Heterometallic Ru-Pt metallacycle for two-photon photodynamic therapy

Advancing Photodynamic Therapy with Nano-Conjugated Hypocrellin: Mechanisms and Clinical Applications

Hypocrellin-based photodynamic therapy (PDT) is developing as a viable cancer therapeutic option, especially when enhanced by nanoconjugation. This review investigates the methods by which nano-conjugated hypocrellin enhances therapeutic efficacy and precision when targeting cancer cells. These nanoconjugates encapsulate or covalently bind hypocrellin photosensitizers (PSs), allowing them to accumulate preferentially in malignancies. When activated by light, the nanoconjugates produce singlet oxygen and other reactive oxygen species (ROS), resulting in oxidative stress that selectively destroys cancer cells while protecting healthy tissues. We look at how they can be used to treat a variety of cancers. Clinical and preclinical studies show that they have advantages such as increased water solubility, improved tumor penetration, longer circulation times, and tailored delivery, all of which contribute to fewer off-target effects and overall toxicity. Ongoing research focuses on improving these nanoconjugates for better tumor targeting, drug release kinetics, and overcoming biological obstacles. Furthermore, the incorporation of developing technologies such as stimuli-responsive nanocarriers and combination therapies opens exciting opportunities for enhancing hypocrellin-based PDT. In conclusion, the combination of hypocrellin and nanotechnology constitutes a significant approach to cancer treatment, increasing the efficacy and safety of PDT. Future research will seek to create conjugates including hypocrellin, herceptin, and gold nanoparticles to induce apoptosis in human breast cancer cells in vitro, opening possibilities for therapeutic applications.

Leveraging the Photofunctions of Transition Metal Complexes for the Design of Innovative Phototherapeutics

Despite the advent of various medical interventions for cancer treatment, the disease continues to pose a formidable global health challenge, necessitating the development of new therapeutic approaches for more effective treatment outcomes. Photodynamic therapy (PDT), which utilizes light to activate a photosensitizer to produce cytotoxic reactive oxygen species (ROS) for eradicating cancer cells, has emerged as a promising approach for cancer treatment due to its high spatiotemporal precision and minimal invasiveness. However, the widespread clinical use of PDT faces several challenges, including the inefficient production of ROS in the hypoxic tumor microenvironment, the limited penetration depth of light in biological tissues, and the inadequate accumulation of photosensitizers at the tumor site. Over the past decade, there has been increasing interest in the utilization of photofunctional transition metal complexes as photosensitizers for PDT applications due to their intriguing photophysical and photochemical properties. This review provides an overview of the current design strategies used in the development of transition metal complexes as innovative phototherapeutics, aiming to address the limitations associated with PDT and achieve more effective treatment outcomes. The current challenges and future perspectives on the clinical translation of transition metal complexes are also discussed.

Enhanced Sonodynamic Therapy for Deep Tumors Using a Self-Assembled Organoplatinum(II) Sonosensitizer

Despite the promising advances in photodynamic therapy (PDT), it remains challenging to target and treat deep-seated solid tumors effectively. Herein, we developed an organoplatinum(II) complex (Pt-TPE) with self-assembly properties for sonodynamic therapy (SDT). Pt-TPE forms a nanofiber network structure through Pt-Pt and π-π stacking interactions. Notably, under ultrasound (US), Pt-TPE demonstrates unique self-assembly-induced singlet oxygen (1O2) generation due to a significantly enhanced singlet-triplet intersystem crossing (ISC). This generation of 1O2 occurs exclusively in the self-assembled state of Pt-TPE. Additionally, Pt-TPE exhibits sono-cytotoxicity against cancer cells by impairing mitochondrial membrane potential (MMP), inhibiting glucose uptake, and aerobic glycolysis. Furthermore, US-activated Pt-TPE significantly inhibits deep solid tumors in mice, achieving remarkable therapeutic efficacy even at penetration depths greater than 10 cm. This study highlights the potential of self-assembled metal complexes to enhance the efficacy of SDT for treating deep tumors.

Polyphosphoester-stabilized cubosomes encapsulating a Ru(II) complex for the photodynamic treatment of lung adenocarcinoma

The clinical translation of photosensitizers based on ruthenium(II) polypyridyl complexes (RPCs) in photodynamic therapy of cancer faces several challenges. To address these limitations, we conducted an investigation to assess the potential of a cubosome formulation stabilized in water against coalescence utilizing a polyphosphoester analog of Pluronic F127 as a stabilizer and loaded with newly synthesized RPC-based photosensitizer [Ru(dppn)2(bpy-morph)](PF6)2 (bpy-morph = 2,2'-bipyridine-4,4'-diylbis(morpholinomethanone)), PS-Ru. The photophysical characterization of PS-Ru revealed its robust capacity to induce the formation of singlet oxygen (1O2). Furthermore, the physicochemical analysis of the PS-Ru-loaded cubosomes dispersion demonstrated that the encapsulation of the photosensitizer within the nanoparticles did not disrupt the three-dimensional arrangement of the lipid bilayer. The biological tests showed that PS-Ru-loaded cubosomes exhibited significant phototoxic activity when exposed to the light source, in stark contrast to empty cubosomes and to the same formulation without irradiation. This promising outcome suggests the potential of the formulation in overcoming the drawbacks associated with the clinical use of RPCs in photodynamic therapy for anticancer treatments.

