Targeted drug delivery to the respiratory tract: Solute permeability of air-interface cultured rabbit tracheal epithelial cell monolayers

The permeability characteristics of air-interfaced rabbit tracheal epithelial cell monolayers to model macromolecules and low molecular weight solutes were evaluated. The model macromolecules were fluorescein isothiocyanate (FITC)-labeled dextrans of varying molecular sizes (4,000 to 70,000 daltons). The model low molecular weight solutes were beta-adrenergic compounds of similar sizes with widely different log octanol/pH 7.4 buffer partition coefficients (log P). FITC-dextrans were assayed spectro-fluorimetrically and beta-adrenergic compounds were assayed by reverse phase HPLC with UV detection. The apparent permeability coefficients (Papp) for FITC alone to FITC-dextran 70,000 in the tracheal epithelial barrier were in the range of 11.2 to 0.3 X 10(-8) cm/sec. A molecular cut-off at about 20,000 daltons, consistent with a single equivalent pore population of about 5 nm in radius, was found. A sigmoidal relationship best described the influence of drug lipophilicity on the Papp of beta-adrenergic compounds, where the log P at the half maximal Papp was 2.08. Thus, the air-interfaced rabbit tracheal epithelial cell culture model has been successfully applied to elucidate the permeability of tracheal epithelial barrier to model macromolecules and small solutes. It appears that the tracheal epithelial cell monolayers absorb drugs in a similar manner as do native excised tracheal and other epithelia.