Targeting Heme-Oxidized Soluble Guanylate Cyclase in Experimental Heart Failure

Fluorescence dequenching makes haem-free soluble guanylate cyclase detectable in living cells

In cardiovascular disease, the protective NO/sGC/cGMP signalling-pathway is impaired due to a decreased pool of NO-sensitive haem-containing sGC accompanied by a reciprocal increase in NO-insensitive haem-free sGC. However, no direct method to detect cellular haem-free sGC other than its activation by the new therapeutic class of haem mimetics, such as BAY 58-2667, is available. Here we show that fluorescence dequenching, based on the interaction of the optical active prosthetic haem group and the attached biarsenical fluorophor FlAsH can be used to detect changes in cellular sGC haem status. The partly overlap of the emission spectrum of haem and FlAsH allows energy transfer from the fluorophore to the haem which reduces the intensity of FlAsH fluorescence. Loss of the prosthetic group, e.g. by oxidative stress or by replacement with the haem mimetic BAY 58-2667, prevented the energy transfer resulting in increased fluorescence. Haem loss was corroborated by an observed decrease in NO-induced sGC activity, reduced sGC protein levels, and an increased effect of BAY 58-2667. The use of a haem-free sGC mutant and a biarsenical dye that was not quenched by haem as controls further validated that the increase in fluorescence was due to the loss of the prosthetic haem group. The present approach is based on the cellular expression of an engineered sGC variant limiting is applicability to recombinant expression systems. Nevertheless, it allows to monitor sGC's redox regulation in living cells and future enhancements might be able to extend this approach to in vivo conditions.

Nitric oxide synthase and cyclic GMP signaling in cardiac myocytes: from contractility to remodeling

Cyclic guanosine 3'5'monophosphate (cGMP) is the common downstream second messenger of natriuretic peptides and nitric oxide. In cardiac myocytes, the physiological effects of cGMP are exerted through the activation of protein kinase G (PKG) signaling, and the activation and/or inhibition of phosphodiesterases (PDEs), providing an integration point between cAMP and cGMP signals. Specificity of cGMP signals is achieved through compartmentalization of cGMP synthesis by guanylate cyclases, and cGMP hydrolysis by PDEs. Increasing evidence suggests that cGMP-dependent signaling pathways play an important role in inhibiting cardiac remodeling, through the inhibition Ca(2+) handling upstream of pathological Ca(2+)-dependent signaling pathways. Thus, enhancing cardiac myocyte cGMP signaling represents a promising therapeutic target for treatment of cardiovascular disease. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

Measuring oxidative burden and predicting pharmacological response in coronary artery disease patients with a novel direct activator of haem-free/oxidised sGC

OBJECTIVE

The soluble guanylate cyclase (sGC) activator Cinaciguat (BAY 58-2667) represents a novel class of drugs that selectively activate oxidised sGC. The extent of oxidised sGC depends on the patient's oxidative burden. We here describe two platelet-based assays that allow determining the extent of oxidised sGC and thus provide a basis for an individualised pharmacotherapy.

METHODS/RESULTS

Platelets obtained from patients with (n=12) and without (n=12) coronary artery disease (CAD) were examined by flow cytometry (P-selectin expression), and Western blots (vasodilator associated phosphoprotein, VASP-phosphorylation). Results were compared to maximal oxidation of sGC achieved by the oxidising agent ODQ (1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one). Treatment of platelets with Cinaciguat resulted in differential activation of oxidised sGC. Platelet P-selectin expression and VASP-phosphorylation revealed significant differences (p=0.012, p=0.039, respectively) between CAD and non-CAD patients.

CONCLUSION

We describe platelet-based assays that allow the determination of patients' oxidative status and thus allow the prediction of pharmacological response to direct sGC activators.

Soluble guanylate cyclase activation with cinaciguat: a new approach to the treatment of decompensated heart failure

Heart failure (HF) remains a major cause of morbidity and mortality in the United States despite recent advances in its treatment. The nitric oxide -soluble guanylate cyclase (sGC)-cyclic 3', 5'-guanosine monophosphate pathway is a key signaling cascade involved in many physiologic processes. Derangements of the cascade may play an important role in the pathophysiology of HF and other diseases. Organic nitrates, which derive their action from their metabolic conversion to nitric oxide, exploit this pathway therapeutically. They are a mainstay of treatment for acute HF, but the development of tolerance with chronic administration limits their long-term efficacy. The development of a novel class of sGC activators has shown in both animal and preliminary clinical trials to improve hemodynamics without tolerance, while preserving renal function in patients with HF. A phase II clinical program using the sGC activator cinaciguat (BAY 58-2667) is now in progress in patients with symptomatic HF to further evaluate the efficacy and safety of this treatment approach.

