Ask half a dozen filtration experts about the latest trend and most will respond with one word: disposables. Single-use filters first caught on in laboratories, then they were used for buffers and other “clean” process fluids, followed by general use at pilot scale. Today, processors tend to specify disposable filtration wherever feasible. The benefits of disposable processing in general apply to filtration as well: reduced validation burden, lower hardware costs, and elimination of cleaning.
Robert Conway, Ph.D., a consultant at Bioprocess Technology Consultants, notes that completely disposable filtration systems, which are established during late downstream sterilization, have recently been moving upstream into clarification and depth filtration. But ultrafiltration remains a challenge for single-use filters.
“Because of issues related to materials of construction and the challenges of ultrafiltration, which is done either with cassettes or hollow fibers, the move toward complete disposable systems in this area has been much slower than for sterilization,” comments Dr. Conway.
Disposable filtration is also establishing itself in specialty applications, e.g., for viral clearance and in membrane adsorption. Bioprocessors increasingly add a filtration step to achieve viral clearance above and beyond what is anticipated from other separations. Membrane adsorption, a hybrid of filtration and chromatography, is routinely applied to viral clearance, removal of DNA during polishing, and virus capture for gene therapy and vaccine work.
“Capacities remain challenging with membranes,” Dr. Conway notes. “But for some applications they compete quite well with gel- or bead-based chromatography resins by providing higher flow. They are also cost-competitive and since they are mostly disposable, they require no cleaning.”
According to Paul Miraglia, director of biotechnology at Integrated Project Services (IPS), disposables are taking over in applications where they have not traditionally been used. “This includes large filters for polishing bioreactor broth that has been clarified with centrifugation,” he says. “Millipore’s POD system is filling this niche quite well.”
Miraglia observes that the trickle of ultrafiltration and nanofiltration products now available as fully disposable flow-paths offers advantages similar to those for other single-use equipment including segregation and a reduction in batch-to-batch or product-to-product cross-contamination. But single-use filtration products will probably not work at large (10–20 kL) scales. “Cartridge filters just don’t have that kind of capacity,” notes Miraglia.
That is not to say that disposable filtration is limited to low volumes. CUNO is developing what it claims will be the first user-friendly, ergonomically designed, disposable lenticular depth filter in a fully encapsulated format.
Because of their size and how they are connected to the process stream, lenticular filters today use disposable or multiple-use media supported within stainless steel holders. Filters are stacked in the same orientation in which they are used, vertically, which makes loading difficult for some operators. The new line will be stacked horizontally at waist height, then rotated into position. When set up, it will utilize the same flow path as with current depth filters, but have the convenience of added ergonomics and complete disposability.
The new products, which will be introduced during the first quarter of 2009, will be part of CUNO’s Zeta Plus™ EXT line of depth filters. Current Zeta Plus EXT filters are supplied with two types of media that perform complementary purifications during post-cell culture harvest. Composed of cellulose and a positively charged resin, EXT SP is an open-grade medium that serves as an upstream or prefilter that can replace centrifugation for small (< 200 L) batches. EXT ZA media are of tighter grade and hold a strongly positive resin for removal of host cell proteins and DNA.
The new filters will be offered in 11 SP grades and 6 ZA grades. They will be self-contained, in single-use cartridges that will eliminate the need for a stainless-steel dome housing and the associated cleaning and cleaning validation between batches.
During the 1990s most vendors of filtration products offered a small number of sterilizing-grade, 0.2-micron membrane filters that the companies positioned for every type of sterile filtration. Over the last five years, bioprocess streams were becoming larger and more complex, and multi-gram-per-liter product titers were just entering the mainstream.
Soy-based cell culture media entering production, notes Jerry Martin, senior vp for scientific affairs at Pall, posed significantly more difficulty for large-scale filtration than serum-based media. Buffer volumes rose as well. Maintaining the same footprint for a process containing two or three times the product, and dozens of additional feed and nutritional constituents, demanded that manufacturing, particularly downstream operations, become more productive.
“Around this time, biotech companies that were ramping up processes to larger scales were seeking to economize on filtration for chromatography and diafiltration buffers,” notes Martin.
Downstream processing of vaccines, a surging biotech sector, presents even more challenges, particularly when the products are formulated with lipids. “There is something about lipid-containing formulations,” says Martin, “that can compromise the ability of some sterilizing-grade 0.2-micron filters to completely retain the high levels of bacteria required for validation of sterilizing filtration.”
Pall has transformed its product line in response to these challenges, augmenting its line of sterilizing-grade 0.2-micron membrane filters with hybrid products that address specific process requirements.
Fluorodyne® EX hybrid double-layer membrane filter cartridges, for example, utilize Pall’s Ultipleat® technology, a design that imparts stability and permits up to twice as much membrane to fit into a given volume. In the hybrid EDF-grade product, the highly asymmetric 0.2-micron polyethersulfone (PES) upstream membrane layer removes particles and debris, for example, during cell culture harvest or vaccine production. The downstream layer is a 0.2-micron PVDF sterilizing membrane for retaining bacteria. “Fluorodyne EX filters exploit the best properties of the two polymers,” adds Martin.
When serum-based media ruled, drug manufacturers dealt with mycoplasma contamination with 0.1-micron rated membranes. Bioprocessors erroneously believed that serum-free soy-based media did not harbor mycoplasma. This misconception was recognized as soon as the mycobacterium Acholoplasma laidlawii, traced back to cell culture media, began showing up during filling line sterility validations. Thus, it became common to use 0.1-micron membranes for aseptic filling validation, as was the practice with mammalian cell culture media.
