Systematic Development of a Scale-Down Model for a 25,000 L Fed-Batch CHO Culture Process

We will present a typical process of developing and fine-tuning a 5 L bioreactor model for a 25,000 L scale CHO fed-batch process. The process was first scaled-down according to engineering principles (same power input per volume and vvm).  We were able to reproduce most of the culture parameters in 5 L bioreactors such as specific growth rate, peak viable cell density, titer and specific productivity. However differences in certain culture indicators (such as viability and lactate metabolic shift) and critical quality attributes (such as glycoform profile) were observed between 5 L and 25,000 L bioreactors.

We carried out a systematic analysis and step-wise experimentations to identify key cell culture parameters (such as CO₂ stripping and base consumption) that contributed to the observed differences. Intrinsically, small scale bioreactors are superior at mixing and O₂/CO₂ transfer and base required for pH control is often a few fold less than that in large scale bioreactors. In experiments, cell inoculum and media were obtained from manufacturing operations to avoid culture source and materials variation. Controlled experiments showed that the glycoform profile differences at 25,000 L scale was mainly due to decrease in cell viability. We were able to fine tune the scale-down model to match the large scale culture process closely by artificially re-creating the 25,000 L osmolality trend in 5 L bioreactors using 1 N NaOH. Protein quality attributes of specific interest, glycoform profiles, were produced at the same levels as seen in 25,000 L bioreactors after the culture processes were aligned. We will discuss the impact of osmolality and pCO₂ on cell viability and protein quality attributes. Relationships between pCO_2, lactate, osmolality and base consumption will be presented. Applications of the improved scale-down model in cell culture performance evaluations (such as the impact of cell population doubling length) will also be discussed.