Nano-palladium is a cellular catalyst for in vivo chemistry

Dissociative Bioorthogonal Reactions

Bioorthogonal reactions that proceed readily under physiological conditions without interference from biomolecules have found widespread application in the life sciences. Complementary to the bioorthogonal reactions that ligate two molecules, reactions that release a molecule or cleave a linker are increasingly attracting interest. Such dissociative bioorthogonal reactions have a broad spectrum of uses, for example, in controlling bio-macromolecule activity, in drug delivery, and in diagnostic assays. This review article summarizes the developed bioorthogonal reactions linked to a release step, outlines representative areas of the applications of such reactions, and discusses aspects that require further improvement.

A Biocompatible Heterogeneous MOF-Cu Catalyst for In Vivo Drug Synthesis in Targeted Subcellular Organelles

As a typical bioorthogonal reaction, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) has been used for drug design and synthesis. However, for localized drug synthesis, it is important to be able to determine where the CuAAC reaction occurs in living cells. In this study, we constructed a heterogeneous copper catalyst on a metal-organic framework that could preferentially accumulate in the mitochondria of living cells. Our system enabled the localized synthesis of drugs through a site-specific CuAAC reaction in mitochondria with good biocompatibility. Importantly, the subcellular catalytic process for localized drug synthesis avoided the problems of the delivery and distribution of toxic molecules. In vivo tumor therapy experiments indicated that the localized synthesis of resveratrol-derived drugs led to greater antitumor efficacy and minimized side effects usually associated with drug delivery and distribution.

Ruthenium-Catalyzed Redox Isomerizations inside Living Cells

Tailored ruthenium(IV) complexes can catalyze the isomerization of allylic alcohols into saturated carbonyl derivatives under physiologically relevant conditions, and even inside living mammalian cells. The reaction, which involves ruthenium-hydride intermediates, is bioorthogonal and biocompatible, and can be used for the "in cellulo" generation of fluorescent and bioactive probes. Overall, our research reveals a novel metal-based tool for cellular intervention, and comes to further demonstrate the compatibility of organometallic mechanisms with the complex environment of cells.

Bioorthogonal nanozymes: progress towards therapeutic applications

Bioorthogonal nanocatalysts in the form of 'nanozymes', are promising tools for generating imaging and therapeutic molecules in living systems. These systems use transformations developed by synthetic chemists to effect transformations that cannot be performed by cellular machinery. This emerging platform is rapidly evolving towards the creation of smart nanodevices featuring the capabilities of their enzyme prototypes, modulating catalytic activity through structure as well as chemical and physical signals. Here we describe different strategies to fabricate these nanocatalysts and their potential in diagnostic and therapeutic applications.

Hollow nanoreactors for Pd-catalyzed Suzuki-Miyaura coupling and O-propargyl cleavage reactions in bio-relevant aqueous media

We describe the fabrication of hollow microspheres consisting of mesoporous silica nanoshells decorated with an inner layer of palladium nanoparticles and their use as Pd-nanoreactors in aqueous media. These palladium-equipped capsules can be used to promote the uncaging of propargyl-protected phenols, as well as Suzuki-Miyaura cross-coupling, in water and at physiologically compatible temperatures. Importantly, the depropargylation reaction can be accomplished in a bioorthogonal manner in the presence of relatively high concentrations of biomolecular components and even in the presence of mammalian cells.

Therapeutic Polymersome Nanoreactors with Tumor-Specific Activable Cascade Reactions for Cooperative Cancer Therapy

Therapeutic nanoreactors are of increasing interest in precise cancer therapy, which have been explored to in situ produce therapeutic compounds from inert prodrugs or intrinsic molecules at the target sites. However, engineering a nanoreactor with tumor activable cascade reactions for efficient cooperative cancer therapy remains a great challenge. Herein, we demonstrate a polymersome nanoreactor with tumor acidity-responsive membrane permeability to activate cascade reactions for orchestrated cooperative cancer treatment. The nanoreactors are constructed from responsive polyprodrug polymersomes incorporating ultrasmall iron oxide nanoparticles and glucose oxidase in the membranes and inner aqueous cavities, respectively. The cascade reactions including glucose consumption to generate H₂O₂, accelerated iron ion release, Fenton reaction between H₂O₂ and iron ion to produce hydroxyl radicals (•OH), and •OH-triggered rapid release of parent drugs can be specifically activated by the tumor acidity-responsive membrane permeability. During this process, the orchestrated cooperative cancer therapy including starving therapy, chemodynamic therapy, and chemotherapy is realized for high-efficiency tumor suppression by the in situ consumed and produced compounds. The nanoreactor design with tumor-activable cascade reactions represents an insightful paradigm for precise cooperative cancer therapy.

