Mitochondria-targeting fluorescent molecules for high efficiency cancer growth inhibition and imaging

Developing dual-responsive quinolinium prodrugs of 8-hydroxyquinoline by harnessing the dual chelating sites

The bidentate metal ion chelator 8-hydroxyquinoline (8-HQ) demonstrates significant potential in anticancer therapy but is hindered by adverse effects due to nonspecific chelation in normal tissues. The phenolic hydroxyl oxygen of 8-HQ has been extensively exploited to develop O-masked 8-HQ prodrugs aimed at achieving on-demand chelation. However, the equally crucial quinoline nitrogen for chelation remains underutilized. By alkylating the quinoline nitrogen of 8-HQ, we synthesized a series of N-masked quinolinium (QUM) prodrugs that release 8-HQ upon activation by various stimuli. Comprehensive in vitro and in vivo studies were conducted with QUM-1 and QUM-4, which are activated by H2O2 and β-glucosidase, respectively. Both QUM-1 and QUM-4 exhibit improved cancer cell selectivity compared to 8-HQ or the O-masked isomeric prodrug, attributed to unique properties such as enhanced mitochondrial targeting and increased glucose transporter-mediated cellular uptake. Additionally, by leveraging both chelating sites, we constructed dual-masked 8-HQ prodrugs that are activated non-sequentially by two stimuli to release 8-HQ. QUM-5 demonstrates anticancer activity upon activation by UV/H2O2 and shows improved safety in mice compared to 8-HQ. Our research presents novel applications for the construction of quaternary ammonium prodrugs utilizing aromatic tertiary amines and underscores the potential of dual-responsive prochelators for targeted cancer therapy.

Superior Tumor Cell Uptake by Mono- and Tri-Nuclear Rhodamine-Gadolinium(III) Agents

The synthesis and characterization of a novel trinuclear rhodamine-Gd(III) complex, along with two analogous mononuclear rhodamine-Gd(III) complexes, are reported. All complexes displayed good selectivity in a human glioma cell line (T98G) when compared to a glial cell line (SVG p12), with low cytotoxicities. Superior tumor cell uptake for these Gd(III) complexes was observed at lower incubation concentrations compared to previously-reported delocalized lipophilic cations such as a rhodamine-lanthanoid(III) probe and Gd(III)-arylphosphonium complexes, with ca. 150 % and 250 % increases in Gd uptake, respectively.

Facile construction of dual-response super-resolution probes for tracking organelles dynamics

Super-resolution imaging techniques, such as structured illumination microscopy (SIM), have enabled researchers to obtain nanoscale organelle-level outputs in living systems, but they impose additional stringent requirements on fluorescence probes. However, high-performance, custom-designed SIM probes that can explain underlying biological processes remain unavailable. Herein, a customizable engineering toolkit is developed for the facile assembly of SIM probes suitable for subcellular component detection. This toolkit is used to customize a fluorescent molecule, CPC (coumarin-phenylhydrazine-carboxyl), capable of simultaneously monitoring peroxynitrite (ONOO-) and polarity distribution in mitochondria and lipid droplets (LDs), respectively, through functional ON-OFF mechanisms. The customized CPC molecule demonstrated excellent imaging capabilities under SIM, enabled the successful localization of multiple organelles, and reliably tracked the distribution of different components, thus facilitating the study of the interplay between organelles. Using CPC, the physical transition of intracellular LDs is demonstrated from heterogeneity to homogeneity. This was specifically observed during ferroptosis where the polarity of the LDs increased and their morphology became more contracted. Furthermore, the loss of LDs functionality could not counteract the accumulation of ONOO- within the mitochondria, leading to the decoupling of mitochondrial LDs during ferroptosis. These results confirmed the potential mechanism of LDs dysfunction and decoupling triggered via cumulative mitochondrial oxidative stress during ferroptosis. To summarize, this toolkit will be a powerful tool for examining subtle variations among components during the interplay between different organelles, thus offering novel avenues for understanding and treating related diseases.

Positive-Charge-Based Small Molecule Dyes for Gut Microbiota Fluorescent Imaging

As one of the research hotspots in recent years, gut microbiota have been proven to be closely related to host metabolism, nutrient absorption, and immune regulation. However, there are still many urgent issues in the research of gut microbiota, such as the localization and tracking of gut microbiota. In this research, two new fluorescent probes, EF and 6F, were developed by optimizing the structure of the positron salt small molecule probe F16. In vitro labeling experiments showed that EF and 6F can quickly label Gram-positive bacteria, Staphylococcus aureus and Lactobacillus reuteri, as well as Gram-negative bacteria, Escherichia coli and Salmonella pullorum. Meanwhile, EF and 6F have little bacterial toxicity and are used at a maximum concentration of 200 μM. Compared with EF, 6F has better hydrophilicity and stronger fluorescence characteristics in aqueous solutions, making it more suitable for imaging within gut microbiota populations. The results of in vivo imaging experiments indicate that EF and 6F can label and image the intestinal microbiota colonized by the mouse intestinal mucosal epithelium without causing any damage to intestinal tissue. Compared with commercially available MitoTracker dyes and fluorescein 5-isothiocyanate (FITC) dyes, EF and 6F exhibit better biocompatibility. Therefore, the compounds EF and 6F synthesized in this study are novel small molecule probes suitable for imaging gut microbiota, providing a better probe selection for exploring complex gut microbiota.

