A narrative review: progress in transition metal-mediated bioorthogonal catalysis for the treatment of solid tumors

Introduction

Background

Bioorthogonal reactions were primarily proposed by Hang et al. [2003] (1). They are chemical reactions performed independently in living cells or tissues without eliciting toxicity. These kinds of reactions have no impact on the surrounding biotic system, and the various substances in the biological system do not interfere with them (Figure 1). The safety and independence of bioorthogonal chemistry make it widely used in repairing tissue (2), regulating biological function (3-5), drug activation (6,7), etc. In the treatment of cancer, they represent an alternative approach to dynamic drug delivery and extending the possibility of applying different drug delivery strategies (8). However, bioorthogonal reactions need to be activated by catalysts, which can be envisioned as in situ chemical “factories” to produce drugs and imaging agents at a highly controlled rate (9). This type of catalysis is a novel therapeutic strategy for targeted therapy to eliminate tumor cells using catalytic drugs in situ. Transition metals are widely used as catalysts for bioorthogonal reactions because of their strong oxidation-reduction properties, owing to their vacant d orbitals that can be used for bonding (Figure 2).

Figure 1 Classification of bioorthogonal catalytic reactions. CuAAC, copper-catalyzed azide-alkyne cycloaddition; SPAAC, strain-promoted azide-alkyne cycloaddition; IEDDA, inverse electron-demand diels-alder.

Figure 2 Transition metal (e.g., Pt, Au, Fe, Cu, Pd, Ru)-mediated bioorthogonal catalytic reactions. Pt, platinum; Au, gold; Fe, iron; Cu, copper; Pd, palladium; Ru, ruthenium.

Rationale and knowledge gap

However, transition metals have poor solubility in the biological environment, limited cell uptake, a lack of targeting ability, and with potential cytotoxicity (10,11); therefore, they cannot be widely used in living systems. These challenges may be overcome by combining transition metals with various nanomaterials. For example, transition metals coated with polymer resin, water gels, nanoenzyme (12,13), microspheres, organometric frames (14) or cell-derived vesicles can increase their biocompatibility and enhance the stability of transition metals in the biological environment. Moreover, transition metal-loaded nanomaterials can be modified to reach the targeted site to accurately and efficiently catalyze therapeutic drugs, reducing serious adverse reactions.

Objective

In recent years, transition metals have been developed and applied in biomedical fields such as enzyme design (15,16), biomolecular labeling (17), activation probes and drug release (18), artificial metal proteins (19) and organometrical complexes (20-23). An increasing number of studies were conducted on the bioorthogonal reaction-based treatment of cancer using transition metals, including Cu, Pd, Pt, Ru, and Au. This review focuses on the bioorthogonal catalytic reactions stimulated by several transition metals, systematically describes their research and development and applications in tumor therapy in vitro and in vivo, and discusses the prospects and challenges of using transition metals in drug development and tumor therapy. We present this article in accordance with the Narrative Review reporting checklist (available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-345/rc).

Methods

We searched literature in PubMed and Web of Science databases to find relevant studies on bioorthogonal reaction catalyzed by transition metals for tumor therapy. The keywords used were “cancer treatment”, “bioorthogonal reaction”, “transition metal”, “bioorthogonal catalysis”, “click chemistry”, “nanomaterials” and “biochemistry”. Only English literature was selected, and relevant references cited in the obtained articles were also retrieved. The search strategy is listed in Table 1.

Major transition metal-mediated bioorthogonal reactions

Palladium (Pd), platinum, gold-mediated depropargylation and dealkylation reactions

In recent years, Pd transition metal catalyzes the removal of groups through deprotection-, cleavage-, Suzuki-Miyaura reactions, and so on, and is involved in most bioorthogonal reactions (24-40). The most common bioorthogonal reactions catalyzed by Pd discovered in recent years are listed in Table 2 including depropargylation and dealkylation. Li et al. (24) were the first to use Pd to activate target proteins in their native cellular environment in 2014. They utilized a biocompatible Pd catalyst to catalyze propargyloxycarbonyl in vitro and in vivo and release (Proc)-“caged” lysine analogs. This analog can be incorporated into intracellular proteins in a gene- and site-specific manner to restore protein activity under physiological conditions. This review provides an experimental basis for using transition metals to manipulate proteins under survival conditions with the example of early Pd depropargylation to activate proteins. In the same year, Weiss et al. (25) hypothesized that Pd(0) could be used to catalyze the bioorthogonal organometallic (BOOM) reaction of allylcarbamate by inserting allyl-, propargyl-, and benzyl moieties at the N1 position of 5-fluorouracil (5FU) to reduce its pharmacological activity. The inactive prodrug was activated in cancer tissues by the BOOM reaction through the dealkylation of Pd⁰-resins following implantation into the tumor. Colon and pancreatic cancer cells in the Pro-5FU/Pd⁰-resin group showed bell-shaped growth curves and eventually induced cancer cell death. The Weiss research group also studied Pd-mediated bioorthogonal depropargylation for tumor treatment. Inspired by their previous work, they further developed the prodrug gemcitabine and confirmed the local activation of Pd⁰-resins and N-Poc-protected precursor drugs in the yolk sac of zebrafish embryos (26). Their study confirmed the ability of Pd to catalyze depropargylation and dealkylation reactions, and evaluated the pharmacodynamics and pharmacokinetics of amino-containing drugs.

Table 2

Examples of Pd-mediated related bioorthogonal catalytic reactions

Reactions	Main reaction	Catalyst	Nano materials	Organism/organelle
Depropargylation		Pd (0) (24,26,28,33,40); Pd (II/IV) (24)	Allyl2Pd2Cl2/Pd(dba)2; Pd0-resins; AS1411@Pd@UiO-66; nanoparticles; metallopeptide 2-Pd; AuPd NPs	Hela cells; BxPC-3 cells; Mia PaCa-2 cells; HCT116 cells; Zebrafish; A549 cells
Dealkylation		Pd (0) (25,27,29-32)	Pd0-resins; cRGDfE-PdNP; Pd-microdevices; Pd-exosomes; Lipo-Pd-pHCPT; CuS@PDA/Pd NP	HCT116 cells; BxPC-3 cells; U87-MG cells; A549 cells; MDA-MB-231 cells; Zebrafish; MCF-7 cells; Mice
Allylcarbamate cleavage		Pd (0) (34-37)	LM-Pd; Nano-Pd; Pd-NP; PT-MNs	A549 cells; HT1080 cells; B16-F10 cells; Mice
Propargyl oxy-benzyl cleavage		Pd (0) (38,39)	Pd0-resins; Pd-Exo	A549 cells; U87G cells; U87 cells
Suzuki-Miyaura		Pd (0) (29,40)	cRGDfE-PdNP	U87-MG cells

Pd, palladium; NP, nanoparticle; LM, liquid metal; PT-MNs, microneedle array patches made of PVA incorporated with Pd-TNSs (designated PT-MNs); PVA, polyvinyl alcohol.

