A water-soluble octahedral molybdenum cluster complex as a potential agent for X-ray induced photodynamic therapy

Octahedral molybdenum iodide clusters with pyrazole or pyrazolate ligands

Due to the combination of useful physicochemical properties (luminescence, X-ray contrast, etc.), octahedral molybdenum halide cluster complexes [Mo6X8L6]n have been the subject of active investigation during the last decades. The most common methods for synthesizing new compounds with organic ligands involve the use of silver salts of organic acids or the substitution of terminal methylate ligands. However, these methods often necessitate the use of special conditions, such as an inert atmosphere, absolute solvents, and silver salts, which can be costly. In contrast, aqua-hydroxo complexes formed by hydrolysis of many complexes are considered final unreactive products, despite the tendency for them to form. This work proposes a simple and affordable method for the preparation of hexaaqua and hexahydroxo iodide clusters of molybdenum from [Mo6I14]2- in a single step. Furthermore, the possibility of using such compounds as starting complexes for the synthesis of clusters with organic ligands such as pyrazole is discussed. The paper presents synthetic approaches, detailed characterization both in solid and in solution, and a study of the reactivity and luminescence properties of the obtained compounds.

Nanomaterials for light-mediated therapeutics in deep tissue

Light-mediated therapeutics, including photodynamic therapy, photothermal therapy and light-triggered drug delivery, have been widely studied due to their high specificity and effective therapy. However, conventional light-mediated therapies usually depend on the activation of light-sensitive molecules with UV or visible light, which have poor penetration in biological tissues. Over the past decade, efforts have been made to engineer nanosystems that can generate luminescence through excitation with near-infrared (NIR) light, ultrasound or X-ray. Certain nanosystems can even carry out light-mediated therapy through chemiluminescence, eliminating the need for external activation. Compared to UV or visible light, these 4 excitation modes penetrate more deeply into biological tissues, triggering light-mediated therapy in deeper tissues. In this review, we systematically report the design and mechanisms of different luminescent nanosystems excited by the 4 excitation sources, methods to enhance the generated luminescence, and recent applications of such nanosystems in deep tissue light-mediated therapeutics.

PEGylated Molybdenum-Iodine Nanocluster as a Promising Radiodynamic Agent against Prostatic Adenocarcinoma

The combination of photodynamic therapy and radiotherapy has given rise to a modality called radiodynamic therapy (RDT), based on reactive oxygen species-producing radiosensitizers. The production of singlet oxygen, O2(1Δg), by octahedral molybdenum (Mo6) clusters upon X-ray irradiation allows for simplification of the architecture of radiosensitizing systems. In this context, we prepared a radiosensitizing system using copper-free click chemistry between a Mo6 cluster bearing azido ligands and the homo-bifunctional linker bis-dPEG11-DBCO. The resulting compound formed nanoparticles, which featured production of O2(1Δg) and efficient cellular uptake, leading to remarkable photo- and radiotoxic effects against the prostatic adenocarcinoma TRAMP-C2 cell line. Spheroids of TRAMP-C2 cells were also used for evaluation of toxicity and phototoxicity. In vivo experiments on a mouse model demonstrated that subcutaneous injection of the nanoparticles is a safe administration mode at a dose of up to 0.08 g kg-1. The reported results confirm the relevancy of Mo6-based radiosensitizing nanosystems for RDT.

Nanomaterials-assisted photothermal therapy for breast cancer: State-of-the-art advances and future perspectives

Breast cancer (BC) remains an enigmatic fatal modality ubiquitously prevalent in different parts of the world. Contemporary medicines face severe challenges in remediating and healing breast cancer. Due to its spatial specificity and nominal invasive therapeutic regime, photothermal therapy (PTT) has attracted much scientific attention down the lane. PTT utilizes a near-infrared (NIR) light source to irradiate the tumor target intravenously or non-invasively, which is converted into heat energy over an optical fibre. Dynamic progress in nanomaterial synthesis was achieved with specialized visual, physicochemical, biological, and pharmacological features to make up for the inadequacies and expand the horizon of PTT. Numerous nanomaterials have substantial NIR absorption and can function as efficient photothermal transducers. It is achievable to limit the wavelength range of an absorbance peak for specific nanomaterials by manipulating their synthesis, enhancing the precision and quality of PTT. Along the same lines, various nanomaterials are conjugated with a wide range of surface-modifying chemicals, including polymers and antibodies, which may modify the persistence of the nanomaterial and diminish toxicity concerns. In this article, we tend to put forth specific insights and fundamental conceptualizations on pre-existing PTT and its advances upon conjugation with different biocompatible nanomaterials working in synergy to combat breast cancer, encompassing several strategies like immunotherapy, chemotherapy, photodynamic therapy, and radiotherapy coupled with PTT. Additionally, the role or mechanisms of nanoparticles, as well as possible alternatives to PTT, are summarized as a distinctive integral aspect in this article.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Silica-Based Materials Containing Inorganic Red/NIR Emitters and Their Application in Biomedicine

