The presence and functionality of P-gp (mdr1) proteins were probed respectively by immunocytochemistry and bi-directional permeability studies with the two established substrates, 3H-digoxin and Rh123. A positive immunocytochemical signal was obtained on the apical surface of RL-65 cell layers cultured in both media for 8 days while no green fluorescence was detected when cells were only incubated with the FITC-labelled secondary antibody (Fig. 6). However, no statistical difference (p > 0.05) between AB and BA transport across 8-day old RL-65 layers was observed for any of the two P-gp substrates investigated ( Fig. 7), suggesting

negligible transporter-mediated drug trafficking ZD6474 in the cell culture model. In vivo and ex vivo absorption studies are frequently conducted in rats to predict the pharmacokinetics of inhaled drug candidates in humans ( Tronde et al., 2003). However, variations in drug disposition in human and rat lungs have yet to be fully appraised. A rat respiratory epithelial cell culture model suitable for permeability screening would aid better understanding of interspecies differences in pulmonary drug absorption, including the role of drug transporters, in addition to providing an ethical alternative to animal testing. This

study evaluates the potential of layers of the bronchial/bronchiolar epithelial rat cell line, RL-65, as an in vitro permeability screening tool. It demonstrates that RL-65 cells

cultured at an AL interface on Transwell® supports formed layers morphologically Venetoclax solubility dmso similar to the upper airway epithelium with a TEER and 14C-mannitol paracellular permeability values in agreement with those in established human bronchial epithelial cell models. Expression of the drug transporters P-gp Oxalosuccinic acid and octn2 was confirmed in the cell layers, although no vectorial transport of widely used P-gp probes was observed. This preliminary characterisation of air-interfaced RL-65 cell layers identifies a potentially useful tool for investigating differences in drug permeability between the human and rat airway epithelia. Morphological analysis of RL-65 cells grown in presence of serum revealed multilayered cultures with an uppermost layer of non-viable cells (Fig. 4), thus providing a poor representation of the native epithelium. This indicated that a serum containing medium is unsuitable for the development of RL-65 cells into polarised layers mimicking the airway epithelium. Likewise, sub-optimal growth of the cell line had previously been described in presence of serum (Roberts et al., 1990). Our study also demonstrated that the sole consideration of markers of epithelial barrier formation such as TEER and paracellular permeability values is potentially misleading for a reliable assessment of cell-based absorption screens, and highlights the importance of morphological examinations in the characterisation of those models.