Heme-dependent and independent soluble guanylate cyclase activators and vasodilation

Since the discovery of nitric oxide (NO), which is released from endothelial cells as the main mediator of vasodilation, its target, the soluble guanylyl cyclase (sGC), has become a focus of interest for the treatment of diseases associated with endothelial dysfunction. NO donors were developed to suppress NO deficiency; however, tolerance to organic nitrates was reported. Non-NO-based drugs targeting sGC were developed to overcome the problem of tolerance. In this review, we briefly describe the process of sGC activation by its main physiological activator NO and the advances in the development of drugs capable of activating sGC in a NO-independent manner. sGC stimulators, as some of these drugs are called, require the integrity of the reduced heme moiety of the prosthetic group within the sGC and therefore are called heme-dependent stimulators. Other drugs are able to activate sGC independent of heme moiety and are hence called heme-independent activators. Because pathologic conditions modulate sGC and oxidize the heme moiety, the heme-independent sGC activators could potentially become drugs of choice because of their higher affinity to the oxidized enzyme. However, these drugs are still undergoing clinical trials and are not available for clinical use.

Pulmonary and systemic vasodilator responses to the soluble guanylyl cyclase activator, BAY 60-2770, are not dependent on endogenous nitric oxide or reduced heme

4-({(4-Carboxybutyl)[2-(5-fluoro-2-{[4'-(trifluoromethyl)biphenyl-4-yl]methoxy}phenyl)ethyl]amino}methyl)benzoic acid (BAY 60-2770) is a nitric oxide (NO)-independent activator of soluble guanylyl cyclase (sGC) that increases the catalytic activity of the heme-oxidized or heme-free form of the enzyme. In this study, responses to intravenous injections of the sGC activator BAY 60-2770 were investigated under baseline and elevated tone conditions induced by the thromboxane mimic U-46619 when NO synthesis was inhibited by N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME), when sGC activity was inhibited by 1H-[1,2,4]-oxadizaolo[4,3]quinoxaline-1-one (ODQ), an agent that oxidizes sGC, and in animals with monocrotaline-induced pulmonary hypertension. The intravenous injections of BAY 60-2770 under baseline conditions caused small decreases in pulmonary arterial pressure, larger decreases in systemic arterial pressure, and no change or small increases in cardiac output. Under elevated tone conditions during infusion of U-46619, intravenous injections of BAY 60-2770 caused larger decreases in pulmonary arterial pressure, smaller decreases in systemic arterial pressure, and increases in cardiac output. Pulmonary vasodilator responses to BAY 60-2770 were enhanced by L-NAME or by ODQ in a dose that attenuated responses to the NO donor sodium nitroprusside. ODQ had no significant effect on baseline pressures and attenuated pulmonary and systemic vasodilator responses to the sGC stimulator BAY 41-8543 2-{1-[2-(fluorophenyl)methyl]-1H-pyrazolo[3,4-b]pyridin-3-yl}-5(4-morpholinyl)-4,6-pyrimidinediamine. BAY 60-2770 and sodium nitroprusside decreased pulmonary and systemic arterial pressures in monocrotaline-treated rats in a nonselective manner. The present data show that BAY 60-2770 has vasodilator activity in the pulmonary and systemic vascular beds that is enhanced by ODQ and NOS inhibition, suggesting that the heme-oxidized form of sGC can be activated in vivo in an NO-independent manner to promote vasodilation. These results show that BAY 60-2770 and sodium nitroprusside decreased pulmonary and systemic arterial pressures in monocrotaline-treated rats, suggesting that BAY 60-2770 does not have selective pulmonary vasodilator activity in animals with monocrotaline-induced pulmonary hypertension.

Regulation of circulating progenitor cells in left ventricular dysfunction

BACKGROUND

Reductions in numbers of circulating progenitor cells (CD34+ cell subsets) have been demonstrated in patients at risk for, or in the presence of, cardiovascular disease. The mediators of these reductions remain undefined. To determine whether neurohumoral factors might regulate circulating CD34+ cell subsets in vivo, we studied complementary canine models of left ventricular (LV) dysfunction.

