Heterocyclic NO prodrugs.

An overview of the different heterocyclic NO-releasing compounds is given. Mesoionic heterocycles like sydnone imines are one example. This class is discussed on the synthesis and the mechanism of NO formation from Molsidomine and its first metabolite SIN-1. Furthermore, 1,2,3,4 oxatriazolium olates and imidates are presented in an example of the synthesis of GEA-3175. Heterocyclic N-oxides are another group of compounds capable of NO release under certain conditions. This class is discussed in the example of furoxane carboxamides like CAS-1609, and some SAR-data show the great impact of intramolecular hydrogen bridges on their in vitro activity. Each class of compounds requires different cofactors for NO release: sydnone imines need oxidants like oxygen, furoxanes are converted to NO via reaction with thioles.

Characterisation of furoxancarbonitriles as a new class of vasodilators.

The synthesis, structural characterization, NO-donor properties, and in vitro vasodilating activities of a series of furoxancarbonitriles 2, 17-22a, b are reported. Some derivatives (2b, 2a, 18b, 21b, 22b) are more potent vasodilating agents than sodium nitroprusside (SNP), the reference compound, some others display similar potency (17b, 19b, 20b). Log EC50 values fit well on the linear correlation log EC50 versus log C0.1(1 min) (namely the logarithm of the concentration able to release 2.6 mumol l-1 min-1 of NO) found in a previous work. The haemodynamic profile in anaesthetised pigs for some selected derivatives (2a, b, 19a, b) is also presented. These profiles are consistent with that known for another furoxan NO-donor (4-hydroxymethyl-3-furoxancarboxamide, CAS 1609) and suggest similar characteristic of in vivo NO-release.

Cardiovascular actions of the furoxan CAS 1609, a novel nitric oxide donor.

1. This study examines the cardiovascular effects of CAS 1609 (4-hydroxymethyl-furoxan-3-carboxamide) in vitro as well as in vivo in various animal models. 2. CAS 1609 relaxed guinea-pig pulmonary artery strips without endothelium with IC50-values of 0.9 microM (phenylephrine contracted) and 15 microM (KCl-depolarized). This effect was inhibited by oxyhaemoglobin. In these arteries CAS 1609 significantly increased (+192%) guanosine 3':5'-cyclic monophosphate levels, which indicates that the compound acts as a donor of nitric oxide (NO). 3. In the anaesthetized pig, CAS 1609 (0.3-1.0 mg kg-1, i.d.) significantly lowered blood pressure and left ventricular end-diastolic pressure. Left ventricular contractility was slightly reduced and heart rate remained almost unchanged. 4. In anaesthetized dogs, i.v. or i.d. administration of CAS 1609 (0.3-3.0 mg kg-1) decreased, in a dose-related fashion, preload and afterload of the heart, cardiac output, left ventricular work and myocardial oxygen consumption. This haemodynamic profile is similar to that of known NO-donors. 5. In anaesthetized dogs with acute heart failure due to intracoronary injection of microspheres, CAS 1609 (0.3 mg kg-1, i.v.) improved the haemodynamic condition and reduced mortality by 80%. 6. In conscious dogs, oral treatment with a dose of 0.5 mg kg-1 given twice daily at 07 h 00 min and 19 h 00 min (each dose had a duration of action > or = 12 h) for 5 days showed no signs of tolerance to the haemodynamic effects of the drug. 7. All these data indicate that CAS 1609 is a potent, long-lasting orally active donor of NO, devoid of tolerance development.

Antiischemic effects of pirsidomine, a new nitric oxide donor.

