Economic Drivers and Trade-Offs in Antibody Purification Processes

ABSTRACT

Increasing titers in mammalian cell culture has recently shifted the focus of process development efforts towards improving the economics of product recovery and purification processes. Some argue that a paradigm shift may be required to ensure timely and cost-effective delivery of future antibody candidates. This paper discusses the key process economic drivers and the impact of scale and titer on downstream processing (DSP) economic trade-offs.

The success of therapeutic monoclonal antibodies (MAbs) in the treatment of indications such as cancer and autoimmune diseases has fuelled clinical trial activities; in recent years MAbs have become the fastest growing segment of the biopharmaceutical industry.^1,2 Attractive returns coupled with the potential losses in revenue resulting from delays in product approval have made companies focus on speed-to-market rather than on improving process economics.^3–5 However, MAbs are amongst the most expensive drugs—the annual cost per patient can reach $35,000 for antibodies that treat cancer conditions. With intensified competition on the way and increasing pressure from healthcare providers, the ultimate success of the next generation of MAb therapeutics will increasingly depend on economic factors.⁶ As a result, production costs and capacity use are becoming critical success factors for the industry.^7,8

Economies of Scale

The relative importance of key process parameters on the overall economic feasibility of a process varies with both the scale of operation and the titer assumed. As annual output and scale increase at a given titer or combined with titer increases, the relative importance of different cost categories are expected to change as follows (Figure 1):

• Overall cost of goods/gram (COG/g) decreases

• Direct:indirect costs:

- Material costs rise considerably and dominate COG/g

- Labor and capital-dependent costs (overheads) represent a less significant proportion of COG/g

• Upstream:downstream processing costs:

- Downstream processing costs become a major component of COG/g.

If the increase in annual output is also accompanied by titer increases, then the trends are expected to become even more pronounced. At small scales, fixed costs tend to dominate and any changes in raw materials will have minimal impact. However, as scale increases, the role of raw materials becomes more critical. It is therefore important to look at the distribution of the costs (upstream to downstream, and direct to indirect costs) and at the scale and titer of interest to prioritize optimization efforts.

Figure 1. Typical cost trends as scale increases for (a) total COG/g, (b) material, labor, and indirect (capital-related) costs, and (c) upstream and downstream processing operating costs.

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 typical 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

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.

Figure 2. Overall yield as a function of individual step yields and the number of steps

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

Chromatography Capacity

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

Process Economic Trade-offs for DSP Bottlenecks

Current efforts to avoid downstream processing becoming a bottleneck when handling larger masses include intensifying existing processes by enhancing capacity and speed. Such approaches can mitigate the need for extra investment in equipment and improve the process economics. However, with each of these approaches there are trade-offs and uncertainties that need to be evaluated to assess the impact on overall process economics.

Chromatography Resin Dynamic Binding Capacity

Increasing resin binding capacity reduces column size requirements with concomitant drops in resin volumes required and buffer consumption [for equilibration, washing, elution, regeneration, and clean-in-place (CIP)] per batch. Novel resins have capacities that are more than twice those of the first generation resins.^24,31 This can lower consumables costs which are a major component of COG/g at higher titers and demands,⁹ although the impact on the consumable costs will depend on the price differential between first and second generation resins. Increasing the binding capacity of affinity resins can have a greater influence on COG/g because affinity resins are more expensive than ion-exchange resins. With the cheaper ion-exchange resins, Sommerfeld and Strube highlight that a trade-off exists between the less pronounced drop in consumables cost and the increase in labor costs, which becomes more important at higher binding capacities, because of the longer processing times. However, given that most of the novel resins also allow higher flow rates, this may not be an issue.

Chromatography Flow Rates

The first generation of cell-culture–based MAb processes used compressible chromatography resins. However, these impose severe limits on usable bed heights and flow rates when considering the expected increases in titer and scale.^30,32 A move away from compressible resins and towards rigid resins that can handle higher flow rates reduces process cycle times and turnaround time and increases productivity.^24,30,31 Flow rates with rigid resins can be three to five times faster than conventional compressible agarose resins.^24,31 However, their use can also lead to increased buffer demands, higher pressure drops, packing complications, and shear stresses.³⁰ New ion-exchange resins have recently been developed that can handle high flow rates (700 cm/h) and low back pressures (<3 bar). If the productivity increases outweigh the increased buffer demands, this will positively influence the COG/g.

Chromatography Resin Cycle Limits

As mentioned earlier, resin and filter re-use can have a significant impact on the process economics at higher scales if the materials are expensive, despite the CIP and cleaning validation costs. Re-use also reduces the frequency of column packing which is time-consuming and costly when carried out on a large-scale.³¹ New resins are becoming available with increased stability when exposed to the harsh chemicals used for CIP; hence, they have longer cycle limits and can contribute to reducing the raw material costs.

