H2S Donors and Their Use in Medicinal Chemistry.

Hydrogen sulfide (H2S) is a ubiquitous gaseous signaling molecule that has an important role in many physiological and pathological processes in mammalian tissues, with the same importance as two others endogenous gasotransmitters such as NO (nitric oxide) and CO (carbon monoxide). Endogenous H2S is involved in a broad gamut of processes in mammalian tissues including inflammation, vascular tone, hypertension, gastric mucosal integrity, neuromodulation, and defense mechanisms against viral infections as well as SARS-CoV-2 infection. These results suggest that the modulation of H2S levels has a potential therapeutic value. Consequently, synthetic H2S-releasing agents represent not only important research tools, but also potent therapeutic agents. This review has been designed in order to summarize the currently available H2S donors; furthermore, herein we discuss their preparation, the H2S-releasing mechanisms, and their -biological applications.

Engineering macromolecular nanocarriers for local delivery of gaseous signaling molecules.

In addition to being notorious air pollutants, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) have also been known as endogenous gaseous signaling molecules (GSMs). These GSMs play critical roles in maintaining the homeostasis of living organisms. Importantly, the occurrence and development of many diseases such as inflammation and cancer are highly associated with the concentration changes of GSMs. As such, GSMs could also be used as new therapeutic agents, showing great potential in the treatment of many formidable diseases. Although clinically it is possible to directly inhale GSMs, the precise control of the dose and concentration for local delivery of GSMs remains a substantial challenge. The development of gaseous signaling molecule-releasing molecules provides a great tool for the safe and convenient delivery of GSMs. In this review article, we primarily focus on the recent development of macromolecular nanocarriers for the local delivery of various GSMs. Learning from the chemistry of small molecule-based donors, the integration of these gaseous signaling molecule-releasing molecules into polymeric matrices through physical encapsulation, post-modification, or direct polymerization approach renders it possible to fabricate numerous macromolecular nanocarriers with optimized pharmacokinetics and pharmacodynamics, revealing improved therapeutic performance than the small molecule analogs. The development of GSMs represents a new means for many disease treatments with unique therapeutic outcomes.

Metal-organic frameworks for therapeutic gas delivery.

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are gaseous signaling molecules (gasotransmitters) that regulate both physiological and pathological processes and offer therapeutic potential for the treatment of many diseases, such as cancer, cardiovascular disease, renal disease, bacterial and viral infections. However, the inherent labile nature of therapeutic gases results in difficulties in direct gases administration and their controlled delivery at clinically relevant ranges. Metal-organic frameworks (MOFs) with highly porous, stable, and easy-to-tailor properties have shown promising therapeutic gas delivery potential. Herein, we highlight the recent advances of MOF-based platforms for therapeutic gas delivery, either by endogenous (i.e., direct transfer of gases to targets) or exogenous (i.e., stimulating triggered release of gases) means. Reports that involve in vitro and/or in vivo studies are highlighted due to their high potential for clinical translation. Current challenges for clinical requirements and possible future innovative designs to meet variable healthcare needs are discussed.

H2S- and NO-releasing gasotransmitter platform: A crosstalk signaling pathway in the treatment of acute kidney injury.

Acute kidney injury (AKI) is a syndrome affecting most patients hospitalized due to kidney disease; it accounts for 15 % of patients hospitalized in intensive care units worldwide. AKI is mainly caused by ischemia and reperfusion (IR) injury, which temporarily obstructs the blood flow, increases inflammation processes and induces oxidative stress. AKI treatments available nowadays present notable disadvantages, mostly for patients with other comorbidities. Thus, it is important to investigate different approaches to help minimizing side effects such as the ones observed in patients subjected to the aforementioned treatments. Therefore, the aim of the current review is to highlight the potential of two endogenous gasotransmitters - hydrogen sulfide (H2S) and nitric oxide (NO) - and their crosstalk in AKI treatment. Both H2S and NO are endogenous signalling molecules involved in several physiological and pathophysiological processes, such as the ones taking place in the renal system. Overall, these molecules act by decreasing inflammation, controlling reactive oxygen species (ROS) concentrations, activating/inactivating pro-inflammatory cytokines, as well as promoting vasodilation and decreasing apoptosis, hypertrophy and autophagy. Since these gasotransmitters are found in gaseous state at environmental conditions, they can be directly applied by inhalation, or in combination with H2S and NO donors, which are compounds capable of releasing these molecules at biological conditions, thus enabling higher stability and slow release of NO and H2S. Moreover, the combination between these donor compounds and nanomaterials has the potential to enable targeted treatments, reduce side effects and increase the potential of H2S and NO. Finally, it is essential highlighting challenges to, and perspectives in, pharmacological applications of H2S and NO to treat AKI, mainly in combination with nanoparticulated delivery platforms.