METHODS AND RESULTS

A pacing model of severe LV dysfunction and a hypertensive renal wrap model in which dogs were randomized to receive deoxycorticosterone acetate (DOCA) were studied. Circulating CD34+ cell subsets including hematopoietic precursor cells (HPCs: CD34+/CD45(dim)/VEGFR2-) and endothelial progenitor cells (EPCs: CD34+/CD45-/VEGFR2+) were quantified. Additionally, the effect of mineralocorticoid excess on circulating progenitor cells in normal dogs was studied. The majority of circulating CD34+ cells expressed CD45dimly and did not express VEGFR2, consistent with an HPC phenotype. HPCs were decreased in response to pacing, and this decrease correlated with plasma aldosterone levels (Spearman rank correlation=-0.67, P=0.03). In the hypertensive renal wrap model, administration of DOCA resulted in decreased HPCs. No changes were seen in EPCs in either model. Normal dogs treated with DOCA exhibited a decrease in HPCs in peripheral blood but not bone marrow associated with decreased telomerase activity.

CONCLUSIONS

This is the first study to demonstrate that mineralocorticoid excess, either endogenous or exogenous, results in reduction in HPCs. These data suggest that mineralocorticoids may induce accelerated senescence of progenitor cells, leading to their reduced survival and decline in numbers.

Role of guanylate cyclase modulators in decompensated heart failure

In this review we investigate the role of particulate and soluble guanylate cyclase (pGC and sGC, respectively) pathways in heart failure, and several novel drugs that modify guanylate cyclase. Nesiritide and ularitide/urodilatin are natriuretic peptides with vasodilating, natriuretic and diuretic effects, acting on pGC, whilst cinaciguat (BAY 58-2667) is a novel sGC activator. Cinaciguat has a promising and novel mode of action because it can stimulate cyclic guanosine-3',5'-monophosphate synthesis by targeting sGC in its nitric oxide-insensitive, oxidised ferric (Fe(3+)) or haem-free state. Thus, cinaciguat may also be effective under oxidative stress conditions resulting in oxidised or haem-free sGC refractory to traditional organic nitrate therapies. Preliminary studies of cinaciguat in patients with acute decompensated heart failure show substantial improvements in haemodynamics and symptoms, whilst maintaining renal function.

A nitric oxide/cysteine interaction mediates the activation of soluble guanylate cyclase

Nitric oxide (NO) regulates a number of essential physiological processes by activating soluble guanylate cyclase (sGC) to produce the second messenger cGMP. The mechanism of NO sensing was previously thought to result exclusively from NO binding to the sGC heme; however, recent studies indicate that heme-bound NO only partially activates sGC and additional NO is involved in the mechanism of maximal NO activation. Furthermore, thiol oxidation of sGC cysteines results in the loss of enzyme activity. Herein the role of cysteines in NO-stimulated sGC activity investigated. We find that the thiol modifying reagent methyl methanethiosulfonate specifically inhibits NO activation of sGC by blocking a non-heme site, which defines a role for sGC cysteine(s) in mediating NO binding. The nature of the NO/cysteine interaction was probed by examining the effects of redox active reagents on NO-stimulated activity. These results show that NO binding to, and dissociation from, the critical cysteine(s) does not involve a change in the thiol redox state. Evidence is provided for non-heme NO in the physiological activation of sGC in context of a primary cell culture of human umbilical vein endothelial cells. These findings have relevance to diseases involving the NO/cGMP signaling pathway.

Pharmacologic management of the cardiorenal syndrome in heart failure

Cardiorenal syndrome describes the impairment of renal function and associated diuretic resistance in patients with heart failure and clinically manifest volume overload. The pathophysiology of this syndrome is poorly understood, but appears to be caused by impairment of tubuloglomerular feedback, neurohormonal activation, and other factors and therapies used in the management of heart failure. Early diagnosis of the cardiorenal syndrome by way of markers of renal injury and function is critical for timely interventions that may attenuate progression. Many novel therapies have been evaluated in the cardiorenal syndrome setting, including agents that block key local factors (eg, adenosine A(I) receptor antagonists), improve diuresis, aquaresis, and natriuresis, and augment natural vasodilator mechanisms to improve renal perfusion. Furthermore, device-based approaches such as ultrafiltration may also play an important therapeutic role.

