Asymmetric transfer hydrogenation by synthetic catalysts in cancer cells

Pressure-induced generation of heterogeneous electrocatalytic metal hydride surfaces for sustainable hydrogen transfer

Metal hydrides are crucial intermediates in numerous catalytic reactions. Intensive efforts have been dedicated to constructing molecular metal hydrides, where toxic precursors and delicate mediators are usually involved. Herein, we demonstrate a facile pressure-induced methodology to generate a cost-effective heterogeneous electrocatalytic metal hydride surface for sustainable hydrogen transfer. Taking carbon dioxide (CO2) electroreduction as a model system and zinc (Zn), a well-known carbon monoxide (CO)-selective catalyst, as a model catalyst, we showcase a homogeneous-type hydrogen atom transfer process induced by heterogeneous hydride surfaces, enabling direct hydrogenation pathways traditionally considered "prohibited". Specifically, the maximal Faradaic efficiency for formate is enhanced by ~fivefold to 83% under ambient conditions. Experimental and theoretical analyses reveal that unlike the distal hydrogenation route for CO2 to CO over pristine Zn, the Zn hydride surface enables direct hydrogenation at the carbon site of CO2 to form formate. This work provides a promising material platform for sustainable synthesis.

In-cell Catalysis by Tethered Organo-Osmium Complexes Generates Selectivity for Breast Cancer Cells

Anticancer agents that exhibit catalytic mechanisms of action offer a unique multi-targeting strategy to overcome drug resistance. Nonetheless, many in-cell catalysts in development are hindered by deactivation by endogenous nucleophiles. We have synthesised a highly potent, stable Os-based 16-electron half-sandwich ('piano stool') catalyst by introducing a permanent covalent tether between the arene and chelated diamine ligand. This catalyst exhibits antiproliferative activity comparable to the clinical drug cisplatin towards triple-negative breast cancer cells and can overcome tamoxifen resistance. Speciation experiments revealed Os to be almost exclusively albumin-bound in the extracellular medium, while cellular accumulation studies identified an energy-dependent, protein-mediated Os accumulation pathway, consistent with albumin-mediated uptake. Importantly, the tethered Os complex was active for in-cell transfer hydrogenation catalysis, initiated by co-administration of a non-toxic dose of sodium formate as a source of hydride, indicating that the Os catalyst is delivered to the cytosol of cancer cells intact. The mechanism of action involves the generation of reactive oxygen species (ROS), thus exploiting the inherent redox vulnerability of cancer cells, accompanied by selectivity for cancerous cells over non-tumorigenic cells.

Dinuclear Tridentate Ru(II) Complex with Strong Near-Infrared Light-Triggered Anticancer Activity

The design of the dinuclear Ru(II) complex (Ru2) with strong near-infrared (NIR) absorption properties has been reported for efficient anticancer phototherapy. Under 700 nm LED light excitation, Ru2 exhibited remarkable synergistic type I/II photosensitization ability and photocatalytic activity toward intracellular biomolecules. Ru2 showed impressive 700 nm light-triggered anticancer activity under normoxia and hypoxia compared with the clinically used photosensitizer Chlorin e6. The mechanistic studies showed that Ru2 induced intracellular redox imbalance and perturbed the energy metabolism and biosynthesis in A549 cancer cells. Overall, this work provides a new strategy for developing efficient metal-based complexes for anticancer phototherapy under NIR light.

Biomimetic Chiral Nanomaterials with Selective Catalysis Activity

Chiral nanomaterials with unique chiral configurations and biocompatible ligands have been booming over the past decade for their interesting chiroptical effect, unique catalytical activity, and related bioapplications. The catalytic activity and selectivity of chiral nanomaterials have emerged as important topics, that can be potentially controlled and optimized by the rational biochemical design of nanomaterials. In this review, chiral nanomaterials synthesis, composition, and catalytic performances of different biohybrid chiral nanomaterials are discussed. The construction of chiral nanomaterials with multiscale chiral geometries along with the underlying principles for enhancing chiroptical responses are highlighted. Various biochemical approaches to regulate the selectivity and catalytic activity of chiral nanomaterials for biocatalysis are also summarized. Furthermore, attention is paid to specific chiral ligands, materials compositions, structure characteristics, and so on for introducing selective catalytic activities of representative chiral nanomaterials, with emphasis on substrates including small molecules, biological macromolecule, and in-site catalysis in living systems. Promising progress has also been emphasized in chiral nanomaterials featuring structural versatility and improved chiral responses that gave rise to unprecedented chances to utilize light for biocatalytic applications. In summary, the challenges, future trends, and prospects associated with chiral nanomaterials for catalysis are comprehensively proposed.

