Guide to validation of terminal sterilization process of drugs (GUI-0074): Phase 1, process design 

A key concern during this stage is to ensure the sterilization process and potential sources of variability are adequately understood and controlled. This page outlines general considerations that apply to all terminal sterilization methods.

On this page

• General considerations
• Sterilization process development
• F₀ and D-value

General considerations

When developing a sterilization process, you should aim to control the pre-sterilization bioburden to an appropriate level.

The level of microbial quality must be critically evaluated first in order for the terminal sterilization process to be properly applied. It's thus important to understand the microbial quality of all the materials, such as:

• raw materials
• bulk materials
• packaging materials
• process components, such as:
  • in-process drugs and finished products

Reducing the microbial bioburden of these materials will generate a more effective terminal sterilization process.

You should:

• establish definition for the product to be sterilized (in terms of physical, chemical, pharmacological properties, packaging, loading configuration and microbiological quality) before validation
  • conduct studies to determine bioburden in the materials to be sterilized
  • for products that promote the growth of microorganisms, include in the studies the impact of process hold times as well as partial or interrupted sterilization cycles on the bioburden
• establish a process definition, which involves setting the maximum and minimum sterilizing conditions for the sterilization process of a defined product
  • conduct studies to determine bioburden of the materials to be sterilized.
  • for products that promote the growth of microorganisms, evaluate the impact of hold times as well as partial or interrupted sterilization process on the bioburden
• determine the required sterility assurance level (SAL)
• evaluate the compatibility of the sterilization process with the material to be sterilized
  • ensure that the primary and secondary packaging can tolerate sterilization process parameters, while maintaining product/material and package/container integrity for the expected life of the product/material
  • for test protocols, consider the maximum tolerances for repeat exposures that the product can withstand
• follow effective and validated premises and equipment cleaning and sanitation procedures to reduce the potential for contamination
  • put controls in place and routinely monitor and document bioburden to ensure the microbiological quality of the product/material that's to be sterilized is controlled and does not compromise the effectiveness of the sterilization process

Note: Use risk assessment(s) to determine the amount of work at the development/prequalification stage. Fully assess and understand the impact of the sterilization process on the drug, including its stability.

Sterilization process development

There are 2 approaches to developing sterilization process parameters:

1. Overkill method: Use when the product/material can withstand prolonged exposure to the sterilization process without adversely affecting the quality of the product/material over the product's life.
   • Sterilization is performed for longer than required to kill the bioburden present on or in the material being sterilized.
   • A moist heat or EO (ethylene oxide) sterilization process designed with the overkill approach is a process that is sufficient to provide at least a 12 log reduction of microorganisms with a minimum specified D-value.
   • For moist heat, the D-value must be a minimum of 1 minute at 121°C.
2. Product-specific or combined BI bioburden microbial approach: Use for products that may be impacted by the sterilization process.
   • For moist heat and ethylene oxide (EO) sterilization, the process for the terminal sterilization is validated to achieve the destruction of pre-sterilization bioburden to a level of 10⁰, with a minimum safety factor of an additional six-log reduction (1x10^-6).
   • The probability that any 1unit is contaminated is no more than 1 in a million (considered an acceptable level of sterility assurance).
     • The probability of survival is determined using a semi-logarithmic microbial death curve, where a plot of the log of the number of survivors versus time at fixed exposure conditions yields a straight line. The linear portion of the curve is extrapolated to project process requirements (exposure times or dose) for various survivorship levels below 10⁰ (including 10^-6 to support sterility assurance requirements).
     • The minimum required process lethality used in the probability of survival approach is determined by the:
       • number of microorganisms (bioburden) found in a given product
       • required sterility assurance level (SAL)
       • resistance of the microorganisms to the sterilization method

F₀ and D-value

F₀ is the amount of time in minutes (equivalent to time at 121°C) to which a unit has been exposed during a sterilization process. One method of calculating the F₀ is to integrate the time the unit is exposed to heat in terms of equivalent time at 121°C.

D-value is the time (in minutes) required to reduce a microbial population by 90% (1 log value) under specified test conditions (such as fixed temperature, single species, specified medium). When heat labile products will not withstand excessive heat treatment, D-value studies of product isolates can determine the minimum lethality factor (F₀) that will provide an acceptable assurance of sterilization.

The minimum F₀ value required by a process can be related to the D-value of the bioburden by the following equation:

D₁₂₁ × (log A − log B)

where:

• D₁₂₁ is equal to the time required at 121°C to reduce the population of the most heat-resistant organisms in the unit by 90%
• "A" is the microbial count per container
• "B" is the maximum acceptable probability of survival (1 x 10^-6 for pharmaceutical dosage forms)

Lab studies that determine the number and resistance of microorganisms for a product (bioburden) serve to calculate the minimum F₀ value required for sterilization.

A more conservative approach would be to use the D-value of a highly heat-resistant spore-forming organism for the bioburden of the product.