Moreover, Pérez-López et al. tackled the problem of catalyst toxicity by exploring and developing a biocompatible catalytic system based on a hydrogel-immobilized Pd nanosheet (27). The Pd nanosheet-hydrogel frameworks minimized metal leakage, and the relatively dispersed Pd nanosheets ensured that the catalytic exposure area sustained efficient and rapid depropargylation reactions, which inhibited cancer cell proliferation through the controlled release of paclitaxel (PTX) in vitro. Their study provided a new method for the safe and efficacious delivery of PTX in a clinical application. However, further work is required to apply their method to the in vivo treatment of animals, such as improving the circulation of materials from epidermal tumor treatment to in vivo tumor treatment.

Nitric oxide (NO) could be potentially used for cancer treatment. Lv et al. (28) envisioned the association of transition metals with NO based on Pd(0)-catalyzed depropargylation and deallylation reactions. A biocompatible Pd(0) catalyst (Pd(dba)₂) activated O²-derived diazeniumdiolates through bioorthogonal bond cleavage, which released NO into living cells and inhibited the proliferation of tumor cells. This was the first study to combine NO and bioorthogonal catalysis. This type of bioorthogonal NO precursor may be more specific in triggering local anti-tumor effects compared with O²-derived diazeniumdiolates, which can be activated by hydrolases or other biochemical stimuli.

Transition metals act as catalysts, and only in situ activation can achieve optimal therapeutic effects and minimal toxic side effects. Clavadetscher et al. coated Pd nanoparticles with polystyrene microspheres. External modification with cyclic-RGD targets tumors and catalyzes the directional activation of two precursors in the cell: the simultaneous dealkylation of pro-5FU to 5FU and Suzuki-Miyaura cross-coupling reactions to form PP-121 (29). This nanomaterial plays a dual role by increasing the cytotoxicity of glioblastoma cells. These synthesized cRGDfE-PdNPs solved the problem of transition metal targeting, which provides a premise for further exploration of transition-metal-oriented catalytic prodrugs for in vivo tumor therapy. Furthermore, Sebastian et al. used exosomes secreted by homologous cells to coat Pd catalysts through CO reduction. This method prevented the destruction of exosome integrity, while improving the loading capacity. The catalytic efficiency was confirmed by intracellular dealkylation (30). This bioorthogonal catalytic reaction mainly achieved the targeted therapy of tumors in vivo by prioritizing exosomes over homologous cells and showed superiority compared to exogenous materials in terms of biocompatibility. However, there remain issues that require further investigation. For example, exosome preparation is a complex process, and different isolation and purification conditions should be formulated for different cell lines. The targeting of homologous cells may be destroyed during processing, and further work is needed to determine whether it is necessary to confer additional targeting mechanisms to exosomes.

Sun et al. developed a nanozyme platform based on a protein scaffold in 2022. They added different transition metal nanoparticles to a ferritin nanocage (FTn)-based protein scaffold to synthesize different nanozymes, which were screened via nanozyme/protective group pairing as the basis for designing lysosomal-related prodrugs for anti-tumor therapy (31). The results showed that Pd nanozymes could effectively induce cleavage of the propargylic ether bond to produce a dealkylation reaction. Thus, they loaded the nanozyme and hydroxycamptothecin prodrug (pro-HCPT) on liposomes to form Lipo-Pd-pHCPT, which was modified with an RGD-targeted peptide on its membrane. Lipo-Pd-pHCPT was actively taken up by tumor cells and accurately reached the lysosomes of tumor cells, playing a catalytic role in inhibiting tumor growth. The team successfully combined a nanoenzyme with a transition metal and demonstrated, for the first time, that Pd nanomaterials possess mutant P450_BM3-like activity. Zhao and his colleagues made breakthroughs in the use of bioorthogonality for comprehensive tumor therapy by loading Pd nanoparticles onto CuS nanoplates to construct a platform for a NIR-II photopromoted, Pd-mediated bioorthogonal cleavage reaction (32). This nanocomposite accelerates the rate of Pd catalyzed depropargylation and CuS catalyzed CuAAC reaction to produce 5-FU and resveratrol by photothermal irradiation. It was demonstrated as an effective anti-tumor therapy in mice. This study combined the two bioorthogonal reactions and fully facilitated the local therapeutic effect of antitumor drugs under the promotion of NIR-II to provide new insights for practical application in clinical combination therapy.

The successful combination of nanozymes and transition metals expands the application of bioorthogonal chemistry based on dealkylation for in situ drug development against specific subcellular organelles. For example, Chen et al. innovatively combined transition metals with metal-organic frameworks. They deposited Pd nanoparticles on UiO-66 metal-organic frameworks (Pd@UiO-66) using a transition metal-organic framework (MOF) binding approach. The MOF was then modified with an AS1411 aptamer that targets nucleolin overexpression in cancer cells. They demonstrated that AS1411@Pd@UiO-66 activates propargylcarbamate-caged 4-OHT to regulate the stability and activity of ER50-fused bacterial effector proteins (33). Finally, they monitored the activation of the MAPK/ERK signaling pathway in cancer cells. Their study expands the scope of research on tumor cell signaling through the targeted bioorthogonal catalytic regulation of bacterial effector protein activities to enable the effective integration of biochemical and biological functions in one bioorthogonal catalytic reaction. Similarly, a novel heterogeneous LM-Pd catalyst using liquid metal as a regulator was designed and constructed. Liquid metal is used as a kind of “ligand” to stabilize Pd⁰ in an electron-rich environment, which improves its catalytic efficiency. The tumor growth in the experimental group was strongly inhibited following the synergistic effect of photothermal therapy (34). In vivo safety evaluation showed that there were no significant side effects in the major organs of mice from the treatment group. Their research opens up the immense role of liquid metals in biological orthogonal catalysis. It is hoped that targeted therapy can be further explored for solid tumors in vivo, and that tumor treatment can become more universal.

