Benzothiazole Sulfinate: a Water-Soluble and Slow-Releasing Sulfur Dioxide Donor

A Thermally Stable SO2-Releasing Mechanophore: Facile Activation, Single-Event Spectroscopy, and Molecular Dynamic Simulations

Polymers that release small molecules in response to mechanical force are promising candidates as next-generation on-demand delivery systems. Despite advancements in the development of mechanophores for releasing diverse payloads through careful molecular design, the availability of scaffolds capable of discharging biomedically significant cargos in substantial quantities remains scarce. In this report, we detail a nonscissile mechanophore built from an 8-thiabicyclo[3.2.1]octane 8,8-dioxide (TBO) motif that releases one equivalent of sulfur dioxide (SO2) from each repeat unit. The TBO mechanophore exhibits high thermal stability but is activated mechanochemically using solution ultrasonication in either organic solvent or aqueous media with up to 63% efficiency, equating to 206 molecules of SO2 released per 143.3 kDa chain. We quantified the mechanochemical reactivity of TBO by single-molecule force spectroscopy and resolved its single-event activation. The force-coupled rate constant for TBO opening reaches ∼9.0 s-1 at ∼1520 pN, and each reaction of a single TBO domain releases a stored length of ∼0.68 nm. We investigated the mechanism of TBO activation using ab initio steered molecular dynamic simulations and rationalized the observed stereoselectivity. These comprehensive studies of the TBO mechanophore provide a mechanically coupled mechanism of multi-SO2 release from one polymer chain, facilitating the translation of polymer mechanochemistry to potential biomedical applications.

Recent advances in sulfur dioxide releasing nanoplatforms for cancer therapy

Sulfur dioxide (SO2), long considered to be a harmful atmospheric pollutant, has recently been posited as the fourth gasotransmitter, as it is produced endogenously in mammals and has important pathophysiological effects. The field of tumor therapy has witnessed a paradigm shift with the emergence of SO2-based gas therapy. This has been possible because SO2 is a potent glutathione consumer that can promote the production of reactive oxygen species, eventually leading to oxidative-stress-induced cancer cell death. Nevertheless, this therapeutic gas cannot be directly administrated in gaseous form. Thus, various nano formulations incorporating SO2 donors or prodrugs capable of storing and releasing SO2 have been developed in an attempt to achieve active/passive intratumoral accumulation and SO2 release in the tumor microenvironment. In this review article, the advances over the past decade in nanoplatforms incorporating sulfur SO2 prodrugs to provide controlled release of SO2 for cancer therapy are summarized. We first describe the synthesis of polypeptide SO2 prodrugs to overcome multiple drug resistance that was pioneered by our group, followed by other macromolecular SO2 prodrug structures that self-assemble into nanoparticles for tumor therapy. Second, we describe nanoplatforms composed of various small-molecule SO2 donors with endogenous or exogenous stimuli responsiveness, including thiol activated, acid-sensitive, and ultraviolet or near-infrared light-responsive SO2 donors, which have been used for tumor inhibition. Combinations of SO2 gas therapy with photodynamic therapy, chemotherapy, photothermal therapy, sonodynamic therapy, and nanocatalytic tumor therapy are also presented. Finally, we discuss the current limitations and challenges and the future outlook for SO2-based gas therapy. STATEMENT OF SIGNIFICANCE: Gas therapy is attracting increasing attention in the scientific community because it is a highly promising strategy against cancer owing to its inherent biosafety and avoidance of drug resistance. Sulfur dioxide (SO2) is recently found to be produced endogenously in mammals with important pathophysiological effects. This review summarizes recent advances in SO2 releasing nanosystems for cancer therapy, including polymeric prodrugs, endogenous or exogenous stimulus-activated SO2 donors delivered by nanoplatform and combination therapy strategies.

Sulfur signaling pathway in cardiovascular disease

Hydrogen sulfide (H2S) and sulfur dioxide (SO2), recognized as endogenous sulfur-containing gas signaling molecules, were the third and fourth molecules to be identified subsequent to nitric oxide and carbon monoxide (CO), and exerted diverse biological effects on the cardiovascular system. However, the exact mechanisms underlying the actions of H2S and SO2 have remained elusive until now. Recently, novel post-translational modifications known as S-sulfhydration and S-sulfenylation, induced by H2S and SO2 respectively, have been proposed. These modifications involve the chemical alteration of specific cysteine residues in target proteins through S-sulfhydration and S-sulfenylation, respectively. H2S induced S-sulfhydrylation can have a significant impact on various cellular processes such as cell survival, apoptosis, cell proliferation, metabolism, mitochondrial function, endoplasmic reticulum stress, vasodilation, anti-inflammatory response and oxidative stress in the cardiovascular system. Alternatively, S-sulfenylation caused by SO2 serves primarily to maintain vascular homeostasis. Additional research is warranted to explore the physiological function of proteins with specific cysteine sites, despite the considerable advancements in comprehending the role of H2S-induced S-sulfhydration and SO2-induced S-sulfenylation in the cardiovascular system. The primary objective of this review is to present a comprehensive examination of the function and potential mechanism of S-sulfhydration and S-sulfenylation in the cardiovascular system. Proteins that undergo S-sulfhydration and S-sulfenylation may serve as promising targets for therapeutic intervention and drug development in the cardiovascular system. This could potentially expedite the future development and utilization of drugs related to H2S and SO2.

