Recent Advances in Antitumor Nanomedicine Based on Covalent Organic Frameworks

Introduction

Cancer remains a leading cause of global morbidity and mortality, posing a serious threat to human health and imposing significant constraints on social and economic development.^1,2 As a promising complement to conventional treatments such as chemotherapy, radiotherapy, and surgery, nanomedicine has garnered considerable attention in recent years.^3–6 Nanoparticles of appropriate size can leverage the enhanced permeability and retention (EPR) effect to selectively accumulate at tumor sites, demonstrating substantial potential for various cancer therapeutics.^7–9 A wide range of nanoparticles (NPs), including inorganic, organic, and organic-inorganic hybrid NPs, have been extensively investigated for anticancer applications.^10–12 Despite their promise, limited functionalization options often restrict their therapeutic efficacy and biocompatibility.^13–16

Covalent organic frameworks (COFs) represent an emerging class of crystalline porous polymers with high specific surface areas.^17,18 Similar to metal–organic frameworks (MOFs), COFs possess regularly structured pores with tunable size and morphology, facilitating functional modification. However, unlike MOFs, COFs are constructed from light elements (eg., H, B, C, N, O) connected via dynamic covalent bonds.^19–21 This structure endows COFs with superior stability compared to conventional self-assembled organic materials and greater resilience than inorganic particles held by ionic or metallic bonds. Moreover, the absence of metal ions mitigates potential toxicity concerns.^22–25 To date, COFs have attracted growing interest in diagnostic and therapeutic applications owing to their stimuli-responsiveness, biodegradability, high surface area, substantial pore volume, tunable porosity, and extensive π-conjugation.^26–28

This review summarizes the latest advancements in the field of COFs synthesis, with a focus on their synthetic methods, their distinctions from other traditional nanomaterials and advantages in responsive drug release within the tumor microenvironment. It also summarizes specific applications of COFs in cancer diagnosis and treatment (Figure 1). Furthermore, we discuss the opportunities and challenges associated with the clinical translation of COFs, aiming to offer a theoretical basis and future directions for nano-strategy-based cancer prevention and therapy research.

Figure 1 Overview of antitumor nanomedicine based on covalent organic frameworks. The red arrow indicates an increase in the degree of hypoxia. The green arrow indicates the increase of GSH.

Synthesis of COFs

Developing a universal synthesis method for COFs that simultaneously achieves high crystallinity, high porosity, low cost, and large-scale production remains a significant challenge. This is primarily because the optimal conditions (such as solvent selection, temperature, catalyst type, and reaction time) required for synthesizing specific COFs often necessitate extensive experimental exploration, sometimes involving dozens or even hundreds of attempts.^29–31 Since Yaghi et al first successfully synthesized COFs via the solvothermal method in 2005,³² researchers have developed numerous alternative or complementary synthetic strategies. This section summarizes and discusses these major COF materials and their corresponding synthesis methods (Table 1).

Table 1 Primary Synthesis Methods of COFs and Their Advantages and Disadvantages

Solvothermal Synthesis

Solvothermal synthesis remains the most classical and widely used method for synthesizing covalent organic frameworks (COFs) to date. The procedure involves placing COF monomers, solvent, and catalyst into a pressure-resistant Pyrex tube or autoclave. Oxygen and moisture are thoroughly removed from the system using freeze–pump–thaw cycles (typically performed three times), and the reaction vessel is sealed to isolate it from air. Subsequently, the mixture is reacted at elevated temperatures (80–150 °C) for 3–7 days. The final product is obtained by filtration and washing. The pioneering application of this method dates back to 2005 when Yaghi’s team first successfully synthesized two COF materials—COF-1 and COF-5—using solvothermal synthesis, establishing the field of crystalline porous covalent organic frameworks. COF-1 is formed by self-condensation of the monomer 1,4-benzenediboronic acid (BDBA), while COF-5 is formed by co-condensation of BDBA and hexahydroxytriphenylene (HHTP) in a 1:1 molar ratio. In their experiment, a solvent system consisting of mesitylene and 1,4-dioxane in a 1:1 volume ratio was used. The reaction was carried out in pressure-resistant Pyrex tubes after freeze–pump–thaw degassing and sealing.

The relatively mild and controllable reaction conditions of solvothermal synthesis are among its key advantages for widespread application. The choice of solvent system (eg., mesitylene/1,4-dioxane, o-dichlorobenzene/n-butanol, acetonitrile/acetic acid) is crucial, as it not only affects monomer solubility but also directly modulates the kinetics of reversible reactions and the crystallinity of the final product. Catalysts, such as acetic acid, are often added to promote the formation and exchange of specific linkages (eg., imine bonds), thereby enhancing crystallinity. Due to these characteristics, this method can yield COFs with good crystallinity and is applicable to various types of dynamic covalent bonds, including imine, boronate ester, hydrazone, and triazine linkages. The method also shows potential for structural control. For instance, Zhao et al reported that using the same monomers but in different solvents during solvothermal synthesis yielded two different COF isomers.³³ More importantly, they observed the transformation between these two isomers under solvothermal conditions. Furthermore, employing a special two-step solvothermal procedure can convert low-crystallinity COFs or amorphous polymers into highly crystalline COFs.³⁴

However, this method also has significant drawbacks: reactions typically require high temperatures (80–150 °C) and pressures (sealed system), along with long reaction times (usually 3–7 days). Simultaneously, precise control over the morphology (eg., size, shape) of the resulting COFs is often difficult.^35–37

Ionothermal Synthesis

Ionothermal synthesis is similar to the solvothermal process but requires more demanding reaction conditions.^38–40 It utilizes molten ionic liquids or deep eutectic mixtures as the reaction medium and catalyst. In 2008, Thomas’s team first achieved the preparation of COFs via ionothermal synthesis.⁴¹ They synthesized a covalent triazine framework (CTF-1) with high thermal stability, chemical stability, and crystallinity by heating a mixture of ZnCl₂ and 1,4-dicyanobenzene (DCB) at 400 °C for 40 hours. In this reaction, molten zinc chloride acted not only as the solvent but also as the catalyst for the trimerization reaction. In 2018, Fang Qianrong’s team developed an ionothermal method for synthesizing 3D COFs under ambient temperature and pressure, significantly reducing the required reaction conditions.⁴¹ Hao Long’s team utilized high-temperature ionothermal conditions to achieve structural transformation of a benzotrithiophene-based COF, enhancing its electrochemical performance.⁴²

In this synthetic approach, the ionic liquid serves as both solvent and catalyst (eg., imidazolium-based ionic liquids catalyzing imine bond formation), avoiding the need for volatile organic solvents and additional acid catalysts. High temperatures (typically above the melting point of the ionic liquid) favor crystallization. The low vapor pressure and thermal stability of ionic liquids allow operation at ambient or low pressure. However, products can be difficult to separate and purify from the viscous ionic liquid. Applicable bond types include imine and triazine linkages.⁴³

Microwave-Assisted Synthesis

Microwave synthesis is a solvothermal approach that utilizes microwave dielectric heating.^44–47 It works by irradiating the reaction mixture with microwaves, causing intense molecular motion and rapid temperature increase. In 2009, Cooper et al first used microwave synthesis to prepare the B–O linked COF-5 in just 20 minutes, approximately 200 times faster than conventional solvothermal synthesis (72 hours).⁴⁸ Furthermore, the BET surface area of COF-5 obtained by this method (2019 m²·g⁻¹) was higher than that of solvothermally synthesized COF-5 (1590 m²·g⁻¹).

Compared to traditional external heating, microwave heating offers significant advantages: internal/volumetric heating, where materials directly absorb microwave energy and convert it into heat, resulting in high efficiency;⁴⁹ rapid and uniform heating, which shortens reaction times and offers significant energy savings; and ease of process control, leading to high reaction efficiency and sometimes improved product crystallinity or different crystal phases. However, this method also has limitations, such as the requirement for specialized microwave reactors and typically small reaction scales. Currently applicable bond types include imine, boronate ester, and triazine linkages.^50,51

Mechanochemical Synthesis

Mechanochemical synthesis involves the direct application of mechanical force (eg., grinding, milling) using equipment like ball mills to induce reactions between solid monomers under solvent-free conditions or with minimal solvent (liquid-assisted grinding, LAG).^52,53 This is a rapid, solvent-free, economical, and environmentally friendly COF synthesis method. Developed by the Banerjee group in 2013, this approach synthesized three highly stable β-ketoenamine COFs (COF-23) based on 1,3,5-triformylphloroglucinol (Tp) via Schiff-base reactions. In a typical synthesis, precursors were placed in a mortar and ground at room temperature using a mortar and pestle under solvent-free conditions.⁵⁴ After 5 minutes, the color of the powder mixture changed to pale yellow, indicating the reaction onset. Grinding for 40 minutes yielded a deep red crystalline powder, signifying complete COF formation.