A Transfer Hydrogenation Approach to Activity-Based Sensing of Formate in Living Cells

Formate is a major reactive carbon species in one-carbon metabolism, where it serves as an endogenous precursor for amino acid and nucleic acid biosynthesis and a cellular source of NAD(P)H. On the other hand, aberrant elevations in cellular formate are connected to progression of serious diseases, including cancer and Alzheimer's disease. Traditional methods for formate detection in biological environments often rely on sample destruction or extensive processing, resulting in a loss of spatiotemporal information. To help address these limitations, here we present the design, synthesis, and biological evaluation of a first-generation activity-based sensing system for live-cell formate imaging that relies on iridium-mediated transfer hydrogenation chemistry. Formate facilitates an aldehyde-to-alcohol conversion on various fluorophore scaffolds to enable fluorescence detection of this one-carbon unit, including through a two-color ratiometric response with internal calibration. The resulting two-component probe system can detect changes in formate levels in living cells with a high selectivity over potentially competing biological analytes. Moreover, this activity-based sensing system can visualize changes in endogenous formate fluxes through alterations of one-carbon pathways in cell-based models of human colon cancer, presaging the potential utility of this chemical approach to probe the continuum between one-carbon metabolism and signaling in cancer and other diseases.

Self-Metallized Whole Cell Vaccines Prepared by Microfluidics for Bioorthogonally Catalyzed Antitumor Immunotherapy

Tumor whole cell, carrying a complete set of tumor-associated antigens and tumor-specific antigens, has shown great potential in the construction of tumor vaccines but is hindered by the complex engineering means and limited efficacy to cause immunity. Herein, we provided a strategy for the self-mineralization of autologous tumor cells with palladium ions in microfluidic droplets, which endowed the engineered cells with both immune and catalytic functions, to establish a bioorthogonally catalytic tumor whole-cell vaccine. This vaccine showed strong inhibition both in the occurrence and recurrence of tumor by invoking the immediate antitumor immunity and building a long-term immunity.

Effect of Ligand Substituents on Spectroscopic and Catalytic Properties of Water-Compatible Cp*Ir-(pyridinylmethyl)sulfonamide-Based Transfer Hydrogenation Catalysts

Transition-metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups were synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable-temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereoinversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable-temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step in the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.

Harnessing Transition Metal Scaffolds for Targeted Antibacterial Therapy

Antimicrobial resistance, caused by persistent adaptation and growing resistance of pathogenic bacteria to overprescribed antibiotics, poses one of the most serious and urgent threats to global public health. The limited pipeline of experimental antibiotics in development further exacerbates this looming crisis and new drugs with alternative modes of action are needed to tackle evolving pathogenic adaptation. Transition metal complexes can replenish this diminishing stockpile of drug candidates by providing compounds with unique properties that are not easily accessible using pure organic scaffolds. We spotlight four emerging strategies to harness these unique properties to develop new targeted antibacterial agents.

Transition Metal Catalysis in Living Cells: Progress, Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.

Lewis acid-driven self-assembly of diiridium macrocyclic catalysts imparts substrate selectivity and glutathione tolerance

Molecular inorganic catalysts (MICs) tend to have solvent-exposed metal centers that lack substrate specificity and are easily inhibited by biological nucleophiles. Unfortunately, these limitations exclude many MICs from being considered for in vivo applications. To overcome this challenge, a strategy to spatially confine MICs using Lewis acid-driven self-assembly is presented. It was shown that in the presence of external cations (e.g., Li+, Na+, K+, or Cs+) or phosphate buffered saline, diiridium macrocycles spontaneously formed supramolecular iridium-cation species, which were characterized by X-ray crystallography and dynamic light scattering. These nanoassemblies selectively reduced sterically unhindered C[double bond, length as m-dash]O groups via transfer hydrogenation and tolerated up to 1 mM of glutathione. In contrast, when non-coordinating tetraalkylammonium cations were used, the diiridium catalysts were unable to form higher-ordered structures and discriminate between different aldehyde substrates. This work suggests that in situ coordination self-assembly could be a versatile approach to enable or enhance the integration of MICs with biological hosts.

Nanocompartment-confined polymerization in living systems

Polymerization in living systems has become an effective strategy to regulate cell functions and behavior. However, the requirement of high concentrations of monomers, the existence of complicated intracorporal interferences, and the demand for extra external stimulations hinder their further biological applications. Herein, a nanocompartment-confined strategy that provides a confined and secluded environment for monomer enrichment and isolation is developed to achieve high polymerization efficiency, reduce the interference from external environment, and realize broad-spectrum polymerizations in living systems. For exogenous photopolymerization, the light-mediated free-radical polymerization of sodium 4-styrenesulfonate induces a 2.7-fold increase in the reaction rate with the protection of a confined environment. For endogenous hydrogen peroxide-responsive polymerization, p‑aminodiphenylamine hydrochloride embedded in a nanocompartment not only performs a 6.4-fold higher reaction rate than that of free monomers, but also activates an effective second near-infrared photoacoustic imaging-guided photothermal immunotherapy at tumor sites. This nanocompartment-confined strategy breaks the shackles of conventional polymerization, providing a universal platform for in vivo synthesis of polymers with diverse structures and functions.