Exploiting cGMP-based therapies for the prevention of left ventricular hypertrophy: NO* and beyond

Left ventricular hypertrophy (LVH), an increased left ventricular (LV) mass, is common to many cardiovascular disorders, initially developing as an adaptive response to maintain myocardial function. In the longer term, this LV remodelling becomes maladaptive, with progressive decline in LV contractility and diastolic function. Indeed LVH is recognised as an important blood-pressure independent predictor of cardiovascular morbidity and mortality. The clinical efficacy of current treatments for LVH is reduced, however, by their tendency to slow disease progression rather than induce its reversal, and thus the development of new therapies for LVH is paramount. The signalling molecule cyclic guanosine-3',5'-monophosphate (cGMP), well-recognised for its role in regulating vascular tone, is now being increasingly identified as an important anti-hypertrophic mediator. This review is focused on the various means by which cGMP can be stimulated in the heart, such as via the natriuretic peptides, to exert anti-hypertrophic actions. In particular we address the limitations of traditional nitric oxide (NO*) donors in the face of the potential therapeutic advantages offered by novel alternatives; NO* siblings, ligands of the cGMP-generating enzymes, soluble (sGC) and particulate guanylyl cyclases (pGC), and phosphodiesterase inhibitors. Further impact of cGMP within the cardiovascular system is also discussed with a view to representing cGMP-based therapies as innovative pharmacotherapy, alone or concurrent with standard care, for the management of LVH.

Pharmacological activation of soluble guanylate cyclase protects the heart against ischemic injury

BACKGROUND

The role of the nitric oxide/cGMP/cGMP-dependent protein kinase G pathway in myocardial protection and preconditioning has been the object of intensive investigations. The novel soluble guanylate cyclase activator cinaciguat has been reported to elevate intracellular [cGMP] and activate the nitric oxide/cGMP/cGMP-dependent protein kinase G pathway in vivo. We investigated the effects of cinaciguat on myocardial infarction induced by isoproterenol in rats.

METHODS AND RESULTS

Rats were treated orally twice a day for 4 days with vehicle or cinaciguat (10 mg/kg). Isoproterenol (85 mg/kg) was injected subcutaneously 2 days after the first treatment at an interval of 24 hours for 2 days to produce myocardial infarction. After 17 hours, histopathological observations and left ventricular pressure-volume analysis to assess cardiac function with a Millar microtip pressure-volume conductance catheter were performed, and levels of biochemicals of the heart tissues were measured. Gene expression analysis was performed by quantitative real-time polymerase chain reaction. Isolated canine coronary arterial rings exposed to peroxynitrite were investigated for vasomotor function, and immunohistochemistry was performed for cGMP and nitrotyrosine. The present results show that cinaciguat treatment improves histopathological lesions, improves cardiac performance, improves impaired cardiac relaxation, reduces oxidative stress, ameliorates intracellular enzyme release, and decreases cyclooxygenase 2, transforming growth factor-beta, and beta-actin mRNA expression in experimentally induced myocardial infarction in rats. In vitro exposure of coronary arteries to peroxynitrite resulted in an impairment of endothelium-dependent vasorelaxation, increased nitro-oxidative stress, and reduced intracellular cGMP levels, which were all improved by cinaciguat. A cardioprotective effect of postischemic cinaciguat treatment was shown in a canine model of global ischemia/reperfusion.

CONCLUSIONS

Pharmacological soluble guanylate cyclase activation could be a novel approach for the prevention and treatment of ischemic heart disease.

Cyclic GMP signaling in cardiovascular pathophysiology and therapeutics

Cyclic guanosine 3',5'-monophosphate (cGMP) mediates a wide spectrum of physiologic processes in multiple cell types within the cardiovascular system. Dysfunctional signaling at any step of the cascade - cGMP synthesis, effector activation, or catabolism - have been implicated in numerous cardiovascular diseases, ranging from hypertension to atherosclerosis to cardiac hypertrophy and heart failure. In this review, we outline each step of the cGMP signaling cascade and discuss its regulation and physiologic effects within the cardiovascular system. In addition, we illustrate how cGMP signaling becomes dysregulated in specific cardiovascular disease states. The ubiquitous role cGMP plays in cardiac physiology and pathophysiology presents great opportunities for pharmacologic modulation of the cGMP signal in the treatment of cardiovascular diseases. We detail the various therapeutic interventional strategies that have been developed or are in development, summarizing relevant preclinical and clinical studies.

Cinaciguat (BAY 58-2667) improves cardiopulmonary hemodynamics in patients with acute decompensated heart failure

BACKGROUND

Cinaciguat (BAY 58-2667) is the first of a new class of soluble guanylate cyclase activators in clinical development for acute decompensated heart failure. We aimed to assess the hemodynamic effects, safety, and tolerability of intravenous cinaciguat in patients with acute decompensated heart failure (pulmonary capillary wedge pressure > or =18 mm Hg).

