Manganese-Based Nanotheranostics for Magnetic Resonance Imaging-Mediated Precise Cancer Management

Introduction

Cancer remains the major threat to human health with the second leading cause of death after heart diseases, and the burden of cancer increasing dramatically worldwide.^1,2 Early diagnosis and targeted therapy are related to the prognosis and survival of patients. Compared with other diagnostic techniques widely used in clinical practice, magnetic resonance imaging (MRI) has the advantages of non-invasiveness, high spatial resolution, non-ionizing radiation, and soft tissue comparison.^3,4 In addition, MRI provides high-resolution anatomical images for the detection of small tumors and metastases.⁵ In brief, the mechanism of MRI is that the hydrogen nucleus in the examined tissue cells resonates under the action of a strong magnetic field. The trajectory of the resonance is recorded by the instrument, and then the data is reconstructed by the computer to form the image for clinical diagnosis.⁶ However, MRI without contrast agents lacks broad imaging quality standards leads to local and regional staging impossible, and results in poor differentiation between normal and pathological tissues which affects disease identification.^7,8 Therefore, MRI contrast agents are essential for disease diagnosis.

The most frequently used contrast agents are gadolinium (Gd)- and iron (Fe)-based metal chelates and nanoparticles (NPs).⁹ However, the typical shortcomings of the two are gradually revealed: Gd-based contrast agents exhibit relatively fast electronic relaxation, high inherent toxicity, and slow water exchange rates;¹⁰ Fe-based contrast agents exhibit negative imaging defects. Recently, the use of iron chelates and derived nanoprobes as positive contrast agents shows ideal relaxivity in MRI.^11,12 This new research frontier may be a breakthrough direction of Fe-based contrast agents in the future. In contrast, manganese (Mn)-based functional nanomaterials have received much attention in MRI due to their relatively high magnetization spin, fast water particle exchange rate, low side effects, good biocompatibility, crystallinity, and uniformity.^13–16

Various Mn-derived contrast agents have played a commendable role in illuminating tumors and directing treatment, however, unsatisfactory targeting and specificity have hampered their further development.¹⁷ An ideal diagnostic and therapeutic approach should be specific targeted to the tumor site while minimizing adverse effect to normal tissues.¹⁸ Accordingly, nanotechnology can effectively guide the design and synthesis of various Mn-based contrast agents, improve their performance, and mediate their applications in cancer theranostics.¹⁹ With the development of nanotechnology, Mn-based multifunctional nanocomposites have become safe nanoplatforms for clinical cancer diagnosis, treatment, and monitoring.²⁰ On the one hand, Mn-based functional nanoplatforms can be expanded by developing novel nanomaterials with high resolution and low toxicity in the future; on the other hand, they are used as carriers to combine with other substances to achieve multi-functionalization, such as MRI-involved multimodal imaging-guided oncotherapy and multimodal synergistic therapy based on biomimetic mineralization strategy.

This review focuses on the Mn-based functional nanoplatforms from the following three aspects: classification of Mn-based contrast agents, preparation methods of various contrast agents, and application of various Mn-based nanotheranostics in MRI-mediated precise diagnosis and targeted treatment of malignancies. Subsequently, some key challenges of nanotoxicity and industrial issues regarding the design of multifunctional nanoagents are discussed. Ultimately, the direction of development and future prospects of Mn-based magnetic nanomaterial-derived cancer nanomedicine are anticipated. These will provide scientific guidance and strategic reference for expanding the construction of Mn nanosystems and their significance in the precise management of cancer.

Classification of MRI Contrast Agents

In recent years, a variety of Mn-based MRI contrast agents, including inorganic and organic contrast agents, have aroused a wide range of basic and clinical research enthusiasm. Inorganic contrast agents include a single Mn metal, Mn oxide, hydroxide, sulfide, Mn ferrite, Mn-based salt, and other material systems. Organic contrast agents are mainly divided into metal-organic small molecule complex and polymer-modified Mn. In this section, representative Mn contrast agents are classified according to the material system. Figure 1 summarized the material types of commonly used Mn-based MRI contrast agents. Table 1 summarized the classification of Mn-based MRI contrast agents.

Table 1 Classification of Mn-Based MRI Contrast Agents

Figure 1 Classification, preparation methods and application of Mn-based MRI contrast agents.

Inorganic Contrast Agents

Single Mn Metal

Mn NPs have been widely used in the biomedical field, especially in MRI because of their suitable size, high biocompatibility, superparamagnetism, modifiable targeting, and excellent photostability.⁴¹ For instance, bispecific antibodies were functionalized and conjugated to Mn NPs surfaces to achieve preferential targeting of HER2 receptor and/or epidermal growth factor receptor (EGFR)-expressing tumor cells.²¹

Mn Oxide

Mn oxide NPs, including MnO, MnO₂, Mn₃O₄, MnO_x, and polymetallic oxides, have attracted attention as T₁-weighted MRI contrast agents because of their controllable circulation time of colloidal NPs.²² Among these NPs, MnO has the advantages of a small volume, simple preparation, and low toxicity. MnO₂ can not only alleviate hypoxia by catalyzing the decomposition of hydrogen peroxide (H₂O₂) into oxygen, but also enhance the T₁-weighted MRI of pH/redox reactions in the tumor microenvironment (TME) and the drug release characteristics after Mn²⁺ decomposition.²³ In addition, considering its good water dispersion, colloidal stability, low toxicity, and responsiveness to TME, MnO₂ has been constructed in various nanoforms, such as nanodots, nanorods, and nanosheets, to expand its application scope.²⁴ Other Mn oxide NPs also exhibit excellent physicochemical properties and MRI potential, such as high T₁-relaxation, good water dispersibility, colloidal stability, and biocompatibility.⁴²

Mn Hydroxide

Previous studies have reported that layered dihydroxides (LDH) are two-dimensional nanomaterials with extensive medical applications.⁴³ It has some exciting properties, including easy doping of various metal ions such as Mn²⁺ to prepare functional nanostructures, high host layer charge density and anion exchange capacity, good acid sensitivity due to the protonation of OH groups around the metal ions,⁴⁴ good structural tunability and biocompatibility.⁴⁵ PH-activated Mn-LDH nanomaterials can accelerate MRI behavior for detecting a variety of cancers.²⁵

Mn Sulfide

Au@MnS@ZnS is a novel therapeutic agent with core/shell/shell structure. The inner shell of MnS has a T₁-contrast capability in MRI. MnS NPs obtained by non-covalent modification have high solubility in physiological solutions and no obvious cytotoxicity.²⁶ In addition, the excellent photothermal conversion efficiency and photothermal stability strengthen the potential of MnS as an MRI contrast agent.⁴⁶

