Abstract
In the last two decades, recombinant protein yields from mammalian cells in batch and extended-batch bioprocesses have increased
dramatically. In general, this has been a result of the utilization of cell lines with high specific productivities, the formulation
of media to allow the suspension cultivation of cells to high densities, a better understanding of bioprocess conditions,
and the enhancement of cell viability in these high-density suspension cultures. This article discusses the contributions
of these factors to yield improvement. It also describes a theoretical model to show how volumetric productivities may increase
in the future. The major consequence of higher volumetric productivities is expected to be the decline in the time or volume
of production runs. This outcome is expected to lead to a substantial improvement in the efficiency of protein manufacturing.
Avecia Biologics Limited
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About half of all recombinant therapeutic proteins currently on the market are produced in mammalian cells, with about 70%
of these being generated in suspension culture-adapted Chinese hamster ovary (CHO) cells.1 CHO was chosen as the host for the production of tissue plasminogen activator (tPA), the first recombinant therapeutic protein
from mammalian cells to gain regulatory approval. Other biopharmaceutical manufacturers continued using CHO cells because
these were then considered an acceptable host system for products intended for intravenous administration in humans, but with
time other mammalian cell lines such as mouse myelomas (NS0 and SP2/0), baby hamster kidney (BHK-21) cells, and human embryonic
kidney (HEK-293) cells became acceptable as hosts for recombinant protein production.2 Besides the approximately 140 recombinant therapeutic proteins already on the market, there are hundreds of protein products
currently in clinical trials with the majority of these being recombinant monoclonal antibodies (MAbs) produced in CHO cells.
Therefore, this cell line will continue to be the workhorse of the biopharmaceutical industry for the coming decades.
Meeting the Market Demand
Although the abundance of candidate therapeutic proteins is good news for the biopharmaceutical industry and for public health,
this success has brought its own set of problems. Undoubtedly, a major one is the so-called "capacity crunch," the expected
limitation in the number of manufacturing facilities to produce the quantities of MAbs and other recombinant proteins necessary
to meet the market demand in the coming years. One obvious solution is building additional manufacturing plants, but this
path is both costly and slow. Alternatively, manufacturers may seek novel solutions to the most widely used type of manufacturing
process—the production of recombinant proteins from suspension-adapted cell lines cultivated in large stainless-steel stirred-tank
bioreactors. One avenue is developing disposable cultivation systems for part or all of the manufacturing process. Another
is refining methods and technologies to allow protein production from cultivated mammalian cells to be more efficient by increasing
the volumetric yields from these processes. Although this has always been one of the major driving forces for the manufacturers
of biopharmaceuticals, there is now a heightened urgency for more rapid progress on yield improvement, given the sheer number
of recombinant proteins in clinical trials.
Volumetric Yields
As discussed earlier, volumetric yields from recombinant mammalian cell lines have increased dramatically over the span of
20 years since the approval of tPA.2 In 1986, the industry standard for production from stable CHO-derived cell lines was a specific productivity of 10 pg protein/cell/day
with volumetric yields of 50 mg/liter for batch processes that lasted up to seven days.2 By 2004, the highest reported specific and volumetric productivities were 90 pg/cell/day and 5 g/L, respectively, for an
extended-batch culture lasting up to three weeks. How have these increases been achieved? Unfortunately, the answer remains
obscure because manufacturers are reluctant to reveal their insights into the generation and cultivation of recombinant cell
lines. Nevertheless, it is clear that four main factors have contributed to the overall improvement in protein yields: (i)
the generation of recombinant cell lines with high specific productivities, (ii) the formulation of media to support high-density
cell cultivation, (iii) the understanding of bioprocess conditions for cell cultivation, and (iv) the sustained viability
of cell lines in high-density batch and extended-batch cultures. We will summarize how each of these factors has supported
the increase in volumetric recombinant protein productivity observed in recent years. It is likely that manufacturers have
taken different routes to establish production principles resulting in multigram per-liter-yields.
