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Abstract

Monoclonal antibodies are biomolecules that are able to bind to a specific antigen. Antibodies are important drugs for the targeted treatment of various types of cancer. Due to their complex molecular structure, monoclonal antibodies are usually produced through the cultivation of genetically modified mammalian cells. At the production scale, the cultivation is typically carried out using sparged stirred tank bioreactors and the product quality as well as the yield depend, among other factors, on the hydrodynamic conditions inside the utilized bioreactor. The optimization of cell cultivation for the production of monoclonal antibodies is of high economical importance for the pharmaceutical industry. The simulation of the flow field inside the utilized bioreactors with computational fluid dynamics enables the prediction of relevant process characteristics, which must be considered during the scale-up of cell culture processes. The focus of the present study is on the hydrodynamic characterization and the selection of the operating conditions during scale-up of the cell culture processes of four single-use bioreactors with varying sizes ranging from the lab-scale to the production scale, the Mobius® CellReady 3 L, the XcellerexTM XDR-10, the XcellerexTM XDR-200, and the XcellerexTM XDR-2000. Additionally, the hydrodynamic characteristics of a miniaturized stirred tank bioreactor, the Ambr®250, and another of the XcellerexTM bioreactors, the XcellerexTM XDR-500, are investigated. The simulations have been carried out with the Euler-Euler and the Euler-Lagrange approaches with the open source software OpenFOAM and the commercial software MixIT. The considered process characteristics include the mixing time, the hydrodynamic stress, the average strain rate in the impeller zone, and the volumetric oxygen mass transfer coefficient. These are representing the homogenization in the liquid phase, the mechanical stress acting on the cultivated cells and the availability of oxygen, which is essential for aerobic organisms. Only through the hydrodynamic characterization of the different bioreactors can the causal relationship of the bioreactor operating conditions like the impeller speed, the working volume, and the sparging strategy with the process performance of the cell cultivation be understood, which is required for the optimization of operating conditions for the different bioreactors. For larger biroeactor volumes an increase in the mixing time cannot be avoided, whereas a similar maximum hydrodynamic stress, a similar average strain rate of the impeller zone, and a similar volumetric oxygen mass transfer coefficient are observed for all investigated bioreactors. To optimize mixing without risking cell damage, the maximum tolerable average strain rate of the impeller zone is selected as the scale-up criterion for the impeller speed. Experimental cell culture results provided by Yuichi Aki from Daiichi-Sankyo Japan support the suitability of this criterion through a successful scale-up of the cell cultivation from the Mobius® CellReady 3 L to the XcellerexTM XDR-200. Other typical scale-up criteria like the volumetric power input and the impeller tip speed result in lower impeller speeds than the with presented strategy, therefore appearing less suitable to optimize the mixing time during scale-up. This emphasizes the advantages of a detailed hydrodynamic analysis over classical scale-up parameters.