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Intrinsic Particle Size Distribution: A New Metric to Guide the Design of HFA Solution pMDIs

Lewis DA.

RDD Europe 2013. Volume 1, 2013: 175-184.

Abstract:

A theoretical method to determine the particle size distribution (PSD) emitted from ethanol based solution hydrofluoroalkane (HFA) metered dose inhalers (MDIs) is described. The scope of this work is restricted to the dose collected by the impaction plates of an Andersen Cascade Impactor (ACI) during characterization. The existence of an intrinsic PSD, i.e., a consistent initial PSD at the point of atomization that is dependent upon propellant choice (or propellant mixture), but independent of formulation composition, metering valve chamber size and the actuator’s atomization orifice (diameter, shape, area and number) is proposed. Residual PSDs have been determined experimentally and mathematically transformed to extrapolate to PSDs at the point of atomization and during the flashing and evaporation processes. This work highlights that when volatile excipient evaporation is incomplete, it may be difficult to experimentally distinguish the PSDs of MDIs that have alternative formulation and/or hardware characteristics. This is of particular relevance to light scattering or abbreviated impactor sizing methodologies.