Like all areas of bioprocessing, downstream purification has its own favored buzzwords—titers, bottlenecks, capacity, and others—meant to evoke a particular response depending on who is speaking or writing them. But, if we could just identify one leading, universal trend in downstream bioprocessing, it would be simplification.
Processes are becoming more streamlined with fewer steps, improved versatility, and rapid changeover between products. Disposable equipment plays a huge role in this evolution, particularly during steps that achieve several purification objectives simultaneously. For example, charged depth (lenticular) filters from several vendors now remove DNA and host-cell proteins as well as cellular debris.
According to Uwe Gottschalk, Ph.D., group vp for purification at Sartorius Stedim Biotech, process intensification has become the leading driver for large-scale chromatography. New processes for monoclonal antibodies, he says, are trending toward utilization of just two columns where three or four columns might have been the norm. Processors generally achieve capture and volume reduction through the initial column, which is followed by a flow-through polishing step typically composed of an anion exchange column or membrane adsorber.
The Real Hurdle
Rising protein titers have been blamed for numerous downstream bottlenecks in monoclonal antibody processes, but according to Dr. Gottschalk, capture is the real hurdle in terms of time and direct and indirect costs.
The titer question revolves around the fact that for a given process volume upstream productivity is driven by cell density, while downstream separations are mass-driven. Put another way: upstream volumetric productivity rises with cell density—the limit has apparently not yet been reached—whereas, capture resin binding capacity is constant. A doubling of cell density produces twice the protein in the same volume, but capturing it requires twice the column volume, twice the buffer, etc.
Protein A capture for mAbs works exquisitely well, but its cost is three to ten times that of nonaffinity resins, which amounts to hundreds of millions of dollars for resin over the lifetime of a successful mAb product. In addition, the protein A ligand leaches into the process and must be quantified at the end.
“Bioprocessors rely too much on chromatography,” explains Dr. Gottschalk. “We are beginning to face facility-fit problems with high-titer processes. It’s not so much the size of the columns but of peripheral activities such as buffer prep, buffer hold, and utilities for column washing.”
Dr. Gottschalk envisions emerging bioprocesses that rely more on precipitation, extraction, or crystallization for initial capture of monoclonal antibodies and other product types. Of the three, he prefers precipitation for its robustness, followed by resolubilization and polishing through a series of media—membranes, perfusion, monoliths, and columns—to clear contaminants sequentially in one operation.
“Processes of the future might have several cartridges linked together, operated by the same pump, that removes all contaminants down to specified limits for each.”
The individual pieces already exist for such a purification train but engineering work needs to be done in the areas of connecting sequential steps, synchronizing flows among them, and the measurement of conductivity and/or pH, all in disposable format.