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Abstract
Oral inhalation of drugs is the classic therapy of obstructive lung diseases. In contrast to the oral route, the link between in vitro and in vivo findings is less well defined and predictive models and parameters for in vitro-in vivo correlations are missing. Frequently used in vitro models and problems in obtaining in vivo values to establish such models and to identify the action of formulations in vivo are discussed. It may be concluded that major obstacles to link in vitro parameters on in vivo action include lack of treatment adherence and incorrect use of inhalers by patients, variation in inhaler performance, changes by humidity, uncertainties about lung deposition, and difficulties to measure drug levels in epithelial lining fluid and tissue. Physiologically more relevant in vitro models, improvement in inhaler performance, and better techniques for in vivo measurements may help to better understand importance and interactions between individual in vitro parameters in pulmonary delivery.
Highlights
Inhalation of aerosols has a long history in the treatment of lung diseases
Variations in the delivery from inhalers and effects of inhaler mishandling by the patient, disease-related changes in airway morphology, pH and viscosity of airway lining fluid and receptor distribution will induce some variation
Several developments may improve the current situation in the future, while for others dramatic changes are not expected (Table 1)
Summary
Inhalation of aerosols has a long history in the treatment of lung diseases. It has the advantage that the drug is directly delivered at the level of the diseased cells. 2. Patient InhaleMraIinnlytetrharecetitoypnes of devices, pressured metered dose inhalers (pMDI), dry powder inhaler (DPI), and nebulizers are used to deliver the aerosol. Drug particles are inhaled by airflow generated by the patient. Soft mist inhalers represent a new type of propellant-free inhaler that delivers a higher proportion of the emitted dose to the lung than pMDI or DPI. In a systemic review of 38 studies reporting inhaler misuse by asthma and COPD patients >18y, it was stated that rates of incorrect use were mostly in the range of 0–20%. Another study on inhaler misuse by adult asthma and COPD patients reported overall error rates of 50–100% with critical errors in 14–92%. Maximal airway velocity was not affected in generation G5 of the bronchial tree (corresponding to the end of the upper airways with 3.5 mm diameter) but strongly affected in G6-1 to G7-3 (smaller airways with 2.2–2.7 mm diameter) at light and moderate exercise [23]
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