Drug absorption in the lungs : studies in the isolated perfused rat lung model combined with physiologically based biopharmaceutics modelling

Abstract: Pulmonary delivery of drugs is the preferred route of administration for treatment of local lung diseases like asthma and chronic obstructive pulmonary disease. Recently, there has also been increased interest in systemic delivery of drugs via the lungs to avoid problems with low and/or variable gastrointestinal absorption, and as a needle-free alternative for drugs that cannot be ingested. Both the pharmacological and the potentially adverse effects of inhaled drugs depend on the drug’s local and systemic concentrations, which in turn depend on the pulmonary absorption of the drug. Pulmonary drug absorption is governed by the dissolution, permeability, tissue retention, and non-absorptive clearance of the drug in the lungs. Predicting systemic and local exposure is necessary for developing an inhaled drug product, and these predictions can be based on data obtained from both in vitro and ex vivo methods, such as cell lines, solubility measurements, and the isolated perfused lung (IPL) model. Data obtained by these methods can then be used to inform physiologically based biopharmaceutics (PBB) models about drug-specific absorption parameters.The overall aim of this thesis was to increase the mechanistic understanding of pulmonary drug absorption, with a special focus on obtaining and analyzing ex vivo absorption parameters for different inhalation drugs and formulations, and evaluating the predictive power of these parameters in simulations of pulmonary drug absorption. In the first two papers of the thesis, drugs were formulated as solutions, suspensions, and dry powders, and pulmonary absorption of these were measured using the IPL model. The data from these experiments were then analyzed to obtain absorption parameters for each drug using a PBB model. Tissue retention was shown to be an important parameter for describing drug absorption in IPL, and particle wetting was shown to greatly affect the absorption of dry powders. Permeability in IPL correlated well with intrinsic permeability measured in cell monolayers, suggesting that passive transcellular transport is the main transport mechanism in the lungs. In the second two papers, the absorption parameters obtained from IPL data were used to simulate rat and human pulmonary drug absorption. The simulations predicted systemic exposure after inhalation well for both rat and human, suggesting that ex vivo parameters can be used to predict rat in vivo and human plasma concentrations. This thesis deepens our understanding of absorption parameters involved in pulmonary drug absorption, and suggests applications for these parameters in predictions of local and systemic exposure after inhalation.