Can the Water Activity Test replace Microbial Testing?

The USP General Chapters - Microbiology (GCM) Expert Committee proposed a revision of USP chapter <1112> Application of Water Activity Determination to Nonsterile Pharmaceutical Products. The proposal has been published for comment in Pharmacopeial Forum (PF) 47(1) with a comment deadline until 31 March 2021.

Reduced microbial testing may be justified through risk assessment

Water activity (Aw) is different from water content and may be considered as the available (free) water to support microbial growth. According to the USP, low water activity has traditionally been used to control microbial deterioration of foodstuffs, e.g. dried fruit.

The draft chapter states that no microbiological testing of low water activity (< 0.6) pharmaceutical products may be needed after a comprehensive risk assessment. Thus, determination of the water activity of nonsterile dosage forms (especially oral liquids, topical ointments, lotions, creams, and nasal sprays) will help to support the rationale for reducing the frequency of microbial testing (e.g. skip-lot testing) for product release and stability testing.

This is particularly the case when:

  • Products are made from ingredients of good quality
  • Manufacturing processes do not foster microbial contamination
  • Packaging systems protect the product from humidity
  • Processes are in place that reduce the microbial load
  • Formulations have antimicrobial activity
  • Manufacturing sites have an established testing history of low bioburden associated with their products

According to the USP, similar arguments could be made for microbial testing of APIs and excipients. However, this would require pharmaceutical manufacturers to have a comprehensive knowledge of the API / excipient manufacturers' production processes, quality programs, and testing records (this could be obtained through a supplier audit program).

In addition, the revised chapter <1112> contains a table with water activities provided for different dosage forms (e.g. nasal inhalant, hair shampoo, topical cream, lip balm). However, companies should test their products using the methods described in USP chapter <922> Water Activity before developing a testing strategy.

The draft chapter <1112> Role of Water Activity in Microbial Risk Assessment of Nonsterile Pharmaceutical Products can be viewed after registration for the Pharmacopeial Forum.

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