Intracellular Catalysis with Selected Metal Complexes and Metallic Nanoparticles: Advances toward the Development of Catalytic Metallodrugs

Platinum-containing drugs (e.g., cisplatin) are among the most frequently used chemotherapeutic agents. Their tremendous success has spurred research and development of other metal-based drugs, with notable achievements. Generally, the vast majority of metal-based drug candidates in clinical and developmental stages are stoichiometric agents, i.e., each metal complex reacts only once with their biological target. Additionally, many of these metal complexes are involved in side reactions, which not only reduce the effective amount of the drug but may also cause toxicity. On a separate note, transition metal complexes and nanoparticles have a well-established history of being potent catalysts for selective molecular transformations, with examples such as the Mo- and Ru-based catalysts for metathesis reactions (Nobel Prize in 2005) or palladium catalysts for C-C bond forming reactions such as Heck, Negishi, or Suzuki reactions (Nobel Prize in 2010). Also, notably, no direct biological equivalent of these transformations exists in a biological environment such as bacteria or mammalian cells. It is, therefore, only logical that recent interest has focused on developing transition-metal based catalytic systems that are capable of performing transformations inside cells, with the aim of inducing medicinally relevant cellular changes. Because unlike in stoichiometric reactions, a catalytically active compound may turn over many substrate molecules, only very small amounts of such a catalytic metallodrug are required to achieve a desired pharmacologic effect, and therefore, toxicity and side reactions are reduced. Furthermore, performing catalytic reactions in biological systems also opens the door for new methodologies to study the behavior of biomolecules in their natural state, e.g., via in situ labeling or by increasing/depleting their concentration at will. There is, of course, an art to the choice of catalysts and reactions which have to be compatible with biological conditions, namely an aqueous, oxygen-containing environment. In this review, we aim to describe new developments that bring together the far-distant worlds of transition-metal based catalysis and metal-based drugs, in what is termed "catalytic metallodrugs". Here we will focus on transformations that have been performed on small biomolecules (such as shifting equilibria like in the NAD⁺/NADH or GSH/GSSG couples), on non-natural molecules such as dyes for imaging purposes, or on biomacromolecules such as proteins. Neither reactions involving release (e.g., CO) or transformation of small molecules (e.g., ¹O₂ production), degradation of biomolecules such as proteins, RNA or DNA nor light-induced medicinal chemistry (e.g., photodynamic therapy) are covered, even if metal complexes are centrally involved in those. In each section, we describe the (inorganic) chemistry involved, as well as selected examples of biological applications in the hope that this snapshot of a new but quickly developing field will indeed inspire novel research and unprecedented interactions across disciplinary boundaries.

Click and release: bioorthogonal approaches to “on-demand” activation of prodrugs

This review summarizes recent developments in using bioorthogonal chemistry in prodrug design for the delivery of traditional small molecule- and gasotransmitter-based therapeutics.

Design and manufacture of functional catalyst-carrier structures for the bioorthogonal activation of anticancer agents

Achieving the activation of a latent prodrug via bio-orthogonal chemistry on the catalytic surface of a tailored Ti-[Pd] device.

Highly biocompatible jujube polysaccharide-stabilized palladium nanoparticles with excellent catalytic performance

Jujube polysaccharide-stabilized palladium nanoparticles provide active sites for efficient catalysis of 4-nitrophenol to 4-aminophenol.

Solid-phase synthesis of biocompatible N-heterocyclic carbene–Pd catalysts using a sub-monomer approach

Biocompatible N-heterocyclic carbene (NHC) based Pd ligands were assembled and loaded with Pd via solid-phase synthesis.

