Measurement of Aerodynamic Particle Size Distribution of Orally Inhaled Products by Cascade Impactor: How to Let the Product Specification Drive the Quality Requirements of the Cascade Impactor

The multi-stage inertial cascade impactor is used to determine the mass-weighted aerodynamic particle size distribution (APSD) as a critical quality attribute for orally inhaled products (OIPs). These apparatuses progressively size-fractionate the aerosol passing through a series of stages containing one or more nozzles, by increasing particle velocity. Nozzle sizes for a given multi-nozzle stage can be described collectively by effective diameter (\( \overset{\sim }{W_0} \)), related to the cut-point size, providing the link to aerodynamic diameter. Users undertake stage mensuration periodically to assure that each stage \( \overset{\sim }{W_0} \) remains within the manufacturer’s tolerance, but there is no guidance on how frequently such checks should be made. We examine the philosophy that particle size-related specifications of the OIP should determine when an impactor is mensurated. Taking an example of a dry powder inhaler-generated aerosol sampled via a Next Generation Impactor with pre-separator, we find that there are only three critical stages that could have a material effect on the measured APSD specified as four groupings of stages following current regulatory practice. Furthermore, \( \overset{\sim }{W_0} \) for the most critical stage having the smallest nozzle sizes could be relaxed by a factor of four or more before risking an inability to measure the mass fraction of API in the group containing the finest particles to a specification within ± 10% of nominal. We therefore conclude that users should consider letting the specification for APSD performance of an OIP in terms of accepted stage groupings drive the impactor quality requirements and frequency that stage mensuration is undertaken.