Application of Terminal Sterilization for Aseptic Manufactured Products
USP <1238> Vaccines for Human Use – Bacterial Vaccines states: “Tests required for each lot-release protocol include potency, general safety, sterility, purity, identity, and constituent materials. Potency and potency-related tests are different for each bacterial vaccine. The inclusion of these tests makes each bacterial vaccine lot-release protocol unique.” Vaccines, like other aseptically manufactured finished products, rely on filtration to remove any organisms. After filtration, the finished products are filled under classified conditions into pre-sterilized vials, stoppered and sealed. At this point aseptically manufactured products are usually assumed to be inappropriate for steam sterilization due to possible heat liability of the finished product.
However, due diligence must be followed in order for the manufacturer to comply with cGMP because with newer thinking and technology – this assumption may be incorrect. It is well accepted that the best practice is to follow aseptic manufacturing with terminal sterilization whenever possible.
As per USP <1229> Sterilization of Compendial Articles, “…a specimen is deemed sterile only when there is a complete absence of viable microorganisms (bacteria, yeasts, and molds)…”. However, sterility cannot be demonstrated with respect to compendial articles and other materials because of the inherent limitations of the current test (see USP <71> Sterility Tests). Sterility can be accomplished only by the use of a validated sterilization process under appropriate current good manufacturing practices and cannot be demonstrated by reliance on aseptic manufacturing, media fills or sterility testing.
Terminal sterilization alone is able to define the probability of a non-sterile unit [PNSU]. Liquids in sealed containers are steam sterilized in programmable air over pressure steam sterilizers. Development cycles are performed to ensure both product potency and sterilization effectiveness are maintained. New thinking does not rely upon high populations of highly resistant spores like Geobacillus stearothermophilus to assure sterilization nor excessively high sterilization temperatures. For example, less resistant spores and/or product bioburden are discussed in USP <1229.2> with the specification of obtaining ≤ 10-6 PNSU.
Utilizing a product’s bioburden dramatically reduces the exposure time for a given sterilization temperature and obtains an acceptable PNSU. For example, a typical Geobacillus stearotherpmophilus spore D value is 1.0 minute. If the natural bioburden for a product survives boiling at 100°C for one minute and not for 10 minutes then its D value is 100 times lower than Geobacillus stearotherpmophilus , i.e., 0.01 minutes.
The accessibility of steam sterilization is illustrated as follows. When F0 is 8 and the bioburden is 100 CFUs, the calculation of the PNSU is:
Log Nu = −F/D + log N0
Nu = PNSU
D = D-value of the natural bioburden
F0 = F0-value of the process (lethality)
N0 = bioburden population per container
Log Nu = −8/0.01 + 2 = −78
Therefore, the PNSU = 10-78.
Product potency can be examined after exposure to development cycles designed to be consistent with sterility and product integrity.
In this example, in addition to sterilizing at 121.1°C, because Z = 10 the D value will rise or fall by a factor of ten when the sterilization temperature is raised or lowered, respectively. To minimize heat lability the sterilization temperature can be lowered to 111.1°C achieving an acceptable PNSU of 10-7.8 when D = 0.1.
In practice, tools available to the sterilization practitioner now include lower sterilization temperatures, longer cycles and bioburden based parameters, all of which may permit a much greater number of aseptically manufactured products to be terminally sterilized to the benefit of all.