Effects of Rifampicin (Rifampin) on the Pharmacokinetics and Safety of Ambrisentan in Healthy Subjects A Single-Sequence, Open-Label Study

Background: Ambrisentan is a once-daily, endothelin (ET) type A receptor-selective antagonist approved for the treatment of pulmonary arterial hypertension. Ambrisentan is primarily metabolized by glucuronidation and undergoes cytochrome P450 (CYP)-mediated oxidation to a lesser extent. Objective: To assess the effects of rifampicin (rifampin), a potent inducer of CYP3A4 and inhibitor of organic anion transporter polypeptides (OATPs), on the steady-state pharmacokinetics, safety and tolerability of ambrisentan. Methods: This was a 14-day, single-sequence, open-label study that was conducted in 24 healthy adults. Subjects were administered oral doses of ambrisentan (10 mg) once daily on days 1 through 5 and were then co-administered ambrisentan (10 mg) plus rifampicin (600 mg) once daily on days 6 through 13. The steady-state pharmacokinetics of ambrisentan and its oxidative metabolite 4-hydroxymethyl ambrisentan were determined in the absence and presence of repeated administration of rifampicin. The main outcome measure was the analysis of ambrisentan pharmacokinetics (area under the plasma concentration-time curve during a dosage interval [AUC(tau)], maximum plasma drug concentration [C(max)] and minimum plasma drug concentration [C(min)]) for steady-state ambrisentan alone (day 5) as compared with steady-state ambrisentan plus steady-state rifampicin (day 13). Adverse events (AEs), ECG recordings, vital signs and clinical laboratory parameters were monitored throughout the study and at follow-up. Results: A transient increase (+87% [95% CI 79, 95]) in ambrisentan steady-state systemic exposure (AUC(tau)) was observed during the first 2 days of rifampicin co-administration. However, in the presence of steady-state rifampicin, ambrisentan C(max) and AUC(tau) values were similar (+2% [95% CI 7, 12] and -4% [-9, 2], respectively) to those observed for ambrisentan alone. Relative systemic exposure of 4-hydroxymethyl ambrisentan was unaffected by either acute or steady-state rifampicin. No serious AEs or AEs leading to withdrawal were reported and there were no clinically significant changes in vital signs, ECG recordings or clinical laboratory parameters with co-administration of ambrisentan and rifampicin. Conclusion: Steady-state rifampicin had no clinically relevant effects on the steady-state pharmacokinetics of ambrisentan. The overall safety profile of ambrisentan was similar in the presence and absence of rifampicin. No dose adjustment of ambrisentan should be required when it is co-administered with rifampicin, a strong inducer of CYP3A4 activity and inhibitor of OATPs.