Hydrogen Sulfide as Potential Regulatory Gasotransmitter in Arthritic Diseases.

The social and economic impact of chronic inflammatory diseases, such as arthritis, explains the growing interest of the research in this field. The antioxidant and anti-inflammatory properties of the endogenous gasotransmitter hydrogen sulfide (H2S) were recently demonstrated in the context of different inflammatory diseases. In particular, H2S is able to suppress the production of pro-inflammatory mediations by lymphocytes and innate immunity cells. Considering these biological effects of H2S, a potential role in the treatment of inflammatory arthritis, such as rheumatoid arthritis (RA), can be postulated. However, despite the growing interest in H2S, more evidence is needed to understand the pathophysiology and the potential of H2S as a therapeutic agent. Within this review, we provide an overview on H2S biological effects, on its role in immune-mediated inflammatory diseases, on H2S releasing drugs, and on systems of tissue repair and regeneration that are currently under investigation for potential therapeutic applications in arthritic diseases.

Hydrogen sulfide: a target to modulate oxidative stress and neuroplasticity for the treatment of pathological anxiety.

Introduction: Anxiety disorders result inhigh patient burden and utilization of healthcare resources. Evidence-based treatments for pathological anxiety include targeted psychotherapy and use of serotonin-augmenting agents. Limitations in access to cognitive behavioral therapy and potential disadvantages to the use of psychotropics make the need for novel approaches to therapeutics for pathological anxiety salient.Areas Covered: Neuroplasticity mechanisms, as well as managing oxidative stress and inflammatory cellular allostatic loads can decrease anxiety. The gasotransmitter hydrogen sulfide (H2S) can impact these mechanisms through a) maintaining intracellular reduced glutathione in the CNS to decrease oxidative stress; b) facilitating neuroplasticity in amygdalar regions via the 2B subunit of n-methyl-d-aspartate (NMDA) receptors, in conjunction with the cAMP messenger system and a CNS kinase, PKC-γ; and c) regulating intracellular Ca2+ homeostasis in neurons and glial cells, among others.Expert Opinion: Given the mounting evidence for the role of H2S in neuronal health and its potential to decrease pathological anxiety, the current challenge in H2S therapeutics remains finding an efficient delivery system of this gasotransmitter in a reliable, safe and nontoxic form to engage in clinical trials. Current efforts include H2S-delivering moieties attached to known drugs, natural sulfide-releasing compounds such as garlic, and the regulation of dysfunctional breathing through breathing retraining.

Intra-articular injection of hydrogen sulfide decreased the progression of gonarthrosis.

Hydrogen sulfide (H2S) is found in both the plasma and synovial fluid of patients with gonarthrosis. In the present study, we investigated whether intra-articular injection of sodium hydrosulfide (NaSH) (1 mM, 30 µL), a H2S donor, might affect gonarthrosis in rats. Gonarthrosis was induced surgically in the left knees of rats and left for 6 weeks for the development of disease. Then, intra-articular injections of NaSH or methylprednisolone (1 mg/kg, 30 µL) were administered to rats. Half of each group was sacrificed at the end of the first day and the other half was sacrificed at the end of 4 weeks to evaluate early and later effects of injections on gonarthrosis. The injury induced by anterior cruciate ligament resection and medial meniscectomy in rats caused the development of gonarthrosis. As the duration lengthened after gonarthrosis induction, the progression of the disease continued. According to the modified Mankin Scoring System, intra-articular injection of NaSH histopathologically slowed the progression of gonarthrosis, whereas methylprednisolone was ineffective. In addition, NaSH decreased apoptosis in rat knees with gonarthrosis. Each treatment did not cause injury to healthy knees. Our results lead to the consideration that intra-articular NaSH administration may be effective in the progression of gonarthrosis.

