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The objective of this paper is to assess the number of drug particles or droplets contained in metered dose inhaler (MDI) aerosols. Equations were developed to estimate this. The number of drug particles was estimated to be as high as about 300 million for QVAR solution MDIs; as low as 670,000 for Beclovent MDIs. The number of particles in MDI aerosols was shown to be highly dependent on the mass median aerodynamic diameter (MMAD); geometric standard deviation,; to a lesser extent the total mass of the aerosol. It was demonstrated that when the number of particles are calculated assuming that the aerosol is monodisperse; using the MMAD as the particle size, the number of particles are significantly underestimated. The number of droplets atomized from HFA-134a MDIs was estimated to range from about 220 million to about 1.1 billion droplets per actuation. For solution MDIs, each of the atomized droplets contains drug; thus the number of drug particles is the same as the number of atomized droplets. However, for suspension MDI formulations many of the droplets do not contain any micronized drug particles; the number of drug particles is much lower than the number of atomized droplets.
Early Pharmaceutics; Technology Department, 3M Drug Delivery Systems, 3M Center—Building 260–3A-05, St. Paul, Minnesota, USA