Water Activity Testing with Pharmaceutical Solid Dosage Product

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A water activity measurement of pharmaceutical solid dosage product can give insight into the shelf-life and microbial properties of the formulation. Water activity measurements have long been standard in the food industry and are now being increasingly used in pharmaceutical applications.

In particular, the FMS-Water Activity Analyzer has been used to define the stability of solid dosage product. In general, water that is associated with a formulation can be classified as 'free' or 'bound' water.  Free water is loosely adsorbed to the formulation and has the ability to interact with the active pharmaceutical ingredients. Bound water is not readily available for interaction with the active pharmaceutical ingredients due to the fact that it is tightly associated with the formulation.  With regards to stability, it is therefore the amount of free water that is critical to the chemical and physical stability of the solid dosage formulation rather than the total amount of water as measured by the standard destructive moisture determination techniques such as Karl Fischer titration.

The FMS-Water Activity Analyzer measures water activity by measuring the relative humidity (RH) of the atmosphere in which the solid dosage product is stored.  Vials are prepared with defined amounts of solid dosage product.  A rapid non-destructive headspace water vapor measurement is made in the vial at a defined temperature returning a Equilibrium Relative Humidity (ERH) value. These measurements can be used to predict the stability of the product against moisture and define a moisture specification. The method has been implemented for the release testing of pharmaceutical solid dosage product and can also be used as an in-process control during the manufacturing process.