Photothermal-Triggered Sulfur Oxide Gas Therapy Augments Type I Photodynamic Therapy for Potentiating Cancer Stem Cell Ablation and Inhibiting Radioresistant Tumor Recurrence

Despite advances in cancer therapy, the existence of self-renewing cancer stem cells (CSC) can lead to tumor recurrence and radiation resistance, resulting in treatment failure and high mortality in patients. To address this issue, a near-infrared (NIR) laser-induced synergistic therapeutic platform has been developed by incorporating aggregation-induced emission (AIE)-active phototheranostic agents and sulfur dioxide (SO2 ) prodrug into a biocompatible hydrogel, namely TBH, to suppress malignant CSC growth. Outstanding hydroxyl radical (·OH) generation and photothermal effect of the AIE phototheranostic agent actualizes Type I photodynamic therapy (PDT) and photothermal therapy through 660 nm NIR laser irradiation. Meanwhile, a large amount of SO2 is released from the SO2 prodrug in thermo-sensitive TBH gel, which depletes upregulated glutathione in CSC and consequentially promotes ·OH generation for PDT enhancement. Thus, the resulting TBH hydrogel can diminish CSC under 660 nm laser irradiation and finally restrain tumor recurrence after radiotherapy (RT). In comparison, the tumor in the mice that were only treated with RT relapsed rapidly. These findings reveal a double-boosting ·OH generation protocol, and the synergistic combination of AIE-mediated PDT and gas therapy provides a novel strategy for inhibiting CSC growth and cancer recurrence after RT, which presents great potential for clinical treatment.

Therapeutic gas-releasing nanomedicines with controlled release: Advances and perspectives

Nanoparticle-based drug delivery has become one of the most popular approaches for maximising drug therapeutic potentials. With the notable improvements, a greater challenge hinges on the formulation of gasotransmitters with unique challenges that are not met in liquid and solid active ingredients. Gas molecules upon release from formulations for therapeutic purposes have not really been discussed extensively. Herein, we take a critical look at four key gasotransmitters, that is, carbon monoxide (CO), nitric oxide (NO), hydrogen sulphide (H2S) and sulphur dioxide (SO2), their possible modification into prodrugs known as gas-releasing molecules (GRMs), and their release from GRMs. Different nanosystems and their mediatory roles for efficient shuttling, targeting and release of these therapeutic gases are also reviewed extensively. This review thoroughly looks at the diverse ways in which these GRM prodrugs in delivery nanosystems are designed to respond to intrinsic and extrinsic stimuli for sustained release. In this review, we seek to provide a succinct summary for the development of therapeutic gases into potent prodrugs that can be adapted in nanomedicine for potential clinical use.

Role of hydrogen sulfide in sulfur dioxide production and vascular regulation

Both hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) are produced endogenously from the mammalian metabolic pathway of sulfur-containing amino acids and play important roles in several vascular diseases. However, their interaction during the control of vascular function has not been fully clear. Here, we investigated the potential role of H₂S in SO₂ production and vascular regulation in vivo and in vitro. Wistar rats were divided into the vehicle, SO₂, DL-propargylglycine (PPG) + SO₂, β-cyano-L-alanine (BCA) + SO₂ and sodium hydrosulfide (NaHS) + SO₂ groups. SO₂ donor was administered with or without pre-administration of PPG, BCA or NaHS for 30 min after blood pressure was stabilized for 1 h, and then, the change in blood pressure was detected by catheterization via the common carotid artery. Rat plasma SO₂ and H₂S concentrations were measured by high performance liquid chromatography and sensitive sulfur electrode, respectively. The isolated aortic rings were prepared for the measurement of changes in vasorelaxation stimulated by SO₂ after PPG, BCA or NaHS pre-incubation. Results showed that the intravenous injection of SO₂ donors caused transient hypotension in rats compared with vehicle group. After PPG or BCA pretreatment, the plasma H₂S content decreased but the SO₂ content increased markedly, and the hypotensive effect of SO₂ was significantly enhanced. Conversely, NaHS pretreatment upregulated the plasma H₂S content but reduced SO₂ content, and attenuated the hypotensive effect of SO₂. After PPG or BCA pre-incubation, the vasorelaxation response to SO₂ was enhanced significantly. While NaHS pre-administration weakened the SO₂-induced relaxation in aortic rings. In conclusion, our in vivo and in vitro data indicate that H₂S negatively controls the plasma content of SO₂ and the vasorelaxant effect under physiological conditions.