Miller and his team also studied Pd in more detail by encapsulating the Pd(II) precatalyst into poly(lactic-co-glycolic acid)-polyethylene glycol (PLGA-PEG) to form a palladium nanoparticles (Pd-NP) in 2017. They used Pd to catalyze this reaction through allylcarbamate cleavage to locally activate proDOX in tumors and allow it to stably bind and damage DNA in tumor cells (35). In vivo imaging results showed that a small amount of Pd-NP accumulated in the liver, spleen, and kidney; however, the degree of substrate activation in the local tumor was greater. Moreover, in vitro results showed that a combined treatment using Pd-NP and proDOX had a more significant synergistic killing effect than either of these agents alone through localized catalysis.

Miller’s group further explored “catalyst + prodrug” targeted tumor therapy with the development of Pd catalyst research. In 2018, Pd-NP was allowed to passively accumulate at the tumor site based on its biological stability and high enhanced permeability and retention (EPR) effect on tumors from previous studies. The caged prodrugs of MMAE and doxorubicin (DOX), Alloc-SIL-C16-MMAE, and Alloc-SIL-C16-DOX were synthesized to enable in situ activation to inhibit tumor growth (36). This study expands the feasibility of prodrug design; hence, more advanced bioorthogonal catalytic processes can be explored in the future based on this technology. This may include analyzing the tumor microenvironment response, molecular targeting, and synergistic therapy. This bioorthogonal reaction was exploited to explore the therapeutic effect of Pd nanosheets encapsulated in microneedles on melanoma in 2021. Polyvinyl alcohol (PVA) was used as the substrate, and TiO₂ nanosheets were used as the carrier to prepare a microneedle patch (PT-MNs) that catalyzes the alloc-DOX. This device exhibits high stability, good biocompatibility, and easy removal. It locally activates DOX in the tumor site in a minimally invasive and spatially controlled manner and achieves a good anti-tumor effect in mice (37). The alloc-DOX/PT-MN group shows a significantly reduced tumor volume compared to the control groups. The experimental results also showed that trace leakage of Pd in their plasma and major organs did not threaten mouse survival. In other words, the device fabricated by Gu has optimal biosafety in vivo, which provides an experimental basis for future clinical trials.

Pd catalyzes the formation of phenolic ether groups at physiological pH, followed by the 1,6-elimination of a 4-hydroxybenzyl group directly connected to the OH of the hydroxamic acid group of a drug (38). Pd-resins significantly cause propargyl oxybenzyl cleavage and activate the prodrug to kill cancer cells compared to the control group. However, many questions remain. For example, whether the catalyst has long-term stability in vivo, how much Pd is needed to achieve bioactive drug concentrations, and whether this catalyst exerts anti-tumor effects in vivo. The resolution of these problems will expand the range of chemically activated functional groups for Pd and provide more possibilities for tumor therapy in the future.

Sancho-Albero’s group proposed using exosomes from A549 cells to load ultrathin Pd nanosheets in situ reduced in CO ambient to form bioartificial vesicles (Pd-Exo), based on their previous research. The preferential tendency of Pd-Exo to progenitor cells allows it to target and catalyze bioorthogonal reactions to activate intracellular anti-tumor prodrugs (39). Their constructed bioorthogonal catalyst had advantages and disadvantages similar to those of the one previously fabricated. Nevertheless, the use of exosome-based catalysts to target specific cell types and mediate bioorthogonal processes in the inner space of the cell offers plenty of possibilities for the combined treatment of cancer in biomedicine, chemical biology, and other disciplines. Zhang’s team used cationic scaffolds as the delivery vector of Pd, so that Pd was effectively protected and retained at the tumor site, which could continue to play a catalytic activity within 7 days (40). Nanozymes could activate pro-5FU locally to produce 5FU, and inhibited tumor growth and development in vitro and in vivo, without obvious hepatotoxicity in vivo. These demonstrated the spatial localization, catalytic activity of nanozymes and the possibility of nanozymes for tumor therapy in vivo. Nanozymes not only provide a smarter strategy for tumor treatment, but also play an important role in the development of adjuvant therapy for postoperative recurrence of cancer.

In the same way, the Das R group used gold nanoparticles (AuNPs) as scaffolds and ligands to load ruthenium on the hydrophobic segment, and a bioorthogonal nanocatalytic structure like a nanozyme was constructed (41). The surfaces of these particles were modified with cations or zwitterions to improve cellular localization. The cationic nanozymes (Pos-NZ) significantly activates the prodrug pro-DOX in HeLa cells (Figure 3A) and kills tumor cells, whereas the zinc ion scaffold located outside the cells does not activate the prodrug and has no killing ability. Next, bioorthogonal NZs loaded with AuNPs were injected into RAW 264.7 macrophages and macrophages bearing internalized bioorthogonal NZs (RAW_NZ) were synthesized (42). They used the innate chemotactic behavior of macrophages to target RAW_NZ to tumor sites to activate prodrug pro-5FU, and achieved certain efficacy in vitro experiments. In addition to metalloenzymes that use proteins as compatible carriers of metals, Perez-Lopez AM and colleagues synthesized a homogeneous Pd peptide catalyst (metallopeptide 2-Pd), consisting of a methyl salicylate tagged hydrophilic peptide (LLEYLKR) complexed to Pd (43). In vitro studies, metallopeptide 2-Pd effectively catalyzed the synthesis of PTX and linifanib (LNF) to kill A549 tumor cells. This treatment catalyzed the conversion of two nontoxic prodrugs, effectively reducing the toxic side effects of drugs while enhancing the killing effect. In future studies, in addition to verify the efficacy of animal experiments in vivo, we can also focus on the research of the binding of catalytic materials and targeted substances. By coupling with targeted substances, we can overcome the difficulties of toxic and side effects of chemotherapy drugs on normal cells in vivo.

Figure 3 Bioorthogonal reactions mediated by Pt and Au. (A) Ruthenium loaded cationic gold complex-based nanozymes that activate the pro-DOX by deallylationt. (B) Platinum catalyzes depropargylation and dealkylation of the prodrug. (C) Au nanoparticles bioorthogonally catalyze propargyl oxybenzyl cleavage of HDAC inhibitors prodrugs to obtain panobinostat. Pt, platinum; Au, gold; DOX, doxorubicin; HDAC, histone deacetylases.