Detouring Self-Assembled 3-Methoxy-pyrrole-Based Nanoparticles into Mitochondria to Induce Apoptosis in Lung Cancer Cells

Lung cancer remains a lethal disease globally. Recently, the development and progression of lung cancer were strongly linked with mitochondrial dysfunction. Hence, targeting mitochondria in lung cancer can be an interesting alternative strategy for therapeutic applications. To address this, we have designed and synthesized a 3-methoxy-pyrrole-enamine-triphenylphosphonium cation-based library through a concise chemical strategy. Upon screening this library in cervical (HeLa), colon (HCT-116), breast (MCF7), and lung (A549) cancer cells, we identified a small molecule that self-assembled into nanoscale spherical particles with a positive surface charge. This nanoparticle was confined to the mitochondria to induce mitochondrial damage and produced reactive superoxide in A549 cells. This small molecule self-assembled nanoparticle-mediated mitochondrial damage triggered apoptosis leading to the remarkable killing of A549 cells. These 3-methoxy-pyrrole-enamine-triphenylphosphonium nanoparticles can be used as a tool to understand the chemical biology of mitochondria in lung cancer for chemotherapeutic applications.

Recent advances in mitochondria-targeting theranostic agents

For its vital role in maintaining cellular activity and survival, mitochondrion is highly involved in various diseases, and several strategies to target mitochondria have been developed for specific imaging and treatment. Among these approaches, theranostic may realize both diagnosis and therapy with one integrated material, benefiting the simplification of treatment process and candidate drug evaluation. A variety of mitochondria-targeting theranostic agents have been designed based on the differential structure and composition of mitochondria, which enable more precise localization within cellular mitochondria at disease sites, facilitating the unveiling of pathological information while concurrently performing therapeutic interventions. Here, progress of mitochondria-targeting theranostic materials reported in recent years along with background information on mitochondria-targeting and therapy have been briefly summarized, determining to deliver updated status and design ideas in this field to readers.

Insights into tumor size-dependent nanoparticle accumulation using deformed organosilica nanoprobes

Deformed organosilica nanoprobes (CDPF) exhibit enhanced accumulation within larger tumors, highlighting the pivotal role of the tumor microenvironment in the optimization of nanoparticle-based therapeutic strategies.

Molecular engineering and bioimaging applications of C2-alkenyl indole dyes with tunable emission wavelengths covering visible to NIR light

As an important member of dyes, small-molecule fluorescent dyes show indispensable value in biomedical fields. Although various molecular dyes have been developed, full-color dyes covering blue to red region derived from a single chromophore are still in urgent demand. In this work, a series of dyes based on C2-alkenyl indole skeleton were synthesized, namely AI dyes, and their photophysical properties, cytotoxicity, and imaging capacity were verified to be satisfactory. Particularly, the maximal emission wavelengths of these dyes could cover a wide range from visible to NIR light with large Stokes shifts. Besides, the optical and structural discrepancies between the C2- and C3- alkenyl AI dyes were discussed in detail, and the theoretical calculations were conducted to provide insights on such structure-activity relationship. Finally, as a proof-of-concept, a fluorescent probe AI-Py-B capable of imaging endogenous ONOO- was presented, demonstrating the bioimaging potentials of these alkenyl indole dyes. This work is anticipated to open up new possibilities for developing dye engineering and bio-applications of natural indole framework.

Toxic Effects of Penetrating Cations

As mitochondria are negatively charged organelles, penetrating cations are used as parts of chimeric molecules to deliver specific compounds into mitochondria. In other words, they are used as electrophilic carriers for such chemical moieties as antioxidants, dyes, etc., to transfer them inside mitochondria. However, unmodified penetrating cations affect different aspects of cellular physiology as well. In this review, we have attempted to summarise the data about the side effects of commonly used natural (e.g., berberine) and artificial (e.g., tetraphenylphosphonium, rhodamine, methylene blue) penetrating cations on cellular physiology. For instance, it was shown that such types of molecules can (1) facilitate proton transport across membranes; (2) react with redox groups of the respiratory chain; (3) induce DNA damage; (4) interfere with pleiotropic drug resistance; (5) disturb membrane integrity; and (6) inhibit enzymes. Also, the products of the biodegradation of penetrating cations can be toxic. As penetrating cations accumulate in mitochondria, their toxicity is mostly due to mitochondrial damage. Mitochondria from certain types of cancer cells appear to be especially sensitive to penetrating cations. Here, we discuss the molecular mechanisms of the toxic effects and the anti-cancer activity of penetrating cations.