Robust and efficient transfer hydrogenation of carbonyl compounds catalyzed by NN-Mn(I) complexes

A series of manganese(I) carbonyl complexes bearing structurally related NN- and NNN-chelating ligands have been synthesized and assessed as catalysts for transfer hydrogenation (TH). Notably, the NN-systems based on N-R functionalized 5,6,7,8-tetrahydroquinoline-8-amines, proved the most effective in the manganese-promoted conversion of acetophenone to 1-phenylethanol. In particular, the N-isopropyl derivative, Mn1, when conducted in combination with t-BuONa, was the standout performer mediating not only the reduction of acetophenone but also a range of carbonyl substrates including (hetero)aromatic-, aliphatic- and cycloalkyl-containing ketones and aldehydes with especially high values of TON (up to 17 200; TOF of 3550 h-1). These findings, obtained through a systematic variation of the N-R group of the NN ligand, are consistent with an outer-sphere mechanism for the hydrogen transfer. As a more general point, this Mn-based catalytic TH protocol offers an attractive and sustainable alternative for producing alcoholic products from carbonyl substrates.

Variations in Intracellular Organometallic Reaction Frequency Captured by Single-Molecule Fluorescence Microscopy

Studies of organometallic reactions in living cells commonly rely on ensemble-averaged measurements, which can obscure the detection of reaction dynamics or location-specific behavior. This information is necessary to guide the design of bioorthogonal catalysts with improved biocompatibility, activity, and selectivity. By leveraging the high spatial and temporal resolution of single-molecule fluorescence microscopy, we have successfully captured single-molecule events promoted by Ru complexes inside live A549 human lung cells. By observing individual allylcarbamate cleavage reactions in real-time, our results revealed that they occur with greater frequency inside the mitochondria than in the non-mitochondria regions. The estimated turnover frequency of the Ru complexes was at least 3-fold higher in the former than the latter. These results suggest that organelle specificity is a critical factor to consider in intracellular catalyst design, such as in developing metallodrugs for therapeutic applications.

Novel Ru(II) complexes with multiple anticancer photoreactivity: ligand exchange, photoredox catalysis, reactive oxygen generation and endoperoxide formation

The hypoxic microenvironment and drug resistance of cancer cells have become a huge threat for clinical anticancer therapy. Anticancer phototherapy providing spatial and temporal control over drug activation may conquer this problem. Herein, we report a novel photoactivated Ru(II) complex (Ru2) with multiple activities including photochemotherapy, photodynamic and photocatalytic therapy, and endoperoxide formation. Upon white light irradiation, Ru2 can dissociate the coordinating ligands and form endoperoxides, produce diverse reactive oxygen species and catalytically oxidize cellular coenzymes. As a result, Ru2 shows promising antiproliferation activity toward cisplatin and 5-fluorouracil resistant tumor cell lines under normoxia and hypoxia. The multifunctional design strategy of metal-based anticancer drugs offers novel efficient therapeutics to combat drug-resistant cancer cells under hypoxia.

Organoiridium Complexes Enhance Cellular Defense Against Reactive Aldehydes Species

Although reactive aldehyde species (RASP) are associated with the pathogenesis of many major diseases, there are currently no clinically approved treatments for RASP overload. Conventional aldehyde detox agents are stoichiometric reactants that get consumed upon reacting with their biological targets, which limits their therapeutic efficiency. To achieve longer-lasting detoxification effects, small-molecule intracellular metal catalysts (SIMCats) were used to protect cells by converting RASP into non-toxic alcohols. It was shown that SIMCats were significantly more effective in lowering cell death from the treatment with 4-hydroxynon-2-enal than aldehyde scavengers over a 72 h period. Studies revealed that SIMCats reduced the aldehyde accumulation in cells exposed to the known RASP inducer arsenic trioxide. This work demonstrates that SIMCats offer unique benefits over stochiometric agents, potentially providing new ways to combat diseases with greater selectivity and efficiency than existing approaches.

An ultrasound-activatable platinum prodrug for sono-sensitized chemotherapy

Despite the great success achieved by photoactivated chemotherapy, eradicating deep tumors using external sources with high tissue penetration depth remains a challenge. Here, we present cyaninplatin, a paradigm of Pt(IV) anticancer prodrug that can be activated by ultrasound in a precise and spatiotemporally controllable manner. Upon sono-activation, mitochondria-accumulated cyaninplatin exhibits strengthened mitochondrial DNA damage and cell killing efficiency, and the prodrug overcomes drug resistance as a consequence of combined effects from released Pt(II) chemotherapeutics, the depletion of intracellular reductants, and the burst of reactive oxygen species, which gives rise to a therapeutic approach, namely sono-sensitized chemotherapy (SSCT). Guided by high-resolution ultrasound, optical, and photoacoustic imaging modalities, cyaninplatin realizes the overall theranostics of tumors in vivo with superior efficacy and biosafety. This work highlights the practical utility of ultrasound to precisely activate Pt(IV) anticancer prodrugs for the eradication of deep tumor lesions and broadens the biomedical uses of Pt coordination complexes.

