Fortunately, the polishing area—unlike the capture area—is beginning to benefit from advances such as disposable chromatography
systems, which have been implemented in a number of processes.4 In one example, a disposable 0.5-L membrane can remove process-derived impurities such as DNA, viruses, and endotoxins from
a 1000-L feed stream just as well as a stainless steel column that is a hundred times larger. Results from a case study examining
the economics of such systems demonstrated that disposable equipment can provide a cost savings up to 70% in MAb polishing.5 The disposable options benefit from lower capital investment and significantly reduced material costs because these small
devices consume 95% less buffer. And for viral clearance, disposable chromatography offers additional advantages beyond cost
savings: it eliminates the need to carry out virus carryover and cleaning validation studies.
In many processes, virus filtration (the other main method for virus removal) is the most expensive downstream step, accounting
for up to 40% of costs. Thus, it is understandable that this step is a target for process optimization. Overall productivity,
however, is not only a matter of initial flow rate, but also of total throughput and sensitivity to aggregate and impurity
level. A viral clearance study with at least four viruses for late-stage development is state-of-the-art, and log reduction
values (LRVs) of 4 and higher are hygienic factors. More interesting, but also less trivial, are LRVs that are independent
of the flow decay of the filter.
Other Current Trends in Bioseparation
A clear trend in bioseparations is to combine steps, even eliminating the need for staging tanks and hold-up volumes between
unit operations. Orthogonal strategies for each objective are key to achieving industry goals for yield and quality.
With increasing biomass in cell culture, high-productivity harvesting procedures are enticing, and there is a lot of interaction
among the three unit operations of centrifugation, cross flow, and depth filtration. Eventually, we will see more integrated
processes, such as expanded bed adsorption, ideally in disposable formats.
For monoclonal antibody manufacturing, for example, we need to develop processes that require only two column steps followed
by a rational filtration train, in which every step addresses the removal of a certain contaminant.
Polishing before capturing sounds provocative, but is becoming more common. This approach protects the protein A column and
prevents the column eluate from precipitating by removing certain lipoproteins from the feed stream. These concepts can be
custom designed with smart membranes that remove individual contaminants like proteases very specifically and early in the
process to yield stable storage forms.
In chromatography, attempts are being made to make resins look more like membranes and vice versa. It makes much more sense
to accept the limitations of both technologies. They each are great in their individual core application areas and deserve
an integrated approach, for example, by using a chromatography train (capturing phase) followed by a filtration train (polishing
phase). Capturing will always require high dynamic capacity and will be dominated by resins (with the exceptions of large
molecules and high dilutions), whereas polishing is going to be the domain of charged membranes.
Thus, we are seeing many paradigm shifts in bioseparations, including the trend toward disposables to save costs. I predict
that in a number of years, polishing applications will be dominated by disposable concepts. But much more is required. If
downstream processing had kept pace with the revolution in fermentation, chromatographic separations would require only minutes,
not hours. Because this is not feasible, we must seek alternatives. Bioseparation is desperate for high-throughput, high-productivity
unit operations. One place to start would be to copy what has been done in food, beverage, and technical enzyme manufacturing.
In sum, robust and reliable methods such as extraction, precipitation, and crystallization deserve a renaissance. Michelangelo,
although considered a master, was said to be constantly dissatisfied with himself. He probably realized that complacency is
a dangerous enemy. We should take our cue from him.
References
1. BioPlan Associates. 3rd annual report & survey on biopharmaceutical manufacturing capacity and production. June 2005.
2. Wurm FM, Production of recombinant protein therapeutics in cultivated mammalian cells. Nature Biotechnol. 2004; 22: 1393–1398.
3. Aldridge S. Downstream processing needs a boost. GEN 2006; 26(1).
4. Zhou JX, Tressel T. Basic concepts in Q membrane chroma-tography for large scale antibody production. Biotechnol. Progress.
2006; 22(2):341–349.
5. Sinclair A. Disposable technologies—impact on biomanu-facturing. London: Institution of Chemical Engineers; 2005.
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