The in vivo drug delivery pattern of the organelle-targeting small molecules

Eukaryotic cell organelles sustain the life of cells. Their structural changes and dysfunctions can cause abnormal physiological activities and lead to various diseases. Molecular imaging technology enables the visualization of subcellular structures, cells, organs, and the whole living body's structure and metabolism dynamic changes. This could help to reveal the pharmacology mechanisms and drug delivery pathway in vivo. This article discusses the relationship between organelles and human disease, reviews recent probes targeting organelles and their behavior in vivo. We found that mitochondria-targeting probes prefer accumulation in the intestine, heart, and tumor. The lysosome-targeting probe accumulates in the intestine and tumor. Few studies on endoplasmic reticulum- or Golgi apparatus-targeting probes have been reported for in vivo imaging. We hope this review could provide new insights for developing and applying organelle-targeting probes.

Fatty Acid-Activated Proton Transport by Bisaryl Anion Transporters Depolarises Mitochondria and Reduces the Viability of MDA-MB-231 Breast Cancer Cells

In respiring mitochondria, the proton gradient across the inner mitochondrial membrane is used to drive ATP production. Mitochondrial uncouplers, which are typically weak acid protonophores, can disrupt this process to induce mitochondrial dysfunction and apoptosis in cancer cells. We have shown that bisaryl urea-based anion transporters can also mediate mitochondrial uncoupling through a novel fatty acid-activated proton transport mechanism, where the bisaryl urea promotes the transbilayer movement of deprotonated fatty acids and proton transport. In this paper, we investigated the impact of replacing the urea group with squaramide, amide and diurea anion binding motifs. Bisaryl squaramides were found to depolarise mitochondria and reduce MDA-MB-231 breast cancer cell viability to similar extents as their urea counterpart. Bisaryl amides and diureas were less active and required higher concentrations to produce these effects. For all scaffolds, the substitution of the bisaryl rings with lipophilic electron-withdrawing groups was required for activity. An investigation of the proton transport mechanism in vesicles showed that active compounds participate in fatty acid-activated proton transport, except for a squaramide analogue, which was sufficiently acidic to act as a classical protonophore and transport protons in the absence of free fatty acids.

Bioimaging and detecting endogenous and exogenous cyanide in foods, living cells and mice based on a turn-on mitochondria-targeted fluorescent probe

A novel fluorescent probe, with advanced features including "turn-on" fluorescence response, high sensitivity, good compatibility, and mitochondria-targeting function, has been synthesized based on structural design for detecting and visualizing cyanide in foods and biological systems. An electron-donating triphenylamine group (TPA) was employed as the fluorescent and an electron-accepting 4-methyl-N-methyl-pyridinium iodide (Py) moiety was used as a mitochondria-targeted localization unit, which formed intramolecular charge transfer (ICT) system. The "turn-on" fluorescence response of the probe (TPA-BTD-Py, TBP) toward cyanide is attributed two reasons, one is the insertion of an electron-deficient benzothiadiazole (BTD) group into the conjugated system between TPA and Py, and the other is the inhibition of ICT induced by the nucleophilic addition of CN^-. Two active sites for reacting with CN^- were involved in TBP molecule and high response sensitivity were observed in tetrahydrofuran solvent containing 3 % H₂O. The response time could be reduced to 150 s, the linear range was 0.25-50 μM, and the limit of detection was 0.046 μM for CN^- analysis. The TBP probe was successfully applied to the detection of cyanide in food samples prepared in aqueous solution, including the sprouting potato, bitter almond, cassava, and apple seeds. Furthermore, TBP exhibited low cytotoxicity, clear mitochondria-localizing capability in HeLa cells and excellent fluorescence imaging of exogenous and endogenous CN^- in living PC12 cells. Moreover, exogenous CN^- with intraperitoneal injection in nude mice could be well monitored visually by the "turn-on" fluorescence. Therefore, the strategy based on structural design provided good prospects for optimizing fluorescent probes.

A review of nemorosone: Chemistry and biological properties

Nemorosone is a bicyclic polyprenylated acylphloroglucinol derivative originally isolated from Clusia spp. and it can be obtained through chemical synthesis employing different synthetic strategies. Since its discovery, it has attracted great attention both from a biological and chemical viewpoint. In the present article, we attempted to review various chemical and biological topics around nemorosone, with an emphasis on its antiproliferative activities. For this purpose, relevant data was collected from different scientific databases including Google Scholar, PubMed, Scopus and ISI Web of Knowledge. This natural compound has shown activity against several types of malignancies such as leukemia, human colorectal, pancreatic, and breast cancer because it modulates multiple molecular pathways. Nemorosone has both cytostatic and cytotoxic activity and it also seems to induce apoptosis and ferroptosis. Additionally, it has antimicrobial capabilities against Gram-positive bacteria and parasites belonging to genus Leishmania. Its promising antiproliferative pre-clinical effects deserve further attention for anticancer and anti-parasitic drug development and translation to the clinic.

