(ILLUSTRATION BY M. MCEVOY. IMAGES: YAMADA TARO, JOEL SARTORE, IMAGE SOURCE/GETTY IMAGES)
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It's become an embarrassment of riches. New, robust cell lines now churn out therapeutic monoclonal antibodies (mAbs) in quantities
so high, biopharmaceutical manufacturers are having a hard time purifying the drugs with state-of-the-art tools and techniques.
The bottleneck, many claim, arises because, simply put: downstream processing technologies have not kept pace with improvements
in upstream production technologies.
"Since the introduction of monoclonal antibodies, upstream productivity has increased about 1000-fold," says Uwe Gottschalk,
group vice-president for purification technologies at Sartorius Stedim Biotech in Gottingen, Germany. "Downstream technologies
have improved only about 10-fold when we compare the productivity of critical steps such as chromatography. There is a gap,
and it's widening. If we keep the existing strategies and technologies, then there is little chance of closing the gap," says
Gottschalk, who edited the recently published book, Process Scale Purification of Antibodies.
State-of-the-art purification
In the current paradigm, therapeutic mAbs are produced by mammalian cells. These cells are maintained in large bioreactors—vats
that can hold tens of thousands of liters. The cells are suspended in a culture medium, essentially a nutritive broth, and
genetically engineered to produce therapeutic mAbs. The cells generally secrete the antibody products into the medium, along
with the molecular waste of cellular metabolism. Many cells—sometimes as many as 50%—die in the bioreactor during the course
of production, and these dead cells often split, or lyse open, and spill their contents into the medium. These contents include
bits of the fatty cell membrane, cellular proteins, virus particles that may have infected the cell or were suspended in the
medium, and the nucleic acids DNA and RNA, which, in this context, are also considered waste products. Purification, therefore,
has two goals.
The first goal is to retain product, and this is accomplished in the "capture" phase of purification. The second goal is to
discard contaminants, achieved during the "polishing" phase. Before either of these phases is initiated, however, the contents
of the bioreactor are "clarified," usually by centrifugation, followed by filtration. The idea is to remove particles, including
the remaining and now unwanted intact cells. Centrifugation separates out components by weight, so the intact cells fall to
the bottom of the centrifuge tube, leaving the lighter components—including the antibody product—suspended in the fluid on
top of the centrifuge tube. The solid matter is discarded, and the supernatant is then filtered to remove any remaining particulate
matter. Capture and polishing then follow. Of these two phases, capture has received the most the attention in terms of technical
innovation and cost reduction.
Downstream burden
Figure 1: The major steps in monoclonal antibody purification. (FIGURE IS COURTESY OF THE AUTHOR, ADAPTED FROM SOURCE 4)
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The current wisdom posits that high-yield bioreactors are producing so much material that biopharmaceutical manufacturers
are having a hard time purifying it all. But not all manufacturers are experiencing the bottleneck, or at least not as severely
as others. Manufacturers producing very large quantities of blockbuster drugs, at quantities that can exceed one metric ton
per year, in fixed facilities may feel the problem more acutely than others. For them, the problem boils down to this: A manufacturer
may have two options to deal with the bottleneck. Either it acquires additional purification equipment, or purifies the product
in batches. But producers using dedicated, fixed facilities may simply not have the space to accommodate additional equipment.
For them, the best option might be to purify the product in batches. Three to five batches may be necessary to purify all
of the protein product now produced by today's high-yield cell-culture systems.