Sulfur Dioxide: Endogenous Generation, Biological Effects, Detection, and Therapeutic Potential

Significance: Previously, sulfur dioxide (SO₂) was recognized as an air pollutant. However, it is found to be endogenously produced in mammalian tissues. As a new gasotransmitter, SO₂ is involved in regulating the structure and function of blood vessels, heart, lung, gastrointestinal tract, nervous system, etc.Recent Advances: Increasing evidence showed that endogenous SO₂ regulates cardiovascular physiological processes, such as blood pressure control, vasodilation, maintenance of the normal vascular structure, and cardiac negative inotropy. Under pathological conditions including hypertension, atherosclerosis, vascular calcification, aging endothelial dysfunction, myocardial injury, myocardial hypertrophy, diabetic myocardial fibrosis, sepsis-induced cardiac dysfunction, pulmonary hypertension, acute lung injury, colitis, epilepsy-related brain injury, depression and anxiety, and addictive drug reward memory consolidation, endogenous SO₂ protects against the pathological changes via different molecular mechanisms and the disturbed SO₂/aspartate aminotransferase pathway is likely involved in the mechanisms for the earlier mentioned pathologic processes. Critical Issues: A comprehensive understanding of the biological effects of endogenous SO₂ is extremely important for the development of novel SO₂ therapy. In this review, we summarized the biological effects, mechanism of action, SO₂ detection methods, and its related prodrugs. Future Directions: Further studies should be conducted to understand the effects of endogenous SO₂ in various physiological and pathophysiological processes and clarify its underlying mechanisms. More efficient and accurate SO₂ detection methods, as well as specific and effective SO₂-releasing systems should be designed for the treatment and prevention of clinical related diseases. The translation from SO₂ basic medical research to its clinical application is also worthy of further study. Antioxid. Redox Signal. 36, 256-274.

Sulphenylation of CypD at Cysteine 104: A Novel Mechanism by Which SO2 Inhibits Cardiomyocyte Apoptosis

Objectives: The study was designed to explore the role of endogenous gaseous signaling molecule sulfur dioxide (SO2) in the control of cardiomyocyte apoptosis and its molecular mechanisms.

Methods: Neonatal mouse cardiac myocytes (NMCMs) and H9c2 cells were used in the cell experiments. The endogenous SO2 pathway including SO2 level and the expression of SO2-generating enzyme aspartate aminotransferase 1/2 (AAT1/2) were detected in NMCMs. The apoptosis of cardiomyocytes was examined by a TUNEL assay. The cleavage and the activity of apoptotic proteins caspase9 and caspase3 were measured. The content of ATP, the opening of mitochondrial permeability transition pore (mPTP), and the cytochrome c (cytc) leakage were detected by immunofluorescence. The sulphenylation of cyclophilin-D (CypD) was detected by biotin switch analysis. The four CypD mutant plasmids in which cysteine sites were mutated to serine were constructed to identify the SO2-affected site in vitro.

Results: ISO down-regulated the endogenous SO2/AAT pathway of cardiomyocytes in association with a significant increase in cardiomyocyte apoptosis, demonstrated by the increases in apoptosis, cleaved-caspase3/caspase3 ratio, and caspase3 activity. Furthermore, ISO significantly reduced ATP production in H9c2 cells, but the supplement of SO2 significantly restored the content of ATP. ISO stimulated mPTP opening, resulting in an increase in the release of cytc, which further increased the ratio of cleaved caspase9/caspase9 and enhanced the protein activity of caspase9. While, the supplementation of SO2 reversed the above effects. Mechanistically, SO2 did not affect CypD protein expression, but sulphenylated CypD and inhibited mPTP opening, resulting in an inhibition of cardiomyocyte apoptosis. The C104S mutation in CypD abolished SO2-induced sulphenylation of CypD, and thereby blocked the inhibitory effect of SO2 on the mPTP opening and cardiomyocyte apoptosis.

Conclusion: Endogenous SO2 sulphenylated CypD at Cys104 to inhibit mPTP opening, and thus protected against cardiomyocyte apoptosis.

