Interactions between endothelial mediators

Current Trends and Future Perspectives in the Treatment of Pulmonary Arterial Hypertension

Pulmonary hypertension continues to be a life-threatening illness with debilitating physical and emotional consequences affecting around 1% of global population. The progression of this devastating disease is characterized by increase in pulmonary vascular resistance resulting in elevated pulmonary pressure, eventually leading to right heart failure and death. Better understanding of pathophysiology has led to substantial improvements in terms of availability of treatment options. The purpose of this review is to summarize the currently available treatment options along with pertinent trials and possible future therapies of pulmonary arterial hypertension.

Prostanoids for Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a severe condition that markedly reduces exercise capacity and survival in the affected patient population. PAH includes primary pulmonary hypertension (PPH) and pulmonary hypertension associated with collagen vascular diseases, congenital systemic-to-pulmonary shunts, portal hypertension and HIV infection. All these conditions share virtually identical obstructive pathologic changes of the pulmonary microcirculation and probably similar pathobiologic processes. The pathophysiology is characterized by a progressive increase in pulmonary vascular resistance, leading to right ventricular failure and death. Prostacyclin is an endogenous substance that is produced by vascular endothelial cells and induces vasodilatation, inhibition of platelet activity, and antiproliferative effects. A dysregulation of prostacyclin metabolic pathways has been shown in patients with PAH and this represents the rationale for the exogenous therapeutic administration of this substance. The clinical use of prostacyclin in patients with PAH has been made possible by the synthesis of stable analogs that possess different pharmacokinetic properties but share similar pharmacodynamic effects. Experience in humans has been initially collected with epoprostenol, which is a synthetic salt of prostacyclin. Epoprostenol has a short half-life in the circulation and requires continuous administration by the intravenous route by means of infusion pumps and permanent tunnelized catheters. In addition, epoprostenol is unstable at room temperature, and the complex delivery system required is associated with several adverse effects and potentially serious complications. For these reasons, alternatives to intravenous epoprostenol have been sought and this has led to the development of analogs that can be administered subcutaneously (treprostinil), orally (beraprost sodium) or by inhalation (iloprost). Three unblinded clinical trials and several uncontrolled trials have shown that treatment with epoprostenol improved symptoms and exercise capacity in New York Heart Association (NYHA) class III and IV PAH patients and also survival in patients with PPH. Subcutaneous treprostinil improved symptoms, exercise, hemodynamics and clinical events in the largest clinical trial ever performed in PAH, but local infusion site reactions limited efficacy in a proportion of patients. Oral beraprost sodium improved exercise capacity only in patients with PPH and is the only prostacyclin analog that has also been tested in NYHA class II patients. Inhaled iloprost has improved symptoms, exercise capacity and clinical events in patients with PAH and inoperable chronic thromboembolic pulmonary hypertension. The favorable effects of prostanoids observed in all studies coupled with different profiles of adverse events and tolerability for each prostacyclin analog allow the unique opportunity to select the most appropriate compound for the individual patient with PAH.

Treprostinil for the treatment of pulmonary hypertension

BACKGROUND

Pulmonary hypertension (PH) is a severely disabling disorder characterized by sustained elevations of pulmonary arterial pressure, ultimately leading to right-heart failure and death. Pulmonary arterial hypertension (PAH) usually occurs in the absence of an evident cause (idiopathic PAH) or may be associated with connective tissue disease, HIV infection, congenital heart disease, chronic liver disease or result from the use of toxic agents and anorexigens.

OBJECTIVE/METHOD

Intravenous epoprostenol has been widely used in patients with PAH, leading to long-term clinical benefits and improved survival. Epoprostenol has to be delivered through a permanently implanted Intravenous catheter. This may expose patients to potentially life-threatening complications. Thus, more stable compounds and alternative modes of prostacyclin delivery have been sought.

CONCLUSION

Treprostinil sodium is a stable prostacyclin analogue, sharing pharmacologic actions similar to epoprostenol with comparable haemodynamic effects. Treprostinil is chemically stable at room temperature and has a long half-life (2 - 4 h), making this drug suitable for subcutaneous administration, with practical benefits in avoiding the risk of line infection and thrombosis, and cardiovascular reactions due to abrupt drug discontinuation.

Natriuretic peptides in cardiovascular diseases

The natriuretic peptide family comprises atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), dendroaspis natriuretic peptide (DNP), and urodilatin. The activities of natriuretic peptides and endothelins are strictly associated with each other. ANP and BNP inhibit endothelin-1 (ET-1) production. ET-1 stimulates natriuretic peptide synthesis. All natriuretic peptides are synthesized from polypeptide precursors. Changes in natriuretic peptides and endothelin release were observed in many cardiovascular diseases: e.g. chronic heart failure, left ventricular dysfunction and coronary artery disease.