METHODS AND RESULTS

After initial dose finding (part A; n=27), cinaciguat was evaluated in the nonrandomized, uncontrolled proof-of-concept part of the study (part B; n=33) using a starting dose of 100 microg/h, which could be titrated depending on hemodynamic response. Patients were categorized as responders if their pulmonary capillary wedge pressure decreased by > or =4 mm Hg compared with baseline. Final doses of cinaciguat after 6 hours of infusion in part B were 50 microg/h (n=2), 200 microg/h (n=12), and 400 microg/h (n=16). Compared with baseline, a 6-hour infusion of cinaciguat led to significant reductions in pulmonary capillary wedge pressure (-7.9 mm Hg), mean right atrial pressure (-2.9 mm Hg), mean pulmonary artery pressure (-6.5 mm Hg), pulmonary vascular resistance (-43.4 dynes . s . cm(-5)), and systemic vascular resistance (-597 dynes . s . cm(-5)), while increasing heart rate by 4.4 bpm and cardiac output by 1.68 L/min. The responder rate was 53% after 2 hours, 83% after 4 hours, and 90% after 6 hours. Cinaciguat was well tolerated, with 13 of 60 patients reporting 14 drug-related treatment-emergent adverse events of mild to moderate intensity, most commonly hypotension.

CONCLUSIONS

Cinaciguat has potent preload- and afterload-reducing effects, increasing cardiac output. Further investigation of cinaciguat for acute decompensated heart failure is warranted.

BAY 58-2667, a nitric oxide-independent guanylyl cyclase activator, pharmacologically post-conditions rabbit and rat hearts

AIMS

BAY 58-2667 (BAY-58) directly activates soluble guanylyl cyclase without tolerance in a nitric oxide (NO)-independent manner, and its haemodynamic effect is similar to that of nitroglycerin. We tested whether BAY-58 could make both rabbit and rat hearts resistant to infarction when given at the end of an ischaemic insult.

METHODS AND RESULTS

All hearts were exposed to 30 min regional ischaemia followed by 120-(isolated hearts) or 180-(in situ hearts) min reperfusion. BAY-58 (1-50 nM) infused for 60 min starting 5 min before reperfusion significantly reduced infarction from 33.0 +/- 3.2% in control isolated rabbit hearts to 9.5-12.7% (P < 0.05). In a more clinically relevant in situ rabbit model, infarct size was similarly reduced with a loading dose of 53.6 microg/kg followed by a 60 min infusion of 1.25 microg/kg/min (41.1 +/- 3.1% infarction in control hearts to 16.0 +/- 4.4% in treated hearts, P < 0.05). BAY-58 similarly decreased infarction in the isolated rat heart, and protection was abolished by co-treatment with a protein kinase G (PKG) antagonist, or a mitochondrial K(ATP) channel antagonist. Conversely, N(omega)-nitro-L-arginine-methyl-ester-hydrochloride, a NO-synthase inhibitor, failed to block BAY-58's ability to decrease infarction, consistent with the latter's putative NO-independent activation of PKG. Finally, BAY-58 increased myocardial cGMP content in reperfused hearts while cAMP was unchanged.

CONCLUSION

When applied at reperfusion, BAY-58 is an effective cardioprotective agent with a mechanism similar to that of ischaemic pre-conditioning and, hence, should be a candidate for treatment of acute myocardial infarction in man.

NO- and haem-independent soluble guanylate cyclase activators

Oxidative stress, a risk factor for several cardiovascular disorders, interferes with the NO/sGC/cGMP signalling pathway through scavenging of NO and formation of the strong intermediate oxidant, peroxynitrite. Under these conditions, endothelial and vascular dysfunction develops, culminating in different cardio-renal and pulmonary-vascular diseases. Substituting NO with organic nitrates that release NO (NO donors) has been an important principle in cardiovascular therapy for more than a century. However, the development of nitrate tolerance limits their continuous clinical application and, under oxidative stress and increased formation of peroxynitrite foils the desired therapeutic effect. To overcome these obstacles of nitrate therapy, direct NO- and haem-independent sGC activators have been developed, such as BAY 58-2667 (cinaciguat) and HMR1766 (ataciguat), showing unique biochemical and pharmacological properties. Both compounds are capable of selectively activating the oxidized/haem-free enzyme via binding to the enzyme's haem pocket, causing pronounced vasodilatation. The potential importance of these new drugs resides in the fact that they selectively target a modified state of sGC that is prevalent under disease conditions as shown in several animal models and human disease. Activators of sGC may be beneficial in the treatment of a range of diseases including systemic and pulmonary hypertension (PH), heart failure, atherosclerosis, peripheral arterial occlusive disease (PAOD), thrombosis and renal fibrosis. The sGC activator HMR1766 is currently in clinical development as an oral therapy for patients with PAOD. The sGC activator BAY 58-2667 has demonstrated efficacy in a proof-of-concept study in patients with acute decompensated heart failure (ADHF), reducing pre- and afterload and increasing cardiac output from baseline. A phase IIb clinical study for the indication of ADHF is currently underway.

