In our February technical bulletin, we provided an update on our recommendation to perform USP <51> testing on measuring the stability of a product. It is important to also consider which products should adhere to USP <51> specifications.
For the purpose of compendial chapter: <51> ANTIMICROBIAL EFFECTIVENESS TESTING, products that must adhere to the specifications are defined as aqueous products which have a water activity of more than 0.6 aW (common Bees’ Honey exhibits a water activity of 0.6 aW).
Reduced water activity assists in the prevention of microbial proliferation in pharmaceutical products; the formulation, manufacturing steps, and testing of nonsterile dosage forms should reflect this parameter. Other product attributes, such as, low or high pH, absence of nutrients, presence of surfactants, antimicrobial agents, in addition to low water activity, help to prevent microbial growth.
Please reference Application of Water Activity Determination to Nonsterile Pharmaceutical Products <1112> for additional details, it should be noted that more resistant microorganisms, including spore-forming Clostridium spp., Bacillus spp., Salmonella spp. and filamentous fungi, although they may not proliferate in a drug product with a low water activity, may persist within the product.
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