Background—Although quinidine is known to elevate plasma digoxin concentrations, the mechanism underlying this interaction is not fully understood. Digoxin is not extensively metabolized, but it is known to be transported by the drug efflux pump P-glycoprotein, which is expressed in excretory tissues (kidney, liver, intestine) and at the blood-brain barrier. Accordingly, we tested the hypothesis that inhibition of P-glycoprotein–mediated digoxin transport by quinidine contributes to the digoxin-quinidine interaction.

Methods and Results—First, we demonstrated active transcellular transport of both digoxin and quinidine in cultured cell lines that express P-glycoprotein in a polarized fashion. In addition, 5 μmol/L quinidine inhibited P-glycoprotein–mediated digoxin transport by 57%. Second, the effect of quinidine on digoxin disposition was studied in wild-type and in mdr1a(−/−) mice, in which the gene expressing the major digoxin-transporting P-glycoprotein has been disrupted. Because the in vitro data showed that quinidine itself is a P-glycoprotein substrate, quinidine doses were reduced in mdr1a(−/−) mice to produce plasma concentrations similar to those in wild-type control animals. Quinidine increased plasma digoxin concentrations by 73.0% (P=0.05) in wild-type animals, compared with 19.5% (P=NS) in mdr1a(−/−) mice. Moreover, quinidine increased digoxin brain concentrations by 73.2% (P=0.05) in wild-type animals; by contrast, quinidine did not increase digoxin brain concentrations in mdr1a(−/−) mice but rather decreased them (−30.7%, P<0.01).

Conclusions—Quinidine and digoxin are both substrates for P-glycoprotein, and quinidine is a potent inhibitor of digoxin transport in vitro. The in vivo data strongly support the hypothesis that inhibition of P-glycoprotein–mediated digoxin elimination plays an important role in the increase of plasma digoxin concentration occurring with quinidine coadministration in wild-type mice and thus support a similar mechanism in humans.