Formation of a low-symmetry Pd8 molecular barrel employing a hetero donor tetradentate ligand, and its use in the binding and extraction of C70

The majority of reported metallo-supramolecules are highly symmetric homoleptic assemblies of M x L y type, with a few reports on assemblies that are obtained using multicomponent self-assembly or using ambidentate ligands. Herein, we report the use of an unsymmetrical tetratopic ligand (Lun) containing pyridyl and imidazole donor sites in combination with a cis-protected Pd(ii) acceptor for the formation of a low-symmetry M8Lun 4 molecular barrel (UNMB). Four potential orientational isomeric (HHHH, HHHT, HHTT, and HTHT) molecular barrels can be anticipated for the M8Lun 4 type metallo-assemblies. However, the formation of an orientational isomer (HHTT) of the barrel was suggested from single-crystal X-ray diffraction and 1H NMR analysis of UNMB. Two large open apertures at terminals and the hydrophobic confined space surrounded by four aromatic panels of Lun make UNMB a potential host for bigger guests. UNMB encapsulates fullerenes C70 and C60 favoured by non-covalent interactions between the fullerenes and aromatic panels of the ligand molecules. Experimental and theoretical studies revealed that UNMB has the ability to bind C70 more strongly than its lower analogue C60. The stronger affinity of UNMB towards C70 was exploited to separate C70 from an equimolar mixture of C70 and C60. Moreover, C70 can be extracted from the C70⊂UNMB complex by toluene, and therefore, UNMB can be reused as a recyclable separating agent for C70 extraction.

Cyclometalated iridium(III) tetrazine complexes for mitochondria-targeted two-photon photodynamic therapy

The fast-moving field of photodynamic therapy (PDT) has provided fresh opportunities to expand the potential of metallodrugs to combat cancers in a light-controlled manner. As such, in the present study, a series of cyclometalated Ir(III) complexes modified with a tetrazine functional group (namely, Ir-ppy-Tz, Ir-pbt-Tz, and Ir-dfppy-Tz) are developed as potential two-photon photodynamic anticancer agents. These complexes target mitochondria but exhibit low toxicity towards HLF primary lung fibroblast normal cells in the dark. When receiving a low-dose one- or two-photon PDT, they become highly potent towards A549 lung cancer cells (with IC50 values ranging from 24.0 nM to 96.0 nM) through the generation of reactive oxygen species (ROS) to induce mitochondrial damage and subsequent apoptosis. Our results indicated that the incorporation of tetrazine with cyclometalated Ir(III) matrices would increase the singlet oxygen (1O2) quantum yield (ΦΔ) and, meanwhile, enable a type I PDT mechanism. Ir-pbt-Tz, with the largest two-photon absorption (TPA) cross-section (σ2 = 102 GM), shows great promise in serving as a two-photon PDT agent for phototherapy.

Iridium(III)-based minor groove binding complexes as DNA photocleavage agents

Transition metal complexes containing the qtpy ligand (2':4,4'':4',4'''-quaterpyridyl) are known to be DNA intercalators or minor groove binders. In this study, new tricationic iridium(III) complexes of qtpy are reported. Both [Ir(bpy)2(qtpy)]3+1 and [Ir(phen)2(qtpy)]3+2 display good water solubility as chloride salts. The complexes possess high-energy excited states, which are quenched in the presence of duplex DNA and even by the mononucleotides guanosine monophosphate and adenosine monophosphate. Further studies reveal that although the complexes bind to quadruplex DNA, they display a preference for duplex structures, which are bound with an order of magnitude higher affinities than their isostructural dicationic RuII-analogues. Detailed molecular dynamics simulations confirm that the complexes are groove binders through the insertion of, predominantly, the qtpy ligand into the minor groove. Photoirradiation of 1 in the presence of plasmid DNA confirms that this class of complexes can function as synthetic photonucleases by cleaving DNA.

Organo-Ptii Complexes for Potent Photodynamic Inactivation of Multi-Drug Resistant Bacteria and the Influence of Configuration

PtII based organometallic photosensitizers (PSs) have emerged as novel potent photodynamic inactivation (PDI) reagents through their enhanced intersystem crossing (ISC) processes. Currently, few PtII PSs have been investigated as antibacterial materials, with relatively poor performances reported and with structure-activity relationships not well described. Herein, a pair of configurational isomers are reported of Bis-BODIPY (4,4-difluoro-boradizaindacene) embedded PtII PSs. The cis-isomer (cis-BBP) displayed enhanced 1O2 generation and better bacterial membrane anchoring capability as compared to the trans-isomer (trans-BBP). The effective PDI concentrations (efficiency > 99.9%) for cis-BBP in Acinetobacter baumannii (multi-drug resistant (MDR)) and Staphylococcus aureus are 400 nM (12 J cm-2) and 100 nM (18 J cm-2), respectively; corresponding concentrations and light doses for trans-BBP in the two bacteria are 2.50 µM (30 J cm-2) and 1.50 µM (18 J cm-2), respectively. The 50% and 90% minimum inhibitory concentration (MIC50 and MIC90) ratio of trans-BBP to cis-BBP is 22.22 and 24.02 in A. baumannii (MDR); 21.29 and 22.36 in methicillin resistant S. aureus (MRSA), respectively. Furthermore, cis-BBP displays superior in vivo antibacterial performance, with acceptable dark and photoinduced cytotoxicity. These results demonstrate cis-BBP is a robust light-assisted antibacterial reagent at sub-micromolecular concentrations. More importantly, configuration of PtII PSs should be an important issue to be considered in further PDI reagents design.

