P320 Digoxin is not a Substrate for Organic Anion Transporting Polypeptide Transporters OATP1A2, OATP1B1, OATP1B3, and OATP2B1 but is a Substrate for a Sodium Dependent Transporter Expressed in HEK293 Cells

Mitchell E. Taub , Drug Metabolism & Pharmacokinetics, Boehringer Ingelheim Pharmaceuticals, Inc., Ridgefield, CT
Kirsten Mease , Drug Metabolism & Pharmacokinetics, Boehringer Ingelheim Pharmaceuticals, Inc., Ridgefield, CT
Rucha S. Sane , Drug Metabolism & Pharmacokinetics, Boehringer Ingelheim Pharmaceuticals, Inc., Ridgefield, CT
Cory A. Watson , Drug Metabolism and Pharmacokinetics, GlaxoSmithKline, King of Prussia, PA
Liangfu Chen , Drug Metabolism and Pharmacokinetics, GlaxoSmithKline, King of Prussia, PA
Harma Ellens , Drug Metabolism and Pharmacokinetics, GlaxoSmithKline, King of Prussia, PA
Brad Hirakawa , Drug Safety Research & Development, Pfizer Global Research & Development, San Diego, CA
Eric L. Reyner , Structural and Computational Biology, Pfizer Global Research & Development, San Diego, CA
Márton Jani , Research & Development, SOLVO Biotechnology, Budaors, Hungary
Caroline A. Lee , Independent, San Diego, CA
Digoxin, an orally administered cardiac glycoside cardiovascular drug, has a narrow therapeutic window.  Circulating digoxin levels (Cmax ~1.5 ng/mL) require careful monitoring, and the potential for drug-drug interactions (DDI) is a concern.  Increases in digoxin plasma exposure caused by inhibition of P-glycoprotein (P-gp) have been reported.  Digoxin has also been described as a substrate of various organic anion transporting polypeptide (OATP) transporters, posing a risk that inhibition of OATPs may result in a clinically relevant DDI similar to what has been observed for P-gp.  While studies in rats have shown that Oatps contribute to the disposition of digoxin, the role of OATPs in the disposition of digoxin in humans has not been clearly defined.   In independent studies, Boehringer-Ingelheim, GlaxoSmithKline, Pfizer, and Solvo observed that digoxin is not a substrate of OATPs 1A2, 1B1, 1B3 and 2B1.  However, digoxin inhibited the uptake of probe substrates of OATP1B1 (IC50 of 47 μM), OATP1B3 (IC50 > 8.1 μM), and OATP2B1 (IC50 > 300 μM), but not OATP1A2 in transfected cell lines.  Interestingly, digoxin is a substrate of a sodium dependent transporter endogenously expressed in HEK293 cells, as uptake of digoxin was significantly greater in cells incubated with sodium-fortified media compared to incubations conducted in media where sodium was absent.  Thus, while digoxin is not a substrate for the human OATP transporters evaluated in this study, in addition to P-gp mediated efflux, its uptake and pharmacokinetic disposition may be partially facilitated by a sodium-dependent transporter.