Modular Nanoparticulate Prodrug Design Enables Efficient Treatment of Solid Tumors Using Bioorthogonal Activation

Prodrug strategies that facilitate localized and controlled activity of small-molecule therapeutics can reduce systemic exposure and improve pharmacokinetics, yet limitations in activation chemistry have made it difficult to assign tunable multifunctionality to prodrugs. Here, we present the design and application of a modular small-molecule caging strategy that couples bioorthogonal cleavage with a self-immolative linker and an aliphatic anchor. This strategy leverages recently discovered in vivo catalysis by a nanoencapsulated palladium compound (Pd-NP), which mediates alloxylcarbamate cleavage and triggers release of the activated drug. The aliphatic anchor enables >90% nanoencapsulation efficiency of the prodrug, while also allowing >10⁴-fold increased cytotoxicity upon prodrug activation. We apply the strategy to a prodrug formulation of monomethyl auristatin E (MMAE), demonstrating its ability to target microtubules and kill cancer cells only after selective activation by Pd-NP. Computational pharmacokinetic modeling provides a mechanistic basis for the observation that the nanotherapeutic prodrug strategy can lead to more selective activation in the tumor, yet in a manner that is more sensitive to variable enhanced permeability and retention (EPR) effects. Combination treatment with the nanoencapsulated MMAE prodrug and Pd-NP safely blocks tumor growth, especially when combined with a local radiation therapy regimen that is known to improve EPR effects, and represents a conceptual step forward in prodrug design.

Synergistic enzymatic and bioorthogonal reactions for selective prodrug activation in living systems

Adverse drug reactions (ADRs) restrict the maximum doses applicable in chemotherapy, which leads to failure in cancer treatment. Various approaches, including nano-drug and prodrug strategies aimed at reducing ADRs, have been developed, but these strategies have their own pitfalls. A renovated strategy for ADR reduction is urgently needed. Here, we employ an enzymatic supramolecular self-assembly process to accumulate a bioorthogonal decaging reaction trigger inside targeted cancer cells, enabling spatiotemporally controlled, synergistic prodrug activation. The bioorthogonally activated prodrug exhibits significantly enhanced potency against cancer cells compared with normal cells. This prodrug activation strategy further demonstrates high tumour inhibition efficacy with satisfactory biocompatibility, pharmacokinetics, and safety in vivo. We envision that integration of enzymatic and bioorthogonal reactions will serve as a general small-molecule-based strategy for alleviation of ADRs in chemotherapy.

Bioorthogonal Uncaging of the Active Metabolite of Irinotecan by Palladium-Functionalized Microdevices

SN-38, the active metabolite of irinotecan, is released upon liver hydrolysis to mediate potent antitumor activity. Systemic exposure to SN-38, however, also leads to serious side effects. To reduce systemic toxicity by controlling where and when SN-38 is generated, a new prodrug was specifically designed to be metabolically stable and undergo rapid palladium-mediated activation. Blocking the phenolic OH of SN-38 with a 2,6-bis(propargyloxy)benzyl group led to significant reduction of cytotoxic activity (up to 44-fold). Anticancer properties were swiftly restored in the presence of heterogeneous palladium (Pd) catalysts to kill colorectal cancer and glioma cells, proving the efficacy of this novel masking strategy for aromatic hydroxyls. Combination with a Pd-activated 5FU prodrug augmented the antiproliferative potency of the treatment, while displaying no activity in the absence of the Pd source, which illustrates the benefit of achieving controlled release of multiple approved therapeutics-sequentially or simultaneously-by the same bioorthogonal catalyst to increase anticancer activity.

Bright insights into palladium-triggered local chemotherapy

The incorporation of transition metal catalysts to the bioorthogonal toolbox has opened the possibility of producing supra-stoichiometric amounts of xenobiotics in living systems in a non-enzymatic fashion. For medical use, such metals could be embedded in implantable devices (i.e. heterogeneous catalyst) to "synthesize" drugs in desired locations (e.g. in a tumour) with high specificity and for extended periods of time, overcoming the useful life limitations of current local therapy modalities directed to specific organ sites (e.g. brachytherapy, controlled release systems). To translate this approach into a bona fide therapeutic option, it is essential to develop clinically-accessible implantation procedures and to understand and validate the activation process in relevant preclinical models. Herein we report the development of a novel Pd-activatable precursor of the red-fluorescent drug doxorubicin and Pd devices of optimized size and activity. Screening in state-of-the-art cancer models provided fundamental insights into the insertion protocols, safety and stability of the devices and into the prodrug distribution profile before and after activation.