Mitochondrial dysfunction-targeted nanosystems for precise tumor therapeutics

Mitochondria play critical roles in the regulation of the proliferation and apoptosis of cancerous cells. Targeted induction of mitochondrial dysfunction in cancer cells by multifunctional nanosystems for cancer treatment has attracted increasing attention in the past few years. Numerous therapeutic nanosystems have been designed for precise tumor therapy by inducing mitochondrial dysfunction, including reducing adenosine triphosphate, breaking redox homeostasis, inhibiting glycolysis, regulating proteins, membrane potential depolarization, mtDNA damage, mitophagy dysregulation and so on. Understanding the mechanisms of mitochondrial dysfunction would be helpful for efficient treatment of diseases and accelerating the translation of these therapeutic strategies into the clinic. Then, various strategies to construct mitochondria-targeted nanosystems and induce mitochondrial dysfunction are summarized, and the recent research progress regarding precise tumor therapeutics is highlighted. Finally, the major challenges and an outlook in this rapidly developing field are discussed. This review is expected to inspire further development of novel mitochondrial dysfunction-based strategies for precise treatments of cancer and other human diseases.

Recent Advances in N-Heterocyclic Small Molecules for Synthesis and Application in Direct Fluorescence Cell Imaging

Nitrogen-containing heterocycles are ubiquitous in natural products and drugs. Various organic small molecules with nitrogen-containing heterocycles, such as nitrogen-containing boron compounds, cyanine, pyridine derivatives, indole derivatives, quinoline derivatives, maleimide derivatives, etc., have unique biological features, which could be applied in various biological fields, including biological imaging. Fluorescence cell imaging is a significant and effective imaging modality in biological imaging. This review focuses on the synthesis and applications in direct fluorescence cell imaging of N-heterocyclic organic small molecules in the last five years, to provide useful information and enlightenment for researchers in this field.

Bioinspired large Stokes shift small molecular dyes for biomedical fluorescence imaging

Long Stokes shift dyes that minimize cross-talk between the excitation source and fluorescent emission to improve the signal-to-background ratio are highly desired for fluorescence imaging. However, simple small molecular dyes with large Stokes shift (more than 120 nanometers) and near-infrared (NIR) emissions have been rarely reported so far. Here, inspired by the chromophore chemical structure of fluorescent proteins, we designed and synthesized a series of styrene oxazolone dyes (SODs) with simple synthetic methods, which show NIR emissions (>650 nanometers) with long Stokes shift (ranged from 136 to 198 nanometers) and small molecular weight (<450 daltons). The most promising SOD9 shows rapid renal excretion and blood-brain barrier passing properties. After functioning with the mitochondrial-targeted triphenylphosphonium (TPP) group, the resulting SOD9-TPP can be engineered for head-neck tumor imaging, fluorescence image-guided surgery, brain neuroimaging, and on-site pathologic analysis. In summary, our findings add an essential small molecular dye category to the classical dyes.

Activation of pyroptosis by specific organelle-targeting photodynamic therapy to amplify immunogenic cell death for anti-tumor immunotherapy

Pyroptosis, a unique lytic programmed cell death, inspired tempting implications as potent anti-tumor strategy in pertinent to its potentials in stimulating anti-tumor immunity for eradication of primary tumors and metastasis. Nonetheless, rare therapeutics have been reported to successfully stimulate pyroptosis. In view of the intimate participation of reactive oxygen species (ROS) in stimulating pyroptosis, we attempted to devise a spectrum of well-defined subcellular organelle (including mitochondria, lysosomes and endoplasmic reticulum)-targeting photosensitizers with the aim of precisely localizing ROS (produced from photosensitizers) at the subcellular compartments and explore their potentials in urging pyroptosis and immunogenic cell death (ICD). The subsequent investigations revealed varied degrees of pyroptosis upon photodynamic therapy (PDT) towards cancerous cells, as supported by not only observation of the distinctive morphological and mechanistic characteristics of pyroptosis, but for the first-time explicit validation from comprehensive RNA-Seq analysis. Furthermore, in vivo anti-tumor PDT could exert eradication of the primary tumors, more importantly suppressed the distant tumor and metastatic tumor growth through an abscopal effect, approving the acquirement of specific anti-tumor immunity as a consequence of pyroptosis. Hence, pyroptosis was concluded unprecedently by our proposed organelles-targeting PDT strategy and explicitly delineated with molecular insights into its occurrence and the consequent ICD.

The Molecular Diversity of 1H-Indole-3-Carbaldehyde Derivatives and Their Role in Multicomponent Reactions

1H-Indole-3-carbaldehyde and related members of the indole family are ideal precursors for the synthesis of active molecules. 1H-Indole-3-carbaldehyde and its derivatives are essential and efficient chemical precursors for generating biologically active structures. Multicomponent reactions (MCRs) offer access to complex molecules. This review highlights the recent applications of 1H-indole-3-carbaldehyde in such inherently sustainable multicomponent reactions from the period, 2014 to 2021 and provides an overview of the field that awaits further exploitation in the assembly of pharmaceutically interesting scaffolds.

Unique assemble of carbonylpyridinium and chromene reveal mitochondrial thiol starvation under ferroptosis and novel ferroptosis inducer

To reveal the delicate function of mitochondrial, precise detection tools in spatiotemporal manner remains highly desirable. However, current probes with positive charge warheads for targeting mitochondria diffuse out of the...

