DSP Economic Drivers at High Titers
Efforts to lower COG/g must be targeted at decreasing the overall batch costs (e.g., reducing raw material costs) or increasing
the overall productivity (e.g., increasing process yields). This section discusses specific downstream processing (DSP) drivers,
which are of prime importance when handling multigram per liter titers.
As titers increase to 10–15g/L, it is expected that this will have a profound effect on reducing COG/g, as long as the purification
costs do not negate the cell culture gains. With increasing titers, the ratio of upstream to downstream costs shifts so that
the downstream costs become more dominant. For example, Sommerfeld and Strube calculated that increasing the fermentation
titer 10-fold from 0.1 to 1 g/L caused the ratio of upstream to downstream costs in their process to drop from 55:45 to 30:70.
This shift reflects the fact that the upstream costs are inversely proportional to titer but the same is not true for the
downstream processing costs.9 Increasing the titer to satisfy larger market demands increases the protein load on chromatography steps resulting in an
increase in the number of cycles or additional investment in larger columns; this also produces larger volume loads on any
subsequent filtration steps leading to longer filtration times or a need for larger areas.
All these factors increase the downstream operating costs per batch. However, the overall COG/g can still fall if the increase
in overall productivity outweighs the increase in downstream costs. Accordingly, as titers increase further, the downstream
processing steps will become major contributors to the overall COG/g and offer greater potential for improvements and cost
savings. Consequently, the downstream yield and material costs become significant cost drivers. DSP bottlenecks can lead to
increased investment in larger equipment and longer batch durations. This results in increased running costs and decreased
productivity and suboptimal COG/g values.
Overall DSP Yield
The overall DSP yield is a function of the individual step yields and the number of downstream processing steps, as has often
been demonstrated using the plot in Figure 2. Improvements in step yields and the reductions in the number of steps have contributed
to overall process yields increasing from 40 to 75% in recent years, with savings in cost of goods and investment, and allowing
for higher facility throughputs.10,11
Figure 2. Overall yield as a function of individual step yields and the number of steps
The impact of increasing step yields has been illustrated by Sommerfeld and Strube where increasing the average step yield
in a seven-step downstream process from 85 to 95%, which increases the overall yield from ~30 to 70%, results in a 40% reduction
in the downstream COG/g.9 Increasing step yields actually increases the equipment size or number of cycles required and, hence, the cost of the DSP
steps, because each step needs to handle a larger load (chromatography) or volume (filtration). However, because more product
is produced per batch, the COG/g typically falls with increasing yields.
To maximize productivity and minimize investment and running costs, it is advisable to keep the number of downstream processing
steps to a minimum.10,12,13 For antibody processes, this has encouraged the elimination of buffer exchange steps (diafiltration) that add little purification
value by designing each chromatography step so that it can take the material eluted from the previous step where possible.10,14,15 Some companies have recently adopted processes that use only two chromatography steps while maintaining the desired purity
levels.16 This requires an anion-exchange step to have additional selectivity to replace the intermediate purification and polishing
steps. In cases where the contaminant profile, pH, and conductivity provide the opportunity to adopt this simpler process,
time and cost savings can be achieved.
Material Reuse and Lifetime
In downstream processing, the distribution of raw material costs is highly dependent on whether or not resins and filters
are reused. When treating these material components as disposables, resins and filters tend to dominate the material costs.
Similar patterns can be seen in clinical manufacturing because these materials remain product specific. The impact of this
operating strategy on the overall COG/g can depend on both the scale and the phase of development. The use of downstream consumables
such as resins and membranes in a disposable fashion for a 200-L antibody process supplying early-phase clinical trials can
provide both financial and operational savings.7,17,18 It has been reported that downstream processing using disposables can become a major disadvantage at the 10,000-L scale.19 This can be attributed to the fact that economies of scale result in a disproportionate effect on raw materials.20 Consequently, raw materials savings become more important for any process as the scale increases.
The reuse of resins and filters involves a trade-off between reduced material costs and increased cleaning validation costs
to determine the number of reuses with consistent performance. The higher the component cost or number of process steps, or
the lower the validation costs, the greater the incentives to adopt filter or resin reuse.21 Chromatography resins, in particular Protein A, are often quoted as dominating purification raw material costs, owing to
the high cost of the resin which is higher than ion-exchange resins. Large bioreactor scales of 10,000 L operating with a
titer of 1 g/L, can result in Protein A resin costs of $4–5 million.22 Consequently, Protein A resins tend to be used in smaller quantities with multiple cycles, despite the complications of
reuse validation.22,23 The reuse of Protein A resins can dramatically reduce their relative contribution to costs, making the costs associated
with filters much more prevalent. In particular, virus filtration can represent a large contributor to purification material
costs because of the costly membranes that are often used in a disposable fashion.24
Buffer and Water for Injection Demands
As the reuse of filters and resins increases, the cost of made-up buffers [chemical reagents and water for injection (WFI)]
can account for a surprisingly large proportion of the costs, which can, in some cases, be greater than the cost of resins
and filters. For example, at a fermentation scale of 20,000 L, approximately 140,000 L of buffer is required.25 The difficulty in estimating buffer costs partly reflects the large differences in the estimated costs of WFI, with values
of approximately $0.20/L suggested for in-house generation and $3/L for vendor or contract manufacturing organization charges.26 Buffer costs have been quoted as varying between $2/L and $12/L.27 Efforts to reduce the volume and number of buffers required can lead to savings, because this naturally occurs when moving
from a three-step chromatography platform to a two-step one.
As increasing titers demand the use of larger downstream equipment or additional cycles, the requirement for buffers and WFI
will also increase. In existing facilities, this can present retrofit challenges if demands exceed the capacity of the buffer
preparation suites and WFI storage tanks or the rate of WFI generation. The use of buffer concentrates and in-line buffer
mixing can help to reduce the tank size and floor space required for buffers.28
The greater the mass load during chromatography, the greater the likelihood that the practical capacity of chromatography
columns will be exceeded where the current limit of column diameters is 2 m.29 Under these circumstances, multiple cycles may be required and there is a risk that the downstream processing time will
exceed the bioreactor time. This will reduce the potential throughput of the facility and impact on the COG/g. These large
columns can also pose installation challenges in existing facilities if there is insufficient floor space and if they cannot
fit through doors.30