Immunostimulation with chemotherapy of a ruthenium-arene complex via blockading CD47 signal in chronic myelogenous leukemia cells

Combination of novel immunomodulation and traditional chemotherapy has become a new tendency in cancer treatment. Increasing evidence suggests that blocking the "don't eat me" signal transmitted by the CD47 can promote the phagocytic ability of macrophages to cancer cells, which might be promising for improved cancer chemoimmunotherapy. In this work, we conjugated CPI-alkyne modified by Devimistat (CPI-613) with ruthenium-arene azide precursor Ru-N₃ by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction to construct Ru complex CPI-Ru. CPI-Ru exhibited satisfactory cytotoxicity towards the K562 cells while nearly non-toxic towards the normal HLF cells. CPI-Ru has been demonstrated to cause severe damage to mitochondria and DNA, ultimately inducing cancer cell death through the autophagic pathway. Moreover, CPI-Ru could significantly downregulate the expression of CD47 on the surface of K562 accompanied by the enhanced immune response by targeting the blockade of CD47. This work provides a new strategy for utilizing metal-based anticancer agents to block CD47 signal to achieve chemoimmunotherapy in chronic myeloid leukemia treatment.

Targeting cancer lactate metabolism with synergistic combinations of synthetic catalysts and monocarboxylate transporter inhibitors

Synthetic anticancer catalysts offer potential for low-dose therapy and the targeting of biochemical pathways in novel ways. Chiral organo-osmium complexes, for example, can catalyse the asymmetric transfer hydrogenation of pyruvate, a key substrate for energy generation, in cells. However, small-molecule synthetic catalysts are readily poisoned and there is a need to optimise their activity before this occurs, or to avoid this occurring. We show that the activity of the synthetic organometallic redox catalyst [Os(p-cymene)(TsDPEN)] (1), which can reduce pyruvate to un-natural D-lactate in MCF7 breast cancer cells using formate as a hydride source, is significantly increased in combination with the monocarboxylate transporter (MCT) inhibitor AZD3965. AZD3965, a drug currently in clinical trials, also significantly lowers the intracellular level of glutathione and increases mitochondrial metabolism. These synergistic mechanisms of reductive stress induced by 1, blockade of lactate efflux, and oxidative stress induced by AZD3965 provide a strategy for low-dose combination therapy with novel mechanisms of action.

Targeting Pathogenic Formate-Dependent Bacteria with a Bioinspired Metallo-Nitroreductase Complex

Nitroreductases (NTRs) constitute an important class of oxidoreductase enzymes that have evolved to metabolize nitro-containing compounds. Their unique characteristics have spurred an array of potential uses in medicinal chemistry, chemical biology, and bioengineering toward harnessing nitro caging groups and constructing NTR variants for niche applications. Inspired by how they carry out enzymatic reduction via a cascade of hydride transfer reactions, we sought to develop a synthetic small-molecule NTR system based on transfer hydrogenation mediated by transition metal complexes harnessing native cofactors. We report the first water-stable Ru-arene complex capable of selectively and fully reducing nitroaromatics into anilines in a biocompatible buffered aqueous environment using formate as the hydride source. We further demonstrated its application to activate nitro-caged sulfanilamide prodrug in formate-abundant bacteria, specifically pathogenic methicillin-resistant Staphylococcus aureus. This proof of concept paves the way for a new targeted antibacterial chemotherapeutic approach leveraging on redox-active metal complexes for prodrug activation via bioinspired nitroreduction.

Liquid Metal as Bioinspired and Unusual Modulator in Bioorthogonal Catalysis for Tumor Inhibition Therapy

Bioorthogonal catalysis mediated by Pd-based transition metal catalysts has sparked increasing interest in combating diseases. However, the catalytic and therapeutic efficiency of current Pd⁰ catalysts is unsatisfactory. Herein, inspired by the concept that ligands around metal sites could enable enzymes to catalyze astonishing reactions by changing their electronic environment, a LM-Pd catalyst with liquid metal (LM) as an unusual modulator has been designed to realize efficient bioorthogonal catalysis for tumor inhibition. The LM matrix can serve as a "ligand" to afford an electron-rich environment to stabilize the active Pd⁰ and promote nucleophilic turnover of the π-allylpalladium species to accelerate the uncaging process. Besides, the photothermal properties of LM can lead to the enhanced removal of tumor cells by photo-enhanced catalysis and photothermal effect. We believe that our work will broaden the application of LM and motivate the design of bioinspired bioorthogonal catalysts.

Role of ROS‑mediated autophagy in melanoma (Review)

Melanoma is the most aggressive form of skin cancer with the poorest prognosis and its pathogenesis has yet to be fully elucidated. As key factors that regulate cellular homeostasis, both reactive oxygen species (ROS) and autophagy are involved in the development of melanoma, from melanomagenesis to progression and drug resistance. However, the interaction between ROS and autophagy in the etiology and treatment of melanoma is not well characterized. The present review examined the production of ROS and the role of oxidative stress in melanoma, and summarized the role of ROS‑mediated autophagy in melanomagenesis and melanoma cell fate decision following treatment with various anticancer drugs. The present findings may lead to a better understanding of the pathogenesis and progression of melanoma, and suggest promising treatment options for this disease.

