Ruthenium(II) Conjugates of Boron-Dipyrromethene and Biotin for Targeted Photodynamic Therapy in Red Light

Ruthenium complexes bearing nile red chromophore and one of its derivative: Theoretical evaluation of PDT-related properties

The outcomes of DFT-based calculations are here reported to assess the applicability of two synthesized polypyridyl Ru(II) complexes, bearing ethynyl nile red (NR) on a bpy ligand, and two analogues, bearing modified-NR, in photodynamic therapy. The absorption spectra, together with the non-radiative rate constants for the S1 - Tn intersystem crossing transitions, have been computed for this purpose. Calculations evidence that the structural modification on the chromophore destabilizes the HOMO of the complexes thus reducing the H-L gap and, consequently, red shifting the maximum absorption wavelength within the therapeutic window, up to 620 nm. Moreover, the favored ISC process from the bright state involves the triplet state closest in energy, which is also characterized by the highest SOC value and by the involvement of the whole bpy ligand bearing the chromophore in delocalising the unpaired electrons. These outcomes show that the photophysical behavior of the complexes is dominated by the chromophore.

Photodynamic Therapy with Targeted Release of Boron-Dipyrromethene Dye from Cobalt(III) Prodrugs in Red Light

Boron-dipyrromethene (BODIPY) dyes are promising photosensitizers for cellular imaging and photodynamic therapy (PDT) owing to their excellent photophysical properties and the synthetically tunable core. Metalation provides a convenient way to overcome the drawbacks arising from their low aqueous solubility. New photo-/redox-responsive Co(III) prodrug chaperones are developed as anticancer PDT agents for efficient cellular delivery of red-light-active BODIPY dyes. The photobiological activity of heteroleptic Co(III) complexes derived from tris(2-pyridylmethyl)amine (TPA) and acetylacetone-conjugated PEGylated distyryl BODIPY (HL1) or its dibromo analogue (HL2), [CoIII(TPA)(L1/L2)](ClO4)2 (1 and 2), are investigated. The Co(III)/Co(II) redox potential is tuned using the Co(III)-TPA scaffold. Complex 1 displays the in vitro release of BODIPY on red light irradiation. Complex 2, having good singlet oxygen quantum yield (ΦΔ ∼ 0.28 in DMSO), demonstrates submicromolar photocytotoxicity to HeLa cancer cells (IC50 ≈ 0.23 μM) while being less toxic to HPL1D normal cells in red light. Cellular imaging using the emissive complex 1 shows mitochondrial localization and significant penetration into the HeLa tumor spheroids. Complex 2 shows supercoiled DNA photocleavage activity and apoptotic cell death through phototriggered generation of reactive oxygen species. The Co(III)-BODIPY prodrug conjugates exemplify new type of phototherapeutic agents with better efficacy than the organic dyes alone in the phototherapeutic window.

Biotin-conjugated Ru(II) complexes with AIE characteristics as mitochondria-targeted photosensitizers for enhancing photodynamic therapy by disrupting cellular redox balance

The potential use of Ru(II) complexes as photosensitizers (PSs) in photodynamic therapy (PDT) has gained significant attention. In comparison with fluorophores with aggregation-caused quenching (ACQ), fluorophores with aggregation-induced emission (AIE) characteristics exhibit sustained fluorescence and dispersibility in aqueous solutions. PSs with AIE characteristics have received much attention in recent years. Herein, we reported two novel biotin-conjugated Ru(II) polypyridyl complexes (Ru1 and Ru2) with AIE characteristics. When exposed to 460 nm (10 mW cm-2) light, Ru1 and Ru2 exhibited outstanding photostability and photocatalytic activity. Ru1 and Ru2 could efficiently generate singlet oxygen and induce pUC19 DNA photolysis when exposed to 460 nm light. Interestingly, both Ru1 and Ru2 also functioned as catalysts for NADH oxidation when exposed to 460 nm light. The presence of biotin fragments in Ru1 and Ru2 enhanced the specific uptake of these complexes by tumor cells. Both complexes showed minimal toxicity to selected cells in the dark. Nevertheless, the phototoxicity of both complexes significantly increased upon 460 nm light irradiation for 15 min. Further experiments revealed that Ru2 primarily accumulated in mitochondria and might bind to mitochondrial DNA. Under 460 nm light irradiation, Ru2 induced the generation of reactive oxygen species (ROS) and NADH depletion disrupting intracellular redox homeostasis in A549 cells, activating the mitochondrial apoptosis pathway resulting in up-regulation of apoptotic marker caspase-3, effectively damaged A549 cell DNA and arrested A549 cell cycle in the S phase. In vivo anti-tumor experiments were conducted to assess the effects of Ru2 on tumor growth in A549 tumor-bearing mice. The results showed that Ru2 effectively inhibited tumor growth under 460 nm light irradiation conditions. These findings indicate that Ru2 has great potential as a targeted photosensitizer for mitochondrial targeting imaging and photodynamic therapy of tumors.

