Sterilization Considerations

STERILIZATION CONSIDERATIONS   

It is axiomatic that whatever method is chosen, the process should not cause damage to the product. By reference mostly to moist heat sterilization processes (the reader should remember that there are parallel approaches to other methods of sterilization) this section illustrates the factors that must be considered in the design of a sterilization process.

The simplest method of sterilization, for an aqueous product, is to expose it to the standard moist heat sterilization conditions, i.e. holding the product at 121 °C for 15 minutes, a process termed overkill. These conditions are quite severe and therefore milder conditions might be considered, i.e. a lower holding temperature than 121 °C, or a shorter holding period than 15 minutes for a product prone to degradation. The minimum holding period for moist heat sterilization might be considered to be 8 minutes at 121 °C. However, in reality a slightly shorter holding period may be satisfactory if the lethality of the whole autoclave cycle (including heat-up and cooling phases) is calculated using F₀ values  and shown to afford the requisite SAL. F₀ values of 8 minutes or more are normally considered satisfactory. Use of lower temperatures and times gives an autoclave process partly based on the initial bioburden and partly on the known stability of the product.

A)                Decision Trees

Where it is not possible to sterilize a product in its final container by terminal heat sterilization at 121 °C for 15 minutes, decisions have to be made to use an alternative method. The options include filtration in combination with aseptic processing, but readers should note that aseptic processing by itself is not a method of sterilization, rather of preventing contamination of the product whilst it is manufactured from individually sterilized components.

The European Agency for the Evaluation of Medicinal Products in 2000 produced an Annex for Guidance on Development Pharmaceuticals (CPMP/QWP/155/96) showing decision trees for the selection of sterilization methods. The tree for the sterilization choices for aqueous products is shown in Figure 22.1. The initial premise is that if the products may be sterilized at 121 °C for 15 minutes, that process should be used. The next alternative is that if the product is stable when an F₀ of 8 minutes or more can be used, then the reduced moist heat process should be undertaken. If heat processes are unsuitable (an F₀ < 8 minutes will not achieve the necessary SAL), then filtration through a microbial filter should be chosen as the process to render the product sterile. If that process cannot be utilized, then presterilizing of stable components and aseptic compounding and filling must be considered. The described methods generally show decreasing levels of sterility assurance on moving down the tree. It is therefore imperative to remember that the highest level of sterility assurance is achieved in conjunction with the lowest presterilization bioburden. The use of inappropriate heat-labile packaging material cannot by itself be the reason for the use of aseptic processing, and any manufacturer should use the best sterilization method achievable for a given formulation before selecting the packaging material. The manufacture of biotechnology products, which are typically heat-labile peptides, proteins or nucleic acids, will provide a challenge as their overall stability dictates their positioning near the bottom of the decision tree. They may require sterilization by submicron (< 0.1 μm) filtration and filling and finishing using aseptic processes. The overall SAL for terminally sterilized products should be less than 10⁻⁶ and for aseptically produced products less than 10⁻³.

B)            Problems Of Drug Stability

Certain issues of product instability may be resolved by formulation or careful selection of vehicle. Aminophylline injection, for example, is a solution of the drug in Water for Injections Injections free from carbon dioxide, as the presence of this gas causes precipitation of the active ingredient. Similarly, promethazine injection is a solution of the active ingredient in Water for free from dissolved air, as the presence of oxygen would cause promethazine oxidation. Removal of these gases can be accomplished by prior boiling; additionally, the product may be packed under an atmosphere of nitrogen to eliminate oxygen from the headspace in the ampoule.

Formulations may be further stabilized by the inclusion of inactive ingredients with specific functions. Although the British Pharmacopoeia (2010) describes chloramphenicol eye drops as a sterile solution of chloramphenicol in purified water, normally the system is buffered for stability with a boric acid/sodium borate buffer (see Table 22.1). Sodium metabisulphite may be found in many products as an antioxidant to prevent degradation of the active, examples being promethazine injection and adrenaline injection. The presence of antimicrobial preservatives may be found in multiple-dose products, to prevent microbial growth following contamination during use. Many of these formulation considerations relate to stability of the product during storage, but an understanding of thermostability is required for the selection of the appropriate sterilization process.

The choice of sterilization method depends on the thermostability of the active ingredient. Moist heat sterilization can only be applied to drugs that are heat-stable in aqueous solution and are not subject to hydrolysis. Where aqueous solutions are so unstable that chemical stabilization is impossible, consideration should be given to sterilization of the drug itself by dry heat processes (160 °C for 2 hours or its equivalent at higher temperatures) in its final container and dissolution immediately before use by the addition of sterile Water for Injections BP. For drugs which are both thermolabile and unstable in aqueous solution, a sterile solution of the drug may be freeze-dried in its final container and is again reconstituted as above just before use. Examples include many antibiotics and Hyaluronidase Injection BP.