Is the Mitochondrial Membrane Potential (∆Ψ) Correctly Assessed? Intracellular and Intramitochondrial Modifications of the ∆Ψ Probe, Rhodamine 123

The mitochondrial membrane potential (∆Ψ) is the driving force providing the electrical component of the total transmembrane potential of hydrogen ions generated by proton pumps, which is utilized by the ATP synthase. The role of ∆Ψ is not limited to its role in bioenergetics since it takes part in other important intracellular processes, which leads to the mandatory requirement of the homeostasis of ∆Ψ. Conventionally, ∆Ψ in living cells is estimated by the fluorescence of probes such as rhodamine 123, tetramethylrodamine, etc. However, when assessing the fluorescence, the possibility of the intracellular/intramitochondrial modification of the rhodamine molecule is not taken into account. Such changes were revealed in this work, in which a comparison of normal (astrocytic) and tumor (glioma) cells was conducted. Fluorescent microscopy, flow cytometry, and mass spectrometry revealed significant modifications of rhodamine molecules developing over time, which were prevented by amiodarone apparently due to blocking the release of xenobiotics from the cell and their transformation with the participation of cytochrome P450. Obviously, an important role in these processes is played by the increased retention of rhodamines in tumor cells. Our data require careful evaluation of mitochondrial ∆Ψ potential based on the assessment of the fluorescence of the mitochondrial probe.

Development of Mitochondria-Targeted Small-Molecule Dyes for Myocardial PET and Fluorescence Bimodal Imaging

Mitochondria-targeting positron emission tomography (PET) and fluorescent dual-modal probes are rarely reported. As one of the most promising lipophilic cations, F16 and its derivatives (F16s) have never been used for myocardial imaging. In this work, 14 F16s are synthesized and evaluated for cardiac imaging. In vitro cell fluorescence imaging revealed that the lead probe 5MEF is precisely localized in the mitochondria of cardiomyocytes. In addition, it shows excellent ex vivo fluorescence imaging quality with the heart-to-muscle and heart-to-liver ratios up to ∼2. Furthermore, the radiofluorinated probe ¹⁸F-5MEF is successfully prepared and shows a high initial heart uptake of 8.66 ± 0.34 % ID/g at 5 min post injection. It displays a high heart imaging performance, a long retention time in the heart, and a low background in the most normal tissues as revealed by PET. To our knowledge, this is the first time novel F16 analogues are designed and developed for myocardial dual-modal imaging.

A bright, red-emitting water-soluble BODIPY fluorophore as an alternative to the commercial Mito Tracker Red for high-resolution mitochondrial imaging

With the emergence and rapid development of super-resolution fluorescence microscopy, monitoring of mitochondrial morphological changes has aroused great interest for exploring the role of mitochondria in the process of cell metabolism. However, in the absence of water-soluble, photostable and low-toxicity fluorescent dyes, ultra-high-resolution mitochondrial imaging is still challenging. Herein, we designed two fluorescent BODIPY dyes, namely Mito-BDP 630 and Mito-BDP 760, for mitochondrial imaging. The results proved that Mito-BDP 760 underwent aggregation-caused quenching (ACQ) in the aqueous matrix owing to its hydrophobicity and was inaccessible to the cells, which restricted its applications in mitochondrial imaging. In stark contrast, water-soluble Mito-BDP 630 readily penetrated cellular and mitochondrial membranes for mitochondrial imaging with high dye densities under wash-free conditions as driven by membrane potential. As a comparison, Mito Tracker Red presented high photobleaching (the fluorescence intensity dropped by nearly 50%) and high phototoxicity after irradiation by a laser for 30 min. However, Mito-BDP 630 possessed excellent biocompatibility, photostability and chemical stability. Furthermore, clear and bright mitochondria distribution in living HeLa cells after incubation with Mito-BDP 630 could be observed by CLSM. Convincingly, the morphology and cristae of mitochondria could be visualized using an ultra-high-resolution microscope. In short, Mito-BDP 630 provided a powerful and convenient tool for monitoring mitochondrial morphologies in living cells. Given the facile synthesis, photobleaching resistance and low phototoxicity of Mito-BDP 630, it is an alternative to the commercial Mito Tracker Red.

Fluorescent styrylpyrylium probes for the imaging of mitochondria in live cells

Eight styrylpyrylium tetrafluoroborate salts have been synthesized and fully optically characterized by UV-vis absorption and fluorescence steady-state/time-resolved spectroscopies. The new dyes exhibit strong emission bands with yellow-orange colours, depending on the substituents present in the structure. Notably, the Stokes shift recorded for some of them exceeds 100 nm, a very valuable feature for biological imaging. Four of them have been assayed as biological imaging agents by confocal laser scanning microscopy (CLSM) in the human hepatoma cell line Hep3B. It has been found that all the compounds efficiently stain intracellular structures which have been identified as mitochondria through colocalization assays with MitoView (a well-known mitochondrial marker) and using carbonyl cyanide m-chlorophenyl hydrazone (CCCP) as a mitochondrial membrane potential uncoupler. Additionally, the potential ability of the studied dyes as cytotoxic drugs has been explored. The inhibitory concentration (IC50) against Hep3B was found to be in the range of 4.2 μM-11.5 μM, similar to other described anticancer drugs for the same hepatoma cell line. The combined features of a good imaging agent and potential anticancer drug make the family of the studied pyrylium salts good candidates for further theranostic studies. Remarkably, despite the extensive use of pyrylium dyes in several scientific areas (from photocatalysis to optics), there is no precedent description of a styrylpyrylium salt with potential theranostic applications.