Biotin and boron-dipyrromethene-tagged platinum(IV) prodrug for cellular imaging and mito-targeted photocytotoxicity in red light

A platinum(IV) prodrug, cis,cis,trans-[Pt(NH3)2Cl2(biotin)(L)] (1), derived from cisplatin, where HL is the PEGylated red-light active boron-dipyrromethene (BODIPY) ligand, was synthesized, characterized and its photocytotoxicity evaluated. The complex showed a near-IR absorption band at 653 nm (ε ∼9.19 × 104 M-1 cm-1) in dimethyl sulfoxide and Dulbecco's phosphate-buffered saline (1 : 1 v/v) at pH 7.2. When excited at 630 nm, it showed an emission band at 677 nm in DMSO with a fluorescence quantum yield of 0.13. The 1,3-diphenylisobenzofuran titration experiment gave a singlet oxygen quantum yield (ΦΔ) of ∼0.32. A mechanistic DNA photocleavage study revealed singlet oxygen as the reactive oxygen species (ROS). The complex with biotin and PEGylated-distyryl-BODIPY showed significantly higher cellular uptake in A549 cancer cells as compared to non-cancerous Beas-2B cells from flow cytometry, indicating selectivity towards cancer cells. A dichlorodihydrofluorescein diacetate assay showed cellular ROS generation. Confocal images revealed predominant internalization in the mitochondria. The prodrug showed remarkable photodynamic therapy (PDT) activity in cancerous A549 and multidrug-resistant MDA-MB-231 cells with a high photocytotoxicity index value (half-maximal inhibitory concentration (IC50): 0.61-1.54 μM in red light), while being non-toxic in the dark. The chemo-PDT activity was significantly less in non-tumorigenic lung epithelial cells (Beas-2B). The prodrug effectively triggered cellular apoptosis, which was confirmed by the Annexin V-FITC/propidium iodide assay, and the alteration of the mitochondrial membrane potential was substantiated by the JC-1 dye assay. The β-tubulin immunofluorescence assay confirmed that incubating the cells with a light-treated complex resulted in the rapture of the cytoskeletal structure and the formation of apoptotic bodies. The results demonstrate that the prodrug triggered apoptosis via DNA damage, a reduction in mitochondrial function and disruption of the cytoskeletal framework.

In Situ Bioconjugation of a Maleimide-Functionalized Ruthenium-Based Photosensitizer to Albumin for Photodynamic Therapy

Maleimide-containing prodrugs can quickly and selectively react with circulating serum albumin following their injection in the bloodstream. The drug-albumin complex then benefits from longer blood circulation times and better tumor accumulation. Herein, we have applied this strategy to a previously reported highly phototoxic Ru polypyridyl complex-based photosensitizer to increase its accumulation at the tumor, reduce off-target cytotoxicity, and therefore improve its pharmacological profile. Specifically, two complexes were synthesized bearing a maleimide group: one complex with the maleimide directly incorporated into the bipyridyl ligand, and the other has a hydrophilic linker between the ligand and the maleimide group. Their interaction with albumin was studied in-depth, revealing their ability to efficiently bind both covalently and noncovalently to the plasma protein. A crucial finding is that the maleimide-functionalized complexes exhibited significantly lower cytotoxicity in noncancerous cells under dark conditions compared to the nonfunctionalized complex, which is a highly desirable property for a photosensitizer. The binding to albumin also led to a decrease in the phototoxicity of the Ru bioconjugates in comparison to the nonfunctionalized complex, probably due to a decreased cellular uptake. Unfortunately, this decrease in phototoxicity was not compensated by a dramatic increase in tumor accumulation, as was demonstrated in a tumor-bearing mouse model using inductively coupled plasma mass spectrometry (ICP-MS) studies. Consequently, this study provides valuable insight into the future design of in situ albumin-binding complexes for photodynamic therapy in order to maximize their effectiveness and realize their full potential.