Strategies to address key challenges of metallacycle/metallacage-based supramolecular coordination complexes in biomedical applications

Owing to their capacity for dynamically linking two or more functional molecules, supramolecular coordination complexes (SCCs), exemplified by two-dimensional (2D) metallacycles and three-dimensional (3D) metallacages, have gained increasing significance in biomedical applications. However, their inherent hydrophobicity and self-assembly driven by heavy metal ions present common challenges in their applications. These challenges can be overcome by enhancing the aqueous solubility and in vivo circulation stability of SCCs, alongside minimizing their side effects during treatment. Addressing these challenges is crucial for advancing the fundamental research of SCCs and their subsequent clinical translation. In this review, drawing on extensive contemporary research, we offer a thorough and systematic analysis of the strategies employed by SCCs to surmount these prevalent yet pivotal obstacles. Additionally, we explore further potential challenges and prospects for the broader application of SCCs in the biomedical field.

Combining Multiple Photosensitizer Modules into One Supramolecular System for Synergetic Enhanced Photodynamic Therapy

Photodynamic therapy (PDT), as an emerging cancer treatment, requires the development of highly desirable photosensitizers (PSs) with integrated functional groups to achieve enhanced therapeutic efficacy. Coordination-driven self-assembly (CDSA) would provide an alternative approach for combining multiple PSs synergistically. Here, we demonstrate a simple yet powerful strategy of combining conventional chromophores (tetraphenylethylene, porphyrin, or Zn-porphyrin) with pyridinium salt PSs together through condensation reactions, followed by CDSA to construct a series of novel metallo-supramolecular PSs (S1-S3). The generation of reactive oxygen species (ROS) is dramatically enhanced by the direct combination of two different PSs, and further reinforced in the subsequent ensembles. Among all the ensembles, S2 with two porphyrin cores shows the highest ROS generation efficiency, specific interactions with lysosome, and strong emission for probing cells. Moreover, the cellular and living experiments confirm that S2 has excellent PDT efficacy, biocompatibility, and biosafety. As such, this study will enable the development of more efficient PSs with potential clinical applications.

Multimodal Therapeutic Platforms Based on Self-Assembled Metallacycles/Metallacages for Cancer Radiochemotherapy

Discrete organometallic complexes with defined structures are proceeding rapidly in combating malignant tumors due to their multipronged treatment modalities. Many innovative superiorities, such as high antitumor activity, extremely low systemic toxicity, active targeting ability, and enhanced cellular uptake, make them more competent for clinical applications than individual precursors. In particular, coordination-induced regulation of luminescence and photophysical properties of organic light-emitting ligands has demonstrated significant potential in the timely evaluation of therapeutic efficacy by bioimaging and enabled synergistic photodynamic therapy (PDT) or photothermal therapy (PTT). This review highlights instructive examples of multimodal radiochemotherapy platforms for cancer ablation based on self-assembled metallacycles/metallacages, which would be classified by functions in a progressive manner. Finally, the essential demands and some plausible prospects in this field for cancer therapy are also presented.

Recent advances on the construction of long-wavelength emissive supramolecular coordination complexes for photo-diagnosis and therapy

Recently, metal-based drugs have attracted relentless interest in the biomedical field. However, their short excitation/emission wavelengths and unsatisfactory therapeutic efficiency limit their biological applications in vivo. Currently, the second near-infrared window (NIR-II, 1000-1700 nm) provides more accurate imaging and therapeutic options. Thus, there has been a constant focus on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. Fortunately, supramolecular coordination complexes (SCCs) formed by the coordination-driven self-assembly of NIR-II emissive ligands can address the above issues. Importantly, metal receptors with chemotherapeutic properties in SCCs can bind to luminescent ligands, thus becoming a versatile therapeutic platform for chemotherapy, imaging and phototherapy. In this context, we systematically summarize the evolution of NIR-II emissive SCCs for biomedical applications and discuss future challenges and prospects.

Theranostics with photodynamic therapy for personalized medicine: to see and to treat

Photodynamic Therapy (PDT) is an approved treatment modality, which is presently receiving great attention due to its limited invasiveness, high selectivity and limited susceptibility to drug resistance. Another related research area currently expanding rapidly is the development of novel theranostic agents based on the combination of PDT with different imaging technologies, which allows for both therapy and diagnosis. This combination can help to address issues of suboptimal biodistribution and selectivity through regional imaging, while therapeutic agents enable an effective and personalized therapy. In this review, we describe compounds, whose structures combine PDT photosensitizers with different imaging probes - including examples for near-infrared optical imaging, magnetic resonance imaging (MRI) and nuclear imaging (PET or SPECT), generating novel theranostic drug candidates. We have intentionally focused our attention on novel compounds, which have already been investigated preclinically in vivo in order to demonstrate the potential of such theranostic agents for clinical applications.

Emodin-Based Nanoarchitectonics with Giant Two-Photon Absorption for Enhanced Photodynamic Therapy

Two-photon-excited photodynamic therapy (TPE-PDT) has significant advantages over conventional photodynamic therapy (PDT). However, obtaining easily accessible TPE photosensitizers (PSs) with high efficiency remains a challenge. Herein, we demonstrate that emodin (Emo), a natural anthraquinone (NA) derivative, is a promising TPE PS with a large two-photon absorption cross-section (TPAC: 380.9 GM) and high singlet oxygen (1 O2 ) quantum yield (31.9 %). When co-assembled with human serum albumin (HSA), the formed Emo/HSA nanoparticles (E/H NPs) possess a giant TPAC (4.02×107  GM) and desirable 1 O2 generation capability, thus showing outstanding TPE-PDT properties against cancer cells. In vivo experiments reveal that E/H NPs exhibit improved retention time in tumors and can ablate tumors at an ultra-low dosage (0.2 mg/kg) under an 800 nm femtosecond pulsed laser irradiation. This work is beneficial for the use of natural extracts NAs for high-efficiency TPE-PDT.