Exploring the therapeutic potential of mitochondrial uncouplers in cancer

BACKGROUND

Mitochondrial uncouplers are well-known for their ability to treat a myriad of metabolic diseases, including obesity and fatty liver diseases. However, for many years now, mitochondrial uncouplers have also been evaluated in diverse models of cancer in vitro and in vivo. Furthermore, some mitochondrial uncouplers are now in clinical trials for cancer, although none have yet been approved for the treatment of cancer.

SCOPE OF REVIEW

In this review we summarise published studies in which mitochondrial uncouplers have been investigated as an anti-cancer therapy in preclinical models. In many cases, mitochondrial uncouplers show strong anti-cancer effects both as single agents, and in combination therapies, and some are more toxic to cancer cells than normal cells. Furthermore, the mitochondrial uncoupling mechanism of action in cancer cells has been described in detail, with consistencies and inconsistencies between different structural classes of uncouplers. For example, many mitochondrial uncouplers decrease ATP levels and disrupt key metabolic signalling pathways such as AMPK/mTOR but have different effects on reactive oxygen species (ROS) production. Many of these effects oppose aberrant phenotypes common in cancer cells that ultimately result in cell death. We also highlight several gaps in knowledge that need to be addressed before we have a clear direction and strategy for applying mitochondrial uncouplers as anti-cancer agents.

MAJOR CONCLUSIONS

There is a large body of evidence supporting the therapeutic use of mitochondrial uncouplers to treat cancer. However, the long-term safety of some uncouplers remains in question and it will be critical to identify which patients and cancer types would benefit most from these agents.

Novel Mitochondria-targeted Drugs for Cancer Therapy

The search for mitochondria-targeted drugs has dramatically risen over the last decade. Mitochondria are essential organelles serving not only as a powerhouse of the cell but also as a key player in cell proliferation and cell death. Their central role in the energetic metabolism, calcium homeostasis and apoptosis makes them an intriguing field of interest for cancer pharmacology. In cancer cells, many mitochondrial signaling and metabolic pathways are altered. These changes contribute to cancer development and progression. Due to changes in mitochondrial metabolism and changes in membrane potential, cancer cells are more susceptible to mitochondria-targeted therapy. The loss of functional mitochondria leads to the arrest of cancer progression and/or a cancer cell death. Identification of mitochondrial changes specific for tumor growth and progression, rational development of new mitochondria-targeted drugs and research on delivery agents led to the advance of this promising area. This review will highlight the current findings in mitochondrial biology, which are important for cancer initiation, progression and resistance, and discuss approaches of cancer pharmacology with a special focus on the anti-cancer drugs referred to as 'mitocans'.

Indole Alkaloids, Synthetic Dimers and Hybrids with Potential In Vivo Anticancer Activity

Indole, a heterocyclic organic compound, is one of the most promising heterocycles found in natural and synthetic sources since its derivatives possess fascinating structural diversity and various therapeutic properties. Indole alkaloids, synthetic dimers and hybrids could act on diverse targets in cancer cells, and consequently, possess potential antiproliferative effects on various cancers both in vitro and in vivo. Vinblastine, midostaurin, and anlotinib as the representative of indole alkaloids, synthetic dimers and hybrids respectively, have already been clinically applied to treat many types of cancers, demonstrating indole alkaloids, synthetic dimers and hybrids are useful scaffolds for the development of novel anticancer agents. Covering articles published between 2010 and 2020, this review emphasizes the recent development of indole alkaloids, synthetic dimers and hybrids with potential in vivo therapeutic application for cancers.

Supramolecular Antagonists Promote Mitochondrial Dysfunction

Mitochondrion-targeting therapy exhibits great potential in cancer therapy but significantly suffers from limited therapeutic efficiency. Here we report on mitochondrion-targeting supramolecular antagonist-inducing tumor cell death via simultaneously promoting cellular apoptosis and preventing survival. The supramolecular antagonist was created via coassembly of a mitochondrion-targeting pentapeptide with its two derivatives functionalized with a BH3 domain or the drug camptothecin (CPT). While drug CPT released from the antagonist induced cellular apoptosis via decreasing the mitochondrial membrane potential, the BH3 domain prevented cellular survival through facilitating the association between the supramolecular antagonists and antiapoptotic proteins, thereby initiating mitochondrial permeabilization. Both in vitro and in vivo studies confirmed the combinatorial therapeutic effect arising from the BH3 domain and CPT drug within the supramolecular antagonist on cell death and thereby inhibiting tumor growth. Our findings demonstrate an efficient combinatorial mechanism for mitochondrial dysfunction, thus potentially serving as novel organelle-targeting medicines.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