Encapsulation of Ru(II) Polypyridine Complexes for Tumor-Targeted Anticancer Therapy

Ru(II) polypyridine complexes have attracted much attention as anticancer agents because of their unique photophysical, photochemical, and biological properties. Despite their promising therapeutic profile, the vast majority of compounds are associated with poor water solubility and poor cancer selectivity. Among the different strategies employed to overcome these pharmacological limitations, many research efforts have been devoted to the physical or covalent encapsulation of the Ru(II) polypyridine complexes into nanoparticles. This article highlights recent developments in the design, preparation, and physicochemical properties of Ru(II) polypyridine complex-loaded nanoparticles for their potential application in anticancer therapy.

Biocompatible Aza-BODIPY-Biotin Conjugates for Photodynamic Therapy of Cancer

With an objective to develop efficient photosensitizers to cancerous tissues, we synthesized two novel biocompatible sensitizers based on aza-BODIPYs incorporated with heavy atoms and biotin moieties. The bioconjugates DPR2a and DPR2b exhibited a favorable absorption range (600-750 nm) with excellent triplet-state quantum yields (up to 79%) and singlet oxygen generation yields (up to 75%). In vitro photobiological investigations employing MDA-MB-231 breast cancer cell lines exhibited rapid cellular uptake, negligible dark toxicity, and high photocytotoxicity. The mechanism of cell death of these systems was predominantly due to the mitochondrial damage, leading to apoptosis mediated via the generation of singlet oxygen-triggered reactive oxygen species. The in vivo studies with the representative conjugate DPR2a employing female NOD/SCID mice models showed inhibition in tumor growth and significantly decreased tumor volume post photodynamic therapy (PDT) treatment. Our results validate that both DPR2a and DPR2b with iodine incorporation exhibit favorable and superior photophysical and photobiological aspects and demonstrate thereby their potential applications in imaging and PDT of cancer.

Identification of a luminescent platinum(II) complex with BODIPY derivative as novel photodynamic therapy agent for triple negative breast cancer cells

Triple-negative breast cancer (TNBC) is one of the most malignant breast tumors for its poor prognosis and high tumor recurrence. It is urgent to develop new strategy or effective agents to overcome resistance and improve therapeutic effectiveness. Boron-dipyrromethene (BODIPY) based photosensitizers possess exciting photophysical features suitable for theranostic applications, namely, photodynamic therapy (PDT). We have designed a luminescent monofunctional platinum(II) complex with BODIPY derivative, namely I₂BC-Pt, as novel high PDT agent against triple negative breast cancer (TNBC). The di-iodinated BODIPY complex I₂BC-Pt showed excellent PDT effect against TNBC cells in green light (520 nm) giving IC₅₀ values of 0.11-0.13 μM in MDA-MB-231 and MDA-MB-468 cells. I₂BC-Pt predominately aggregated in the mitochondria of MDA-MB-231 cells and emitted green fluorescence. Besides, the anticancer mechanism studies demonstrated that I₂BC-Pt could help DNA repair through attenuating RAD51, FoxM1 and BRCA1/2, and induce p53-mediated apoptosis of TNBC cells. Taken together, I₂BC-Pt could be potentially developed as a BODIPY-based photosensitizers for TNBC therapy.

Red-to-Near-Infrared Emitting PyrrolylBODIPY Dyes: Synthesis, Photophysical Properties and Bioimaging Application

Near-infrared (NIR) fluorophores with characteristics such as deep tissue penetration, minimal damage to the biological samples, and low background interference, are highly sought-after materials for in vivo and deep-tissue fluorescence imaging. Herein, series of 3-pyrrolylBODIPY derivatives and 3,5-dipyrrolylBODIPY derivatives have been prepared by a facile regioselective nucleophilic aromatic substitution reaction (S_N Ar) on 3,5-halogenated BODIPY derivatives (3,5-dibromo or 2,3,5,6-tetrachloroBODIPYs) with pyrroles. The installation of a pyrrolic unit onto the 3-position of the BODIPY chromophore leads to a dramatic red shift of both the absorption (up to 160 nm) and the emission (up to 260 nm) in these resultant 3-pyrrolylBODIPYs with respect to that of the BODIPY chromophore. Their further 5-positional functionalization provides a facile way to fine tune their photophysical properties, and these resulting dipyrrolylBODIPYs and functionalized pyrrolylBODIPYs show strong absorption in the deep red-to-NIR regions (595-684 nm) and intense NIR fluorescence emission (650-715 nm) in dichloromethane. To demonstrate the applicability of these functionalized pyrrolylBODIPYs as NIR fluorescent probes for cell imaging, pyrrolylBODIPY 6 a containing mitochondrion-targeting butyltriphenylphosphonium cationic species was also prepared. It selectively localized in mitochondria of HeLa cells, with low cytotoxicity and intense deep red fluorescence emission.