Mn and Other Inorganic Nonmetallic Compounds

This type of material mainly refers to Mn-based nanocomposites with other inorganic nonmetals, such as silicon (Si), carbon (C), phosphorus (P), and halogen. Monodisperse Mn-P nanocomposites with a size of approximately 100 nm exhibit strong Mn dissociation elasticity, structural stability, internal biosafety, and structure-dependent T₁ and T₂ relaxation, which could be used for MRI-guided pharmacokinetic improvement in vivo.²⁷ As a representative halide, MnCl₂ can be used in the long-term tracing of tumors in vivo. In a previous study, MnCl₂-labeled cells were found to be highly tumorigenic and could be used to identify early malignant lesions by observing tumor volume development prior to palpation.²⁸

Mn Ferrite

Mn ferrite and Mn-doped iron oxide have good biocompatibility and high loading capacity, and can be easily functionalized with various silane reagents.⁴⁷ They can not only react with elevated H₂O₂ in cells as a catalyst, to generate O₂ in situ, but also magnetically guide tumors to achieve accurate cancer treatment.⁴⁸ It has been proven that Mn-doped ferric oxides show enhanced saturation magnetization and higher thermal efficiency than iron oxide NPs, which has good application prospects in size-dependent MRI and hyperthermia.²⁹ The synthesized anisotropic Mn-Zn ferrite NPs have an ideal magnetic core lipid shell structure, high colloidal stability, good permeability, and low core loss, which show great potential in MRI-enhanced cancer theranostics by regulating the physical appearance of nanomaterials.³⁰

Mn Salt

Mn salts with MRI potential include Mn silicate, carbonate, and phosphate. Among these, Mn silicate has good dispersion, colloidal stability, biocompatibility, persistent luminescence, long in vivo circulation, and high longitudinal relaxation, making it a potential T₁ contrast agent for cancer diagnosis.^31–33For example, a novel two-dimensional Mn silicate with a high signal-to-noise ratio and degradability has been used in tumor-specific MRI.³⁴ Mn carbonate NPs exhibit paramagnetism at room temperature and weak ferromagnetism at low temperatures, good water dispersion and colloidal stability, and high r₁ relaxation efficiency, making them excellent contrast agents for T₁ MRI.³⁵ Mn phosphate NPs have porosity, versatility, strong pH responsiveness, MRI signal amplification, and drug release properties, and can be used for MRI-enhanced cancer therapy in response to acidic tumor environments.³⁶

Organic Contrast Agents

Metal-Organic Small Molecule Complex

Represented by metal-organic frameworks (MOFs), metal-organic nanocomplexes have uniform porosity, good dispersibility and stability, large specific surface area, and inherent properties of Lewis acid metals and organic groups, which have been widely used in biomedicine.³⁷ For example, surface modification of the Mn-quantum dot nanocomplex showed strong fluorescence, low toxicity, and a high T₁ relaxation rate, which could specifically fluorescently label cancer cells and be used for MRI-enhanced renal cancer diagnosis.³⁸

Polymer Modified Mn

As a representative example, the novel Mn (II)-ethylenediaminetetraacetic acid complex nanocage exhibits good stability of the chelate benzothiazole aniline (BTA), high lipotropy BTA derivatives, sufficient protein binding affinity, excellent stability, and relaxivity, which makes it a promising candidate for clinical liver imaging.^39,40 In addition, as a new type of biocompatible material, nanogel loaded with Mn(II) chelates shows high relaxivity values, stability and diagnostic efficiency.⁴⁹ Therefore, there is still a lot of room for the development of this type of material by loading various functional nanogels.

The classification of Mn-based MRI contrast agents by inorganic and organic is only one of current forms. In fact, with the increasing number of Mn-based MRI contrast agents being developed, not only to enrich the existing agent system, but also new classifications and sub-classifications will emerge. It should be noted that different material systems often lead to different characteristics, including size, stability, biocompatibility and many other aspects. These characteristics largely determine the application scope or situation of Mn-based MRI contrast agents. Polymer modified Mn may be an attractive research direction. In addition, the combination of these agents with other nanomaterials such as nanogels may result in a more ideal effect.

Preparation Methods of Contrast Agents

The key to preparing good nanomaterials is the control of the technological conditions of these preparation methods, wherein the control of particle size and dispersion is particularly important. The commonly used preparation methods are solvent exchange, one-step reduction, one-step bioinspired crystallization, oxidation-reduction, co-precipitation, isomorphic substitution, salt solution replacement, hydrothermal, ligand exchange, co-condensation and replacement and mixed deionization methods. This is outlined in the following subsections. Table 2 described the synthesis methods of Mn-based NPs and the respective advantages and disadvantages.

Table 2 Synthesis Methods of Mn-Based NPs and the Respective Advantages and Disadvantages Description

Solvent Exchange Method

The solvent exchange method is based on the solubility difference of a solute in different solvents, so that in the solution, a good solvent is introduced with a nonsolvent, and the good solvent is removed to obtain nanoprecipitation. For this method, high temperature is avoided, and it is suitable for preparing thermally unstable or volatile nanomaterials such as Mn oxide NPs. However, this method is expensive and requires a large amount of organic solvent. Residual solvents that are difficult to remove completely are harmful to the human body and can easily cause drug recrystallization and reduce product dispersion. For instance, Savla et al⁵ heated a chloroform solution of hydrophobic Mn₃O₄ nuclei, 1.2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino (polyethylene glycol) 2000]ammonium salt, and 1.2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[maleimide (polyethylene glycol)2000] ammonium salt to 60°C and then slowly added them to dimethyl sulfoxide. After the evaporation of chloroform, uniform and stable polymer-modified Mn₃O₄ NPs were obtained.

One-Step Reduction Method

The reduction method involves reducing the soluble salt in a solution with a strong reducing agent to obtain nanoprecipitation. This method is commonly used to prepare Mn oxide NPs of several nanometers. It is simple and easy to control; however, the resulting products are not easy to transfer and assemble, and they easily agglomerate and contain impurities. To overcome the clinical application limitations due to the large size of MnO₂ NPs prepared by the multi-step method, Fu et al²⁴ directly used hydraulic acid oligomer as a reducing agent to reduce sodium permanganate and generate MnO₂ NPs in one step.

One-Step Bioinspired Crystallization Method

The crystallization method involves precipitation of solute crystals by cooling a thermally saturated solution. It can separate highly purified crystals from a solution containing impurities or from a multicomponent molten mixture. This method has a low energy consumption. To simulate the disinfection process of potassium permanganate (KMnO₄), Yang et al⁵⁰ used a natural biomacromolecule silk fibroin as a reductant and template, and monodispersed MnO₂ NPs were obtained after the redox reaction between KMnO₄ and silk fibroin, followed by in situ mineralization.

Oxidation-Reduction Method

The redox method obtains uniform nanostructures by changing the valence state of substances in a solution with an oxidizer or reductant. This procedure is convenient and reliable; however, the storage and transportation of products are troublesome. For example, Foroushani et al⁴² stirred a certain amount of a water/ethanol mixture of heated, cooled, purified several times, and centrifuged KMnO₄ to obtain uniform Mn₃O₄ NPs.

