Powder Characterization For Inhaled Drug Delivery

By Tim Freeman, Director of Operations, Freeman Technology

Manipulating and controlling powder behaviour is a demanding but essential element of pharmaceutical manufacturing. The development and production of dry powder inhalers (DPIs) exemplifies and highlights the difficulties faced, arguably presenting the industry with its toughest powder engineering challenge. Fine by necessity, to ensure deposition in the lung, dry powder formulations tend to be highly cohesive and difficult to handle. The need to understand the aerosolisation processes that ensure successful drug delivery adds an additional and substantial layer of complexity.

Over many decades the pharmaceutical industry has developed a number of powder testing methods. These include bulk density techniques such as Carr's Index and Hausner ratio as well as methods such as shear testing. While these techniques have a place in describing powder behaviour they do not, in isolation, provide sufficient information for today's dry powder formulator. It is reasonable to suggest that the optimal analytical toolkit for dry powder applications is still being assembled.

One contender for inclusion is dynamic powder characterization using a powder rheometer. This approach is unique in its ability to measure a powder's response to air directly - an intuitively relevant characteristic for DPI design. Researchers working at the cutting edge of DPI development have reported strong correlations between dynamic parameters and the success of drug delivery, confirming the potential of the technique.

This paper examines the use of dynamic powder characterization, in combination with shear and bulk property measurement, in the development of DPI formulations. Addressing issues associated both with delivery of the dose and manufacture of the product, it highlights how universal powder testers that incorporate all three methodologies have a significant role to play in supporting the advancement of this important technology.