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.

Stimuli-responsive platinum and ruthenium complexes for lung cancer therapy

Lung cancer is the most common cause of cancer-related deaths worldwide. More efficient treatments are desperately needed. For decades, the success of platinum-based anticancer drugs has promoted the exploration of metal-based agents. Four ruthenium-based complexes have also entered clinical trials as candidates of anticancer metallodrugs. However, systemic toxicity, severe side effects and drug-resistance impeded their applications and efficacy. Stimuli-responsiveness of Pt- and Ru-based complexes provide a great chance to weaken the side effects and strengthen the clinical efficacy in drug design. This review provides an overview on the stimuli-responsive Pt- and Ru-based metallic anticancer drugs for lung cancer. They are categorized as endo-stimuli-responsive, exo-stimuli-responsive, and dual-stimuli-responsive prodrugs based on the nature of stimuli. We describe various representative examples of structure, response mechanism, and potential medical applications in lung cancer. In the end, we discuss the future opportunities and challenges in this field.

Red-Absorbing Ru(II) Polypyridyl Complexes with Biotin Targeting Spontaneously Assemble into Nanoparticles in Biological Media

Four new ruthenium(II) polypyridyl complexes were synthesized to study the effect of poly(ethylene glycol) and/or biotin conjugation on their physical and biological properties, including their hydrophilicity, their cellular uptake, and their phototoxicity. Unexpectedly, these complexes self-assembled into nanoparticles upon dilution in biological media. This behavior leads to their accumulation in lysosomes following their internalization by cells. While a significant increase in cellular uptake was observed for the biotin-conjugated complexes, it did not result in an increase in their phototoxicity. However, their high phototoxicity upon irradiation at long wavelengths (645-670 nm) and their self-assembling behavior make them a promising backbone for the development of new lysosome-targeted photosensitizers for photodynamic therapy.

Bichromophoric ruthenium(II) bis-terpyridine-BODIPY based photosensitizers for cellular imaging and photodynamic therapy

Two multichromophoric homoleptic ruthenium(II) complexes [Ru(tpy-BODIPY)₂]Cl₂ (complexes 1 and 2, tpy = 4-phenyl-2,2:6,2-terpyridine, BODIPY = boron-dipyrromethene) were prepared, characterized and their phototherapeutic activity and bioimaging properties were studied. The complexes having structural similarity differ only by a phenylethynyl linker, and its overall influence on their physicochemical and photobiological behavior was evaluated. The terpyridine-BODIPY ligand L¹ was structurally characterized by X-ray crystallography. The complexes showed intense absorption near 500 nm (ε: ∼1.5 × 10⁵ M^-1 cm^-1 in DMSO), have a high singlet oxygen quantum yield (Φ_Δ: ∼0.6 in DMSO), and displayed low photobleaching thus making them suitable for PDT applications. The complexes showed high DNA binding affinity and induced DNA damage on light activation via multiple types of ROS production. Confocal laser scanning microscopy experiments revealed their incorporation in the cancer cells and complex 1 predominantly accumulated in lysosomes. The complexes displayed a significant PDT effect in cancerous cells with visible light activation with a high photocytotoxicity index (PI) value in HeLa cells. Both type-I and type-II photosensitization processes were involved in the PDT effect. The photodynamic action of complex 2 initiated cellular apoptosis. Finally, their diagnostic potential was evaluated against clinically relevant 3D multicellular tumor spheroids (MCTs).

