Soluble guanylate cyclase (sGC), a key enzyme of the nitric oxide (NO) signaling pathway, is attracting rapidly growing interest as a therapeutic target in cardiopulmonary disease, with several sGC agonists currently in clinical development. On binding of NO to a prosthetic heme group on sGC, the enzyme catalyzes synthesis of the second messenger 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 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 Because 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 2 categories based on their modes of action: sGC stimulators and sGC activators. Stimulators sensitize sGC to low levels of bioavailable NO by stabilizing the nitrosylheme complex and thus maintaining the enzyme in its active configuration; 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 In 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 activity. 18 Levels of oxidized or hemefree sGC are increased in animal models of hypertension and hyperlipidemia, as well as in certain cardiovascular diseases and type 2 diabetes mellitus 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 2 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 the ability to act preferentially in conditions associated with oxidative …