A membrane intercalating metal-free conjugated organic photosensitizer for bacterial photodynamic inactivation

Photodynamic inhibition (PDI) of bacteria represents a powerful strategy for dealing with multidrug-resistant pathogens and infections, as it exhibits minimal development of antibiotic resistance. The PDI action stems from the generation of a triplet state in the photosensitizer (PS), which subsequently transfers energy or electrons to molecular oxygen, resulting in the formation of reactive oxygen species (ROS). These ROS are then able to damage cells, eventually causing bacterial eradication. Enhancing the efficacy of PDI includes the introduction of heavy atoms to augment triplet generation in the PS, as well as membrane intercalation to circumvent the problem of the short lifetime of ROS. However, the former approach can pose safety and environmental concerns, while achieving stable membrane partitioning remains challenging due to the complex outer envelope of bacteria. Here, we introduce a novel PS, consisting of a metal-free donor-acceptor thiophene-based conjugate molecule (BV-1). It presents several advantageous features for achieving effective PDI, namely: (i) it exhibits strong light absorption due to the conjugated donor-acceptor moieties; (ii) it exhibits spontaneous and stable membrane partitioning thanks to its amphiphilicity, accompanied by a strong fluorescence turn-on; (iii) it undergoes metal-free intersystem crossing, which occurs preferentially when the molecule resides in the membrane. All these properties, which we rationalized via optical spectroscopies and calculations, enable the effective eradication of Escherichia coli, with an inhibition concentration that is below that of current state-of-the-art treatments. Our approach holds significant potential for the development of new PS for controlling bacterial infections, particularly those caused by Gram-negative bacteria.

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Engineered Metallacycle-Based Supramolecular Photosensitizers for Effective Photodynamic Therapy

Although metallacycle-based supramolecular photosensitizers (PSs) have attracted increasing attention in biomedicine, their clinical translation is still hindered by their inherent dark toxicity. Herein, we report what to our knowledge is the first example of a molecular engineering approach to building blocks of metallacycles for constructing a series of supramolecular PSs (RuA-RuD), with the aim of simultaneously reducing dark toxicity and enhancing phototoxicity, and consequently obtaining high phototoxicity indexes (PI). Detailed in vitro investigations demonstrate that RuA-RuD display high cancer cellular uptake and remarkable antitumor activity even under hypoxic conditions. Notably, RuD exhibited no dark toxicity and displayed the highest PI value (≈406). Theoretical calculations verified that RuD has the largest steric hindrance and the lowest singlet-triplet energy gap (ΔE_ST , 0.61 eV). Further in vivo studies confirmed that RuD allows safe and effective phototherapy against A549 tumors.

Promising heterometallic compounds as anticancer agents: Recent studies in vivo

Over the past decade, interest on multitarget anticancer drugs -including heterometallic compounds-has increased considerably. Heterometallic species display improved efficacy and physicochemical properties compared to the individual metallic fragments for a variety of metal pair combinations. By 2018, several compounds had emerged as promising candidates against cisplatin resistant cancers. Here, we summarize research contributions to this topic over the past four years (July 2018-July 2022). In particular, we highlight five articles reporting on the in vivo activity and preliminary mechanisms of action for five groups of compounds. From this selection, we further feature two families of compounds based on Pt(IV)-Gd(III) and Ti(IV)-Au(I) metal combinations, given their potential for clinical translation.

Light-related activities of metal-based nanoparticles and their implications on dermatological treatment

Metal-based nanoparticles (MNPs) represent an emerging class of materials that have attracted enormous attention in many fields. By comparison with other biomaterials, MNPs own unique optical properties which make them a potential alternative to conventional therapeutic agents in medical applications. Especially, owing to the easy access to the skin, the use of MNPs based on their optical properties has gained importance for the treatment of a variety of skin diseases. This review provides an insight into the different optical properties of MNPs, including photoprotection, photocatalysis, and photothermal, and highlights their implications in treating skin disorders, with a special emphasis on their use in infection control. Finally, a perspective on the safety concern of MNPs for dermatological use is discussed and analyzed. The information gathered and presented in this review will help the readers have a comprehensive understanding of utilizing the photo-triggered activity of MNPs for the treatment of skin diseases.

Cyanine-Functionalized 2,2'-Bipyridine Compounds for Photocatalytic Cancer Therapy

Recently, interest has been given to developing photocatalytic anticancer drugs. This area of research is dominated by metal complexes. Here, we report the potential of lysosome/mitochondria targeting cyanine appended bipyridine compounds as the organic photocatalytic anticancer agents. The organocatalyst (bpyPCN) not only exhibits light-induced NADH oxidation but also generates intracellular ROS to demonstrate anticancer activity. This is the first example of organic compound induced catalytic NADH photo-oxidation in an aqueous solution and in cancer cells.

Bioinorganic supramolecular coordination complexes and their biomedical applications

The field of Bioinorganic Supramolecular Chemistry is an emerging research area including metal-based supramolecules resulting from coordination-driven self-assembly (CDSA), whereby metal ions and organic ligands can be easily linked by metal-ligand bonds via Lewis' acid/base interactions. The focus of this 'In a Nutshell' review will be on the family of supramolecular coordination complexes, discrete entities formed by CDSA, which have recently captured widespread attention as a new class of versatile multifunctional materials with broad biological applications including molecular recognition, biosensing, therapy, imaging and drug delivery. Herein, we provide a summary of the state-of-the-art use of these systems in biomedicine, with some selected representative examples, as well as our visions of the challenges and possible directions in the field.

