Development and Application of Carbonyl Sulfide-Based Donors for H2S Delivery

Bodipy-Based highly sensitive hydrogen sulfide fluorescent probe and its fluorescence imaging in cells and zebrafish

Hydrogen sulfide (H2S) is a crucial endogenous gasotransmitter that plays a role in various physiological and pathological processes. Therefore, accurate and rapid monitoring of H2S in organisms is highly significant for understanding the underlying pathological mechanisms and facilitating early diagnosis of related diseases. In this study, we developed a novel fluorescent probe, B-CHO-NO2, based on a bodipy fluorophore, which exhibits excellent sensitivity and selectivity towards H2S. The design of the probe exploits the nucleophilicity of H2S by introducing a formyl group as the ortho-participating moiety, significantly enhancing the reaction rate with H2S. In cellular and zebrafish models, the probe B-CHO-NO2 successfully achieved fluorescence imaging of endogenous and exogenous H2S. The development of probe B-CHO-NO2 provides a powerful tool for biological studies of H2S and diagnosis of related diseases.

Gas-Responsive and Gas-Releasing Polymer Assemblies

In order to explore the unique physiological roles of gas signaling molecules and gasotransmitters in vivo, chemists have engineered a variety of gas-responsive polymers that can monitor their changes in cellular milieu, and gas-releasing polymers that can orchestrate the release of gases. These have advanced their potential applications in the field of bio-imaging, nanodelivery, and theranostics. Since these polymers are of different chain structures and properties, the morphology of their assemblies will manifest distinct transitions after responding to gas or releasing gas. In this review, we summarize the fundamental design rationale of gas-responsive and gas-releasing polymers in structure and their controlled transition in self-assembled morphology and function, as well as present some perspectives in this prosperous field. Emerging challenges faced for the future research are also discussed.

H2S-Responsive NIR-II Fluorescent Nanozyme that Regulates Tumor Microenvironment for Activatable Synergistic CO Therapy/Catalytic Therapy/Immunotherapy

Nanozyme catalytic therapy triggered by the tumor microenvironment (TME)-responsive enzyme-like catalytic activities is an emerging approach for tumor treatment. However, the poor catalytic efficiency of nanozymes in tumors and the toxic side effects on normal tissues limit their further development, primarily due to the limited uptake and penetration depth of nanozyme in tumor tissues. Here, a tumor-targeting TME and electric field stimuli-responsive nanozyme (AgPt@CaCO3-FA) is developed, which is capable of catalyzing the generation of ROS to induce cell death and releasing carbon monoxide (CO) specifically in tumor tissues for on-demand CO therapy and immunotherapy. Benefiting from the endogenous H2S activated NIR-II fluorescence (FL) imaging guidance, AgPt@CaCO3-FA can be delivered into the deeper site of tumor tissues resulted from the TME regulation via generated CO during the electrolysis process to improve the catalytic efficiency of nanozymes in tumors. Moreover, CO effectively relieve immunosuppression TME via reeducating tumor-supportive M2-like macrophages to tumoricidal M1-like macrophages and induce mitochondrial dysfunction by reducing mitochondrial membrane potential, triggering tumor cells apoptosis. The enzyme-like activities combined with CO therapy arouse distinct immunogenic cell death (ICD) effect. Therefore, AgPt@CaCO3-FA permits synergistic CO gas, catalytic therapy and immunotherapy, effectively eradicating orthotopic breast tumors and preventing tumor metastasis and recurrence.

Fluorescent small molecule donors

Small molecule donors (SMDs) play subtle roles in the signaling mechanism and disease treatments. While many excellent SMDs have been developed, dosage control, targeted delivery, spatiotemporal feedback, as well as the efficiency evaluation of small molecules are still key challenges. Accordingly, fluorescent small molecule donors (FSMDs) have emerged to meet these challenges. FSMDs enable controllable release and non-invasive real-time monitoring, providing significant advantages for drug development and clinical diagnosis. Integration of FSMDs with chemotherapeutic, photodynamic or photothermal properties can take full advantage of each mode to enhance therapeutic efficacy. Given the remarkable properties and the thriving development of FSMDs, we believe a review is needed to summarize the design, triggering strategies and tracking mechanisms of FSMDs. With this review, we compiled FSMDs for most small molecules (nitric oxide, carbon monoxide, hydrogen sulfide, sulfur dioxide, reactive oxygen species and formaldehyde), and discuss recent progress concerning their molecular design, structural classification, mechanisms of generation, triggered release, structure-activity relationships, and the fluorescence response mechanism. Firstly, from the large number of fluorescent small molecular donors available, we have organized the common structures for producing different types of small molecules, providing a general strategy for the development of FSMDs. Secondly, we have classified FSMDs in terms of the respective donor types and fluorophore structures. Thirdly, we discuss the mechanisms and factors associated with the controlled release of small molecules and the regulation of the fluorescence responses, from which universal guidelines for optical properties and structure rearrangement were established, mainly involving light-controlled, enzyme-activated, reactive oxygen species-triggered, biothiol-triggered, single-electron reduction, click chemistry, and other triggering mechanisms. Fourthly, representative applications of FSMDs for trackable release, and evaluation monitoring, as well as for visible in vivo treatment are outlined, to illustrate the potential of FSMDs in drug screening and precision medicine. Finally, we discuss the opportunities and remaining challenges for the development of FSMDs for practical and clinical applications, which we anticipate will stimulate the attention of researchers in the diverse fields of chemistry, pharmacology, chemical biology and clinical chemistry. With this review, we hope to impart new understanding thereby enabling the rapid development of the next generation of FSMDs.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Glycosidase-activated H2S donorsto enhance chemotherapy efficacy