Endothelial dysfunction in a primate model of cerebral vasospasm

OBJECT

Although abnormalities in the control of endothelial vasomotility have been reported in both experimental and clinical studies, the mechanism of the endothelial dysfunction that occurs following subarachnoid hemorrhage (SAH) remains unclear. Because of the absence of previous in vivo studies of endothelial function in cerebral vessels in response to SAH or cerebral vasospasm, the authors investigated endothelium-dependent responses in an established primate model of vasospasm after SAH. Endothelial function was assessed by examining vascular responses to intracarotid injections of various drugs known to act via the endothelium. Drugs that have a rapid total body clearance were selected so that their pharmacological effects would be limited to the cerebral circulation after an intracarotid infusion.

METHODS

Seventeen adult male cynomolgus monkeys were used. Cerebrovascular endothelium-dependent responses were examined in control animals and in animals with SAH 7, 14, and 21 days after placement of a subarachnoid clot around the right middle cerebral artery. Cortical cerebral blood flow (CBF) and cerebrovascular resistance (CVR) were recorded continuously during 5-minute intracarotid infusions of 5% dextrose vehicle, acetylcholine, histamine, bradykinin, or Calcimycin. In control animals the intracarotid infusion of acetylcholine produced a significant (7.8 +/- 9.5%) increase in CBF and a 9.3 +/- 8.7% reduction in CVR in comparison with a control infusion of dextrose vehicle. The responses to acetylcholine disappeared in animals 7 days post-SAH, specifically in the subset of animals in which arteriography confirmed the presence of vasospasm. Infusion of Calcimycin produced no significant changes in CBF or CVR in control animals, but resulted in a significant reduction in CBF and increase in CVR in animals 7 days after SAH and in animals with vasospasm. An infusion of histamine or bradykinin had no significant effect on CBF or CVR.

CONCLUSIONS

An intracarotid infusion of acetylcholine, but not one of histamine, bradykinin, or Calcimycin, produced a measurable physiological response in the normal primate cerebrovasculature. Cerebral vasospasm that occurred after SAH produced a pathophysiological effect similar to the endothelial denudation shown in the in vitro experiments of Furchgott and Zawadzki, in which acetylcholine constricted the vessels via activation of receptors on smooth-muscle cells. Changes in vascular responses to acetylcholine and Calcimycin in animals with vasospasm, compared with control animals, provide evidence that endothelial dysfunction plays a key role in the development and/or sustenance of vasospasm after SAH.

Effects of beraprost sodium, an oral prostacyclin analogue, in patients with pulmonary arterial hypertension: a randomized, double-blind, placebo-controlled trial

OBJECTIVES

The purpose of this study was to assess the efficacy and safety of beraprost sodium, an orally active prostacyclin analogue, in New York Heart Association (NYHA) functional class II and III patients with pulmonary arterial hypertension (PAH).

BACKGROUND

Pulmonary arterial hypertension is a life-threatening disease for which continuous intravenous infusion of prostacyclin has been proven effective. However, this treatment is associated with serious complications arising from the complex delivery system.

METHODS

In this double-blind, placebo-controlled study, 130 patients with PAH were randomized to the maximal tolerated dose of beraprost (median dose 80 microg four times a day) or to placebo for 12 weeks. The primary end point was the change in exercise capacity assessed by the 6-min walk test. Secondary end points included changes in Borg dyspnea index, cardiopulmonary hemodynamics and NYHA functional class.

RESULTS

Patients treated with beraprost improved exercise capacity and symptoms. The difference between treatment groups in the mean change of 6-min walking distance at week 12 was 25.1 m (95% confidence interval [CI]: 1.8 to 48.3, p = 0.036). The difference in the mean change of Borg dyspnea index was -0.94 (95% CI: -1.63 to -0.24, p = 0.009). In the sub-group of patients with primary pulmonary hypertension, the difference in the mean change of 6-min walking distance was 46.1 m (95% CI: 3.0 to 89.3, p = 0.035). Cardiopulmonary hemodynamics and NYHA functional class had no statistically significant changes. Drug-related adverse events were common in the titration phase and decreased in the maintenance period.

CONCLUSIONS

Beraprost improves exercise capacity and symptoms in NYHA functional class II and III patients with PAH and, in particular, in those with primary pulmonary hypertension.

Diabetes and macrovascular disease

Cardiovascular disease (CVD) is the major cause of morbidity and mortality in patients with diabetes. Macrovascular events, including stroke, myocardial infarction (MI), and peripheral arterial disease (PAD), occur earlier than in nondiabetics and the underlying pathologies are often more diffuse and severe. Diabetic arteriopathy, which encompasses endothelial dysfunction, hypercoagulability, changes in blood flow, and platelet abnormalities, contributes to the early evolution of these events. Tight glucose and blood pressure control improves the vascular status of these patients by varying degrees. Antiplatelet agents have also been shown to be effective in the secondary prevention of cardiovascular events. In the ideal world, every risk factor would be addressed and each diabetic would have excellent glycemic control, a low normal blood pressure, a low LDL, and be prescribed an ACE inhibitor, together with aspirin and clopidogrel. If this is done, this emerging epidemic of macrovascular disease will be contained.

