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Call for Collaboration: Terminal Sterilization for Aseptic Manufactured Products Based on Bioburden, Sterilization Temperature F0 to accomplish a PNSU ≥ 10-6

Liquid filled products manufactured aseptically are usually assumed to be inappropriate for steam sterilization due to possible heat liability of the finished product. However, these products must be sterile. Data is needed to verify that new approaches in the current USP <1229.2> can provide due diligence to justify standard practices or to supplement aseptic manufacturing practices with terminal sterilization.

As written in USP <1238> Vaccines for Human Use – Bacterial Vaccines, 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. Manufacturing relies on the use of a controlled clean environment and verification of the process cleanliness by testing 3,000 to 5,000 units by media fill. Sterility testing is used to justify that the aseptic manufacturing is under control.

A critical deficit of media fill based aseptic manufacturing is there is no assurance of sterility based on probability. A critical advantage of terminal sterilization is that it does provide a probability of sterilization.  Therefore, 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). 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.

Because of new thinking [USP 1229.2> a fresh look can be taken for products aseptically manufactured that is both realistic and practical.  By using bioburden as the basis of the challenge and lower temperatures more products than before can be successfully terminally steam sterilized.

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 and maintain package integrity without leaks or damage.  Development cycles are performed to ensure both product potency and sterilization effectiveness is maintained.

New thinking does not rely upon high populations of highly resistant spores like Geobacillus stearothermophilus to assure sterilization nor excessively high sterilization temperatures.

Instead for heat labile products that cannot withstand the extreme conditions imposed on products by “overkill” sterilization USP <1229.2> discusses  product bioburden with the specification of obtaining ≥ 10-6 PNSU.

Utilizing a product’s bioburden dramatically reduces the exposure time for a given sterilization temperature because the natural bioburden is usually less resistant than bacterial spores used in overkill cycles. For example, a typical Geobacillus stearothermophilus spore D value is 1.0 minute. As per USP <1229.2> one may test the resistance of the product’s natural bioburden. If it survives boiling at 100°C for one minute and not for 10 minutes then, by definition, its D121 value is taken to be 100 times lower than Geobacillus stearothermophilus, i.e., 0.01 minutes.

The application and utility of steam sterilization at 121.1C is illustrated as follows. When F0 is 8 and the product’s natural bioburden is 100 CFUs, the calculation of the PNSU is:

Log Nu = −F + Log N0

D = D-value of the natural bioburden
F0 = F0-value of the process (lethality)
N0 = bioburden population per container


Log Nu = −8 + 2 = −798

Therefore, the PNSU = 10-798.  Product potency is critical for heat labile products. Clearly this level of assurance is excessive and is not justified if product integrity is compromised, e.g., diminished potency.

Potency can be examined after exposure to development cycles designed to be consistent with sterility and product integrity. Thus, a product may or may not maintain its potency and purity if exposed to 121.1C.

However, by lowering the sterilization exposure temperature from 121.1°C the D value will become 0.1 because Z = 10.  Thus, to minimize possible damage to the product, whilst in the sterilizer, the sterilization temperature can be lowered to 111.1°C and still achieve [more than] acceptable PNSU of 10-78.



Still there are more options to protect the product and achieve the required PNSU. Specifically, by lowering the temperature from 121.1 to 101.1 one obtains the desired PNSU of 10-6

Log Nu = −8 + 2 = −6

Therefore, the PNSU = 10-6

Sterilization Temperature F D Log N0 PNSU
121.1 8 0.01 2 10-798.
111.1 8 0.1 2 10-78
101.1 8 1.0 2 10-6
All three approaches achieve a valid sterilization process.


As seen in the table below, 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  at a PNSU of ≥ 10-6 to the benefit of all.

In summary, a PNSU of 10-6 is essential for regulatory and consumer confidence and has never been scientifically assignable with respect to aseptic processing. However, using terminal sterilization and the approaches discussed a PNSU assignment is available for aseptically manufactured products and other heat sensitive products at relatively low sterilization exposure temperatures.

As a community we can explore together the potential application of the terminal sterilization strategies. Gibraltar Laboratories invites your participation to submit products to be evaluated under defined temperature, bioburden, N0 and F0 limits versus heat lability and overall product integrity.


You may contact,

Daniel Prince,Ph.D.

973 227 6882 x. 519






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