Hydrogen sulfide (H2S) plays a critical role in cancer biology. Herein, we developed a series of glycosidase-triggered hydrogen sulfide (H2S) donors by connecting sugar moieties (including glucose, galactose and mannose) to COS donors via a self-immolative spacer. In the presence of corresponding glycosidases, H2S was gradually released from these donors in PBS buffer with releasing efficiencies from 36 to 67 %. H2S release was also detected by H2S probe WSP-1 after treatment HepG2 cells with Man1. Cytotoxicities of these glycosylated H2S donors were evaluated against HepG2 by MTT assay. Among them, Man1 and Man2 exhibited an obvious reduction of cell viability in HepG2 cells, with cell viability as 37.6 % for 80 μM of Man. Consistently, significant apoptosis was observed in HepG2 cells after treatment with Man1 and Man2. Finally, We evaluated the potential of Man1 for combination therapy with doxorubicin. A synergistic effect was observed between Man1 and Doxorubicin in HepG2 and Hela cells. All these results indicated glycosidase-activated H2S donorshave promising potential for cancer therapy.

Activatable organic probes for in situ imaging of biomolecules

Biomolecules are fundamental for various chemical and biological processes of living organisms. High-resolution in situ imaging of the dynamics and local distribution of biomolecules may facilitate better interpretation of diverse complex cell events in the biomedicine field. In different advanced imaging tools, fluorescence imaging-based activatable organic probes can be noninvasively and effortlessly internalized into cells and can be easily modified, which is essential for the in situ imaging of targets in living organisms. We here briefly summarize the existing general design strategies of activatable organic probes for retaining the fluorescence signal inside cells. We particularly describe the bioapplication of these probes for the in situ bioimaging. This review is expected to promote the development of new molecular tools for extending the application of these in situ imaging strategies to other biomolecules.

Hydrogels for Gasotransmitter Delivery: Nitric Oxide, Carbon Monoxide, and Hydrogen Sulfide

Gasotransmitters, gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2 S), maintain myriad physiological processes. Low levels of gasotransmitters are often associated with specific problems or diseases, so NO, CO, and H2 S hold potential in treating bacterial infections, chronic wounds, myocardial infarction, ischemia, and various other diseases. However, their clinical applications as therapeutic agents are limited due to their gaseous nature, short half-life, and broad physiological roles. One route toward the greater application of gasotransmitters in medicine is through localized delivery. Hydrogels are attractive biomedical materials for the controlled release of embedded therapeutics as they are typically biocompatible, possess high water content, have tunable mechanical properties, and are injectable in certain cases. Hydrogel-based gasotransmitter delivery systems began with NO, and hydrogels for CO and H2 S have appeared more recently. In this review, the biological importance of gasotransmitters is highlighted, and the fabrication of hydrogel materials is discussed, distinguishing between methods used to physically encapsulate small molecule gasotransmitter donor compounds or chemically tether them to a hydrogel scaffold. The release behavior and potential therapeutic applications of gasotransmitter-releasing hydrogels are also detailed. Finally, the authors envision the future of this field and describe challenges moving forward.

Chemical Strategies Toward Prodrugs and Fluorescent Probes for Gasotransmitters

Three gaseous molecules are widely accepted as important gasotransmitters in mammalian cells, namely NO, CO and H2S. Due to the pharmacological effects observed in preclinical studies, these three gasotransmitters represent promising drug candidates for clinical translation. Fluorescent probes of the gasotransmitters are also in high demand; however, the mechanisms of actions or the roles played by gasotransmitters under both physiological and pathological conditions remain to be answered. In order to bring these challenges to the attention of both chemists and biologists working in this field, we herein summarize the chemical strategies used for the design of both probes and prodrugs of these three gasotransmitters.

Bioorthogonal Bond Cleavage Chemistry for On-demand Prodrug Activation: Opportunities and Challenges

Time- and space-resolved drug delivery is highly demanded for cancer treatment, which, however, can barely be achieved with a traditional prodrug strategy. In recent years, the prodrug strategy based on a bioorthogonal bond cleavage chemistry has emerged with the advantages of high temporospatial resolution over drug activation and homogeneous activation irrespective of individual heterogeneity. In the past five years, tremendous progress has been witnessed in this field with one such bioorthogonal prodrug entering Phase II clinical trials. This Perspective aims to highlight these new advances (2019-2023) and critically discuss their pros and cons. In addition, the remaining challenges and potential strategic directions for future progress will also be included.

