Soluble guanylate cyclase (sGC), a key enzyme of the nitric oxide (NO) signaling pathway, is attracting a rapidly growing interest as a therapeutic target in cardiopulmonary disease, with several sGC agonists currently in clinical development. Upon binding of NO to a prosthetic heme group on sGC, the enzyme catalyzes synthesis of the second messenger cyclic guanosine monophosphate (cGMP), which produces vasorelaxation and inhibits smooth muscle proliferation, leukocyte recruitment and platelet aggregation through a number of downstream mechanisms.1,2
Impaired NO and cGMP signaling has been implicated in the pathogenesis of cardiovascular disease, including systemic arterial and pulmonary hypertension (PH), coronary artery disease, peripheral vascular disease (including erectile dysfunction), and atherosclerosis.1,3–5 Organic nitrates that target the NO signaling pathway have been used to treat cardiovascular disease for more than 150 years. More recently, gaseous NO administered by inhalation has been approved for the treatment of persistent PH of the newborn.3,6 These agents nonetheless have several important limitations. Cardiovascular disease is associated with resistance to NO and organic nitrates.7 This may be due to the oxidative-stress-induced alteration of the redox state of the prosthetic heme on sGC (from ferrous to ferric) that weakens the binding of heme to the enzyme and renders sGC unresponsive to NO.1,8 Furthermore, the long-term efficacy of organic nitrates is limited by the development of tolerance.9 Nitric oxide may also have numerous cytotoxic effects, mostly attributed to the reactive oxidant peroxynitrite (formed from the diffusion-controlled reaction of NO with superoxide).3,10 Peroxynitrite interacts with proteins and lipids, altering cellular signaling, disrupting mitochondrial function, and damaging DNA, which can eventually culminate in cellular dysfunction and/or death.3
As the beneficial effects of NO appear to be mediated through the sGC-cGMP-dependent downstream mechanisms, whereas most of its detrimental effects occur independently,11 recent efforts have centered on identifying pharmacological agents that could target sGC-cGMP signaling directly. Compounds that act directly on sGC can be divided into two categories based on their modes of action – sGC stimulators and sGC activators. Stimulators sensitize sGC to low levels of bioavailable NO by stabilizing the nitrosyl-heme complex and thus maintaining the enzyme in its active configuration, and they can also increase sGC activity in the absence of NO.11,12 Their action is dependent on the presence of a reduced (ferrous) prosthetic heme.13–15 By contrast, sGC activators preferentially and effectively activate sGC when it is in an oxidized or, finally, a heme-free state (Figure 1).11,16,17 Oxidation of the heme group on sGC results in its dissociation from the enzyme and the generation of NO-insensitive sGC with only basal levels of activity.18 Levels of oxidized or heme-free sGC are increased in animal models of hypertension and hyperlipidemia, as well as in certain cardiovascular diseases and type 2 diabetes in humans.19,20 The detrimental effects of high levels of heme-free sGC were recently demonstrated in a study of genetically modified mice that express only the heme-free version of the enzyme. The mice had systemic hypertension with a loss of smooth muscle relaxation responses to NO and a shortened lifespan.21 The two categories of sGC agonists may thus have utility in different groups of diseases, depending on the relative importance of synergistic action with NO (sGC stimulators) compared with ability to act preferentially in conditions associated with oxidative stress (sGC activators).
The first sGC activator, an amino dicarboxylic acid known as cinaciguat (BAY 58-2667), was discovered in a high-throughput screening less than a decade ago.22 Cinaciguat enabled scientists to demonstrate the presence of heme-free sGC in vivo for the first time.20 It activates oxidized/heme-free sGC by binding in the sGC heme pocket and mimicking the heme group; it also protects heme-free sGC from oxidation-induced proteasomal degradation. Cinaciguat therefore opens up the possibility of new mechanism-based therapies for cardiovascular diseases associated with oxidative stress,8,23 and is currently in clinical development for the treatment of acute decompensated heart failure.24–26 A more recently discovered sGC activator, the anthranilic acid derivative ataciguat (HMR 1766),27 has also been studied in clinical trials in healthy volunteers,28 in patients with intermittent claudication due to peripheral arterial disease and in patients with neuropathic pain. However, its clinical development in these patients appears to have been stopped.29,30 Another activator of sGC, BAY 60-2770, has been newly characterized in preclinical studies.31
The development of sGC stimulators began in the mid-1990s with the synthetic benzylindazole compound YC-1.32–34 Binding of YC-1 to sGC is thought to stabilize the enzyme in its active configuration by maintaining stability of the nitrosyl-heme complex.35,36 YC-1 increases the activity of purified sGC by approximately 10-fold, an effect that is enhanced by approximately one-to-two orders of magnitude in the presence of NO.33,34,37 Although the precise mechanism by which YC-1 stimulates sGC remains to be elucidated, evidence to date suggests that YC-1 interacts with the catalytic domain of sGC and implicates both subunits of sGC in the action of YC-1.12 YC-1 has been shown to have additional, cGMP-independent effects38–40 and to inhibit phosphodiesterase (PDE) 5,41,42 thus limiting its usefulness as a sGC stimulator. A structurally unrelated class of sGC stimulators (the acrylamide analogs A-350619, A-344905 and A-778935) was also discovered in recent years, but the vast majority of publications have focused on YC-1 and its successors (the indazole family).12,43–45 Another sGC stimulator, CFM-1571, was developed based on YC-1 as a lead structure,46 but has low oral bioavailability and potency.
A separate chemical and pharmacological optimization program yielded the pyrazolopyridine derivatives BAY 41-2272 and BAY 41-8543.13,14,47 The mode of action of these two compounds is similar to that of YC-1, but they have greater potency and specificity for sGC than YC-1. BAY 41-2272 stimulates the activity of sGC by approximately 20-fold13 and BAY 41-8543 stimulates it by up to 92-fold,14 and both compounds strongly synergize with NO to stimulate sGC activity by up to 200-fold.15 Unlike YC-1, BAY 41-8543 is devoid of PDE5 inhibition14 and BAY 41-2272 does not cause any significant inhibition of PDE5 at the concentrations needed to stimulate sGC.13,48–50 In addition, BAY 41-2272 and BAY 41-8543 do not inhibit other cGMP-specific or cGMP-metabolizing PDEs, such as PDE1, PDE2 and PDE9.13,14,51
Further pharmacokinetic optimization with an investigation of over 800 pyrimidine derivatives finally yielded the orally bioavailable sGC stimulator riociguat (BAY 63-2521).52 Riociguat increases the activity of sGC in vitro by up to 73-fold and acts in synergy with NO to increase sGC activity up to 122-fold.53 It does not inhibit cGMP-specific or cGMP-metabolizing PDEs, such as PDE1, PDE2, PDE5 and PDE9, at concentrations up to 10 μM.53 It has vasodilator properties similar to BAY 41-2272 and BAY 41-8543, and is the first sGC stimulator to make the transition into clinical research, showing promising results in patients with PH in uncontrolled trials.54,55
While clinical research is focusing on PH at present, disrupted NO signaling is a common pathogenic feature in many forms of cardiovascular disease, and the therapeutic potential of sGC stimulators has been and continues to be explored in a wide range of animal models. Research to identify and optimize new compounds in this drug class (e.g. the aminopyrimidines)56 is also ongoing. The remainder of this review will evaluate the potential of sGC stimulation across the broad spectrum of cardiovascular disease, explain the rationale behind the current clinical focus on PH, and discuss the implications of the initial clinical results.