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

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.

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.

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.

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.

Molecular engineering of organic-based agents for in situ bioimaging and phototherapeutics

In situ monitoring of the location and transportation of bioactive molecules is essential for deciphering diverse biological events in the field of biomedicine. In addition, obtaining the in situ information of lesions will provide a clear perspective for surgeons to perform precise resection in clinical surgery. Notably, delivering drugs or operating photodynamic therapy/photothermal therapy in situ by labeling the lesion regions of interest can improve treatment and reduce side effects in vivo. In various advanced imaging and therapy modalities, optical theranostic agents based on organic small molecules can be conveniently modified as needed and can be easily internalized into cells/lesions in a non-invasive manner, which are prerequisites for in situ bioimaging and precision treatment. In this tutorial review, we first summarize the in situ molecular immobilization strategies to retain small-molecule agents inside cells/lesions to prevent their diffusion in living organisms. Emphasis will be focused on introducing the application of these strategies for in situ imaging of biomolecules and precision treatment, particularly pertaining to why targeting therapy in situ is required.

The Path to Controlled Delivery of Reactive Sulfur Species

Reactive sulfur species (RSS) play regulatory roles in many physiological and pathological processes. Since the discovery of hydrogen sulfide (H₂S) as a nitric oxide (NO)-like signaling molecule, understanding the chemical biology of H₂S and H₂S-related RSS, such as hydropersulfides (RSSH) and polysulfides (H₂S_n), has become a fast-growing research field. However, the research on these RSS has technical difficulties due to their high reactivity and instability. To solve this problem, considerable efforts have been put into the development of unique RSS releasing compounds (e.g., donors) or in situ RSS generation systems. This Account tells the story of our research group's effort to develop novel RSS donors.We began with exploring molecular entities that were stable by themselves but could be triggered by biologically relevant factors, such as pH, thiols, light, or enzymes, to release H₂S in a controllable fashion. These studies led to the discovery of a series of novel H₂S donors. We later expanded our interests to other RSS including RSSH, H₂S_n, RSeSH, HSNO, RSOH, etc. The fundamental chemistry of these RSS was studied and applied to the development of the corresponding donors. In addition to small molecule donors, we also worked on H₂S-releasing biomaterials and their applications. This Account summarizes our work and systematically explains how each RSS donor template was proposed and evaluated. The Account covers the following key points: (1) rational chemistry design of each RSS donor template, (2) evaluation and mechanistic insights of each donor template, and (3) properties and biological applications of the donors.

On the origins of the mechanistic variants in the thermal reactions of Sx+ (x = 1-3) with benzene

The S-π interaction between sulfur atom(s) and aromatic ring prevails in chemical and biochemical processes. The thermal gas-phase reactions of the S_n⁺ (n = 1-3) ions with benzene have been explored by using Quadrupole-Ion Trap (Q-IT) mass spectrometry complemented by quantum chemical calculations. Charge transfer was found to be the only reaction channel for S₂⁺/C₆H₆, while both charge transfer and bond activation are available for the S⁺/C₆H₆ and S₃⁺/C₆H₆ couples. Upon interrogating the associated electronic origins, multiple factors were found to matter for these processes. In contrast to the σ-type two-center three-electron (2c-3e) S-π hemibond as reported previously, unusual S-π hemibonds were addressed for the S_n⁺/C₆H₆ couples, i.e. the 2c-3e π(S061Eπ) and the three-center three-electron (3c-3e) σ(S₂061Eπ) hemibonds. Such S-π interaction was found to be responsible for the charge transfer processes in S⁺/C₆H₆ and S₂⁺/C₆H₆, but uninvolved in any transformation for S₃⁺/C₆H₆.

Amino acid-based H2S donors: N-thiocarboxyanhydrides that release H2S with innocuous byproducts

A library of N-thiocarboxyanhydrides (NTAs) derived from natural amino acids with benign byproducts and controlled H2S-release kinetics is reported. Minimal acute in vitro toxicity was observed in multiple cell lines, while longer-term toxicity in cancer cells was observed, with slow-releasing donors exhibiting the greatest cytotoxic effects.

