Systemic treatment by inhalation of macromolecules--principles, problems, and examples

Aerosol inhalation is an established route of medical administration for the treatment of pulmonary diseases. In contrast, aerosol inhalation for treatment of systemic diseases is a novel therapeutic approach. Clinical use of the latter therapy for many years has been limited by the lack of accuracy, efficiency, and reproducibility of the administered doses. Usually, only a small fraction of inhaled drug reached the target region within the lungs. Further problems were the risk of potential allergic reactions in the respiratory tract and a potential variability of drug absorption from the alveoli into the circulation. These problems have been solved in the last years by modern aerosol delivery systems allowing the production of an aerosol with a defined and optimised aerosol particle size combined with an optimized breathing maneuver and optimization of the efficacy of the technology. Furthermore, there were no observations of relevant allergic reactions after inhalation of systemically active drugs in numerous studies. Studies demonstrated that only a small number of morphological factors influence alveolar drug deposition (e.g., exogen allergic alveolitis, active sarcoidosis, active smoking). In consequence, an increasing number of studies investigated the systemic effect of inhaled high molecular weight substances (e.g., insulin, heparin, interleukin-2) and demonstrated that controlled aerosol therapy may serve as a non-invasive alternative for drug application by means of a syringe. Our review briefly summarizes the mechanisms for pulmonary absorption of macromolecules and gives an overview on prior research in the field of inhalant treatment of systemic diseases.