A novel fluorescent probe for the detection of sulfur dioxide derivatives and its application in biological imaging

A new probe CA-SO2 to efficiently and specifically detect SO2 was designed. The probe showed a fast response time (<50 s), low detection limit (LOD = 75 nM), large Stokes shift (129 nm) and was applied to detect SO2 in living cells and zebrafish.

Endogenous SO2 Controls Cell Apoptosis: The State-of-the-Art

SO2, previously known as the product of industrial waste, has recently been proven to be a novel gasotransmitter in the cardiovascular system. It is endogenously produced from the metabolism pathway of sulfur-containing amino acids in mammalians. Endogenous SO2 acts as an important controller in the regulation of many biological processes including cardiovascular physiological and pathophysiological events. Recently, the studies on the regulatory effect of endogenous SO2 on cell apoptosis and its pathophysiological significance have attracted great attention. Endogenous SO2 can regulate the apoptosis of vascular smooth muscle cells, endothelial cells, cardiomyocytes, neuron, alveolar macrophages, polymorphonuclear neutrophils and retinal photoreceptor cells, which might be involved in the pathogenesis of hypertension, pulmonary hypertension, myocardial injury, brain injury, acute lung injury, and retinal disease. Therefore, in the present study, we described the current findings on how endogenous SO2 is generated and metabolized, and we summarized its regulatory effects on cell apoptosis, underlying mechanisms, and pathophysiological relevance.

Therapeutic gas delivery strategies

Gas molecules with pharmaceutical effects offer emerging solutions to diseases. In addition to traditional medical gases including O₂ and NO, more gases such as H₂ , H₂ S, SO₂ , and CO have recently been discovered to play important roles in various diseases. Though some issues need to be addressed before clinical application, the increasing attention to gas therapy clearly indicates the potentials of these gases for disease treatment. The most important and difficult part of developing gas therapy systems is to transport gas molecules of high diffusibility and penetrability to interesting targets. Given the particular importance of gas molecule delivery for gas therapy, distinguished strategies have been explored to improve gas delivery efficiency and controllable gas release. Here, we summarize the strategies of therapeutic gas delivery for gas therapy, including direct gas molecule delivery by chemical and physical absorption, inorganic/organic/hybrid gas prodrugs, and natural/artificial/hybrid catalyst delivery for gas generation. The advantages and shortcomings of these gas delivery strategies are analyzed. On this basis, intelligent gas delivery strategies and catalysts use in future gas therapy are discussed. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Self-Propelled Asymmetrical Nanomotor for Self-Reported Gas Therapy

Gas therapy has emerged as a new therapeutic strategy in combating cancer owing to its high therapeutic efficacy and biosafety. However, the clinical translation of gas therapy remains challenging due to the rapid diffusion and limited tissue penetration of therapeutic gases. Herein, a self-propelled, asymmetrical Au@MnO₂ nanomotor for efficient delivery of therapeutic gas to deep-seated cancer tissue for enhanced efficacy of gas therapy, is reported. The Au@MnO₂ nanoparticles (NPs) catalyze endogenous H₂ O₂ into O₂ that propels NPs into deep solid tumors, where SO₂ prodrug is released from the hollow NPs owing to the degradation of MnO₂ shells. Fluorescein isothiocyanate (FITC) is conjugated onto the surface of Au via caspase-3 responsive peptide (DEVD) and the therapeutic process of gas therapy can be optically self-reported by the fluorescence of FITC that is turned on in the presence of overexpressed caspase-3 as an apoptosis indicator. Au@MnO₂ nanomotors show self-reported therapeutic efficacy and high biocompatibility both in vitro and in vivo, offering important new insights to the design and development of novel nanomotors for efficient payload delivery into deep tumor tissue and in situ monitoring of the therapeutic process.

Sulfur Dioxide: An Endogenous Protector Against Myocardial Injury

Sulfur dioxide (SO2) was previously known as a harmful gas in air pollution. Recently, it was reported that SO2 can be endogenously generated in cardiovascular tissues. Many studies have revealed that endogenous SO2 has important physiological and pathophysiological significance and pharmacological potential. As a novel gasotransmitter, SO2 has important regulatory effects on the heart. It has a dose-dependent negative inotropic effect on cardiac function, in which L-type calcium channels are involved. SO2 can also attenuate myocardial injury caused by various harmful stimuli and play an important role in myocardial ischemia-reperfusion injury and myocardial hypertrophy. These effects are thought to be linked to its ability to reduce inflammation and as an antioxidant. In addition, SO2 regulates cardiomyocyte apoptosis and autophagy. Therefore, endogenous SO2 plays an important role in maintaining cardiovascular system homeostasis. In the present review, the literature concerning the metabolism of endogenous SO2, its cardiac toxicological effects and physiological regulatory effects, mechanisms for SO2-mediated myocardial protection and its pharmacological applications are summarized and discussed.