Hydrogen sulfide and autophagy: A double edged sword.

Hydrogen sulfide (H2S) has been considered the third gaseous signaling molecule that plays important roles in a wide range of physiological and pathological conditions. However, there has been some controversy on the role of H2S in autophagy. Recent studies indicate that a number of signaling pathways are involved in the pro-autophagy effect of H2S, such as PI3K/Akt/mTOR, AMPK/mTOR, LKB1/STRAD/MO25, and miR-30c signaling pathways. On the other hand, there are many signaling pathways that play important roles in the anti-autophagy effect of H2S, including SR-A, PI3K/SGK1/GSK3ß, PI3K/AKT/mTOR, Nrf2-ROS-AMPK, AMPK/mTOR, and JNK1 signaling pathways. Novel H2S-releasing donors/drugs could be designed and identified in order to increase the therapeutic effects by mediating autophagy in human diseases. In this review, the H2S metabolism in mammals is summarized and the effects of signaling pathways in H2S-mediated autophagy are further discussed.

A two-photon excitable and ratiometric fluorogenic nitric oxide photoreleaser and its biological applications.

We report for the first time the development of a two-photon excitable NO photoreleaser, CNNO, for ratiometric imaging and tracking of NO release in live cells. CNNO exhibits the merits of spatiotemporal control in both the site-specific NO release in the selected cell culture region and the controllable vasodilation of mouse aorta ex vivo.

A Long-Term and Slow-Releasing Hydrogen Sulfide Donor Protects against Myocardial Ischemia/Reperfusion Injury.

Hydrogen sulfide (H2S) has been recognized as an important gasotransmitter exerting various physiological effects, especially in the cardiovascular system. Herein we investigated the cardioprotective effects of a novel long-term and slow-releasing H2S donor, DATS-MSN, using in vivo myocardial ischemia/reperfusion (I/R) models and in vitro hypoxia/reoxygenation cardiomyocyte models. Unlike the instant-releasing pattern of sodium hydrosulphide (NaHS), the release of H2S from DATS-MSN was quite slow and continuous both in the cell culture medium and in rat plasma (elevated H2S concentrations during 24 h and 72 h reperfusion). Correspondingly, DATS-MSN demonstrated superior cardioprotective effects over NaHS in I/R models, which were associated with greater survival rates, reduced CK-MB and troponin I levels, decreased cardiomyocyte apoptosis index, increased antioxidant enzyme activities, inhibited myocardial inflammation, greater reduction in the infarct area and preserved cardiac ejection fraction. Some of these effects of DATS-MSN were also found to be superior to classic slow-releasing H2S donor, GYY4137. In in vitro experiments, cardiomyocytes injury was also found to be relived with the use of DATS-MSN compared to NaHS after the hypoxia/reoxygenation processes. The present work provides a novel long-term and slow-releasing H2S donor and an insight into how the release patterns of H2S donors affect its physiological functionality.

Hydrogen sulfide improves intestinal recovery following ischemia by endothelial nitric oxide-dependent mechanisms.

