Growing mammalian cell cultures in bioreactors is a complex activity - scaling up such cultures must consider both biological and engineering aspects. Cell lines, when they are grown in shake-flasks, exhibit growth characteristics that are vastly different from those observed when such cultures are grown in agitated bioreactors. It would almost seem like the cells somehow “know” that they need to behave differently when they are transferred from one vessel to another!
In reality, this difference in behaviour is largely due to a change in the environment inside the bioreactor that the cells “experience”. In other words, when the cell cultures are in a shake flask, the kind of environment that they experience in the shake flask is significantly different from the environment that they experience when they are in a larger bioreactor. Ultimately, cells respond to stimuli in their environment and if these are different (and they are when one changes reactors), then cell behaviour also changes. The direct consequence of this fact is that if these differences are not properly accounted for, moving cell culture processes from one vessel to another to result in significant changes in the product quality. This is a problem of critical significance in the biotech industry across different modalities - MAbs, C> therapy, vaccines etc.
So how does one account for these differences properly? The first step in this direction is to identify what constitutes the “cell culture environment”. The cell culture environment, from an engineering viewpoint, consists of the following three aspects:
- Mixing: The better mixed or “homogeneous” the environment, the more uniform the behaviour of the cells. This is because, a well-mixed environment ensures that all cells have access to nearly the same amount of nutrients, oxygen etc. When mixing behaviour between two vessels is the same, then the access to nutrients, oxygen etc. is also nearly the same for cell cultures growing in both the vessels.
- Mass-transfer: Most mammalian cell cultures of interest need oxygen to grow which is typically sparged via mechanical means into the growing cell culture. While well mixedness ensures that the sparged gas is made available to the cells equally in all parts of the bioreactor, it does not guarantee that the oxygen in the gas phase can dissolve in the medium. Mass-transfer from the gas to the liquid medium is therefore a second critical aspect that controls how well the cell cultures grow. As before, when mass-transfer aspects between two vessels are the same, then oxygen availability to the cells is also the same in both the vessels.
- Shear-sensitivity: It is fairly well-known that mammalian cell cultures can be shear sensitive, especially in the presence of agitation and sparging. Growing cells can be destroyed by subjecting them to intense shear - which is sometimes likely in an agitated bioreactor - thus decreasing cell viability. It is important to understand the magnitude of shear that cells are subjected to on average and ensure that the amount of shear always remains within the acceptable limits. It also follows as before that if the shear environment in two vessels are the same, then one can expect that the cell viability in the two vessels will likely be the same.
We have thus seen that the cell culture environment, characterized by “mixing”, “mass-transfer” and “shear”, controls the behaviour of cell cultures as they are transferred from one vessel to another. Successful scale-up and scale-down of such cell cultures will depend quite heavily on how well these three aspects are matched between two scales. Fortunately, methodologies have been developed to quantify these aspects so that the matching principle can be implemented in an efficient manner. These methods, which will be described in our next blog in this series, aim to achieve this “equivalence of cell culture environments” between scales in a manner that is not only rooted in the fundamentals but also easy to demonstrate to regulatory authorities.