Highlights of New Strategies to Increase the Efficacy of Transition Metal Complexes for Cancer Treatments

Although important progress has been made, cancer still remains a complex disease to treat. Serious side effects, the insurgence of resistance and poor selectivity are some of the problems associated with the classical metal-based anti-cancer therapies currently in clinical use. New treatment approaches are still needed to increase cancer patient survival without cancer recurrence. Herein, we reviewed two promising-at least in our opinion-new strategies to increase the efficacy of transition metal-based complexes. First, we considered the possibility of assembling two biologically active fragments containing different metal centres into the same molecule, thus obtaining a heterobimetallic complex. A critical comparison with the monometallic counterparts was done. The reviewed literature has been divided into two groups: the case of platinum; the case of gold. Secondly, the conjugation of metal-based complexes to a targeting moiety was discussed. Particularly, we highlighted some interesting examples of compounds targeting cancer cell organelles according to a third-order targeting approach, and complexes targeting the whole cancer cell, according to a second-order targeting strategy.

Hydrazone- and imine-containing [PdPtL4]4+ cages: a comparative study of the stability and host-guest chemistry

A new [PdPtL₄]⁴⁺ heterobimetallic cage containing hydrazone linkages has been synthesised using the sub-component self-assembly approach. ¹H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the formation of the [PdPtL₄]⁴⁺ architecture. The cage was stimulus-responsive and could be partially disassembled and reassembled by the addition of dimethylaminopyridine (DMAP) and p-tolenesulfonic acid (TsOH), respectively. Additionally, the stability of the hydrazone cage against hydrolysis in the presence of water and nucleophilic decomposition in the presence of guest molecules was compared to a previously synthesised imine-containing [PdPtL₄]⁴⁺ cage. It was established that the hydrazone linkage was more resistant to hydrolysis. Furthermore, the host-guest (HG) chemistry with a series of drug and drug-like molecules was examined. The hydrazone cage was shown to interact with cisplatin while the smaller imine cage was shown to interact with 5-fluorouracil and oxaliplatin in CD₃CN. No HG interactions were observed in the more polar d₆-DMSO. In vitro antiproliferative activity studies demonstrated both cages were active against the cancer cell lines tested and displayed half-maximal inhibitory (IC₅₀) values in the range of 25-35 μM. Most [PdPtL₄]⁴⁺-drug mixtures tested had higher IC₅₀ values than the hosts. However, the [PdPtL₄]⁴⁺ cages, and [PdPtL₄]⁴⁺:drug mixtures were less cytotoxic than the well established anticancer drugs cisplatin, oxaliplatin and 5-fluorouracil.

Ru(II) CONTAINING PHOTOSENSITIZERS FOR PHOTODYNAMIC THERAPY: A CRITIQUE ON REPORTING AND AN ATTEMPT TO COMPARE EFFICACY

Ruthenium(II)-based coordination complexes have emerged as photosensitizers (PSs) for photodynamic therapy (PDT) in oncology as well as antimicrobial indications and have great potential. Their modular architectures that integrate multiple ligands can be exploited to tune cellular uptake and subcellular targeting, solubility, light absorption, and other photophysical properties. A wide range of Ru(II) containing compounds have been reported as PSs for PDT or as photochemotherapy (PCT) agents. Many studies employ a common scaffold that is subject to systematic variation in one or two ligands to elucidate the impact of these modifications on the photophysical and photobiological performance. Studies that probe the excited state energies and dynamics within these molecules are of fundamental interest and are used to design next-generation systems. However, a comparison of the PDT efficacy between Ru(II) containing PSs and 1st or 2nd generation PSs, already in clinical use or preclinical/clinical studies, is rare. Even comparisons between Ru(II) containing molecular structures are difficult, given the wide range of excitation wavelengths, power densities, and cell lines utilized. Despite this gap, PDT dose metrics quantifying a PS's efficacy are available to perform qualitative comparisons. Such models are independent of excitation wavelength and are based on common outcome parameters, such as the photon density absorbed by the Ru(II) compound to cause 50% cell kill (LD50) based on the previously established threshold model. In this focused photophysical review, we identified all published studies on Ru(II) containing PSs since 2005 that reported the required photophysical, light treatment, and in vitro outcome data to permit the application of the Photodynamic Threshold Model to quantify their potential efficacy. The resulting LD50 values range from less than 1013 to above 1020 [hν cm-3], indicating a wide range in PDT efficacy and required optical energy density for ultimate clinical translation.

Precision Nanomedicines: Targeting Hot Mitochondria in Cancer Cells

Mitochondrion is a multifunctional organelle in a cell, and it is one of the important targets of antitumor therapy. Conventional mitochondrial targeting strategies can hardly distinguish the mitochondria in cancer cells from those in normal cells, which might raise a concern about the biosafety. Recent studies suggest that a relatively high temperature of mitochondria exists in cancer cells. We named it tumor intrinsic mitochondrial overheating (TIMO). By taking advantage of the difference in mitochondrial temperatures between cancer cells and normal cells, therapeutic agents can be specifically delivered to the mitochondria in cancer cells. Here we will briefly overview the mitochondria-targeted delivery strategies. In addition, the recent discovery of hot mitochondria in cancer cells and the development of mitochondrial temperature-responsive delivery systems for antitumor therapy will be reviewed.

Rationally designed Ru(II) metallacycles with tunable imidazole ligands for synergistical chemo-phototherapy of cancer

Herein, we construct a series of Ru(II) metallacycles with multimodal chemo-phototherapeutic properties, which exhibited much higher anticancer activity and better cancer-cell selectivity than cisplatin. The antitumor mechanism could be ascribed to the activation of caspase 3/7 and the resulting apoptosis. These results open new possibilities for Ru(II) metallacycles in biomedicine.