Moving Past Quinone-Methides: Recent Advances toward Minimizing Electrophilic Byproducts from COS/H2S Donors

Hydrogen sulfide (H2S) is an important biomolecule that plays key signaling and protective roles in different physiological processes. With the goals of advancing both the available research tools and the associated therapeutic potential of H2S, researchers have developed different methods to deliver H2S on-demand in different biological contexts. A recent approach to develop such donors has been to design compounds that release carbonyl sulfide (COS), which is quickly converted to H2S in biological systems by the ubiquitous enzyme carbonic anhydrase (CA). Although highly diversifiable, many approaches using this general platform release quinone methides or related electrophiles after donor activation. Many such electrophiles are likely scavenged by water, but recent efforts have also expanded alternative approaches that minimize the formation of electrophilic byproducts generated after COS release. This mini-review focuses specifically on recent examples of COS-based H2S donors that do not generate quinone methide byproducts after donor activation.

A H2 S-Specific Ultrasensitive Fluorogenic Probe Reveals TMV-Induced H2 S Production to Limit Virus Replication

Understanding the role of H₂ S in host defense mechanisms against RNA viruses may provide opportunities for the development of antivirals to combat viral infections. Here, we have developed a green-emitting fluorogenic probe, which exhibits a large fluorescence response at 520 nm (>560-fold) when treated with 100 μM H₂ S for 1 h. It is highly selective for H₂ S over biothiols (>400-fold F/F₀ ) and has a detection limit of 12.9 nM. We demonstrate the application of the probe for endogenous H₂ S detection in vivo for the understanding of its roles in antiviral host defense. Such virus-induced H₂ S inhibits viral replication by reducing gene expression of RNA-dependent RNA polymerase (RdRp) and coat protein (CP). Additionally, a H₂ S donor GYY4137 showed significantly antiviral activity as ribavirin, a broad-spectrum drug against RNA viruses. Furtherly, we propose a possible molecular mechanism for the TMV-induced H₂ S biogenesis. This work provides a proof-of-principle in support of further studies identifying endogenous H₂ S and its donors as potential antivirals toward RNA viruses.

N-Methylation of Self-Immolative Thiocarbamates Provides Insights into the Mechanism of Carbonyl Sulfide Release

Hydrogen sulfide (H₂S) is an important biomolecule, and self-immolative thiocarbamates have shown great promise as triggerable H₂S donors with suitable analogous control compounds; however, thiocarbamates with electron-deficient payloads are less efficient H₂S donors. We report here the synthesis and study of a series of N-methylated esterase-triggered thiocarbamates that block the postulated unproductive deprotonation-based pathway for these compounds. The relative reaction profiles for H₂S release across a series of electron-rich and electron-poor N-Me aniline payloads are examined experimentally and computationally. We show that thiocarbamate N-methylation does block some side reactivity and increases the H₂S release profiles for electron-poor donors. Additionally, we show that isothiocyanate release is not a competitive pathway, and rather that the reduced efficiency of electron-poor donors is likely due to other side reactions.

Copper-Catalyzed Cycloaddition of Heterobicyclic Alkenes with Diaryl Disulfides to Synthesize Dihydrobenzo[b]thiophene Derivatives

A novel copper-catalyzed cycloaddition of diaryl disulfides to heterobicyclic alkenes has been developed. The C-S and C-C bonds can be formed simultaneously on the C═C bond of the olefins via a single-step cycloaddition to afford a series of 2,3-dihydrobenzo[b]thiophene derivatives. This reaction exhibits excellent diastereoselectivity and relatively broad substrate scope. Various functional groups attached to the substrates are tolerated in this protocol to give the corresponding exo adducts in moderate yields.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Novel turn-on fluorescence sensor for detection and imaging of endogenous H2S induced by sodium nitroprusside

Currently, fluorescence analysis method has a good application in the detection and imaging of biomarkers and has become an important analytical method. Although there are many fluorescent probes for detecting hydrogen sulfide(H₂S), they are mostly based on fluorophores which already existed, such as 1,8-naphthalimide, coumarin, rhodamine and their derivatives. Here, a new type of fluorescent molecule (BOTD) was synthesized and applied to the detection of H₂S. The probe BOTD could quickly and sensitively detect H₂S and turn on fluorescence. Moreover, the probe BOTD was successfully applied to the detection of exogenous and endogenous H₂S in living cells, and may be expected to become a research tool for studying H₂S-induced drugs.

