Considerations on Re-Use of Sterilizing-Grade Filters - Pharmaceutical Technology

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Considerations on Re-Use of Sterilizing-Grade Filters
The author examines re-use of hydrophilic- or hydrophobic-membrane sterilizing-grade filters in liquid sterilizing applications.


Pharmaceutical Technology


FDA Enforcement. FDA has actively enforced its guidance on re-use of filters under GMP. An FDA Warning Letter to a sterile pharmaceutical manufacturing facility has stated, "Failure to validate the extended re-use period of the (redacted) filters used for many different injectable product formulations and batches. Validation rinse data was inadequate to support that ingredient residues from the previous batch are removed... Your response (to the 483) failed to ensure that products (redacted) do not contain unacceptable residues from prior batches of different products that used the same filters. The inspection found inadequate validation rinse data to support that drug residue from the previous batch was removed. We are concerned of the possibility of cross-contamination" (7).

Another FDA Warning Letter to a sterile ophthalmics manufacturer stated, "According to the establishment inspection report you re-use sterilizing filters as long as they perform to the manufacturer's filter integrity standards of (redacted) or for a maximum of your internal specification of 50 re-uses. We are concerned about the effectiveness of your filters after being re-used and autoclaved 50 times. We understand that you conduct a filter integrity test by the (redacted) method before and after each batch. However, in order to justify 50 re-uses, we would like to see bacterial retention validation studies using product both upon the initial use of the filter and after the 50th re-use. Further, it is unclear to us whether you have conducted filter extractable and leachable testing with product. If you have this data, provide it to us. If not, let us know when you will be able to provide it to us" (8).

An FDA reviewer has stated publicly that reuse of sterilizing filters is "discouraged for sterile drug product," while simultaneously recognizing that sterilizing-grade filter re-use is "common for API production" (9). The reviewer recommended, "Microbial retention validation should incorporate multiple filtration and sterilization processes" and "correlate microbial retention validation, filter integrity testing following multiple sterilization/filtration cycles, and sterility of API/product" (10). FDA is now requesting that batch records indicate when sterile filters have been previously re-used, as stated in an October 2008 483 observation (10).

Validation of filter re-use

Following PDA and FDA recommendations, a multiphase bacterial retention study program is indicated for validation of sterilizing filtration with re-used filters. PDA Technical Report 26 (7, 8) and FDA's aseptic processing guidance (4) currently call for preliminary bioburden studies of product or process fluid to determine suitability of conducting bacterial retention studies on production filter membrane discs using either the standard bacterial challenge organism Brevundimonas diminuta (ATCC 19146) or a bioburden isolate under worst-case product and process conditions. These bacterial challenge tests should include multiple filter membrane lots (typically three) and "at least one of the three membrane lots used for the bacterial retention validation study should have a pre-study or pre-use physical integrity test value at or near the filter manufacturer's test specification" (i.e., representing the least retentive membrane) (6).

A study of this extent would be sufficient to demonstrate the filter membrane's capability to sterilize the drug product or process fluid on a single usage. The study would not, however, predict filter performance after multiple cleaning, resterilization, and re-use cycles. Bacterial challenge should be conducted on production filters subject to actual, or preferably, worst-case process conditions incorporating the full extent of multiple cleaning, rinsing, or resterilization and re-use cycles.

Re-use processes entailing multiple batches, either without inter-filter batch rinsing or with solvent rinsing between batches, can often be scaled down and modeled at the bench. More complex re-use processes, however, such as those entailing multiple cleaning and resterilization cycles, or drying between campaigns, are difficult to simulate in the laboratory with filter discs, capsules, or even cartridges. In such cases, it is preferable to supplement the single batch disc challenge tests with a series of bacterial challenges conducted on production filter cartridges that have been exposed to the full extent of actual process use, cleaning, resterilization, and re-use cycles. Typical production cartridges are appropriate for this second phase as "worst-case membrane" is assessed in the initial disc study and testing of used production cartridges serves to confirm process compatibility as measured by maintenance of bacterial retention properties. Such tests may be the only means to determine whether a filter's sterilizing properties remain unaffected by the multiple re-use process cycle conditions. Integrity tests may be insufficient as demonstrated by the following case study.


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