Laser Diffractometry as a Technique for the Rapid Assessment of Aerosol Particle Size from Inhalers

The rapid assessment of aerosols produced by medicinal inhalers is highly desirable from several standpoints, including the assurance of product quality, the development of new delivery systems, and the need to meet an increasing requirement by regulatory bodies for reliable in vitro performance data. Particle size analysis has traditionally been undertaken by cascade impactor on account of the direct assessment of active pharmaceutical ingredient(s) (APIs) that is possible by this method. However, laser diffractometry is less labor-intensive, more rapid, and can be a less invasive procedure. The technique provides meaningful results; as long as precautions are taken to validate that the measurements are an accurate reflection of the distribution of API mass as a function of particle or droplet size. We begin the review by examining the underlying theory of the laser diffraction method. After a brief description of current laser diffractometers used in inhaler measurements, we continue by examining the range of applications by inhaler class. We then examine the basis upon which inhaler measurements made by laser-diffractometry can be compared with equivalent particle size distribution data from compendial techniques. We conclude the assessment of the technique by developing guidelines for its valid application as a component of the range of in vitro methods that are available for inhaler performance assessment.