Cisplatin is a commonly used anticancer drug. It was hypothesized that the platinum complex could be used as a catalyst to cleave chemical bonds. The dealkylation and depropargylation reactions of alkyne and platinum complexes can activate the prodrugs of monomethyl auristatin E (MMAE) and 5-FU (Figure 3B) (44). A significant combined anti-tumor effect was observed following pFU + CisPt treatment compared with those of the other groups using a zebrafish xenograft model. In summary, this treatment is cytotoxic to some extent; however, some biomolecules and nucleophiles can destabilize the platinum (Pt) complex under physiological conditions. Further studies are needed to improve its biocompatibility and stability.

The thiol reactivity of gold(I) is related to the nonspecific binding of serum thiols and the targeted inhibition of the intracellular thiol-enzyme complex. Physiological nucleophilic substances (such as thiols) inactivate gold via competitive binding before it reaches a specific site. This poses a challenge to the development of gold(I) catalysts and anticancer drugs (45-47). Currently, there are few studies on AuNPs as bioorthogonal catalysts. It is worth mentioning that AuNPs were used that mediate the propargyl oxybenzyl cleavage reaction under physiological conditions to prepare a bioorthogonal prodrug for the activation of HDAC inhibitors (HDACi) by gold catalysts using amino-functionalized TentaGel®HL resins (Figure 3C). The first case of a bioorthogonal HDACi prodrug was reported with a wide therapeutic window (48). The gold catalyst is unaffected by thiols in the presence of serum and serum proteins, and still has a good catalytic effect. This experiment is still at the cellular level and is understudied. However, its good catalytic transformation performance and killing ability of tumor cells lays the foundation for using gold as a lone catalyst to initiate bioorthogonal reactions. The Rubio-Ruiz team further tested multiple noble-metal based nanoalloys. In their preliminary study, different encapsulation methods for Pd and Au nanoparticles (AuPd NPs) were explored. The results show that both materials promoted AuPd catalysis by different means: nondegradable mesoporous silica nanorods augmented the catalytic performance of the nanoalloys, whereas biodegradable PLGA matrixes enhanced transcellular NP delivery (49). In general, Pd, as a catalyst of bioorthogonality, has been relatively mature in the aspect of anti-tumor therapy, especially in the combination of tumor therapy with good ability to kill cancer cells, which provides a solid theoretical and experimental basis for the transformation of clinical application in the future.

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions

The CuAAC reaction is a classical bioorthogonal reaction that has the advantages of mild reaction conditions, fast reaction speed, and high product yield (50-52). It was extensively studied for the activation of prodrugs and in in situ drug synthesis (53,54).

In 2012, Nairn et al. used dithiothreitol (DTT) as a reducing agent to optimize the CuAAC reaction (Figure 4A) (55) for the first time, effectively increasing the amount of air required for the reaction redox potential. Branched PEGylated IFNb conjugate of different molecular weights greatly improved pharmacokinetics and produced a marked increase in drug efficacy in vivo. These results demonstrate the unique site specificity of PEGylated proteins. This technology was used to bioengineer molecules that can be optimally selected for various conjugation sites with desired physicochemical and biological properties to lay the foundation for developing protein-coupling complexes. A series of studies on the bioorthogonal catalysis of Cu was conducted by using the CuAAC reaction to construct a heterogeneous copper catalyst targeting the mitochondria by Wang and You’s groups (56-60). An MOF was used as a scaffold to protect and load copper nanoparticles and modified with triphenylphosphine (TPP) to allow its accumulation in the mitochondria of living cells (56). Cell experiments and in vivo tumor therapy showed that the synthesized resveratrol-derived drugs have anti-tumor effects (Figure 4B). The catalytic effect of Cu(0) on CuAAC was improved by combining photodynamic and photothermal therapy with bioorthogonal catalysis to produce copper nanoparticles (CuNPs) encapsulated in mesoporous carbon nanospheres (57). Reactive oxygen species (ROS) can be induced to promote the conversion of Cu(0) to Cu(I) and accelerate the CuAAC reaction in mesoporous carbon nanospheres (MCNs) irradiated with near-infrared (NIR) light (Figure 4B). Second, it converts near-infrared light into thermal energy to increase the regional temperature and further accelerate the catalysis. The tumor tissue of the prodrug + MCNs-Cu + NIR group was seriously damaged in in vivo experiments. The number of tumor cells significantly decreased, and the number of apoptotic cells increased. The good tumor cell-killing effect of resveratrol catalyzed by near-infrared and MCNs-Cu was supported by the minimal side effects according to hematoxylin and eosin (H&E) staining and body weight analysis. Improved CuAAC reactions in the body have achieved excellent results; however, their lack of targeting ability could influence CuNP-specific areas in catalysis. A platform for DNA was constructed to test whether exploiting the programmability of nucleic acids leads to DNA-based nanocatalysts with different sequences that are specific and universal (58). Modular DNA CuNPs (Figure 4C) were synthesized using DNAzyme as the basic template, and aptamers were utilized to endow the materials with targeting properties. This method used high H₂O₂ concentrations in tumor cells without external stimulation to generate reactive free radicals on the surface of DNA-templated CuNPs to promote the conversion of Cu(0) to Cu(I). In addition, the AS1411 aptamer specifically recognizes overexpressed radionuclides on cancer cell surfaces, which enhances the bioorthogonal catalytic reaction and in situ activation of prodrugs. In the same year, a similar bioorthogonal nanomaterial was synthesized using DNA as a template (Figure 4C). This further confirms the versatility of the DNA nanocatalyst in vitro and in vivo. Moreover, the conversion efficiency of this catalyst was improved by an order of magnitude compared to that of the commonly used CuSO₄/sodium ascorbate catalyst (59). Therefore, this DNA-based targeting bioorthogonal nanocatalyst has high efficiency, accurate targeting, and universality, with broad prospects in personalized tumor treatment. Lately, this team turned their attention to Cu(I) inside tumors and envisioned using the Cu(I) of cancer cells to catalyze the CuAAC reaction. However, the catalysis is inefficient. Thus, they constructed an adaptive bioorthogonal catalytic system by wrapping prodrug and sodium ascorbate (NaAsc) in metal-organic framework nanoparticles modified with adenosine triphosphate aptamers (ZIF-90@P-A NPs) (60). NaAsc promotes the transformation of Cu(II) to Cu(I) in tumor cells and enhances the endogenous level of Cu(I) in situ to ensure the efficient reaction of CuAAC. The in vivo experimental results confirmed the feasibility of this method. This safe and efficient bioorthogonal therapy using the internal tumor environment is worthy of further exploration.