The low absorption of biological substances and living tissues in the red/near-infrared region (therapeutic window) makes luminophores emitting in the range of ~650-1350 nm favorable for in vitro and in vivo imaging. In contrast to commonly used organic dyes, inorganic red/NIR emitters, including ruthenium complexes, quantum dots, lanthanide compounds, and octahedral cluster complexes of molybdenum and tungsten, not only exhibit excellent emission in the desired region but also possess additional functional properties, such as photosensitization of the singlet oxygen generation process, upconversion luminescence, photoactivated effects, and so on. However, despite their outstanding functional applicability, they share the same drawback-instability in aqueous media under physiological conditions, especially without additional modifications. One of the most effective and thus widely used types of modification is incorporation into silica, which is (1) easy to obtain, (2) biocompatible, and (3) non-toxic. In addition, the variety of morphological characteristics, along with simple surface modification, provides room for creativity in the development of various multifunctional diagnostic/therapeutic platforms. In this review, we have highlighted biomedical applications of silica-based materials containing red/NIR-emitting compounds.

Multifunctional Oxidized Dextran as a Matrix for Stabilization of Octahedral Molybdenum and Tungsten Iodide Clusters in Aqueous Media

Due to their high abundance, polymeric character, and chemical tunability, polysaccharides are perfect candidates for the stabilization of photoactive nanoscale objects, which are of great interest in modern science but can be unstable in aqueous media. In this work, we have demonstrated the relevance of oxidized dextran polysaccharide, obtained via a simple reaction with H₂O₂, towards the stabilization of photoactive octahedral molybdenum and tungsten iodide cluster complexes [M₆I₈}(DMSO)₆](NO₃)₄ in aqueous and culture media. The cluster-containing materials were obtained by co-precipitation of the starting reagents in DMSO solution. According to the data obtained, the amount and ratio of functional carbonyl and carboxylic groups as well as the molecular weight of oxidized dextran strongly affect the extent of stabilization, i.e., high loading of aldehyde groups and high molecular weight increase the stability, while acidic groups have some negative impact on the stability. The most stable material based on the tungsten cluster complex exhibited low dark and moderate photoinduced cytotoxicity, which together with high cellular uptake makes these polymers promising for the fields of bioimaging and PDT.

Catalytic nanotechnology of X-ray photodynamics for cancer treatments

Photodynamic therapy (PDT) has been applied in cancer treatment because of its high selectivity, low toxicity, and non-invasiveness. However, the limited penetration depth of the light still hampers from reaching deep-seated tumors. Considering the penetrating ability of high-energy radiotherapy, X-ray-induced photodynamic therapy (X-PDT) has evolved as an alternative to overcome tissue blocks. As the basic principle of X-PDT, X-rays stimulate the nanoparticles to emit scintillating or persistent luminescence and further activate the photosensitizers to generate reactive oxygen species (ROS), which would cause a series of molecular and cellular damages, immune response, and eventually break down the tumor tissue. In recent years, catalytic nanosystems with unique structures and functions have emerged that can enhance X-PDT therapeutic effects via an immune response. The anti-cancer effect of X-PDT is closely related to the following factors: energy conversion efficiency of the material, the radiation dose of X-rays, quantum yield of the material, tumor resistance, and biocompatibility. Based on the latest research in this field and the classical theories of nanoscience, this paper systematically elucidates the current development of the X-PDT and related immunotherapy, and highlights its broad prospects in medical applications, discussing the connection between fundamental science and clinical translation.

