cGMP modulation therapeutics for sickle cell disease

Abstract

Sickle cell disease (SCD) is an inherited disease caused by the production of abnormal hemoglobin (Hb) S, whose deoxygenation-induced polymerization results in red blood cell (RBC) sickling and numerous pathophysiological consequences. SCD affects approximately 300,000 newborns worldwide each year and is associated with acute and chronic complications, including frequent painful vaso-occlusive episodes that often require hospitalization. Chronic intravascular hemolysis in SCD significantly reduces vascular nitric oxide (NO) bioavailability, consequently decreasing intracellular signaling via cyclic guanosine monophosphate (cGMP), in turn diminishing vasodilation and contributing to the inflammatory mechanisms that trigger vaso-occlusive processes. Oxidative stress may further reduce NO bioavailability in SCD and can oxidize the intracellular enzyme target of NO, soluble guanylate cyclase (sGC), rendering it inactive. Increasing intracellular cGMP-dependent signaling constitutes an important pharmacological therapeutic approach for SCD with a view to augmenting vasodilation, and reducing inflammatory mechanisms, as well as for increasing the production of anti-polymerizing fetal Hb in erythroid cells. Pharmacological agents under pre-clinical and clinical investigation for SCD include NO-based therapeutics to augment NO bioavailability, as well as heme-dependent sGC stimulators and heme-independent sGC activators that directly stimulate native and oxidized sGC, respectively, therefore bypassing the need for vascular NO delivery. Additionally, the phosphodiesterases (PDEs) that degrade intracellular cyclic nucleotides with specific cellular distributions are attractive drug targets for SCD; PDE9 is highly expressed in hematopoietic cells, making the use of PDE9 inhibitors, originally developed for use in neurological diseases, a potential approach that could rapidly amplify intracellular cGMP concentrations in a relatively tissue-specific manner. Impact statement Sickle cell disease (SCD) is one of the most common inherited diseases and is associated with a reduced life expectancy and acute and chronic complications, including frequent painful vaso-occlusive episodes that often require hospitalization. At present, treatment of SCD is limited to hematopoietic stem cell transplant, transfusion, and limited options for pharmacotherapy, based principally on hydroxyurea therapy. This review highlights the importance of intracellular cGMP-dependent signaling pathways in SCD pathophysiology; modulation of these pathways with soluble guanylate cyclase (sGC) stimulators or phosphodiesterase (PDE) inhibitors could potentially provide vasorelaxation and anti-inflammatory effects, as well as elevate levels of anti-sickling fetal hemoglobin.