Fluorescent half-sandwich iridium picolinamidate complexes for in-cell visualization

In this work, we report on the development of fluorescent half-sandwich iridium complexes using a fluorophore attachment strategy. These constructs consist of pentamethylcyclopentadienyl (Cp*) iridium units ligated by picolinamidate donors conjugated to green-emitting boron-dipyrromethene (bodipy) dyes. Reaction studies in H2O/THF mixtures showed that the fluorescent Ir complexes were active as catalysts for transfer hydrogenation, with activities similar to that of their non-fluorescent counterparts. The iridium complexes were taken up by NIH-3T3 mouse fibroblast cells, with 50% inhibition concentrations ranging from ~20-70 μM after exposure for 3 h. Visualization of the bodipy-functionalized Ir complexes in cells using fluorescence microscopy revealed that they were localized in the mitochondria and lysosome but not the nucleus. These results indicate that our fluorescent iridium complexes could be useful for future biological studies requiring intracellular catalyst tracking.

Elucidation of Structure-Activity Relationships in Indolobenzazepine-Derived Ligands and Their Copper(II) Complexes: the Role of Key Structural Components and Insight into the Mechanism of Action

Indolo[3,2-d][1]benzazepines (paullones), indolo[3,2-d][2]benzazepines, and indolo[2,3-d][2]benzazepines (latonduines) are isomeric scaffolds of current medicinal interest. Herein, we prepared a small library of novel indolo[3,2-d][2]benzazepine-derived ligands HL¹-HL⁴ and copper(II) complexes 1-4. All compounds were characterized by spectroscopic methods (¹H and ¹³C NMR, UV-vis, IR) and electrospray ionization (ESI) mass spectrometry, while complexes 2 and 3, in addition, by X-ray crystallography. Their purity was confirmed by HPLC coupled with high-resolution ESI mass spectrometry and/or elemental analysis. The stability of compounds in aqueous solutions in the presence of DMSO was confirmed by ¹H NMR and UV-vis spectroscopy measurements. The compounds revealed high antiproliferative activity in vitro in the breast cancer cell line MDA-MB-231 and hepatocellular carcinoma cell line LM3 in the low micromolar to nanomolar concentration range. Important structure-activity relationships were deduced from the comparison of anticancer activities of HL¹-HL⁴ and 1-4 with those of structurally similar paullone-derived (HL⁵-HL⁷ and 5-7) and latonduine-derived scaffolds (HL⁸-HL¹¹ and 8-11). The high anticancer activity of the lead drug candidate 4 was linked to reactive oxygen species and endoplasmic reticulum stress induction, which were confirmed by fluorescent microscopy and Western blot analysis.

Enantioselective Cytotoxicity of Chiral Diphosphine Ruthenium(II) Complexes Against Cancer Cells

The chiral cationic complex [Ru(η1 -OAc)(CO)((R,R)-Skewphos)(phen)]OAc (2R ), isolated from reaction of [Ru(η1 -OAc)(η2 -OAc)(R,R)-Skewphos)(CO)] (1R ) with phen, reacts with NaOPiv and KSAc affording [RuX(CO)((R,R)-Skewphos)(phen)]Y (X=Y=OPiv 3R ; X=SAc, Y=OAc 4R ). The corresponding enantiomers 2S -4S have been obtained from 1S containing (S,S)-Skewphos. Reaction of 2R and 2S with (S)-cysteine and NaPF6 at pH=9 gives the diastereoisomers [Ru((S)-Cys)(CO)(PP)(phen)]PF6 (PP=(R,R)-Skewphos 2R -Cys; (S,S)-Skewphos 2S -Cys). The DFT energetic profile for 2R with (S)-cysteine in H2 O indicates that aquo and hydroxo species are involved in formation of 2R -Cys. The stability of the ruthenium complexes in 0.9 % w/v NaCl solution, PBS and complete DMEM medium, as well as their n-octanol/water partition coefficient (logP), have been evaluated. The chiral complexes show high cytotoxic activity against SW1736, 8505 C, HCT-116 and A549 cell lines with EC50 values of 2.8-0.04 μM. The (R,R)-Skewphos derivatives show higher cytotoxicity compared to their enantiomers, 4R (EC50 =0.04 μM) being 14 times more cytotoxic than 4S against the anaplastic thyroid cancer 8505 C cell line.

Single-Cell Quantification of a Highly Biocompatible Dinuclear Iridium(III) Complex for Photocatalytic Cancer Therapy

Quantifying the content of metal-based anticancer drugs within single cancer cells remains a challenge. Here, we used single-cell inductively coupled plasma mass spectrometry to study the uptake and retention of mononuclear (Ir1) and dinuclear (Ir2) Ir^III photoredox catalysts. This method allowed rapid and precise quantification of the drug in individual cancer cells. Importantly, Ir2 showed a significant synergism but not an additive effect for NAD(P)H photocatalytic oxidation. The lysosome-targeting Ir2 showed low dark toxicity in vitro and in vivo. Ir2 exhibited high photocatalytic therapeutic efficiency at 525 nm with an excellent photo-index in vitro and in tumor-bearing mice model. Interestingly, the photocatalytic anticancer profile of the dinuclear Ir2 was much better than the mononuclear Ir1, indicating for the first time that dinuclear metal-based photocatalysts can be applied for photocatalytic anticancer treatment.