Hydrogen sulfide (H2S) is an endogenous gasotransmitter that has vasodilatory properties. It may be a novel therapy for intestinal ischemia-reperfusion (I/R) injury. We hypothesized that 1) H2S would improve postischemic survival, mesenteric perfusion, mucosal injury, and inflammation compared with vehicle and 2) the benefits of H2S would be mediated through endothelial nitric oxide. C57BL/6J wild-type and endothelial nitric oxide synthase knockout (eNOS KO) mice were anesthetized, and a midline laparotomy was performed. Intestines were eviscerated, the small bowel mesenteric root identified, and baseline intestinal perfusion was determined using laser Doppler. Intestinal ischemia was established by temporarily occluding the superior mesenteric artery. Following ischemia, the clamp was removed, and the intestines were allowed to recover. Either sodium hydrosulfide (2 nmol/kg or 2 µmol/kg NaHS) in PBS vehicle or vehicle only was injected into the peritoneum. Animals were allowed to recover and were assessed for mesenteric perfusion, mucosal injury, and intestinal cytokines. P values < 0.05 were significant. H2S improved mesenteric perfusion and mucosal injury scores following I/R injury. However, in the setting of eNOS ablation, there was no improvement in these parameters with H2S therapy. Application of H2S also resulted in lower levels of intestinal cytokine production following I/R. Intraperitoneal H2S therapy can improve mesenteric perfusion, intestinal mucosal injury, and intestinal inflammation following I/R. The benefits of H2S appear to be mediated through endothelial nitric oxide-dependent pathways.NEW & NOTEWORTHY H2S is a gaseous mediator that acts as an anti-inflammatory agent contributing to gastrointestinal mucosal defense. It promotes vascular dilation, mucosal repair, and resolution of inflammation following intestinal ischemia and may be exploited as a novel therapeutic agent. It is unclear whether H2S works through nitric oxide-dependent pathways in the intestine. We appreciate that H2S was able to improve postischemic recovery of mesenteric perfusion, mucosal integrity, and inflammation. The beneficial effects of H2S appear to be mediated through endothelial nitric oxide-dependent pathways.

Gasotransmitter delivery via self-assembling peptides: Treating diseases with natural signaling gases.

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are powerful signaling molecules that play a variety of roles in mammalian biology. Collectively called gasotransmitters, these gases have wide-ranging therapeutic potential, but their clinical use is limited by their gaseous nature, extensive reactivity, short half-life, and systemic toxicity. Strategies for gasotransmitter delivery with control over the duration and location of release are therefore vital for developing effective therapies. An attractive strategy for gasotransmitter delivery is though injectable or implantable gels, which can ideally deliver their payload over a controllable duration and then degrade into benign metabolites. Self-assembling peptide-based gels are well-suited to this purpose due to their tunable mechanical properties, easy chemical modification, and inherent biodegradability. In this review we illustrate the biological roles of NO, CO, and H2S, discuss their therapeutic potential, and highlight recent efforts toward their controlled delivery with a focus on peptide-based delivery systems.

Hydrogen Sulfide Protects Renal Grafts Against Prolonged Cold Ischemia-Reperfusion Injury via Specific Mitochondrial Actions.

Ischemia-reperfusion injury is unavoidably caused by loss and subsequent restoration of blood flow during organ procurement, and prolonged ischemia-reperfusion injury IRI results in increased rates of delayed graft function and early graft loss. The endogenously produced gasotransmitter, hydrogen sulfide (H2 S), is a novel molecule that mitigates hypoxic tissue injury. The current study investigates the protective mitochondrial effects of H2 S during in vivo cold storage and subsequent renal transplantation (RTx) and in vitro cold hypoxic renal injury. Donor allografts from Brown Norway rats treated with University of Wisconsin (UW) solution + H2 S (150 µM NaSH) during prolonged (24-h) cold (4°C) storage exhibited significantly (p < 0.05) decreased acute necrotic/apoptotic injury and significantly (p < 0.05) improved function and recipient Lewis rat survival compared to UW solution alone. Treatment of rat kidney epithelial cells (NRK-52E) with the mitochondrial-targeted H2 S donor, AP39, during in vitro cold hypoxic injury improved the protective capacity of H2 S >1000-fold compared to similar levels of the nonspecific H2 S donor, GYY4137 and also improved syngraft function and survival following prolonged cold storage compared to UW solution. H2 S treatment mitigates cold IRI-associated renal injury via mitochondrial actions and could represent a novel therapeutic strategy to minimize the detrimental clinical outcomes of prolonged cold IRI during RTx.

