Investigation of the influence of large scale occurring pH heterogeneities on CHO cells

Abstract

This study investigated the impact of pH heterogeneities on large scale CHO fed batch cultivations by the usage of a two-compartment scale down system. It is known that base addition from the top of bioreactors leads to a pH gradient as a consequence of long mixing times. These pH heterogeneities are assumed to have an influence on the physiology and the productivity of recombinant protein producing mammalian cells. Therefore, an experimental approach was developed to determine the size and the average pH of an alkaline zone, which can occur in a 10 m3 bioreactor, with a bench scale reactor. To determine the size of the alkaline zone, a bromthymolblue indicator was used to display the formation of the alkaline zone. The average pH of the alkaline zone was determined by using several pH probes placed in different areas of the bioreactor. We found that base addition from the top of a bioreactor leads to an alkaline zone with a volume of 5% of the initial reactor volume and an average pH amplitude of 0.25 units at a mixing time of 120 s. This experimental method for the determination of alkaline zone parameters could be used as an addition to CFD simulations. An alkaline zone with the obtained parameters was subsequently mimicked in CHO Fed Batch cultivations which were carried out with a two-compartment scale down system. In comparison to a control cultivation without an alkaline zone, no obvious changes in growth behaviour nor in the productivity of the cells could be observed. Further it was demonstrated that the formation of metabolites like lactate, ammonium or the uptake of glucose are not affected by the presence of an alkaline of this size and pH. This work extends the findings of previous studies in the research field of pH heterogeneities since it was the first time that an alkaline zone of such a low size and pH amplitude was mimicked. Furthermore, the results suggest that submerse addition of the base in large scale cultivations might not be necessary.