Carrier-mediated transport of tiotropium in bronchial smooth muscle cells: The role of pH-dependent organic cation transporters

Organic cation transporters (OCTs) play an important role in drug absorption, tissue distribution and elimination. Cationic drug transport mechanisms in the airway tissue, however, are poorly understood. To assess the role of OCTs in airway drug transport, uptake of the inhaled bronchodilator tiotropium (a cationic, quaternary ammonium compound) by human bronchial smooth muscle cells was measured. Primary bronchial smooth muscle cells showed a carrier-mediated, concentration-, time-, and temperature-dependent uptake of tritiated tiotropium. Tiotropium uptake was reduced by various OCT inhibitors, including L-carnitine, the pH-dependent organic cation/carnitine transporter 2 (OCTN2)-carried zwitterion. OCTN2 expression was confirmed by RT-PCR and immunofluorescence analysis. Selective suppression of OCTN2 expression using lentivirus-mediated production of shRNA reduced tiotropium uptake in smooth muscle cells. Although bronchial smooth muscle cells also express corticosteroid-sensitive OCTs, tiotropium uptake was insensitive to corticosteroids in these cells. In contrast, tiotropium uptake was pH-dependent, with nearly 50% lower rates at acidic (pH 5.7) than at alkaline (pH 8.2) extracellular pH. These findings demonstrate the existence and functional role of a pH-dependent uptake machinery for tiotropium in human bronchial smooth muscle cells. We suggest that inflammation-associated airway acidification, through OCTN2 inhibition, could interfere with the disposal of tiotropium by smooth muscle in the airway and increase its concentration at muscarinic receptor sites.