This synthesis method is arguably the greenest, using almost no or minimal solvent. It is operationally simple, rapid (typically minutes to hours), and suitable for large-scale production.⁵⁵ Nevertheless, the crystallinity of the obtained COFs is generally lower than that achieved by solvothermal methods (though high crystallinity examples exist), and sometimes requires post-synthesis solvent-assisted treatment to improve crystallinity. The reaction mechanism still requires in-depth study. Currently applicable bond types include imine, boronate ester, hydrazone, and β-ketoenamine linkages.⁵⁶

Interfacial Synthesis

Interfacial synthesis is a strategy that utilizes dynamic covalent chemistry (DCvC) to directly prepare COF films at interfaces (gas–liquid, liquid–liquid, or solid–liquid) through confined space reactions.⁵⁷ This synthesis route was first used to prepare COF films in 2013 by the team of Wang Dong and Wan Lijun at the University of Chinese Academy of Sciences.⁵⁸ They employed a solid–gas interfacial reaction strategy: pre-loading the triamine monomer (TAPB) onto a highly oriented pyrolytic graphite (HOPG) surface, then introducing gaseous terephthalaldehyde (TPA) to react at 150 °C, forming an imine-linked honeycomb monolayer COF (SCOF-IC1). Subsequently, the Banerjee group synthesized free-standing COF films at the interface between dichloromethane and water via Schiff-base reactions in 2017. Taking the Tp-Bpy COF film as an example, a solution of monomer 1,3,5-triformylphloroglucinol (Tp) in dichloromethane was poured into a glass beaker. A spacer layer of water was added on top of the Tp solution, followed by slow addition of an aqueous solution of 2,2′-bipyridine-5,5′-diamine (Bpy) on top of the water layer. After 72 hours at room temperature, a free-standing COF film formed at the interface, exhibiting good crystallinity and a high BET surface area.

This synthesis method offers structural controllability. Film thickness can be tuned from nanometers to micrometers, and anisotropic structures can be designed (eg., one side with microspheres / smooth morphology on both sides).⁵⁹ Reaction conditions are mild, requiring only ambient pressure and room or low temperature, reducing the use of high-boiling-point toxic solvents (eg., replacing mesitylene with water/oil systems), making it more environmentally friendly and safer. Self-supporting films can be generated in situ at the interface, eliminating powder processing and transfer steps and simplifying the process flow. However, synthesized films are fragile upon peeling, and achieving large-area uniform films (>100 cm²) remains a major challenge. Currently used bond types include imine, boronate ester, and hydrazone linkages.^60–62

Sonochemical Synthesis

Ultrasound, capable of generating uniform high temperature and pressure, is an excellent energy source for COF synthesis. The core of sonochemical synthesis is the acoustic cavitation effect.⁶³ High-frequency ultrasound (typically ≥20 kHz) applied to a liquid medium causes periodic compression and rarefaction, forming micrometer-sized bubbles that collapse violently. This collapse instantaneously creates extreme microenvironments with local high temperature (>5000 K), high pressure (>1000 atm), and high cooling rates (~10⁹ K·s⁻¹). In 2022, Andy Cooper’s team first synthesized imine-linked COFs via sonochemical synthesis in an aqueous phase.⁶⁴ By optimizing ultrasound parameters (power, time), the resulting sonoCOF-3 exhibited superior specific surface area and photocatalytic hydrogen evolution performance compared to samples made by traditional solvothermal methods (solvoCOF-3). Separately, Zhao developed an efficient, simple, and scalable sonochemical method to obtain ultrathin 2D COFs on the gram scale. Apart from replacing conventional heating with ultrasound, the reaction system is similar to solvothermal synthesis.

Using this method, reaction times are reduced from days to hours, and the ultrathin COFs show excellent dispersibility. The synthesis operation is simple: reactions proceed at room temperature and ambient pressure, requiring no special equipment (like autoclaves). It is highly efficient and rapid: reaction times are shortened from the 3–7 days typical of solvothermal methods to within 1 hour, with gram-scale production achievable. Additionally, the synthesis is environmentally friendly. However, ultrasound power/duration requires fine optimization; otherwise, it may cause structural defects or fragmentation of nanosheets. In large-scale production, uneven distribution of the acoustic field might affect product uniformity. Currently applicable bond types include imine, boronate ester, and β-ketoenamine linkages.⁶⁵

In summary, solvothermal synthesis remains the benchmark method for obtaining high-quality, high-crystallinity COFs. Alternative or complementary methods like microwave-assisted synthesis, ionothermal synthesis, and mechanochemical synthesis have been developed to overcome the drawbacks of solvothermal synthesis (long duration, high energy consumption, use of large amounts of organic solvent). Each alternative method offers unique advantages (speed, greenness, solvent-free operation, etc)., although crystallinity control may still require optimization in some cases. The choice of a suitable synthesis method depends on factors such as the target COF structure, the desired bond type, requirements for crystallinity/yield, and environmental friendliness.

Structural Differences and Performance Comparison Between Covalent Organic Frameworks and Conventional Nanocarriers

To systematically elucidate the distinctive advantages of COFs in biomedical applications, Table 1 compares COFs with several conventional nanocarriers, including metal-organic frameworks (MOFs), mesoporous silica nanoparticles (MSNs), liposomes, and polymeric micelles, across key parameters such as drug loading capacity, pore size tunability, biodegradability, stimuli-responsiveness, and in vivo stability.

As summarized in Table 2, COFs offer a unique combination of features that distinguish them from other nanocarriers. Compared with MOFs, COFs are entirely metal-free, substantially reducing the risk of long-term metal-associated toxicity while maintaining high crystallinity and structural versatility. Unlike inorganic mesoporous silica nanoparticles (MSNs), which often exhibit limited biodegradability and potential long-term retention concerns, COFs can be engineered with labile covalent bonds to achieve controlled degradation and favorable clearance profiles. Relative to conventional organic nanocarriers such as liposomes and polymeric micelles, COFs possess rigid, well-defined porous architectures that enable precise pore size tuning and exceptionally high drug loading capacities, avoiding the burst release issues commonly associated with amorphous or self-assembled systems. Furthermore, the dynamic covalent linkages in COFs allow for the rational design of stimuli-responsive backbones, enabling sophisticated drug release profiles in response to tumor microenvironment cues. Collectively, these attributes position COFs as a next-generation nanoplatform that bridges the gap between inorganic and organic nanocarriers, offering unparalleled structural programmability, biocompatibility, and multifunctionality for precision cancer theranostics.

Table 2 Comparison of COFs with Conventional Nanocarriers for Antitumor Applications

COFs for Smart Responsive Drug Delivery

The effective delivery of drugs within tumor tissues faces challenges due to the complex intracellular environment. The tumor microenvironment (TME) comprises various components, such as stromal cells,⁶⁶ endothelial cells, fibroblasts,⁶⁷ immune cells, and macromolecules (eg., cytokines, microRNAs, and other epigenetic factors) that regulate cancer cell fate.^68–71 Additionally, hypoxia^72–74 and low pH⁷⁵ are significant hallmarks of the TME. In recent years, strategies targeting the TME have emerged as highly promising cancer treatments, given its crucial role in regulating tumor progression and therapeutic response. Covalent organic frameworks (COFs), leveraging their designable pore structures, precise functionalization capabilities, and adaptive dynamic covalent bonds, have become ideal carriers for constructing TME-responsive drug delivery systems. This represents the core advantage of COFs as drug delivery systems. They can accurately recognize characteristic signals of the TME (such as acidic pH, hypoxia, high specific enzyme activity, or abnormal redox levels) to achieve spatiotemporally controlled drug release, thereby significantly enhancing therapeutic efficacy and reducing systemic toxicity.

Single-Stimulus-Responsive Drug Delivery Systems

Through functional modification or framework design, COFs can respond to specific signals within the TME to trigger drug release.