Smart Lipid Nanoparticle that Remodels Tumor Microenvironment for Activatable H2S Gas and Photodynamic Immunotherapy

Hydrogen sulfide (H2S) has shown promise for gas therapy. However, it is still controversial whether H2S can remodel the tumor microenvironment (TME) and induce robust antitumor immunity. Here, a tumor-targeting and TME-responsive "smart" lipid nanoparticle (1-JK-PS-FA) is presented, which is capable of delivering and releasing H2S specifically in tumor tissues for on-demand H2S gas and photodynamic immunotherapy. 1-JK-PS-FA enables a burst release of H2S in the acidic TME, which promptly reduces the embedded organic electrochromic materials and consequently switches on near-infrared fluorescence and photodynamic activity. Furthermore, we found that high levels of H2S can reprogram the TME by reducing tumor interstitial fluid pressure, promoting angiogenesis, increasing vascular permeability, ameliorating hypoxia, and reducing immunosuppressive conditions. This leads to increased tumor uptake of 1-JK-PS-FA, thereby enhancing PDT efficacy and eliciting strong immunogenic cell death during 808 nm laser irradiation. Therefore, 1-JK-PS-FA permits synergistic H2S gas and photodynamic immunotherapy, effectively eradicating orthotopic breast tumors and preventing tumor metastasis and recurrence. This work showcases the capacity of H2S to reprogram the TME to enhance H2S gas and immunotherapy.

An ROS-Responsive Donor That Self-Reports Its H2S Delivery by Forming a Benzoxazole-Based Fluorophore

Hydrogen sulfide (H2S), an endogenous signaling molecule, is known to play a pivotal role in neuroprotection, vasodilation, and hormonal regulation. To further explore the biological effects of H2S, refined donors that facilitate its biological delivery, especially under specific (patho) physiological conditions, are needed. In the present study, we demonstrate that ortho-substituted, aryl boronate esters provide two unique and distinct pathways for H2S release from thioamide-based donors: Lewis acid-facilitated hydrolysis and reactive oxygen species (ROS)-induced oxidation/cyclization. Through a detailed structure-activity relationship study, donors that resist hydrolysis and release H2S solely via the latter mechanism were identified, which have the added benefit of providing a potentially useful heterocycle as the lone byproduct of this novel chemistry. To highlight this, we developed an ROS-activated donor (QH642) that simultaneously synthesizes a benzoxazole-based fluorophore en route to its H2S delivery. A distinct advantage of this design over earlier self-reporting donors is that fluorophore formation is possible only if H2S has been discharged from the donor. This key feature eliminates the potential for false positives and provides a more accurate depiction of reaction progress and donor delivery of H2S, including in complex cellular environments.

Prodrugs of sulfide and persulfide species: Implications in their different pharmacological activities

Reactive sulfur species (RSS), such as H₂S, hydrogen polysulfide (H₂S_n, n ≥ 2), and hydropersulfides (RSS_nH, n ≥ 1), are known to mediate diverse signaling pathways and possess a plethora of exciting therapeutic opportunities. Historically, due to the rapid inter-conversion among those species in vivo, the biological differences of distinct sulfur species were often overlooked. These species were considered to enrich the global sulfur pool in almost an equal fashion. However, advancement in this field has revealed that sulfur species at different oxidation states result in different pharmacological effects including scavenging reactive oxygen species (ROS), activating ion channels, and exhibiting analgesic effects. Here, we summarize recent advances in studying the biological and pharmacological differences of distinct sulfur species; discuss this phenomenon from the view of chemical properties and sulfur signaling pathways; and lay out a roadmap to transforming such new knowledge into general principles in developing sulfur-based therapeutics.

Insights into self-degradation of cysteine esters and amides under physiological conditions yield new cleavable chemistry

An unprecedented H₂S release from cysteine esters and amides (CysO/NHR) under physiological conditions was discovered and the plausible mechanism was proposed. Alkylation of the amino moiety of cysteine esters enables the H₂S release to be tuned and further provides support to the mechanistic insights. This discovery not only provides new insights into several fundamental science issues including non-enzymatic H₂S-produced pathways, but also inspires new tunable cleavable motifs for sustained release of arylthiols and even for prodrug design.

Fluorescent probes for ferroptosis bioimaging: advances, challenges, and prospects

Ferroptosis is a form of regulatory cell death distinct from caspase-dependent apoptosis and plays an important role in life entities. Since ferroptosis involves a variety of complex regulatory factors, the levels of certain biological species and microenvironments would change during this process. Thus, the investigation of the level fluctuation of key target analytes during ferroptosis is of great significance for disease treatment and drug design. Toward this aim, multiple organic fluorescent probes with simple preparation and non-destructive detection have been developed, and research over the past decade has uncovered a vast array of homeostasis and other physiological characteristics of ferroptosis. However, this significant and cutting-edge topic has not yet been reviewed. In this work, we aim to highlight the latest breakthrough results of fluorescent probes for monitoring various bio-related molecules and microenvironments during ferroptosis at the cellular, tissue and in vivo levels. Accordingly, this tutorial review has been organized according to the target molecules identified by the probes including ionic species, reactive sulfur species, reactive oxygen species, biomacromolecules, microenvironment, and others. In addition to providing new insights into the findings of each fluorescent probe in ferroptosis studies, we also discuss the defects and limitations of the probes developed, and highlight the potential challenges and further prospects in this domain. We anticipate that this review will convey profound implications for designing powerful fluorescent probes to decrypt changes in key molecules and microenvironments during ferroptosis.

