Intensification of upstream cell culture-based processes leads to higher cell densities and product titers, which in turn have resulted in lower process volumes and smaller, more compact processes. Yet these efficiencies stress downstream operations constructed for different times and different product streams. A study resulting from a recent collaboration between scientists from the University of Utah and Thermo Fisher Scientific evaluates two connected processes, harvest and purification, and how improvements therein might improve overall process efficiency and economy, while streamlining the transition from upstream to downstream operations.

“Upstream changes have downstream implications,” says study co-author Mark Smith, PhD, senior manager, bioprocess technology research, Thermo Fisher Scientific. “For example, single-use depth filtration is generally used for harvesting mammalian cultures expressing recombinant proteins. But depth filtration becomes less technically and economically tenable as upstream processes intensify because the increased cell density requires substantially more filters. Thus, it behooves processors to consider alternatives to depth filtration for single-use harvest, especially if they seek to intensify upstream operations.”

Disk stack centrifuges (DSCs) excel at supporting a wide range of volumes, from about 200 L to more than 20,000 L, but the technology is limited at scales below 200 L, which limits scale-down and process development opportunities. Smith explains that this limitation arises from the volume limitation of about three liters for a typical DSC bowl, and the relatively large volumetric requirements of continuous centrifugation at development or pilot scales.

“By contrast, representative scale-down depth filtration runs as low as 100 mL, a thousandfold-smaller volume,” Smith explains.

Single-use centrifugation (SUCF) has similar low-volume limitations for scaledown, although some models or modes are more scalable and flexible than others.

Critical difference

A critical difference between DSC and SUCF is the ability to have a single-use product-contact flow path, and all the benefits that provides, especially to a multi-product facility or one with shortened production campaigns.

“The performance of SUCF is limited not on size, per se, but the precision and material robustness of the single-use rotor,” Smith says. “As a point of reference, a spinning single-use rotor might experience pressures that are equivalent to several adult elephants stepping on it. These high-performance demands make a SUCF a technically difficult engineering challenge.”

Several technologies available today or in the works aim at addressing the performance, efficiency, and economy of harvest. Many depth filter vendors already provide charged depth filter membranes to improve the clearance of detritus in the harvest feed stream.

“An example of using charge vs sizing is 3M’s recently released Harvest RC product, for which Thermo Fisher partnered with 3M,” Smith says. Harvest RC has almost no traditional filter properties, but relies on a charged membrane to capture and clarify harvest fluid.”

The cited paper on the study mentions flocculation as a means of increasing harvest capacity. Other physical methods, e.g. acid precipitation, works similarly. These techniques, followed by depth filtration or flocculation, work as long as the product remains in solution. But even here, process designers face tradeoffs.

These include product loss in the flocculant, undesired product aggregation, and flocculation agents ending up in the product. Plus an extra process step complicates process optimization and deployment during manufacturing.

“Finally,” Smith adds, “certain flocculants can add substantial cost, which can offset the economic benefits garnered by reducing filters or other processes downstream of the flocculation.”

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