A preclinical evaluation of cytarabine prodrug nanofibers assembled from cytarabine-lauric acid conjugate toward solid tumors

Cytarabine (Ara-C) has become cornerstones for the treatment of hatmatological malignancies for several decades; however, it still faces serious challenges in clinical applications due to its side effects such as hand foot syndrome (HFS) and stomatitis. Therefore, considerable researchers have devoted to looking for the new derivative with desirable activity and low toxicity. A new prodrug based on the conjugation of cytarabine with lauric acid (LA-Ara) was synthesized in our group, and it could self-assemble into nanofibers (NFs) in aqueous solution with high drug loading (57 wt%). The lauric acid moiety protects NH₂ group of from the enzymatic attachment and simultaneously raises the lipophilicity of Ara-C, thus obviously prolongs its plasma half-life. The oil/water partition coefficient (lg P) and the permeability of cell membrane of LA-Ara were obviously increased compared with Ara-C. Furthermore, the in vitro gastrointestinal stability results indicated the prodrug was suitable to be administrated orally. In the current study, the in vitro cytotoxicity and in vivo anti breast cancer experimental results indicate LA-Ara markedly improved antitumor activity compared with free Ara-C. The favorable safety evaluations elucidated its potentiality for oral alternative treatment to Ara-C. Importantly, LA-Ara can effectively decrease the incidence of toxic effects (HFS and stomatitis) of Ara-C, thereby exhibiting favorable skin safety profile. Overall, these results indicated the LA-Ara would be an excellent candidate for further clinical investigation and simultaneously highlight the prospects of Ara-C prodrug strategies in solid tumors therapy.

Intracellular Deprotection Reactions Mediated by Palladium Complexes Equipped with Designed Phosphine Ligands

Discrete palladium(II) complexes featuring purposely designed phosphine ligands can promote depropargylation and deallylation reactions in cell lysates. These complexes perform better than other palladium sources, which apparently are rapidly deactivated in such hostile complex media. This good balance between reactivity and stability allows the use of these discrete phosphine palladium complexes in living mammalian cells, whereby they can mediate similar transformations. The presence of a phosphine ligand in the coordination sphere of palladium also provides for the introduction of targeting groups, such as hydrophobic phosphonium moieties, which facilitate the accumulation of the complexes in mitochondria.

A cell-penetrating artificial metalloenzyme regulates a gene switch in a designer mammalian cell

Complementing enzymes in their native environment with either homogeneous or heterogeneous catalysts is challenging due to the sea of functionalities present within a cell. To supplement these efforts, artificial metalloenzymes are drawing attention as they combine attractive features of both homogeneous catalysts and enzymes. Herein we show that such hybrid catalysts consisting of a metal cofactor, a cell-penetrating module, and a protein scaffold are taken up into HEK-293T cells where they catalyze the uncaging of a hormone. This bioorthogonal reaction causes the upregulation of a gene circuit, which in turn leads to the expression of a nanoluc-luciferase. Relying on the biotin-streptavidin technology, variation of the biotinylated ruthenium complex: the biotinylated cell-penetrating poly(disulfide) ratio can be combined with point mutations on streptavidin to optimize the catalytic uncaging of an allyl-carbamate-protected thyroid hormone triiodothyronine. These results demonstrate that artificial metalloenzymes offer highly modular tools to perform bioorthogonal catalysis in live HEK cells.

A thioether-directed palladium-cleavable linker for targeted bioorthogonal drug decaging

We describe the development of a bifunctional linker that simultaneously allows site-specific protein modification and palladium-mediated bioorthogonal decaging.

We describe the development of a bifunctional linker that simultaneously allows site-specific protein modification and palladium-mediated bioorthogonal decaging. This was enabled by a thioether binding motif in the propargyl carbamate linker and a readily available palladium complex. We demonstrate the efficiency of this reaction by controlled drug release from a PEGylated doxorubicin prodrug in cancer cells. The linker can be easily installed into cysteine bearing proteins which we demonstrated for the construction of an anti-HER2 nanobody–drug conjugate. Targeted delivery of the nanobody drug conjugate showed effective cell killing in HER2+ cells upon palladium-mediated decaging.

