated the application of statistics and approached the method development as an early-stage, continuously updated process, by building iterative datasets and statistically interpreting their responses. We capitalized on the statistical models defined by the DOE-feed methodology to study the influence of feeds on daily productivity and growth and to extrapolate feeding-schemes that improved the cell line screening. The DOE-feed became a methodology suited for CLD needs at AbbVie, and optimized the early stage screening, reduced the operational hands-on time and improved the overall workstream efficiency.

Strategic Framework for Parameterization of Cell Culture Models

Global Sensitivity Analysis (GSA) is a technique that numerically evaluates the significance of model parameters with the aim of reducing the number of parameters that need to be estimated accurately from experimental data. In the work presented herein, we explore different methods and criteria in the sensitivity analysis of a recently developed mathematical model to describe Chinese hamster ovary (CHO) cell metabolism in order to establish a strategic, transferable framework for parameterizing mechanistic cell culture models. For that reason, several types of GSA employing different sampling methods (Sobol’, Pseudo-random and Scrambled-Sobol’), parameter deviations (10%, 30% and 50%) and sensitivity index significance thresholds (0.05, 0.1 and 0.2) were examined. The results were evaluated according to the goodness of fit between the simulation results and experimental data from fed-batch CHO cell cultures. Then, the predictive capability of the model was tested against four different feeding experiments. Parameter value deviation levels proved not to have a significant effect on the results of the sensitivity analysis, while the Sobol’ and Scrambled-Sobol’ sampling methods and a 0.1 significance threshold were found to be the optimum settings. The resulting framework was finally used to calibrate the model for another CHO cell line, resulting in a good overall fit. The results of this work set the basis for the use of a single mechanistic metabolic model that can be easily adapted through the proposed sensitivity analysis method to the behavior of different cell lines and therefore minimize the experimental cost of model development.

Mechanisms driving the lactate switch in Chinese hamster ovary cells

The metabolism of Chinese Hamster Ovary (CHO) cells in a production environment has been extensively investigated. However, a key metabolic transition, the switch from lactate production to lactate consumption, remains enigmatic. Though commonly observed in CHO cultures, the mechanism(s) by which this metabolic shift is triggered is unknown. Despite this, efforts to control the switch have emerged due to the association of lactate consumption with improved cell growth and productivity. This review aims to consolidate current theories surrounding the lactate switch. The influence of pH, NAD⁺ /NADH, pyruvate availability and mitochondrial function on lactate consumption are explored. A hypothesis based on the cellular redox state is put forward to explain the onset of lactate consumption. Various techniques implemented to control the lactate switch, including manipulation of the culture environment, genetic engineering, and cell line selection are also discussed.

Exploring cellular behavior under transient gene expression and its impact on mAb productivity and Fc-glycosylation

Transient gene expression (TGE) is a methodology employed in bioprocessing for the fast provision of recombinant protein material. Mild hypothermia is often introduced to overcome the low yield typically achieved with TGE and improve specific protein productivity. It is therefore of interest to examine the impact of mild hypothermic temperatures on both the yield and quality of transiently expressed proteins and the relationship to changes in cellular processes and metabolism. In this study, we focus on the ability of a Chinese hamster ovary cell line to galactosylate a recombinant monoclonal antibody (mAb) product. Through experimentation and flux balance analysis, our results show that TGE in mild hypothermic conditions led to a 76% increase in q_P compared to TGE at 36.5°C in our system. This increase is accompanied by increased consumption of nutrients and amino acids, together with increased production of intracellular nucleotide sugar species, and higher rates of mAb galactosylation, despite a reduced rate of cell growth. The reduction in biomass accumulation allowed cells to redistribute their energy and resources toward mAb synthesis and Fc‐glycosylation. Interestingly, the higher capacity of cells to galactosylate the recombinant product in TGE at 32°C appears not to have been assisted by the upregulation of galactosyltransferases (GalTs), but by the increased expression of N‐acetylglucosaminyltransferase II (GnTII) in this cell line, which facilitated the production of bi‐antennary glycan structures for further processing.

Selection of chemically defined media for CHO cell fed-batch culture processes

Two CHO cell clones derived from the same parental CHOBC® cell line and producing the same monoclonal antibody (BC-G, a low producing clone; BC-P, a high producing clone) were tested in four basal media in all possible combinations with three feeds (=12 conditions) in fed-batch cultures. Higher amino acid feeding did not always lead to higher mAb production. The two clones showed differences in cell physiology, metabolism and optimal medium-feed combinations. During the phase transitions of all cultures, cell metabolism showed a shift represented by lower specific consumption and production rates, except for the specific glucose consumption rate in cultures fed by Actifeed A/B. The BC-P clone fed by Actifeed A/B showed a threefold cell volume increase and an increase of the specific consumption rate of glucose in the stationary phase. Since feeding was based on glucose this resulted in accumulation of amino acids for this feed, while this did not occur for the poorer feed (EFA/B). The same feed also led to an increase of cell size for the BC-G clone, but to a lesser extent.