Co-Precipitation Method and Isomorphic Substitution Method

Co-precipitation is the process by which a precipitant is added to a solution containing multiple cations to precipitate all ions completely. The product obtained by this method is single, but because of the different pH values required for the precipitation of different ions, not all ions can be precipitated simultaneously, resulting in uneven stratification of the products. Mn hydroxide can be prepared via co-precipitation and isomorphic substitution. For instance, Zhang et al²⁵ prepared a Mg₃Al-LDH suspension by the co-precipitation method and then partially substituted Mg (II) with Mn (II) ions to obtain Mn-LDH NPs. In the solution of pH 8.5, perfluoropolyether and Mn-LDH NPs self-assembled to form hybrid Mn-LDH@perfluoropolyether nanostructures through electrostatic interactions.

Salt Solution Replacement Method

The salt solution substitution reaction is a chemical process in which an element reacts with a salt solution to form another element and a salt solution. This method is mainly used for preparing Mn sulfide contrast agent. In a typical example, controllable size and biodegradable metastable γ-phase Mn sulfide nanotubes were synthesized using a simple salt solution replacement method for traceable gas therapy-initiated pH-responsive chemodynamic therapy (CDT) of cancer.⁵¹

Hydrothermal Method

The hydrothermal method is used for preparing Mn salt nanomaterials by heating and pressurizing the reaction system in water, the medium, in a special closed reactor, to form a relatively high-temperature and high-pressure environment, so that the insoluble reactants can be dissolved and recrystallized. The obtained product has high purity, good dispersibility, and controllable size; however, it has a long reaction cycle and is highly dependent on the production equipment. For example, Chen et al⁵² ultrasonically mixed an aqueous solution of FeSO₄.7H₂O, MnCl₂.4H₂O, and NH₄Cl with the water dispersion of silica colloidal spheres, heated mixture to 140°C for 16 h in a Teflon-lined stainless-steel autoclave, to prepare homogeneous and controllable FeMn(SiO₄) hollow nanospheres.

Ligand Exchange Method

The ligand-exchange method generates additional single- or multiple-ligand-protected nanoclusters by exchanging peripheral protective ligands. This is a simple and novel method for preparing hydrophilic Mn ferrite NPs based on superparamagnetic iron oxide NPs. Leal et al⁵³ prepared water-soluble Mn ferrite NPs smoothly and instantaneously by ligand exchanging with nucleophilic bases, such as triethylamine and gallol-PEG-OH.

Co-Condensation and Replacement Method

Condensation reaction is a process by which two or more organic compounds bond and release smaller molecules to form a new larger molecule. This method is suitable for the synthesis of metal-organic small-molecule complexes. Bao et al³⁷ directly performed a co-condensation reaction to synthesize an organic tetrakis (4-carboxyphenyl) porphyrin (TCPP) linker and further synthesized a hafnium (Hf)-based Mn porphyrin nanomaterial by inserting Mn into the center through a substitution reaction.

Mixed Deionization Method

The deionization method utilizes an ion exchange function of the ion exchange resin and the selective transmission effect of ion exchange membranes to guide ion directional migration under a direct current electric field to complete the continuous and deep desalination of water. This method can be used to prepare polymer-modified Mn contrast agents, such as PEGylated Mn²⁺-chelated dopamine NPs. Miao et al⁵⁴ added a MnSO₄·H₂O solution to a polydopamine (PDA) NPs suspension and reacted mixture to obtain Mn²⁺-PDA nanocomposites, which were used for MRI-mediated photothermal therapy (PTT) of cancer. The operational cost of this method is low, but the initial equipment investment is large.

Choosing the appropriate preparation method is important for the production of the expected contrast agent. Researchers should first comprehensively consider the characteristics of the target agent and take into account operability and repeatability. In addition, cost control, product purity, safety, and equipment limitations are also factors that cannot be ignored in the selection of preparation methods.

Mn-Based MRI Nanoprobe-Mediated Targeted Cancer Theranostics

Inorganic Mn Contrast Agent-Mediated Cancer Theranostics

Single Mn Metal

Mn metal-based contrast agents have been widely used in cancer theranostics. In a typical example, Sonis et al⁵⁵ developed avasopasem Mn by synthesizing small-molecule dismutase mimics, to effectively intervene in severe oral mucositis caused by radiotherapy and chemotherapy in patients with head and neck cancer, which has been confirmed in clinical trials. Cheng et al⁵⁶ designed a Mn-deposited iron oxide nanoplatform to trigger an intracellular cascade reaction. The nanosystem effectively induced cell apoptosis accompanied by iron cell droop, and was considered an ideal candidate for the treatment of tumor cytoplasmic iron-binding apoptosis (Figure 2). Zou et al⁵⁷ developed a TME-responsive hollow mesoporous Mn-doped silica shell loaded with doxorubicin (DOX), which had ultrasensitive multimodal diagnostic capability, enhanced anticancer efficacy, and improved biodegradability, thereby improving the comprehensive efficacy. Furthermore, Wu et al⁵⁸ prepared mesoporous PDANPs highly loaded with Mn ions. The nanocomposites showed good MRI contrast, high photothermal conversion efficiency, and encouraging antitumor activity in human cervical epithelial carcinoma (HeLa) tumor-bearing mice, making them an ideal choice for MRI-guided photothermal tumor ablation. In an exploratory study, Sishc et al⁵⁹ demonstrated that avasopasem Mn increased intracellular H₂O₂ concentration in non-small cell lung cancer and breast adenocarcinoma, and its superoxide scavenging activity had a palliative effect on radiation-induced mucositis in head and neck cancer. This anticancer synergy is currently undergoing phases I to II clinical validation. Furthermore, Yi et al⁶⁰ tested the synergistic effect of Mn²⁺ and YM101 in vitro by unidirectional mixed lymphocyte reaction, carboxyfluorescein diacetate succinimidyl ester dilution, and cytokine analysis. Mn²⁺ combined with YM101 synergistically inhibited the growth of multiple cogene mouse tumors and effectively prolonged their survival. A previous study showed that Mn is essential for enhancing the body’s innate immune perception and adaptive immune response to tumors. Tumor growth and metastasis were significantly enhanced in Mn-deficient mice and tumor-infiltrating CD⁸⁺ T cells were sharply reduced.⁶¹ Xiao et al⁶² prepared functional nanoplatforms by coating nano selenium onto self-assembled pseudomonas geniculate cell membrane and loading Mn²⁺ ions and DOX, demonstrating the efficacy of mitochondrial and nuclear dual-targeted tumor cell death induction. After modification, Zhu et al⁶³ developed exosome nanovesicles loaded with a Mn carbonyl group, which showed good performance in tumor targeting, mitochondrial damage, and radiosensitization therapy.