Figure 4 Cu- or Ru-mediated CuAAC reactions. (A) CuAAC reaction was optimized and extended by using dithiothreitol as reducing agent. (B) Heterogeneous CuNPs-mediated CuAAC reaction generates the Resveratrol analog. (C) DNA-templated CuNPs catalyze the CuAAC reaction to produce toxic the Resveratrol analog. (D) Bioorthogonally labeled ruthenium (II) complex specifically kill cells under two-photon irradiation. PEG, polyethylene glycol; DTT, dithiothreitol; TBTA, tris(benzyltriazolylmethyl)amine; SDS, sodium dodecyl sulfate; MOF, metal-organic framework; TPP, triphenylphosphine; MCNs, mesoporous carbon nanospheres; NIR, near-infrared; THPTA, tris(3-hydroxypropyltriazolylmethyl) amine; Cu, copper; Ru, ruthenium; CuAAC, copper-catalyzed azide-alkyne cycloaddition; CuNPs, copper nanoparticles.

In recent studies, ruthenium(II) is widely utilized as a photosensitizer in treating triple-negative breast cancers, but it lacks specificity and does not efficiently accumulate at the tumor site. A ruthenium(II) complex with an alkyne group was developed to address the differences in highly expressed proteins between tumors and normal cells (61). Tetraacylated N-azidoacetylmannosamine (Ac4Man-Naz) was used to specifically introduce an azide group on the cell membrane so that Ru-alkyne-2 reacts with the bioorthogonally labeled cell membrane under two-photon excitation, resulting in cell death (Figure 4D). This is the first bioorthogonal two-photon photosensitizer based on a ruthenium(II) complex. Two-photon irradiation has better tissue penetration and a more effective reduction of light-induced damage compared to single-photon irradiation. Ac4Man-Naz selectively and bioorthogonally labels the cell membrane. This high selectivity is crucial for specific killing of tumor cells by Ru-alkyne-2. The cell membrane is severely damaged under two-photon irradiation, and a large amount of lactate dehydrogenase is released. This study was restricted to the in vitro level; however, it paves the way to develop more integrated therapies based on bioorthogonal metal complexes.

Transition metal-mediated and other related bioorthogonal reactions

Other bioorthogonal reactions related to gold

Bioorthogonal activation to modulate the biological activity of gold complexes in living systems was performed in 2021 by Long et al. by designing an HRGDH-Pd complex containing an RGD-targeting peptide. An HRGDH-Pd complex containing RGD-targeted peptides that selectively recognizes and accumulates within cancer cells was designed to trigger a metal transport reaction through Pd(II) to break the Au-C bond (Figure 5A) and convert organogold(I) to active gold(I) (62). This metal transfer activation method reduces the influence of thiols on gold catalysis, exerts an inhibitory effect, shows cytotoxicity [gold(I)] on thioredoxin reductase, and demonstrates an anti-angiogenic ability in a zebrafish model. This was the first example of bioorthogonal activation to modulate the biological activity of gold complexes in living systems to solve the problem of off-target catalysis of organogold. Hence, this study has provided a new direction for the targeted therapy of tumors and an opportunity to develop new anticancer drugs.

Figure 5 Au, Pt, Ru, Fe related bioorthogonal reactions. (A) Palladium can trigger the Au-C bond fracture and activate organogold (I) complexes. (B) An internal photoswitch inside rodanplatin converts the inert Pt (IV) to the Pt (II) drugs after visible light irradiation. (C) Photosensitizer activates the Pt (IV) complex and generates ¹O₂ under photocatalysis. (D) Ruthenium-based bioorthogonal polyzymes catalyze pro-Mit to exert anticancer effects in vitro. (E) Bioorthogonal catalytic central section of tES-HRP and activated IAA reduce iron (III) to iron (II). IR, infrared; HRPc, horseradish peroxidase center; Au, gold; Pt, platinum; Ru, ruthenium; Fe, iron; tES-HRP, porous exoshells encapsulating horseradish peroxidase; IAA, indole-3-acetic acid.

Deng’s group developed a novel pre-targeting system in which tetrazine (Tz) was attached to PEGylated AuNPs with a diameter of about 30 nm, enabling AuNPs to be conjugated to trans-cyclooctene (TCO) functionalized a monoclonal antibody in vivo (63). The results showed that Tz-AuNPs could bind to CC49-TCO effectively in tumor tissues. Over time, this pretargeting system accumulates at the tumor site. The proof-of-concept study has potential and broad application prospects in the application of prodrug activation, improving the internalization ability of nanoparticles, and overcoming tumor heterogeneity.

Photoreduction of platinum prodrugs

Platinum prodrugs are activated by the combined effects of photocatalysis and bioorthogonality. For example, platinum prodrugs are developed and activated by photoswitches in vivo. Rhodamine B (RhB) was coupled with carboplatin- and oxaliplatin-based Pt(IV) complexes to decrease the distance between the photoswitch and the Pt(IV) center (64). Rhodaplatin was combined with sodium ascorbate in a physiological environment for photoreduction under visible-light irradiation to generate Pt(II) and free RhB ligands (Figure 5B). One of the prodrugs strongly targeted the mitochondria, which may be related to its lipophilicity. In addition, rhodaplatin induces mitochondrial DNA damage and initiates the endogenous apoptotic pathway induced by non-nuclear DNA damage to overcome drug resistance. This approach improves the intracellular activation efficiency of the drug and drastically increases photocytotoxicity compared to the parameters following treatment with the photocatalyst and substrate in vitro. This suggests a possible strategy to overcome cancer drug resistance by activating pathways unrelated to nuclear DNA damage. However, the targeting mode and therapeutic effect of this prodrug needs to be explored in depth and improved in vivo.

Ruthenium-catalyzed bond cleavage

A ruthenium-based photosensitizer (PS-1) was designed by Norman DJ’s group, which could target the mitochondria to activate Pt(IV) complexes (Pt-c) while generating singlet oxygen under light conditions (Figure 5C) (65). This provides spatial and temporal control of their cytotoxic effects. The study explored the resistance to this prodrug. The results showed that Pt-c can still be activated in large amounts under photocatalysis and achieved dual killing of tumor cells, even in drug-resistant SKOV-3 cells. Poly(oxanorborneneimide) (PONI) polymer-based nanoparticles were used to wrap ruthenium and synthesize bioorthogonal “multienzymes” in 2020 (66). The hydrophobic environment inside the polymer guarantees the stability and activity of this ruthenium-based catalyst in a biological environment. Conversion of the non-toxic mitoxantrone prodrug into toxic anticancer drugs via bioorthogonal catalysis results in cell death in HeLa cells (Figure 5D). Therefore, the efficient generation of drugs by multienzymes in vitro provides an alternative method for the diagnosis and treatment of cancer.