Modulation of cGMP in heart failure: a new therapeutic paradigm

Heart failure (HF) is a common disease that continues to be associated with high morbidity and mortality warranting novel therapeutic strategies. Cyclic guanosine monophosphate (cGMP) is the second messenger of several important signaling pathways based on distinct guanylate cyclases (GCs) in the cardiovascular system. Both the nitric oxide/soluble GC (NO/sGC) as well as the natriuretic peptide/GC-A (NP/GC-A) systems are disordered in HF, providing a rationale for their therapeutic augmentation. Soluble GC activation with conventional nitrovasodilators has been used for more than a century but is associated with cGMP-independent actions and the development of tolerance, actions which novel NO-independent sGC activators now in clinical development lack. Activation of GC-A by administration of naturally occurring or designer natriuretic peptides is an emerging field, as is the inhibition of enzymes that degrade endogenous NPs. Finally, inhibition of cGMP-degrading phosphodiesterases, particularly phosphodiesterase 5 provides an additional strategy to augment cGMP-signaling.

Vasodilators in the treatment of acute heart failure: what we know, what we don't

Although we have recently witnessed substantial progress in management and outcome of patients with chronic heart failure, acute heart failure (AHF) management and outcome have not changed over almost a generation. Vasodilators are one of the cornerstones of AHF management; however, to a large extent, none of those currently used has been examined by large, placebo-controlled, non-hemodynamic monitored, prospective randomized studies powered to assess the effects on outcomes, in addition to symptoms. In this article, we will discuss the role of vasodilators in AHF trying to point out which are the potentially best indications to their administration and which are the pitfalls which may be associated with their use. Unfortunately, most of this discussion is only partially evidence based due to lack of appropriate clinical trials. In general, we believe that vasodilators should be administered early to AHF patients with normal or high blood pressure (BP) at presentation. They should not be administered to patients with low BP since they may cause hypotension and hypoperfusion of vital organs, leading to renal and/or myocardial damage which may further worsen patients' outcome. It is not clear whether vasodilators have a role in either patients with borderline BP at presentation (i.e., low-normal) or beyond the first 1-2 days from presentation. Given the limitations of the currently available clinical trial data, we cannot recommend any specific agent as first line therapy, although nitrates in different formulations are still the most widely used in clinical practice.

Pharmacokinetics, pharmacodynamics, tolerability, and safety of the soluble guanylate cyclase activator cinaciguat (BAY 58-2667) in healthy male volunteers

Preclinical data indicate that the nitric oxide-independent soluble guanylate cyclase activator cinaciguat (BAY 58-2667), which is a new drug in development for patients with heart failure, induces vasodilation preferentially in diseased vessels. This study aimed to assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of cinaciguat. Seventy-six healthy volunteers were included in this randomized, placebo-controlled study. Cinaciguat (50-250 microg/h) was administered intravenously for up to 4 hours in a maximum of 6 individuals per dose group. No serious adverse events were reported. Four-hour infusions (50-250 microg/h) decreased diastolic blood pressure and increased heart rate (all P values < .05) versus placebo, without significantly reducing systolic blood pressure (P between 0.07 and 0.56). At higher doses (150-250 microg/h), 4-hour infusions decreased mean arterial pressure and increased plasma cyclic guanosine monophosphate levels (all P values < .05). Pharmacokinetics showed dose-proportionality with low interindividual variability. Plasma concentrations declined below 1.0 microg/L within 30 minutes of cessation of infusion. Cinaciguat had potent cardiovascular effects reducing preload and afterload, warranting further investigation in patients with heart failure.