Nuclease-like metalloscissors: Biomimetic candidates for cancer and bacterial and viral infections therapy

Despite the extensive and rapid discovery of modern drugs for treatment of cancer, microbial infections, and viral illnesses; these diseases are still among major global health concerns. To take inspiration from natural nucleases and also the therapeutic potential of metallopeptide antibiotics such as the bleomycin family, artificial metallonucleases with the ability of promoting DNA/RNA cleavage and eventually affecting cellular biological processes can be introduced as a new class of therapeutic candidates. Metal complexes can be considered as one of the main categories of artificial metalloscissors, which can prompt nucleic acid strand scission. Accordingly, biologists, inorganic chemists, and medicinal inorganic chemists worldwide have been designing, synthesizing and evaluating the biological properties of metal complexes as artificial metalloscissors. In this review, we try to highlight the recent studies conducted on the nuclease-like metalloscissors and their potential therapeutic applications. Under the light of the concurrent Covid-19 pandemic, the human need for new therapeutics was highlighted much more than ever before. The nuclease-like metalloscissors with the potential of RNA cleavage of invading viral pathogens hence deserve prime attention.

Factors that influence singlet oxygen formation vs. ligand substitution for light-activated ruthenium anticancer compounds

This review focuses on light-activated ruthenium anticancer compounds and the factors that influence which pathway is favored. Photodynamic therapy (PDT) is favored by π expansion and the presence of low-lying triplet excited states (e.g. ³MLCT, ³IL). Photoactivated chemotherapy (PACT) refers to light-driven ligand dissociation to give a toxic metal complex or a toxic ligand upon photo substitution. This process is driven by steric bulk near the metal center and weak metal-ligand bonds to create a low-energy ³MC state with antibonding character. With protic dihydroxybipyridine ligands, ligand charge can play a key role in these processes, with a more electron-rich deprotonated ligand favoring PDT and an electron-poor protonated ligand favoring PACT in several cases.

Construction of emissive ruthenium(II) metallacycle over 1000 nm wavelength for in vivo biomedical applications

Although Ru(II)-based agents are expected to be promising candidates for substituting Pt-drug, their in vivo biomedical applications are still limited by the short excitation/emission wavelengths and unsatisfactory therapeutic efficiency. Herein, we rationally design a Ru(II) metallacycle with excitation at 808 nm and emission over 1000 nm, namely Ru1085, which holds deep optical penetration (up to 6 mm) and enhanced chemo-phototherapy activity. In vitro studies indicate that Ru1085 exhibits prominent cell uptake and desirable anticancer capability against various cancer cell lines, especially for cisplatin-resistant A549 cells. Further studies reveal Ru1085 induces mitochondria-mediated apoptosis along with S and G2/M phase cell cycle arrest. Finally, Ru1085 shows precise NIR-II fluorescence imaging guided and long-term monitored chemo-phototherapy against A549 tumor with minimal side effects. We envision that the design of long-wavelength emissive metallacycle will offer emerging opportunities of metal-based agents for in vivo biomedical applications.

Ruthenium (Ru(II)) compounds are of interest as platinum drug replacements but have suffered from suboptimal therapeutic efficiency. Here, the authors design a Ru(II) metallacycle with NIR excitation and emission wavelengths and demonstrate application for deep tumour imaging and chemo-photo therapy.

BODIPY-dipicolylamine complexes of platinum(II): X-ray structure, cellular imaging and organelle-specific near-IR light type-II PDT

Dipicolylamine (dpa) based platinum(II) complexes [Pt(L1-3)Cl]Cl (1-3), where L2 and L3 are green and red light BODIPY-tagged dpa ligands and L1 is a benzyl derivative of dpa, were synthesized and characterized and their in vitro cytotoxicity was studied. The perchlorate salt of complex 2 was structurally characterized. It showed a PtN₃Cl core with a deformed square-planar geometry. At pH 7.2, complexes 2 and 3 showed strong absorption bands at 500 nm (ε ∼6.8 × 10⁴ dm³ mol^-1 cm^-1) and 653 nm (ε ∼1.0 × 10⁵ dm³ mol^-1 cm^-1) in a 1 : 1 (v/v) mixture of dimethyl sulfoxide and Dulbecco's phosphate-buffered saline (DMSO/DPBS), respectively. They displayed respective emission bands at 515 and 677 nm having fluorescence quantum yield values of 0.36 and 0.25. Complex 3 generated singlet oxygen, as evidenced from the 1,3-diphenylisobenzofuran titration experiments and mechanistic DNA photocleavage study. It showed high photocytotoxicity in red light (600-720 nm) with half-maximal inhibitory concentration (IC₅₀) values of 1.73 and 2.67 μM in HeLa and A549 cells. The complexes showed significantly reduced chemo-PDT activity in a non-cancerous HPL1D cell line and in the dark. The 2',7'-dichlorofluorescein diacetate assay revealed reactive oxygen species-mediated type-II photodynamic therapy (PDT) activity. Cellular imaging of A549 cancer cells using complexes 2 and 3 revealed their preferential localization in mitochondria and endoplasmic reticulum. The annexin V-FITC/PI assay confirmed apoptotic cell damage. Cell cycle analysis indicated arrest in the G1 phase upon red light irradiation. Pt-DNA adduct formation was proposed from a DNA binding experiment with green light active complex 2 and 9-ethylguanine as a nucleobase from the mass spectral study.