The antiischemic effect of pirsidomine (CAS 936 (3-(cis-2,6-dimethylpiperidino)-N-(4-methoxybenzoyl))-sydnon imine), a new nitric oxide donor, was investigated in a model of myocardial infarction in the dog. Dogs were anaesthetised, thoracotomized, and the left descending coronary artery was occluded for 6 h. Pirsidomine was given intraduodenally (i.d.) at the dose of 1.0 mg/kg to 11 dogs 30 min prior to coronary occlusion. Eleven dogs received the solvent i.d. and served as controls. Pirsidomine administration completely prevented the increase in left ventricular end-diastolic pressure and pulmonary artery pressure induced by the coronary occlusion and resulted in a marked decrease in systolic and diastolic blood pressure, cardiac output, left ventricular contractility, left ventricular work and left ventricular oxygen consumption. Additionally, pirsidomine completely prevented the occlusion-induced increase in flow in the non-occluded circumflex coronary artery. Regional blood flow measurements (with radioactive microspheres) revealed that pirsidomine induced a significant reduction in blood flow in the non-ischemic areas (both epi- and endocardial) but in the course of the ischemia, significantly increased flow in the ischemic epicardial areas. Infarct-size (triphenyltetrazolium chloride technique) in control dogs was 45% of the area at risk, but only 26% (P < 0.05) in pirsidomine-treated dogs. Thus, pirsidomine had a marked antiischemic effect in this model. This was probably due to the hemodynamic unloading of the heart as well as to redistribution of blood from the non-ischemic to the ischemic areas of the myocardium.

Thiol-mediated generation of nitric oxide accounts for the vasodilator action of furoxans.

Furoxans (1,2,5-oxadiazole-2-oxides) are widely used in organic chemistry as intermediate compounds for the synthesis of various heterocycles. Despite the fact that some furoxans have been found to possess remarkable biological activities, up to now no systematic study on their mode of action has been reported. The aim of the present study was to investigate the molecular mode of the vasodilator action of furoxans. Furoxans, but not the corresponding furazans, concentration-dependently increased coronary flow in an isolated working rat heart preparation. This effect was blunted upon coinfusion with methylene blue. All tested furoxans were demonstrated to increase potently the activity of soluble guanylate cyclase. Enzyme stimulation was found to be mediated by the generation of nitric oxide (NO) following chemical reaction of the furoxans with sulfhydryl groups of low molecular weight thiols and proteins. Furoxans are thus prodrugs which increase the level of cyclic GMP via formation of NO and may therefore be classified as nitrovasodilators. Along with the generation of NO, nitrite and nitrate ions and S-nitrosothiols were formed. The rates of formation of these metabolites, however, did not appear to be related to enzyme stimulation. A tentative reaction scheme that fits the obtained experimental data is proposed. Recently reported cytotoxic, mutagenic, immunosuppressive and anticancer effects of furoxans are discussed in the light of their ability to release NO upon reaction with thiols.

Cardiovascular effects of the new nitric oxide donor, pirsidomine. Hemodynamic profile and tolerance studies in anesthetized and conscious dogs.

The hemodynamic profile of pirsidomine, a new donor of NO (nitric oxide), was evaluated in dogs. In anesthetized dogs, the intravenous or intraduodenal administration of pirsidomine (0.3-10 mg/kg) decreased dose relatedly the preload and afterload of the heart, total peripheral resistance, cardiac output, left ventricular work and myocardial oxygen consumption. In conscious renal-hypertensive dogs, oral administration of pirsidomine (1.0-10 mg/kg) caused a marked and sustained decrease in systolic blood pressure and left ventricular end-diastolic pressure, which was accompanied by a slight and transient increase in heart rate and contractility. The diastolic blood pressure was affected less than in anesthetized dogs. Similar hemodynamic effects were obtained with M1 (3-(1-(2,6-dimethylpiperidino))-sydnonimine; 0.3-1 mg/kg), the main metabolite of pirsidomine, and with the known NO donor, isosorbide-5-mononitrate (IS-5-MN; 2-10 mg/kg). Tolerance development after repeated administration of pirsidomine and IS-5-MN was also investigated. In anesthetized dogs, repeated intraduodenal administrations of pirsidomine did not attenuate the response whereas tolerance occurred with hemodynamically equieffective doses of IS-5-MN. In conscious dogs, long term oral treatment, three times daily every 8th h for 5 days, revealed tolerance to IS-5-MN, slight or no tolerance to pirsidomine, and no cross-tolerance between the two agents. The results indicate that pirsidomine possesses an antianginal hemodynamic profile similar to that of its main metabolite, M1, and of IS-5-MN. This suggests a common mode of action via the release of NO.(ABSTRACT TRUNCATED AT 250 WORDS)