Targeted Delivery of Persulfides to the Gut: Effects on the Microbiome

Persulfides (R-SSH) have been hypothesized as potent redox modulators and signaling compounds. Reported herein is the synthesis, characterization, and in vivo evaluation of a persulfide donor that releases N-acetyl cysteine persulfide (NAC-SSH) in response to the prokaryote-specific enzyme nitroreductase. The donor, termed NDP-NAC, decomposed in response to E. coli nitroreductase, resulting in release of NAC-SSH. NDP-NAC elicited gastroprotective effects in mice that were not observed in animals treated with control compounds incapable of persulfide release or in animals treated with Na2 S. NDP-NAC induced these effects by the upregulation of beneficial small- and medium-chain fatty acids and through increasing growth of Turicibacter sanguinis, a beneficial gut bacterium. It also decreased the populations of Synergistales bacteria, opportunistic pathogens implicated in gastrointestinal infections. This study reveals the possibility of maintaining gut health or treating microbiome-related diseases by the targeted delivery of reactive sulfur species.

Post-translational Modifications of IκBα: The State of the Art

The nuclear factor-kappa B (NF-κB) signaling pathway regulates a variety of biological functions in the body, and its abnormal activation contributes to the pathogenesis of many diseases, such as cardiovascular and respiratory diseases and cancers. Therefore, to ensure physiological homeostasis of body systems, this pathway is strictly regulated by IκBα transcription, IκBα synthesis, and the IκBα-dependent nuclear transport of NF-κB. Particularly, the post-translational modifications of IκBα including phosphorylation, ubiquitination, SUMOylation, glutathionylation and hydroxylation are crucial in the abovementioned regulatory process. Because of the importance of the NF-κB pathway in maintaining body homeostasis, understanding the post-translational modifications of IκBα can not only provide deeper insights into the regulation of NF-κB pathway but also contribute to the development of new drug targets and biomarkers for the diseases.

Prodrugs of Persulfides, Sulfur Dioxide, and Carbon Disulfide: Important Tools for Studying Sulfur Signaling at Various Oxidation States

Significance: Bioactive sulfur species such as hydrogen sulfide (H₂S), persulfide species (R-S_nSH, n ≥ 1), hydrogen polysulfide (H₂S_n, n ≥ 2), sulfur dioxide (SO₂), and carbon disulfide (CS₂) participate in various physiological and/or pathological pathways such as vasodilation, apoptosis, inflammation, and energy metabolism regulation. The oxidation state of the individual sulfur species endows them unique biological activities. Recent Advances: There have been great strides made in achieving molecular understanding of the sulfur-signaling processes. Critical Issues: The development of various chemical tools that deliver reactive sulfur species in a controllable manner has played an important role in understanding the different roles of various sulfur species. In this review, we focus on three types of sulfur species, including persulfide, SO₂, and CS₂. Starting with a brief introduction of their physiological functions, we will then assess the various drug delivery strategies to generate persulfide species, SO₂, and CS₂ as research tools and potentially as therapeutic agents. Future Directions: Development of donors of various sulfur species that respond to distinct stimulus is critical for this field. Another key to the long-term success of this field is the identification of an area of unmet medical need that can be addressed with these sulfur species.

Acidity-Triggered Tumor-Targeted Nanosystem for Synergistic Therapy via a Cascade of ROS Generation and NO Release

Nitric oxide (NO) gas therapy has aroused intense interest in recent years. l-Arginine (l-Arg) reacts with reactive oxygen species (ROS) in tumor cells to generate NO. This phenomenon represents an effective method for tumor therapy. However, endogenous ROS levels in most types of tumor cells cannot enable an effective reaction. β-Lapachone is generally used to increase H₂O₂, which can oxidize guanidine derivatives to form nitric oxide in tumor cells. In addition, based on the ferrocene (Fc)-catalyzed Fenton reaction, ·OH is generated from H₂O₂, and the ONOO^- could be generated from an interaction between ·O₂^- (generated through the Haber-Weiss reaction) and NO. Arg-rich poly(ε-caprolactone) (PCL)-b-PArg, a macromolecular NO donor, was accurately synthesized to avoid premature l-Arg leakage during in vivo transport. In this design, the self-assembled PCL-b-PArg nanoparticles were dressed with the tumor-shreddable masking (PEG-b-PDMA, a negatively charged pH-sensitive hydrophilic diblock polymer), to prepare P-lapa-Fc nanoparticles and hide penetrative capability in the circulation. The experimental results confirmed that this synergistic therapy based on ROS and NO had a significant inhibitory effect on cancer cells, thereby providing new inspiration for NO gas treatment.