Spatially Selective Monitoring of Subcellular Enzyme Dynamics in Response to Mitochondria-Targeted Photodynamic Therapy

Tracking spatial and temporal dynamics of bioactive molecules such as enzymes responding to therapeutic treatment is highly important for understanding of the related functions. However, in situ molecular imaging at subcellular level during photodynamic therapy (PDT) has been hampered by the limitations of existing methods. Herein, we present a multifunctional nanoplatform (termed as UR-HAPT) that is able to simultaneously monitor subcellular dynamics of human apurinic/apyrimidinic endonuclease 1 (APE1) during the near-infrared (NIR) light-mediated PDT. UR-HAPT was constructed by the combination of an upconversion nanoparticle-based PDT design and a mitochondria-targeting strategy with an APE1-responsive DNA reporter. Benefiting from the gain-of-function approach, activatable mitochondrial accumulation of APE1 in response to the oxidative stress was observed during the NIR light-triggered, mitochondria-targeted PDT process. We envision that this nanoplatform can be applicable to screen and evaluate potential enzyme inhibitors to improve the PDT efficacy.

A Near-Infrared BODIPY-Based Rhomboidal Metallacycle for Imaging-Guided Photothermal Therapy

Although boron dipyrromethene (BODIPY)-based metallacycles are expected to be promising candidates for imaging probes and therapeutic agents, their biomedical applications are restricted by their short absorption/emission wavelengths. In this work, we report a rhombic metallacycle M with broad absorption in the near-infrared (NIR) range and emissions at wavelengths >800 nm, which exhibits an efficient photothermal conversion capacity. Metallacycle M was encapsulated via Pluronic F127 to fit the biotic environment, resulting in the generation of F127/M nanoparticles (NPs) with high hydrophilicity and biocompatibility. In vitro studies demonstrated that the F127/M NPs underwent efficient cellular uptake and exhibited satisfactory photothermal therapeutic activity. Furthermore, in vivo experiments revealed that tumor growth was effectively inhibited, and the degree of undesirable biological damage was minimal in treatment with F127/M NPs and laser irradiation. Finally, the F127/M NPs could be visualized through NIR fluorescence imaging in living mice, thereby allowing their distribution to be monitored in order to enhance treatment accuracy during photothermal therapy. We envision that such BODIPY-based metallacycles will provide emerging opportunities for the development of novel therapeutic agents for biomedical applications.

Rationally designed Ru(ii)-metallacycle chemo-phototheranostic that emits beyond 1000 nm

Ruthenium complexes are emerging as potential complements to platinum drugs. They also show promise as photo-diagnostic and therapeutic agents. However, most ruthenium species studied to date as potential drugs are characterized by short excitation/emission wavelengths. This limits their applicability for deep-tissue fluorescence imaging and light-based therapeutic treatments. Here, we report a Ru(ii) metallacycle (Ru1100) that emits at ≥1000 nm. This system possesses excellent deep-tissue penetration capability (∼7 mm) and displays good chemo-phototherapeutic performance. In vitro studies revealed that Ru1100 benefits from good cellular uptake and produces a strong anticancer response against several cancer cell lines, including a cisplatin-resistant A549 cell line (IC50 = 1.6 μM vs. 51.4 μM for cisplatin). On the basis of in vitro studies, it is concluded that Ru1100 exerts its anticancer action by regulating cell cycle progression and triggering cancer cell apoptosis. In vivo studies involving the use of a nanoparticle formulation served to confirm that Ru1100 allows for high-performance NIR-II fluorescence imaging-guided precise chemo-phototherapy in the case of A549 tumour mouse xenografts with no obvious side effects. This work thus provides a paradigm for the development of long-wavelength emissive supramolecular theranostic agents based on ruthenium.

NIR-II Emissive Ru(II) Metallacycle Assisting Fluorescence Imaging and Cancer Therapy

Despite the success of emissive Ruthenium (Ru) agents in biomedicine, problems such as the visible-light excitation/emission and single chemo- or phototherapy modality still hamper their applications in deep-tissue imaging and efficient cancer therapy. Herein, an second nearinfrared window (NIR-II) emissive Ru(II) metallacycle (Ru1000, λ_em  = 1000 nm) via coordination-driven self-assembly is reported, which holds remarkable deep-tissue imaging capability (≈6 mm) and satisfactory chemo-phototherapeutic performance. In vitro results indicate Ru1000 displays promising cellular uptake, good cancer-cell selectivity, attractive anti-metastasis properties, and remarkable anticancer activity against various cancer cells, including cisplatin-resistant A549 cells (IC₅₀  = 3.4 × 10^-6  m vs 92.8 × 10^-6  m for cisplatin). The antitumor mechanism could be attributed to Ru1000-induced lysosomal membrane damage and mitochondrial-mediated apoptotic cell death. Furthermore, Ru1000 also allows the high-performance in vivo NIR-II fluorescence imaging-guided chemo-phototherapy against A549 tumors. This work may provide a paradigm for the development of long-wavelength emissive metallacycle-based agents for future biomedicine.

Platinum-Based Two-Photon Photosensitizer Responsive to NIR Light in Tumor Hypoxia Microenvironment

Platinum-based photosensitizers are promising anticancer agents in photodynamic therapy. The cytotoxic effects primarily arise from the production of singlet oxygen and platination of DNA. However, their efficacy is limited by drug resistance and hypoxic tumor microenvironment. A naphthalimide-modified cyclometalated platinum(II) complex PtPAN [PA = N-(2-(diethylamino)ethyl)picolinamide, N = N-(2'-ethylhexyl)-4-ethynyl-1,8-naphthalimide] is designed to conquer these problems. PtPAN generates ROS efficiently under both normoxia and hypoxia. It does not interact with DNA and shows low cytotoxicity in the dark, while it kills tumor cells via ROS under near-infrared light irradiation; moreover, it inhibits tumor growth in mice at a low light dose with negligible side effects. PtPAN is the first reported platinum-based photosensitizer that is unreactive to DNA in the dark but highly cytotoxic upon near-infrared (NIR) irradiation for oxygen-independent photodynamic therapy. Owing to its two-photon excitation property (λ = 825 nm), PtPAN may be suitable for the treatment of deep solid tumors.