Hypoxia-Responsive Drug Delivery System

Hypoxia is a core feature of the TME, closely associated with rapid tumor cell proliferation, dysregulated angiogenesis, and metabolic disorders. Studies show that even under oxygen-sufficient conditions, tumor cells primarily metabolize glucose via glycolysis at rates up to 20 times faster than normal cells, consuming large amounts of oxygen and producing lactic acid accumulation, which further inhibits mitochondrial oxidative phosphorylation.^76,77 Furthermore, abnormal tumor vasculature (eg., disordered endothelial cell arrangement, incomplete pericyte coverage) leads to vessel tortuosity, increased permeability, and uneven blood perfusion. Overexpression of vascular endothelial growth factor (VEGF) forms numerous dysfunctional blind-end vessels, reducing oxygen delivery efficiency.⁷⁸ Simultaneously, excessive collagen deposition in the tumor extracellular matrix (ECM) thickens interstitial fluid pressure, restricting oxygen diffusion and creating a progressive hypoxic gradient away from blood vessels. These factors collectively cause and exacerbate hypoxia within tumor tissues. Sustained hypoxia activates hypoxia-inducible factor HIF-1α and its downstream pathways, further promoting angiogenesis, enhancing glycolysis, and inducing immunosuppression, forming a vicious cycle that ultimately fosters tumor progression, invasion, metastasis, and poor prognosis.⁷⁹

Based on the tumor-specific hypoxic environment, researchers have developed various hypoxia-responsive nanomedicines. Such designs aim to utilize the hypoxic signal to enhance targeting, minimize adverse effects on normal tissues, and achieve precise treatment of lesions. For example, Jiang et al developed a nano-sized hypoxia-responsive azobenzene-containing COF, within whose pores photosensitizer chlorin e6 (Ce6) and the hypoxia-activated prodrug tirapazamine (TPZ) were immobilized (Figure 2).⁸⁰ When this COF enters hypoxic tumor regions, its structure ruptures, releasing the loaded drugs. Concurrently, under near-infrared (NIR) light irradiation, Ce6 consumes oxygen to generate reactive oxygen species (ROS), further intensifying local hypoxia. These two hypoxia-triggered steps sequentially induce COF disintegration, drug release, and TPZ activation.

Figure 2 (a) Synthesis of TA-COF-P@CT; (b) Schematic illustration of hypoxia-sensitive TA-COF-P@CT for drug/PDT therapy. The red arrow indicates an increase in the degree of hypoxia; (c and d) Drug release under hypoxic and normoxic conditions. Scale bar is 20 μm. ***p < 0.001 analysed by Student’s t test, one-tailed. Reproduced with permission from ref.80 Copyright 2021, American Chemical Society.

In addition, he and collaborators synthesized a multifunctional COF platform with high porphyrin loading capacity by crosslinking photosensitizer tetrakis(4-hydroxyphenyl)porphyrin (THPP) with O-cleavable thioketal (TK) linkers (Figure 3).⁸¹ This platform significantly improved ROS generation efficiency, facilitating photodynamic therapy (PDT) applications. Under 660 nm laser irradiation, the nanoplatform generates substantial singlet oxygen to kill tumor cells. Meanwhile, oxygen consumption exacerbates tumor hypoxia, subsequently activating the co-loaded hypoxia-activated prodrug AQ4N, enabling hypoxia-activated cascade chemotherapy. This innovative nano-sized COF platform, combined with laser-controlled drug release, greatly enhanced tumor suppression efficacy.

Figure 3 (a) Schematic illustration for the synthesis of AQ4N@THPP TK -PEG NPs and the mechanism of PDT and hypoxia-activated cascade chemotherapy. *p < 0.05, **p < 0.01, ***p < 0.001; (b) AQ4N@THPP TK-PEG NPs demonstrate potent tumor growth inhibition; (c) 4T1 tumor-bearing mice’s cumulative survival in treated groups; (d) Tumor slides stained with H&E, TUNEL, and HIF-1 α in different treatment groups. Reproduced with permission from ref.81 Copyright 2022, Elsevier BV.

pH-Responsive Drug Delivery System

Tumor cells primarily metabolize glucose via glycolysis even under aerobic conditions, producing large amounts of lactic acid. Simultaneously, disordered tumor vasculature hinders lactate/H⁺ efflux, causing accumulation within the extracellular matrix (ECM), forming the characteristic acidic microenvironment of the TME (pH typically 6.5–7.0). Studies indicate that tumor cells can adapt and even proliferate in acidic environments, making them more resistant to therapy and potentially more invasive.^82–84

Currently, COFs can achieve pH-responsive drug release through mechanisms like protonation effects, acid-labile bond cleavage, and hydrophilic–hydrophobic transitions. Sun et al successfully encapsulated the photothermal agent hypocrellin B (HPB) into a COF via a one-pot method (Figure 4).⁸⁵ The HPB@COF platform not only exhibits excellent biocompatibility and high photothermal conversion efficiency, but also demonstrates precise drug release control at tumor sites due to its unique pH-responsive release properties, thereby achieving outstanding tumor growth inhibition capabilities. Additionally, Yang et al designed a polyethylene glycol (PEG)-modified one-dimensional nanoscale coordination polymer (1D-NCP) carrier with inherent biodegradability, large specific surface area, and pH-responsive behavior.⁸⁶ In the representative system Mn-ICG@pHis-PEG/GA, the carrier demonstrated effective pH-responsive retention within tumors following systemic administration. This targeted accumulation was critically enhanced by its pH-sensitive design, which also facilitated the controlled release of glycyrrhetinic acid (GA), a natural Hsp90 inhibitor. Under mild heating (∼43 °C) induced by NIR irradiation, this pH-guided targeting and release strategy efficiently triggered tumor cell apoptosis, achieving highly effective mild-temperature photothermal therapy (PTT). This work not only presents a facile method for constructing tumor-specific, pH-responsive 1D-NCPs but also proposes a unique mild-temperature PTT strategy, fundamentally enabled by the pH-responsive release mechanism.

Figure 4 (a) The synthesis process of HPB@COF and Schematic illustration of the mechanism of HPB@COF acting to protect the normal cells and kill the cancer cells; (b) The relative tumor volume changes of mice for all groups: (I) PBS as the control; (II) NIR; (III) HPB; (IV) HPB@COF, (V) HPB + NIR; (VI) HPB@COF + NIR, respectively. The yellow dotted circle indicates the tumor focus that has been completely cleared by naked eyes; (c) SEM images of the HPB@COF; (d) Semi-quantitative statistics of relative tumor volume changes in all groups of mice; (e) Release profiles of HPB@COF under different pH conditions. Reproduced with permission from ref.85 Copyright 2022, Royal Society of Chemistry.

Glutathione (GSH)-Responsive Drug Delivery System

The concentration of reduced glutathione (GSH) is significantly elevated in the tumor microenvironment. This primarily stems from: high expression of γ-glutamylcysteine synthetase (γ-GCS) and glutathione synthetase (GSS) in tumor cells, catalyzing GSH synthesis from glutamate/cysteine/glycine; overexpression of the cystine/glutamate antiporter (System Xc⁻, containing the SLC7A11 subunit), accelerating extracellular cystine uptake; rapid tumor proliferation leading to excessive accumulation of reactive oxygen species (ROS) (concentration >50 μM), activating the NRF2/KEAP1 pathway and upregulating antioxidant genes (eg., GCLC, GCLM) to promote GSH synthesis for ROS scavenging; mitochondrial dysfunction (eg., mutant p53 inhibiting SCO2) forcing cells to rely on the glutathione peroxidase 4 (GPX4)-GSH pathway to maintain redox homeostasis. These mechanisms collectively shape the persistently evolving reductive environment within the TME.

The high intracellular GSH levels in tumor cells can reduce disulfide bonds (–S–S–) within COFs, causing carrier disintegration. Therefore, designing GSH-responsive nanodelivery systems is an effective strategy. Dong et al first reported a disulfide bond-linked porphyrin COF (Figure 5).⁸⁷ This COF can undergo biodegradation triggered by endogenous GSH within tumor cells. After loading the chemotherapeutic drug 5-fluorouracil (5-Fu), the formed multifunctional COF nanoreagent could be effectively dissociated by the high intracellular GSH concentration in tumor cells, efficiently releasing 5-Fu to achieve selective killing of tumor cells. Combined with GSH depletion-enhanced PDT effects, this system achieved ideal synergistic therapy against MCF-7 breast cancer cells.

