Current management of primary pulmonary hypertension

eNOS-beta-actin interaction contributes to increased peroxynitrite formation during hyperoxia in pulmonary artery endothelial cells and mouse lungs

Oxygen toxicity is the most severe side effect of oxygen therapy in neonates and adults. Pulmonary damage of oxygen toxicity is related to the overproduction of reactive oxygen species (ROS). In the present study, we investigated the effect of hyperoxia on the production of peroxynitrite in pulmonary artery endothelial cells (PAEC) and mouse lungs. Incubation of PAEC under hyperoxia (95% O(2)) for 24 h resulted in an increase in peroxynitrite formation. Uric acid, a peroxynitrite scavenger, prevented hyperoxia-induced increase in peroxynitrite. The increase in peroxynitrite formation is accompanied by increases in nitric oxide (NO) release and endothelial NO synthase (eNOS) activity. We have previously reported that association of eNOS with β-actin increases eNOS activity and NO production in lung endothelial cells. To study whether eNOS-β-actin association contributes to increased peroxynitrite production, eNOS-β-actin interaction were inhibited by reducing β-actin availability or by using a synthetic peptide (P326TAT) containing a sequence corresponding to the actin binding site on eNOS. We found that disruption of eNOS-β-actin interaction prevented hyperoxia-induced increases in eNOS-β-actin association, eNOS activity, NO and peroxynitrite production, and protein tyrosine nitration. Hyperoxia failed to induce the increases in eNOS activity, NO and peroxynitrite formation in COS-7 cells transfected with plasmids containing eNOS mutant cDNA in which amino acids leucine and tryptophan were replaced with alanine in the actin binding site on eNOS. Exposure of mice to hyperoxia resulted in significant increases in eNOS-β-actin association, eNOS activity, and protein tyrosine nitration in the lungs. Our data indicate that increased association of eNOS with β-actin in PAEC contributes to hyperoxia-induced increase in the production of peroxynitrite which may cause nitrosative stress in pulmonary vasculature.

Nitric oxide and pulmonary hypertension

Pulmonary hypertension is a serious complication of a number of lung and heart diseases that is characterized by peripheral vascular structural remodeling and loss of vascular tone. Nitric oxide can modulate vascular injury and interrupt elevation of pulmonary vascular resistance selectively; however, it can also produce cytotoxic oxygen radicals and exert cytotoxic and antiplatelet effects. The balance between the protective and adverse effects of nitric oxide is determined by the relative amount of nitric oxide and reactive radicals. Nitric oxide has been shown to be clinically effective in the treatment of congenital heart disease, mitrial valvular disease combined with pulmonary hypertension and in orthotropic cardiac transplantation patients. Additionally, new therapeutic modalities for the treatment of pulmonary hypertension, phosphodiesterase inhibitors, natriuretic peptides and aqueous nitric oxide are also effective for treatment of elevated pulmonary vascular resistance.

Advances in the Medical Treatment of Pulmonary Hypertension

Increased pulmonary precapillary vascular resistance due to vasoconstriction and vasoproliferative processes is the basic pathophysiological mechanism in the development of pulmonary hypertension (PH). With the exception of pulmonary venous hypertension, where the primary cause of PH is left ventricular failure or mitral valvular disease, all the other PH categories will benefit to a greater or lesser extent from pulmonary vasodilator and antivasoproliferative therapy. Today, for this purpose, in addition to intravenous prostacyclin (epoprostenol), which is restricted to severe pulmonary arterial hypertension (NYHA class IV and late class III), other therapeutic options such as treatment with more stable prostacyclin analogs (oral beraprost, aerosolized iloprost), endothelin-receptor antagonists (bosentan) or phosphodiesterase inhibitors (sildenafil) are also available and these are especially useful for the treatment of the early stages of the disease. The recent progress in medical therapy has markedly increased the life expectancy in patients with pulmonary arterial hypertension and substantially improved their quality of life. Chronic hemodialysis (HD) patients show higher endothelin-1 (ET-1) activity in comparison to healthy individuals and there is evidence that the increase of pulmonary vascular resistance in these patients is at least in part mediated by ET-1. Recent data show good results after PH therapy with the endothelin-receptor antagonist bosentan in HD patients. Also prostacyclin and its analogs, as well as phosphodiesterase inhibitors, can be useful for the treatment of pulmonary hypertension in patients with chronic renal failure.