H2S Donors with Cytoprotective Effects in Models of MI/R Injury and Chemotherapy-Induced Cardiotoxicity

Hydrogen sulfide (H2S) is an endogenous signaling molecule that greatly influences several important (patho)physiological processes related to cardiovascular health and disease, including vasodilation, angiogenesis, inflammation, and cellular redox homeostasis. Consequently, H2S supplementation is an emerging area of interest, especially for the treatment of cardiovascular-related diseases. To fully unlock the medicinal properties of hydrogen sulfide, however, the development and refinement of H2S releasing compounds (or donors) are required to augment its bioavailability and to better mimic its natural enzymatic production. Categorizing donors by the biological stimulus that triggers their H2S release, this review highlights the fundamental chemistry and releasing mechanisms of a range of H2S donors that have exhibited promising protective effects in models of myocardial ischemia-reperfusion (MI/R) injury and cancer chemotherapy-induced cardiotoxicity, specifically. Thus, in addition to serving as important investigative tools that further advance our knowledge and understanding of H2S chemical biology, the compounds highlighted in this review have the potential to serve as vital therapeutic agents for the treatment (or prevention) of various cardiomyopathies.

Protein persulfidation: Rewiring the hydrogen sulfide signaling in cell stress response

The past few decades have witnessed significant progress in the discovery of hydrogen sulfide (H₂S) as a ubiquitous gaseous signaling molecule in mammalian physiology, akin to nitric oxide and carbon monoxide. As the third gasotransmitter, H₂S is now known to exert a wide range of physiological and cytoprotective functions in the biological systems. However, endogenous H₂S concentrations are usually low, and its potential biologic mechanisms responsible have not yet been fully clarified. Recently, a growing body of evidence has demonstrated that protein persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH) elicited by H₂S, is a fundamental mechanism of H₂S-mediated signaling pathways. Persulfidation, as a biological switch for protein function, plays an important role in the maintenance of cell homeostasis in response to various internal and external stress stimuli and is also implicated in numerous diseases, such as cardiovascular and neurodegenerative diseases and cancer. In this review, the biological significance of protein persulfidation by H₂S in cell stress response is reviewed providing a framework for understanding the multifaceted roles of H₂S. A mechanism-guided perspective can help open novel avenues for the exploitation of therapeutics based on H₂S-induced persulfidation in the context of diseases.

Observing hydrogen sulfide in the endoplasmic reticulum of cancer cells and zebrafish by using an activity-based fluorescent probe

A crucial endogenous signaling chemical, hydrogen sulfide, is involved in many physiological actions. In this work, we created the fluorescent probe ER-Nap-NBD using a naphthalimide fluorophore as the signal reporter, a 7-nitro-1,2,3-benzoxadiazole amine as the responsive moiety, and a sulfonamide part for endoplasmic reticulum targeting. ER-Nap-NBD could be detected the H₂S levels in solution and in living systems (cells and zebrafish).

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

Photoclick and Release: Co-activation of Carbon Monoxide and a Fluorescent Self-reporter, COS or Sulfonamide with Fast Kinetics

Bioorthogonal prodrugs with both fast reaction kinetics and multiple outputs are highly desirable but are only found sporadically. Herein, we report a novel photoclick-and-release strategy for the co-activation of carbon monoxide and a self-reporter, carbonyl sulfide, or sulfonamide with fast reaction kinetics (k: 1.4-22.6 M^-1  s^-1 ). Such a photoclick-and-release strategy was successfully applied in live cells to deliver carbon monoxide and a fluorescent self-reporter, both of which exhibited pronounced antiproliferative activity against 4T1 cancer cells. It is conceivable that this photoclick-and-release strategy could find applications in other fields, in which a controlled bond cleavage is preferred.

On-demand therapeutic delivery of hydrogen sulfide aided by biomolecules

Hydrogen sulfide (H₂S), known as the third gasotransmitter, exerts various physiological functions including cardiac protection, angiogenesis, anti-inflammatory, and anti-cancer capability. Given its promising therapeutic potential as well as severe perniciousness if improper use, the sustained and tunable H₂S delivery systems are highly required for H₂S-based gas therapy with enhanced bioactivity and reduced side effects. To this end, a series of stimuli-responsive compounds capable of releasing H₂S (termed H₂S donors) have been designed over the past two decades to mimic the endogenous generation of H₂S and elucidate the biological functions. Further to improve the stability of H₂S donors and achieve the targeted delivery, various delivery systems have been constructed. In this review, we focus on the recent advances of an emerging subset, biomolecular-based H₂S delivery systems, which combine H₂S donors with biomolecular vectors including polysaccharide, peptide, and protein. We demonstrated their basic structures, building strategies, and therapeutic applications respectively to unfold their inherent merits endued by biomolecules including biocompatibility, biodegradability as well as expansibility. The varied development potentials of biomolecular-based H₂S delivery systems based on their specific properties are also discussed. At the end, brief future outlooks and upcoming challenges are presented as well.

Hydrogen sulfide-releasing polyurethane/gelatin/keratin-TA conjugate mats for wound healing

Hydrogen sulfide (H₂S) has indispensable effects on wound healing promotion, such as anti-inflammatory, cell proliferation enhancement, and angiogenesis improvement. However, small-molecular donors have drawbacks of burst release and low biocompatibility. In this study, a novel bio-macromolecular donor of H₂S based on a keratin-thiobenzoic acid conjugate (KTC) was first synthesized by a thiol-disulfide exchange reaction. Then, the KTC was incorporated into polyurethane and gelatin to fabricate nanofibrous mats by electrospinning. Interestingly, both the KTC donor and biocomposite mats were capable of releasing H₂S triggered by glutathione (GSH) under physiological conditions. These mats had excellent water absorption capacity, demonstrating their great ability to absorb tissue exudate. In addition, these mats possessed a notable antioxidant ability, thereby protecting cell membranes from oxidation. Cell assays in vitro demonstrated that these mats could promote cell proliferation and migration. Furthermore, the in vivo full-thickness defect model verified that these H₂S-releasing wound dressing mats could accelerate granulation tissue formation, enhance collagen deposition, and promote angiogenesis. Based on these advantages, these wound dressing mats had great potential applications for wound healing.

