Filtration, sterilization method

Five sterilization processes are described in the USP steam, dry-heat, filtration, gas, and ionizing radiation. All are commonly used for parenteral products, except gas and ionizing radiation, which are widely used for devices and surgical materials. To assist in the selection of the sterilization method, certain basic information and data must be gathered. This includes determining... 

The USP also recommends the use of biological indicators, whenever possible, to monitor all sterilization methods except sterile filtration. Biological indicators are generally of two types. If a product to be sterilized is a liquid, microorganisms are added directly to carefully identified representative samples of the product. When this is not practical, as with solids or equipment to be sterilized, the culture is added to strips of filter paper. The organism chosen varies with the method of sterilization. 

Where terminal processing is not possible, the justification for alternative sterilization methods will be included in the EPAR, or at least a statement to the effect that sterile filtration/aseptic processing will be used. Presterilization bioburden issues that arose during the assessment will be included in the EPAR. 

Table 3 lists the sterilization methods used for sterile products. There are five basic methods—heat, gas, radiation, light, and filtration. The first four methods destroy microbial life, while filtration removes micro-organisms. Vali-... 

Medium for use in a process simulation test can be rendered sterile using either moist heat (autoclaving) or filtration. The method chosen depends on the availability of suitable equipment and the information desired from the study. 

For parenteral IV formulations, a sterile solution of the compound is required. A terminal sterilization method is preferred, rather than aseptic filtration, because there is a greater assurance of achieving sterility. As noted by Moldenhauer (1998), the FDA requires a written justification to explain why a product is not terminally sterilized. Therefore, it is mandatory to assess whether the candidate drug is stable to autoclaving as part of any preformulation selection process. Autoclaving (usually 15 min at 121°C) at various pHs is undertaken, after which the solutions should be evaluated for impurities, colour, pH and degradation products. Clearly, if one compound shows superior stability after autoclaving, then this will be the one to choose. 

Sterile filtration may not be considered sufficient if a product can be sterilized in its final container. Steam sterilization is preferred among available sterilization methods. 

The preparation of sterile ophthalmic suspensions, as required by all official compendia, presents some difficulties since filtration as a sterilization method cannot be used. Even if heat sterilization is possible in some cases, it is to be avoided since it might cause partial dissolution of the drug at high temperature and separation of larger crystals on cooling. Therefore, aseptic methods are preferable. In some cases, final sterilization by y irradiation is possible. A detailed picture of different techniques for aseptic preparation of suspensions is given in Refs. 4.1 and 44. 

Preparation of BdUrd absorbed on activated charcoal was by the method of Russev and Tsanev modified by Kanda, Kato and Morales-Ramirez (4). Activated charcoal was washed several times with 2.5% NaOH, distilled H2O, 2.5% HCl and finally distilled HjO until the original water pH was attained. The charcoal was then dried for 2 hrs at 105°C. An aqueous solution of BdUrd was prepared (25 mg/ml) and sterilized by filtration. Sterilized activated charcoal was added to the BdUrd solution at a concentration of 100 mg/ml and stirred for 2 hr at room temperature before use. 

Generally, sterility is synonym with the absence of any viable microorganisms including their spores. Currently, the available sterilization methods include heat sterilization (steam and hot-air sterilization), cold sterilization (gas sterilization, sterilization by ionizing radiation), sterilization by aqueous solution (aldehydes, peracetic acid, hypochlorite, hydrogen peroxide), and sterilization by filtration methods. The choice of method is based on recommendations in medicinal literature, legal requirements, and the compatibility of a medical product with the method used. The decision for a particular method has to take the following factors into consideration [954] ... 

Sterile aqueous D-sorbitol solutions are fermented with y cetobacter subo >gichns in the presence of large amounts of air to complete the microbiological oxidation. The L-sorbose is isolated by crystallisation, filtration, and drying. Various methods for the fermentation of D-sorbitol have been reviewed (60). A.cetobacter suboyydans is the organism of choice as it gives L-sorbose in >90% yield (61). Large-scale fermentations can be carried out in either batch or continuous modes. In either case, stefihty is important to prevent contamination, with subsequent loss of product. 