Hydrogen Sulfide Inhalation Improves Neurological Outcome via NF-κB-Mediated Inflammatory Pathway in a Rat Model of Cardiac Arrest and Resuscitation.

BACKGROUND/AIMS: The effects of H2S on cerebral inflammatory reaction after cardiac arrest (CA) and cardiopulmonary resuscitation (CPR) remain poorly understood. In this study, we investigated the effects of exogenous 40 ppm and 80 ppm H2S gas on inflammatory reaction and neurological outcome after CA/CPR. METHODS: CA was induced by ventricular fibrillation and followed by CPR. Forty or 80 ppm H2S was inhaled for 1 h immediately following CPR. The levels of IL-1ß, IL-6 and TNF-α, the myeloperoxidase (MPO) activity, the expression of iNOS and ICAM-1, and the phosphorylation and translocation of NF-κB were evaluated at 24 h after CA/ CPR. The tape removal test, survival rate and hippocampal neuronal counts were investigated at 14 d after CA/CPR. RESULTS: CA/CPR induced significant increases in IL-1ß, IL-6, TNF-α and MPO activity. The phosphorylation and translocation of NF-κB, and the expression of iNOS and ICAM-1 were increased significantly. Inhalation of 40 or 80 ppm H2S gas decreased these inflammatory cytokines. Furthermore, 40 or 80 ppm H2S inhibited the activation of NF-κB and the downstream proinflammatory mediators iNOS and ICAM-1. H2S inhalation also improved neurological function, 14-d survival rate, and reduced hippocampal neuronal loss. CONCLUSION: These results indicated that inhalation of H2S protected against brain injury after CA/CPR. The mechanisms underlying protective effects of H2S were associated with the inhibition of CA/ CPR-induced inflammation reactions by reducing IL-1ß, IL-6 and TNF-α, and concomitantly inhibiting the activation and infiltration of neutrophils. The beneficial effects of H2S might be mediated by downregulation of NF-κB and the downstream proinflammatory signaling pathway.

Effects of H2S on the central regulation of respiration in adult rats.

Hydrogen sulfide (H2S) is a gasotransmitter synthesized from cysteine (Cys) by pyridoxal-5'-phosphate-dependent enzymes. We investigated the potential roles of H2S in the regulation of central rhythmic respiration in adult rats in vivo. Sodium hydrosulfide (NaHS: 2.5 mM, 10 mM, and 5 mM) as a source of exogenous H2S, Cys (2.5 mM, 10 mM and 5 mM) as a source of endogenous H2S, 2.5 mM Cys+10 mM hydroxylamine (NH2OH), and 10 mM NH2OH, respectively, were intracerebroventricularly injected into rats. The rhythmic discharge of the diaphragm, including burst duration (BD), burst interval (BI), burst frequency (BF), and integrated amplitude (IA), and arterial blood pressure (BP) were measured at different time points. The results were analyzed by analysis of variance. A total of 2.5 mM NaHS did not significantly affect changes in BD, BI, BF, IA, or BP (P>0.05), whereas 2.5 mM Cys significantly altered BD, BI, and BF (P<0.05); however, there was no change in IA and BP (P>0.05). A concentration of 5 mM Cys had effects similar to those of 5 mM NaHS; both induced biphasic respiratory responses and changed the BF (P<0.05). A concentration of 10 mM NH2OH irreversibly inhibited rhythmic discharge of the diaphragm except for IA. No change was seen in BI, BF, IA, or BP (P>0.05) except for BD was temporarily decreased (P<0.05) in the 2.5 mM Cys+10 mM NH2OH group. These results suggest that exogenous and endogenous H2S may participate in the regulation of respiratory activity in adult rats.

Hydrogen sulfide attenuates neurodegeneration and neurovascular dysfunction induced by intracerebral-administered homocysteine in mice.