Hydrodechlorination of Aryl Chlorides Under Biocompatible Conditions

Developing nonenzymatic chemistry that is nontoxic to microbial organisms creates the potential to integrate synthetic chemistry with metabolism and offers new remediation strategies. Chlorinated organic compounds known to bioaccumulate and cause harmful environmental impact can be converted into less damaging derivatives through hydrodehalogenation. The hydrodechlorination of substituted aryl chlorides using Pd/C and ammonium formate in biological media under physiological conditions (neutral pH, moderate temperature, and ambient pressure) is reported. The reaction conditions were successful for a range of aryl chlorides with electron-donating and -withdrawing groups, limited by the solubility of substrates in aqueous media. Soluble substrates gave good yields (60-98%) of the reduction product within 48 h. The relative toxicities of each reaction component were tested separately and together against bacteria, and the reaction proceeded in bacterial cultures containing an aryl chloride with robust cell growth. This work offers an initial step toward the removal of aryl chlorides from waste streams that currently use bacterial degradation to remove pollutants.

Os(II) complexes for catalytic anticancer therapy: recent update

The recent dramatic enhancement in cancer-related mortality and the drawbacks (side effects and resistance) of Pt-based first-generation chemotherapeutics have escalated the need for new cancer medicines with unique anticancer activities for better human life. To overcome the demerits of Pt-based cancer drugs, the concept of catalytic anticancer agents has recently been presented in the field of anticancer metallodrug development research. Many intracellular transformations in cancer cells are catalyzed by metal complexes, including pyruvate reduction to lactate, NAD(P)⁺ reduction to NAD(P)H and vice versa, and the conversion of ³O₂ to reactive oxygen species (ROS). These artificial in-cell changes with non-toxic and catalytic dosages of metal complexes have been shown to disrupt several essential intracellular processes which ultimately cause cell death. This new approach could develop potent next-generation catalytic anticancer drugs. In this context, recently, several 16/18 electron Os(II)-based complexes have shown promising catalytic anticancer activities with unique anticancer mechanisms. Herein, we have delineated the catalytic anticancer activity of Os(II) complexes from a critical viewpoint. These catalysts are reported to induce the in-cell catalytic transfer hydrogenation of pyruvate and important quinones to create metabolic disorder and photocatalytic ROS generation for oxidative stress generation in cancer cells. Overall, these Os(II) catalysts have the potential to be novel catalytic cancer drugs with new anticancer mechanisms.

New Triazolyl N^N Bidentate Rh(III), Ir(III), Ru(II) and Os(II) Complexes: Synthesis and Characterization, Probing Possible Relations between Cytotoxicity with Transfer Hydrogenation Efficacy and Interaction with Model Biomolecules

Cisplatin and other metallodrugs have realised great success in clinical chemotherapeutic applications as anticancer drugs. However, severe toxicity to healthy cells and non-selectivity to cancer cells remains a challenge, warranting the further search for alternative agents. Herein, we report the anticancer potential of a series of complexes of the general formula [MCl(p-cym)(k2-N^N-L)]+ X− and [MCl(Cp*)(k2-N^N-L)]+ X−, where M is the metal centre (Ru(II), Os(II), Rh(III) or Ir(III)), L = 1-benzyl-4-pyridinyl-1-H-1,2,3-triazole for L1 and 1-picolyl-4-pyridinyl-1-H-1,2,3-triazole for L2 and X− = Cl−, BF4−, BPh4−. When evaluated for activity against some cancerous and non-cancerous cell lines (namely, HeLa, HEK293, A549 and MT4 cancer cells and the normal healthy kidney cells (BHK21)), most of the compounds displayed poor cytotoxicities against cancer cells except for complexes C2 ([RuCl(p-cym)(k2-N^N-L1)]+ BPh4−, EC50 = 9−16 µM and SI = 14), C7 ([RuCl(p-cym)(k2-N^N-L2)]+ BPh4−, EC50 = 17−53 µM and SI = 4) and C11 ([IrCl(Cp*)(k2-N^N-L2)]+ BF4−, EC50 < 5 µM and SI > 10). Selected complexes C1 ([RuCl(p-cym)(k2-N^N-L1)]+ BF4−), C5 ([IrCl(Cp*)(k2-N^N-L1)]+ BF4−) and C11 showed significant interactions with model biomolecules such as guanosine-5′-monophosphate (5′-GMP), bovine serum albumin (BSA) and amino acids under physiological conditions, possibly through carbenylation and N-coordination with 5′-GMP, N-coordination with L-Histidine and L-proline. While the compounds showed good activities in reducing pyruvate to lactate, there was no direct correlation between catalytic transfer hydrogenation of pyruvate and the observed cytotoxic activities. As observed in this work, the marked influence of single atom replacement in ligand may provide a pivotal approach to improving the cytotoxicity and fine-tuning the selectivity to cancer cells.

