In absence of a convenient and truly representative model of the alveolar epithelium, bronchial systems have been favoured [3] and [4]. Among these, the human bronchial cell line Calu-3 and normal human bronchial epithelial (NHBE) cells are gaining in popularity due to their capacity to develop polarised cell layers morphologically similar to the native epithelium and suitability for permeability measurements when

cultured at an air–liquid interface (ALI) [4], [5] and [6]. However, although the presence of active drug transport mechanisms has been confirmed in Calu-3 and NHBE layers [1], [6], [7], [8] and [9], an overview of the range of transporters being expressed and functional in these models is still lacking. P-glycoprotein/multidrug resistance protein 1 (P-gp/MDR1) is a member of the ATP-binding cassette (ABC) efflux transporter family and plays a major role in drug–drug interactions check details [10], limitation of oral drug absorption and poor drug penetration Wnt inhibitor in the central nervous system [11]. As it has been reported several drugs administered by the pulmonary route might be MDR1 substrates [1], targeting the transporter present in the epithelium has been envisaged as a strategy to increase the residence time of inhaled drugs in the lung tissue. Consequently, MDR1 is by far the most extensively studied drug

transporter in the lung [1]. Although weakly expressed in the lung as compared to other major organs [12], the presence

of the MDR1 protein has been demonstrated in the bronchial epithelium [1]. However, its actual impact on the pulmonary absorption of established substrates is a matter of debate [1]. Similarly, reports on the expression, localisation and functionality of MDR1 in bronchial epithelial cell culture models are Tryptophan synthase conflicting [1]. Passage number and time in culture have recently been shown to impact on MDR1 expression or activity in ALI bronchial epithelial layers [6], [13] and [14], which may partly explain discrepancies between studies. Identifying the transporter protein involved in carrier-mediated drug trafficking is highly challenging in biological systems expressing multiple transporters with broad and overlapping substrate specificities. For instance, the cardiac glycoside digoxin has been well characterised as an MDR1 substrate and is largely used for evaluating the risk of drug–drug interactions with new chemical entities consequent to their modulation of MDR1 activity [15] and [16]. Accordingly, although not an inhaled drug, digoxin has been used for probing MDR1 activity in bronchial cell culture models [13] and in rodent lungs [13] and [17]. However, digoxin has also been described as a substrate for carriers other than MDR1.