Figure 2 (A) Schema of the preparation of magnetic Pt-FMO NPs and its mechanism of inducing apoptosis and ferroptosis for the combined anti-tumor effect. (B) The FMO vector sustainably releases Mn2+ and Fe2+/Fe3+ ions into the acidic environment, which promotes ferroptosis in cells via the Fenton and Fenton-like reactions. Endogenous GSH activates Pt (IV) prodrug and generates cytotoxic cisplatin, thus triggering cellular apoptosis. Meanwhile, cisplatin also mediates conversion of oxygen (O2) to generate downstream H2O2 that further elevates Fenton reactions. (C) Tumor volume growth curves. The BALB/c-Nude mice bearing HeLa tumors were treated with various agents. (D) Body weight of nude mice. (E) Hemolysis assay of Pt-FMO at various concentrations. The UV-vis spectra absorbance of the supernatant of red blood cells at 540 nm incubated with Pt-FMO from 0.125 to 1 mg/mL. (F-J) Histological microscopic images. The dissected tumors were stained with H&E (F; scale bar represents 100 µm), Tunel (G; blue fluorescence: Hoechst; green fluorescence: Tunel; scale bar represents 25 µm), Ki67 (H; scale bar represents 25 µm), GPX4 (I; scale bar represents 25 µm) and caspase-3 (J; scale bar represents 25 µm). Reprinted from Cheng J, Zhu Y, Xing X, et al. Manganese-deposited iron oxide promotes tumor-responsive ferroptosis that synergizes the apoptosis of cisplatin. Theranostics. 2021;11(11):5418–5429. Copyright 2021.56

Therefore, Mn metal-based contrast agents have shown excellent anticancer potential in a variety of preclinical animal models and some clinical cases, effectively inhibiting tumor growth and prolonging the survival of patients. For the record, they are pretty cytotoxic. This typical defect limits the application of Mn metal-based contrast agent. However, there is still no effective solution to the cytotoxicity of Mn metal-based contrast agent. At present, most of the biosafety studies of Mn-based nanomaterials rely on in vitro cell viability tests, while a comprehensive in vivo toxicological mechanism analysis of single-Mn metal will undoubtedly enhance the existing understanding.

Mn Oxide

Mn oxide NPs can significantly amplify MRI signals at the tumor site, thus improving the sensitivity of cancer recognition and tumor margin delineation, and enhancing the visibility of metastases. For example, Xu et al⁶⁴ constructed a tumor cell membrane coated with bismuth/Mn oxide NPs as a targeted therapy for triple-negative breast cancer. Zhu et al⁶⁵ reported a nanotheranostic agent with Mn oxide nanoflowers as the core, PDA as the shell, and indocyanine green (ICG) as the photosensitizer. The release of Mn²⁺ ions made it a TME-sensitive MRI contrast agent for a highly specific diagnosis of liver cancer. Liu et al⁶⁶ reported a photoactivated oxygenase hydrogel consisting of Mn oxide and singlet oxygen-reactive preenzyme NPs as a photodynamic therapy (PDT)-mediated combination therapy to effectively inhibit lung metastasis in breast cancer. Furthermore, Tan et al⁶⁷ combined oleic acid-modified Mn oxide with temozolomide-obtained nanotherapeutics, showing great potential for MRI-visualized glioma therapy. Wang et al⁶⁸ designed cisplatin-loaded mesoporous PDA/MnO₂ NPs mediated by human EGFR2 (HER2)-specific dimers for MRI-enhanced chemotherapy of HER2-positive ovarian cancer. Luo et al⁶⁹ constructed nanohybrids composed of superparamagnetic iron oxide, MnO₂, and DOX. The nanoagents acted as inducers for cancer chemotherapy and hyperthermia, and significantly inhibited tumor growth with minimal side effects, demonstrating their great potential as cancer treatments.

In a fascinating report, Zhang et al⁷⁰ reported a mesoporous MnO₂ cascade catalytic nanoplatform loaded with acoustic sensitizer and surface-modified with glucose oxidase, which provided attractive opportunities as cancer therapy. Furthermore, pH-sensitive MnO₂@bovine serum albumin (BSA) NPs showed MR contrast signal amplification stimulated by the acidic extracellular environment of solid tumors.⁷¹ Hu et al⁷² chemically synthesized a drug delivery vector, PEG coated hollow MnO₂, which significantly inhibited the proliferation of endometrial cancer. In other cases of clinical synergy, Hou et al⁷³ synthesized a biocompatible MnO₂-containing agarose hydrogel using the sol-gel method for an enhanced PTT/PDT combination. Shen et al⁷⁴ self-assembled the ICG derivative CyI and chitosan to form nanosized MnO₂ through electrostatic interaction and Mn-N coordination, effectively alleviating hypoxia in the TME. Another PEG-MnO₂-osteopontin siRNA nanocomplex was constructed using a modular streptavidin bridge, which effectively inhibited the progression of renal cancer in vitro and in vivo in 786-O tumor-bearing mice.⁷⁵ Furthermore, Nie et al⁷⁶ designed MOF-coated MnO₂ nanosheets that strongly inhibited survivin after loading DNAzyme (DZ) and DOX, thus achieving a chemotherapy cogene therapy for cancer (Figure 3). Wang et al⁷⁷ constructed a glutathione (GSH)-triggered Au@MnO₂ nanoplatform for photoacoustic/MRI dual-enhanced synergistic photothermal cancer chemotherapy. Fan et al⁷⁸ used surface-grown MnO₂ to assist in the synthesis of Fe₃O₄ NPs and realized near-infrared fluorescence/photoacoustic imaging-monitored synergistic anticancer efficiency. Jain et al⁷⁹ synthesized Mn oxide nanocubes by a chemical precipitation method and modified their surfaces with serotonin-stearic acid biocoupling to achieve liver cancer targeting. Through an ingenious design, Ding et al⁸⁰ coated chromium-doped zinc gallate NPs with Mn³⁺ oxides. Through a fenton-like reaction, endogenous H₂O₂ was transformed into highly toxic hydroxyl radicals, which could accurately diagnose deep tumors and effectively inhibit tumor growth without external activation energy.

Figure 3 (A)The schematic illustration the construction of MnO2/DOX@DZ and its synergistic antitumor mechanism. Upon endocytosis, the MnO2/DOX@DZ is degraded by intracellular GSH and acidic environment to release DOX and DZ, and the produced Mn2+ exerts excellent Fenton-like activity to generate highly reactive •OH. In addition, Mn2+ could also act as metal cofactor to activate DZ to silence survivin expression, resulting in synergistic antitumor with DOX. (B) UV-vis spectra monitoring the reaction over a time-course from 0 to 60 min. Inset: The appearance of each tube with different reaction time. (C) transmission electron microscopy (TEM) and dynamic size of MnO2 nanosheets. (D) The atomic force microscopy (AFM) of MnO2 nanosheets. (E) The fluorescence of DOX after adding different concentrations of MnO2 nanosheet. (F) The loading capacity of DOX on MnO2 as a function of DOX concentration. (G) The loading capacity and loading efficiency of DZ as a function of DZ concentration. (H) The TEM and (I) AFM microscopic images of MnO2/DOX@DZ. (J) The stability of MnO2/DOX@DZ under different conditions. Reprinted from Int J Pharm, 585, Nie Y, Li D, Peng Y, et al. Metal organic framework coated MnO(2) nanosheets delivering doxorubicin and self-activated DNAzyme for chemo-gene combinatorial treatment of cancer. Reprinted from Int J Pharm, 585, Nie Y, Li D, Peng Y, et al. Metal organic framework coated MnO(2) nanosheets delivering doxorubicin and self-activated DNAzyme for chemo-gene combinatorial treatment of cancer. 119513, Copyright 2020, with permission from Elsevier.76

