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.

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.

Strategies to convert organic fluorophores into red/near-infrared emitting analogues and their utilization in bioimaging probes

Organic fluorophores aided by current microscopy imaging modalities are essential for studying biological systems. Recently, red/near-infrared emitting fluorophores have attracted great research efforts, as they enable bioimaging applications with reduced autofluorescence interference and light scattering, two significant obstacles for deep-tissue imaging, as well as reduced photodamage and photobleaching. Herein, we analyzed the current strategies to convert key organic fluorophores bearing xanthene, coumarin, and naphthalene cores into longer wavelength-emitting derivatives by focussing on their effectiveness and limitations. Together, we introduced typical examples of how such fluorophores can be used to develop molecular probes for biological analytes, along with key sensing features. Finally, we listed several critical issues to be considered in developing new fluorophores.

Facile synthesis of sulfinate esters from aryl iodides via direct oxidation of thioesters

A practical method to synthesize sulfinate esters from aryl iodides is disclosed. Direct oxidation of thioesters prepared by copper-catalyzed C-S formation of aryl iodides realized the efficient synthesis of sulfinate esters. Due to the good accessibility of aryl iodides, a wide variety of sulfinate esters were prepared from easily available starting materials such as carboxylic acids and anilines.

An Update Review of Approaches to Multiple Action-Based Antibacterials

Many approaches are being pursued to address the major global health challenge posed by the increasing resistance of pathogenic bacteria to antibacterial agents. One of the promising approaches being investigated includes the design and development of multiple action-based small-molecule antibacterials. Aspects of this broad area have been reviewed previously, and recent developments are addressed in this update review covering the literature mainly over the past three years. Considerations encompassing drug combinations, single-molecule hybrids and prodrugs are summarised in regard to the intentional design and development of multiple-action agents with a focus on potential triple or greater activities in bacteria. The hope for such single agents or combinations of single agents is that resistance development will be significantly hindered, and they may be useful in tackling bacterial disease caused by both resistant and non-resistant bacteria.

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.

A Review of Chemical Tools for Studying Small Molecule Persulfides: Detection and Delivery

Hydrogen sulfide (H₂S) has gained significant attention as a potent bioregulator in the redox metabolome, but it is just one of many reactive sulfur species (RSS). Recently, small molecule persulfides (structure RSSH) have emerged as RSS of particular interest due to their enhanced antioxidant abilities compared to H₂S and their ability to directly convert protein thiols into protein persulfides, suggesting that persulfides may have distinct physiological functions from H₂S. However, persulfides exhibit instability and cross-reactivity that hampers the elucidation of their precise biological roles. As such, chemists have designed chemical tools and techniques to facilitate the study of persulfides under various conditions. These molecules and methods include persulfide trapping reagents and sensors, as well as compounds that degrade in response to various triggers to release persulfides, termed persulfide donors. There now exist a variety of persulfide donor classes, some of which possess tissue-targeting capabilities designed to mimic localized endogenous production of RSS. This Review briefly covers the physicochemical properties of persulfides, the endogenous production of small molecule persulfides, and their reactions with protein thiols and other reactive species. These introductory sections are followed by a discussion of chemical tools used in persulfide chemical biology, with critical analysis of recent advancements in the field and commentary on potential directions for future research.

Recent Advances on Stimuli-Responsive Combination Therapy against Multidrug-Resistant Bacteria and Biofilm

The widespread occurrence of infections from multidrug-resistant (MDR) bacteria is a global health problem. It has been amplified over the past few years due to the increase in adaptive traits in bacteria and lack of advanced treatment strategies. Because of the low bioavailability and limited penetration at infected sites, the existing antibiotics often fail to resist bacterial growth. Recently, developed stimuli-responsive drug delivery systems and combinatorial therapeutic systems based on nanoparticles, metal-organic frameworks, hydrogels, and organic chromophores offer the ability to improve the therapeutic efficacy of antibiotics by reducing drug resistance and other side effects. These therapeutic systems have been designed with the relevant chemical and physical properties that respond to specific triggers resulting in spatiotemporal controlled release and site-specific transportability. This review highlights the latest development of single and dual/multistimuli-responsive antibiotic delivery systems for combination therapies to treat MDR bacterial infections and biofilm eradication.

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.

Real-time monitoring of a photoactivated hydrogen persulfide donor for biological entities

Hydrogen persulfide (H2S2) plays an important role in sulfur-based redox signaling mechanisms. Herein, we developed a visible light activated ESIPT based H2S2 donor using a p-hydroxyphenacyl phototrigger. The unique feature of the designed H2S2 donor system is the ability to monitor the H2S2 release in real time through a non-invasive fluorescence color change approach, with the color changing from green to blue. Next, we demonstrated the detection and quantification of H2S2 using a fluorescein based "turn-on" fluorescent probe. Furthermore, in vitro studies of the designed H2S2 donor demonstrated the real-time monitored H2S2 release and cytoprotective ability in the highly oxidizing cellular environment of MDA-MB-468 cells.

Photolysis of dimethoxynitrobenzyl-"caged" acids yields fluorescent products

Carboxylic acids conjugated with 4,5-dimethoxy-2-nitrobenzyl photoremovable protecting group are well known and widely used for biological studies. In this paper, we study the photolysis of likewise “caged” acetic, caprylic and arachidonic acids. Unexpectedly, we observed huge growth of fluorescence emission at ~430 nm during photolysis. Following further UV irradiation, a product with fluorescence at longer wavelength was formed (470 nm excitation / ~500–600 nm emission). While it may be used to monitor the “uncaging”, these fluorescent products may interfere with widespread dyes such as fluorescein in biomedical experiments. This effect might be negligible if the photolysis products dissolve in the medium. On the other hand, we observed that arachidonic and caprylic acids derivatives self-organize in emulsion droplets in water environment due to long lipophilic chains. Illumination of droplets by UV rapidly induces orange fluorescence excited by 488 nm light. This fluorescence turn-on was fast (~0.1 s) and apparently caused by the accumulation of water-insoluble fluorescent residuals inside droplets. These self-organized lipophilic structures with fluorescence turn-on capability may be of interest for biomedical and other application. We have identified and hypothesized some compounds which may be responsible for the observed fluorescense.