CAS 936, a novel syndnonimine with direct vasodilating and nitric oxide-donating properties: effects on isolated blood vessels.

Organic nitrates and sydnonimines exert their vasorelaxant activity by a common mechanism of action, i.e., release of nitric oxide (NO) and stimulation of the soluble guanylate cyclase of vascular smooth muscle cells. We wished to investigate the vasodilating activity of the novel sydnonimine CAS 936 in guinea pig isolated pulmonary arteries without endothelium. CAS 936 had no effect on contractions induced by norepinephrine (NE) or by the PGF2 alpha-analogue U46 619, but induced a longlasting relaxation of potassium depolarized arteries and of A23 187-contracted vessels. This effect was concentration-dependent (IC50 approximately 16 microM). Oxyhemoglobin and methylene blue had no inhibitory effect on CAS 936, whereas they inhibited the relaxations induced by SIN-1, a sydnonimine which acts by releasing NO. These results suggest that the vasodilating activity of CAS 936 is not related to NO. On the other hand, in vivo metabolites of CAS 936 inhibited NE- and U46 619-induced contractions. Oxyhemoglobin inhibited this effect. Therefore, we conclude that the CAS 936 molecule possesses a vasodilating activity of its own, whereas the metabolites may function as NO donors. The primary target of the intrinsic vasodilating activity of CAS 936 is very likely the vascular smooth muscle cell membrane. To determine which mechanism of action (NO unrelated or NO related) contributes mainly to the in vivo effects of CAS 936, studies of the metabolic fate of CAS 936 may be crucial.

[Nitric oxide dependence of the anti-thrombotic effect of sydnonimines in coronary arteries of swine]. / NO-Abhängigkeit der antithrombotischen Wirkung von Sydnoniminen in Koronararterien von Schweinen.

The metabolites of sydnonimine-derivatives generate nitric oxide (NO) and inhibit, like NO, platelet aggregation and adhesion. The purpose of the present study in anesthetized pigs was to examine if sufficient NO is generated by 3 sydnonimine-derivatives in vivo in order to prevent platelet-dependent arterial thrombosis. Stenosis of mechanically damaged coronary arteries resulted in cyclical blood flow reductions due to recurrent formation and embolization of platelet-thrombi. Injection of molsidomine (0.3 mg/kg) significantly reduced the frequency of cyclical flow reductions by 66%: 1.8 +/- 0.8/30 min versus 5.3 +/- 0.5/30 min before injection (p less than 0.01, n = 6). Molsidomine decreased arterial blood pressure by 34 +/- 7 mmHg (systolic) and 23 +/- 6 mmHg (diastolic) in these animals. Two further sydnonimine-derivatives were studied at a dose with similar hemodynamic effects. The onset of action of CAS 936 (2 mg/kg) was slow as compared to molsidomine. Therefore, CAS 936 reduced the frequency of cyclical flow reductions by 29% in the first hour following injection, but by 50% (p less than 0.05, n = 7) during the second hour. The remaining flow cycles were very small (mean size reduction 88%). C 89-4095 (0.6 mg/kg) prevented cyclical flow reductions nearly completely (reduction by 93%, p less than 0.01, n = 6). The subsequent injection of oxyhemoglobin reinitiated cyclical flow reductions in all three treatment groups. The antihypertensive drug dihydralazine (0.25 mg/kg) reduced blood pressure in 6 pigs by 38 +/- 3 (systolic) and 35 +/- 5 (diastolic) mmHg but had no effect on the frequency of cyclical coronary flow reductions.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen and oxidation promote the release of nitric oxide from sydnonimines.