Flexible and Transparent Luminescent Cellulose-Transition Metal Cluster Composites

Red-NIR luminescent polymers are principally obtained from petroleum-based derivatives in which emitters, usually a critical raw material such as rare-earth or platinum group metal ions, are embedded. Considering the strong ecological impact of their synthesis and the major risk of fossil fuel energy shortage, there is an urgent need to find alternatives. We describe a luminescent nanocomposite based on red-NIR phosphorescent molybdenum nanoclusters, namely Cs₂Mo₆I₈(OCOC₂F₅)₆, embedded in an eco-friendly cellulose biopolymer matrix that is obtained by a simple solvent casting technique. While homogeneity is kept up to 20 wt% of cluster complex doping, annealing hybrids leads to a large increase of their emission efficiency, as demonstrated by quantum yield measurements.

Molybdenum-Iodine Cluster Loaded Polymeric Nanoparticles Allowing a Coupled Therapeutic Action with Low Side Toxicity for Treatment of Ovarian Cancer

The ability of cancer cells to develop resistance to anti-cancer drugs, known as multidrug resistance, remains a major cause of tumor recurrence and cancer metastasis. This work explores the double mechanism of toxicity of (D, L-lactide-co-glycolide) acid (PLGA) nanoparticles encapsulating a molybdenum cluster compound, namely Cs₂[{Mo₆I₈}(OOCC₂F₅)₆] (CMIF). Hemocompatibility and biocompatibility assays show the safe potential of CMIF loaded nanoparticles (CNPs) as delivery systems intended for tumor targeting for PDT of ovarian cancer with a slight hemolytic activity and a lack of toxicity up to 50 µM CMIF concentration. Cellular uptake shows a preferential uptake of CNPs in lysosomes, which is not interfering with CMIF activity. The double mechanism of CNPs consists in a production of ROS and a DNA damage activity, from 5 µM and 0.5 µM respectively (CMIF concentration). The cellular death mechanism comprises 80% of necrosis and 20% of direct apoptosis by direct DNA damages. This work confirms CMIF loaded PLGA nanoparticles as an efficient and relevant delivery system for PDT.

A Neutral Heteroleptic Molybdenum Cluster trans-[{Mo6I8}(py)2I4]

Despite that the chemistry of octahedral cluster complexes has been actively developed recently, there are still a lot of unexplored areas. For example, to date, only a few halide M6-clusters with N-heterocycles are known. Here, we obtained an apically heteroleptic octahedral iodide molybdenum cluster complex with pyridine ligands—trans-[{Mo6I8}(py)2I4] by the direct substitution of iodide apical ligands of [{Mo6I8}I6]2– in a pyridine solution. The compound co-crystalized with a monosubstituted form [{Mo6I8}(py)I5]– in the ratio of 1:4, and thus, can be described by the formula (pyH)0.2[{Mo6I8}(py)1.8I4.2]·1.8py. The composition was studied using XRPD, elemental analyses, and 1H-NMR and IR spectroscopies. According to the absorption and luminescence data, the partial substitution of apical ligands weakly affects optical properties.

Oxygen-Sensitive Photo- and Radioluminescent Polyurethane Nanoparticles Modified with Octahedral Iodide Tungsten Clusters

The development of cancer treatment techniques able to cure tumors located deep in the body is an urgent task for scientists and physicians. One of the most promising methods is X-ray-induced photodynamic therapy (X-PDT), since X-rays have unlimited penetration through tissues. In this work, octahedral iodide tungsten clusters, combining the properties of a scintillator and photosensitizer, are considered as a key component of nanosized polyurethane (pU) particles in the production of materials promising for X-PDT. Cluster-containing pU nanoparticles obtained here demonstrate bright photo- and X-ray-induced emission in both solid and water dispersion, great efficiency in the generation of singlet oxygen, and high sensitivity regarding photoluminescence intensity in relation to oxygen concentration. Additionally, incorporation of the cluster complex into the pU matrix greatly increases its stability against hydrolysis in water and under X-rays.

Octahedral Molybdenum Cluster-Based Nanomaterials for Potential Photodynamic Therapy

Photo/radiosensitizers, such as octahedral molybdenum clusters (Mo₆), have been intensively studied for photodynamic applications to treat various diseases. However, their delivery to the desired target can be hampered by its limited solubility, low stability in physiological conditions, and inappropriate biodistribution, thus limiting the therapeutic effect and increasing the side effects of the therapy. To overcome such obstacles and to prepare photofunctional nanomaterials, we employed biocompatible and water-soluble copolymers based on N-(2-hydroxypropyl)methacrylamide (pHPMA) as carriers of Mo₆ clusters. Several strategies based on electrostatic, hydrophobic, or covalent interactions were employed for the formation of polymer-cluster constructs. Importantly, the luminescent properties of the Mo₆ clusters were preserved upon association with the polymers: all polymer-cluster constructs exhibited an effective quenching of their excited states, suggesting a production of singlet oxygen (O₂(¹Δ_g)) species which is a major factor for a successful photodynamic treatment. Even though the colloidal stability of all polymer-cluster constructs was satisfactory in deionized water, the complexes prepared by electrostatic and hydrophobic interactions underwent severe aggregation in phosphate buffer saline (PBS) accompanied by the disruption of the cohesive forces between the cluster and polymer molecules. On the contrary, the conjugates prepared by covalent interactions notably displayed colloidal stability in PBS in addition to high luminescence quantum yields, suggesting that pHPMA is a suitable nanocarrier for molybdenum cluster-based photosensitizers intended for photodynamic applications.