Inhaled iloprost: in primary pulmonary hypertension

Iloprost is a stable prostacyclin analogue with a pharmacokinetic profile allowing nebulised administration in patients with primary pulmonary hypertension (PPH). Inhaled iloprost is a potent acute pulmonary vasodilator with a duration of action of about 60 minutes. It may exert additional long-term benefit through antiproliferative and antithrombotic effects. Inhaled iloprost 2.5 or 5 microg six or nine times daily for 12 weeks (n = 101) significantly (p < 0.01) improved a combined clinical endpoint of a > or =10% increase in distance walked in 6 minutes and an improvement of > or =1 class in New York Heart Association functional class without clinical deterioration or death (16.8 versus 4.9% of placebo recipients, n = 102) in patients with severe PPH or selected forms of nonprimary pulmonary hypertension. Statistical analysis of the response for the PPH subgroup (20.8 versus 5.5% with placebo; n = 51 and 51) was not reported. Improvements from baseline in exercise capacity and haemodynamic/gas exchange variables have been reported in patients with PPH with continued use of inhaled iloprost. In addition, improvement in preinhalation vascular resistance occurred after 12 weeks of inhaled iloprost (p < 0.01 versus placebo) in a large randomised trial. Increased cough, headache, flushing and an influenza-like syndrome were the most common adverse events in the largest trial of patients receiving inhaled iloprost. Headache, flushing and jaw pain occurred significantly more frequently with inhaled iloprost than with placebo.

Understanding primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a severe and progressive disease. It is characterized by persistent elevation of pulmonary artery pressure without any known cause. The hemodynamics demonstrates high pulmonary artery pressure and resistance to blood flow through the lung circulation. PPH is a diagnosis of exclusion. The diagnosis is often delayed because the initial symptoms may be mild and the exam findings subtle. The mean age of survival is 2.8 years without treatment. With treatment patients can survive for more than 10 years. Current lifetime treatment includes anticoagulation, oxygen, digoxin, calcium channel blockers, vasodilators, and diuretics. Promising results have been obtained with prostacyclin and endothelin receptor antagonists. Lung transplantation can be considered for patients with advances PPH and those not responding to medical therapy.

Bosentan

Bosentan is a nonpeptide, specific, competitive, dual antagonist at both endothelin receptor subtypes (ET(A) and ET(B)). Orally administered bosentan effectively prevents endothelin 1-induced vasoconstriction in pulmonary vessels in patients with pulmonary arterial hypertension. Improvement in exercise capacity from baseline was significantly greater with bosentan than with placebo in two phase III trials in patients with WHO functional class III or IV pulmonary arterial hypertension (primary or associated with connective tissue disease) despite treatment with vasodilators, diuretics, anticoagulants, cardiac glycosides, or supplemental oxygen. The beneficial effects of bosentan on exercise capacity were maintained for at least 20 weeks. Compared with placebo, bosentan led to a significantly greater improvement from pretreatment values in secondary efficacy endpoints such as the Borg dyspnea index, WHO functional class, and cardiopulmonary hemodynamic parameters (cardiac index, pulmonary vascular resistance, pulmonary artery pressure, pulmonary capillary wedge pressure, mean right atrial pressure). Bosentan significantly reduced the incidence, and delayed the onset, of clinical worsening of pulmonary arterial hypertension compared with placebo. In published clinical trials, adverse events that occurred with similar or greater frequency with bosentan 125 mg twice daily than with placebo included headache, syncope, flushing and abnormal hepatic function. Those that occurred less frequently with bosentan 125 mg twice daily than with placebo included dizziness, worsening of symptoms of pulmonary arterial hypertension, cough and dyspnea.