It is worth noting that new nanomaterials based on MnO₂ have been widely developed in recent years. These materials are often multifunctional. For example, Guan et al⁸¹ provided a biodegradable mesoporous zeolitic-imidazolate-framework@MnO₂/doxorubicin hydrochloride nanocomposites to achieve enhanced sonodynamic therapy/CDT/chemotherapy by promoting oxidative stress and overcoming the multidrug resistance, and Liu et al⁸² constructed an ultrathin‐FeOOH‐coated MnO₂ nanospheres as sonosensitizers to achieve effective anti-tumor efficacy by simultaneously increasing the yield of reactive oxygen species and tuning TME. There are also some nanomaterials that show the effect of tumor immunotherapy, such as TME responsive MnO₂-melittin nanoparticles and MnO₂ nanoparticles with high anti programmed death ligand 1 encapsulation efficiency, which synthesized by Tang et al⁸³ and Deng et al⁸⁴ respectively. Another interesting discovery is about the diabetic patients after tumor surgical resection. Specifically, a degradable self-enhanced cascade catalysts CS/MnO₂-GOx nanocatalysts proposed by Wang et al⁸⁵ for antitumor/antibacterial therapy and promotion of wound healing.

In short, although the synthesis and therapeutic potential of various Mn oxides are different, existing reports have clearly demonstrated that TME-sensitive active targeting NPs can preferentially concentrate at the tumor sites, thereby improving the sensitivity of MRI and anticancer efficacy. As a controllable drug delivery system, Mn oxide-derived tumor-targeting nanomaterials have a wide therapeutic window, low toxicity, good biocompatibility, strong adsorption capacity, and excellent therapeutic effects. In addition, the inherent pH sensitivity of Mn oxides has prompted scientists to devote considerable effort to explore their novel clinical potential as an intelligent cancer treatment in the future. Table 3 described the applications of Mn oxide-based nanomaterials in cancer diagnosis and treatment.

Table 3 Applications of Mn Oxide-Based Nanomaterials in Cancer Diagnosis and Treatment

Mn Hydroxide

In recent years, multifunctional NPs based on Mn hydroxide have been widely used for effective detection and treatment of cancer. Zuo et al⁴⁵ prepared dual-functionalized layered Mn dihydroxide NPs, which proved to be an effective anticancer drug/gene delivery system for T₁-weighted MRI-guided brain cancer treatment. Xie et al⁸⁶ prepared Mn-doped layered NaOH NPs through a simple two-step synthesis method and demonstrated excellent photothermal properties and T₁-weighted MRI capability, thus providing a potentially effective platform for biodegradable inorganic nanotherapy. Enlightened by this, Wen et al⁸⁷ self-assembled layered coal seam dihydroxy compounds and MnO₂ to prepare a multifunctional nanocervator as a selective combination cancer therapy. After co-loading hydrophilic DOX and hydrophobic paclitaxel (PTX), the nanosystem showed an effective combination of chemotherapy against liver cancer. Liao et al⁸⁸ synthesized a Mn hydroxide nanocapsule with a high loading capacity and strong catalytic activity, which could effectively relieve hypoxia in tumor tissues and improve the synergistic anticancer effect. Yan et al⁸⁹ synthesized CoMn-LDH nanosheets using a bottom-up approach and surface modification with photosensitizer chloride e6 (Ce6). The nanosystem showed satisfactory anticancer activity and promoted the complete apoptosis of cancer cells to remove tumors.

In brief, Mn hydroxide nanomaterials are simple to synthesize, biocompatible, and sensitive to TME. Meanwhile, they enhance T₁/T₂ relaxation and improve MRI accuracy, showing great potential for biomedical applications. However, these nanomaterials still mainly in the research stage, and their advantages and potentialities have not been fully transformed in clinical practice. How to systematically obtain a large amount of reliable biosafety-related data is also a major challenge for Mn hydroxide nanomaterials.

Mn Sulfide

Conventional radiotherapy has been widely used in the clinical treatment of cancer. The effect of MnS contrast agents combined with radiotherapy is remarkable. Li et al²⁶ synthesized PEG-functionalized Au@MnS@ZnS core/shell/shell NPs. The NPs effectively accumulated and were retained at the tumor site after intravenous injection, showing T₁-weighted MRI-guided significant inhibition of tumor growth. Chen et al⁴⁶ designed CuS-MnS₂ nanoflowers as promising nanotheranostic agents for the combined PTT/PDT of ovarian cancer. He et al⁵¹ synthesized metastable γ-phase MnS nanotubes with controllable sizes and biodegradability using a simple wet chemical method and protected them with BSA. After intravenous injection, MnS@BSA exhibited T₁-weighted MRI-activated tumor growth inhibition and significantly prolonged the lifetime of the tumor-bearing mice (Figure 4). Ke et al⁹⁰ synthesized two-dimensional Cu₂MnS₂ nanoplates using a simple and environmentally friendly process for MRI/multispectral photoacoustic tomography dual-mode imaging-guided PTT for cancer in the NIR-II window. Further introducing the radiosensitizer, a novel nanoprobe was developed and its application prospects in imaging-enhanced radiotherapy for cancer were explored.

Figure 4 (A) MnS@BSA for tumor pH-responsive traceable hydrogen sulfide (H2S) gas therapy primed CDT of cancer. The MnS@BSA can be degraded in response to the mildly acidic TME, releasing H2S for gas therapy and Mn2+ for MRI and CDT of cancer. (B) Cell viability of 4T1 cells after incubation with Na2S, MnCl2 and MnS@BSA at different concentrations for 24 h. (C) Mortality of 4T1 cells caused by •OH radicals and H2S gas after incubated with MnS@BSA for 24 h. (D) H2O2 concentrations in 4T1, MCF10A, HeLa, A375 cells detected by a H2O2 kit. Viability of 4T1 cells incubated with (E) MnCl2, MnCl2+10 µM L-ascorbic acid (AA) (F) MnS@BSA, MnS@BSA+10 µM AA. (G) Cytotoxicity of 4T1 cells incubated with 200 µM of MnS@BSA and various concentrations of AA: 0, 10, 20, 40, 80 µM for 24 h. (H) Confocal images of 4T1 cells incubated with various concentration of MnS@BSA: 0, 50, 100, 200 µM and MnCl2: 100 µM for 4 h and stained with DCFH-DA fluorescence probe. (I) Fluorescence images of 4T1 cells incubated with WSP-5 H2S fluorescence probe for 30 min, subsequently adding various concentration of MnS@BSA: 0, 50, 100, 200 µM and Na2S 100 µM for 30 min, scale bar 100 µm. Reprinted from He T, Qin X, Jiang C, et al. Tumor pH-responsive metastable-phase manganese sulfide nanotheranostics for traceable hydrogen sulfide gas therapy primed chemodynamic therapy. Reprinted from He T, Qin X, Jiang C, et al. Tumor pH-responsive metastable-phase manganese sulfide nanotheranostics for traceable hydrogen sulfide gas therapy primed chemodynamic therapy. Theranostics. 2020;10(6):2453–2462, Creative Commons.51

Due to the positive effect of Mn sulfide contrast agent in conventional radiotherapy, the combination of the two should be given priority in future clinical treatment. At the same time, researchers are inspired to develop targeted treatment programs based on the characteristics of different nanomaterials, which firstly depends on the further enrichment and development of Mn-based functional nanoplatforms.