Stabilization of Octahedral Metal Halide Clusters by Host-Guest Complexation with γ-Cyclodextrin: Toward Nontoxic Luminescent Compounds

γ-Cyclodextrin (γ-CD) interacts in aqueous solution with octahedral halide clusters Na₂[{M₆X₈}Cl₆] (M = Mo, W; X = Br, I) to form robust inclusion supramolecular complexes [{M₆X₈}Cl₆@2γ-CD]^2-. Single-crystal X-ray diffraction analyses revealed two conformational organizations within the adduct depending on the nature of the inner halide X within the {M₆X₈} core. Using ³⁵Cl NMR and UV-vis as complementary techniques, the kinetics of the hydrolysis process were shown to increase with the following order: {W₆I₈} < {W₆Br₈} ≈ {Mo₆I₈} < {Mo₆Br₈}. The complexation with γ-CD drastically enhances the hydrolytic stability of luminescent [{M₆X₈}Cl₆]^2- cluster-based units, which was quantitatively proved by the same techniques. The resulting host-guest complexation provides a protective shell against contact with water and offers promising horizons for octahedral clusters in biology as revealed by the low dark cytotoxicity and cellular uptake.

Progress of Nanomaterials in Photodynamic Therapy Against Tumor

Photodynamic therapy (PDT) is an advanced therapeutic strategy with light-triggered, minimally invasive, high spatiotemporal selective and low systemic toxicity properties, which has been widely used in the clinical treatment of many solid tumors in recent years. Any strategies that improve the three elements of PDT (light, oxygen, and photosensitizers) can improve the efficacy of PDT. However, traditional PDT is confronted some challenges of poor solubility of photosensitizers and tumor suppressive microenvironment. To overcome the related obstacles of PDT, various strategies have been investigated in terms of improving photosensitizers (PSs) delivery, penetration of excitation light sources, and hypoxic tumor microenvironment. In addition, compared with a single treatment mode, the synergistic treatment of multiple treatment modalities such as photothermal therapy, chemotherapy, and radiation therapy can improve the efficacy of PDT. This review summarizes recent advances in nanomaterials, including metal nanoparticles, liposomes, hydrogels and polymers, to enhance the efficiency of PDT against malignant tumor.

Avenue to X-ray-induced photodynamic therapy of prostatic carcinoma with octahedral molybdenum cluster nanoparticles

X-Ray-induced photodynamic therapy represents a suitable modality for the treatment of various malignancies. It is based on the production of reactive oxygen species by radiosensitizing nanoparticles activated by X-rays. Hence, it allows overcoming the depth-penetration limitations of conventional photodynamic therapy and, at the same time, reducing the dose needed to eradicate cancer in the frame of radiotherapy treatment. The direct production of singlet oxygen by octahedral molybdenum cluster complexes upon X-ray irradiation is a promising avenue in order to simplify the architecture of radiosensitizing systems. One such complex was utilized to prepare water-stable nanoparticles using the solvent displacement method. The nanoparticles displayed intense red luminescence in aqueous media, efficiently quenched by oxygen to produce singlet oxygen, resulting in a substantial photodynamic effect under blue light irradiation. A robust radiosensitizing effect of the nanoparticles was demonstrated in vitro against TRAMP-C2 murine prostatic carcinoma cells at typical therapeutic X-ray doses. Injection of a suspension of the nanoparticles to a mouse model revealed the absence of acute toxicity as evidenced by the invariance of key physiological parameters. This study paves the way for the application of octahedral molybdenum cluster-based radiosensitizers in X-ray-induced photodynamic therapy and its translation to in vivo experiments.