BODIPY Conjugates as Functional Compounds for Medical Diagnostics and Treatment

Fluorescent dyes absorbing and emitting in the visible and near-IR regions are promising for the development of fluorescent probes for labeling and bio-visualization of body cells. The ability to absorb and emit in the long-wavelength region increases the efficiency of recording the spectral signals of the probes due to the higher permeability of the skin layers. Compared to other fluorescent dyes, BODIPYs are attractive due to their excellent photophysical properties-narrow absorption and emission, intense fluorescence, simple signal modulation for the practical applications. As part of conjugates with biomolecules, BODIPY could act as a biomarker, but as therapeutic agent, which allows solving several problems at once-labeling or bioimaging and treatment based on the suppression of pathogenic microflora and cancer cells, which provides a huge potential for practical application of BODIPY conjugates in medicine. The review is devoted to the discussion of the recent, promising directions of BODIPY application in the field of conjugation with biomolecules. The first direction is associated with the development of BODIPY conjugates with drugs, including compounds of platinum, paclitaxel, chlorambucil, isoxazole, capsaicin, etc. The second direction is devoted to the labeling of vitamins, hormones, lipids, and other biomolecules to control the processes of their transport, localization in target cells, and metabolism. Within the framework of the third direction, the problem of obtaining functional optically active materials by conjugating BODIPY with other colored and fluorescent particles, in particular, phthalocyanines, is being solved.

Construction of synergistic pH/H2O2-responsive prodrug for prolonging blood circulation and accelerating cellular internalization

We have developed a real-time and multifunctional doxifluridine-conjugate prodrug (LYX), which involved the preliminary methylfluorescein with 5-fluorouracil linker as protecting group, the targeting biotin unit, and a model therapeutic drug (doxifluridine). The shielding group (5'-DFUR) was found to be effective in prolonging circulation at physiological pH 7.4 and improving accumulation in the acidic microenvironment of the tumor. Based on this strategy, the stability and stimulus responsive properties of prodrug could enhance drug release efficiency and exhibit fewer side effects, thereby providing a unique opportunity for diagnosis and imaging additional analytes or enzymatic activities.

BODIPY-Ruthenium(II) Bis-Terpyridine Complexes for Cellular Imaging and Type-I/-II Photodynamic Therapy

A series of multichromophoric ruthenium(II) complexes with the formulation [Ru(tpy-BODIPY)(tpy-R)]Cl2 (1-4), having a heteroleptic Ru(II)-bis-tpy (tpy = 4'-phenyl-2,2':6',2″-terpyridine) moiety covalently linked to a boron-dipyrromethene (BODIPY) pendant, have been prepared and characterized and their application as a phototherapeutic and photodetection agent in cancer therapy has been explored. Ligand L1 with a terpyridine-BODIPY moiety and complex 1 as its PF6 salt (1a) have been structurally characterized by a single-crystal X-ray diffraction study. Complex 1a has a distorted-octahedral RuN6 core with a Ru(II)-bis-terpyridine unit that is covalently linked to one photoactive BODIPY unit. The complexes exhibit strong absorbance near 502 nm (ε ≈ (3.7-7.8) × 104 M-1 cm-1) and high singlet oxygen sensitization ability, giving singlet oxygen quantum yield (ΦΔ) values ranging from 0.57 to 0.75 in DMSO. An emission-based study using complex 4 and Singlet Oxygen Sensor Green (SOSG) displays the formation of singlet oxygen inside the cells and also in the buffer medium upon light irradiation. DNA (pUC19) photocleavage experiments using ROS scavengers/stabilizers reveal photoinduced generation of singlet oxygen by a type-II process and of the superoxide anion radical by a type-I process. Complex 4 having a pendant biotin moiety as a cancer cell targeting group shows high photocytotoxicity with a remarkable phototherapeutic index (PI) value of >1400 in HeLa cancer cells with a low light dose activation (400-700 nm, 2.2 J cm-2). The complexes display reduced activity in noncancerous HPL1D cells. The emission property of the complexes is used for cellular imaging, thus making them suitable as next-generation theranostic PDT agents.