Mn and Other Inorganic Nonmetallic Compounds

Contrast-enhanced MRI based on Mn and inorganic non-metallic composites is essential for the diagnosis and monitoring of cancer. For instance, Irmania et al⁹¹ synthesized Mn-doped Cquantum dots via a one-pot hydrothermal method using waste green tea as the raw material. By coupling the terminal amino groups with folic acid and Ce6, it specifically targeted folate receptor-overexpressing HeLa cells and exerted a magnetic fluorescence PDT effect. Atif et al⁹² prepared Mn-doped cerium oxide nanocomposites by a hydrothermal method, which exhibited considerable cytotoxicity and was considered as a promising candidate for the targeted therapy breast cancer. Sha et al⁹³ constructed a multifunctional nanoplatform by in situ doping of Mn in gold-core mesoporous silica NPs and loading DOX to specifically target and efficiently kill osteosarcoma (Figure 5). Similarly, Tang et al²⁷ constructed Mn-doped silica NPs loaded with sorafenib using an optimized one-pot Stobers method, which fought tumor cells by disrupting intracellular redox homeostasis via GSH consumption. Furthermore, Keca et al⁹⁴ described a novel Mn-texaphyrin-phospholipid building block and its Mn-nanotexaphyrin assembly for the MRI-enhanced visualization of lymphatic drainage of proximal lymph nodes in the head and neck of VX-2 tumors. Jin et al⁴⁷ explored monolayer double-anchored Mn boride nanosheets as a novel metallic boride, MBene. The nanosystem exhibited unique NIR photothermal and photoacoustic effects, X-ray absorption, and MRI properties, having excellent potential in multimodal cancer imaging-induced PTT for cancer.

Figure 5 (A) Synthesis steps of DOX@Au@MMSN-Ald. (B) Upon targeted endocytosis, DOX@Au@MMSN-Ald treated by H2O2 and GSH can release Fenton-like initiator Mn2+ from the Mn–O–Si framework, which achieved CT/MR dual-modality imaging guided chemo-chemodynamic combination therapy. (C) Cell viability assay of (C) HUVECs and (D) RAW264.7 cells incubated with Au@MMSN-Ald suspensions at different concentrations for 24 h and 48 h. (E) Hemolytic percentage of RBCs after incubation with Au@MMSN-Ald suspensions. (F) H&E stained sections of major organs after 7 days of treatment for the control group and Au@MMSN-Ald group. Reprinted from Sha Z, Yang S, Fu L, et al. Manganese-doped gold core mesoporous silica particles as a nanoplatform for dual-modality imaging and chemo-chemodynamic combination osteosarcoma therapy. Reprinted from Sha Z, Yang S, Fu L, et al. Manganese-doped gold core mesoporous silica particles as a nanoplatform for dual-modality imaging and chemo-chemodynamic combination osteosarcoma therapy. Nanoscale. 2021;13(9):5077–5093, permission converyed through Copyright Clearance Center, Inc.93

Based on the above, multifunctional Mn-doped inorganic nonmetallic nanoreagents have been used as multimode imaging probes or effective therapeutics for remote detection and eradication of tumors. In addition, this kind of nanoreagents still have a lot of exploration space, and the subsequent generation of new nanomaterials is likely to be accompanied by noteworthy characteristics. Such nano-compounds will pave the way for other MRI-related cancer diagnosis and treatment.

Mn Ferrite

As a typical representative of Mn ferrite composites, MnFe₂O₄ NPs exhibit ideal magnetic targeting and remarkable NIR light responsiveness to promote their high tumor localization. Therefore, this magnetic nanosystem has great potential for simultaneous cancer diagnosis and accurate phototherapy. In a previous study, Kim et al⁴⁷ designed biocompatible Mn ferrite NPs and encapsulated them into mesoporous silica NPs to overcome hypoxia, thus improving in vivo PDT outcomes for cancer. After further modification, Deng et al⁹⁵ functionalized MnFe₂O₄ with an NIR dye, IR806, to obtain a nanosystem with excellent magnetic targeting and MRI capabilities. These functionalized nanomaterials concentrated specifically at the tumor site under an external magnetic field and 808 nm laser irradiation and completely destroyed subcutaneous solid tumors. To confirm this phenomenon, Ding et al⁴⁸ doped PEG-modified MnFe₂O₄ loaded with a photosensitizer into macroporous silica NPs. The nanocomposite achieved both magnetic targeting and the ability to overcome tumor hypoxia and was used for NIR-excitation and O₂ adaptive photodynamic cancer treatment.

In addition to inducing phototherapy, Yang et al⁹⁶ reported Mn ferrite nanocapsules as a delivery vehicle for acoustic sensitizer to release active cargoes at specific tumor sites and enhance the efficacy of sonodynamic therapy in cancer.

Based on monotherapy, MRI-guided multiple therapies derived from Mn ferrite nanostructures have also achieved considerably in research. For example, Rio et al⁹⁷ and Rodrigues et al⁹⁸ developed multifunctional liposomes loaded with gold-modified Mn-ferrite NPs for combined chemotherapy and phototherapy in cancer, respectively. Ribeiro et al⁹⁹ synthesized mixed magnetic liposomes of calcium-Mn ferrite by citrate co-precipitation and the sol-gel method, which were used for pH and temperature dual-sensitive chemotherapy and magnetic hyperthermia synergistic anticancer. In another ingenious design, He et al¹⁰⁰ prepared PEG-functionalized Mn ferrite NPs. The nanoplatform continuously catalyzed the decomposition of endogenous H₂O₂ to achieve oxygen self-sufficiency, while consuming GSH to reduce the loss of reactive oxygen species during radiotherapy, thus achieving an exciting combination of radiotherapy and CDT. Furthermore, Wang et al¹⁰¹ prepared block copolymer micelles containing HA and Mn-Zn ferrite NPs using a two-step method, which significantly improved the efficiency of hyperthermia and radiotherapy against lung cancer. These findings confirm the great potential of Mn ferrite as an effective theranostic agent for cancer treatment.

In brief, Mn ferrite nanocomposites, such as MnFe₂O₄ or anisotropic Mn-Zn ferrite NPs, exhibit strong NIR light absorption, good photothermal stability, excellent MRI ability, and potential advantages in MRI-enhanced tumor observation and treatment evaluation. Table 4 described the applications of Mn ferrite in cancer diagnosis and treatment.