Functionalized Au@Cu-Sb-S Nanoparticles for Spectral CT/Photoacoustic Imaging-Guided Synergetic Photo-Radiotherapy in Breast Cancer

Background

Radiotherapy (RT) is clinically well-established cancer treatment. However, radioresistance remains a significant issue associated with failure of RT. Phototherapy-induced radiosensitization has recently attracted attention in translational cancer research.

Methods

Cu-Sb-S nanoparticles (NPs) coated with ultra-small Au nanocrystals (Au@Cu-Sb-S) were synthesized and characterized. The biosafety profiles, absorption of near-infrared (NIR) laser and radiation-enhancing effect of the NPs were evaluated. In vitro and in vivo spectral computed tomography (CT) imaging and photoacoustic (PA) imaging were performed in 4T1 breast cancer-bearing mice. The synergetic radio-phototherapy was assessed by in vivo tumor inhibition studies.

Results

Au@Cu-Sb-S NPs were prepared by in situ growth of Au NCs on the surface of Cu-Sb-S NPs. The cell viability experiments showed that the combination of Au@Cu-Sb-S+NIR+RT was significantly more cytotoxic to tumor cells than the other treatments at concentrations above 25 ppm Sb. In vitro and in vivo spectral CT imaging demonstrated that the X-ray attenuation ability of Au@Cu-Sb-S NPs was superior to that of the clinically used Iodine, particularly at lower KeV levels. Au@Cu-Sb-S NPs showed a concentration-dependent and remarkable PA signal brightening effect. In vivo tumor inhibition studies showed that the prepared Au@Cu-Sb-S NPs significantly suppressed tumor growth in 4T1 breast cancer-bearing mice treated with NIR laser irradiation and an intermediate X-ray dose (4 Gy).

Conclusion

These results indicate that Au@Cu-Sb-S integrated with spectral CT, PA imaging, and phototherapy-enhanced radiosensitization is a promising multifunctional theranostic nanoplatform for clinical applications.

The role of hydrolysis in biological effects of molybdenum cluster with DMSO ligands

Biological applications of octahedral molybdenum cluster complexes are complicated by their hydrolytic instability, since hydrolysis leads to irreversible changes in the structure and properties of these compounds. On the other hand, if such changes are thoroughly investigated and understood, the hydrolysis process can become an important tool for regulating specific biological effects of the clusters. In this work, we demonstrate how the luminescence and biological properties (cellular uptake, cytotoxicity in the dark and photodynamic effect) of highly unstable cluster complex [{Mo₆I₈}(DMSO)₆](NO₃)₄ change along with the degree of hydrolysis. Particularly, cluster solution preliminarily aged in water demonstrated lower dark and higher photoinduced cytotoxicity and higher cellular uptake in comparison with fresh solution.

From molecules to nanovectors: Current state of the art and applications of photosensitizers in photodynamic therapy

Photodynamic therapy (PDT) is a concept based on a selective activation by light of drugs called photosensitizers (PS) leading to reactive oxygen species production responsible for cell destruction. Mechanisms of photodynamic reaction and cell photo-destruction following direct or indirect mechanisms will be presented as well as PS classification, from first generation molecules developed in the 1960 s to third generation vectorized PS with improved affinity for tumor cells. Many clinical applications in dermatology, ophthalmology, urology, gastroenterology, gynecology, neurosurgery and pneumology reported encouraging results in human tumor management. However, this interesting technique needs improvements that are currently investigated in the field of PS excitation by the design of new PS intended for two-photon excitation or for X-ray excitation. The former excitation technique is allowing better light penetration and preservation of healthy tissues while the latter is combining PDT and radiotherapy so that external light sources are no longer needed to generate the photodynamic effect. Nanotechnology can also improve the PS to reach the tumor cells by grafting addressing molecule and by increasing its aqueous solubility and consequently its bioavailability by encapsulation in synthetic or biogenic nanovector systems, ensuring good drug protection and targeting. Co-internalization of PS with magnetic nanoparticles in multifunctional vectors or stealth nanoplatforms allows a theranostic anticancer approach. Finally, a new category of inorganic PS will be presented with promising results on cancer cell destruction.

The role of gold nanoparticles' aspect ratio in plasmon-enhanced luminescence and the singlet oxygen generation rate of Mo6 clusters

Here we present a study on the effect of the aspect ratio (AR) of gold nanoparticles on the emission intensity and singlet oxygen production rate of hexamolybdenum cluster-doped silica particles. It was shown that these parameters can be enhanced gradually up to 6.7- and 13-fold with the AR.