Abstract
Digoxin is a commonly used drug to treat congestive heart failure. With its narrow therapeutic index, patients are susceptible to drug-drug interaction-mediated cardiotoxicity. Digoxin is primarily renally cleared; however, a significant component of clearance is due to multidrug resistance 1 (MDR1)-mediated transport into bile. Digoxin is reported to be actively transported into human hepatocytes by the organic anion transporting polypeptide 1B3 (OATP1B3); however, further characterization has not been fully described. The purpose of the present study was to investigate the hepatic uptake mechanisms of [3H]digoxin using sandwich-cultured human hepatocytes (SCHH) and transporter-expressing cells. Digoxin uptake in SCHH involves both a saturable (carrier-mediated) process and a passive (non-saturable) process. At low concentrations, the saturable component exhibited an apparent Km of 2.39 µM and a Vmax of 4.49 pmol/min/mg protein. The calculated passive diffusion clearance was 1.25 µL/min/mg protein. Uptake of [3H]digoxin in SCHH was not inhibited by a variety of substrates or inhibitors for OATP1B1, OATP1B3, OATP2B1, organic anion transporter 2 (OAT2), organic cation transporter 1 (OCT1) and monocarboxylate transporter 8 (MCT8). Cytochalasin B, which inhibits glucose transporters (GLUT), did not significantly inhibit digoxin uptake, whereas the flavonoids quercetin and rutin inhibited uptake by ~50%. Nonlabeled digoxin inhibited [3H]digoxin uptake by ~50%. Studies with OATP-transfected HEK cells or oocytes showed that digoxin is not a substrate of OATP1B1, 2B1 or 1B3. In conclusion, the data suggest that digoxin uptake in SCHH involves both saturable and passive processes. The saturable process is mediated by an as yet undetermined digoxin transporter(s).
- Received May 11, 2010.
- Accepted October 6, 2010.
- The American Society for Pharmacology and Experimental Therapeutics