Sterile filter systems

The complexity of the sterile filtration operation and the CGMP regulations require the validation of sterilizing filter systems. The validation of a sterile filtration operation can be complex, with many operational parameters and their interactions needing to be identified, controlled, and predicted for each end product to demonstrate that sterility is adequately achieved by the filtration process. In the commonly used steam sterilization process, the heat parameters are identified and in-process controls specified such that a level of sterility assurance can be reproducibly obtained. In steam sterilization, the important parameter of heat, measured by temperature, can be accurately measured and continuously monitored to ensure the operational integrity of the autoclave however, unlike steam sterilization, filtration sterilization cannot be monitored on a continuous basis throughout the process. 

Polyarylsulfones offer materials with good thermal-oxidative stability, solvent resistance, creep resistance, and good hydrolytic stability. Their low flammability and smoke evolution encourage their use in aircraft and transportation applications. They hold up to repeated steam sterilization cycles and are used in a wide variety of medical applications such as life support parts, autoclavable tray systems, and surgical and laboratory equipment. Blow-molded products include suction bottles, surgical hollow shapes, and tissue culture bottles. PPS has a number of automotive uses including as an injection-molded fuel line coimector and as part of the fuel filter system. 

However, large holes or tears can be introduced into an otherwise effective sterilizing filter and performance tests have to be designed to ensure that the complete assembly of the filter membrane and its supporting equipment are devoid of imperfections. These include bubble testing in which gas is forced under pressure through the wetted filter and the pressure required for bubbles to first appear measured. In principle this identifies the largest hole present in the complete system. 

This simple technique may also be applied to the sterilization of nonau-toclavable materials such as protein and nucleic acid solutions or heat-labile reagents. Bacterial contamination can be removed from these solutions by passing them through filter systems that have been sterilized by autoclaving. 

Therefore, glass fibers are favorable as filter medium because they give a lower pressure drop and are less liable to wetting or combustion. Modern fibrous filter systems are cylinders made from bonded borosilicate microfibers sheathed in reinforcing polypropylene mesh in which the layers increase in fineness and density from the center outward. This type of design can deliver over 3 m3/s of sterile air at 0.1 bar of pressure drop (Quesnel, 1987). 

The search for the ideal nondestructive test of a sterilizing fdter system is still proceeding. One new suggestion has been proposed to use test gas pressures above the bubble point. In the meantime, the wise user of filter systems for sterilization will test in as many ways as possible and correlate for physical tests with bacterial challenge tests. 

Sterilize the filter system. Figure 13 shows a hypothetical test system for a disk filter medium. 

The entire volume of the challenge filtrate is subsequently forced through a sterility test filter system and incubated in the same manner as the negative control filtrate. 

For example, if a 293-mm-diameter disk filter system having an EFA of 530 cm2 is challenged and the 107/cm2 level is used, the total challenge to the filter is 5.3 x 109 organisms. If a sterile filtrate is assumed the LRV would be calculated and reported as follows ... 

For fast and simple purification of RNA oligonucleotides from crude transcription reactions, we use an AKTA prime FPLC system equipped with a 50-ml superloop and three 5 ml HiTrap diethylaminoethyl (DEAE) sepharose Fast-Flow columns (GE Healthcare) connected in series. The DEAE columns are equilibrated with 3 CV of buffer A (50 mM sodium phosphate, pH 6.5, 150 mMsodium chloride, and 0.2 mMEDTA) at room temperature. Buffer B contains the same components with 2 Msodium chloride. Both buffers can be prepared in large quantities, sterile filtered and stored at 4 °C (buffer A) or room temperature (buffer B) to avoid precipitation of sodium chloride. 

If a vacuum exhaust system is used to remove the air or steam from a vessel, it is necessary to clean and disinfect all fittings regularly. This prevents residues which may be drawn into them from supporting microbial growth, which may later be returned to the vessel in the form of condensate and contaminate subsequent batches of product. If air is bled back into the vessel it should be passed through a sterilizing filter. 

Steam Sterilization. Cartridge filter systems discussed for cell harvest applications present no difficulties in steam sterilization. They may be sterilized by autoclaving, with vents open (and filtered) for 1 hr. at 121QC. The same units coupled to fermentation equipment may be sterilized in situ for the same time. 

The automatic filter integrity tester will be operationally qualified, and all test parameters and acceptable test results will be verified in the Operational and Performance Qualifications of the equipment, for example, the Sterilizing Filter and the Media Preparation System. 

A suitably sized solution preparation system similar to that mentioned under the previous sections can be used to provide material for bulk freeze drying. (Since product solutions can be sterile-filtered directly into the final container, microbial and particulate exposure will be minimized.) The sterile solution is subdivided into trays and placed into a sterilized freeze dryer. Aseptic transfer of sterile product in trays to the freeze dryer must be validated. After tray drying, the sterile product is aseptically transferred through a mill into suitably designed sterile containers. The preparation of sterile bulk material is usually reserved for those cases where the product cannot be isolated by more common and relatively less expensive crystallization methods. Due to recent advances in this field, a freeze drying process should be considered as a viable option. ... 

Spray drying processes can be batch or continuous depending on production needs and the stability of the solutions to be spray dried. Because of reduced product manipulation, microbial and particulate burden can be reduced. Normally there is a solution vessel, a filtration system with prefilters and sterile filters, a pressure vessel to feed the spray dryer at a controlled rate, the spray dryer itself, and bulk containers. 

In selecting a membrane material, its pH compatibility and wettability should be considered. Some hydrophobic membranes require prewetting with a low-surface-tension solvent such as alcohol, whereas cartridges containing membranes are often presterilized using gamma irradiation. Such filter systems do not require assembly and steam sterilization. 

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