Guest Colulmn: Powder Characterization Techniques For Dry Powder Inhaler Applications

By Tim Freeman, Freeman Technology

At the very heart of dry powder inhaler technology lies a demanding powder engineering challenge. Particles in the sub-5 micron range - the ideal for deposition in the lung - tend to be cohesive, making them difficult to handle and disperse. Achieving efficient dispersion of the active to a respirable size is a primary goal of formulators. However, addressing manufacturing issues at an early stage is also beneficial. Accurate and precise manufacture is vital for consistent drug delivery, a critical step being the extraction of a very small, representative dose into a capsule or blister pack. Developing formulations that disperse efficiently during inhalation and which process well is a daunting task but certain tools can help. One such is the powder rheometer which combines bulk, shear and dynamic powder testing to give the fullest insight into the nature of a DPI formulation.

Fine particle dose (FPD) is the amount of active that, on the basis of size will tend to penetrate into the lung, and is a commonly applied in vitro measure of delivery efficiency. Developing a formulation that disperses easily and effectively is the key to achieving a high FPD since with most DPIs the energy available for aerosolization of the dose is simply that provided by the inhaling patient. As many DPI formulations use a carrier to improve flowability characteristics, this dispersion step is complex and involves stripping the active from the larger excipient particle.