Iron-mediated bioorthogonal catalysis

Iron (Fe) has excellent biocompatibility and exists in various oxidized forms in organisms, the most common being ferrous [Fe(II)] and ferric iron [Fe(III)]. Its compatibility was used to produce a heat-resistant, self-assembled single-enzyme nanoparticle centered on an iron-containing catalytic reaction comprising an inner layer of horseradish peroxidase (HRP) and an outer layer of a high-temperature-resistant porous shell. These nanoparticles reach the hypoxic tumor site and selectively activate indole-3-acetic acid (IAA) in the tumor according to the active nature of HRP under low oxygen tension. IAA then enters the catalytic center from the pore, and biologically active intermediates and free radicals are generated to induce oxidative degradation and apoptosis of target cells when iron(III) is reduced to iron(II) (Figure 5E) (67). The tumor volume of the porous exoshells (tESs) encapsulating Renilla luciferase (tES-rLuc) group significantly decreased in vivo, and the treatment effect was statistically different across groups. However, these are only preliminary experiments. Furthermore, the authors mentioned that the required dosage of IAA and tESs encapsulating HRP (tES-HRP) is the biggest problem for clinical application. This needs to be evaluated in further studies.

Conclusions

This review summarized the research progress of bioorthogonal reactions mediated by transition metals in solid tumors and discussed the application of bioorthogonal catalysis in tumor therapy. In recent years, researchers have continued to explore and optimize the biocompatibility of transition metals and expand their applications. Compared with metal-free bioorthogonal reactions, such as Strain-promoted Azide-Alkyne Cycloaddition (SPAAC) reaction and the inverse electron-demand Diels-Alder (IED-DA) reaction, the low toxicity and reaction rate of these reactions are superior to those mediated by transition metals. Therefore, there are three main points of consideration. First, it is necessary to reduce the toxic side effects and improve the biostability and compatibility of transition metals. Second, it is necessary to improve the specificity of transition metals to their catalytic sites. The targeting ability of transition metals can be increased by modification with antibodies, aptamers, targeted peptides, or the tumor microenvironment to ensure that they bind to their targets and achieve the maximum therapeutic effect. Finally, other synthetic approaches need to be explored. More emerging metal catalysts can be developed while reducing the difficulty of utilizing transition metals by designing simple synthesis methods.

The development of bioorthogonal catalysis not only requires the development of efficient transition metal catalysts, but also is closely related to the screening and design of drugs. The speed and accuracy of compound synthesis, screening, and optimization all directly affect the outcome of the drug research process. Therefore, effective strategies for building chemical reaction libraries and screening chemicals remain to be developed. We believe that more transition-metal-mediated bioorthogonal reactions will be applied for precision tumor therapy with the continuous development of materials chemistry and biochemistry. Future work may involve deeply coupling bioorthogonal therapy with immunotherapy, photodynamic therapy, and photochemotherapy to continuously innovate and develop the treatment modes available against cancer.

Acknowledgments

We would like to thank Editage for their help in polishing the language of our review.

Funding: This work was supported by the Scientific and Technological Innovation Major Base of Guangxi (No. 2022-36-Z05), and the National Natural Science Foundation of China (Nos. 82060562 and 81703082).

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-345/rc

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Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tcr.amegroups.com/article/view/10.21037/tcr-23-345/coif). The authors have no conflicts of interest to declare.

