Characterization of Fibra-Cel® packed-bed bioreactors for the culture of CHO cells

Today, more than 30 products using a manufacturing process based on mammalian cell culture have been approved for human therapy. Most of these products are currently supplied with stirred tank bioreactors operated in batch or fed-batch mode. However, the bioprocess industry needs to constantly increase the productivity of its cell culture processes in order to supply more material with minimal investments in additional equipments. Therefore, alternative production techniques must also be considered, such as the perfusion culture of mammalian cells immobilized in packed-bed bioreactors (PBRs). PBRs can reach very high cell density and hence very high volumetric productivity; so their potential for bioprocess applications must be further evaluated. A review of current state-of-the-art in PBRs development (Chapter 2) showed that the latest generation of PBRs used for bioprocess applications have achieved very high cell densities (i.e. 107-108 cells per milliliter) leading to outstandingly high volumetric productivity. However, the major bottleneck for bioprocess PBRs is their relatively small volume due to the impossibility to avoid nutrient concentration gradients in PBRs of large volume. The current maximal volume seems to be in the range 10-30 liters, and more than 10-fold scale-up would still be required make the PBRs a competitive production technique. Beside their use for bioprocessing applications, PBRs have proven to be an excellent tool to fulfill the requirements of compact bioartificial organs in biomedical applications: they can reach the cell density and volume of an organ. However, as observed during the development of bioartificial livers, a decrease of metabolic activity is frequently observed after 1-2 weeks of culture. Therefore, the main challenge in this field is to develop cell lines that grow consistently to high cell density in vitro, and that maintain a stable phenotype for a minimum of 1-2 months to make them applicable to the PBR technology and to fulfill the clinical demand. PBRs are characterized by high cell density levels, thus by high metabolic rates, and accurate control of the cultures is required. Furthermore cell number cannot be determined directly in PBRs so an indirect method is needed. An indirect method based on the Glucose Consumption Rate (GCR) was developed to monitor a PBR process using recombinant Chinese Hamster Ovarian (CHO) cells cultured on Fibra-Cel® disk carriers (Chapter 3). A key step in this process was the switch from the cell growth phase to the production phase triggered by a reduction of the temperature. In this system, a GCR-based criterion was defined for the switch, and this control strategy proved to be robust, very simple, and was applied successfuly in routine operations for the monitoring and control of an industrial process at pilot-scale. The process operated with this GCR-based strategy yielded consistent, reproducible process performance across numerous bioreactor runs performed on