Figure 5 (a) Synthesis of 5-Fu3 Nanoparticle DSPP-COF, Drug Release Triggered by Endogenous GSH, and GSH-Depletion-Enhanced Photodynamic Therapy Combined with Antitumor Activity of 5-Fu3 Nanoparticle DSPP-COF. In the figure, the white upward arrow represents the increase of index concentration, and the white downward arrow represents the decrease of index concentration; (b) Calcein-AM/PI double staining. (i) control, (ii) nano DSPP-COF, (iii) 5-Fu3nano DSPP-COF, (iv) nano DSPP-COF + light, and (v) 5-Fu3nano DSPP-COF + light, respectively. The scale bar is 100 μm. Reproduced with permission from ref.87 Copyright 2023, Royal Society of Chemistry.

Tang’s team constructed a porphyrin-containing nano-COF platform, where the porphyrin units simultaneously serve as GSH-responsive nitric oxide (NO) donors.⁸⁸ The platform was further modified with hyaluronic acid (HA) to enhance biocompatibility and tumor targeting. Under laser irradiation, the porphyrin units within the COF effectively generate ROS to kill cancer cells. Simultaneously, after internalization by tumor cells, high GSH levels trigger the platform (BFX@COF-HA) to rapidly and completely release NO for gas therapy (GT). NO release is accompanied by GSH depletion, which further elevates ROS levels, thereby achieving synergistic enhancement of gas therapy (GT) and photodynamic therapy (PDT).

Enzyme-Responsive Drug Delivery System

The tumor microenvironment often harbors overexpressed specific enzymes (such as β-glucuronidase, matrix metalloproteinases (MMPs), etc). These enzymes can specifically hydrolyze enzyme-sensitive groups (eg., specific peptide chains or glycosidic bonds) modified on the COF surface, thereby triggering drug release. Designing drug delivery systems based on enzyme responsiveness is another effective strategy for achieving tumor-targeted release.^89–93

Researchers have designed COFs with surface modifications of glucuronic acid groups that can be specifically cleaved by β-glucuronidase highly expressed in the TME. In normal tissues with low enzyme activity, the drug remains stably encapsulated; once reaching the tumor site, enzymatic hydrolysis cleaves the linker, enabling targeted drug release.

Multi-Stimulus-Responsive Drug Delivery

Multi-stimulus-responsive drug delivery systems represent an advanced platform designed to respond to multiple signals or triggers within the TME to release therapeutic agents. By responding to various environmental or physiological factors (such as pH, GSH, enzymes, light, temperature, magnetic fields, or specific biomarkers), these systems significantly improve the precision, efficiency, and safety of drug delivery, achieving synergistic effects (1+1 > 2).

Zhou’s team synthesized a covalent organic framework based on diselenide-bridged porphyrin (DiSe-Por-DOX), exhibiting pH/GSH/photothermal triple sensitivity (Figure 6).⁹⁴ It utilizes specific characteristics of the tumor TME (high intracellular GSH content, acidic pH and photothermal) to achieve controlled drug release. DiSe-Por-DOX is internalized by tumor cells and gradually degrades, releasing the encapsulated chemotherapeutic drug doxorubicin (DOX) in response to stimuli. Its unique responsiveness manifests in: cleavage of the diselenide bond (–Se–Se–) under acidic and high GSH conditions in the TME, promoting intracellular ROS generation and exerting chemodynamic therapy (CDT) effects; concurrently, the highly extended two-dimensional porous structure of this COF, under the high temperature generated by the photothermal effect, accelerates drug molecule movement, further promoting drug penetration into cells. Benefiting from its multi-responsive properties, the DiSe-Por-DOX system achieves near-complete drug release at the tumor site and maximizes the synergistic antitumor effects of chemotherapy and phototherapy.

Figure 6 (a) Schematic representation of the preparation of DiSe-Por-DOX and its anti-tumor application; (b) Fluorescence images of PC-3 cells incubated with DiSe-Por-DOX with different treatments. The bar is 100 μm. Reproduced with permission from ref.94 Copyright 2022, Royal Society of Chemistry.

COFs has attracted extensive attention in the field of nano-biomedicine in recent years because of its excellent specific surface area, ordered pore structure and flexible and adjustable chemical modification. As a new type of organic porous material, COFs can be used as an ideal core carrier to realize the effective integration of various diagnosis and treatment functions. Next, we further discuss the unique advantages of the nano-platform based on COF in the application of cancer diagnosis and treatment: on the one hand, the platform can realize accurate imaging of tumor areas by loading or modifying contrast agents; On the other hand, it also shows rich functional diversity at the therapeutic level, covering chemical kinetic therapy (such as inducing tumor cell death based on Fenton reaction) and various therapeutic modes driven by external physical fields such as light, microwave and ultrasound. What is particularly important is that the above-mentioned single treatment methods do not exist in isolation, but can achieve synergistic combination through reasonable design, enhance the anti-tumor effect from different mechanisms, and reduce the possible side effects of single treatment. COFs has shown broad application prospects and important research value in building an accurate, efficient and low toxic and side effects cancer treatment system (Figure 7).

Figure 7 Schematic representation of theranostic applications (ie., cancer imaging, CDT, MWTT, PDT, PTT, SDT, and combination therapy) of COF NPs.

Application of COFs in Cancer Diagnosis

Nanoprobes

Non-invasive imaging is a crucial tool for tumor diagnosis. Although widely used techniques such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasonography (US) are prevalent, their contrast agents face challenges including rapid in vivo metabolism and potential side effects, prompting researchers to explore safer and more effective alternatives.^95–97 Covalent organic frameworks (COFs), as an emerging class of porous crystalline materials, exhibit tremendous potential in tumor diagnosis due to their highly ordered pore structures, extremely large specific surface area, excellent structural designability, good chemical stability, and ease of functionalization. COFs can serve as ideal carriers for fluorescent molecules or tracer molecules, or can be used for their own fluorescent tracing capabilities, and can be applied to a variety of diagnostic strategies.^98,99

N. Li et al developed a carbonized COF (C-COF)-based nanoprobe for cancer cell imaging (Figure 8a).¹⁰⁰ COF nanoparticles (NPs) were first synthesized at room temperature, followed by pyrolysis under an inert atmosphere to obtain C-COF. Subsequently, dye-labeled specific recognition sequences (targeting survivin mRNA and TK1 mRNA) were physically adsorbed onto the C-COF surface, yielding the C-COF@survivin and C-COF@tk1 probes. These probes specifically illuminate target tumor biomarkers in cancer cells while exhibiting high biocompatibility and low background fluorescence. Compared to conventional COF NPs, C-COFs demonstrated superior dispersibility, enhanced fluorescence quenching efficiency, and higher photothermal conversion capability. Owing to their exceptional specificity, good stability, and high biocompatibility, these probes proved to be effective tools for intracellular cancer imaging.

The same team (Li et al) further developed a porphyrin-based COF-DNA dual-color fluorescent probe for the diagnosis of various cancers (Figure 8b).¹⁰¹ They prepared two COF-DNA probes (COF-BP-RT and COF-BP-FZ) under room temperature and cryogenic conditions, respectively. The resulting COFs exhibited good crystallinity and biocompatibility. After loading TAMRA-labeled survivin-mRNA and Cy5-labeled TK1-mRNA recognition sequences onto the porphyrin-based COF NPs, the dye fluorescence signals were initially quenched by the COF NPs and subsequently recovered upon encountering the target nucleic acids, enabling specific fluorescence imaging. The study revealed that the cryogenic preparation condition (COF-BP-FZ) effectively increased the DNA loading capacity of the COF NPs and improved their fluorescence quenching efficiency, without compromising the target recognition ability of the probes. More importantly, the nanoprobes prepared via the cryogenic method exhibited a significantly improved signal-to-noise ratio, providing a more reliable approach for cancer detection.

Figure 8 (a) Synthesis of carbonized COF NPs and its application for cancer cell imaging. Reproduced with permission from ref.100 Copyright 2021, American Chemical Society. (b) Synthesis of COF and the Multicolor Nanoprobe and Its Application for Cell Imaging. Reproduced with permission from ref.101 Copyright 2022, American Chemical Society. (c) Synthesis of COF@survivin/MUC1 and schematic representation of its cancer imaging applications. Reproduced with permission from ref.102 Copyright 2021, American Chemical Society. (d) Preparation, tumor cell specific imaging and therapeutic applications of TpDh-DT. Reproduced with permission from ref.101 Copyright 2021, American Chemical Society.

