"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 purificationIn 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.