High levels of homocysteine (Hcy), known as hyperhomocysteinemia are associated with neurovascular diseases. H2S, a metabolite of Hcy, has potent anti-oxidant and anti-inflammatory activities; however, the effect of H2S has not been explored in Hcy (IC)-induced neurodegeneration and neurovascular dysfunction in mice. Therefore, the present study was designed to explore the neuroprotective role of H2S on Hcy-induced neurodegeneration and neurovascular dysfunction. To test this hypothesis we employed wild-type (WT) males ages 8-10 weeks, WT+artificial cerebrospinal fluid (aCSF), WT+Hcy (0.5 µmol/µl) intracerebral injection (IC, one time only prior to NaHS treatment), WT+Hcy+NaHS (sodium hydrogen sulfide, precursor of H2S, 30 µmol/kg, body weight). NaHS was injected i.p. once daily for the period of 7 days after the Hcy (IC) injection. Hcy treatment significantly increased malondialdehyde, nitrite level, acetylcholinestrase activity, tumor necrosis factor-alpha, interleukin-1 beta, glial fibrillary acidic protein, inducible nitric oxide synthase, endothelial nitric oxide synthase and decreased glutathione level indicating oxidative-nitrosative stress and neuroinflammation as compared to control and aCSF-treated groups. Further, increased expression of neuron-specific enolase, S100B and decreased expression of (post-synaptic density-95, synaptosome-associated protein-97) synaptic protein indicated neurodegeneration. Brain sections of Hcy-treated mice showed damage in the cortical area and periventricular cells. Terminal deoxynucleotidyl transferase-mediated, dUTP nick-end labeling-positive cells and Fluro Jade-C staining indicated apoptosis and neurodegeneration. The increased expression of matrix metalloproteinase (MMP) MMP9, MMP2 and decreased expression of tissue inhibitor of metalloproteinase (TIMP) TIMP-1, TIMP-2, tight junction proteins (zonula occulden 1) in Hcy-treated group indicate neurovascular remodeling. Interestingly, NaHS treatment significantly attenuated Hcy-induced oxidative stress, memory deficit, neurodegeneration, neuroinflammation and cerebrovascular remodeling. The results indicate that H2S is effective in providing protection against neurodegeneration and neurovascular dysfunction.

Treatment with hydrogen sulfide alleviates streptozotocin-induced diabetic retinopathy in rats.

BACKGROUND AND PURPOSE: Retinopathy, as a common complication of diabetes, is a leading cause of reduced visual acuity and acquired blindness in the adult population. The aim of present study was to investigate the therapeutic effect of hydrogen sulfide on streptozotocin (STZ)-induced diabetic retinopathy in rats. EXPERIMENTAL APPROACH: Rats were injected with a single i.p. injection of STZ (60 mg·kg⁻¹) to induce diabetic retinopathy. Two weeks later, the rats were treated with NaHS (i.p. injection of 0.1 mL·kg⁻¹·d⁻¹ of 0.28 mol·L⁻¹ NaHS, a donor of H2S) for 14 weeks. KEY RESULTS: Treatment with H2S had no significant effect on blood glucose in STZ-induced diabetic rats. Treatment with exogenous H2S enhanced H2S levels in both plasma and retinas of STZ-induced diabetic rats. Treatment with H2S in STZ-treated rats improved the retinal neuronal dysfunction marked by enhanced amplitudes of b-waves and oscillatory potentials and expression of synaptophysin and brain-derived neurotrophic factor, alleviated retinal vascular abnormalities marked by reduced retinal vascular permeability and acellular capillary formation, decreased vitreous VEGF content, down-regulated expressions of HIF-1α and VEGFR2, and enhanced occludin expression, and attenuated retinal thickening and suppressed expression of extracellular matrix molecules including laminin ß1 and collagen IVα3 expression in retinas of STZ-induced diabetic rats. Treatment with H2S in retinas of STZ-induced diabetic rats abated oxidative stress, alleviated mitochondrial dysfunction, suppressed NF-κB activation and attenuated inflammation. CONCLUSIONS AND IMPLICATIONS: Treatment with H2S alleviates STZ-induced diabetic retinopathy in rats possibly through abating oxidative stress and suppressing inflammation.