Ambrisentan: A New Drug For Pulmonary Arterial Hypertension

A New Drug for PAH: Clinical Pharmacology of Ambrisentan

In June 2007, the FDA approved ambrisentan (LETAIRIS) for the once-daily treatment of PAH to improve exercise capacity and delay clinical worsening.

Ambrisentan, a diphenyl propionic acid derivative ( Figure 1. PubChem CID:6918493), represents a valuable addition to the treatment alternatives for this orphan disease, which include prostacyclin (Flolan), inhaled nitric oxide, iloprost (a prostacyclin analog), sildenafil (a PDE5 inhibitor), and conventional Ca++ channel blockers (e.g., verapamil and nifedipine). Ambrisentan is orally active and similar to an existing drug, bosentan;[5] both are competitive inhibitors of endothelin receptors and are termed endothelin receptor antagonists (ERAs). However, bosentan lacks the receptor selectivity of ambrisentan. Ambrisentan exhibits ∼100-fold greater selectivity for endothelin receptor A (ETA) as compared to endothelin receptor B (ETB) [Ki = 0.6 nM for the human ETA receptor, 49 nM for the ETB receptor].[4] This moderate selectivity for the ET receptor may provide therapeutic advantages compared to bosentan. A common adverse effect of ERAs is the propensity to impair liver function in some patients. Thus, ambrisentan is not recommended for use in patients with moderate or severe hepatic impairment since it may cause additional liver damage.[6] Liver aminotransferase enzymes (ALT and AST) must be measured before initiation of therapy and at least monthly thereafter during therapy. The drug is an FDA category X pregnancy drug and is contraindicated in women who are or who may become pregnant. It is not known whether the drug is secreted in milk, therefore nursing mothers should not breastfeed while receiving the drug. Because of the risks of liver injury and birth defects, ambrisentan is only available through the Letairis Education and Access Program (LEAP); only prescribers and pharmacies registered with LEAP may prescribe and dispense the drug, and only patients enrolled in and meeting all conditions of LEAP may receive the drug (for enrollment information, call 866-663-5327).

Figure 1.  (click image to zoom)

       

Adult dosing is initiated with 5 mg once daily, and if tolerated, dosing is increased to 10 mg once daily. Although early clinical trials showed no significant differences in beneficial effects using different doses of ambrisentan (1-10 mg) for patients with PAH,[7] the results of two recent clinical trials, ARIES-1 and ARIES-2, showed a dose-dependent improvement in 6-minute walk distance with once a day ambrisentan after 12 weeks. The absolute bioavailability of ambrisentan in humans is unknown, but is ∼90% in dogs.[7a] Bioavailability is unaffected by food. The drug is rapidly absorbed after oral administration with peak plasma concentrations occurring between 1.7 and 3.3 hours. The terminal half-life is about 15 hours, but after long-term daily dosing the effective half-life is about 9 hours.[49] At steady-state, mean trough concentrations of drug are ∼15% of mean peak concentrations. The drug is highly bound to plasma proteins and elimination is primarily by non-renal pathways. In vitro data indicate ambrisentan is a potential substrate of P-glycoprotein (P-gp), the Organic Anion Transport Protein (OATP), CYP3A4, CYP2C19, and uridine glucuronosyltransferases (UGTs) 1A9S, 2B7S, and 1A3S. Because in vivo data regarding drug interactions of ambrisentan with strong inhibitors or inducers of these transporters and metabolizing enzymes are very limited, caution is advised in co-administering drugs known to affect these entities. Concomitant administration of ambrisentan with warfarin in PAH patients had no effect on PT or INR; no adjustments in dose of warfarin or ambrisentan are required when co-administered.[49] Similarly, no dose adjustments are required when ambrisentan is co-administered with sildenafil.[49] Available in vitro and in vivo pharmacokinetic data indicate hepatic impairment is likely to prolong the elimination of ambrisentan. The drug is not recommended for patients with moderate to severe liver disease, and patients with mild pre-existing hepatic impairment may require reduced dosing. Based on clinical data, no dose adjustment is required in patients with mild to moderate renal impairment, but data in patients with severe renal disease are currently unavailable.[49]

