To evaluate the impact of CYP2C19 polymorphisms on ambrisentan exposure and to assess its modification by St. John's wort (SJW), 20 healthy volunteers (10 CYP2C19 extensive, four poor and six ultrarapid metabolizers) received therapeutic doses of ambrisentan (5 mg qd po) for 20 days and concomitantly SJW (300 mg tid po) for the last 10 days. To quantify changes of CYP3A4 activity, midazolam (3 mg po) as a probe drug was used. Ambrisentan pharmacokinetics was assessed on days 1, 10 and 20, and midazolam pharmacokinetics before and on days 1, 10, 17 and 20. At steady state, ambrisentan exposure was similar in extensive and ultrarapid metabolizers but 43% larger in poor metabolizers (p < 0.01). In all volunteers, SJW reduced ambrisentan exposure and the relative change (17-26%) was similar in all genotype groups. The extent of this interaction did not correlate with the changes in CYP3A activity (midazolam clearance) (r s = 0.23, p = 0.34). Ambrisentan had no effect on midazolam pharmacokinetics. In conclusion, SJW significantly reduced exposure with ambrisentan irrespective of the CYP2C19 genotype. The extent of this interaction was small and thus likely without clinical relevance.Ambrisentan is an ET A -selective endothelin-1 receptor antagonist (ERA) approved for the treatment of pulmonary arterial hypertension (PAH), which is a devastating disease characterized by elevated pulmonary arterial pressure, right ventricular failure and ultimately death. Ambrisentan improved the quality of life and exercise capacity as determined by the 6-min. walking distance, decreased dyspnoea and delayed the time to clinical worsening in patients with PAH. Also haemodynamic (e.g. pulmonary vascular resistance) and hormonal (e.g. N-terminal-pro-hormone-brain natriuretic peptide) endpoints were improved [1,2].Common side effects of ERA include headache, hepatotoxicity, peripheral oedema and nasal congestion, which dose dependently increase in frequency; for example, doubling the dose doubled ambrisentan-associated nasal congestion [1]. Higher doses of ambrisentan also lead to more pronounced beneficial effects [1], suggesting that effects and side effects of ambrisentan are dose dependent, and substantial changes in exposure are likely of clinical relevance.Clearance of ambrisentan is mainly determined by the uridine diphosphate glucuronosyltransferase (UGT) isozymes UGT1A9, UGT2B7 and UGT1A3. A minor part of the drug (20%) undergoes phase I oxidative metabolism mainly by cytochrome P450 (CYP) 3A4 and to a lesser extent by the polymorphic CYPs 3A5 and 2C19 to form the pharmacologically inactive 4-hydroxymethyl ambrisentan [3]. Organic anion-transporting polypeptides (OATP) partly mediate the hepatic uptake of ambrisentan, while efflux transporters do not appear to be involved in its kinetics [4,5].More than 50% of PAH patients show depressive symptoms [6] with every fifth patient being in need of pharmacological therapy [7]. Self-medication of St John's wort (Hypericum perforatum, SJW) is often unrecognized [8] and may de...

SJW increased CYP3A activity but had no consistent effect on bosentan clearance. However, inter-individual changes of the interaction were large, suggesting that close monitoring of bosentan effects may be advisable. The contribution of CYP2C9 to this interaction seems to be minor.