Gap example # 3: primary recovery equipment
In the primary-recovery step of the process, the cell culture fluid from the production bioreactor is fed via a rotary pump
to a multi-membrane tangential flow filtration (TFF) system. The filtrate (i.e., permeate) is collected in the harvest tank
while the retentate is recycled back to the production bioreactor, as shown in Figure 5. The facility fit analysis indicated
that the TFF feed pump was different between the sending and receiving sites. Because the receiving site was a multi-product
facility and the feed pump was used for several products, it was highly desirable to adapt the process to the existing equipment
at the receiving site if possible. The difference in the TFF feed pump could potentially change the turbulent-eddy size distribution
to which the cells were exposed in the TFF flow path and thereby cause different levels of cell lysis during the primary recovery
operation (13, 14). The performance of the primary recovery step, such as step yield and processing time, could be affected
as a result. Additionally, cellular enzymes, such as glycosidases, proteases, or reductases, may be released as a result of
varied cell lysis and could potentially affect product quality. An extremely high energy-dissipation rate due to the feed
pump difference could also affect product quality. However, this was considered unlikely given the type of pump at the receiving
site and available information from the literature on the effect of high shear on proteins (15, 16). Therefore, this gap
in the TFF equipment was classified as a medium risk.
Figure 5: Process flow diagram of the tangential flow filtration step.
Laboratory-scale TFF systems typically do not represent the performance of manufacturing-scale systems well. Additionally,
manufacturing-scale pump performance cannot be reproduced in the laboratory. Therefore, the risk-mitigation plan was to perform
full-scale engineering runs to assess the potential effect on process performance and modify any TFF process parameters if
The TFF operation is run by controlling the filtrate flow rate at a predetermined target until the trans-membrane pressure
(TMP) reaches a maximum limit, after which the filtrate-flow rate is reduced to maintain the TMP at the maximum limit. The
switch between the concentration phase and the diafiltration phase is based on a target-concentration factor. TFF operation
ends once a target diafiltration volume has been reached. The TMP and filtrate turbidity profiles of the engineering runs
were compared with two typical manufacturing runs from the sending site in Figures 6a and 6b, respectively. It was evident
from the first two engineering runs that the equipment difference likely resulted in a higher level of cell lysis and thus
caused higher filtrate turbidity and TMP. In fact, the first engineering run was terminated before reaching the target diafiltration
volume because of the low filtrate flowrate. For the third engineering run, the initial filtrate flow-rate target and the
maximum TMP limit were reduced to improve the performance of the TFF step.
Figure 6a: Tangential flow microfiltration trans-membrane pressure (TMP) profiles of the engineering runs. Gray and back lines
are typical TMP profiles from the sending site.