Filter Validation

Biopharmaceutical processes are validated processes to assure a reproducible product quality within set specifications. Equally important is the validation of the filters used within the process, especially the sterilizing grade filters, which often enough are used before filling or final processing of the drug product. In its Guideline on General Principles of Process Validation (1985) and Guideline on Sterile Drug Products Produced by Aseptic Processing (2004), the Food and Drug Administration (FDA) makes plain that the validation of sterile processes is required by the manufacturers of sterile products. Similar demands can be found in ISO 13408-2 (2003), EudraLex Vol 4 Annex 1 (2008) and PDA Technical Report #26 (2008), the later being the most comprehensive in the description of filter validation needs.
Sterilizing grade filters are determined by the bacteria challenge tests. This test is performed under strict parameters and a defined solution (ASTM F 838-05). Since the ASTM challenge test assesses the filter only under standard conditions, regulatory authorities require also evidence, that the sterilizing grade filter will create a sterile filtrate, no matter of the process, fluid or bioburden found. This means that bacteria challenge tests have to be performed with the actual drug product, bioburden, if different or known to be smaller than Brevundimonas diminuta and the process parameters. The reason for the requirement of a product bacteria challenge test is threefold. First of all the influence of the product and process parameters to the microorganism has to be tested. There may be cases of either shrinkage of organisms due to a higher osmolarity of the product or prolonged processing times. Secondly the filters compatibility with the product and the parameters has to be tested. The filter should not show any sign of degradation due to the product filtered. Additionally rest assurance is required that the filter used will withstand the process parameters, e.g. pressure pulses, if happening, should not influence the filters performance. Thirdly, there are two separation mechanisms involved in liquid filtration; sieve retention and retention by adsorptive sequestration. In sieve retention the smallest particle or organism size is retained by the biggest pore within the membrane structure. The contaminant will be retained, no matter of the process parameters. This is the ideal and best retention assurance. Retention by adsorptive sequestration depends on the filtration conditions. Contaminants smaller than the actual pore size penetrate such and may be captured by adsorptive attachment to the pore wall or by bridging effects. This effect is enhanced using highly adsorptive filter materials, for example glass fiber or diatomaceous earth as a prefilter or Polyamide as a membrane. Nevertheless certain liquid properties, e.g. pH or surfactant content, can minimize the adsorptive effect, which could mean penetration of organisms. Whether the fluid has such properties and will lower the effect of adsorptive sequestration and may eventually cause penetration has to be evaluated in specific product bacteria challenge tests.
Before performing a product bacteria challenge test, it has to be assured that the liquid product does not have any detrimental, bactericidal or bacteriostatic, effects on the challenge organisms. This is done utilizing viability tests. The organism is inoculated into the product to be filtered at a certain bioburden level. At specified times the log value of this bioburden is tested. If the bioburden is reduced due to the fluid properties different bacteria challenge test mode become applicable. If the reduction is a slow process the challenge test can be performed with a higher bioburden level, bearing in mind that the challenge level has to reach 107 per square centimeter filtration area at the end of the processing time. If the mortality rate is too high the toxic substance is either removed or product properties are changed, always having the viability results at hand to explain such changes. This challenge fluid is called a placebo. Another methodology would circulate the fluid product through the filter at the specific process parameters as long as the actual processing time would be. Afterwards the filter is flushed extensively with water and the challenge test, as described in ASTM F838-05, performed. Nevertheless such challenge test procedure would be more or less a filter compatibility test.
Besides the product bacteria challenge test, tests of extractable/leachable substances and/or particulate releases have to be performed. Extractable measurements and the resulting data are available from filter manufacturers for the individual filters. Nevertheless depending on the process conditions and the solvents used, product and process specific extractable tests have to be performed. These tests are commonly done only with the solvent used with the drug product, but not with the drug ingredients themselves, because the drug product usually covers any extractables profile during measurement. Such tests are conducted by the validation services of the filter manufacturers using sophisticated separation and detection methodologies, as GC-MS, FTIR and RP-HPLC. These methodologies are required due to the fact that the individual components possibly released from the filter have to be identified and quantified. Elaborated studies, performed by filter manufacturers showed that there is neither a release of high quantities of extractables (the range is ppb to max. ppm per 10" element) nor have been toxic substances been found.
Particulates are critical in sterile filtration, specifically of injectables. The USP (United States Pharmacopeia) and BP (British Pharmacopeia) quote specific limits of particulate level contaminations for defined particle sizes. These limits have to be met. Filters are routinely tested, evaluating the filtrate with laser particle counters. Such tests are also performed with the actual product under process conditions to proof that the product, but especially process conditions do not result in an increased level of particulates within the filtrate.
Additionally with certain products loss of yield or product ingredients due to adsorption shall be determined. For example preservatives, like benzalkoniumchloride or chlorhexadine, can be adsorbed by specific filter membrane polymers. Such membranes need to be saturated with the preservative to avoid preservative loss within the actual product. This preservative loss, e.g. in contact lense solutions or nasal sprays, can be detrimental due to long-term use of such solutions. Similarly problematic would be the adsorption of required proteins within a biological solution. To optimize the yield of such proteins within an application, small scale adsorption trials are recommended to find the optimal membrane material and filter construction.
Other routine validation or qualification tests would be flow rate and throughput determination, thermal and mechanical stability for example during in-line steam sterilization or any pulsations during the filtration process.