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

Transition metal luminophores are emerging as important tools for intracellular imaging and sensing. Their putative suitability for such applications has long been recognised but poor membrane permeability and cytotoxicity were significant barriers that impeded early progress. In recent years, numerous effective routes to overcoming these issues have been reported, inspired in part, by advances and insights from the pharmaceutical and drug delivery domains. In particular, the conjugation of biomolecules but also other less natural synthetic species, from a repertoire of functional motifs have granted membrane permeability and cellular targeting. Such motifs can also reduce cytotoxicity of transition metal complexes and offer a valuable avenue to circumvent such problems leading to promising metal complex candidates for application in bioimaging, sensing and diagnostics. The advances in metal complex probes permeability/targeting are timely, as, in parallel, over the past two decades significant technological advances in luminescence imaging have occurred. In particular, super-resolution imaging is enormously powerful but makes substantial demands of its imaging contrast agents and metal complex luminophores frequently possess the photophysical characteristics to meet these demands. Here, we review some of the key vectors that have been conjugated to transition metal complex luminophores to promote their use in intra-cellular imaging applications. We evaluate some of the most effective strategies in terms of membrane permeability, intracellular targeting and what impact these approaches have on toxicity and phototoxicity which are important considerations in a luminescent contrast or sensing agent.

Enhancing the tumor cell selectivity of a rhodamine-decorated iridium(III) complex by conjugating with indomethacin for COX-2 targeted photodynamic therapy

A rhodamine-iridium (III) complex bearing indomethacin moiety, named IM-rho-Ir, was synthesized and evaluated for COX-2 targetable photodynamic therapy. By integrating COX-2 directing group, IM-rho-Ir exhibited enhanced cellular uptake in cancer cells than in normal cells compared to rhodamine-iridium (III) complex without indomethacin moiety.

Photodynamic Therapy of Inorganic Complexes for the Treatment of Cancer†

Photodynamic therapy (PDT) is a medicinal tool that uses a photosensitizer and a light source to treat several conditions, including cancer. PDT uses reactive oxygen species such as cytotoxic singlet oxygen (¹ O₂ ) to induce cell death in cancer cells. Chemotherapy has historically utilized the cytotoxic effects of many metals, especially transition metal complexes. However, chemotherapy is a systemic treatment so all cells in a patient's body are exposed to the same cytotoxic effects. Transition metal complexes have also shown high cytotoxicity as PDT agents. PDT is a potential localized method for treating several cancer types by using inorganic complexes as photosensitizing agents. This review covers several in vitro and in vivo studies, as well as clinical trials that reported on the anticancer properties of inorganic pharmaceuticals used in PDT against different types of cancer.

Emerging Molecular Receptors for the Specific-Target Delivery of Ruthenium and Gold Complexes into Cancer Cells

Cisplatin and derivatives are highly effective in the treatment of a wide range of cancer types; however, these metallodrugs display low selectivity, leading to severe side effects. Additionally, their administration often results in the development of chemoresistance, which ultimately results in therapeutic failure. This scenario triggered the study of other transition metals with innovative pharmacological profiles as alternatives to platinum, ruthenium- (e.g., KP1339 and NAMI-A) and gold-based (e.g., Auranofin) complexes being among the most advanced in terms of clinical evaluation. Concerning the importance of improving the in vivo selectivity of metal complexes and the current relevance of ruthenium and gold metals, this review article aims to survey the main research efforts made in the past few years toward the design and biological evaluation of target-specific ruthenium and gold complexes. Herein, we give an overview of the inorganic and organometallic molecules conjugated to different biomolecules for targeting membrane proteins, namely cell adhesion molecules, G-protein coupled receptors, and growth factor receptors. Complexes that recognize the progesterone receptors or other targets involved in metabolic pathways such as glucose transporters are discussed as well. Finally, we describe some complexes aimed at recognizing cell organelles or compartments, mitochondria being the most explored. The few complexes addressing targeted gene therapy are also presented and discussed.