Tang et al developed a COF-based triple-color fluorescent nanoprobe (COF@survivin/MUC1) capable of simultaneously imaging spatially distinct biomarkers in living cells (Figure 8c).¹⁰² They synthesized intrinsically fluorescent COF NPs with a diameter of approximately 80 nm and an emission wavelength near 510 nm. Using cryogenic treatment to enhance interactions, Cy5-labeled MUC1 aptamer and TAMRA-labeled survivin mRNA antisense oligonucleotides were adsorbed onto the COF NP surface. This probe demonstrated high specificity, restoring its fluorescence signal only upon encountering specific targets. Furthermore, the intrinsic fluorescence of the COF itself enabled real-time observation of NP cellular uptake and intracellular distribution, offering a new and promising direction for in vivo imaging of cancer cells.

Subsequently, the Tang team reported an intelligent nucleic acid-gated COF nanosystem integrating cancer-specific imaging and microenvironment-responsive drug release (Figure 8d).¹⁰¹ This system utilized Cy5 dye-labeled single-stranded DNA (ssDNA), adsorbed onto the surface of doxorubicin (Dox)-loaded COF nanoparticles (NPs), to recognize specific mRNA (eg., TK1 mRNA). The Dox loaded within the pores enhanced the interaction between the ssDNA and the COF, as well as the quenching effect on Cy5 fluorescence, while the encapsulated ssDNA effectively prevented Dox leakage. In normal cells, this nanoplatform exhibited weak fluorescence signals and low Dox release. In tumor cells, however, the overexpressed TK1 mRNA competitively bound to and released the ssDNA, leading to the recovery of the Cy5 fluorescence signal and triggering Dox release for chemotherapy. This nanoplatform provides a novel approach for cancer theranostics and an effective strategy for modulating interactions between COFs and biomacromolecules.

Nanoscale Separation and Enrichment Platform

The regular pore structures and tailorable pore-wall chemistry of COFs endow them with high selective adsorption capacity for specific tumor-related biomarkers, such as phosphorylated peptides, glycosylated proteins, and small-molecule metabolites. This property can be utilized for the pre-treatment enrichment of trace targets in complex biological samples (eg., serum, tissue fluid), significantly enhancing the accuracy and reliability of subsequent highly sensitive detection techniques like mass spectrometry.

Luo et al developed bifunctional magnetic covalent organic framework nanospheres (MCNC@Polymer@COF-MUBA) for the simultaneous enrichment of phosphopeptides and glycopeptides.¹⁰³ The material features a magnetic core for rapid separation, a highly ordered COF shell with uniform mesopores (~3.2 nm) for size-exclusion of large proteins, and surface-grafted MUBA molecules providing dual-affinity interactions: hydrogen bonding for phosphopeptides and hydrophilic interactions for glycopeptides. This design enabled highly selective and simultaneous capture of both types of peptides even in the presence of 1000-fold interfering proteins (BSA), with detection limits as low as 0.05 fmol/μL. The material was successfully applied to enrich phosphopeptides and glycopeptides from complex samples such as rat liver tissue digests and plasma-derived exosomes from liver cancer patients, demonstrating its great potential for proteomic profiling and biomarker discovery in clinical samples (Figure 9).

Figure 9 Schematic Diagram of the Binding Modes and Controllable Selective Enrichment of MCNC@Polymer@COF-MUBA Nanospheres for Phosphopeptides and Glycopeptides. Reproduced with permission from ref.103 Copyright 2021, Elsevier.

Nanobiosensors

The high specific surface area and porous structure of COFs enable the efficient loading of signal probes such as fluorescent dye molecules (eg., fluorescein, rhodamine B) or quantum dots.¹⁰⁴ When target tumor biomarkers (eg., microRNA, proteins, enzymes, small-molecule metabolites) interact specifically with the COFs or their surface-loaded recognition probes (eg., via nucleic acid hybridization, antigen-antibody binding, enzyme-catalyzed reactions), this can cause significant enhancement (“Turn-On”) or reduction (“Turn-Off”) of the fluorescence signal, thereby enabling highly sensitive detection of the targets. Concurrently, the COF structure provides excellent protection for the loaded probes, enhancing their stability within complex biological environments.¹⁰⁵

Liang et al developed magnetic covalent organic framework nanospheres (MCOF) for the highly sensitive detection of glioma-associated miRNA-182. The MCOF consists of a magnetic Fe₃O₄ core and a highly crystalline COF shell, which facilitates efficient adsorption of hairpin DNA probes via hydrogen bonding and π–π interactions. Upon hybridization with the target miRNA, the formation of double-stranded DNA leads to a “Turn-On” fluorescence signal due to reduced adsorption on the MCOF surface.¹⁰⁶ This biosensor achieved a detection limit of 20 fM in human serum, with a linear range of 0.1–10 pM and excellent specificity against single-base mismatches. Moreover, the MCOF-based sensor enabled visual detection of miRNA-182 in micro-samples using a capillary chip system, demonstrating its potential for point-of-care glioma diagnosis and postoperative monitoring (Figure 10).

Figure 10 Schematic diagram of synthesis and detection. (a) The dispersed and high-crystalline COF-coated Fe3O4 magnetic nanospheres (MCOF) prepared by monomer-mediated in-situ interface growth strategy. (b) The fluorescent signal amplified miRNA biosensor constructed by using two hairpin DNA probes. Reproduced with permission from ref.106 Copyright 2021, China Science Press and Elsevier.

Application of COFs in Cancer Therapy

Chemodynamic Therapy

Chemodynamic therapy (CDT) is an emerging tumor treatment strategy. Its core principle involves utilizing the overexpressed hydrogen peroxide (H₂O₂, concentration reaching 50–100 μM) in the tumor microenvironment (TME) as a reaction substrate. Under the catalysis of endogenous transition metal ions (eg., Fe²⁺, Cu⁺), Fenton or Fenton-like reactions occur locally within the tumor, generating highly toxic hydroxyl radicals (·OH). This concept was first systematically proposed and named by Professor Buabeau’s team in 2016. CDT demonstrates significant unique advantages: its tumor selectivity stems from dependence on the TME’s characteristic weak acidity, high H₂O₂ environment, and specific metal ions;¹⁰³ the treatment process requires no external energy input (eg., light, sound, radiation), simplifying operation; by consuming H₂O₂ and glutathione (GSH), it also holds the potential to partially alleviate the tumor’s immunosuppressive state;¹⁰⁷ moreover, the treatment cost is relatively low. However, the efficiency of single CDT can be limited by factors such as insufficient H₂O₂ concentration or excessive GSH consumption depleting metal catalysts.

To overcome these challenges, researchers actively develop synergistic therapeutic strategies, combining CDT with chemotherapy (enhancing cytotoxicity), radiotherapy (promoting radical generation via radiation), phototherapy (photothermal/photodynamic therapy, PTT/PDT, providing heat or additional reactive oxygen species (ROS),¹⁰⁸ potentially ameliorating hypoxia),¹⁰⁹ sonodynamic therapy (SDT, ultrasound activating sonosensitizers to produce ROS), and immunotherapy (activating anti-tumor immune responses by leveraging CDT-induced immunogenic cell death). This multimodal synergy significantly enhances the overall anti-cancer efficacy and expands the application scenarios and clinical translation potential of CDT. Covalent organic frameworks (COFs) exhibit remarkable advantages in CDT, primarily due to their customizable pore structures, high specific surface area, surface functional diversity, and tunable catalytic activity.¹¹⁰

Zhou’s team developed the RSL3@COF-Fc nanosystem, which innovatively co-loads ferrocene (Fc) and the glutathione peroxidase 4 (GPX4) inhibitor RSL3 onto a COF carrier. This system synergistically disrupts tumor cell redox balance through a dual pathway, significantly enhancing CDT efficacy (Figure 11).¹¹¹ The mechanism of action is as follows: after the nanosystem is endocytosed by tumor cells, the acidic TME triggers the release of RSL3, which specifically inhibits GPX4 activity, blocking the intracellular lipid peroxidation repair pathway; simultaneously, the highly dispersed ferrocene units within the COF skeleton continuously convert endogenous tumor H₂O₂ into highly toxic ·OH via Fenton-like reactions, exacerbating lipid peroxidation accumulation. The RSL3-mediated collapse of antioxidant defenses and the Fc-driven burst of ·OH form a positive feedback loop, ultimately leading to irreversible ferroptosis—characterized by plasma membrane rupture, increased lysosomal membrane permeability, and mitochondrial cristae destruction.¹¹² This achieves highly selective cancer cell elimination while avoiding systemic toxicity to normal tissues. This study first proposed the “COF-redox dyshomeostasis” therapeutic strategy, providing a new paradigm for developing highly efficient and low-toxicity nanomedicine catalytic platforms.