H2S and reactive sulfur signaling at the host-bacterial pathogen interface

Bacterial pathogens that cause invasive disease in the vertebrate host must adapt to host efforts to cripple their viability. Major host insults are reactive oxygen and reactive nitrogen species as well as cellular stress induced by antibiotics. Hydrogen sulfide (H2S) is emerging as an important player in cytoprotection against these stressors, which may well be attributed to downstream more oxidized sulfur species termed reactive sulfur species (RSS). In this review, we summarize recent work that suggests that H2S/RSS impacts bacterial survival in infected cells and animals. We discuss the mechanisms of biogenesis and clearance of RSS in the context of a bacterial H2S/RSS homeostasis model and the bacterial transcriptional regulatory proteins that act as "sensors" of cellular RSS that maintain H2S/RSS homeostasis. In addition, we cover fluorescence imaging- and MS-based approaches used to detect and quantify RSS in bacterial cells. Last, we discuss proteome persulfidation (S-sulfuration) as a potential mediator of H2S/RSS signaling in bacteria in the context of the writer-reader-eraser paradigm, and progress toward ascribing regulatory significance to this widespread post-translational modification.

Progress toward colorimetric and fluorescent detection of carbonyl sulfide

We report here that a fluorescent benzobisimidazolium salt (TBBI) can be used for the fluorescent and colorimetric detection of carbonyl sulfide (COS) over related heterocumulenes including CO2 and CS2 in wet MeCN. The reaction between TBBI and COS in the presence of fluoride yields a highly fluorescent (λem = 354 nm) and colored product (λmax = 321, 621 nm), that is readily observed by the naked eye. We view these results as a first step toward developing activity-based probes for COS detection.

Highly efficient H2S scavengers via thiolysis of positively-charged NBD amines

H2S is a well-known toxic gas and also a gaseous signaling molecule involved in many biological processes. Advanced chemical tools that can regulate H2S levels in vivo are useful for understanding H2S biology as well as its potential therapeutic effects. To this end, we have developed a series of 7-nitro-1,2,3-benzoxadiazole (NBD) amines as potential H2S scavengers. The kinetic studies of thiolysis reactions revealed that incorporation of positively-charged groups onto the NBD amines greatly increased the rate of the H2S-specific thiolysis reaction. We demonstrate that these reactions proceed effectively, with second order rate constants (k 2) of >116 M-1 s-1 at 37 °C for NBD-S8. Additionally, we demonstrate that NBD-S8 can effectively scavenge enzymatically-produced and endogenous H2S in live cells. Furthering the biological significance, we demonstrate NBD-S8 mediates scavenging of H2S in mice.

Insights into the Mechanism of Thiol-Triggered COS/H2S Release from N-Dithiasuccinoyl Amines

The hydrolysis of carbonyl sulfide (COS) to form H₂S by carbonic anhydrase has been demonstrated to be a viable strategy to deliver H₂S in a biological system. Herein, we describe N-dithiasuccinoyl amines as thiol-triggered COS/H₂S donors. Notably, thiol species especially GSH and homocysteine can trigger the release of both COS and H₂S directly from several specific analogues via an unexpected mechanism. Importantly, two representative analogues Dts-1 and Dts-5 show intracellular H₂S release, and Dts-1 imparts potent anti-inflammatory effects in LPS-challenged microglia cells. In conclusion, N-dithiasuccinoyl amine could serve as promising COS/H₂S donors for either H₂S biological studies or H₂S-based therapeutics development.

