Effects of itraconazole and diltiazem on the pharmacokinetics of fexofenadine, a substrate of P-glycoprotein

Aims: Fexofenadine is a substrate of several drug transporters including P-glycoprotein. Our objective was to evaluate the possible effects of two P-glycoprotein inhibitors, itraconazole and diltiazem, on the pharmacokinetics of fexofenadine, a putative probe of P-glycoprotein activity in vivo, and compare the inhibitory effect between the two in healthy volunteers. Methods: In a randomized three-phase crossover study, eight healthy volunteers were given oral doses of 100 mg itraconazole twice daily, 100 mg diltiazem twice daily or a placebo capsule twice daily (control) for 5 days. On the morning of day 5 each subject was given 120 mg fexofenadine, and plasma concentrations and urinary excretion of fexofenadine were measured up to 48 h after dosing. Results: Itraconazole pretreatment significantly increased mean (± SD) peak plasma concentration (C_max) of fexofenadine from 699 (± 366) ng ml^-1 to 1346 (± 561) ng ml^-1 (95% CI of differences 253, 1040; P < 0.005) and the area under the plasma concentration-time curve [AUC(0,∞)] from 4133 (± 1776) ng ml^-1 h to 11287 (± 4552) ng ml^-1 h (95% CI 3731, 10575; P < 0.0001). Elimination half-life and renal clearance in the itraconazole phase were not altered significantly compared with those in the control phase. In contrast, diltiazem pretreatment did not affect C_max (704 ± 316 ng ml^-1, 95% CI -145, 155), AUC(0, ∞) (4433 ± 1565 ng ml^-1 h, 95% CI -1353, 754), or other pharmacokinetic parameters of fexofenadine. Conclusions: Although some drug transporters other than P-glycoprotein are thought to play an important role in fexofenadine pharmacokinetics, itraconazole pretreatment increased fexofenadine exposure, probably due to the reduced first-pass effect by inhibiting the P-glycoprotein activity. As diltiazem pretreatment did not alter fexofenadine pharmacokinetics, therapeutic doses of diltiazem are unlikely to affect the P-glycoprotein activity in vivo.