Dry powder inhalers may be employed for the delivery of drugs to the lungs for the treatment of pulmonary and systemic diseases. Dry powder inhalers consist of a formulation, metering system and delivery device. The formulation consists of small drug particles (median aerodynamic diameter <5 J.lm) usually combined with large excipient particles (30-125 J.lm). The physico-chemical properties of particles influence flow, fluidization and dispersion. An aerosol is passively inhaled by means of the

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... inspiratory effort of the patient. Active devices use independent energy sources to deliver the aerosol. The aerodynamic particle size of an aerosol significantly effects lung deposition. Inertial deposition of particles, as a function of their aerodynamic diameter, allows measurement of the drug mass in a specific size range. The fine particle dose, or fine particle fraction (FPF) of the metered dose, is the simplest expression of the aerosol particle size. Passive inhalers have been reported to administer up to 50% FPF and active inhalers up to 70% FPF. The criteria for optimal inhaler performance must be defined to facilitate the rapid growth in research and development in this field.