Comparison of output particle size distributions from pressurized aerosols formulated as solutions or suspensions

The delivery of particles as small as possible (preferably less than 5 microns) to the respiratory tract should be the aim of those formulating metered dose inhalers (MDIs). This may be facilitated by the formulation of solution, rather than suspension-type, pressurized aerosol units. Two series of MDIs were compared; one contained suspended micronized disodium fluorescein (0.1%, w/v), while the other contained the same concentration of dissolved salicylic acid. Either oleic acid, L-alpha-phosphatidylcholine, or sorbitan trioleate was incorporated at 0.15% (w/v) as suspending agent (disodium fluorescein) or solubilizing agent (salicylic acid). The propellant blend was 70% (w/w) Freon 12 and 30% (w/w) Freon 11 in all cases. This exhibited a vapor pressure of 50.6 psig (444.7 kPa) at 21 degrees C. The output particle size distribution of the aerosol reaching the cascade impactor showed a mass median aerodynamic diameter (MMAD) of approximately 4 and 2 microns for the suspension and solution formulations respectively, regardless of the surfactant used. Larger MMADs were observed for solution aerosols formulated with oleic acid (2.32 microns) compared to those containing L-alpha-phosphatidylcholine (1.93 microns) or sorbitan trioleate (2.07 microns). Possible reasons for these observations are discussed.