Controllable Cycloadditions between 2H-(Thio)pyran-2-(thi)ones and Strained Alkynes: A Click-and-Release Strategy for COS/H2S Generation

In this work, we carried out computational studies to predict the cycloaddition efficiency of strained alkynes with 2H-pyran-2-one and its three sulfur-containing analogues: 2H-pyran-2-thione, 2H-thiopyran-2-one, and 2H-thiopyran-2-thione. It was predicted that the decreased aromaticity of the substrate would yield higher reactivity. Experimental studies confirmed the calculation results, and 2H-pyan-2-thiones were found to be the most reactive substrates. This reaction proceeded effectively in aqueous buffers and in cellular environments. It also produced COS as the byproduct, which could be converted into hydrogen sulfide (H₂S) in the presence of carbonate anhydrase. This click-and-release approach may serve as a unique way to deliver COS/H₂S to specific locations.

Subcellular Delivery of Hydrogen Sulfide Using Small Molecule Donors Impacts Organelle Stress

Hydrogen sulfide (H₂S) is an endogenously produced gaseous signaling molecule with important roles in regulating organelle function and stress. Because of its high reactivity, targeted delivery of H₂S using small molecule H₂S donors has garnered significant interest to minimize off-target effects. Although mitochondrially targeted H₂S donors, such as AP39, have been reported previously and exhibit significantly higher potency than nontargeted donors, the expansion of targeted H₂S delivery to other subcellular organelles remains largely absent. To fill this key unmet need, we report a library of organelle-targeted H₂S donors that localize H₂S delivery to specific subcellular organelles, including the Golgi apparatus, lysosome, endoplasmic reticulum, and mitochondria. We measured H₂S production in vitro from each donor, confirmed the localization of H₂S delivery using organelle-specific H₂S responsive fluorescent probes, and demonstrated enhanced potency of these targeted H₂S donors in providing protection against organelle-specific stress. We anticipate this class of targeted H₂S donors will enable future studies of subcellular roles of H₂S and the pathways by which H₂S alleviates subcellular organelle stress.

External stimuli-responsive gasotransmitter prodrugs: Chemistry and spatiotemporal release

Gasotransmitters like nitric oxide, carbon monoxide, and hydrogen sulfide with unique pleiotropic pharmacological effects in mammals are an emerging therapeutic modality for different human diseases including cancer, infection, ischemia-reperfusion injuries, and inflammation; however, their clinical translation is hampered by the lack of a reliable delivery form, which delivers such gasotransmitters to the action site with precisely controlled dosage. The external stimuli-responsive prodrug strategy has shown tremendous potential in developing gasotransmitter prodrugs, which affords precise temporospatial control and better dose control compared with endogenous stimuli-sensitive prodrugs. The promising external stimuli employed for gasotransmitter activation range from photo, ultrasound, and bioorthogonal click chemistry to exogenous enzymes. Herein, we highlight the recent development of external stimuli-mediated decaging chemistry for the temporospatial delivery of gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide and sulfur dioxide, and discuss the pros and cons of different designs.

Thiol-Activated 1,2,4-Thiadiazolidin-3,5-diones Release Hydrogen Sulfide through a Carbonyl-Sulfide-Dependent Pathway

Recent efforts have expanded the development of small molecule donors that release the important biological signaling molecule hydrogen sulfide (H₂S). Previous work on 1,2,4-thiadiazolidin-3,5-diones (TDZNs) reported that these compounds release H₂S directly, albeit inefficiently. However, TDZNs showed promising efficacy in H₂S-mediated relaxation in ex vivo aortic ring relaxation models. Here, we show that TDZNs release carbonyl sulfide (COS) efficiently, which can be converted to H₂S by the enzyme carbonic anhydrase (CA) rather than releasing H₂S directly as previously reported.

Reactive sulfur and selenium species in the regulation of bone homeostasis

Reactive oxygen species (ROS) are important modulators of physiological signaling and play important roles in bone tissue regulation. Both reactive sulfur species (RSS) and reactive selenium species (RSeS) are involved in ROS signaling, and recent work suggests RSS and RSeS involvement in the regulation of bone homeostasis. For example, RSS can promote osteogenic differentiation and decrease osteoclast activity and differentiation, and the antioxidant activity of RSeS play crucial roles in balancing bone remodeling. Here, we outline current research progress on the application of RSS and RSeS in bone disease and regeneration. Focusing on these investigations, we highlight different methods, tools, and sources of RSS and RSeS, and we also highlight future opportunities for delivery of RSS and RSeS in biological environments relating to bone.

Reactive sulfur species and their significance in health and disease

Reactive sulfur species (RSS) have been recognized in the last two decades as very important molecules in redox regulation. They are involved in metabolic processes and, in this way, they are responsible for maintenance of health. This review summarizes current information about the essential biological RSS, including H₂S, low molecular weight persulfides, protein persulfides as well as organic and inorganic polysulfides, their synthesis, catabolism and chemical reactivity. Moreover, the role of RSS disturbances in various pathologies including vascular diseases, chronic kidney diseases, diabetes mellitus Type 2, neurological diseases, obesity, chronic obstructive pulmonary disease and in the most current problem of COVID-19 is presented. The significance of RSS in aging is also mentioned. Finally, the possibilities of using the precursors of various forms of RSS for therapeutic purposes are discussed.

