March 15, 2017 (Vol. 37, No. 6)
Continuous bioprocessing may be defined the same way it is often implemented—in stages. The stage-by-stage approach to a full definition was demonstrated at the 2014 International Symposium on Continuous Manufacturing of Pharmaceuticals, where a landmark paper was delivered by Charles L. Cooney, Ph.D., and Konstantin B. Konstantinov, Ph.D. These presenters began by describing unit operations, and they worked their way up to a fully integrated, final system:
“A unit operation is continuous if it is capable of processing a continuous flow input for prolonged periods of time. A continuous unit operation has minimal internal hold volume. The output can be continuous or discretized in small packets produced in a cyclic manner.
Dr. Cooney, a professor of chemical engineering at MIT, and Dr. Konstantinov, then a vice president of late-stage process development at Genzyme, proceeded to build on this idea. “A process is continuous if it is composed of integrated (physically connected) continuous unit operations with zero or minimal hold volume in between. To emphasize that all the unit operations are continuous and integrated, such processes are also referred to as fully continuous or end-to-end continuous.”
The presenters not only also distinguished between continuity in upstream and downstream operations, but they also described hybrid operations: “A process is hybrid if it is composed of both batch and continuous unit operations.”
The previous year, at the bioProcessUK conference in London, Dr. Konstantinov (who later became senior vice president of manufacturing and process sciences at Codiak Biosciences) addressed a plenary session. Dr. Konstantinov’s lecture was summarized in a paper, “Continuous Bioprocessing: The Real Thing This Time?” that was later published in the journal Mabs.
In his talk, Dr. Konstantinov suggested that despite its initial fits and starts, continuous bioprocessing was poised to take bioprocessing by storm, as it had in many low-technology process industries. Yet at the 2016 bioProcessUK meeting, there were just two talks on continuous bioprocessing, one an award lecture.
While progress in the continuous manufacture of small-molecule drugs continues (albeit slowly), the FDA has issued a smattering of decrees and guidance, but not much in the way of direction on continuously manufactured biotherapeutics.
Treading Water Upstream
GEN has extensively covered innovations and adaptations of perfusion cell culture. The systems are simple in concept: As Dr. Cooney has noted, all that is required is a cell-retention system that allows the product to leave while keeping cells in place. Leading bioprocess vendors offer some variations on this theme:
- Pall’s iCELLis single-use, fixed-bed, bioreactors use microcarriers to immobilize continuously perfused cells.
- GE Healthcare’s Wave bioreactors are adaptable to batch and perfusion culture.
- 3D Biotek’s 3D Insert, a small-scale cell-culture system, is based on constantly perfused porous 3D-polymer scaffolds.
Then there’s Repligen’s XCell ATF, a cell-retention device suitable for perfusion cultures. By continuously removing waste products from the fermentor, the alternating tangential flow filtration (ATF) system achieves cell densities to two or three times higher than those reached in conventional batch fermentation. All things being equal, higher cell density equals higher volumetric yield, which improves facility utilization and reduces the size of the bioreactor required to manufacture a given volume of biologic drug product.
With the recent (October 2016) introduction of a single-use version, the XCell ATF is now available in both stainless-steel (reusable stainless housing, replaceable filter) and single-use (single-use housing/filter combination) configurations.
“The availability of XCell ATF in a single-use format eliminates the pre-use workflow associated with autoclaving, leading to an 80% reduction in implementation time,” says Christine Gebski, Repligen’s vice president for product management and applications.
Hand in Hand
The potential advantages of continuous bioprocessing sound as if they were lifted from a marketing sheet for single-use equipment: reductions in facility size, utility requirements, and capital expenditure for equipment and maintenance of capital equipment.
“As ancillary equipment associated with preparation and cleaning of reusable (as opposed to single-use) equipment becomes vestigial, facility size can be reduced, and the equipment and utilities required for support can be minimized,” Gebski adds. “Process areas can even be modularized and made more portable.”
Imagine, then, how a continuous, single-use, modularized process could be described. “The equipment required to support a given unit operation can reside in a given module, and the module becomes self-sufficient,” Gebski suggests. “For example, if you reduce the complexity of your home and use standard equipment, a prefab house can be built at point A and fabricated/installed at point B in short order. Single-use equipment enables this same concept for bioprocessing.”
The complexity of continuous processing demands a level of sensing, monitoring, and control unknown in batch operations. Real-time release in continuous biomanufacturing demands real-time online quality monitoring—a higher order of process analytics—to assure that data captured at one point can be aligned with the end result.
Drawing on its experience in nonpharmaceutical process industries, Siemens offers the Simatic PCS 7 system, which controls and monitors both continuous and batch processes. Simatic PCS 7 is scalable, covering basic control functions in a laboratory environment up to advanced process control at manufacturing scale. The package provides virtual commissioning and process simulation while monitoring plant performance.
Siemens also offers Sipat, which is a process analytic technology (PAT)-oriented software solution for quality monitoring.
Combined, Sipat and Simatic PCS 7 provide control and oversight for continuous bioprocessing, explains Pamela Docherty, life sciences industry manager, Siemens Industry US. “Continuous operation of purification systems can, for instance, be achieved by cascading various chromatography, filtration, and purification skids, each with its dedicated control platform. PCS 7 orchestrates the functionality of these individual units to behave like a production line.”
