The appropriate combination of harvest and clarification unit operations for mammalian cell culture harvest is obviously scale-
and facility-dependent. Preferences also have evolved with increasing experience with scale-up of certain unit operations.
The early 1990s saw significant focus on MF-based harvest strategies because centrifugation was considered a significant capital
investment and experience with controlling the shear exerted on mammalian cells was limited. Over time, better centrifuge
design and efforts by both leading vendors (Westfalia and Alfa Laval) to optimize operating conditions for biopharmaceutical
cell culture broths have resulted in this becoming the predominant harvest technique for cell culture facilities over a 2,000-L
Also, the capital investment in large-scale centrifuges is no longer seen as cost prohibitive in multiproduct production facilities
because the operating expenses with membranes for MF are significantly higher and the centrifuges can be easily changed over
from one product to another.32 Disk stack centrifuges, on the other hand, are easier to clean and operate in a sanitary fashion than large-scale MF housings
are. Bioburden control and cleaning validation also are simpler for centrifuges than for MF systems because the latter have
complex flow paths which lead to the possibility of dead zones that can harbor microbial growth.
Unlike MF, which provides a clean filtrate stream, the use of centrifuges does, however, imply the need for a secondary clarification
step. This niche is most commonly filled in MAb manufacturing processes by depth filtration. Large-scale depth filtration
now can be readily scaled up with process-scale housings and disposable filter modules. Recent work with flocculants and filter
aids might further increase the throughput of this step. It is rare to see depth filtration as the sole harvest technique
beyond smaller production scales (e.g., a few hundred liters).
Terminal clarification is almost always provided by in-line filtration through microfilters with an absolute pore size rating.
These terminal filters ensure a particle-free feedstock for the capture chromatographic step.
Harvest and clarification schemes for MAb production processes today (Figure 5) are the product of much evolution and evaluation
carried out over the last 15 years. Harvest techniques for mammalian cell culture systems are now routinely expected to operate
with high yields (>98%) and minimal cell disruption. The high titers that can now be achieved in cell culture operations mean
that the challenge has now moved further downstream to improve purification throughput. This also implies that the current
cell culture scales are likely to stay with us over the next decade. Instead of radical changes to the way harvest and clarification
are carried out, improvements are likely to be in the form of gains in efficiency and throughput and improved understanding
of existing unit operations.
Figure 5. Common harvest and clarification schemes for large-scale mammalian cell culture harvest
The authors would like to acknowledge many Bristol-Myers Squibb employees in the departments of manufacturing sciences, process
sciences, and manufacturing operations at the Syracuse, NY, site for contributing to the development of robust harvest operations
for several mammalian cell culture products. We also thank Dr. Steven S. Lee, VP and GM, for supporting this review.
This is an excerpt from the chapter in the forthcoming John Wiley and Sons book, Process Scale Purification of Antibodies, edited by Uwe Gottschalk.
Abhinav A. Shukla is an associate director of Manufacturing Sciences, and Jagannadha Rao Kandula is a process engineer of Manufacturing, both at Bristol-Myers Squibb, Co., East Syracuse, NY, 315.431.7926, firstname.lastname@example.org