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By Daniel Banov, RPh, MS, PCCA Director of Research and Development
This article was updated: 08/31/2020
Water represents the largest component in most compounds, pharmaceuticals, cosmetics and foods. In pharmacy compounding, any preparation containing water requires preservation against microbial contamination and growth in order to increase its beyond-use date (BUD) and to ensure patient safety. However, anhydrous compounded preparations, which contain no water or a negligible amount, can have longer BUDs by default because the lack of water reduces the risk of microbial growth and chemical degradation. This is why the concept of water activity in various products is becoming increasingly important in the pharmacy compounding world.
Water Activity and Water Content Water activity (Aw) is a concept introduced in the 1920s by Levis and Randall.1 It is a measure of water in a substance that is available to react with or attach itself to a material, and it ranges from 0.03–1.0. This available water, or free water, can be responsible for the growth of molds, yeasts and bacteria as well as toxin production, or it may participate in chemical or biochemical reactions. Most topical products have water activity above 0.85, which is sufficient to support the growth of bacteria, yeasts, and molds, and accounts for the need for preservatives.2,3,4
Water activity is different from water content (or moisture content), which is a measure of the total amount of moisture in a material and is usually expressed as a percentage of the total amount (% of total weight). The table below explains the important differences between them.4
The Differences Between Water Content and Water Activity
Water (Moisture) Content
Water Activity
Total amount of water present in a product
The water that is available for reactions in a product
Measured by initial weight of the sample
Independent of initial weight of the sample
Not a reliable indicator of microbial growth or chemical degradation potential
Good indicator of microbial growth or chemical degradation potential
Water Activity and Microorganism Growth
While temperature, pH, and several other factors can influence whether an organism will grow in a product and the rate at which it will grow, water activity is often the most important factor. Bacteria usually require at least Aw 0.75, whereas yeast and mold require at least Aw 0.61. The water activity level that limits the growth of the vast majority of pathogenic microorganisms is Aw 0.60, so if a product has Aw below 0.60, it is considered anhydrous. The table below shows the water-activity levels necessary for microbial growth.2,5,6
Minimum Aw Values for Growth of Microorganisms
Organism Group
Water Activity (Aw)
Gram-negative bacteria
0.90
Gram-positive bacteria
0.75
Yeast
0.61
Molds
However, it is important to understand that the microbiological risk cannot be based on only one parameter. Ultimately, it depends on a multifactor analysis that takes into account physicochemical parameters linked to the product (pH, package, bioburden, chemical quality, compounding environment).3,5
Water Activity and Chemical Reactivity
Water activity influences not only microbial growth but also chemical and enzymatic reactivity. It influences non-enzymatic browning, lipid oxidization, degradation of vitamins, enzymatic reactions and protein denaturation. Typically, when water activity is lower, the rate of chemical degradative reactions decreases. Figure 1 offers a visual representation of the relationship between reaction rates and water activity.3
Figure 1: Water Activity - Stability Diagram (adapted from Decagon)3
What Water Activity Means for Compounding Pharmacies
This means that water activity is crucial to understanding BUDs. Lower water activity in compounded preparations allows compounders to assign longer BUDs to them by default, which is reflected in USP guidelines. For compounders, this can lower operating costs because they do not have to pay for costly stability studies to assign a longer BUD, and it can also increase operational efficiency when compounders need to make anticipatory batches.
For patients, compounded medications with longer BUDs mean that they do not have to refill their prescriptions as often, so they are more convenient and efficient for them as well.
The best way to ensure low water activity in compounds and get the longest default BUDs is to use anhydrous bases and ingredients. Fortunately, PCCA offers a wide range of anhydrous bases.
PCCA Bases with Low Water Activity
PCCA carries a selection of anhydrous bases that have low water activity:
In recent years,, PCCA has also launched new bases characterized by low water activity (Aw < 0.6) in order to improve stability and to reduce microbial growth and chemical degradation in the corresponding compounded medications:
Daniel Banov, RPh, MS, PCCA Director of Research and Development, has over 20 years of hands-on experience in pre-formulation, formulation and reformulation of a variety of dosage forms. He has extensively worked on developing novel techniques for skin-permeation enhancement. Daniel currently has 17 granted U.S. patents and many others pending. Before joining the PCCA team, Daniel was the Director of Fórmula Médica Compounding Pharmacy in São Paulo, Brazil, and was a university teacher and a cosmetic developer and consultant for physicians, spas and aestheticians. He also is the founder of the Anti-aging Society in Brazil.
A version of this article was originally published in the winter 2019 issue of the Apothagram, PCCA’s members-only magazine.
References
These statements are provided for educational purposes only. They have not been evaluated by the Food and Drug Administration, and are not to be interpreted as a promise, guarantee or claim of therapeutic efficacy or safety. The information contained herein is not intended to replace or substitute for conventional medical care, or encourage its abandonment.