Click Synthesis Enabled Sulfur Atom Strategy for Polymerization-Enhanced and Two-Photon Photosensitization

Facile tailoring of photosensitizers (PSs) with advanced and synergetic properties is highly expected to broaden and deepen photodynamic therapy (PDT) applications. Herein, a catalyst-free thiol-yne click reaction was employed to develop the sulfur atom-based PSs by using the in situ formed sulfur "heavy atom effect" to enhance the intersystem crossing (ISC), while such an effect can be remarkably magnified by the polymerization. The introduction of a tetraphenylpyrazine-based aggregation-induced emission (AIE) unit was also advantageous in PS design by suppressing their non-radiative decay to facilitate the ISC in the aggregated state. Besides, the resulting sulfur atom electron donor, together with a double-bond π bridge and AIE electron acceptor, created a donor-π-acceptor (D-π-A) molecular system with good two-photon excitation properties. Combined with the high singlet oxygen generation efficiency, the fabricated polymer nanoparticles exhibited an excellent in vitro two-photon-excited PDT towards cancer cells, therefore possessing a huge potential for the deep-tissue disease therapy.

Photodecaging of a Mitochondria-Localized Iridium(III) Endoperoxide Complex for Two-Photon Photoactivated Therapy under Hypoxia

Despite the clinical success of photodynamic therapy (PDT), the application of this medical technique is intrinsically limited by the low oxygen concentrations found in cancer tumors, hampering the production of therapeutically necessary singlet oxygen (¹O₂). To overcome this limitation, we report on a novel mitochondria-localized iridium(III) endoperoxide prodrug (2-O-IrAn), which, upon two-photon irradiation in NIR, synergistically releases a highly cytotoxic iridium(III) complex (2-IrAn), singlet oxygen, and an alkoxy radical. 2-O-IrAn was found to be highly (photo-)toxic in hypoxic tumor cells and multicellular tumor spheroids (MCTS) in the nanomolar range. To provide cancer selectivity and improve the pharmacological properties of 2-O-IrAn, it was encapsulated into a biotin-functionalized polymer. The generated nanoparticles were found to nearly fully eradicate the tumor inside a mouse model within a single treatment. This study presents, to the best of our knowledge, the first example of an iridium(III)-based endoperoxide prodrug for synergistic photodynamic therapy/photoactivated chemotherapy, opening up new avenues for the treatment of hypoxic tumors.

BODIPY based Metal-Organic Macrocycles and Frameworks: Recent Therapeutic Developments

Boron dipyrromethene, commonly known as BODIPY, based metal-organic macrocycles (MOCs) and metal-organic frameworks (MOFs) represent an interesting part of materials due to their versatile tunability of structure and functionality as well as significant physicochemical properties, thus broadening their applications in various scientific domains, especially in biomedical sciences. With increasing concern over the efficacy of cancer drugs versus quality of patient's life dilemma, scientists have been trying to fabricate novel comprehensive therapeutic strategies along with the discovery of novel safer drugs where research with BODIPY metal complexes has shown vital advancements. In this review, we have exclusively examined the articles involving studies related to light harvesting and photophysical properties of BODIPY based MOCs and MOFs, synthesized through self-assembly process, with a special focus on biomolecular interaction and its importance in anti-cancer drug research. In the end, we also emphasized the possible practical challenges involved during the synthetic process, based on our experience on dealing with BODIPY molecules and steps to overcome them along with their future potentials. This review will significantly help our fellow research groups, especially the budding researchers, to quickly and comprehensively get the near to wholesome picture of BODIPY based MOCs and MOFs and their present status in anti-cancer drug discovery.

Design of a Metallacycle-Based Supramolecular Photosensitizer for In Vivo Image-Guided Photodynamic Inactivation of Bacteria

Bacterial infection is one of the greatest threats to public health. In vivo real-time monitoring and effective treatment of infected sites through non-invasive techniques, remain a challenge. Herein, we designed a PtII metallacycle-based supramolecular photosensitizer through the host-guest interaction between a pillar[5]arene-modified metallacycle and 1-butyl-4-[4-(diphenylamino)styryl]pyridinium. Leveraging the aggregation-induced emission supramolecular photosensitizer, we improved fluorescence performance and antimicrobial photodynamic inactivation. In vivo studies revealed that it displayed precise fluorescence tracking of S. aureus-infected sites, and in situ performed image-guided efficient PDI of S. aureus without noticeable side effects. These results demonstrated that metallacycle combined with host-guest chemistry could provide a paradigm for the development of powerful photosensitizers for biomedicine.

Self-assembled nanoparticles based on supramolecular-organic frameworks and temoporfin for an enhanced photodynamic therapy in vitro and in vivo

Water-soluble three-dimensional supramolecular-organic frameworks (SOFs) and temoporfin (mTHPC) are discovered to form uniform self-assembled nanoparticles. These nanoparticles demonstrate an improved ¹O₂ generation efficiency due to the reduced aggregation-caused quenching effect. SOFs and self-assembled nanoparticles are biocompatible. Self-assembled nanoparticles display an improved photo cytotoxicity toward four types of human cancer cells. The tumor model in mice shows that self-assembled nanoparticles could efficiently suppress tumor growth in vivo.