A dual-labeling probe to track functional mitochondria-lysosome interactions in live cells

Mitochondria–lysosome interactions are essential for maintaining intracellular homeostasis. Although various fluorescent probes have been developed to visualize such interactions, they remain unable to label mitochondria and lysosomes simultaneously and dynamically track their interaction. Here, we introduce a cell-permeable, biocompatible, viscosity-responsive, small organic molecular probe, Coupa, to monitor the interaction of mitochondria and lysosomes in living cells. Through a functional fluorescence conversion, Coupa can simultaneously label mitochondria with blue fluorescence and lysosomes with red fluorescence, and the correlation between the red–blue fluorescence intensity indicates the progress of mitochondria–lysosome interplay during mitophagy. Moreover, because its fluorescence is sensitive to viscosity, Coupa allowed us to precisely localize sites of mitochondria–lysosome contact and reveal increases in local viscosity on mitochondria associated with mitochondria–lysosome contact. Thus, our probe represents an attractive tool for the localization and dynamic tracking of functional mitochondria–lysosome interactions in living cells.

Dynamic labeling and tracking of organelle–organelle contacts is essential to understand the formation and function of these interactions. Here the authors present a small molecule probe, Coupa, that labels mitochondria and lysosomes with blue and red fluorescence, respectively.

Cyanine mitochondrial dye with slightly selective cytotoxicity against A549 cancerous cells

Delocalized lipophilic cations (DLCs) are known as mitochondria-addressed molecules. Mitochondria targeting may provide opportunities for tumor detection. DLCs may have antioxidant or anticancer properties. In this study, we focused on the toxicity and localization of 2-[(E)-2-(5-fluoro-2-methyl-1H-indol-3-yl)ethenyl]-1,6-dimethylpyridin-1-ium iodide (62E2), which has recently been found as a novel cytotoxic fluorescent compound. The excitation maximum of 62E2 is 452 ± 10 nm and its emission maximum is 579 ± 10 nm. It is accumulated in the cells and stains mitochondria in nanomolar concentrations. 62E2 is cytotoxic and mitotoxic in low micromolar concentrations, and it demonstrates some selectivity of cytotoxicity against A549 cancer cells. The closest analog of 62E2 is F16, which is the fluorescent mitotoxic agent that has been described earlier as a potential anticancer agent. We hope that 62E2 described here is useful in expanding the diversity of cyanine fluorescent mitochondrial dyes and the analysis of their structure-activity relationships.

All-in-one mitochondria-targeted NIR-II fluorophores for cancer therapy and imaging

Small-molecule subcellular organelle-targeting theranostic probes are crucial for early disease diagnosis and treatment. The imaging window of these molecules is mainly focused on the visible and near-infrared region (below ∼900 nm) which limits the tissue penetration depth and therapeutic effects. Herein, a novel NIR-II small-molecule probe H4–PEG-Glu with a thiopyrylium cation was synthesized. H4–PEG-Glu not only can quickly and effectively image mitochondria in acute myeloid leukemia (AML) cells, and induce G₀/G₁ phase arrest by the intrinsic mitochondrial apoptosis pathway w/o irradiation, but also exhibit moderate cytotoxicity against AML cancer cells in a dose dependent-manner without laser irradiation. The THP-1 cells treated with H4–PEG-Glu upon NIR laser irradiation showed enhanced chemo- and photothermal therapy (CPTT) with 93.07% ± 6.43 apoptosis by Annexin V staining. Meanwhile, H4–PEG-Glu displayed high synergistic CPTT effects in vivo, as well as specific NIR-II tumor imaging in AML patient derived PDX mouse models for the first time. Our work lays down a solid foundation for designing small-molecule NIR-II mitochondria-selective theranostic probes.

Small-molecule subcellular organelle-targeting theranostic probes are crucial for early disease diagnosis and treatment.

Mitochondrial Targeting of Probes and Therapeutics to the Powerhouse of the Cell

Mitochondria, colloquially known as "the powerhouse of the cell", play important roles in production, but also in processes critical for cellular fate such as cell death, differentiation, signaling, metabolic homeostasis, and innate immunity. Due to its many functions in the cell, the mitochondria have been linked to a variety of human illnesses such as diabetes, cancer, and neurodegenerative diseases. In order to further our understanding and pharmaceutical targeting of this critical organelle, effective strategies must be employed to breach the complex barriers and microenvironment of mitochondria. Here, we summarize advancements in mitochondria-targeted probes and therapeutics.