Simultaneous Sensing of H2S and ATP with a Two-Photon Fluorescent Probe in Alzheimer's Disease: toward Understanding Why H2S Regulates Glutamate-Induced ATP Dysregulation

Energy deprivation and reduced levels of hydrogen sulfide (H₂S) in the brain is closely associated with Alzheimer's disease (AD). However, there is currently no fluorescent probe for precise exploration of both H₂S and adenosine triphosphate (ATP) to directly demonstrate their relationship and their dynamic pattern changes. Herein, we developed a two-photon fluorescent probe, named AD-3, to simultaneously image endogenous H₂S and ATP from two emission channels of fluorescent signals in live rat brains with AD. The probe achieved excellent selectivity and good detection linearity for H₂S in the 0-100 μM concentration range and ATP in the 2-5 mM concentration range, respectively, with a detection limit of 0.19 μM for H₂S and 0.01 mM for ATP. Fluorescence imaging in live cells reveals that such probe could successfully apply for simultaneous imaging and accurate quantification of H₂S and ATP in neuronal cells. Further using real-time quantitative polymerase chain reaction and Western blots, we confirmed that H₂S regulates ATP synthesis by acting on cytochrome C, cytochrome oxidase subunit 3 of complex IV, and protein 6 of complex I in the mitochondrial respiratory chain. Subsequently, we constructed a high-throughput screening platform based on AD-3 probe to rapidly screen the potential anti-AD drugs to control glutamate-stimulated oxidative stress associated with abnormal H₂S and ATP levels. Significantly, AD-3 probe was found capable of imaging of H₂S and ATP in APP/PS1 mice, and the concentration of H₂S and ATP in the AD mouse brain was found to be lower than that in wild-type mice.

Supramolecular Activation of S8 by Cucurbiturils in Water and Mechanism of Reduction to H2S by Thiols: Insights into Biological Sulfane Sulfur Trafficking

Reactive sulfur species (RSS) play critical roles in diverse chemical environments. Molecules containing sulfane sulfur (S⁰) have emerged as key species involved in cellular redox buffering as well as RSS generation, translocation, and action. Using cucurbit[7]uril (CB[7]) as a model hydrophobic host, we demonstrate here that S₈ can be encapsulated to form a 1:1 host guest complex, which was confirmed by solution state experiments, mass spectrometry, and X-ray crystallography. The solid state structure of CB[7]/S₈ shows that the encapsulated S₈ is available to nucleophiles through the carbonyl portals of the host. Treatment of CB[7]/S₈ with thiols results in efficient reduction of S₈ to H₂S in water at physiological pH. We establish that encapsulated S₈ is attacked by a thiol within the CB[7] host and that the resultant soluble hydropolysulfide is ejected into solution, where it reacts further with thiols to generate soluble sulfane sulfur carriers and ultimately H₂S. The formation of these intermediate is supported by observed kinetic saturation behavior, competitive inhibition experiments, and alkylative trapping experiments. We also demonstrate that CB[7]/S₈ can be used to increase sulfane sulfur levels in live cells using fluorescence microscopy. More broadly, this work suggests a general activation mechanism of S₈ by hydrophobic motifs, which may be applicable to proteins, membranes, or other bimolecular compartments that could transiently bind and solubilize S₈ to promote reaction with thiols to solubilize and shuttle S₈ back into the redox labile sulfane sulfur pool. Such a mechanism would provide an attractive manifold in which to understand the RSS translocation and trafficking.

Chemiexcitation-Triggered Prodrug Activation for Targeted Carbon Monoxide Delivery

Photolysis-based prodrug strategy can address some critical drug delivery issues, which otherwise are very challenging to tackle with traditional prodrug strategy. However, the need for external light irradiation significantly hampers its in vivo application due to the poor light accessibility of deep tissue. Herein, we propose a new strategy of chemiexcitation-triggered prodrug activation, wherein a photoresponsive prodrug is excited for drug payload release by chemiexcitation instead of photoirradiation. As such, the bond-cleavage power of photolysis can be employed to address some critical drug delivery issues while obviating the need for external light irradiation. We have established the proof of concept by the successful development of a chemiexcitation responsive carbon monoxide delivery platform, which exhibited specific CO release at the tumor site and pronounced tumor suppression effects. We anticipate that such a concept of chemiexcitation-triggered prodrug activation can be leveraged for the targeted delivery of other small molecule-based drug payloads.

Stimuli-responsive prodrug of non-steroidal anti-inflammatory drug diclofenac: self-immolative drug release with turn-on near-infrared fluorescence

Reactive oxygen species (ROS)-responsive near infrared (NIR) fluorogenic prodrug DCI-ROS is developed for the self-immolative release of diclofenac (DCF) with turn-on fluorescence. The non-toxic prodrug exhibited turn-on red fluorescence with endogenous ROS in cancer cells and inhibited COX-2 expression in the inflammation-induced macrophage cells. The prodrug strategy thus would be helpful for the controlled fluorogenic delivery of DCF for inflammatory diseases.

