Drug Interaction Report

MONITOR: Coadministration with levacetylleucine may increase the plasma concentrations (AUC) and adverse effects of drugs that are substrates of the P-glycoprotein (P-gp) efflux transporter. Levacetylleucine is a P-gp inhibitor in vitro. However, clinical data are not available.

MANAGEMENT: If coadministration is clinically necessary, more frequent clinical monitoring for P-gp substrate related adverse reactions is advised and dose adjustments may be required, particularly when levacetylleucine is initiated or withdrawn from concomitant therapy. Consultation with package labeling of the concomitant medication may be advisable.

Administration of digoxin with a high-fiber meal has been shown to decrease its bioavailability by almost 20%. Fiber can sequester up to 45% of the drug when given orally. Patients should be advised to maintain a regular diet without significant fluctuation in fiber intake while digoxin is being titrated.

Grapefruit juice may modestly increase the plasma concentrations of digoxin. The mechanism is increased absorption of digoxin due to mild inhibition of intestinal P-glycoprotein by certain compounds present in grapefruits. In 12 healthy volunteers, administration of grapefruit juice with and 30 minutes before, as well as 3.5, 7.5, and 11.5 hours after a single digoxin dose (0.5 mg) increased the mean area under the plasma concentration-time curve (AUC) of digoxin by just 9% compared to administration with water. Moreover, P-glycoprotein genetic polymorphism does not appear to influence the magnitude of the effects of grapefruit juice on digoxin. Thus, the interaction is unlikely to be of clinical significance.