H2S-Donating trisulfide linkers confer unexpected biological behaviour to poly(ethylene glycol)-cholesteryl conjugates

Inspired by the properties of the naturally occurring H₂S donor, diallyl trisulfide (DATS, extracted from garlic), the biological behaviour of trisulfide-bearing PEG-conjugates was explored. Specifically, three conjugates comprising an mPEG tail and a cholesteryl head were investigated: conjugates bridged by a trisulfide linker (T), a disulfide linker (D) or a carbamate linker (C), and a fourth comprising two mPEG tails bridged by a trisulfide linker (P). H₂S testing using both a fluorescent chemical probe in HEK293 cells and an amperometric sensor to monitor release in suspended cells, demonstrated the ability of the trisulfide conjugates, T and P, to release H₂S in the presence of cellular thiols. Cytotoxicity and cyto-protective capacity on HEK293 cells showed that T was the best tolerated of the conjugates studied, and remarkably more so than D or C. Moreover, it was noted that application of T conferred a protective effect to the cells, effectively abolishing the toxicity associated with co-administered C. The interaction of conjugates and combinations thereof with the cell membrane of HEK cells, as well as ROS generation were also investigated. It was found that C caused significant membrane perturbation, correlating with high losses in cell viability and pronounced generation of ROS, especially in the mitochondria. T, however, did not disturb the membrane and was able to mitigate the generation of ROS, especially in the mitochondria. The interplay of the cholesteryl group and H₂S donation for conferring cytoprotective effects was clearly demonstrated as P did not display the same beneficial characteristics as T.

Use of Dithiasuccinoyl-Caged Amines Enables COS/H2 S Release Lacking Electrophilic Byproducts

The enzymatic conversion of carbonyl sulfide (COS) to hydrogen sulfide (H₂ S) by carbonic anhydrase has been used to develop self-immolating thiocarbamates as COS-based H₂ S donors to further elucidate the impact of reactive sulfur species in biology. The high modularity of this approach has provided a library of COS-based H₂ S donors that can be activated by specific stimuli. A common limitation, however, is that many such donors result in the formation of an electrophilic quinone methide byproduct during donor activation. As a mild alternative, we demonstrate here that dithiasuccinoyl groups can function as COS/H₂ S donor motifs, and that these groups release two equivalents of COS/H₂ S and uncage an amine payload under physiologically relevant conditions. Additionally, we demonstrate that COS/H₂ S release from this donor motif can be altered by electronic modulation and alkyl substitution. These insights are further supported by DFT investigations, which reveal that aryl and alkyl thiocarbamates release COS with significantly different activation energies.

A Fast-Response Red Shifted Fluorescent Probe for Detection of H2S in Living Cells

Near-infrared (NIR) fluorescent probes are attractive tools for bioimaging applications because of their low auto-fluorescence interference, minimal damage to living samples, and deep tissue penetration. H2S is a gaseous signaling molecule that is involved in redox homeostasis and numerous biological processes in vivo. To this end, we have developed a new red shifted fluorescent probe 1 to detect physiological H2S in live cells. The probe 1 is based on a rhodamine derivative as the red shifted fluorophore and the thiolysis of 7-nitro 1,2,3-benzoxadiazole (NBD) amine as the H2S receptor. The probe 1 displays fast fluorescent enhancement at 660 nm (about 10-fold turn-ons, k2 = 29.8 M-1s-1) after reacting with H2S in buffer (pH 7.4), and the fluorescence quantum yield of the activated red shifted product can reach 0.29. The probe 1 also exhibits high selectivity and sensitivity towards H2S. Moreover, 1 is cell-membrane-permeable and mitochondria-targeting, and can be used for imaging of endogenous H2S in living cells. We believe that this red shifted fluorescent probe can be a useful tool for studies of H2S biology.

Visualization of the cysteine level during Golgi stress using a novel Golgi-targeting highly specific fluorescent probe

A novel Golgi-targeting highly specific fluorescent probe was developed to visualize the level of cysteine during Golgi stress.