Future Improvements in Protein Yield: A Modeling Approach
To anticipate how much recombinant protein production yields may improve in the near-and long-term future, we have modeled
volumetric productivities for hypothetical CHO-based production processes, using a recombinant monoclonal antibody as the
product. We began by assuming a doubling time of 18 hours and a maximal cell density of 10 x 106 cells/mL in an extended-batch process lasting up to 21 days (Figure 1). The volumetric productivity was then determined
for this process based on four different specific productivities ranging from 20 to 200 pg/cell/day. We are aware that a specific
productivity of 200 pg/cell/day may be entirely unrealistic, even from a purely fundamental biological perspective. With the
lowest specific productivity assumed, the volumetric productivity was about 5 g/L for a three-week production run (Figure
2). We note that a specific productivity of 20 pg/cell/day is already a quite reasonable value, which may be the maximum
achievable for certain molecules, even antibodies. By increasing the specific productivity to 100 pg/cell/day, the highest
productivity yet reported for an extended-batch process, the volumetric productivity was about 10 g/L for a two-week process
and about 15 g/L for a three-week process. With another doubling of the specific productivity to a theoretical value of 200
pg/cell/day, a final product titer of about 40 g/L was observed after 21 days of culture.
Next, we increased the cell density to 20 x 106 cells/mL for a production process lasting up to three weeks (Figure 1). Again, the specific productivities were varied from
20–200 pg/cell/day, as described above. Given these parameters, a cell line with the lowest specific productivity yielded
about 10 g/L of recombinant antibody during a three-week process (Figure 3). When the specific productivity was increased
to 50 pg/cell/day, final product concentrations of 10 and 20 g/L were reached with a two-week and a three-week process, respectively.
By doubling the specific productivity to 100 pg/cell/day, the volumetric productivity was about 30 g/L for a two-week process
and 40 g/L for a three-week process. Yields of 80 g/L were obtained for a 21-day process when assuming a specific productivity
of 200 pg/cell/day. At this specific productivity, a titer of 20 g/L was achieved in a theoretical process lasting seven days
Based on the modeling results shown here, it is clear that doubling the volumetric productivity from the current record level
reported (5 g/L) to 10 g/L will not be difficult to achieve for cultures at a density of 10 x 106 cells/mL and with cell lines with specific productivities in the range of 50–100 pg/cell/day. By doubling the cell density
to 20 x 106 cells/mL, the 10 g/L final product yield can be reached in about two weeks for a cell line with a specific productivity
of 50 pg/cell/day, in about five days for one with a specific productivity of 100 pg/cell/day, and in about three days at
a specific productivity of 200 pg/cell/day (Figure 3).
In view of the widely known capacity limitations in downstream processing, mostly with respect to the first product capture
step, it is questionable whether higher product concentrations (than 2–5 g/L) in harvest fluids are even desirable.8 This may change in the future if radically new product recovery principles become available and acceptable. However, as
the above model calculations show, shortening of production runs will be a very useful approach to economize the valuable
cell-culture infrastructure without reducing overall product yield derived from a manufacturing facility. More individual
runs for a given product in a given timeframe can liberate the facility for other products to be subsequently made in the
same facility. Of course, if capital investment is required for construction of a new facility by small and mid-sized biotech
companies, the obtainable yields from cell-culture processes will provide options for the construction of smaller facilities,
thus reducing the financial burden. As an example, a cell line at a density of 10 x 106 cells/mL and a specific productivity of 50 pg/cell/day will yield 10 g/L in a three-week process. If the cell density is
doubled, then the same amount of recombinant protein can be produced in half the volume in three weeks.
In the model presented here, we have assumed that protein quality, particularly the extent of glycosylation and aggregation,
remains the same no matter what the specific productivity or the cell density is. However, as experience has shown, such an
assumption is surely an oversimplification. Typically, careful adjustments to media compositions and numerous process details
must be implemented to maintain product equivalence when extending or shortening run times and or changing the scale of operation
in the final production vessel.