The antianginal effect of sydnonimines is believed to be due to the release of nitric oxide (NO) from inactive prodrugs. The first step in the transformation of the sydnonimine SIN-1 is the hydroxyl ion-dependent conversion to SIN-1A that occurs almost immediately at neutral pH. However, the subsequent process of NO release from SIN-1A is not yet understood. Therefore, the spontaneous degradation of SIN-1A in aqueous solutions was studied by chemical and pharmacological means. Measurement of the oxygen content of a freshly prepared SIN-1A solution revealed a rapid decrease of the oxygen concentration. The oxygen-consuming process was accompanied by a decrease in SIN-1A concentration and a progressive formation of NO and SIN-1C. SIN-1A was stable under nitrogen, labile under air, and very unstable under pure oxygen. Thus, oxygen is an important reactant in the decomposition of SIN-1A. The question of whether or not oxygen is essential for NO release was studied by investigating the effect of other oxidants. The oxidants potassium hexacyanoferrate, cupric sulfate, and lead tetraacetate augmented the conversion of SIN-1A to SIN-1C and increased the relaxing effect on isolated pulmonary arteries used to bioassay NO. Therefore, an oxidative process is essential for the release of NO from sydnonimines.

[New cardiovascular acting 2-aryl-2-imidazolinyl-acetic acids. Synthesis and pharmacologic effects]. / Neue kardiovaskulär wirksame 2-Aryl-2-imidazolinyl-essigsäuren. Synthesen und pharmakologische Wirkungen.

The syntheses of new 2-aryl-2-imidazolinyl-acetic acids and esters are reported together with pharmacological results concerning structure-activity relationship. In contrast to the already known but inactive N(1)-alkylated derivatives of 2-arylamino-2-imidazolines the new compounds with an aryl acetic acid substitution in N(1) position of the imidazoline nucleus show the bradykardic activity of the exocyclic alkylated clonidine derivatives such as alinidine. The majority of these new substances reduce blood pressure and heart rate in anesthetized rats significantly with a long duration of action. 2-[2-(2,6-Dichloro-phenylamine)-2-imidazoline-1-yl]-2-(2-thienyl)- acetic acid 8a is the most potent compound. Blood pressure is lowered by 30-40 mmHg and heart rate by 175 beats/min with a duration of action greater than 60 min. The effect is weakened if the thienyl radical is replaced by other heterocycles or if the 2,6-dichlorphenyl group is replaced by a hydrogen atom. The variation of the substituents in the phenyl nucleus shows that the 2,6-dichlorphenyl structure is the optimum substitution pattern, as in the case of clonidine. In the series comprising the 2-[2-(2,6-dichlorphenylamino)-2-imidazoline-1-yl]-2-phenyl acetic acids (8j-8n) the incorporation of different substituents into the phenyl nucleus in the 2-position results in derivates of different activity. The esters 9a-9c show a faster onset of action as compared to the corresponding carboxylic acid 8a. The most interesting effect of compound 8a in normotensive conscious dogs is a strong and long-lasting decrease in heart rate accompanied by a moderate LVP dp/dtmax decrease, weak lowering of blood pressure and slight increase in LVEDP. After blocking beta-receptors by atenolol the compound 8a still reduces heart rate. Results in ganglion-blocked and pithed rats indicate a presynaptic and postsynaptic alpha 2-agonistic effect. A decrease in sympathetic tone as well as an increase in vagal activity in conscious dogs are considered as possible causes for bradycardic activity. Unlike alinidine, however, compound 8a does not affect directly sinus node function. The bradycardic effect of compound 8a which results in a decrease in oxygen consumption is supposed to be the cause of the reduction of infarct size in anesthetized dogs by 28%.