Figure 11 (a and b) Synthesis of RSL3@COF-Fc and oncological applications. Reaction conditions: (i) CH3COOH, acetonitrile, 25 °C, 12 h; (ii) RSL3, ethanol, 25 °C, 12 h; (iii) CH3COOH, acetonitrile, 25 °C, 12 h; and (iv) RSL3, ethanol, 25 °C, 12 h. In the figure, the green up arrow represents inducing lipid peroxidation, and the red down arrow represents blocking Gpx4; (c) Laser scanning confocal fluorescence microscopy images of JC-1, LysoTracker Red DND-99, and AO staining for determining mitochondrial membrane potential, lysosomal deacidification, and lysosomal membrane permeabilization, respectively. Scale bar: 50 µm. Reproduced with permission from ref.111 Copyright 2021, WILEY.

Photothermal Therapy

Photothermal therapy (PTT) is a non-invasive tumor treatment modality that utilizes photothermal agents to convert light energy into heat under near-infrared (NIR) light irradiation to kill tumor cells. Compared to traditional therapies (surgery, radiotherapy, chemotherapy), PTT offers advantages such as strong targeting capability, low systemic toxicity, short treatment time (approximately minutes), and significant efficacy. It is also frequently combined with chemotherapy, photodynamic therapy, gene therapy, and immunotherapy for synergistic tumor treatment. COFs, owing to their customizable light absorption properties, high specific surface area, and flexible stimulus-responsive design, have become ideal platforms for PTT.^113–116

Bo Tang’s team developed the COF-GA nanoplatform, which innovatively integrates the photothermal conversion capability of COFs with the molecular regulatory functions of glycyrrhizic acid (GA), overcoming the limitations of traditional high-temperature PTT (Figure 12).¹¹⁷ The mechanism of this system is as follows: under NIR laser irradiation, the COF carrier efficiently absorbs light energy and converts it into heat, gently elevating the local tumor temperature to 42–45 °C (below the traditional PTT threshold of >50 °C), selectively inducing cancer cell apoptosis; the loaded GA, acting as an HSP90 protein inhibitor, effectively blocks the tumor heat shock response pathway, significantly lowering the heat resistance threshold of cancer cells, thereby enhancing the efficacy of mild PTT by 2.3-fold. The tumor-targeting accumulation property of COF-GA combined with the mild heating strategy (<45 °C) achieves significant tumor inhibition (91% tumor volume inhibition rate in tumor-bearing mice) while minimizing non-specific thermal damage to adjacent normal tissues. This study not only confirms the potential of COFs as efficient PTT carriers but also, through the synergistic paradigm of “photothermal conversion–molecular regulation,” provides a new perspective for developing low-toxicity, high-efficiency tumor precision thermotherapy.

Figure 12 (a) Schematic of preparing COF-GA and enhancing mild-temperature photothermal therapy. The red upward arrow indicates that the heat shock protein is up-regulated. The red downward arrow indicates that the tumor’s heat resistance is inhibited; (b)) The standard curve of GA was measured by HPLC; (c)) Temperature variation curves for COFs with different concen-trations. The irradiation laser is 635 nm (0.3 W cm−2); (d)) Photostability test for COFs. Reproduced with permission from ref.117 Copyright 2021, Royal Society of Chemistry.

Photodynamic Therapy

As a minimally invasive cancer treatment, photodynamic therapy (PDT) has garnered significant attention in recent years.^118,119 This therapy utilizes light of specific wavelengths to irradiate tumor tissue, exciting photosensitizers to produce cytotoxic reactive oxygen species (ROS), such as superoxide anions, free radicals, and singlet oxygen. This induces tumor cell death (including apoptosis, necrosis, and excessive autophagy) to achieve therapeutic effects. ROS generation primarily occurs through two photochemical pathways: Type I reactions involve the photosensitizer transferring an electron to molecular oxygen or other electron acceptors; Type II reactions involve the photosensitizer transferring energy to ground-state molecular oxygen, generating singlet oxygen.

Metal–organic framework (MOF)-based nanoplatforms, with their high porosity and ease of functionalization, serve as excellent nanocarriers for efficiently delivering photosensitizer molecules or nanoparticles (NPs) to tumor sites. This not only significantly enhances the stability and targeting of photosensitizers but also improves therapeutic efficacy. Furthermore, smart MOFs constructed from porphyrin-based organic ligands can directly serve as PDT photosensitizers, offering a promising option for tumor photodynamic therapy.

Tang et al constructed a porphyrin-based covalent organic framework (COF) enzyme carrier loaded with glucose oxidase (GOx) and catalase (CAT) to achieve long-term starvation therapy and enhance PDT (Figure 13).¹²⁰ In this system, CAT catalyzes the decomposition of overexpressed H₂O₂ in the tumor microenvironment to produce O₂. This simultaneously accelerates GOx-mediated glucose consumption (enhancing starvation therapy) and provides ample oxygen for PDT. Under laser irradiation, the porphyrin-based COF efficiently generates cytotoxic ROS, significantly boosting PDT efficiency. They further investigated the synergistic anti-cancer effect of COF@GOx and CAT on a mouse 4T1 xenograft model, confirming that the combined therapeutic effect of PDT and long-term starvation therapy under experimental conditions surpassed that of monotherapy. W. Pan et al developed a modification-promoted exfoliation strategy for the one-step preparation of ultrathin two-dimensional functionalized COF nanosheets (COF NSs).¹²¹ The obtained hyaluronic acid-functionalized ultrathin porphyrin COF NSs (thickness ~5–8 nm) exhibit enhanced ROS generation effects and are easily prepared for tumor-targeted PDT.

Figure 13 (a) Schematic illustration for the preparation and therapeutic application of the activatable nanosensitizers RCMP; (b and c)) Bio-transmission electron microscopy images of cells treated with cell medium (control) and RCMP+US irradiation (GSH-activated SDT). Mitochondria and chromatin are indicated by Orange and blue arrows respectively. Scale bar: 1 μm. Mitochondria and chromatin are indicated by orange and blue arrows, respectively. Reproduced with permission from ref.118 Copyright 2023, Wiley.

Sonodynamic Therapy

Sonodynamic therapy (SDT) is a novel, non-invasive, and precise tumor treatment method. Its principle relies on the high-concentration aggregation of ultrasound sensitizers with “targeting” properties within tumor cells and tumor neovascular endothelial cells. Although photodynamic therapy (PDT) is one of the most successful and widely used cancer eradication technologies, its potential is limited by the insufficient tissue penetration depth of light, leading to a short effective half-life and small action radius of the generated ROS. In contrast, SDT, under non-invasive conditions and combined with precise imaging localization, utilizes specific ultrasound waves to activate sonosensitizers enriched at the tumor site, triggering sonochemical reactions to produce ROS such as singlet oxygen. These ROS damage tumor cell organelles by seizing electrons, ultimately leading to tumor cell death. Simultaneously, SDT can destroy tumor vascular endothelial cells, induce thromboxane release and thrombus formation within tumor blood vessels, causing ischemic necrosis of tumor tissue. This enables precise and thorough tumor killing without damaging normal cells.^122–125

Pang et al successfully synthesized COF–titanium dioxide nanoparticles (COF–TiO₂ NPs) using a covalent organic framework (COF) as a template. Compared to pure TiO₂ NPs, COF–TiO₂ NPs exhibit a significantly narrowed band gap,¹²⁶ resulting in markedly enhanced SDT performance. Both in vitro and in vivo experiments confirmed the significant tumor suppression effect of COF–TiO₂-mediated SDT. A novel molecular etching strategy based on imine exchange reactions was employed to develop nanoscale COF sonosensitizers. This strategy is suitable for etching bulk imine-linked COFs into nanoparticles. The regular COF structure effectively prevents the loss of sonosensitizing properties caused by porphyrin molecular aggregation and enhances the chemical stability of porphyrin units.¹²⁷ Additionally, the coordination of Fe³⁺ with the COF skeleton endows the nanoparticles with chemodynamic therapy (CDT) capabilities and glutathione (GSH) depletion ability. Enhanced SDT combined with α-programmed death ligand 1 (PD-L1) antibody achieves favorable anti-tumor effects. This innovative strategy for preparing nanoscale COF sonosensitizers provides a new approach for clinical anti-tumor therapy.