A NIR-I light-responsive superoxide radical generator with cancer cell membrane targeting ability for enhanced imaging-guided photodynamic therapy

Photodynamic therapy (PDT), as an emerging treatment modality, which takes advantage of reactive oxygen species (ROS) generated upon light illumination to ablate tumours, has suffered from a limited treatment depth, strong oxygen dependence and short ROS lifespan. Herein, we developed a highly efficient NIR-I light (808 nm laser) initiated theranostic system based on a fluorescent photosensitizer (EBD-1) with cancer cell membrane targeting ability, which can realize large penetration depth in tissue, generate superoxide radicals (O2 -˙) to relieve the oxygen-dependence, confine the ROS oxidation at the cell membrane, and self-report the cell viability during the PDT process. In vivo experiments demonstrated that EBD-1 under 808 nm light successfully accomplished remarkable cancer ablation. This work will be beneficial for the design of novel photosensitizers for PDT-based theranostic systems.

Soft Mesoporous Organosilica Nanoplatforms Improve Blood Circulation, Tumor Accumulation/Penetration, and Photodynamic Efficacy

To date, the ability of nanoplatforms to achieve excellent therapeutic responses is hindered by short blood circulation and limited tumor accumulation/penetration. Herein, a soft mesoporous organosilica nanoplatform modified with hyaluronic acid and cyanine 5.5 are prepared, denoted SMONs-HA-Cy5.5, and comparative studies between SMONs-HA-Cy5.5 (24.2 MPa) and stiff counterparts (79.2 MPa) are conducted. Results indicate that, apart from exhibiting a twofold increase in tumor cellular uptake, the soft nanoplatforms also display a remarkable pharmacokinetic advantage, resulting in considerably improved tumor accumulation. Moreover, SMONs-HA-Cy5.5 exhibits a significantly higher tumor penetration, achieving 30-μm deeper tissue permeability in multicellular spheroids relative to the stiff counterparts. Results further reveal that the soft nanoplatforms have an easier extravasation from the tumor vessels, diffuse farther in the dense extracellular matrix, and reach deeper tumor tissues compared to the stiff ones. Specifically, the soft nanoplatforms generate a 16-fold improvement (43 vs. 2.72 μm) in diffusion distance in tumor parenchyma. Based on the significantly improved blood circulation and tumor accumulation/penetration, a soft therapeutic nanoplatform is constructed by loading photosensitizer chlorin e6 in SMONs-HA-Cy5.5. The resulting nanoplatform exhibits considerably higher therapeutic efficacy on tumors compared to the stiff ones.

IR820 functionalized melanin nanoplates for dual-modal imaging and photothermal tumor eradication

Melanin as an endogenous biomolecule is widely applied in the biomedical field, focusing especially on diagnostic imaging and photothermal therapy in cancer treatment. However, its photothermal conversion efficiency, a benchmark in tumor photothermal therapy (PTT), often could not satisfy PTT requirements to some degree, and this greatly influenced its use in photothermal cancer therapy. As for fluorescence imaging, a small-molecule NIR dye as a fluorescence probe is easily and rapidly metabolized in vivo, resulting in low accumulation in a tumor. To overcome these problems, we attempt to use melanin as a carrier to conjugate a fluorochrome, a recombinant small NIR dye IR820 nanoplatform containing melanin (MNP-PEG-IR820 abbreviated to MPI). The addition of IR820 not only enhances the PTT ability of the nanoplatform, but also endows the material with excellent NIR fluorescence behavior. Most importantly, the integration of fluorescence dye and melanin improves the circulation and stability performance of IR820 while reducing its toxicity in vivo, owing to the protectivity of melanin. Thus, the diagnostic capability is enhanced. Meanwhile, the behavior of the nanoplatform in PAI/PTT is significantly improved. The in vitro investigations reveal that the MPI NPs afford a potent PTT effect and ideal resistance to photobleaching. After intravenous injection, the MPI NPs display effective PTT tumor eradication in a Hep-2 tumor bearing mouse model with excellent dual NIR-I fluorescence/photoacoustic imaging guided phototherapy. Hence, our work shows the potential of MPI NPs as nano-theranostics for biomedical application to laryngocarcinoma.

Extraspecific Manifestation of Nanoheater's Position Effect on Distinctive Cellular Photothermal Responses

Subcellular localization of nanoparticles plays critical roles in precision medicine that can facilitate an in-depth understanding of disease etiology and achieve accurate theranostic regulation via responding to the aiding stimuli. The photothermal effect is an extensively employed strategy that converts light into heat stimulation to induce localized disease ablation. Despite diverse manipulations that have been investigated in photothermal nanotheranostics, influences of nanoheaters' subcellular distribution and their molecular mechanism on cellular heat response remain elusive. Herein, we disclose the biological basis of distinguishable thermal effects at subcellular resolution by localizing photothermal upconversion nanoparticles into specific locations of cell compartments. Upon 808 nm light excitation, the lysosomal cellular uptake initialized by poly(ethylenimine)-modified nanoheaters promoted mitochondria apoptosis through the activation of Bid protein, whereas the cell surface nanoheaters anchored via metabolic glycol biosynthesis triggered necrosis by direct perturbation of the membrane structure. Intriguingly, these two different thermolyses revealed similar levels of heat shock protein expression in live cells. This study stipulates insights underlying the different subcellular positions of nanoparticles for the selective thermal response, which provides valuable perspectives on optimal precision nanomedicine.