Density functional theory investigation of Ru(II) and Os(II) asymmetric transfer hydrogenation catalysts

Transition metal ions have a unique ability to organise and control the steric and electronic effects around a substrate in the active site of a catalyst. We consider half-sandwich Ru(II) (Noyori-type) and Os(II) sulfonyldiamine 16-electron active catalysts [Ru/Os(η⁶-p-cymene)(TsDPEN-H₂)], where TsDPEN is N-tosyl-1,2-diphenylethylenediamine containing S,S or R,R chiral centres, which catalyse the highly efficient asymmetric transfer hydrogenation of aromatic ketones to chiral alcohols using formic acid as a hydride source. We discuss the recognition of the prochiral ketone acetophenone by the catalyst, the protonation of a ligand NH and transfer of hydride from formate to the metal, subsequent transfer of hydride to one enantiotopic face of the ketone, followed by proton transfer from metal-bound NH₂, and regeneration of the catalyst. Our DFT calculations illustrate the role of the two chiral carbons on the N,N-chelated sulfonyldiamine ligand, the axial chirality of the π-bonded p-cymene arene, and the chirality of the metal centre. We discuss new features of the mechanism, including how a change in metal chirality of the hydride intermediate dramatically switches p-cymene coordination from η⁶ to η². Moreover, the calculations suggest a step-wise mechanism involving substrate docking to the bound amine NH₂ followed by hydride transfer prior to protonation of the O-atom of acetophenone and release of the enantio-pure alcohol. This implies that formation and stability of the M-H hydride intermediate is highly dependent on the presence of the protonated amine ligand. The Os(II) catalyst is more stable than the Ru(II) analogue, and these studies illustrate the subtle differences in mechanistic behaviour between these 4d⁶ and 5d⁶ second-row and third-row transition metal congeners in group 8 of the periodic table.

Photocatalytic glucose-appended bio-compatible Ir(III) anticancer complexes

Rationally-designed glucose-appended Ir(III) photo-catalysts ([Ir(N,C)2(N,N-Glc)]+, Ir1-Ir3) show visible light-induced catalytic NAD(P)H oxidation in aqueous solution. Highly in-vivo biocompatible complex, Ir3, shows lysosome and mitochondria targeting necro-apoptotic photo-cytotoxicity against various cancer...

Efficient transfer hydrogenation of ketones by molybdenum complexes through comprehensively verifying auxiliary ligands

Molybdenum complexes, ligated with N1,N1-dialkyl-N2-(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines along with auxiliary ligands, provide various structural features as [NNH/NNHN]Mo(CO)4/3 (Mo1 – Mo3), [NNHN]Mo(CO)2Br] (Mo4 – Mo5), [NNH]Mo(CO)(η3-C3H5)Br](Mo6) and [NNHN/S] Mo(CO)(PPh3)2] (Mo7 – Mo8). All...

Metal substrate catalysis in the confined space for platinum drug delivery

Catalysis-based approaches for the activation of anticancer agents hold considerable promise. These principally rely on the use of metal catalysts capable of deprotecting inactive precursors of organic drugs or transforming key biomolecules available in the cellular environment. Nevertheless, the efficiency of most of the schemes described so far is rather low, limiting the benefits of catalytic amplification as strategy for controlling the therapeutic effects of anticancer compounds. In the work presented here, we show that flavin reactivity within a hydrogel matrix provides a viable solution for the efficient catalytic activation and delivery of cisplatin, a worldwide clinically-approved inorganic chemotherapy agent. This is achieved by ionically adsorbing a flavin catalyst and a Pt(iv) prodrug as substrate into porous amino-functionalized agarose beads. The hydrogel chassis supplies high local concentrations of electron donating groups/molecules in the surrounding of the catalyst, ultimately boosting substrate conversion rates (TOF >200 min-1) and enabling controlled liberation of the drug by light or chemical stimuli. Overall, this approach can afford platforms for the efficient delivery of platinum drugs as demonstrated herein by using a transdermal diffusion model simulating the human skin.

NMR studies of group 8 metallodrugs: 187Os-enriched organo-osmium half-sandwich anticancer complex

We report the synthesis of the organo-osmium anticancer complex [Os(η⁶-p-cym)(N,N-azpy-NMe₂)Br]PF₆ (1) containing natural abundance ¹⁸⁷Os (1.96%), and isotopically-enriched (98%) [¹⁸⁷Os]-1. Complex 1 and [¹⁸⁷Os]-1 contain a π-bonded para-cymene (p-cym), a chelated 4-(2-pyridylazo)-N,N-dimethylaniline (azpy-NMe₂), and a monodentate bromide as ligands. The X-ray crystal structure of 1 confirmed its half-sandwich 'piano-stool' configuration. Complex 1 is a member of a family of potent anticancer complexes, and exhibits sub-micromolar activity against A2780 human ovarian cancer cells (IC₅₀ = 0.40 μM). Complex [¹⁸⁷Os]-1 was analysed by high-resolution ESI-MS, 1D ¹H and ¹³C NMR, and 2D ¹H COSY, ¹³C-¹H HMQC, and ¹H-¹⁸⁷Os HMBC NMR spectroscopy. Couplings of ¹H and ¹³C nuclei from the azpy/p-cym ligands to ¹⁸⁷Os were observed with J-couplings (¹J to ⁴J) ranging between 0.6-8.0 Hz. The ¹⁸⁷Os chemical shift of [¹⁸⁷Os]-1 (-4671.3 ppm, determined by 2D ¹H-¹⁸⁷Os HMBC NMR) is discussed in relation to the range of values reported for related Os(II) arene and cyclopentadienyl complexes (-2000 to -5200 ppm).

