Making the Move to Continuous Chromatography
Continuous chromatography processes use the same resin and buffer systems as batch counterparts, but in a smaller footprint with less resin and buffer consumption. Switching from batch to continuous allows for improved resin capacity utilization through continuous loading (overloading, in fact) of the column or membrane for increased process efficiency. It is particularly attractive for sensitive biomolecules that need to be rapidly separated from the other components in bioprocess solutions.
Some regulatory aspects must still be addressed, however, and for companies with extensive existing infrastructure, switching to continuous chromatography might not be cost-effective.
Continuous vs. batch chromatography
In batch chromatography, one large column is used. In continuous multicolumn chromatography, the large column is effectively split into a number of much smaller columns that operate in series over a larger number of cycles. While product is loaded on some columns, other columns in the set are going through the wash, elution, and regeneration phases. By allowing simultaneous operation on many columns, continuous chromatography is more efficient with time and resin, according to Michelle Najera, senior development scientist for downstream product development for AGC Biologics. “The load phase can be split into two columns in series as well, allowing more product to be applied to a given volume of resin since unbound product from the first column is easily collected by the second column in the load phase,” she adds.
Different vendors offer different formats; in some cases up to eight columns can be used, according to Peter Levison, senior marketing director for downstream processing at Pall Biotech. “Multicolumn chromatography has long been an accepted continuous purification practice by small-molecule manufacturers, but is just now gaining traction in the biomanufacturing space,” he says.
Continuous chromatography systems are designed to be loaded continuously, which is a useful approach when the purification step is connected to an upstream perfusion bioreactor. There are several different techniques available, including simulated moving bed (SMB) and periodic counter current chromatography (PCC). The aim is the same for all: to achieve a continuous (over) loading of the columns or membranes, and thereby improve the process efficiency through increased resin capacity utilization, according to Lotta Molander, senior product manager with GE Healthcare Life Sciences. “This approach also allows a chromatography step to be run with less safety margins than would normally apply in a traditional batch process without loss of product,” she says.
Some systems come with flow paths that are designed to be cleaned and reused, while others are flow paths that are single-use. Some systems come with dynamic control functionality—process analytical technology (PAT)—that enables the continuous chromatography unit operation to better adapt to changes in feed concentration (when connected to a bioreactor) or differences between columns in real-time, assuring steady state operations and a robust/stable output from the chromatography step, Molander notes.
Why should you move to continuous chromatography?
Regardless of the technical solution used for continuous chromatography, it can offer advantages compared to traditional batch chromatography through the improved utilization of resin capacity, according to Molander. “Data indicate that more than one column offers better efficiency, and greater yield of product per unit volume of chromatography resin per unit of time. That means that because the columns are smaller, less resin volume is used, and it can be used much more efficiently, thereby saving cost,” Levison states.
In turn, a decrease in the equipment footprint and floor space requirements in classified areas is possible, because unit operations can be downsized, particularly when two or more unit operations are connected in a fully or semi-continuous/hybrid production process, according to Molander. Buffer consumption is also reduced.
Manufacturers are always trying to optimize their processes, particularly as product titers increase, according to Levison. In the case of antibodies, the industry is now working well above 3 g/L, even up to 5 g/L, with titers of 10 g/L being described. “To really maximize productivity, the kinetic effects of adsorption:desorption of protein to the resin become important, and drive demand for more columns in order to operate at high flow rates while maximizing the productivity benefits of continuous chromatography,” Levison observes.
In the clinical manufacturing setting, Levison notes that users are looking for more efficient use of the resin lifetime by cycling more frequently compared to batch. “Clinical manufacturers are motivated to cycle as much as possible using fairly short residence times in order to use less resin early on when the future outcome for a given molecule is still unknown,” adds Najera. She points out that commercial manufacturers, on the other hand, may use a strategy where slightly longer residence times are used to reach high resin loading because the maximum number of cycles will eventually be reached for a commercialized molecule with steady demand for the drug. “The real benefit,” asserts Levison, “is overall greater productivity and improved process economics based on reduced resin requirements and reduced buffer usage.”
As a contract manufacturer that produces a significant amount of clinical material, AGC Biologics’ current goal with continuous chromatography is to more efficiently use resin so that less is required to make clinical material, according to Najera. “The cost for resin makes up a significant portion of the cost to manufacture clinical material. Resin reduction is achieved by cycling the resin more intensively than traditional chromatography,” she explains.
In addition to cost savings, Molander notes that the steady state operation that can be achieved with continuous chromatography can also give additional process robustness and control and lead to improved product quality levels, which can be particularly useful when processing unstable molecules.
Access to single-use continuous chromatography systems is also advantageous, according to Levison. “With the ability to operate at higher titers or cell densities, there is a drive to reduce bioreactor volume. If a manufacturer can reduce its process to a 2000-L bioreactor, or multiple 2000-L bioreactors, facilities become much easier to build with shorter construction times and reduced cost. In this case, single-use chromatography becomes a very desirable component of the entire single-use process,” he explains.
Importantly, according to Jim Sulzberger, process chromatography consultant with Bio-Rad Laboratories, the advantages of continuous chromatography—increased utilization of chromatography media binding capacity, significant reduction in buffer consumption, increased adoption of single-use buffer pathways and pre-packed large columns, decreased capital costs, and lower real estate costs—can be achieved using all of the same types of media originally developed for batch processes. He also notes that the advent of new membrane chromatography devices and chromatography media with hydrophilic base beads and high ligand densities will maximize the impact of continuous operations.
When should you move to continuous chromatography?