Red-Light-Responsive Ru Complex Photosensitizer for Lysosome Localization Photodynamic Therapy

Photoresponsive ruthenium (Ru) complexes have been extensively studied in the photodynamic therapy (PDT) of cancer. The metal-to-ligand charge transfer (MLCT) absorption maximum of most Ru complexes is located in the short-wavelength visible region, which is well suited for superficial tumors but shows inefficient therapeutic effects for more deep-seated ones. Moreover, Ru complexes are primarily located in the mitochondria or nucleus, always resulting in high levels of dark toxicity and DNA mutation. Herein, we reported a new ruthenium complex (Ru-I) for red-light-triggered PDT. The activation wavelength of Ru-I is successfully extended to 660 nm. Importantly, the complex photosensitizer can be quickly taken up by cancer cells and selectively accumulated in the lysosome, an ideal localization for PDT purposes. Intratumoral injection of Ru-I into tumor-bearing mice achieved excellent therapeutic effects and thus holds great promise for applications in lysosome localization photodynamic therapy.

Maloplatin-B, a Cisplatin-Based BODIPY-Tagged Mito-Specific "Chemo-PDT" Agent Active in Red Light

Maloplatin-B, a cisplatin-based complex, namely [Pt(A-BOD)(NH₃)₂](NO₃) (Pt-A-BOD) with a pendant boron-dipyrromethene (BODIPY) moiety, where HA-BOD is a methyl malonyl chloride derived monostyryl BODIPY ligand, was designed and developed as near-IR light (600-720 nm) organelle-targeting photodynamic therapy agent. The complex [Pt(acac)(NH₃)₂](NO₃) (Pt-Ac) was used as a control. Pt-A-BOD displayed an absorption band at 616 nm (ε = 2.9 × 10⁴ M^-1 cm^-1) in 10% dimethyl sulfoxide/Dulbecco's Modified Eagle's Medium (DMSO/DMEM, pH 7.2). This complex displayed a broad emission band within 650-850 nm with a λ_em value of 720 nm in 10% DMSO-DMEM (pH 7.2) upon excitation (λ_ex) at 615 nm with a large Stokes shift. The fluorescence quantum yield (Φ_F) value for Pt-A-BOD is 0.032 and for the ligand HA-BOD is 0.24. The BODIPY complex and ligand showed the formation of singlet oxygen as the ROS (reactive oxygen species) on irradiation with near-IR red light of 660 nm, as evidenced from a 1,3-diphenylisobenzofuran (DPBF) assay. The complex displayed remarkable apoptotic NIR light-induced PDT activity with half-maximum inhibitory concentration values (IC₅₀) of 1.6-2.4 μM in A549 lung and HeLa cervical cancer cells, while it was less active in the dark. The cellular ROS generation by the complex in red light was ascertained by a DCFDA (2',7'-dichlorofluorescein diacetate) assay. Cellular imaging showed its localization primarily in the mitochondria of A549 cancer cells. The JC1 and Annexin-V FITC/PI assays carried out for A549 cancer cells treated with the BODIPY complex showed the alteration of mitochondrial membrane potential and apoptotic cell death on near-IR red light (600-720 nm) irradiation, respectively.

Polymeric Encapsulation of a Ru(II)-Based Photosensitizer for Folate-Targeted Photodynamic Therapy of Drug Resistant Cancers

The currently used photodynamic therapy (PDT) photosensitizers (PSs) are generally associated with a poor cancer cell selectivity, which is responsible for some undesirable side effects. To overcome these problems, there is an urgent need for a selective drug delivery system for PDT PSs. Herein, the encapsulation of a promising Ru(II) polypyridine complex in a polymer with terminal folate groups to form nanoparticles is presented. While the Ru(II) complex itself has a cytotoxic effect in the dark, the encapsulation is able to overcome this drawback. Upon light exposure, the nanoparticles were found to be highly phototoxic in 2D monolayer cells as well as 3D multicellular tumor spheroids upon 480 or 595 nm irradiation. Importantly, the nanoparticles demonstrated a high selectivity for cancerous cells over noncancerous cells and were found to be active in drug resistant cancer cells lines, indicating that they are able to overcome drug resistances.