Table 4 Applications of Mn Ferrite Theranostics in Cancer Diagnosis and Treatment

Mn Salt

A variety of Mn salt NPs, such as Mn silicate, Mn carbonate, Mn phosphate, and Mn phthalate, have the advantages of multi-responsive space-biological distribution and low systemic toxicity, which provide them with great clinical potential in time-dependent bioimaging-mediated cancer theranostics. For example, Nafiujjaman et al¹⁰² developed a nanocomposite composed of hollow Mn silicate NPs and Ce6, which generated a large amount of oxygen under irradiation to alleviate tumor hypoxia and improve the effect of PDT against breast cancer. Liu et al¹⁰³ coated mesoporous copper/Mn silicate nanospheres with a biodegradable cancer cell membrane, showing good CDT/PDT synergistic efficacy against lung cancer. Furthermore, Xu et al¹⁰⁴ first proposed a tumor catalytic therapy strategy based on an immunomodulatory nanase to realize the synergistic regulation of nanases and TME. The nanase was composed of PEGylated iron Mn silicate NPs loaded with a transforming growth factor (TGF)-β inhibitor and showed effective anti-colorectal cancer activity. Wang et al³² synthesized an iron inducer based on arginine-rich Mn silicate nanospheres using a one-pot reaction method, which demonstrated efficient GSH depletion capacity and T₁-weighted MRI-enhanced on-demand chemotherapeutic drug delivery behavior.

The excellent performance of Mn silicate NPs in imaging-guided antitumor activity has encouraged scientists to explore other Mn salt nanomaterials for biomedical applications. Xiao et al¹⁰⁵ prepared iron oxide NPs coated with nanoselenium, deposited it with Mn carbonate, and showed CDT/laser therapy combination-induced apoptosis of triple-negative breast cancer cells. Hou et al¹⁰⁶ constructed TME-sensitive nanohybrids by encapsulating DOX and phospholipids into amorphous porous Mn-phosphate NPs, which showed good antitumor activity and effectively improved the survival of tumor-bearing mice. Zhu et al¹⁰⁷ designed a protocell-like nanoreactor in which hydrophilic glucose oxidase was loaded into the pores of mesoporous silica NPs and hydrophobic Mn phthalate was loaded into the membrane layer of liposomes. The nanosystem synergistically enhanced the antitumor effects of PDT as starvation therapy for breast cancer (Figure 6).

Figure 6 (A) Synthesis of a dual functional nanoreactor. (B) Consumption of glucose-controlled generation of reactive oxygen species upon laser irradiation for synergistic cancer treatment. (C) Fluorescence imaging of live/dead cells. 4T1 cells were treated with 200 μg/mL NPs and stained with 5 μM fluorescein diacetate (FDA) (green for live cells) and 10 μM propidium iodide (PI) (red for dead cells) (scale bar= 200 μm). (D) Cytotoxicity measurement by MTT assay. 4T1 cells were treated with different NPs, and the cells were analyzed by cytometry measurement. Error bars represent standard deviation from five measurements. (E) Annexin V/PI assay. Cells were treated with 200 μg/mL NPs at a concentration of 0.02 μg/mL GOx and 15 μg/mL MnPc (n = 3; *p < 0.05, **p < 0.01, ***p < 0.001 (t-test). Reprinted from Zhu Y, Shi H, Li T, et al. A dual functional nanoreactor for synergistic starvation and photodynamic therapy. Reprinted with permission from Zhu Y, Shi H, Li T, et al. A dual functional nanoreactor for synergistic starvation and photodynamic therapy. ACS Appl Mater Interfaces. 2020;12(16):18309–18318. Copyright © 2020 American Chemical Society.107

Therefore, the construction and application of Mn-salt-based nanomaterials may provide new insights into targeted cancer therapy. However, there is still a need to improve the specificity, biocompatibility, and targeting efficiency of diagnostic CDT/PDT nano-agents.

At present, inorganic Mn contrast agents and their applications in cancer theranostics have been widely studied and developed. These exciting results are gradually known and recognized by peers in the field. However, it is necessary to further investigate their long-term stability, biodistribution and metabolic kinetics characteristics to promote the clinical transformation process of Mn-based MRI contrast agents. The solution of these problems is inseparable from the collaborative exploration of various disciplines including chemistry, biology and biomedical engineering.

Organic Mn Contrast Agent-Mediated Cancer Theranostics

Metal-Organic Small Molecule Complex

Mn-based nanoMOFs are effective biomedical materials that can be easily synthesized and used for cancer detection and treatment. For example, Wang et al¹⁰⁸ designed a spindle copper (II) TCPP nanometal-organic skeleton (PCN-224(Cu)-GOD@MnO₂) to reduce the antioxidant activity of tumors and improve CDT efficacy. Zeng et al¹⁰⁹ prepared a hollow monodisperse MOF multifunctional nanoplatform containing Mn oxide NPs and FA, which has great potential as an intelligent cancer therapy. Furthermore, Wang et al¹¹⁰ proposed an Mn MOF-derived multifunctional mesoporous nanase to generate endogenous O₂ in situ under the guidance of bioimaging and improved the efficacy of PDT (Figure 7). In addition, Bao et al³⁷ designed a nanoMOF based on a Hf cluster and a Mn (III)-porphyrin ligand, which showed significant tumor growth inhibition as a high-performance therapeutic agent.

Figure 7 (A) Schematic illustration of MOF-derived mesoporous nanoenzyme (NE) for enhanced PDT of cancer. (B) Viability of 4T1 cells treated with MnCoO–PDA–PEG (MCOPP) NE at different concentrations. (C and D) Cell viability assay of 4T1 cells treated under different conditions in normoxic (C) and hypoxic (D) environments before and after 671 nm laser light irradiation. (E) Live/dead cell assay for cells treated with PBS, free Ce6 and MCOPP-Ce6 under normoxic and hypoxic conditions. (F) Confocal laser scanning microscopy (CLSM) images of 4T1 cells stained by O2 indicator, ie, (Ru(dpp)3)Cl2, after different treatments. (G) Western blots of HIF-1α expression in 4T1 cells treated with MCOPP NE. (H) CLSM images of HIF-1α (green) and F-actin (red) in cells incubated with MCOPP NE under normoxic and hypoxic conditions. Statistical analysis was performed using the Student’s two-tailed t-test (*p < 0.05 and **p < 0.01). Reprinted with permission from Wang D, Wu H, Lim WQ, et al. A mesoporous nanoenzyme derived from metal-organic frameworks with endogenous oxygen generation to alleviate tumor hypoxia for significantly enhanced photodynamic therapy. Adv Mater. 2019;31(27):e1901893. Copyright © 2019 John Wiley and Sons.110

In summary, Mn-based nanoMOFs have achieved excellent clinical and preclinical outcomes as adjuncts in TME-stimulated multimodal biomedical diagnosis and real-time treatment monitoring of cancer. However, the therapeutic effect of PDT is still limited by excitation light depth, high levels of GSH, and hypoxia in the TME.