Polymeric persulfide prodrugs: Mitigating oxidative stress through controlled delivery of reactive sulfur species

Related biologically to the known gasotransmitter hydrogen sulfide (H₂S), persulfides (R-SSH) have recently been recognized as native signaling compounds and redox regulators in their own right. Reported here is the synthesis, characterization, and in vitro evaluation of a small molecule persulfide donor and its polymeric counterpart, both of which release N-acetyl cysteine persulfide (NAC-SSH) in response to esterases. The donors, termed EDP-NAC and poly(EDP-NAC), underwent controlled decomposition in response to porcine liver esterase, resulting in pseudo-first-order release half-lives of 1.6 h ± 0.3 h and 36.0 h ± 0.6 h, respectively. In cell experiments, slow-releasing poly(EDP-NAC) rescued H9C2 cardiomyocytes more effectively than EDP-NAC when cells were treated with 5-fluorouricil (5-FU), which induces sustained production of ROS. Neither EDP-NAC nor poly(EDP-NAC) rescued MCF-7 breast cancer cells from 5-FU-induced oxidative stress, suggesting that polymeric persulfide donors could be used as adjuvants to reduce the deleterious cardiotoxic effects of many chemotherapeutics.

Synthesis of Cyclic Sulfite Diesters and their Evaluation as Sulfur Dioxide (SO2 ) Donors

Although sulfur dioxide (SO₂ ) finds widespread use in the food industry as its hydrated sulfite form, a number of aspects of SO₂ biology remain to be completely understood. Of the tools available for intracellular enhancement of SO₂ levels, most suffer from poor cell permeability and a lack of control over SO₂ release. We report 1,2-cyclic sulfite diesters as a new class of reliable SO₂ donors that dissociate in buffer through nucleophilic displacement to produce SO₂ with tunable release profiles. We provide data in support of the suitability of these SO₂ donors to enhance intracellular SO₂ levels more efficiently than sodium bisulfite, the most commonly used SO₂ donor for cellular studies.

The ratiometric fluorescent probes for monitoring the reactive inorganic sulfur species (RISS) signal in the living cell

RSSs (reactive sulfur species) and their metabolites, such as H₂S, S_n^2-, SO₃^2-/HSO₃^-, S₂O₄^2- and S₂O₅^2- (Reactive Inorganic Sulfur Species, RISSs), play a crucial role in the cushion against oxidative stress and the other physiological events. The molecular mechanisms how they affect cellular signaling and other physiological events remain largely unknown. To address their physiological functions, the techniques that can track their levels should be invaluable. Herein, six coumarin hemicyanine scaffolds (CH-RISSs) were synthesized and their fast and strong responses upon H₂S, S_n^2-, SO₃^2-, HSO₃^-, S₂O₄^2- and S₂O₅^2- (Reactive Inorganic Sulfur Species, RISSs) were clarified in the absorption (colorimetric) and fluorescence (ratiometric) spectra, which showed good stability in the physiological pH (7.4). Upon the analytes, the maxima absorption of CH-RISSs switched from ~580 nm to ~400 nm in the absorption spectra. The fluorescence of CH-RISSs depleted at 650-660 nm and increased at 480-505 nm upon the RISSs. Both of coumarin hemicyanine structures with C12 alkyl chain (CH-RISS-3 and CH-RISS-6) showed quick and robust ratiometric fluorescence switch in the living cell imaging. Access to the fluorescent probes for RISSs sets the stage for applying the developing technologies to probe reactive sulfur biology in living systems.

A near-infrared ratiometric fluorescent probe based on the CN double bond for monitoring SO2 and its application in biological imaging

A near-infrared ratiometric fluorescent probe based on CN double bond was developed for monitoring SO₂ and its application in biological imaging.

Gas-Mediated Cancer Bioimaging and Therapy

Gas-involving cancer theranostics have attracted considerable attention in recent years due to their high therapeutic efficacy and biosafety. We have reviewed the recent significant advances in the development of stimuli-responsive gas releasing molecules (GRMs) and gas nanogenerators for cancer bioimaging, targeted and controlled gas therapy, and gas-sensitized synergistic therapy. We have focused on gases with known anticancer effects, such as oxygen (O₂), carbon monoxide (CO), nitric oxide (NO), hydrogen sulfide (H₂S), hydrogen (H₂), sulfur dioxide (SO₂), carbon dioxide (CO₂), and heavy gases that act via the gas-generating process. The GRMs and gas nanogenerators for each gas have been described in terms of the stimulation method, followed by their applications in ultrasound and multimodal imaging, and finally their primary and synergistic actions with other cancer therapeutic modalities. The current challenges and future possibilities of gas therapy and imaging vis-à-vis clinical translation have also been discussed.

