The impact of pH inhomogeneities on CHO cell physiology and fed‐batch process performance – two‐compartment scale‐down modelling and intracellular pH excursion

Due to high mixing times and base addition from top of the vessel, pH inhomogeneities are most likely to occur during large-scale mammalian processes. The goal of this study was to set-up a scale-down model of a 10–12 m3 stirred tank bioreactor and to investigate the effect of pH perturbations on CHO cell physiology and process performance. Short-term changes in extracellular pH were hypothesized to affect intracellular pH and thus cell physiology. Therefore, batch fermentations, including pH shifts to 9.0 and 7.8, in regular one-compartment systems were conducted. The short-term adaption of the cells intracellular pH showed an immediate increase due to elevated extracellular pH. With this basis of fundamental knowledge, a two-compartment system was established which was capable of simulating defined pH inhomogeneities. In contrast to state-of-the-art literature, the scale-down model included parameters (e.g. volume of the inhomogeneous zone) as they might occur during large-scale processes. pH inhomogeneity studies in the two-compartment system were performed with simulation of temporary pH zones of pH 9.0. The specific growth rate especially during the exponential growth phase was strongly affected resulting in a decreased maximum viable cell density and final product titer. The gathered results indicate that even short-term exposure of cells to elevated pH values during large-scale processes can affect cell physiology and overall process performance. In particular, it could be shown for the first time that pH perturbations, which might occur during the early process phase, have to be considered in scale-down models of mammalian processes.