RESOURCES

Abstract
Besides deposition, pulmonary bioavailability is determined by dissolution of particles in the scarce epithelial fluid and by cellular API uptake. In the present work, we have developed an experimental in vitro model, which is combining the state-of-the-art next generation impactor (NGI), used for aerodynamic performance assessment of inhalation products, with a culture of human alveolar A549 epithelial cells to study the fate of inhaled drugs following lung deposition. The goal was to investigate five previously developed nano-milled and spray-dried budesonide formulations and to examine the suitability of the in vitro test model. The NGI dissolution cups of stages 3, 4, and 5 were transformed to accommodate cell culture inserts while assuring minimal interference with the air flow. A549 cells were cultivated at the air–liquid interface on Corning® Matrigel® -coated inserts. After deposition of aerodynamically classified powders on the cell cultures, budesonide amount was determined on the cell surface, in the interior of the cell monolayer, and in the basal solution for four to eight hours. Significant differences in the total deposited drug amount and the amount remaining on the cell surface at the end of the experiment were found between different formulations and NGI stages. Roughly 50% of budesonide was taken up by the cells and converted to a large extent to its metabolic conjugate with oleic acid for all formulations and stages. Prolonged time required for complete drug dissolution and cell uptake in case of large deposited powder amounts suggested initial drug saturation of the surfactant layer of the cell surface. Discrimination between formulations with respect to time scale of dissolution and cell uptake was possible with the present test model providing useful insights into the biopharmaceutical performance of developed formulations that may be relevant for predicting local bioavailability. The absolute quantitative result of cell uptake and permeation into the systemic compartment is unreliable, though, because of partly compromised cell membrane integrity due to particle impaction and professed leakiness of A549 monolayer tight junctions, respectively.