But at this pore size, membranes can quickly clog due to the high particle load of soy-based preparations. For these applications, Pall plans to introduce a new member of the Fluorodyne EX product line—the hybrid EDT filter—in January 2009.
EDT-grade construction resembles that of EDF, but with two 0.1-micron downstream PVDF layers instead of one 0.2-micron layer. The upstream PES layer removes particles while the downstream PVDF membranes eliminate the mycoplasma with greater than 10 LRV efficiency.
Because filtration represents a significant cost in downstream bioprocessing, companies now pay extra attention to optimizing each filtration step. The secret to greater economy, says Martin, is not to cut corners with less-expensive general-purpose sterilizing-grade filters, but to improve performance specifying filters dedicated to a particular task. “These may appear more expensive on the surface, but in the long run they can be more cost-efficient.”
Earlier this year, Sartorius Stedim Biotech launched new versions of its high-performance Sartopore 2 polyethersulfone (PES) membrane filter cartridge. Sartopore 2 XLG and Sartopore 2 XLI are the company’s first-generation filter cartridges that follow an application-focused development approach.
“With its superior throughput, the XLG is ideal for sterile-filtering plant-derived media and bioprocess intermediates,” says Maik Jornitz, group vp at Sartorius Stedim Biotech.
In addition to its prefilter layer/membrane pore size design, the XLG has low nonspecific adsorption, which can enhance yield, explains Jornitz, adding that the Sartopore 2 XLG will replace the Sartopore 2 is some applications, especially on the upstream side, due to its far higher capacity. Sartopore 2 remains in the Sartorius Stedim Biotech product portfolio to serve especially formulation/fill applications.
The Sartopore 2 XLI is designed for filtration of complex fluids in the ophthalmic products industry, and for large-volume parenteral drugs.
Sartorius Stedim Biotech is also planning to introduce advanced versions of its Sartobran P product portfolio to support vaccine manufacturing. A cellulose acetate membrane filter, Sartobran P is Sartorius Stedim Biotech’s flagship product for vaccine filtration.
“The very low nonspecific adsorption of cellulose acetate membranes enables some of the highest yield recoveries for sterilizing-grade membrane filters,” notes Jornitz. Sartobran P’s heterogeneous double-layer membrane design provides high throughput avoiding premature filter clogging, which often results in yield loses, he points out.
No Preparation Required
Through its ReadyToProcess™ (RTP) initiative, GE Healthcare improves multiple-use products by transforming them into a single-use, preconditioned, ready-to-use format. The idea is to eliminate for the end-user the flushing, sanitization, cleaning, and related validation, which Gerard Gach, director of marketing for biotechnology, describes as “wasted time” because they do not add value to core activities like making drugs. “All prep steps do is consume labor, assets, and consumables.”
As their name implies, RTP products are sold ready-to-use out of the box, for single processes or campaigns, thereby addressing needs of speed, simplicity, intuitiveness, safety, and less risk.
GE recently teamed with Novavax on a manufacturing platform that includes preconditioned hollow-fiber filter assemblies, which significantly reduced the time and cost associated with setting up a vaccine manufacturing facility. The filters, which are sold under the ReadyToProcess Hollow Fiber brand, address the need in vaccines for aseptic processing. GE will be introducing hollow-fiber products that have an even higher validation claim for sterility some time in 2009.
Hollow-fiber filtration is, of course, not new, and many users already employ them as de facto disposables. The new RTP line will reflect the added need for speed and reliability, since the membranes come preconditioned (factory flushed with water-for-injection and gamma-irradiated), and generally require no preparation or user validation. Typical applications include ultrafiltration/diafiltration, concentration, fractionation, and clarification. Both ultrafiltration and microfiltration cut-offs are available as ReadyToProcess.
GE is also re-casting its normal-flow filters, particularly its 0.2-micron sterilizing-grade products, in RTP format and increasing their capacity for suitability to large-scale bioprocessing. The ULTA™ line of capsules will incorporate a prefilter plus the sterilizing-grade membrane in one unified RTP kit.
Similarly, the company has developed a line of bioburden-reduction filters, the ULTA-CG line, for applications where sterilizing-grade is overkill, for example right before a chromatography step. “There are many situations where the performance and cost of a 0.2-micron filter is unnecessary, where five-log reduction is enough,” observes Gach.
Recognizing that some users may be unfamiliar with the intricacies of modern filtration, particularly when using new products, GE now offers a free, online filtration-optimization service. The website, www.filterbrain.com, allows users to optimize their filtration selection process in about 15 minutes.
A lot has been written about platform process operations, which seek to standardize a company’s manufacturing. Companies are particularly interested in template monoclonal antibody purifications, particularly for early clinical trial manufacturing. Given the advances in upstream productivity, companies are facing bottlenecks during purification.
“Customers are increasingly concerned about cost of ownership and are constantly looking to improve efficiencies by improving yield, reducing CIP/SIP requirements, and minimizing equipment downtime,” says Mani Krishnan, director of downstream processing at Millipore.
Millipore’s 2006 acquisition of Newport Biosystems provided access to the disposable, integrated Mobius product line that incorporates filtration, containers, and connectors. The company’s disposable Millistak+ POD depth filters can “significantly improve” overall cost of ownership for primary and/or secondary clarification steps, according to Krishnan. The company has also launched the Express PES (polyethersulfone) sterilizing-grade filters that deliver high flow rate and capacity, high microorganism retention, and broad pH compatibility.