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References

1. Hang HC, Yu C, Kato DL, et al. A metabolic labeling approach toward proteomic analysis of mucin-type O-linked glycosylation. Proc Natl Acad Sci U S A 2003;100:14846-51. [Crossref] [PubMed]
2. Li Z, Shen D, Hu S, et al. Pretargeting and Bioorthogonal Click Chemistry-Mediated Endogenous Stem Cell Homing for Heart Repair. ACS Nano 2018;12:12193-200. [Crossref] [PubMed]
3. Okamoto Y, Kojima R, Schwizer F, et al. A cell-penetrating artificial metalloenzyme regulates a gene switch in a designer mammalian cell. Nat Commun 2018;9:1943. [Crossref] [PubMed]
4. Plunk MA, Alaniz A, Olademehin OP, et al. Design and Catalyzed Activation of Tak-242 Prodrugs for Localized Inhibition of TLR4-Induced Inflammation. ACS Med Chem Lett 2020;11:141-6. [Crossref] [PubMed]
5. Tomás-Gamasa M, Martínez-Calvo M, Couceiro JR, et al. Transition metal catalysis in the mitochondria of living cells. Nat Commun 2016;7:12538. [Crossref] [PubMed]
6. Li J, Chen PR. Development and application of bond cleavage reactions in bioorthogonal chemistry. Nat Chem Biol 2016;12:129-37. [Crossref] [PubMed]
7. van de L'Isle MON. Ortega-Liebana MC, Unciti-Broceta A. Transition metal catalysts for the bioorthogonal synthesis of bioactive agents. Curr Opin Chem Biol 2021;61:32-42. [Crossref] [PubMed]
8. Wang W, Zhang X, Huang R, et al. In situ activation of therapeutics through bioorthogonal catalysis. Adv Drug Deliv Rev 2021;176:113893. [Crossref] [PubMed]
9. Fedeli S, Im J, Gopalakrishnan S, et al. Nanomaterial-based bioorthogonal nanozymes for biological applications. Chem Soc Rev 2021;50:13467-80. [Crossref] [PubMed]
10. Zhang X, Huang R, Gopalakrishnan S, et al. Bioorthogonal nanozymes: progress towards therapeutic applications. Trends Chem 2019;1:90-8. [Crossref] [PubMed]
11. Bai Y, Chen J, Zimmerman SC. Designed transition metal catalysts for intracellular organic synthesis. Chem Soc Rev 2018;47:1811-21. [Crossref] [PubMed]
12. Tonga GY, Jeong Y, Duncan B, et al. Supramolecular regulation of bioorthogonal catalysis in cells using nanoparticle-embedded transition metal catalysts. Nat Chem 2015;7:597-603. [Crossref] [PubMed]
13. Cao-Milán R, Gopalakrishnan S, He LD, et al. Thermally Gated Bio-orthogonal Nanozymes with Supramolecularly Confined Porphyrin Catalysts for Antimicrobial Uses. Chem 2020;6:1113-24. [Crossref]
14. Martínez R, Carrillo-Carrión C, Destito P, et al. Core-Shell Palladium/MOF Platforms as Diffusion-Controlled Nanoreactors in Living Cells and Tissue Models. Cell Rep Phys Sci 2020;1:100076. [Crossref] [PubMed]
15. Jeschek M, Reuter R, Heinisch T, et al. Directed evolution of artificial metalloenzymes for in vivo metathesis. Nature 2016;537:661-5. [Crossref] [PubMed]
16. Tsubokura K, Vong KK, Pradipta AR, et al. In Vivo Gold Complex Catalysis within Live Mice. Angew Chem Int Ed Engl 2017;56:3579-84. [Crossref] [PubMed]
17. Spicer CD, Triemer T, Davis BG. Palladium-mediated cell-surface labeling. J Am Chem Soc 2012;134:800-3. [Crossref] [PubMed]
18. Sánchez MI, Penas C, Vázquez ME, et al. Metal-catalyzed uncaging of DNA-binding agents in living cells. Chem Sci 2014;2014:1901-7. [Crossref] [PubMed]
19. Eda S, Nasibullin I, Vong K, et al. Biocompatibility and therapeutic potential of glycosylated albumin artificial metalloenzymes. Nature Catalysis 2019;2:780-92. [Crossref]
20. Vidal C, Tomás-Gamasa M, Destito P, et al. Concurrent and orthogonal gold(I) and ruthenium(II) catalysis inside living cells. Nat Commun 2018;9:1913. [Crossref] [PubMed]
21. Miguel-Ávila J, Tomás-Gamasa M, Olmos A, et al. Discrete Cu(i) complexes for azide-alkyne annulations of small molecules inside mammalian cells. Chem Sci 2018;9:1947-52. [Crossref] [PubMed]
22. Martínez-Calvo M, Couceiro JR, Destito P, et al. Intracellular Deprotection Reactions Mediated by Palladium Complexes Equipped with Designed Phosphine Ligands. ACS Catal 2018;8:6055-61. [Crossref] [PubMed]
23. Völker T, Dempwolff F, Graumann PL, et al. Progress towards bioorthogonal catalysis with organometallic compounds. Angew Chem Int Ed Engl 2014;53:10536-40. [Crossref] [PubMed]
24. Li J, Yu J, Zhao J, et al. Palladium-triggered deprotection chemistry for protein activation in living cells. Nat Chem 2014;6:352-61. [Crossref] [PubMed]
25. Weiss JT, Dawson JC, Macleod KG, et al. Extracellular palladium-catalysed dealkylation of 5-fluoro-1-propargyl-uracil as a bioorthogonally activated prodrug approach. Nat Commun 2014;5:3277. [Crossref] [PubMed]
26. Weiss JT, Dawson JC, Fraser C, et al. Development and bioorthogonal activation of palladium-labile prodrugs of gemcitabine. J Med Chem 2014;57:5395-404. [Crossref] [PubMed]
27. Pérez-López AM, Rubio-Ruiz B, Valero T, et al. Bioorthogonal Uncaging of Cytotoxic Paclitaxel through Pd Nanosheet-Hydrogel Frameworks. J Med Chem 2020;63:9650-9. [Crossref] [PubMed]
28. Lv T, Wu J, Kang F, et al. Synthesis and Evaluation of O(2)-Derived Diazeniumdiolates Activatable via Bioorthogonal Chemistry Reactions in Living Cells. Org Lett 2018;20:2164-7. [Crossref] [PubMed]
29. Clavadetscher J, Indrigo E, Chankeshwara SV, et al. In-Cell Dual Drug Synthesis by Cancer-Targeting Palladium Catalysts. Angew Chem Int Ed Engl 2017;56:6864-8. [Crossref] [PubMed]
30. Sebastian V, Sancho-Albero M, Arruebo M, et al. Nondestructive production of exosomes loaded with ultrathin palladium nanosheets for targeted bio-orthogonal catalysis. Nat Protoc 2021;16:131-63. [Crossref] [PubMed]
31. Sun Z, Liu Q, Wang X, et al. Bioorthogonal catalytic nanozyme-mediated lysosomal membrane leakage for targeted drug delivery. Theranostics 2022;12:1132-47. [Crossref] [PubMed]
32. Zhao H, Liu Z, Wei Y, et al. NIR-II Light Leveraged Dual Drug Synthesis for Orthotopic Combination Therapy. ACS Nano 2022;16:20353-63. [Crossref] [PubMed]
33. Chen X, Cai W, Liu J, et al. Integration of Palladium Nanoparticles with Surface Engineered Metal-Organic Frameworks for Cell-Selective Bioorthogonal Catalysis and Protein Activity Regulation. ACS Appl Mater Interfaces 2022;14:10117-24. [Crossref] [PubMed]
34. Zhang L, Sang Y, Liu Z, et al. Liquid Metal as Bioinspired and Unusual Modulator in Bioorthogonal Catalysis for Tumor Inhibition Therapy. Angew Chem Int Ed Engl 2023;62:e202218159. [Crossref] [PubMed]