Other Organic-Modified Mn

The high dynamic inertia and r₁ relaxation of organic-modified Mn nanocomplexes make them clinically approved as tumor-specific MRI contrast agents. For instance, Cao et al¹¹¹ synthesized a bimetallic complex containing Mn (II) and Cu (II) for CDT of lung cancer. Yang et al¹¹² modified photoresponsive NPs with aptamers (labeled Mn-D@BPFe-A) for lactic acid oxidation and cancer phototherapy. Mn-D@BPFe-A was constructed by assembling a functional complex with BSA followed by surface metal coordination to identify Fe³⁺ with a GAG sequence. Tabatabayi et al¹¹³ synthesized the Mn (II) complex of N,N-bispyridoxine (1,4-butyleneamine) Schiff base, which showed effective pro-apoptotic and antioxidant activities against HepG2 and MCF7 cells, demonstrating its excellent potential for treating liver and breast cancers.

Mn-porphyrin compounds are another candidate for cancer diagnosis and treatment. Thamilarasan et al¹¹⁴ demonstrated that redox-activated Mn porphyrins have significant effects on the treatment of vascular occlusion in humanized sickle mice with acute vascular occlusion crisis (Figure 8). Shrishrimal et al¹¹⁵ found that tetraporphyrins and other Mn (III) porphyrins were effective radioprotectors that prevented radiation-induced fibrosis and activated the NRF2 signaling pathway, partially explaining the mechanism of radiation protection in prostate fibroblasts. Furthermore, Chatterjee et al¹¹⁶ demonstrated that Mn porphyrin, MnTE-2-PYP, significantly reduced prostate tumor size and improved survival. In addition, it lowered blood glucose levels and inhibited pro-fibrotic signaling in diabetic models.

Figure 8 (A) Summarizes the Mn porphyrin sickle cell (SC) treatment with TNFα. (B) Intravital microscopy was performed on anesthetized treated TS mice. Representative images of postcapillary venules (20× magnification) from treated TS mice are presented. Leukocyte and RBC adhesion and vaso-occlusion are indicated with arrows. Scale bars, 50 μm. (C-E) Video frames showing vessel segments were used to quantify fluorescence-labeled cell (leukocyte and RBC) adhesion (C and D) and leukocyte rolling flux (E) in all venules and arterioles were recorded, and numbers were averaged among groups of animals. Cell adhesion presented as number of adherent cells per 100 μm vessel length (C), and fluorescence unit (FU) (D). Leukocyte rolling flux presented as number of leukocytes per minute (E). TS mice injected simultaneously with 1 dose of 0.5 mg/kg MnE and TNF-α showed sporadic cell adhesion (C and D) and reduced leukocyte rolling (E) as opposed to the vehicle group. (F) Blood flow in vivo. Percentage of vessels with normal, slow, and no blood flow (occluded vessels) are shown. Average vessel diameter was almost identical (∼21 μm) in all treatment groups. Blood stasis in TS mice treated with 0.5 mg/kg MnE was significantly reduced compared with the vehicle group. Error bars show SEM of 6 different experiments for each treatment group. **P < 0.01 compared with vehicle treatment regardless of the vessel diameter within the ranges specified. Reprinted from Thamilarasan M, Estupinan R, Batinic-Haberle I, et al. Mn porphyrins as a novel treatment targeting sickle cell NOXs to reverse and prevent acute vaso-occlusion in vivo. Blood Adv. 2020;4(11):2372–2386. Permission conveyed through Copyright Clearance Center, Inc.114

Therefore, organic-functionalized Mn nanomaterials exhibit high drug loading, strong tumor targeting, good blood circulation stability, fast removal, low toxicity, and significant sensitivity in TME-responsive MRI-enhanced cancer treatment. However, synergistic CDT reduction of GSH levels with bimetallic complexes has not been reported, which is expected to improve the efficiency of CDT and promote clinical translation.

From the overall scale, the current research on organic Mn contrast agents is less than that of inorganic Mn, but this does not affect its important component as a Mn-based contrast agent. Inevitable technical barriers may be one of the factors leading to this result. Admittedly, this field needs to be further studied, but the excellent performance of the existing organic Mn contrast agents has attracted great interest from subsequent researchers.

Conclusion and Outlook

MRI is a rapidly developing molecular imaging technology owing to its non-invasiveness, high spatial and soft tissue resolution, non-ionizing radiation, and other qualities. In particular, the introduction of contrast agents alleviates the inherently low sensitivity of MRI, which has aroused widespread concern among researchers. Moreover, owing to their high relaxation efficiency, rapid water proton exchange rate, and good biocompatibility, various Mn-based nanomaterials have been developed as high-quality T₁ contrast agents, showing great clinical significance in cancer diagnosis and monitoring. In this review, the material properties, preparation strategies, and progress of Mn-related contrast agents in MRI-mediated cancer diagnosis and targeted therapy are discussed based on material classification.

However, there are still some challenges in translating Mn-contrast agent-associated nanomedicines into clinical practice. First, the limited understanding of the molecular mechanisms of tumorigenesis and development, especially immune networks in tumor progression, has largely hindered the application of Mn-based NPs in MRI-guided cancer therapy. Second, tumor heterogeneity and patient physical differences lead to individual variations in the therapeutic efficacy of the same nanodrug. Third, the toxicological mechanisms of NPs are not yet fully understood. Furthermore, given that the physicochemical properties of nanomedicines vary with the internal effects of biological substances, the ultimate dosage form must be carefully screened and evaluated. From a practical perspective, large-scale production of nanotheranostics of a uniform size, controllable morphology, and good reproducibility is difficult to transform from laboratory to commercial preparation.

Therefore, it is necessary to screen competitive tumor markers to guide the efficient enrichment and targeted delivery of NPs at specific sites in the body. Furthermore, the design of robust and repeatable experimental protocols is essential to comprehensively characterize and cross-verify the properties and functions of nanoprobe components in reasonable in vitro and in vivo models. In addition, the long-term toxicity, immunotoxicity, and neurotoxicity of NPs should be emphasized. With breakthroughs in interdisciplinary technology, these bottlenecks will be overcome smoothly, hence promoting the bright clinical prospect of Mn-based nanomaterials for precise cancer theranostics in the near future.

According to the characteristics and applications of Mn-based nanomaterials that have been reported at this stage, the future research trends should focus on the following five aspects. First of all, to enrich the current system by synthesizing of new Mn chelates agents, candidate nanotheranostics with different characteristics are of positive significance for the formulation of personalized solutions. Secondly, further improving the imaging contrast to enhance the resolution is always one of the key incentives for the research and development of this field. Thirdly, exploring solutions to achieve lower cytotoxicity and good biocompatibility is undoubtedly conducive to promoting the safe application of manganese-based nanomaterials. Subsequently, in-depth analysis of the mechanism of these nanomaterials is necessary for a broad understanding of their specific biological characteristics, but most of the current research is not sufficient in this regard. Last but not the least, to increase the clinical conversion rate of current research results, this may be initially improved through more representative animal models, such as patient derived xenograft model and humanized mice model, as well as extensive clinical trials in the later period.