The Future of Downstream Processing - The author reviews the state of downstream processing, including a look at the streamlining of full processes and borrowed technologies. - BioPharm International


The Future of Downstream Processing
The author reviews the state of downstream processing, including a look at the streamlining of full processes and borrowed technologies.

BioPharm International
Volume 24, Issue 9, pp. 38-47


Although process redesigns and traditional technologies can contribute to the development of downstream processes, they provide only incremental improvements that marginally increase process efficiency. Incremental or evolutionary technologies have been the mainstay of the bioprocessing industry for the past 20 years, and column chromatography provides one of the best examples of this phenomenon in action (29). However, these slow marginal gains are already beginning to decline, and it is becoming difficult to envisage how sustainable processing can continue without a major injection of downstream-processing capacity.

One way to address this concern is to embrace genuinely novel technological approaches that change the rules of the game. The fringes of the biopharmaceutical industry are populated by companies that survive on innovation, and some of these innovations are disruptive in the sense that their influence on the industry is unpredictable and could contribute to a radical change in bioprocessing.

Most technological innovations in bioprocessing have been incremental, but there are several recent examples of disruptive innovations that have challenged the established business model and caused real grass-roots change in the industry. Again, many of these changes have affected upstream productivity first (e.g., disposable bioreactors and buffer/media storage bags), but we are also seeing examples in downstream processing (e.g., the introduction of simulated moving bed chromatography, expanded bed chromatography, monoliths, and membrane adsorbers) (1). These innovations have taken hold in niche markets but are now beginning to adopt mainstream positions. Disposable modules for downstream processing occupy a more mature status in the development cycle (30). The use of disposable filter modules is now an industry standard and where filters first left their footprint, membrane adsorbers are set to follow (31).

Membrane technology

Table I: Broader polishing operation window with salt-tolerant membrane chromatography.
Disposable membranes adsorbers are beginning to replace traditional chromatography in a number of settings, just as disposable membrane filters replaced steel mesh filters. Indeed, membrane filters have evolved into charged filtration devices that use the principles of both sieving and chemical selection to improve filtration performance, thereby creating a precedent for the use of membrane chromatography in downstream processing. After a period of inertia, the benefits of membrane chromatography are now fairly well established, and manufacturers are willing to consider them as a genuine alternative to fixed columns rather than a step in the dark (32, 33). In contrast to resin-based flow-through processes, membrane chromatography involves the use of thin, synthetic microporous or macroporous membranes stacked in layers within a disposable cartridge (34). The footprint of such devices is much smaller than columns with a similar output. A range of membranes is available with functional groups equivalent to the corresponding resins (e.g., membranes containing activated quaternary ammonium groups for anion exchange, or phenyl groups for hydrophobic interaction chromatography [HIC]), and a relatively new variant also allows salt tolerant interaction chromatography (STIC) in high-salt buffers (35, 36). The availability of STIC membranes is an important and innovative advance in biomanufacturing because even the most recent generation of membrane adsorbers fall short of some manufacturing requirements when challenged with the high-conductivity feed streams often produced in high-titer processes. STIC ensures more flexibility in process design and improves the clearance of host-cell proteins and viruses in buffers containing high concentrations of salt (see Table I). These new adsorbers therefore allow polishing to be carried out at higher load densities without an interstitial dilution step after product capture, reducing process time and circumventing the need for additional buffer preparation and holding.

Table I demonstrates that Sartobind STIC provides higher binding capacities for BSA, DNA, and model viruses compared with a Q anion exchanger under high salt conditions (150 mM NaCl), thereby increasing the design space for polishing operating conditions.

In a recent example described by the Italian biopharmaceutical company Philogen, membrane adsorbers were substituted for the flow-through and bind-and-elute steps for the polishing of a new monoclonal antibody fusion protein in Phase I–II clinical development, achieving 90% recovery and 99.9% purity (37). The performance benefits of membranes provide value for the user, but the complete elimination of cleaning and validation requirements is often cited as the major advantage because this avoids the costs of the chemicals, personnel, and record-keeping, and more importantly avoids the inevitable process down time while cleaning takes place. Spent modules are simply replaced with prevalidated new ones, available in a range of sizes and configurations for maximum flexibility (32).

Figure 1: (a) Mechanistic comparison of solute transport in bead resins (left) and membrane adsorbers (right), where thicker arrows represent bulk convection, thinner arrows represent film diffusion and curved arrows represent pore diffusion. (b) Comparison of bed height in columns (left) and membrane adsorbers (right). Using membrane adsorbers is functionally equivalent to shortening columns to near-zero length, resulting in a similarly small pressure drop that allows extremely high flow rates, thereby reducing overall process times up to a 100-fold. In this example, both formats have a 1350 cm2 frontal surface; the column has a bed height of 15 cm; and the membrane adsorber has a bed height of 0.4 cm. The height to frontal surface ratio is approximately 100 for the column and nearer to 3500 for the membrane device. (ALL FIGURES ARE COURTESY OF THE AUTHORS)
The performance advantage of membranes over resins reflects the transport of solutes to their binding sites mainly by convection, while pore diffusion is minimal (see Figure 1a). Because of these hydrodynamic benefits, membrane adsorbers can operate at much greater flow rates than columns, thereby considerably reducing buffer consumption and shortening the overall process time by up to 100-fold. The use of membrane adsorbers can be viewed as the equivalent of shortening traditional columns to near zero length to allow large-scale processes to run with only a small pressure drop at high flow rates. For example, polishing with an anion exchange membrane can be conducted with a bed height of 4 mm at flow rates of more than 600 cm/h, and provide a high frontal surface area-to-bed height ratio (see Figure 1b). Small-volume disposable membrane chromatography devices can now handle up to 50 L /min/bar/m2 . Even at these high flow rates, the membrane pores provide adequate binding capacity for large molecules such as viruses and DNA, so they can play an important role in the overall viral clearance strategy for antibody purification (38, 39).

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