35. Miller MA, Askevold B, Mikula H, et al. Nano-palladium is a cellular catalyst for in vivo chemistry. Nat Commun 2017;8:15906. [Crossref] [PubMed]
36. Miller MA, Mikula H, Luthria G, et al. Modular Nanoparticulate Prodrug Design Enables Efficient Treatment of Solid Tumors Using Bioorthogonal Activation. ACS Nano 2018;12:12814-26. [Crossref] [PubMed]
37. Chen Z, Li H, Bian Y, et al. Bioorthogonal catalytic patch. Nat Nanotechnol 2021;16:933-41. [Crossref] [PubMed]
38. Rubio-Ruiz B, Weiss JT, Unciti-Broceta A. Efficient Palladium-Triggered Release of Vorinostat from a Bioorthogonal Precursor. J Med Chem 2016;59:9974-80. [Crossref] [PubMed]
39. Sancho-Albero M, Rubio-Ruiz B, Pérez-López AM, et al. Cancer-derived exosomes loaded with ultrathin palladium nanosheets for targeted bioorthogonal catalysis. Nat Catal 2019;2:864-72. [Crossref] [PubMed]
40. Zhang X, Liu Y, Doungchawee J, et al. Bioorthogonal nanozymes for breast cancer imaging and therapy. J Control Release 2023;357:31-9. [Crossref] [PubMed]
41. Das R, Landis RF, Tonga GY, et al. Control of Intra- versus Extracellular Bioorthogonal Catalysis Using Surface-Engineered Nanozymes. ACS Nano 2019;13:229-35. [Crossref] [PubMed]
42. Das R, Hardie J, Joshi BP, et al. Macrophage-Encapsulated Bioorthogonal Nanozymes for Targeting Cancer Cells. JACS Au 2022;2:1679-85. [Crossref] [PubMed]
43. Pérez-López AM, Belsom A, Fiedler L, et al. Dual-Bioorthogonal Catalysis by a Palladium Peptide Complex. J Med Chem 2023;66:3301-11. [Crossref] [PubMed]
44. Oliveira BL, Stenton BJ, Unnikrishnan VB, et al. Platinum-Triggered Bond-Cleavage of Pentynoyl Amide and N-Propargyl Handles for Drug-Activation. J Am Chem Soc 2020;142:10869-80. [Crossref] [PubMed]
45. Zou T, Lum CT, Lok CN, et al. Chemical biology of anticancer gold(III) and gold(I) complexes. Chem Soc Rev 2015;44:8786-801. [Crossref] [PubMed]
46. Berners-Price SJ, Filipovska A. Gold compounds as therapeutic agents for human diseases. Metallomics 2011;3:863-73. [Crossref] [PubMed]
47. Yang Z, Jiang G, Xu Z, et al. Advances in alkynyl gold complexes for use as potential anticancer agents. Coordination Chemistry Reviews 2020;423:213492. [Crossref]
48. Rubio-Ruiz B, Pérez-López AM, Sebastián V, et al. A minimally-masked inactive prodrug of panobinostat that is bioorthogonally activated by gold chemistry. Bioorg Med Chem 2021;41:116217. [Crossref] [PubMed]
49. Rubio-Ruiz B, Pérez-López AM, Uson L, et al. In Cellulo Bioorthogonal Catalysis by Encapsulated AuPd Nanoalloys: Overcoming Intracellular Deactivation. Nano Lett 2023;23:804-11. [Crossref] [PubMed]
50. Porte K, Riomet M, Figliola C, et al. Click and Bio-Orthogonal Reactions with Mesoionic Compounds. Chem Rev 2021;121:6718-43. [Crossref] [PubMed]
51. Meldal M, Tornøe CW. Cu-catalyzed azide-alkyne cycloaddition. Chem Rev 2008;108:2952-3015. [Crossref] [PubMed]
52. Tornøe CW, Christensen C, Meldal M. Peptidotriazoles on solid phase: [1,2,3]-triazoles by regiospecific copper(i)-catalyzed 1,3-dipolar cycloadditions of terminal alkynes to azides. J Org Chem 2002;67:3057-64. [Crossref] [PubMed]
53. Dong Y, Tu Y, Wang K, et al. A General Strategy for Macrotheranostic Prodrug Activation: Synergy between the Acidic Tumor Microenvironment and Bioorthogonal Chemistry. Angew Chem Int Ed Engl 2020;59:7168-72. [Crossref] [PubMed]
54. Yao Q, Lin F, Fan X, et al. Synergistic enzymatic and bioorthogonal reactions for selective prodrug activation in living systems. Nat Commun 2018;9:5032. [Crossref] [PubMed]
55. Nairn NW, Shanebeck KD, Wang A, et al. Development of copper-catalyzed azide-alkyne cycloaddition for increased in vivo efficacy of interferon β-1b by site-specific PEGylation. Bioconjug Chem 2012;23:2087-97. [Crossref] [PubMed]
56. Wang F, Zhang Y, Liu Z, et al. A Biocompatible Heterogeneous MOF-Cu Catalyst for In Vivo Drug Synthesis in Targeted Subcellular Organelles. Angew Chem Int Ed Engl 2019;58:6987-92. [Crossref] [PubMed]
57. You Y, Cao F, Zhao Y, et al. Near-Infrared Light Dual-Promoted Heterogeneous Copper Nanocatalyst for Highly Efficient Bioorthogonal Chemistry in Vivo. ACS Nano 2020;14:4178-87. [Crossref] [PubMed]
58. You Y, Liu H, Zhu J, et al. A DNAzyme-augmented bioorthogonal catalysis system for synergistic cancer therapy. Chem Sci 2022;13:7829-36. [Crossref] [PubMed]
59. You Y, Deng Q, Wang Y, et al. DNA-based platform for efficient and precisely targeted bioorthogonal catalysis in living systems. Nat Commun 2022;13:1459. [Crossref] [PubMed]
60. Zhu J, You Y, Zhang W, et al. Boosting Endogenous Copper(I) for Biologically Safe and Efficient Bioorthogonal Catalysis via Self-Adaptive Metal-Organic Frameworks. J Am Chem Soc 2023;145:1955-63. [Crossref] [PubMed]
61. Lin M, Zou S, Liao X, et al. Ruthenium(II) complexes as bioorthogonal two-photon photosensitizers for tumour-specific photodynamic therapy against triple-negative breast cancer cells. Chem Commun (Camb) 2021;57:4408-11. [Crossref] [PubMed]
62. Long Y, Cao B, Xiong X, et al. Bioorthogonal Activation of Dual Catalytic and Anti-Cancer Activities of Organogold(I) Complexes in Living Systems. Angew Chem Int Ed Engl 2021;60:4133-41. [Crossref] [PubMed]
63. Deng Z, Li C, Chen S, et al. An intramolecular photoswitch can significantly promote photoactivation of Pt(iv) prodrugs. Chem Sci 2021;12:6536-42. [Crossref] [PubMed]
64. Poulie CBM, Sporer E, Hvass L, et al. Bioorthogonal Click of Colloidal Gold Nanoparticles to Antibodies In vivo. Chemistry 2022;28:e202201847. [Crossref] [PubMed]
65. Norman DJ, Gambardella A, Mount AR, et al. A Dual Killing Strategy: Photocatalytic Generation of Singlet Oxygen with Concomitant Pt(IV) Prodrug Activation. Angew Chem Int Ed Engl 2019;58:14189-92. [Crossref] [PubMed]
66. Zhang X, Landis RF, Keshri P, et al. Intracellular Activation of Anticancer Therapeutics Using Polymeric Bioorthogonal Nanocatalysts. Adv Healthc Mater 2021;10:e2001627. [Crossref] [PubMed]
67. Sadeghi S, Masurkar ND, Vallerinteavide Mavelli G, et al. Bioorthogonal Catalysis for Treatment of Solid Tumors Using Thermostable, Self-Assembling, Single Enzyme Nanoparticles and Natural Product Conversion with Indole-3-acetic Acid. ACS Nano 2022;16:10292-301. [Crossref] [PubMed]