Click and release: bioorthogonal approaches to “on-demand” activation of prodrugs

This review summarizes recent developments in using bioorthogonal chemistry in prodrug design for the delivery of traditional small molecule- and gasotransmitter-based therapeutics.

One- and two-photon responsive sulfur dioxide (SO2) donors: a combinatorial drug delivery for improved antibiotic therapy

Light-activated sulfur dioxide donors have been developed and explored their applicability for combinatorial antibiotic therapy with self-monitoring ability.

Dual-site functionalized NIR fluorescent material for a discriminative concentration-dependent response to SO2 in cells and mice

Sulfur dioxide (SO₂), as an important anti-oxidant and gaseous signaling molecule, plays fundamental roles in the regulation of intracellular signaling and cell death cellular bioenergetics.

SO2 Donors and Prodrugs, and Their Possible Applications: A Review

SO2 is widely recognized as an air pollutant and is a known cause of acid rain. At a sufficiently high level, it also causes respiratory diseases. A much lesser known side of SO2 is its endogenous nature and possible physiological roles. There is mounting evidence that SO2 is produced during normal cellular metabolism and may possibly function as a signaling molecule in normal physiology. The latter aspect is still at the stage of being carefully examined as to the validity of classifying SO2 as a gasotransmitter with endogenous signaling roles. One difficulty in studying the biological and pharmacological roles of SO2 is the lack of adequate tools for its controllable and precise delivery. Traditional methods of using SO2 gas or mixed sulfite salts do not meet research need for several reasons. Therefore, there has been increasing attention on the need of developing SO2 donors or prodrugs that can be used as tools for the elucidation of SO2's physiological roles, pharmacological effects, and possible mechanism(s) of action. In this review, we aim to review basic sulfur chemistry in the context of sulfur signaling and various chemical strategies used for designing SO2 donors. We will also discuss potential pharmacological applications of SO2 donors, lay out desirable features for such donors and possibly prodrugs, analyze existing problems, and give our thoughts on research needs.

Small molecule generators of biologically reactive sulfur species

Sulfur metabolism is integral to cellular growth and survival. The presence of a wide range of oxidation states of sulfur in biology coupled with its unique reactivity are some key features of the biology of this element. In particular, nearly all oxidation states of sulfur not only occur but are also inter-convertible. In order to study the chemical biology of reactive sulfur species, tools to reliably detect as well as generate these species within cells are necessary. Herein, an overview of strategies to generate certain reactive sulfur species is presented. The donors of reactive sulfur species have been organized based on their oxidation states. These interesting small molecules have helped lay a strong foundation to study the biology of reactive sulfur species and some may have therapeutic applications in the future as well.

A Persulfide Donor Responsive to Reactive Oxygen Species: Insights into Reactivity and Therapeutic Potential

Persulfides (RSSH) have been hypothesized as critical components in sulfur-mediated redox cycles and as potential signaling compounds, similar to hydrogen sulfide (H2 S). Hindering the study of persulfides is a lack of persulfide-donor compounds with selective triggers that release discrete persulfide species. Reported here is the synthesis and characterization of a ROS-responsive (ROS=reactive oxygen species), self-immolative persulfide donor. The donor, termed BDP-NAC, showed selectivity towards H2 O2 over other potential oxidative or nucleophilic triggers, resulting in the sustained release of the persulfide of N-acetyl cysteine (NAC) over the course of 2 h, as measured by LCMS. Exposure of H9C2 cardiomyocytes to H2 O2 revealed that BDP-NAC mitigated the effects of a highly oxidative environment in a dose-dependent manner over relevant controls and to a greater degree than common H2 S donors sodium sulfide (Na2 S) and GYY4137. BDP-NAC also rescued cells more effectively than a non-persulfide-releasing control compound in concert with common H2 S donors and thiols.

Esterase-sensitive sulfur dioxide prodrugs inspired by modified Julia olefination

Sulfur dioxide (SO₂) is an endogenously produced gaseous molecule, and is emerging as a potential gasotransmitter. Herein, we describe the first series of esterase-sensitive prodrugs inspired by modified Julia olefination as SO₂ donors.