C-Nitrosothioformamide: A Donor Template for Dual Release of HNO and H2S

C-Nitrosothioformamide was demonstrated to be a donor template for dual release of HNO and COS triggered by a retro-Diels-Alder reaction. COS is an H2S precursor in the presence of carbonic anhydrase. This process produces HNO and H2S in a slow but steady manner. As such, the direct reaction between HNO and H2S under this situation appears to be minor. This may provide a useful tool to study the synergistic effects of HNO and H2S.

Cysteine-Activated Small-Molecule H2Se Donors Inspired by Synthetic H2S Donors

The importance of selenium (Se) in biology and health has become increasingly clear. Hydrogen selenide (H₂Se), the biologically available and active form of Se, is suggested to be an emerging nitric oxide (NO)-like signaling molecule. Nevertheless, the research on H₂Se chemical biology has technique difficulties due to the lack of well-characterized and controllable H₂Se donors under physiological conditions, as well as a robust assay for direct H₂Se quantification. Motivated by these needs, here, we demonstrate that selenocyclopropenones and selenoamides are tunable donor motifs that release H₂Se upon reaction with cysteine (Cys) at pH 7.4 and that structural modifications enable the rate of Cys-mediated H₂Se release to be tuned. We monitored the reaction pathways for the H₂Se release and confirmed H₂Se generation qualitatively using different methods. We further developed a quantitative assay for direct H₂Se trapping and quantitation in an aqueous solution, which should also be operative for investigating future H₂Se donor motifs. In addition, we demonstrate that arylselenoamide has the capability of Cys-mediated H₂Se release in cellular environments. Importantly, mechanistic investigations and density functional theory (DFT) calculations illustrate the plausible pathways of Cys-activated H₂Se release from arylselenoamides in detail, which may help understand the mechanistic issues of the H₂S release from pharmacologically important arylthioamides. We anticipate that the well-defined chemistries of Cys-activated H₂Se donor motifs will be useful for studying Se biology and for development of new H₂Se donors and bioconjugate techniques.

Hydrogen Sulfide: An Emerging Precision Strategy for Gas Therapy

Advances in nanotechnology have enabled the rapid development of stimuli-responsive therapeutic nanomaterials for precision gas therapy. Hydrogen sulfide (H₂ S) is a significant gaseous signaling molecule with intrinsic biochemical properties, which exerts its various physiological effects under both normal and pathological conditions. Various nanomaterials with H₂ S-responsive properties, as new-generation therapeutic agents, are explored to guide therapeutic behaviors in biological milieu. The cross disciplinary of H₂ S is an emerging scientific hotspot that studies the chemical properties, biological mechanisms, and therapeutic effects of H₂ S. This review summarizes the state-of-art research on H₂ S-related nanomedicines. In particular, recent advances in H₂ S therapeutics for cancer, such as H₂ S-mediated gas therapy and H₂ S-related synergistic therapies (combined with chemotherapy, photodynamic therapy, photothermal therapy, and chemodynamic therapy) are highlighted. Versatile imaging techniques for real-time monitoring H₂ S during biological diagnosis are reviewed. Finally, the biosafety issues, current challenges, and potential possibilities in the evolution of H₂ S-based therapy that facilitate clinical translation to patients are discussed.

Cell-Trappable BODIPY-NBD Dyad for Imaging of Basal and Stress-Induced H2S in Live Biosystems

H₂S is a gaseous signaling molecule that is involved in many physiological and pathological processes. In general, the level of intracellular H₂S (<1 μM) is much lower than that of GSH (∼1-10 mM), leading to the remaining challenge of selective detection and differentiation of endogenous H₂S in live biosystems. To this end, we quantitatively demonstrate that the thiolysis of NBD amine has much higher selectivity for H₂S over GSH than that of the reduction of aryl azide. Subsequently, we developed the first NBD-based cell-trappable probe 1 (AM-BODIPY-NBD) for highly selective and ultrasensitive imaging of intracellular H₂S. Probe 1 demonstrates a 207-fold fluorescence enhancement at 520 nm after reaction with H₂S/esterase to produce a bright BODIPY (quantum yield 0.42) and a detection limit of 15.7 nM. Probe 1 is water-soluble, cell-trappable, and not cytotoxic. Based on this excellent chemical tool, relative levels of basal H₂S in different cell lines were measured to reveal a positive correlation between endogenous H₂S and the metastatic potential of colon and breast cancer cells. In addition, H₂S biogenesis in vivo was also validated by probe 1 both in tobacco leaves under viral infection and in zebrafish after tail amputation. It is anticipated that probe 1 will have widespread applications in imaging and for investigating different H₂S-related biological processes and diseases.

A fluorogenic H2S donor activated by reactive oxygen species for real-time monitoring in cells and in vivo

Hydrogen sulfide (H₂S) is an important gasotransmitter in biological system, and plays a crucial role in varied physiological and pathological processes. Exogenous H₂S is widely employed as a positive control in H₂S related biological study. Herein, we develop a reactive oxygen species (ROS) triggered donor HSD545 that delivers H₂S and simultaneously generates a fluorophore to real-time monitoring the process of H₂S release in vitro and in vivo. The donor exhibits low cytotoxicity and strong cytoprotection against ROS-induce oxidative stress.