Platform Processes

Platform processes provide a generic approach to antibody production that greatly reduces the development time while streamlining the regulatory aspects of processing. They represent a useful starting point for customization depending on the antibody being manufactured. Advances in resin properties have also allowed platform processes to emerge with two rather than three chromatography steps.^16,28 This can help to alleviate DSP bottlenecks in existing facilities because a two-chromatography–step process occupies less floor space and consumes less buffer. Through such process intensification methods, Kelley predicted that a platform consisting of two chromatography steps with high capacity resins would be able to handle an annual output of 10 tons.¹⁶ Newer resins with the combined attributes of longer lifetimes, higher flow rates, and improved dynamic binding capacities will lead to improved platform processes for antibodies and contribute to significant reductions in downstream costs.^28,31

Alternatives to Chromatography

Research into alternatives for column chromatography focuses on methods that have the potential to effectively handle increased amounts of both the product and impurities (e.g., host cell proteins and antibody aggregates or isomers). Ideally, these alternatives should achieve a separation power equal to that of column chromatography while reducing the COG/g.³³ When assessing the cost-effectiveness of these alternatives, it is important to consider not only the equipment sizes and resource consumption, but also the development and validation costs required.

Membrane chromatography operating in flow-through mode is emerging as a popular alternative to anion-exchange chromatography steps in MAb purification,^{11,26,34–36} because of its rapid operation, ease of scale-up, and cost savings (Table 1). The dominant component in the distribution of raw material costs shifts from buffer costs in packed-bed chromatography to membrane costs; a membrane suitable for processing a batch of several thousand liters can cost several thousands of dollars and is disposable and not reusable. The key process economic trade-offs for anion-exchange applications therefore depends on whether the savings in buffer, labor, and overheads outweigh the high cost of the membranes. Critical variables that will affect the outcome of this cost comparison are the relative differences in the handling capacities assumed between anion-exchange membranes and resins, which dictate the sizes required, and the assumed WFI and buffer costs; higher values of these variables increase the economic attractiveness of membrane chromatography.²⁶ The pace at which resin and membrane capacities improve will contribute to which operation secures its place in future platform processes. In cases where packed-bed and membrane chromatography offer similar COG/g, the real cost advantages may be in the development and validation costs that are significantly reduced with membrane chromatography because there is no column packing or cleaning validation.²⁶

Table 1. Example of downstream process economic trade-offs

Summary and Outlook

As demand and titers continue to increase for MAbs, the DSP costs will become an increasingly dominant proportion of the COG/g with the DSP handling capacity representing a potential bottleneck that could reduce productivity. These factors have encouraged a shift in development efforts towards new DSP solutions that improve the process economics and alleviate bottlenecks. Consequently, the industry is taking advantage of improvements that affect the critical process economic drivers by looking to: improve the overall DSP yield and reduce the batch duration using platform processes based on two chromatography operations without intermediate buffer exchange steps; increase DSP capacity by taking advantage of improvements in chromatography resins that allow increased throughput over shorter times; and lower buffer demands and validation costs using new technologies such as membrane chromatography.

These improvements will be important for facilities that already have large bioreactor capacities installed, and also for newer facilities that will probably be built with smaller capacities with flexibility in mind to allow rapid turnaround between campaigns for multiple products. Furthermore, if cheaper and faster expression technologies, such as glycoengineered Pichia pastoris, become more widespread, there will be an even greater spotlight on DSP costs. Although new DSP approaches may present complex and challenging problems to tackle, it is anticipated that this line of enquiry will dominate studies in the near future so that more cost-effective MAb platform technologies can evolve. However, with each of these new approaches there are trade-offs and potential risks that need to be evaluated to assess the impact on process economics. The capacity to cost such alternatives provides a common basis for such decision-making and will prove a vital tool for bioprocess designs in the future. Process economics can also be dramatically improved if the potency of MAbs is increased; recent efforts in this area are an encouraging sign for the future. BP

This is an excerpt from the chapter entitled "Process Economic Drivers in Industrial Monoclonal Antibody Manufacture" in the forthcoming John Wiley and Sons book Process Scale Purification of Antibodies edited by Uwe Gottschalk.

SUZANNE S. FARID, PhD, is a lecturer (assistant professor) at the Advanced Centre for Biochemical Engineering, Department of Biochemical Engineering, University College London, London, UK, +44 20 7679 4415, s.farid@ucl.ac.uk

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