Sipat incorporates drivers for most third-party spectral analyzers, including near-infrared process sensors (from Mettler, Bruker, and Thermo Fisher Scientific), Raman sensors (from Kaiser), and high-performance liquid chromatography detectors.
Siemens has been updating and expanding its control software drivers since getting involved with the continuous manufacturing of solid dosage forms in 2008. Its expertise in continuous biologics goes back only about five years.
Scale up is as challenging with process controls as it is with cell culture, chromatography, and other unit operations. Here, the issue is not mass or heat transfer, feed volumes, or dilution, but rather the shortcomings of standard laboratory practice. Software control is not often used at bench-scale. Operators instead take measurements and turn knobs manually, developing the process with specialty or one-off tools, without anticipating the eventual transition to commercial manufacturing scale.
The complications that may arise from manual control constitute another argument for scalability. To avoid these outcomes, try implementing automated control at the earliest stages of process development. “Even in a laboratory-scale continuous process,” notes Docherty, “an unmanageable amount of manual interactions would be needed in case of a disturbance.”
In with the Old
Conventional chromatography resins are routinely adapted for use in continuous purification. For example, GE Healthcare has shown how its MabSelect SuRe LX protein A resin may serve as the medium for a three-column periodic countercurrent chromatography (PCC) operation. This particular operation is based on the company’s ÄKTA platform.
In PCC, columns are switched between loading and nonloading steps consisting of column wash, elution, clean-in-place, and equilibration. At breakthrough, the first column in the loading zone is disconnected. Then, the load is redirected to the next column.
Similarly, Thermo Fisher Scientific has been pushing its Poros capture and ion-exchange chromatography media in several continuous chromatography environments. “Perfusion and alternating tangential filtration have unlocked significant productivity improvements upstream,” asserts Kevin Tolley, the company’s senior field application specialist, “but downstream operations remain stuck in a batch mode and have difficulty keeping up.”
Thermo Fisher Scientific has not been developing hardware or software for continuous processing on its own, preferring to collaborate with companies possessing those specialized skills. Tolley believes that Poros resins have a lot to contribute, however, by enabling shorter bed heights while maintaining high binding capacities.
In the absence of a chromatography resin specifically dedicated to continuous processing, advances in continuous chromatography will arise from the marriage of innovative hardware and the application (as we saw with Siemens) of control software.
Banking on Hardware
Bioprocesses traditionally divide cleanly into upstream and downstream components. An analogous delineation exists for continuous processing. Thanks to perfusion cell culture and its many variants, upstream continuous operation is feasible at large scale. Downstream purification steps, however, have yet to catch up.
To upgrade downstream processing, LEWA Bioprocess Technologies, a subsidiary of LEWA (itself a subsidiary, of Nikkiso), entered into a collaboration with ChromaCon in 2014. LEWA Bioprocess Technologies is working on the all-important capture step and eventually expects to tackle polishing chromatography operations in continuous formats.
Continuous chromatography will likely not be adopted for established processes, but LEWA Bioprocess Technologies’ chief marketing officer Gerard Gach believes that “most” developers of new protein manufacturing processes, including biosimilars, are considering continuous chromatography to save time and cut costs.
“Biomanufacturers are taking a pragmatic approach,” he explains. “They’re looking to incorporate one continuous step and get that going reliably, then a second step, then linking unit operations together.”
Product capture is the most attractive entry point for continuous downstream processing because of the high volume/value of the product at that stage, and the fact that conventional antibody purification employs protein A, which is arguably the most expensive “reagent” in the purification train.
“Decreasing the volume of protein A used, and addressing potential debottlenecking,” notes Gach, “will significantly improve process economics.”
He is referring here to the capacity mismatch that sometimes results from purification not maintaining parity with protein expression.
Gach describes his firm’s continuous chromatography team as “hardware nerds,” meaning they operate for the most part behind the scenes on the hardware side of the business instead of by supplying chromatography resins.
“We have a great corporate heritage of excellence in fluid dynamics and applying those highly precise fluid systems at GMP scale,” boasts Gach. “Our history is as a metering and pump company.”
Gach has a point. Approximately 14,000 LEWA pumps (by his estimation) are in use today in chromatography systems. He believes that on that basis alone, plus recent acquisition of GMP technology, LEWA has already produced chromatography systems that improve productivity by a few percentage points, even without continuous processing. This was achieved, Gach says, by applying digital control to chromatography system operation.
In June 2016, LEWA installed a scaled-up version of ChromaCon’s Contichrom Cube system at the Fraunhofer Institute in Stuttgart, Germany. The system was incorporated into a twin-column GMP-grade purification skid for continuous pilot-scale chromatography. According to Gach, the twin-column unit’s design significantly reduces system complexity.
“Other companies use up to eight columns, which may involve the incorporation of up to 300 valves,” states Gach. “The two-column system uses just 20% of the valves, reducing complexity and risk by 80%.”
LEWA also sells the EcoPrime Chromatography System, which doubles as a buffer dilution system.