Another study synthesized various COF nanobowls in a controllable manner and designed them as “activatable” nanosonosensitizers with tumor-specific sonodynamic activity.¹²⁸ The high crystallinity of the COF nanobowls ensures their ordered porous structure, enabling efficient loading of the small-molecule sonosensitizer Rose Bengal (RB). To avoid non-specific damage to normal tissues, manganese dioxide (MnO) was grown in situ on the RB-loaded COF nanobowls to specifically inhibit their sonosensitizing effect. When the nanosonosensitizers reach the tumor site, the surface MnO reacts with the highly expressed GSH in the tumor microenvironment and rapidly decomposes, thereby “unlocking” the ability of the COF nanosonosensitizers to generate ROS under ultrasound irradiation. The synergistic action of elevated intracellular ROS levels and GSH depletion concurrently induces ferroptosis, further enhancing the efficacy of SDT. Additionally, the unconventional bowl-like morphology facilitates enhanced accumulation and retention of the nanosonosensitizers in tumor tissue. This combined strategy of tumor-specific sonodynamic therapy and ferroptosis induction demonstrates high efficiency in killing cancer cells and inhibiting tumor growth. This study not only paves the way for the biomedical application of COF nanosonosensitizers with unconventional morphologies but also provides an example for achieving activatable, ferroptosis-enhanced sonodynamic tumor therapy.

Microwave Thermal Therapy

Tumor microwave thermal therapy (MWTT) is attracting increasing attention due to its minimal damage to body functions, simple operation, and few complications.^129–132 As an exogenous physical therapy, its strong penetration ability and high thermal efficiency overcome the limited treatment depth (only a few millimeters) of photodynamic therapy (PDT) and photothermal therapy (PTT). Combined with the microwave absorption capability of ionic liquids, it can be used as a microwave thermosensitizer to improve tumor ablation effects. However, incomplete MWTT ablation and the elevated expression of pro-tumor angiogenesis factors (such as vascular endothelial growth factor, VEGF) post-ablation inducing recurrence remain challenges, especially for tumors prone to recurrence and metastasis, such as colorectal cancer.

To address this issue, researchers proposed a nanoscale capsule consisting of a covalent organic framework coating a metal–organic framework (MOF@COF), which combines microwave (MW) thermodynamically sensitizing and anti-angiogenic effects.⁸³ A Bi–Mn–porphyrin (BM) MOF was designed as a microwave sensitizer capable of generating cytotoxicity synergizing with the heat from MWTT, enabling microwave dynamic therapy (MWDT). Covalently coating the BM with COF further enhances these two sensitizing properties while allowing the loading of the hydrophobic apatinib inhibitor to downregulate VEGF expression, thereby inhibiting tumor recurrence. The contained Bi and porphyrin also confer computed tomography (CT) and fluorescence imaging (FI) capabilities to the system. In vivo experiments confirmed that this combination therapy significantly inhibits colorectal cancer growth without recurrence. Therefore, this work proposes a comprehensive strategy derived from MOF@COF to significantly enhance single MWTT and reduce tumor recurrence.

Applications of COFs in Combination Therapy

Although various single therapies based on COFs are developing rapidly, meeting drug delivery needs in cancer therapy, many problems remain to be solved. For instance, the inherent limitations of each single therapy, tumor heterogeneity, and the complex internal environment collectively promote the emergence of COF-based multimodal synergistic therapies, offering beneficial insights for cancer therapy diagnostics. It has been reported that combinations of multiple anti-tumor strategies (eg., PDT and PTT) and combination chemotherapy can yield satisfactory anti-cancer effects.

Photodynamic Therapy/Chemotherapy

The hypoxic conditions of the solid tumor microenvironment limit the production of singlet oxygen (¹O₂), constraining the efficacy of PDT for cancer treatment and often exacerbating hypoxia. To address this problem, Jiang Xiaoming’s team combined photo-guided phototherapy with a hypoxia-activated prodrug as a potential clinical cancer treatment modality. By crosslinking the photosensitizer tetra(4-hydroxyphenyl)porphyrin (THPP) and a thioketone (TK) linker capable of scavenging ¹O₂, they synthesized a multifunctional nano-COF platform (THPP(TK)-PEG) with high porphyrin loading capacity, significantly improving ROS generation efficiency and facilitating PDT. The synthesized THPP(TK)-PEG nanoparticles possess high THPP photosensitizer content and mesoporous structures, allowing further loading of the hypoxia-responsive prodrug banoxantrone dihydrochloride (AQ4N) into the COF.⁸⁰ The nanocarrier surface is coated with a thick PEG layer to promote its dispersibility and therapeutic properties in physiological environments. When exposed to 660 nm radiation, this nanoplatform efficiently generates cytotoxic ¹O₂ for local PDT. Concurrently, oxygen consumption exacerbates tumor hypoxia, inducing the activation of AQ4N, thereby achieving hypoxia-activated cascade chemotherapy and improving overall efficacy. This study provides a pathway to obtain ultrasensitive drug release and effective cascade therapy for cancer treatment.

Photodynamic Therapy/Photothermal Therapy

Emerging evidence suggests that the combination of PDT and PTT may be effective due to their different mechanisms of action potentially producing synergy and their non-overlapping toxicity profiles. Furthermore, the development of multimodal nanoplatforms incorporating both photodynamic and photothermal agents has given strong momentum to PDT/PTT combinations. The efficacy of traditional single-mode PDT or PTT in cancer treatment is often limited by the extreme complexity and heterogeneity of lethal tumors. However, combined PDT/PTT therapy not only improves therapeutic efficiency but also avoids unnecessary damage to normal tissues caused by the higher power and longer irradiation durations required for single-mode phototherapy.

Dong and coworkers prepared a nanoscale COF (NCOF) via a simple synthesis method. Subsequently, using a stepwise synthesis and guest encapsulation process, they successfully fabricated a PDT/PTT dual-mode therapeutic nanoagent, VONc@COF-Por (3) (Figure 14a).¹⁷ The covalently grafted porphyrin resin (Por) and non-covalently loaded vanadyl naphthalocyanine (VONc) are responsible for the PDT and PTT functions of the nanoagent, respectively. Under visible light (red LED) and near-infrared (808 nm laser) irradiation, VONc@COF-Por (3) displays high ¹O₂ generation and a photothermal conversion efficiency (55.9%), providing excellent combined PDT/PTT therapeutic effects that inhibit the proliferation and metastasis of MCF-7 tumor cells, as well-confirmed in both in vitro and in vivo experiments.

Figure 14 (a) Material design and synthesis of VONc@COF-Por. (i) 1,3,5-tris(4-aminophenyl)benzene, 2,5-dimethoxyterephthaldehyde, PVP, acetic acid, CH3CN, 25°C; (ii) COF (1), Por, acetic acid solution (3 M), 1,4-dioxane, reflux; (iii) COF-Por (2), VONc, N,N-dimethylacetamide (DMAC), 25°C.Reproduced with permission from ref.132 Copyright 2019, American Chemical Society. (b) Schematic illustrations of the formation of COF-366 NPs and photoacoustic imaging-guided phototherapy under single wavelength light irradiation. Reproduced with permission from ref.83 Copyright 2019, Elsevier BV.

Typically, combining PDT and PTT requires different components activated by different excitation lights. Developing a single-component photoactive agent capable of simultaneously enhancing both PDT and PTT under a single wavelength light source by adjusting the spatial arrangement of photoactive units is a more desirable strategy for tumor phototherapy. Li Xiaodong’s team synthesized porphyrin-based covalent organic framework nanoparticles (COF-366 NPs), controlling the ordered spatial arrangement of photoactive building blocks, and first applied them for in vivo anti-tumor therapy (Figure 14b).¹³³ COF-366 NPs can simultaneously perform PDT and PTT under a single wavelength light source and possess photoacoustic (PA) imaging capabilities for monitoring, simplifying operation. COF-366 NPs achieve good phototherapeutic effects even against large tumors. The prepared multifunctional COF-366 NPs open a new avenue for phototherapy materials and expand the application scope of covalent organic frameworks.