Sulfur dioxide prodrugs: triggered release of SO2via a click reaction

Sulfur dioxide (SO₂) is being recognized as a possible endogenous gasotransmitter with importance on par with that of NO, CO, and H₂S. Herein we describe a series of SO₂ prodrugs that are activated for SO₂ release via a bioorthogonal click reaction. The release rate can be tuned by adjusting the substituents on the prodrug.

Esterase Sensitive Self-Immolative Sulfur Dioxide Donors

A series of cell-permeable esterase-sensitive sulfonates that undergo self-immolation to produce sulfur dioxide (SO₂), a gaseous pollutant with new and emerging biological roles, is reported. These compounds should facilitate the study SO₂ biology and will lay the platform for newer stimuli-responsive donors of this gas.

Visible-Light-Triggered Uncaging of Carbonyl Sulfide for Hydrogen Sulfide (H2S) Release

Generation of hydrogen sulfide (H₂S) is challenging and few methods are capable of localized delivery of this gas. Here, a boron dipyrromethene-based carbamothioate (BDP-H₂S) that is uncaged by visible light of 470 nm to generate carbonyl sulfide (COS), which is rapidly hydrolyzed to H₂S in the presence of carbonic anhydrase, a widely prevalent enzyme, is reported.

Benzothiazole Sulfinate: A Sulfinic Acid Transfer Reagent under Oxidation-Free Conditions

Sulfinic acids are commonly encountered intermediates found in natural product synthesis and medicinal chemistry. However, because of high reactivity, instability, and harsh reaction conditions, they are difficult to synthesize. Herein we have developed an oxidation-free method to produce sulfinic acids and sulfinate salts using 2-sulfinyl benzothiazole (BTS). We have also demonstrated the synthetic usefulness by developing one-pot syntheses of sulfones and sulfonamides.

Click and Release: SO2 Prodrugs with Tunable Release Rates

Employing an intramolecular cycloaddition reaction, we have developed a series of SO₂ prodrugs with tunable release rates with half-lives ranging from minutes to days.

Esterase Activated Carbonyl Sulfide/Hydrogen Sulfide (H2S) Donors

Hydrogen sulfide (H₂S) is a mediator of a number of cellular processes, and modulating cellular levels of this gas has emerged as an important therapeutic area. Localized generation of H₂S is thus very useful but highly challenging. Here, we report pivaloyloxymethyl-based carbonothioates and carbamothioates that are activated by the enzyme, esterase, to generate carbonyl sulfide (COS), which is hydrolyzed to H₂S.

Therapeutic Delivery of H2S via COS: Small Molecule and Polymeric Donors with Benign Byproducts

Carbonyl sulfide (COS) is a gas that may play important roles in mammalian and bacterial biology, but its study is limited by a lack of suitable donor molecules. We report here the use of N-thiocarboxyanhydrides (NTAs) as COS donors that release the gas in a sustained manner under biologically relevant conditions with innocuous peptide byproducts. Carbonic anhydrase converts COS into H₂S, allowing NTAs to serve as either COS or H₂S donors, depending on the availability of the enzyme. Analysis of the pseudo-first-order H₂S release rate under biologically relevant conditions revealed a release half-life of 75 min for the small molecule NTA under investigation. A polynorbornene bearing pendant NTAs made by ring-opening metathesis polymerization was also synthesized to generate a polymeric COS/H₂S donor. A half-life of 280 min was measured for the polymeric donor. Endothelial cell proliferation studies revealed an enhanced rate of proliferation for cells treated with the NTA over untreated controls.

Role of Endogenous Sulfur Dioxide in Regulating Vascular Structural Remodeling in Hypertension

Sulfur dioxide (SO₂), an emerging gasotransmitter, was discovered to be endogenously generated in the cardiovascular system. Recently, the physiological effects of endogenous SO₂ were confirmed. Vascular structural remodeling (VSR), an important pathological change in many cardiovascular diseases, plays a crucial role in the pathogenesis of the diseases. Here, the authors reviewed the research progress of endogenous SO₂ in regulating VSR by searching the relevant data from PubMed and Medline. In spontaneously hypertensive rats (SHRs) and pulmonary hypertensive rats, SO₂/aspartate aminotransferase (AAT) pathway was significantly altered. SO₂ inhibited vascular smooth muscle cell (VSMC) proliferation, promoted apoptosis, inhibited the synthesis of extracellular collagen but promoted its degradation, and enhanced antioxidative capacity, thereby playing a significant role in attenuating VSR. However, the detailed mechanisms needed to be further explored. Further studies in this field would be important for the better understanding of the pathogenesis of systemic hypertension and pulmonary hypertension. Also, clinical trials are needed to demonstrate if SO₂ would be a potential therapeutic target in cardiovascular diseases.