Selection of chemically defined media for CHO cell fed-batch culture processes

Two CHO cell clones derived from the same parental CHO^BC® cell line and producing the same monoclonal antibody (BC-G, a low producing clone; BC-P, a high producing clone) were tested in four basal media in all possible combinations with three feeds (=12 conditions) in fed-batch cultures. Higher amino acid feeding did not always lead to higher mAb production. The two clones showed differences in cell physiology, metabolism and optimal medium-feed combinations. During the phase transitions of all cultures, cell metabolism showed a shift represented by lower specific consumption and production rates, except for the specific glucose consumption rate in cultures fed by Actifeed A/B. The BC-P clone fed by Actifeed A/B showed a threefold cell volume increase and an increase of the specific consumption rate of glucose in the stationary phase. Since feeding was based on glucose this resulted in accumulation of amino acids for this feed, while this did not occur for the poorer feed (EFA/B). The same feed also led to an increase of cell size for the BC-G clone, but to a lesser extent.

Effect of amino acid supplementation on titer and glycosylation distribution in hybridoma cell cultures-Systems biology-based interpretation using genome-scale metabolic flux balance model and multivariate data analysis

Genome-scale flux balance analysis (FBA) is a powerful systems biology tool to characterize intracellular reaction fluxes during cell cultures. FBA estimates intracellular reaction rates by optimizing an objective function, subject to the constraints of a metabolic model and media uptake/excretion rates. A dynamic extension to FBA, dynamic flux balance analysis (DFBA), can calculate intracellular reaction fluxes as they change during cell cultures. In a previous study by Read et al. (2013), a series of informed amino acid supplementation experiments were performed on twelve parallel murine hybridoma cell cultures, and this data was leveraged for further analysis (Read et al., Biotechnol Prog. 2013;29:745-753). In order to understand the effects of media changes on the model murine hybridoma cell line, a systems biology approach is applied in the current study. Dynamic flux balance analysis was performed using a genome-scale mouse metabolic model, and multivariate data analysis was used for interpretation. The calculated reaction fluxes were examined using partial least squares and partial least squares discriminant analysis. The results indicate media supplementation increases product yield because it raises nutrient levels extending the growth phase, and the increased cell density allows for greater culture performance. At the same time, the directed supplementation does not change the overall metabolism of the cells. This supports the conclusion that product quality, as measured by glycoform assays, remains unchanged because the metabolism remains in a similar state. Additionally, the DFBA shows that metabolic state varies more at the beginning of the culture but less by the middle of the growth phase, possibly due to stress on the cells during inoculation. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1163-1173, 2016.

How does mild hypothermia affect monoclonal antibody glycosylation?

The application of mild hypothermic conditions to cell culture is a routine industrial practice used to improve recombinant protein production. However, a thorough understanding of the regulation of dynamic cellular processes at lower temperatures is necessary to enhance bioprocess design and optimization. In this study, we investigated the impact of mild hypothermia on protein glycosylation. Chinese hamster ovary (CHO) cells expressing a monoclonal antibody (mAb) were cultured at 36.5°C and with a temperature shift to 32°C during late exponential/early stationary phase. Experimental results showed higher cell viability with decreased metabolic rates. The specific antibody productivity increased by 25% at 32°C and was accompanied by a reduction in intracellular nucleotide sugar donor (NSD) concentrations and a decreased proportion of the more processed glycan structures on the mAb constant region. To better understand CHO cell metabolism at 32°C, flux balance analysis (FBA) was carried out and constrained with exometabolite data from stationary phase of cultures with or without a temperature shift. Estimated fluxomes suggested reduced fluxes of carbon species towards nucleotide and NSD synthesis and more energy was used for product formation. Expression of the glycosyltransferases that are responsible for N-linked glycan branching and elongation were significantly lower at 32°C. As a result of mild hypothermia, mAb glycosylation was shown to be affected by both NSD availability and glycosyltransferase expression. The combined experimental/FBA approach generated insight as to how product glycosylation can be impacted by changes in culture temperature. Better feeding strategies can be developed based on the understanding of the metabolic flux distribution.