Bioprocess Filtration: Debating Post-sterilization Integrity Testing

At industry trade shows, some of the best discussions happen far from the din of the main conference hall. At the PDA 2011 annual meeting in San Antonio, a case in point was the gathering of PDA’s Filtration Interest Group in a remote room of the J.W. Marriott resort.

The session was hosted by consultant Russell Madsen, president of the Williamsburg Group. There were several speakers on the docket (see below) and a few dozen interested parties in the audience. The most heated discussion related to a stipulation in EMA’s “Annex 1: Manufacture of Sterile Medicinal Products,” regarding the integrity testing of bioprocess filters, post-sterilization and pre-use.

Annex 1 was revised in 2008, with Paragraph 113 reading as follows:

The integrity of the sterilised filter should be verified before use and should be confirmed immediately after use by an appropriate method such as a bubble point, diffusive flow or pressure hold test. The time taken to filter a known volume of bulk solution and the pressure difference to be used across the filter should be determined during validation and any significant differences from this during routine manufacturing should be noted and investigated. Results of these checks should be included in the batch record. . . .

At issue, Madsen noted, was whether or not the act of verifying a filter’s integrity after it has been sterilized might do more harm than good. “You have to look at it on a risk basis,” Madsen said. If the manufacturer has validated the sterilization process for the filter train, and can show that a given process does not damage a filter, then that should suffice. “If someday you did damage the filter, you would pick it up in your post-filtration integrity testing,” he added. “The risk of damaging the process train is greater . . . it doesn’t make sense to me to do a post-sterilization integrity test.”

Michael Moussourakis, filtration expert at Pall Corp., chimed in: There’s a mistaken idea, he said, that a filter can be damaged or its pore size might change during the filtration process as a result, for example, of being plugged with bacteria.

Flawed Hypothesis
Some “early literature,” Madsen noted, suggested that steam sterilization may “relax” the filter and perhaps change pore structure. But this has proven unfounded for current biopharma sterilizing filters, he noted.

The filters referred to in the literature “are never validated as sterilizing filters,” added Jerold Martin, Pall’s senior VP for Global Scientific Affairs, Biopharmaceuticals, and current chair of the Bio-Process Systems Alliance. “So none of the sterilizing filters available on the market do this thing that [EMA is] afraid might happen.”

“The requirement to do this is not based on science,” Martin added. “It’s based on a flawed hypothesis.”

“Integrity test, autoclave, then integrity test again, and you’ll find that it’s not an issue,” added Madsen.

 “It’s worse to bring a human into the process to do filter integrity testing,” said Ross Acucena, global marketing manager for EMD Millipore. Acucena noted that he sometimes recommends to customers to monitor process flow upstream of the filter. “If there was a significant drop [in pressure], it would should a compromise of filter integrity,” he said.  

Another challenge for manufacturers, Martin noted, is the inconsistency with which the regulation has been enforced by the various regional and national authorities within EMA. “It’s not universally enforced throughout Europe,” he said. Some inspectors are rigid, while others can be reasoned with. One company using gamma-irradiated rather than autoclaved filters, for instance, was able to convince inspectors that post-sterilization testing was not required, Martin said.

For global manufacturers, the issue has ripple effects on how and whether post-sterilization integrity testing is performed. At the time of the conference in mid-April, representatives of PDA had planned to sit down with EMA soon to discuss a potential change in policy.

Surface Tension
Also on the agenda, Pall’s Moussourakis spoke on studies his company has done to measure the correlation between filter surface pressure and bacterial penetration. “When we talk about penetration, folks get worried, though bacterial penetration is very rare,” Moussourakis noted. Nevertheless, “the whole reason for doing a validation program is that we know it can happen on occasion.”

Pall looked at the past six years of data that it had from performing validations for clients. Roughly three percent of those validations showed significant penetration, Moussourakis said. However, he continued, there is a correlation between low surface tension and penetration. And products associated with low surface tension tend to be “lipid and lipid-like,” and some vaccine formulations therefore fall into these higher risk categories.

Pall tested a variety of its own and competitors’ filters, and Moussourakis offered the following suggestions for addressing the surface tension issue. Consider, he said:

• a tighter filter (0.1 µm if possible, he said)
• serial (bioburden reduction) filtration
• double sterilizing grade filtration
• filtration prior to product formulation

There’s no one solution for every application, he said, so manufacturers and filter providers must work together to thoroughly evaluate a given product and process, and find a filter that works best.

Next, Mandar Dixit, director of product management, filtration technologies, for Sartorius Stedim North America, spoke about studies his company has done to compare “classical” and contemporary membranes. There is a dizzying array of membrane possibilities on the market, he acknowledged. In short, PES and PES/PVDF combination membranes exhibit higher throughputs compared to classical PVDF or CA membranes. It is essential for end users need to run thorough filter optimization trials with membranes from various manufacturers to ultimately settle on those that are best.

Another issue Dixit addressed was the impact that new filter media may have on the success of filtration efforts. Sartorius is partnering with SAFC Global (a media provider) to study the topic. “Even though cell culture scientists are making advances in cell growth, it may affect filterability,” he noted, In other words, he said, end users must consider the suitability of their current filter types when they introduce new cell culture media.

Finally, EMD Millipore’s Acucena spoke about the tendency of regulatory agencies to be increasingly stringent in regards to extractables and leachables, noting recent FDA Warning Letters have cited manufacturers for failing to perform adequate E/L studies for filters.

Roundup: New on the Filter Front
What follows are some of the new technologies on the market related to bioprocess filtration:

Sartorius Stedim North America has introduced the Sartocheck 4 plus, what it considers its most advanced filter integrity tester. The product has been designed for robustness and ease of use, the company says. The machine has an optional barcode scanner for data entry. After scanning a filter cartridge, only test program relevant to the particular scanned cartridge are displayed. The product is well suited to contract manufacturing, the company says, in that it allows efficient testing of various filters.