Continuous bioprocessing is beginning to overtake traditional batch processing in popularity as new, supportive technologies enter the marketplace. In a continuous bioprocess, unit operations are integrated into a single flow. Currently, a majority of the industry manufactures products in batch mode, isolating each operation from the next. However, hold steps between operations are inefficient, and industry is trending toward continuous bioprocessing for more efficiency and greater yield of product. Additional drivers include better process control, improved process robustness, and reduced operational and capital expenditures (OPEX/CAPEX). A key technology in this transition is single-use technology.
Single-use technology
A variety of bioprocessing units and components are now available in single-use, disposable form, including bioreactors. The main advantage of single-use technology is that it does not require extensive cleaning and sterilization, but is rather discarded at the end of the process run. In the case of bioreactors, single-use perfusion reactors have become available and have shown advantages for continuous bioprocessing. Single-use technology is highly adaptive and much more modular and portable than reusable batch equipment, resulting in a great deal of flexibility in designing the process, as well as simple and rapid product changeover.
The availability of this technology has encouraged many manufacturers to take the plunge into continuous bioprocessing, particularly on the upstream side. Methods for continuous upstream bioprocessing have now been developed and put into use for a range of molecules.
Continuous downstream processing remains a somewhat more aspirational goal, due to challenges such as integrating multiple chromatographic columns and scaling up to commercial quantities of product.
Process intensification
Process intensification is seen as a practical intermediate step between batch processing and continuous bioprocessing. In process intensification, a manufacturer reduces process steps and implements fewer holds in between steps. One example of process intensification comes from Sanofi. It’s called accelerated seamless antibody purification and is based on three chromatography steps: protein A capture, cation exchange or mixed-mode ligand, and flow-through anion-exchange. This process is differentiated from batch processing in that all columns are run continuously without holds or adjustments. Eluate from each column flows directly onto the next column. The intensified process supports the use of single-use bioreactors by enabling the downstream process to keep up with the reactor’s daily output.
There are some challenges when it comes to implementing a continuous bioprocess. The reliability of the process must be confirmed when compared to the batch process.
Novartis is another company that has worked on intensifying its processes toward the goal of a continuous bioprocess. Gerben Zijlstra, platform marketing manager at Sartorius Stedim Biotech, says that Novartis was able to achieve the same output from a Sartorius single-use 1000 L bioreactor as from a conventional stainless steel 14,000 L reactor using process intensification. “You start with intensifying individual unit operations, as Novartis has been doing. The next step is to integrate certain unit operations to combine them in a connected way,” says Zijlstra.
Continuous chromatography
For downstream continuous bioprocessing, it is usually necessary to integrate multiple chromatographic separations. Bio-Rad is one company working to meet the need for resins amenable to a continuous workflow. It offers rigid polymer-based resins with large, open pore structures that are critical properties for performance in a continuous chromatography process. Continuous chromatography provides improved productivity versus batch processing based on increased dynamic binding capacity at a high flow rate through the use of multiple columns in a series, according to Hana Kim, senior global product manager of process chromatography at Bio-Rad.
“Thus the limiting factor for productivity can become pressure drop across multiple columns during the loading stage,” she explains. “The rigidity of the resins in continuous chromatography provides favorable pressure-flow properties. Resins with open and large pore structure provide excellent mass transfer kinetics enabling excellent dynamic binding capacity at a high flow rate, which is beneficial in maximizing productivity from continuous chromatography process.”
Sartorius also makes use of chromatography resins with larger pores, called membrane adsorbers. Typical chromatography resins are small beads with a bound ligand. The product diffuses into the pores of the beads and binds to the ligand. Diffusion determines the rate of product or contaminant biding to the resin when the product flow passes a column packed with resin beads. Meanwhile, unwanted molecules flow through. The product is then eluted using a buffer that releases the product from the ligand. Membrane adsorbers operate on a similar principle, but their larger pores allow more efficient binding of product to ligand, resulting in a quicker process. “It takes a long time for product to diffuse into the small pores. Membrane adsorbers can do the same as resins, but they perform this action much more quickly,” Zijlstra says. “That’s why we see that membrane adsorbers are starting to compete with resins for certain applications.”
Challenges
There are some challenges when it comes to implementing a continuous bioprocess. The reliability of the process must be confirmed when compared to the batch process. Continuous bioprocesses requires additional monitoring and process control because media and other materials are being added and wastes removed. Contamination is also a greater risk with continuous bioprocessing. Lastly, the complexity of the overall process is increased when it is adapted to a continuous format.
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According to Zijlstra, a major challenge in continuous bioprocessing is connecting all of the unit operations into one flow. “Let’s say in step two there is an issue, that means that this will translate all the way to the back of the process,” Zijlstra says. “Because the process is interconnected, if you don’t compensate for or correct variations in the beginning of the process, they can be catastrophic for yield or product quality in the end.” In contrast, with a batch operation, samples are taken after each step, and if there’s a problem, the next step can be adjusted based on the test results. That opportunity is not available in a single continuous flow.
Atul Mohindra heads the mammalian process research and technology team for Lonza, which incorporates elements of continuous bioprocessing as part of its service offerings within mammalian manufacturing. He says that GMP regulations can be an additional, hidden hurdle in implementing continuous bioprocessing. “In GMP, upstream and downstream suites have different room classifications whereby the level of gowning and air handling is different. We do have to take into account moving from one room to another room. Therefore, we need to think smartly in order to limit the number of interruptions to the continuous flow of materials,” Mohindra adds.
Virus filtration and inactivation
Viral contaminants are the biggest quality risk to biological products, and they can be very dangerous for patients. In batch processing, viruses are removed by filtration, and this process is still based on and validated using constant pressure and a decaying flow through the filter, according to Zijlstra. “Working with constant flow and therefore changing pressure drop across viral filters is something that needs to be further developed,” explains Zijlstra. He highlights continuous concentration and diafiltration as another area of technology needing updates for continuous bioprocessing.
Trends toward continuous bioprocessing are gaining momentum in the biopharmaceutical industry, supported by key technologies like single-use equipment, perfusion reactors, and newer chromatography resins. Continuing advances in areas like virus filtration and inactivation, automation, and process design will enable fully continuous end-to-end bioprocesses in the future. Meanwhile, many companies are taking steps in that direction through process intensification and retrofitting of established fixed stainless-steel based facilities.
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