Water Activity Determination

Water Activity of Oral Solid Dosage Product

Controlling water content in oral solid dosage (OSD) product is essential to maintaining efficacy and safety of the drug product. Water activity (aw) is the amount of free or loosely bound water available to participate in unwanted chemical, physical and biological reactions. Monitoring water activity throughout the product life cycle is important to control critical quality attributes such as microorganism growth, product degradation, product dissolution and product hardness or friability.

LIGHTHOUSE offers solutions for water activity determination. Using Tunable Diode Laser Absorption Spectroscopy (TDLAS) it is possible to non-destructively and directly measure the water activity of samples in a sealed glass vial. The draft of the new general chapter USP <922> includes a description of TDLAS and states that it is the only method to reliably and accurately measure water vapor pressure in the presence of other volatiles.


Primary Packaging Selection

USP<922> states that water activity determination can be used to assess the level of protection provided by primary packaging for an oral solid dosage (OSD) product over its shelf life. Primary packaging must maintain a low relative humidity for as long as necessary to ensure the pharmaceutical product maintains its key quality attributes up to its expiration date. These key quality attributes include potency, purity, dissolution rate, tablet hardness, and capsule swelling.

Chemical and Physical Stability

Direct water activity measurements can determine the impact of moisture on potency and dissolution, and consequently help determine the stability and quality of dry pharmaceutical products. It is crucial to gain insight into how temperature and humidity impact powder flow, compression, adhesion, and the physical characteristics of the tablet, pill, or capsule. Water activity is strongly correlated to these critical factors and provides unique and critical information at each stage in the product life cycle.

Reduction of Microbial Contamination

Oral solid dosage pharmaceutical products are subject to microbiological examination prior to release and during stability testing. USP <1112>, Application of Water Activity Determination to Non-sterile Pharmaceutical Products, describes how water activity determination can be used to reduce the need for frequent microbial limit testing and screening, especially for those products with water activities below 0.75. By controlling the water activity of a product, beginning with its raw materials, it is possible to greatly reduce, or even eliminate, the viability of microbial growth.

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