Traditional chromatography steps can create bottlenecks in the process because they can be time and labor intensive. Continuous multicolumn chromatography, according to Levison, overcomes this barrier and can be applied to pretty much any batch process. “The transition is not difficult. The manufacturer just needs the right equipment and expertise; the processes use the same buffers and resins as used in their current batch processes,” he says. “Ultimately, if you want to work faster, continuous multicolumn chromatography is an excellent solution for both small- and large-molecule processes,” he states.
Continuous chromatography can be an excellent choice when working with sensitive molecules, where continuously removing the molecule from a potentially harmful environment into the first purification step can be the only viable option, according to Molander. She adds that in scenarios where there is limited product demand (in kg), continuous chromatography using a higher number of cycles in the available time can allow significant reduction of the needed resin volume.
For Sulzberger, continuous chromatography should be of interest for all processes that start with a capture step and where there is a desire for straight through processing. “The premise behind continuous bioprocessing is a decrease in consumable costs and an increase in process productivity. Process development is the same in terms of selectivity, but there are changes in terms of engineering concerns surrounding efficiency and capacity. For current commercial processes, however, many continuous chromatography platforms have software that aids in converting a batch process to a continuous process,” he comments.
Protein A capture steps are the most common application for continuous chromatography at AGC Biologics. “The resin is expensive and therefore the potential cost of goods savings is substantial,” Najera observes.
When considering continuous chromatography for a new process, Molander does add that the benefits from continuous processing should be balanced with the risk of delayed regulatory process applications and thereby time to clinical trials and market introduction. “Even though regulatory agencies are very supportive and collaborative when it comes to companies introducing different process intensification initiatives, there is still, to my knowledge, no approved biopharmaceutical product with a continuous chromatography step, and some aspects/questions in the area that could still benefit from further clarifications,” she says.
Why you should not move to continuous chromatography
Not all chromatography steps are suitable for continuous processing. For instance, when a manufacturer is using a chromatography adsorbent that requires a longer contact time between the ligand and ligate to get effective interaction, continuous multicolumn chromatography may not be the best option, according to Levison. He points to some affinity processes that need to have extended contact times and need to run slowly; the efficiencies of continuous chromatography would not be realized in these cases. Switching from batch to continuous chromatography would also not make sense for processes that are minimal or only run once per year.
Continuous chromatography is not recommended when a chromatography step is not well understood, is expected to experience significant variability, or may be modified later, according to Najera. “One thing we like to remind ourselves is that we should fully understand all aspects of a process step run in batch mode before it is modified to a continuous step,” she notes.
For companies with a large installed base of equipment and facilities plus well-established platforms and infrastructure, GE Healthcare sees an increased interest in addressing process efficiency initiatives in different ways,” according to Molander. “The producers want to maximize the use of existing resources with few changes to standard operating procedures, rather than invest in new equipment for continuous chromatography,” she says.
Contract manufacturers face other issues. AGC Biologics often has less ability to make modifications toward continuous chromatography for commercial processes, according to Najera. “Modifications to commercial processes may require additional regulatory filing activities. Making the switch to continuous chromatography at this stage is therefore less appealing for our clients,” she observes.
Analytical and regulatory questions
Outside of increasing early adopters of continuous chromatography, there are two improvements that need to be addressed to keep continuous processing on the path to become the new purification paradigm, according to Sulzberger.
The first issue relates to sensor and PAT for enhanced process -monitoring and real-time release of the product to next-unit operation, according to Molander. “PAT and analytical methods that allow a process to continue running while providing feedback on critical quality attributes of a particular purification step have not evolved at the same rate as their process science counterparts,” Sulzberger says. Since PAT is not yet a viable reliable option, he notes that how often and when to have hold steps to perform quality checks is a required discussion.
Reporting and analysis tools for time-based data (i.e., chromatograms) need some improvements as well, according to Najera. “We are going from chromatograms for just a few cycles to often over 100 eluate peaks per continuous sublot. There needs to be a way to analyze and report these data seamlessly, as well as detect adverse trends in performance,” she explains.
Regulatory aspects are the second area of concern. Here, Levison believes that advances can be made as more data are generated. “In this instance, there is a great benefit of starting from the process development step to document scalability,” he notes.
The discussion topic for Sulzberger focuses on whether there is sufficient guidance from regulatory agencies on how a ‘batch’ is defined. “If there is a truly continuous process, when are lot numbers assigned during the process? If there is ever a recall of a drug product, how large of a batch would need to be recalled given lot definition and how far back in the process should the proper investigation be extended?” he asks.
“Collaboration between industry leaders, early adopters of continuous chromatography and continuous processing in general, and regulatory agencies will be paramount in appropriately defining such guidelines and requirements,” Sulzberger asserts.
Finally, Molander notes that the industry is in the early days of adoption of connected processing techniques in the biomanufacturing workflow. “More modular, plug-and-play equipment designs that can fit in seamlessly to different automation backbones would help accelerate that transition,” she observes.
As with all new technologies, Sulzberger notes that there will be a learning curve, with some scientists/companies quick to adopt continuous chromatography. “Early adopters of continuous chromatography will see more efficient production, and by extension lower cost of goods for their final products. Additionally, they will be able to construct smaller production sites due to the reduction of skids and columns required to produce a given number of batches of a particular pharmaceutical product,” he says.
“It comes down to adoption,” agrees Levison. “Open access to real-world experimental data that demonstrate the efficiencies of continuous multicolumn chromatography is essential. As more large-scale applications get published, there will be more demonstrations of scalability, which will advance adoption,” he concludes.