Medical therapy of pulmonary hypertension. The prostacyclins

Prostacyclin is a substance produced by endothelial cells that induces vasodilation and inhibition of platelet aggregation and of vascular cell migration and proliferation. A dysregulation of the prostacyclin metabolic pathways has been shown in patients with pulmonary arterial hypertension. The clinical use of prostacyclin has been made possible by the synthesis of stable analogues that possesses different pharmacokinetic properties but share similar pharmacodynamic effects. The greatest experience has been collected with intravenous epoprostenol while other compounds like subcutaneous UT-15, inhaled iloprost and oral beraprost are currently in different stages of clinical development. Although favorable results have been reported for each compound, different benefit-to-side effects profiles characterize the various modalities of the administration.

Thiol repletion prevents venous thrombosis in rats by nitric oxide/prostacyclin-dependent mechanism: relation to the antithrombotic action of captopril

Clinical and experimental data have recently accumulated for antithrombotic action of angiotensin-converting enzyme inhibitors (ACE-1s). We have shown previously that captopril (which contains a thiol group in the moiety) exerts more pronounced antithrombotic activity than does an equipotent dose of enalapril (the drug devoid of the thiol group). To clarify the relative importance of the presence of the thiol group in the molecule versus angiotensin-converting enzyme (ACE) inhibitory properties in the antithrombotic action of captopril, rats were treated with captopril (5 mg/kg twice daily; CAP), epicaptopril (stereoisomer of captopril devoid of ACE-inhibitory properties; 5 mg/kg twice daily; EPI), N-acetylcysteine (3.75 mg/kg twice daily; ACC), enalapril (3 mg/kg once daily; ENA), or distilled water (VEH) for 10 days, per os. After ligation of the vena cava, the incidence of the venous thrombosis and/or the thrombus weight decreased significantly in all but the ENA-treated groups when compared with control rats. The effect of CAP, EPI, and ACC was accompanied by a marked reduction of euglobulin clot lysis time and, with the exception of ACC, by an increase in prothrombin time in the blood collected from the site of the thrombus formation. Antithrombotic activity of EPI was completely abolished by nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) or indomethacin, with the parallel reversal of fibrinolytic and coagulation parameters toward normal. Activated partial thromboplastin time, mean blood pressure, and bleeding time were not altered by either of the administered drugs. Thus, we demonstrated that thiol compounds exert antithrombotic activity by increasing fibrinolysis and/or suppression of the extrinsic pathway of the coagulation cascade in a nitric oxide/prostacyclin-dependent manner.

Major vasodilator role for nitric oxide in the gastrointestinal circulation of the mid-gestation fetal lamb

As nitric oxide (NO) may be a particularly important vasodilator in early life, we investigated its role in the regulation of the gastrointestinal (GI) circulation at mid-gestation. Cardiac output and GI blood flow were measured by the radioactive microsphere technique in eight chronically instrumented and unanesthetized mid-gestation fetal sheep. Mean arterial pressure (MAP), heart rate, blood flow, oxygen delivery, and vascular resistance were determined before and after infusion of the specific NO synthase inhibitor, Nomega-nitro-L-arginine (L-NNA) at doses of 10 and 25 mg/kg. In response to L-NNA infusion, MAP increased (p < 0.01) and combined ventricular output decreased (p < 0.001). GI blood flow and oxygen delivery decreased and vascular resistance increased in the stomach and all segments of the small and large intestine (all p < 0.001). The greatest reduction in blood flow was in the small intestine (p < 0.01) and the basal differential pattern of small intestinal blood flow exceeding large intestinal flow was completely abolished. These changes were much greater than those previously described in late-gestation fetuses. Our results suggest that, at mid-gestation, NO plays a major role in the regulation of blood flow and vascular tone across all segments of the fetal GI tract, with its effects being more pronounced than later in development.

Hypotensive and vasorelaxing activities of a lectin from the edible mushroom Tricholoma mongolicum

A lectin, which exerted a hypotensive action in rats after intravenous injection via the jugular vein, was isolated from the mycelia of the edible mushroom Tricholoma mongolicum. The lectin possessed a molecular weight of 37 K and its hypotensive activity was dose-dependent. Administration of the lectin at a dose of 10 mg/kg body weight caused a mean arterial blood pressure reduction of 95.3 +/- 7.4 mmHg. The lectin's hypotensive action was not mediated via autonomic ganglion transmission, alpha-adrenoceptors, beta-adrenoceptors, cholinergic receptors, histaminergic receptors, nor the renin-angiotensin system, but it was probably mediated through vasorelaxation via adenosine A2 receptors and/or nitric oxide production.