Development of a novel rat airway epithelial cell culture model for permeability screening

Purpose: To evaluate the suitability of the rat airway epithelial cell line, RL65 as an in vitro model for permeability screening of inhaled drugs.
Methods: RL65 cells were grown on 12 well Transwell® inserts under different culture conditions. Morphology was characterised using histology and microscopy. Formation of functional tight junctions was verified by trans-epithelial electrical resistance (TEER) measurements, permeability of the paracellular marker 14C-mannitol and immunohistochemistry. The presence of P-glycloprotein (P-gp) was assessed by gene expression analysis and permeability studies with the established P-gp substrates 3H-digoxin and Rhodamine 123.
Results: RL65 cell layers were successfully cultured at an air-liquid interface for up to 21 days. They displayed morphological characteristics typical of the bronchial epithelium. TEER above 400 ohm.cm2 and 14C-mannitol permeability values below 2 x 10-6 cm/s were obtained after optimisation of the culture conditions. No asymmetric transport of 3H-digoxin or Rhodamine 123 was observed.
Conclusions: RL65 layers grown at an air-liquid interface offer a suitable rat respiratory in vitro model for permeability measurements with TEER values and 14C-mannitol permeability in the range of established human bronchial epithelial models. The presence of P-gp could not be confirmed under the culture conditions investigated. Evaluation of other drug transporter systems is required to further characterise the model.