Catalytically Active Single-Chain Polymeric Nanoparticles: Exploring Their Functions in Complex Biological Media

Dynamic single-chain polymeric nanoparticles (SCPNs) are intriguing, bioinspired architectures that result from the collapse or folding of an individual polymer chain into a nanometer-sized particle. Here we present a detailed biophysical study on the behavior of dynamic SCPNs in living cells and an evaluation of their catalytic functionality in such a complex medium. We first developed a number of delivery strategies that allowed the selective localization of SCPNs in different cellular compartments. Live/dead tests showed that the SCPNs were not toxic to cells while spectral imaging revealed that SCPNs provide a structural shielding and reduced the influence from the outer biological media. The ability of SCPNs to act as catalysts in biological media was first assessed by investigating their potential for reactive oxygen species generation. With porphyrins covalently attached to the SCPNs, singlet oxygen was generated upon irradiation with light, inducing spatially controlled cell death. In addition, Cu(I)- and Pd(II)-based SCPNs were prepared and these catalysts were screened in vitro and studied in cellular environments for the carbamate cleavage reaction of rhodamine-based substrates. This is a model reaction for the uncaging of bioactive compounds such as cytotoxic drugs for catalysis-based cancer therapy. We observed that the rate of the deprotection depends on both the organometallic catalysts and the nature of the protective group. The rate reduces from in vitro to the biological environment, indicating a strong influence of biomolecules on catalyst performance. The Cu(I)-based SCPNs in combination with the dimethylpropargyloxycarbonyl protective group showed the best performances both in vitro and in biological environment, making this group promising in biomedical applications.

In vivo catalyzed new-to-nature reactions

Bioorthogonal chemistry largely relies on the use of abiotic metals to catalyze new-to-nature reactions in living systems. Over the past decade, metal complexes and metal-encapsulated systems such as nanoparticles have been developed to unravel the reactivity of transition metals, including ruthenium, palladium, iridium, copper, iron, and gold in biological systems. Thanks to these remarkable achievements, abiotic catalysts are able to fluorescently label cells, uncage or form cytotoxic drugs and activate enzymes in cellulo/vivo. Recently, strategies for the delivery of such catalysts to specific cell types, cell compartments or proteins were established. These studies reveal the enormous potential of this emerging field and its application in both medicinal chemistry and in synthetic biology.

Highly stable and biocompatible zwitterionic dendrimer-encapsulated palladium nanoparticles that maintain their catalytic activity in bacterial solution

This study offers a method for constructing an artificial enzyme (Pd_n-G5MC), which maintains its catalytic efficiency in bacterial solution.

Polymer Prodrug-Based Nanoreactors Activated by Tumor Acidity for Orchestrated Oxidation/Chemotherapy

Therapeutic nanoreactors have been proposed to treat cancers through in situ transformation of low-toxicity prodrugs into toxic therapeutics in the body. However, the in vivo applications are limited by low tissue-specificity and different tissue distributions between sequentially injected nanoreactors and prodrugs. Herein, we construct a block copolymer prodrug-based polymersome nanoreactor that can achieve novel orchestrated oxidation/chemotherapy of cancer via specific activation at tumor sites. The block copolymers composed of poly(ethylene glycol) (PEG) and copolymerized monomers of camptothecin (CPT) and piperidine-modified methacrylate [P(CPTMA-co-PEMA)] were optimized to self-assemble into polymersomes in aqueous solution for encapsulation of glucose oxidase (GOD) to obtain GOD-loaded polymersome nanoreactors (GOD@PCPT-NR). GOD@PCPT-NR maintained inactive in normal tissues upon systemic administration. After deposition in tumor tissues, tumor acidity-triggered protonation of PPEMA segments resulted in high permeability of the polymersome membranes and oxidation reaction of diffused glucose and O₂ under the catalysis of GOD. The activation of the reaction generated H₂O₂, improving the oxidative stress in tumors. Simultaneously, a high level of H₂O₂ further activated PCPTMA prodrugs, releasing active CPT drugs. High tumor oxidative stress and released CPT drugs synergistically killed cancer cells and suppressed tumor growth via oxidation/chemotherapy. Our study provides a new strategy for engineering therapeutic nanoreactors in an orchestrated fashion for cancer therapy.

Imaging of anticancer drug action in single cells

Imaging is widely used in anticancer drug development, typically for whole-body tracking of labelled drugs to different organs or to assess drug efficacy through volumetric measurements. However, increasing attention has been drawn to pharmacology at the single-cell level. Diverse cell types, including cancer-associated immune cells, physicochemical features of the tumour microenvironment and heterogeneous cell behaviour all affect drug delivery, response and resistance. This Review summarizes developments in the imaging of in vivo anticancer drug action, with a focus on microscopy approaches at the single-cell level and translational lessons for the clinic.