Photothermal Therapy/Chemotherapy

Researchers proposed and prepared a water-dispersible nanocomposite (COF@IR783) assembled from anatase and COFs.¹³⁴ Firstly, compared to pure COF, this nanocomposite exhibits better dispersibility and water stability. Its nanoscale morphology and negative charge favor improved blood circulation and enhanced permeability, enabling in vivo retention-mediated tumor-targeted delivery. Secondly, the nanocomposite possesses strong photothermal therapy capability in the near-infrared region. It also exhibits photoacoustic imaging capability to guide in vivo anti-tumor therapy. Finally, the nanocomposite can further serve as a drug carrier, loading the anticancer drug cisplatin-aconitine-doxorubicin (CAD) prodrug. Compared to PTT or chemotherapy alone, the combination of PTT and chemotherapy achieved by COF@IR783@CAD synergistically induced cancer cell death in vitro. Intravenous injection of COF@IR783@CAD in mice significantly ablated tumors. This work demonstrates that through rational design, the issues of COF dispersibility and water stability can be appropriately overcome, further expanding the biomedical applications of COFs.

Current Status and Challenges in Clinical Translation of COFs

Covalent Organic Frameworks (COFs) demonstrate significant potential in the field of cancer diagnosis and treatment. However, their development remains largely confined to laboratory and preclinical research stages, and they have not yet entered clinical trials. Nevertheless, research in this area is progressing rapidly, highlighting considerable promise for clinical translation. For COFs to truly transition into clinical applications, several challenges must be overcome.

Synthesis Capacity and Drug Loading Capacity

Drug carriers of covalent organic materials typically have a small loading capacity that may not be adequate for the delivery of larger amounts of drug at one time. Moreover, so far, COFs can only be prepared on a milligram to gram scale in the laboratory. It is still a challenge to synthesize COFs on a large scale and at low cost due to the strict reaction conditions and expensive raw materials, which limits the clinical translation of COFs. On the other hand, obtaining highly crystalline COFs with large surface areas and uniform nanoparticle size distribution remains difficult. Developing new methods to obtain low-cost and high-quality COFs is obviously necessary.^135–140

Toxicity and Biocompatibility of COFs

As novel nanomaterials, biocompatibility and toxicity studies of COFs are only at the laboratory level, and clinical aspects have not been sufficiently investigated. Although COFs themselves may be non-toxic, the synthesized monomers can be toxic. Moreover, while COFs are metal-free and a large number of experiments have shown low short-term toxicity, current studies have only been conducted at the cellular level, and their long-term non-toxicity should be demonstrated in multiple ways (eg., pharmacokinetics).^30,141–143

The biocompatibility of COFs is crucial for their further biomedical applications. The metabolic mechanisms and pathways of nanomaterials in vivo also need to be further investigated. Finally, the current application of nanomaterials in tumor diagnosis and treatment is mainly based on the EPR effect, which is complex and heterogeneous and varies with the composition of COFs, tumor type, and patient’s status.

Standardization and Regulatory Frameworks Require Further Development

Standardization and regulatory frameworks are still in their early stages. As an emerging class of therapeutic materials, COFs currently lack unified pharmacopeial standards, clear quality control guidelines, and well-established safety and efficacy evaluation systems. To advance their clinical translation, close collaboration with drug regulatory agencies (such as the FDA and NMPA) is essential to jointly develop these critical standards and regulations.^144,145

Targeting Efficiency and Therapeutic Efficacy of COFs

Targeting strategies and therapeutic efficacy represent core challenges in the clinical translation of COFs. Although numerous studies have achieved active targeting through surface modifications with targeting ligands (such as antibodies, peptides, or aptamers), these strategies face significant limitations within the complex human physiological environment.^146–150 Targeting efficiency is constrained by multiple factors, including the shielding of targeting molecules by the “protein corona” formed through blood protein adsorption, non-specific clearance by the reticuloendothelial system, and impeded drug penetration due to abnormal vascular structures and high interstitial fluid pressure within tumor tissues. The so-called “protein crown” refers to the phenomenon where, once a nanocarrier enters the bloodstream, plasma proteins rapidly adsorb onto its surface. This protein crown physically shields the surface-modified targeting ligands (such as antibodies, peptides, or aptamers) on the carrier, hindering their specific recognition and binding with receptors on the surface of tumor cells through steric hindrance, thereby leading to a significant decrease in active targeting efficiency. Furthermore, the formation of the protein crown alters the nanoparticle’s recognition characteristics in vivo, leading to its nonspecific uptake by the reticuloendothelial system and further impairing its targeted accumulation at the tumor site. Therefore, the actual in vivo tumor enrichment capability, penetration depth, and cellular internalization efficiency of COFs still require systematic validation using more advanced in vivo models (such as patient-derived organoids or humanized mouse models). Future efforts should focus on optimizing targeting ligand design, developing stimulus-responsive drug release systems, and exploring combined physical approaches (such as ultrasound or magnetic navigation) to synergistically enhance the targeting precision and therapeutic utility of COFs.^151,152

Summary and Perspective

In summary, this review has systematically surveyed recent advances in the synthesis of covalent organic frameworks (COFs) and their emerging applications in tumor diagnosis and therapy. Compared with other nanocarriers, COFs exhibit a compelling combination of attributes—including high porosity, exceptional structural tunability, favorable biocompatibility, and versatile stimuli-responsive drug release capabilities—that collectively position them as a highly promising platform for precision cancer nanomedicine. Despite significant progress, the field of COF-based antitumor nanoplatforms remains at a relatively early stage compared to more established nanocarrier systems. Looking forward, several emerging directions will shape the next generation of COF-based cancer nanomedicine.

Artificial intelligence-assisted design. The vast chemical space accessible through COF chemistry presents both opportunities and challenges for materials discovery. The integration of artificial intelligence (AI) and machine learning (ML) can accelerate the identification of optimal monomer combinations, predict framework crystallinity and porosity, and enable inverse design approaches that work backward from desired therapeutic outcomes to propose optimal COF structures. The convergence of COF chemistry with AI-guided discovery holds immense promise for drastically shortening development timelines and unlocking previously unattainable functional complexities.

Personalized nanomedicine. The inherent modularity of COFs renders them exceptionally well-suited for personalized nanomedicine applications. Advances in rapid, scalable synthesis methodologies may eventually enable on-demand fabrication of patient-specific nanomedicines, wherein tumor-targeting ligands, therapeutic payloads, and imaging agents can be tailored to individual patient profiles. This degree of customization aligns closely with the principles of precision oncology.

Integration with immunotherapy. The intersection of COF-based nanoplatforms with cancer immunotherapy represents a particularly exciting frontier. COFs are uniquely positioned to amplify immunotherapeutic potential through co-delivery of immunomodulatory agents alongside conventional cytotoxic therapies. The combination of COF-mediated tumor destruction with synergistic immunomodulation could establish durable antitumor immunity, addressing challenges related to tumor recurrence and metastasis.

Multimodal theranostic platforms. The convergence of diagnostic and therapeutic functionalities within a single COF-based nanoplatform enables real-time monitoring of drug biodistribution, tumor accumulation, and therapeutic response. The structural programmability of COFs makes them ideal scaffolds for constructing sophisticated theranostic systems that facilitate adaptive treatment strategies.

Clinical translation and regulatory considerations. The successful translation of COF-based nanomedicines will depend on comprehensive evaluation of long-term biocompatibility, pharmacokinetics, and clearance mechanisms. Development of standardized characterization protocols and safety assessment frameworks will be essential to support regulatory submissions. Close collaboration among materials scientists, pharmacologists, and regulatory experts will be necessary to navigate the pathway toward clinical approval.

In conclusion, while significant challenges remain on the path to clinical translation, the unique structural features, functional versatility, and emerging integration with advanced technologies—including artificial intelligence, personalized medicine, and immunotherapeutic modalities—position COFs as a transformative platform for next-generation cancer diagnosis and therapy. Continued interdisciplinary collaboration will be essential to translate the exceptional properties of COFs into tangible clinical benefits for cancer patients.