Revisiting soluble guanylate cyclase pharmacology: Additive potential of stimulators and activators.

Nitric oxide (NO) is a key gasotransmitter that binds to soluble guanylate cyclase (sGC), thereby stimulating cGMP production. Under disease conditions, reactive oxygen species disrupt this signaling by either: (1) scavenging NO, or (2) promoting heme-free, NO-insensitive apo-sGC due to impaired heme incorporation or oxidative damage. To counteract these conditions, two pharmacological approaches have emerged: sGC stimulators, which allosterically enhance sGC sensitivity to low NO levels; and sGC activators, which directly activate apo-sGC (heme-free sGC) by binding to its empty heme pocket, inducing a conformation that mimics NO-bound sGC. Thus, sGC stimulators are thought to target only sGC. Instead, we here show both in vitro and in vivo that sGC stimulators also modulate apo-sGC and - together with sGC activators - exert additive effects on apo-sGC. This newly identified activity of sGC stimulators on apo-sGC appears to be redox-sensitive. Indeed, when inactive sGC (ferric heme sGC) is generated by the commonly used oxidant 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), the stimulatory effect of sGC stimulators is abolished. The potential in vivo relevance of this mechanism was further demonstrated in a mouse model of ischemic stroke, wherein sGC is known to be predominantly NO-insensitive, yet sGC stimulators remain protective. Altogether, these findings challenge the current pharmacological paradigm of sGC modulation, revealing that both sGC and apo-sGC can be stimulated by sGC stimulators, whereas sGC activators remain specific to apo-sGC. This expanded understanding highlights the therapeutic potential of sGC stimulators, both as standalone treatments and in combination with sGC activators.