Near-infrared fluorescent probe for hydrogen sulfide: high-fidelity ferroptosis evaluation in vivo during stroke

Ferroptosis is closely associated with cancer, neurodegenerative diseases and ischemia-reperfusion injury and the detection of its pathological process is very important for early disease diagnosis. Fluorescence based sensing technologies have become excellent tools due to the real-time detection of cellular physiological or pathological processes. However, to date the detection of ferroptosis using reducing substances as markers has not been achieved since the reducing substances are not only present at extremely low concentrations during ferroptosis but also play a key role in the further development of ferroptosis. Significantly, sensors for reducing substances usually consume reducing substances, instigating a redox imbalance, which further aggravates the progression of ferroptosis. In this work, a H₂S triggered and H₂S releasing near-infrared fluorescent probe (HL-H₂S) was developed for the high-fidelity in situ imaging of ferroptosis. In the imaging process, HL-H₂S consumes H₂S and releases carbonyl sulfide, which is then catalyzed by carbonic anhydrase to produce H₂S. Importantly, this strategy does not intensify ferroptosis since it avoids disruption of the redox homeostasis. Furthermore, using erastin as an inducer for ferroptosis, the observed trends for Fe²⁺, MDA, and GSH, indicate that the introduction of the HL-H₂S probe does not exacerbate ferroptosis. In contrast, ferroptosis progression was significantly promoted when the release of H₂S from HL-H₂S was inhibited using AZ. These results indicate that the H₂S triggered and H₂S releasing fluorescent probe did not interfere with the progression of ferroptosis, thus enabling high-fidelity in situ imaging of ferroptosis.

A H₂S triggered and H₂S releasing near-infrared fluorescent probe (HL-H₂S) was developed. HL-H₂S does not interfere with the progression of ferroptosis by consuming H₂S, thus enabling high-fidelity in situ imaging of ferroptosis.

Rational design of a dual-reactive probe for imaging the biogenesis of both H2S and GSH from L-Cys rather than D-Cys in live cells

Biothiols and their interconversion are involved in cellular redox homestasis as well as many physiological processes. Here, a dual-reactive dual-quenching fluorescent probe was rationally developed based on thiolysis reactions of...

Thiobenzophenones: tunable hydrolysis-based donors for intracellular H2S delivery

H2S, the third gasotransmitter, is involved in many physiological and pathological processes. Compounds that can release H2S slowly under physiological conditions are useful chemical tools for studying H2S biology as...

Progress and Perspective of Solid-State Organic Fluorophores for Biomedical Applications

Fluorescent organic dyes have been extensively used as raw materials for the development of versatile imaging tools in the field of biomedicine. Particularly, the development of solid-state organic fluorophores (SSOFs) in the past 20 years has exhibited an upward trend. In recent years, studies on SSOFs have focused on the development of advanced tools, such as optical contrast agents and phototherapy agents, for biomedical applications. However, the practical application of these tools has been hindered owing to several limitations. Thus, in this Perspective, we have provided insights that could aid researchers to further develop these tools and overcome the limitations such as limited aqueous dispersibility, low biocompatibility, and uncontrolled emission. First, we described the inherent photophysical properties and fluorescence mechanisms of conventional, aggregation-induced emissive, and precipitating SSOFs with respect to their biomedical applications. Subsequently, we highlighted the recent development of functionalized SSOFs for bioimaging, biosensing, and theranostics. Finally, we elucidated the potential prospects and limitations of current SSOF-based tools associated with biomedical applications.

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.

A dual lock-and-key two photon fluorescence probe in response to hydrogen peroxide and viscosity: Application in cellular imaging and inflammation therapy

Here, a dual lock-and-key fluorescence probe was developed for visualizing the inflammatory process in myocardial H9C2 cells. The probe possessed two-photon properties, viscosity sensitivity, and hydrogen peroxide (H₂O₂) responsiveness. A thiocarbamate spacer between fluorophore and H₂O₂ responsive unit enabled the release of carbonyl sulfide (COS). This rapidly converts to the anti-inflammatory hydrogen sulfide (H₂S) by the ubiquitous enzyme carbon anhydrase. The probe displayed a dual response towards hydrogen peroxide and viscosity in vitro. No obvious fluorescence changes were observed towards either hydrogen peroxide or viscosity alone. In cellular experiments, the probe demonstrated good biocompatibility, low toxicity, and was shown responses towards exogenous and endogenous hydrogen peroxide under viscosity conditions. LPS induced cell inflammation showed it was able to effectively alleviate the inflammation-caused damage by releasing H₂S and eliminating H₂O₂. The new protocol demonstrates its promising to achieve diagnosis and treatment of cellular inflammatory process.

Design and Development of a Bioorthogonal, Visualizable and Mitochondria-Targeted Hydrogen Sulfide (H2 S) Delivery System

Hydrogen sulfide (H₂ S) is an important endogenous gasotransmitter, but the targeted delivery and real-time feedback of exogenous H₂ S are still challenging. With the aid of density functional theory (DFT) calculations, we designed a new 1,3-dithiolium-4-olate (DTO) compound, which can react with a strained alkyne via the 1,3-dipolar cycloaddition and the retro-Diels-Alder reaction to generate carbonyl sulfide (COS) as the precursor of H₂ S, and a thiophene derivative with turn-on fluorescence. Moreover, the diphenylamino substituent in DTO greatly increases the mitochondrial targeting of this H₂ S delivery system. Such a bioorthogonal click-and-release reaction has integrated three functions in one system for the first time: (1) in situ controllable H₂ S release, (2) concomitant fluorescence response, and (3) mitochondria-targeted delivery. In addition, we investigated the mitochondrial membrane potential loss alleviation by using this system in H9c2 cells under oxidative stress.