Evaluation of bosentan, pinacidil and nitroprusside on human pulmonary arteries: comparison with rat pulmonary arteries

Dual impact of a nitric oxide donor, GEA 3175, in human pulmonary smooth muscle

Nitric oxide (NO) donors could constitute an alternative to inhaled NO as treatment in some patients with pulmonary hypertension. Therefore, the present study investigated the relaxation mechanisms of a novel NO donor, 3-(3-chloro-2-methylphenyl)-5-[[4-methylphenyl)sulphonyl]amino]-)hydroxide (GEA 3175) in segments of human pulmonary arteries and bronchioles, which were mounted in microvascular myographs. GEA 3175 induced concentration-dependent relaxations and was more potent in pulmonary arteries than in bronchioles. A blocker of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ), and iberiotoxin, a blocker of large-conductance calcium-activated K channels, both reduced relaxations induced by GEA 3175 in pulmonary arteries and bronchioles. Combining of ODQ and iberiotoxin did not produce additional inhibition. GEA 3175 relaxation is mediated through guanylyl cyclase-dependent mechanisms followed by activation of large-conductance calcium-activated K(+) channels. The dilatation of both pulmonary small arteries and airways by GEA 3175 seems advantageous, if it is considered administered as inhalation therapy for pulmonary hypertension.

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.

KRN2391: dual action on rat pulmonary artery and no loss of potency in pulmonary hypertension

1. The pulmonary vasorelaxant properties of KRN2391 (N-cyano-N'-(2-nitroxyethyl)-3-pyridinecarboximidamide) were examined in isolated ring preparations of main (MPA) and intralobar (IPA) pulmonary artery from control and pulmonary hypertensive rats (exposure to hypoxia, 10% oxygen, for 1 week). 2. On both MPA and IPA, pulmonary vasorelaxant responses were inhibited by methylene blue (10 micromol/L) or glibenclamide (1 or 10 micromol/L). Thus, KRN2391 has the properties of both a nitric oxide (NO) donor and a K(ATP) channel opener on rat pulmonary arteries. 3. KRN2391 was more potent and gave a greater maximum relaxation on MPA (-log EC(50) 6.47; maximum 92% reversal of induced contraction) than on IPA (-log EC(50) 6.09; maximum 58% reversal of induced contraction). Comparable differences between MPA and IPA were seen for SIN-1 (NO donor) and levcromakalim (K(ATP) channel opener). 4. KRN2391 was equipotent in MPA from control and pulmonary hypertensive rats but, when glibenclamide (10 micromol/L) was present, KRN2391 was six-fold less potent in preparations from pulmonary hypertensive than control rats. An eight-fold reduction in potency was seen for SIN-1 (no glibenclamide) in arteries from pulmonary hypertensive rats, confirming previous findings with other NO donors. 5. It is concluded that the dual mechanism of action of KRN2391 accounts for the finding that this drug is equally potent in pulmonary arteries from pulmonary hypertensive and control rats. In the context of pulmonary hypertension, this property of the drug could give it an advantage over drugs that act solely as NO donors because these decline in potency, at least in animal models of this disease.

In vitro comparison of two NONOates (novel nitric oxide donors) on rat pulmonary arteries

The pulmonary vasorelaxant properties of two NONOates (diazeniumdiolates) were examined because this novel group of nitric oxide (NO) donors may be useful in pulmonary hypertension. MAHMA NONOate ((Z)-1-¿N-Methyl-N-[6-(N-methylammoniohexyl)amino]¿ diazen-1-ium-1,2-diolate) and spermine NONOate ((Z)- 1-¿N-[3-aminopropyl]-N-[4-(3-aminopropylammonio)butyl]-amino¿di azen-1-ium-1,2-diolate) decomposed at different rates (half-lives 1.3 min and 73 min, respectively; 37 degrees C, pH 7.3) but generated the same total amount of NO. They fully relaxed submaximally contracted ring preparations of main and intralobar pulmonary arteries from rats. Responses were inhibited by the guanylate cyclase inhibitor, ODQ (1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one). Potency was not affected by choice of contractile spasmogen (phenylephrine, endothelin-1, thromboxane-mimetic) or endothelium removal, and tolerance did not develop; thus the drugs had properties important for use in pulmonary hypertension. MAHMA NONOate was 10-40-fold more potent than spermine NONOate but responses to spermine NONOate were more sustained (spermine NONOate > 60 min; MAHMA NONOate < 7 min). It is concluded that the differences in potency and time-course reflect the different rates of NO generation by these NONOates.