Sustained soluble guanylate cyclase stimulation offsets nitric-oxide synthase inhibition to restore acute cardiac modulation by sildenafil

Phosphodiesterase type 5 (PDE5) inhibitors are used to treat erectile dysfunction, and growing evidence supports potential cardiovascular utility. Their efficacy declines with reduced nitric-oxide synthase (NOS) activity common to various diseases. We tested whether direct soluble guanylate cyclase (sGC) stimulation restores in vivo cardiovascular modulation by PDE5 inhibition despite acute or chronically suppressed NOS activity. Mice (C57/Bl6; n = 62) were studied by in vivo pressure-volume analysis to assess acute modulation by the PDE5 inhibitor sildenafil (SIL; 100 microg/kg/min) of the cardiac response to isoproterenol (ISO) with or without NOS inhibition [N(omega)-nitro-L-arginine methyl ester (L-NAME)] and cotreatment by the sGC stimulator 2-[1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-5-(4-morpholinyl)pyrimidine-4,6-diamine (BAY 41-8543). SIL induced mild vasodilation but no basal cardiac effects and markedly blunted ISO-stimulated contractility. Acute BAY 41-8543 at a dose lacking cardiovascular effects did not alter ISO responses. However, after acute L-NAME, SIL ceased to influence cardiovascular function, but adding BAY 41-8543 fully restored SIL effects. After 1 week of L-NAME, neither SIL nor SIL + BAY 41-8543 acutely induced vasodilation or blunted ISO responses. However, sustained BAY 41-8543 despite concurrent NOS inhibition restored the cardiovascular efficacy of SIL. The disparity between acute and chronic NOS inhibition related to diffusion of PDE5 away from myocyte z-bands coupled with reduced protein kinase G activation. Both were restored by sustained sGC costimulation. Thus, PDE5 regulation of adrenergic reserve and systemic vasodilation depends upon NOS-induced cGMP/protein kinase G and can be enhanced by sustained low-level stimulation of sGC. This may prove beneficial for enhancing the efficacy of PDE5 inhibitors in conditions with chronically reduced NOS activity.

Platelet hyperaggregability: impaired responsiveness to nitric oxide ("platelet NO resistance") as a therapeutic target

Platelet hyperaggregability and associated thrombosis have been documented in a number of cardiovascular disease states. While one of the current mainstays of anti-thrombotic treatment (i.e. aspirin, clopidogrel, glycoprotein IIb/IIIa antagonists) has been directed at reducing platelet activation and aggregation, it is apparent that there are limitations to the effectiveness of these therapies. Nitric oxide (NO) plays an important role in platelet physiology. The ability of NO to regulate cyclic guanosine-3,'5'-monophosphate (cGMP), via activation of soluble guanylate cyclase, is the principal mechanism of negative control over platelet activity. NO is not only of the endothelial source, it is also released from activated platelets, providing a negative feedback. Studies in patients with symptomatic ischemia, chronic heart failure, diabetes and various risk factors for cardiovascular disease have demonstrated that platelets from these subjects exhibit reduced responsiveness to the anti-aggregating efficacy of NO: a phenomenon termed "platelet NO resistance". It constitutes an impaired physiological response to endogenous NO (endothelium-derived relaxing factor or EDRF), and as such may contribute to the increased risk of ischemic events. NO resistance also accounts for reduced pharmaco-activity of exogenous NO donors, e.g. organic nitrates. Platelet NO resistance results largely from a combination of "scavenging" of NO by superoxide anion radical and inactivation of soluble guanylate cyclase. NO resistance has both diagnostic and prognostic implications. The current review examines the association of platelet NO resistance with pathological hyperaggregability and discusses potential therapeutic strategies targeting this abnormality.

Meeting report: cGMP matters

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) controls many cellular functions ranging from growth to contractility. Generated from guanylyl cyclases in response to natriuretic peptides or nitric oxide, cGMP transduces its effects through a number of cGMP effectors, including cGMP-regulated phosphodiesterases and protein kinases. Drugs that modulate cGMP levels are emerging as promising therapies, particularly for cardiovascular disorders. This report summarizes new data on the molecular mechanisms, (patho)physiological relevance, and therapeutic potential of the cGMP signaling system that were presented at the 3rd cGMP meeting held in June 2007 in Dresden, Germany.

Probing the presence of the ligand-binding haem in cellular nitric oxide receptors

BACKGROUND AND PURPOSE

Nitric oxide (NO) acts on receptors coupled to guanylyl cyclase (GC), leading to cGMP accumulation. The NO binding site is a haem group, oxidation or loss of which diminishes NO-stimulated activity. Agonists reportedly engaging both these NO-insensitive forms have emerged. Here we characterize the effect of a prototype compound (BAY 58-2667) and use it to assess the haem status of cellular GC.

