Targeted Delivery of Inhaled Drugs: Current Challenges and Future Goals

Past success of topical delivery to the lung stems more from the therapeutic nature of drugs used rather than the delivery device. Both the pressurized metered dose inhaler (pMDI) and the dry powder inhaler (DPI) are inefficient, placing a small fraction of the dose at the site of action. Most of the drug deposits in the mouth and is swallowed. Modifications to the pMDI may redress this by improving the coordination of dose generation and inspiration, or by the use of spacers. The DPI can be improved by the use of special excipients. Nevertheless, fundamental weaknesses that limit improvements in targeting drugs to the lung remain. Ideally, a stationary or slow-moving cloud of a selected particle size distribution should be generated at the source by the device. The rate of cloud generation should be such that a patient can accommodate it during a slow inspiration. Many new devices are in development, harnessing one or more of these principles. Respimat (Boehringer Ingelheim, Ingelheim am Rhein, Germany), a soft mist inhaler, shows particular promise. It generates a fine slow-moving cloud over a period exceeding 1 second and incorporates many design features to win patient acceptance.