CiystaUization is the preferred method of forming many final prod-uc ts because veiy high purification is possible. High purity antibiotic ciystals can be produced from colored, rather impure solutions if the filter cake is uniform and amenable to good washingto remove the mother hquor. When a sterile pharmaceutical produc t is desired, ciystals are formed from liquid streams that have been sterihzed by filtration. 

Pharmaceuticals for injection must be presented in a sterile form. Sterility may be achieved by filtration through 0.22 pm filters under aseptic conditions, or by steam, dry heat, radiation or gas sterilisation methods, which may be applied to packaged products. Irrespective of the method, the process must be validated and monitored to assure its effectiveness. As discussed in Chapter 2, this is an example of a process that cannot be assured by verification testing because of its destructive nature. 

The British Pharmacopoeia (1993) recognizes five methods for the sterilization of pharmaceutical products. These are (i) dry heat (ii) heating in an autoclave (steam sterilization) (iii) filtration (iv) ethylene oxide gas and (v) gamma or electron radiation. In addition, other approaches involving steam and formaldehyde and ultraviolet (UV) light have evolved for use in certain situations. For each method, the possible permutations of exposure conditions are numerous, but experience and product stability... 

Principles of the methods employed to sterilize pharmaceutical products are described in Chapter 20. The British Pharmacopoeia (1993) recommends autoclaving and filtration as suitable methods applicable to aqueous liquids, and dry heat for non-aqueous and dry sohd preparatiorrs. The choice is determined largely by the ability of the formulation and container to withstand the physical stresses apphed by moist heat... 

Sterile pharmaceutical preparations must be tested for the presence of fungal and bacterial contamination before use (see Chapters 18 and 23). If the preparation contains an antibiotic, it must be removed or inactivated. Membrane filtration is the usual recommended method. However, this technique has certain disadvantages. Accidental contamination is a problem, as is the retention of the antibiotic on the filter and its subsequent liberation into the nutrient medium. 

The purpose of a sterility test is to determine the probable sterility of a specific batch. The USP lists the procedural details for sterility testing and the sample sizes required [1], The USP official tests are the direct (or culture tube inoculation) method and the membrane filtration method. 

In general, aqueous ophthalmic solutions are manufactured by methods that call for the dissolution of the active ingredient and all or a portion of the excipients into all or a portion of the water and the sterilization of this solution by heat or by sterilizing filtration through sterile depth or membrane filter media into a sterile receptacle. If incomplete at this point, this sterile solution is then mixed with the additional required sterile components, such as previously sterilized solutions of viscosity-imparting agents, preservatives, and so on, and the batch is brought to final volume with additional sterile water. 

Particular attention should be paid to nonstandard production technologies including nonstandard methods of sterilization, sterile filtration and aseptic processing, lyophilization, microencapsulation, and certain critical mixing and coating operations. With such processes pilot-scale manufacture may not be predictive of industrial scale manufacture, and data on three full-scale production batches may be required in the application. 

Justifications for the use of nonstandard (i.e., nonpreferred or nonpharmacopeial) methods of sterilization may include the heat instability of the active ingredient or an essential excipient. The choice of a method based on filtration through a microbial retentive filter and/or aseptic assembly should be justified, and the appropriate in process controls (including bioburden controls on active ingredients, excipients, bulk solutions, process time constraints etc) discussed in detail in the application. Commercial considerations should not form part of the argument for the application of a nonstandard sterilization process. The highest possible sterility assurance level should be achieved. 

Because membrane filtration is the only currently acceptable method of sterilizing protein pharmaceuticals, the adsorption and inactivation of proteins on membranes is of particular concern during formulation development. Pitt [56] examined nonspecific protein binding of polymeric microporous membranes typically used in sterilization by membrane filtration. Nitrocellulose and nylon membranes had extremely high protein adsorption, followed by polysulfone, cellulose diacetate, and hydrophilic polyvinylidene fluoride membranes. In a subsequent study by Truskey et al. [46], protein conformational changes after filtration were observed by CD spectroscopy, particularly with nylon and polysulfone membrane filters. The conformational changes were related to the tendency of the membrane to adsorb the protein, although the precise mechanism was unclear. 

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