EXPERIMENTAL APPROACH

GC activity measurements were made on the purified protein and on rat platelets.

KEY RESULTS

Experiments on purified GC showed that the target for BAY 58-2667 is the haem-free GC, not the haem-oxidized form. The efficacy of BAY 58-2667 was about half that shown normally by NO. In platelets, BAY 58-2667 was a potent GC activator (EC50 approximately 15 nM) but the maximum effect was only about 1% of that achievable with NO. Nevertheless, it was enough to evoke cGMP-dependent protein phosphorylation. Profound (85 %) desensitization of NO-evoked GC activity did not alter the effectiveness of BAY 58-2667. Haem oxidation, however, increased the efficacy of BAY 58-2667 by 22-fold, implying that about half the cellular GC was then haem-free. Oxidation appeared to enhance the rate of haem dissociation from purified GC.

CONCLUSIONS AND IMPLICATIONS

Compounds such as BAY 58-2667 are useful for probing the occupancy of the haem pocket of NO receptors in cells but not for distinguishing oxidized from reduced haem. In vivo, such compounds are likely to be particularly effective in conditions where there is deficient haem incorporation or enhanced haem loss.

Distinct roles for renal particulate and soluble guanylyl cyclases in preserving renal function in experimental acute heart failure

Worsening renal function in the setting of human acute heart failure (AHF) predicts poor outcomes, such as rehospitalization and increased mortality. Understanding potential renoprotective mechanisms is warranted. The guanylate cyclase (GC) enzymes and their second messenger cGMP are the target of two important circulating neurohumoral systems with renoprotective properties. Specifically, natriuretic peptides (NP) released from the heart with AHF target particulate GC in the kidney, while the nitric oxide (NO) system is an activator of renal soluble GC. We hypothesized that both systems are essential to preserve renal excretory and hemodynamic function in AHF but with distinct roles. We investigated these roles in three groups of anesthetized dogs (6 each) with AHF induced by rapid ventricular pacing. After a baseline AHF clearance, each group received intrarenal vehicle (control), N(G)-monomethyl-l-arginine (l-NMMA), a competitive NO inhibitor (50 microg.kg(-1).min(-1)) or a specific NP receptor antagonist, HS-142-1 (0.5 mg/kg). We observed that intrarenal l-NMMA decreased renal blood flow (RBF) without significant decreases in glomerular filtration rate (GFR), urinary sodium excretion (UNaV), or urinary cGMP. In contrast, HS-142-1 resulted in a decrease in UNaV and cGMP excretion together with a reduction in GFR and an increase in distal fractional tubular sodium reabsorption. We conclude that in AHF, the NP system plays a role in maintaining sodium excretion and GFR, while the function of NO is in the maintenance of RBF. These studies have both physiological and therapeutic implications warranting further research into cardiorenal interactions in this syndrome of AHF.

Nitric oxide-independent stimulation of soluble guanylate cyclase with BAY 41-2272 in cardiovascular disease

The nitric oxide (NO)-soluble guanylate cyclase (sGC)-cyclic 3',5'-guanosine monophosphate (cGMP) pathway plays an important role in cardiovascular regulation by promoting vasodilation and inhibiting vascular smooth muscle cell growth, platelet aggregation, and leukocyte adhesion. In pathophysiological states with endothelial dysfunction this signaling pathway is impaired. Activation of sGC has traditionally been achieved with nitrovasodilators; however, these drugs are associated with the development of tolerance and potentially deleterious cGMP-independent actions. In this review the actions of BAY 41-2272, the prototype of a new class of NO-independent sGC stimulators, in cardiovascular disease models is discussed. BAY 41-2272 binds to a regulatory site on the alpha-subunit of sGC and stimulates the enzyme synergistically with NO. BAY 41-2272 had antihypertensive actions and attenuated remodeling in models of systemic arterial hypertension. It also unloaded the heart in experimental congestive heart failure. BAY 41-2272 reduced pulmonary vascular resistance in acute and chronic experimental pulmonary arterial hypertension. Furthermore, BAY 41-2272 inhibited platelet aggregation in vitro and leukocyte adhesion in vivo. These findings make direct sGC stimulation with BAY 41-2272 a promising new therapeutic strategy for cardiovascular diseases and warrant further studies. Finally, the significance of the novel NO- and heme-independent sGC activator BAY 58-2667, which activates two forms of NO-insensitive sGC, is briefly discussed.