Clinical studies have indicated ambrisentan and other ERAs cause a decrease in hemoglobin and hematocrit during the first few weeks of therapy, which then stabilizes. The cause of the decrease in hemoglobin is not known, but is not due to bleeding or hemolysis. Hemoglobin measurements are required prior to ambrisentan therapy and monthly thereafter. Another ERA class effect is peripheral edema, but this is also a consequence of PAH. In the clinical studies with ambrisentan, there was an increase in peripheral edema in patients taking 5 or 10 mg daily compared to placebo, but this was typically mild to moderate in severity. Most other adverse drug reactions seen in the clinical studies were mild to moderate; discontinuation of treatment due to adverse events other than those related to pulmonary hypertension was similar for drug and placebo (∼2%).[49]Pathophysiology of PAH

PAH is defined hemodynamically as a mean pulmonary arterial pressure of =25 mmHg with a pulmonary capillary wedge pressure of =15 mmHg, both measured at rest by right-heart catheterization. Elevated pulmonary arterial pressure in PAH patients is caused by increased pulmonary vascular resistance (PVR) as a result of severe pulmonary vascular remodeling, sustained pulmonary vasoconstriction, and in situ thrombosis in small arteries. Excessive vascular remodeling, manifested by obliteration of small arteries, muscularization of precapillary arterioles, intimal lesions and fibrosis, and the plexiform lesion, is the major contributor to the elevated PVR in most patients, while sustained vasoconstriction contributes to the elevated PVR in 15-20% of PAH patients. Vasoconstriction, by itself, is also a stimulus of pulmonary artery smooth muscle cell hypertrophy and hyperplasia. Loss of pulmonary vascular compliance and increased PVR due to pulmonary vascular remodeling leads to progressively pronounced pulmonary hypertension and has indeed been found to be the predominate pathological finding in idiopathic PAH.[1]Endothelin and Endothelin Receptors in PAH

Endothelin (ET) is a 21-amino acid peptide exhibiting three isoforms ET-1, ET-2, and ET-3 ( Figure 2, Figure 3, PDB ID: 1EDN). ET-1 is a potent vasoconstrictor, mitogen, and proinflammatory mediator. In the pulmonary circulation, increased ET-1 plasma levels correlate with changes in arterial pressure.[9,10] In patients with PAH, the level of ET-1 in the plasma and pulmonary microvasculature is increased as much as 10-fold and its concentrations correlate with pulmonary artery pressure and severity of disease.[10-13] The actions of endothelins in the vasculature are mediated by at least three different G-protein coupled receptors, the ETA receptor and two ETB receptor subtypes, ETB1 and ETB2.[20,21] ETA receptors bind ET-1 and ET-2 with a higher affinity than ET-3 (order of potency: ET-1 = ET-2 > ET-3), while ETB receptors have similar affinities for all three endothelin isoforms (i.e., order of potency is ET-1 = ET-2 = ET-3). ETA and ETB are functionally expressed in various tissues with different proportions. There are at least two studies demonstrating that expression of smooth muscle cell ETB receptors is increased in PAH,[13,16] while other studies demonstrate that the endothelial ETB receptors are downregulated or dysfunctional.[17,18] Furthermore, the endothelial ETB receptor-mediated ET clearance remains intact in patients with idiopathic PAH, and only modestly reduced in patients with PAH associated with connective tissue diseases (e.g., scleroderma).[19] These studies seem to suggest that increased ET concentration in the plasma and lung tissue serves as an important stimulus for sustained pulmonary vasoconstriction and excessive vascular remodeling in patients with PAH. Selective blockade of the endothelin receptors (e.g., ETA) that mediate contractile and mitogenic effects on PASMC, while maintaining (or preserving) the function of the endothelin receptors (e.g., ETB) that cause vasodilative effects and induce ET-1 clearance, is therefore a good strategy for designing therapeutic approaches for patients with PAH, and may offer more benefits than nonselective ETA/ETB antagonists.

Figure 2.  (click image to zoom)

Amino acid sequence of ET-1. Dotted lines show arrangement of intramolecular disulfide bonds.      

Figure 3.  (click image to zoom)

The 3-D structure of human ET-1 determined by x-ray crystallography.      

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