Relationship Between Geometric and Aerodynamic Particle Size Distributions in the Formulation of Solution and Suspension Metered-Dose Inhalers

The relationship between the geometric particle size distribution (GPSD) and the aerodynamic particle size distribution (APSD) of commercial solution and suspension metered-dose inhaler (MDI) formulations was assessed to clarify the use of GPSD to estimate the APSD. The size distribution of particles discharged from four suspension and four solution MDIs was measured using the Inas®100 light-scattering spectrometer and a Next Generation Impactor. The conversion factor was calculated by measuring the GPSD and APSD of MDIs. The morphology and physical properties of MDIs were studied using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Six of the eight MDIs showed similar conversion factor profiles, irrespective of their composition and formulation types. Applying the conversion factor obtained from one of the six MDIs resulted in a particle size distribution comparable to each APSD except for some formulations. The two other solution MDIs, which contained citric acid, had much higher and variable conversion factors. SEM images and DSC scans of the solids obtained by nebulization of the solutions containing beclomethasone and/or citric acid showed the formation of a paste-like amorphous solid. These results indicated that APSD of solution and suspension MDIs that form rigid particles may be estimated by using the conversion factor and GPSD. Contrarily, the estimation is more difficult in formulations that tend to lose the particle structure during the measurement.