Ferroptosis promotes sonodynamic therapy: a platinum(ii)–indocyanine sonosensitizer

Sonodynamic therapy (SDT) has unique advantages in deep tumour ablation due to its deep penetration depth, showing great preclinical and clinical potential. Herein, a platinum(ii)–cyanine complex has been designed to investigate its potential as a SDT anticancer agent. It generates singlet oxygen (¹O₂) under ultrasound (US) irradiation or light irradiation, and exhibits US-cytotoxicity in breast cancer 4T1 cells but with negligible dark-cytotoxicity. Mechanistic investigations reveal that Pt-Cy reduces the cellular GSH and GPX4, and triggers cancer cell ferroptosis under US irradiation. The metabolomics analysis illustrates that Pt-Cy upon US treatment significantly dysregulates glutathione metabolism, and finally induces ferroptosis. In vivo studies further demonstrate that Pt-Cy inhibits tumor growth under US irradiation and its efficiency for SDT is better than that for PDT in vivo. This is the first example of platinum(ii) complexes for sonodynamic therapy. This work extends the biological applications of metal complexes from PDT to SDT.

A novel platinum(ii)–cyanine complex showed a greater excellent sonodynamic therapeutic effect than photodynamic therapy in vivo. This work expands the biological applications of metal complexes from traditional photodynamic therapy to sonodynamic therapy.

Development of nanotechnology-mediated precision radiotherapy for anti-metastasis and radioprotection

Radiotherapy (RT), including external beam RT and internal radiation therapy, uses high-energy ionizing radiation to kill tumor cells.

Recent Approaches towards the Development of Ru(II) Polypyridyl Complexes for Anticancer Photodynamic Therapy

Photodynamic therapy (PDT) is a remarkable alternative or complementary technique to chemotherapy, radiotherapy or immunotherapy to treat certain forms of cancer. The synergistic effect of light, photosensitizer (PS) and oxygen allows for the treatment of tumours with an extremely high spatio-tumoral control, therefore minimizing the severe side effects usually observed in chemotherapy. The currently employed PDT PSs based on porphyrins have, in some cases, some limitations, which include a low absorbance in the therapeutic window, a low body clearance, photobleaching, among others. In this context, Ru(ii) polypyridyl complexes are interesting alternatives. They have low lying excited energy states and the presence of a heavy metal increases the possibility of spin-orbit coupling. Moreover, their photophysical properties are relatively easy to tune and they have very low photobleaching rates. All of these make them attractive candidates for further development as therapeutically suitable PDT PSs. In this review, after having presented this field of research, we discuss the developments made by our group in this field of research since 2017. We notably describe how we tuned the photophysical properties of our complexes from the visible region to the therapeutically suitable red region. This was accompanied by the preparation of PSs with enhanced phototoxicity and high phototoxicity index. We also discuss the use of two-photon excitation to eradicate tumours in nude mice. Furthermore, we describe our approach for the selective delivery of our complexes using targeting agents. Lastly, we report on our very recent synergistic approach to treat cancer using bimetallic Ru(ii)-Pt(iv) prodrug candidates.

A mitochondrion-targeted BODIPY-Ir(III) conjugate as a photoinduced ROS generator for the oxidative destruction of triple-negative breast cancer cells

Photodynamic therapy (PDT) provides an alternative option to root out localized triple-negative breast cancer (TNBC) and has been experiencing a surge of research interest over recent years. In this study, we put forward a paradigm of designing novel transition metal-based PSs with the following characteristics: favorable cell-permeability, significant light-harvesting ability and prominent ROS yield. A novel BODIPY-Ir(III) conjugate has been designed as a photoinduced ROS (¹O₂, ˙OH and ˙O₂^-) generator. BODIPY-Ir is highly photoactive in subduing cancer cells in the PDT regimen with PI values ranging from 172 to 519 and EC₅₀ in the nanomolar regime. Among various cancerous cell lines, TNBC was especially sensitive to BODIPY-Ir-mediated PDT, with a stunning EC₅₀ value of 4.32 nM (PI = 519) under a moderate flux of visible-light irradiation (500 nm, 10.5 mW cm^-2). BODIPY-Ir mainly accumulates in mitochondria and induces cell apoptosis under irradiation. Furthermore, the nanomolar antiproliferative activity of BODIPY-Ir is retained under hypoxia (2.5% O₂). This work sheds light on instilling the O₂-independent type I mechanism and conferring a red-shift absorption to metal-based PSs which fundamentally facilitate the clinical translation of PSs.

Mito-Bomb: Targeting Mitochondria for Cancer Therapy

Cancer has been one of the most common life-threatening diseases for a long time. Traditional cancer therapies such as surgery, chemotherapy (CT), and radiotherapy (RT) have limited effects due to drug resistance, unsatisfactory treatment efficiency, and side effects. In recent years, photodynamic therapy (PDT), photothermal therapy (PTT), and chemodynamic therapy (CDT) have been utilized for cancer treatment owing to their high selectivity, minor resistance, and minimal toxicity. Accumulating evidence has demonstrated that selective delivery of drugs to specific subcellular organelles can significantly enhance the efficiency of cancer therapy. Mitochondria-targeting therapeutic strategies are promising for cancer therapy, which is attributed to the essential role of mitochondria in the regulation of cancer cell apoptosis, metabolism, and more vulnerable to hyperthermia and oxidative damage. Herein, the rational design, functionalization, and applications of diverse mitochondria-targeting units, involving organic phosphine/sulfur salts, quaternary ammonium (QA) salts, peptides, transition-metal complexes, guanidinium or bisguanidinium, as well as mitochondria-targeting cancer therapies including PDT, PTT, CDT, and others are summarized. This review aims to furnish researchers with deep insights and hints in the design and applications of novel mitochondria-targeting agents for cancer therapy.