Photoactivated Osmium Arene Anticancer Complexes

Half-sandwich Os-arene complexes exhibit promising anticancer activity, but their photochemistry has hardly been explored. To exploit the photocytotoxicity and photochemistry of Os-arenes, O,O-chelated complexes [Os(η⁶-p-cymene)(Curc)Cl] (OsCUR-1, Curc = curcumin) and [Os(η⁶-biphenyl)(Curc)Cl] (OsCUR-2), and N,N-chelated complexes [Os(η⁶-biphenyl)(dpq)I]PF₆ (OsDPQ-2, dpq = pyrazino[2,3-f][1,10]phenanthroline) and [Os(η⁶-biphenyl)(bpy)I]PF₆ (OsBPY-2, bpy = 2,2'-bipyridine), have been investigated. The Os-arene curcumin complexes showed remarkable photocytotoxicity toward a range of cancer cell lines (blue light IC₅₀: 2.6-5.8 μM, photocytotoxicity index PI = 23-34), especially toward cisplatin-resistant cancer cells, but were nontoxic to normal cells. They localized mainly in mitochondria in the dark but translocated to the nucleus upon photoirradiation, generating DNA and mitochondrial damage, which might contribute toward overcoming cisplatin resistance. Mitochondrial damage, apoptosis, ROS generation, DNA damage, angiogenesis inhibition, and colony formation were observed when A549 lung cancer cells were treated with OsCUR-2. The photochemistry of these Os-arene complexes was investigated by a combination of NMR, HPLC-MS, high energy resolution fluorescence detected (HERFD), X-ray adsorption near edge structure (XANES) spectroscopy, total fluorescence yield (TFY) XANES spectra, and theoretical computation. Selective photodissociation of the arene ligand and oxidation of Os(II) to Os(III) occurred under blue light or UVA excitation. This new approach to the design of novel Os-arene complexes as phototherapeutic agents suggests that the novel curcumin complex OsCUR-2, in particular, is a potential candidate for further development as a photosensitizer for anticancer photoactivated chemotherapy (PACT).

In situ activation of therapeutics through bioorthogonal catalysis

Bioorthogonal chemistry refers to any chemical reactions that can occur inside of living systems without interfering with native biochemical processes, which has become a promising strategy for modulating biological processes. The development of synthetic metal-based catalysts to perform bioorthogonal reactions has significantly expanded the toolkit of bioorthogonal chemistry for medicinal chemistry and synthetic biology. A wide range of homogeneous and heterogeneous transition metal catalysts (TMCs) have been reported, mediating different transformations such as cycloaddition reactions, as well as bond forming and cleaving reactions. However, the direct application of 'naked' TMCs in complex biological media poses numerous challenges, including poor water solubility, toxicity and catalyst deactivation. Incorporating TMCs into nanomaterials to create bioorthogonal nanocatalysts can solubilize and stabilize catalyst molecules, with the decoration of the nanocatalysts used to provide spatiotemporal control of catalysis. This review presents an overview of the advances in the creation of bioorthogonal nanocatalysts, highlighting different choice of nano-scaffolds, and the therapeutic and diagnostic applications.

Bioorthogonal catalytic patch

Bioorthogonal catalysis mediated by transition metals has inspired a new subfield of artificial chemistry complementary to enzymatic reactions, enabling the selective labelling of biomolecules or in situ synthesis of bioactive agents via non-natural processes. However, the effective deployment of bioorthogonal catalysis in vivo remains challenging, mired by the safety concerns of metal toxicity or complicated procedures to administer catalysts. Here, we describe a bioorthogonal catalytic device comprising a microneedle array patch integrated with Pd nanoparticles deposited on TiO₂ nanosheets. This device is robust and removable, and can mediate the local conversion of caged substrates into their active states in high-level living systems. In particular, we show that such a patch can promote the activation of a prodrug at subcutaneous tumour sites, restoring its parent drug's therapeutic anticancer properties. This in situ applied device potentiates local treatment efficacy and eliminates off-target prodrug activation and dose-dependent side effects in healthy organs or distant tissues.

Bioorthogonal strategies for the in vivo synthesis or release of drugs

The site-specific delivery of antitumor agents is a rapidly developing field that relies on prodrug activation and uncaging strategies. For this purpose, a wide range of homogeneous and heterogeneous biocompatible activators/catalysts have been developed to convert caged drugs with low toxicity and high stability in physiological settings into active substances in a bioorthogonal manner. The current methods allow for the site-specific delivery of activators and prodrugs to organelles, target cells, or tumors in living organisms. Here, we present an overview of the latest advances in catalytic drugs, highlighting the expanding toolbox of bioorthogonal activation strategies made possible by transition metals acting as activators or catalysts.