17.01.3 Drug Delivery by Inhalation

Unfortunately, with inhalation, not all the drug goes to the lungs. For instance, it has been shown that if an aerosol is used to deliver particles of 1-5 μm, after inhalation, only 10% of the drug ends up in the lungs to have its beneficial effect (Figure 17.1). The rest of the drug, 90% is swallowed, and has the potential to be absorbed, circulated and to cause adverse effects (Figure 17.1). This means that when drugs are designed for inhalation, it is still necessary to prevent systemic effects, if possible. One approach is to limit the absorption from the gastrointestinal tract, which will reduce levels in the systemic circulation and adverse effects (Figure 17.1). Another approach is to have a drug that is metabolised in the gut or undergo extensive first pass liver metabolism, as this will also reduce the levels in the systemic circulation and adverse effects (Figure 17.1). Both of these approaches are used with glucocorticoids, where it important to limit access to the circulation, and systemic adverse effects.

Figure 17.1 Fate of inhaled drugs (Copyright QUT, Sheila Doggrell)