Pharmacokinetic/pharmacodynamic approaches to drug delivery design for inhalation drugs

Introduction: Inhaled drugs are important in the treatment of many lung pathologies, but to be therapeutically effective they must reach unbound concentrations at their effect site in the lung that are adequate to interact with their pharmacodynamic properties (PD) and exert the pharmacological action over an appropriate dosing interval. Therefore, the evaluation of pharmacokinetic (PK)/PD relationship is critical to predict their possible therapeutic effect. Areas covered: We review the approaches used to assess the PK/PD relationship of the major classes of inhaled drugs that are prescribed to treat pulmonary pathologies. Expert opinion: There are still great difficulties in producing data on lung concentrations of inhaled drugs and interpreting them as to their ability to induce the desired therapeutic action. The structural complexity of the lungs, the multiplicity of processes involved simultaneously and the physical interactions between the lungs and drug make any PK/PD approach to drug delivery design for inhalation medications extremely challenging. New approaches/methods are increasing our understanding about what happens to inhaled drugs, but they are still not ready for regulatory purposes. Therefore, we must still rely on plasma concentrations based on the axiom that they reflect both the extent and the pattern of deposition within the lungs.