Self-Internal Standard Fluorescence for Ultrasensitive Detecting of mtDNA to Evaluate Matrilineal Genetic Defect Levels

Mitochondrial DNA (mtDNA) is a unique genetic material characterized by maternal inheritance. It possesses a circular structure devoid of histone protection and exhibits low cellular abundance, which poses great challenges for its sensitive and selective detection at the living cell level. Herein, we have designed three bis-naphthylimide probes with varying linker lengths (NANn-OH, n = 0, 2, 6), facilitating the formation of distinct twisted or folded molecular conformations in the free state. These probes emit the red fluorescence around 627 nm with different fluorescence quantum yields (ΦNAN0-OH = 0.0016, ΦNAN2-OH = 0.0136, and ΦNAN6-OH = 0.0125). When encountering mtDNA (0.4-3.4 μg/mL), these probes undergo conformational changes depending on the length of the attached C-strand and exhibit a gradually increasing fluorescence signal around 453 nm. The fluorescence intensity increased to 13.5-fold, 1.9-fold, and 8.2-fold, respectively. Notably, the red fluorescence intensities around 627 nm remain constant throughout this process, thus serving as an inherent correction mechanism for proportional fluorescence signal enhancement to improve selectivity and sensitivity. NAN0-OH, NAN2-OH, and NAN6-OH showed good linearity for mtDNA in the range of 0.4-3.4 μg/mL with detection limits of LODNAN0-OH = 1.04 μg/mL, LODNAN2-OH = 1.10 μg/mL, and LODNAN6-OH = 1.15 μg/mL. Cellular experiments reveal that NAN6-OH effectively monitors curcumin-induced mtDNA damage in HepG-2 cells while enabling monitoring of genetic mtDNA damage. We anticipate that this tool holds significant potential for the precise evaluation of maternal genetic defects, thereby enhancing hypersensitive assessment in clinical medicine.

Recent trends in the design and delivery strategies of ruthenium complexes for breast cancer therapy

As the most frequent and deadly type of cancer in women, breast cancer has a high propensity to spread to the brain, bones, lymph nodes, and lungs. The discovery of cisplatin marked the beginning of the development of anticancer metal-based medications, although the drug's severe side effects have limited its usage in clinical settings. The remarkable antimetastatic and anticancer activity of different ruthenium complexes such as NAMI-A, KP1019, KP1339, etc. reported in the 1980s has bolstered the discovery of ruthenium complexes with various types of ligands for anticancer applications. The review meticulously elucidates the cytotoxic and antimetastatic potential of reported ruthenium complexes against breast cancer cells. Notably, arene-based and cyclometalated ruthenium complexes emerge as standout candidates, showcasing remarkable potency with notably low IC50 values. These findings underscore the promising therapeutic avenues offered by ruthenium-based compounds, particularly in addressing the challenges posed by conventional treatments in refractory or aggressive breast cancer subtypes. Moreover, the review comprehensively integrates a spectrum of ruthenium complexes, spanning traditional metal complexes to nano-based formulations and light-activated variants, underscoring the versatility and adaptability of ruthenium chemistry in breast cancer therapy.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Cell viability imaging in tumor spheroids via DNA binding of a ruthenium(II) light-switch complex

The famous ''light-switch'' ruthenium complex [Ru(bpy)2(dppz)](PF6)2 (1) has been long known for its DNA binding properties in vitro. However, the biological utility of this compound has been hampered by its poor cellular uptake in living cells. Here we report a bioimaging application of 1 as cell viability probe in both 2D cells monolayer and 3D multi-cellular tumor spheroids of various human cancer cell lines (U87, HepG2, A549). When compared to propidium iodide, a routinely used cell viability probe, 1 was found to enhance the staining of dead cells in particular in tumor spheroids. 1 has high photostability, longer Stokes shift, and displays lower cytotoxicity compared to propidium iodide, which is a known carcinogenic. Finally, 1 was also found to displace the classical DNA binding dye Hoechst in dead cells, which makes it a promising dye for time-dependent imaging of dead cells in cell cultures, including multi-cellular tumor spheroids.

Unlocking the Mitochondria for Nanomedicine-based Treatments: Overcoming Biological Barriers, Improving Designs, and Selecting Verification Techniques

Enhanced targeting approaches will support the treatment of diseases associated with dysfunctional mitochondria, which play critical roles in energy generation and cell survival. Obstacles to mitochondria-specific targeting include the presence of distinct biological barriers and the need to pass through (or avoid) various cell internalization mechanisms. A range of studies have reported the design of mitochondrially-targeted nanomedicines that navigate the complex routes required to influence mitochondrial function; nonetheless, a significant journey lies ahead before mitochondrially-targeted nanomedicines become suitable for clinical use. Moving swiftly forward will require safety studies, in vivo assays confirming effectiveness, and methodologies to validate mitochondria-targeted nanomedicines' subcellular location/activity. From a nanomedicine standpoint, we describe the biological routes involved (from administration to arrival within the mitochondria), the features influencing rational design, and the techniques used to identify/validate successful targeting. Overall, rationally-designed mitochondria-targeted-based nanomedicines hold great promise for precise subcellular therapeutic delivery.

Unprecedented enantio-selective live-cell mitochondrial DNA super-resolution imaging and photo-sensitizing by the chiral ruthenium polypyridyl DNA "light-switch"

Mitochondrial DNA (mtDNA) is known to play a critical role in cellular functions. However, the fluorescent probe enantio-selectively targeting live-cell mtDNA is rare. We recently found that the well-known DNA 'light-switch' [Ru(phen)2dppz]Cl2 can image nuclear DNA in live-cells with chlorophenolic counter-anions via forming lipophilic ion-pairing complex. Interestingly, after washing with fresh-medium, [Ru(phen)2dppz]Cl2 was found to re-localize from nucleus to mitochondria via ABC transporter proteins. Intriguingly, the two enantiomers of [Ru(phen)2dppz]Cl2 were found to bind enantio-selectively with mtDNA in live-cells not only by super-resolution optical microscopy techniques (SIM, STED), but also by biochemical methods (mitochondrial membrane staining with Tomo20-dronpa). Using [Ru(phen)2dppz]Cl2 as the new mtDNA probe, we further found that each mitochondrion containing 1-8 mtDNA molecules are distributed throughout the entire mitochondrial matrix, and there are more nucleoids near nucleus. More interestingly, we found enantio-selective apoptotic cell death was induced by the two enantiomers by prolonged visible light irradiation, and in-situ self-monitoring apoptosis process can be achieved by using the unique 'photo-triggered nuclear translocation' property of the Ru complex. This is the first report on enantio-selective targeting and super-resolution imaging of live-cell mtDNA by a chiral Ru complex via formation and dissociation of ion-pairing complex with suitable counter-anions.

Recent Advances in Mitochondria-Localized Luminescent Ruthenium(II) Metallodrugs as Anticancer Agents

Presently, the most effective way to transport drugs specifically to mitochondria inside the cells is of pharmacophoric interest, as mitochondria are recognized as one of the most important targets for new drug design in cancer diagnosis. To date, there are many reviews covering the photophysical, photochemical, and anticancer properties of ruthenium(II) based metallodrugs owing to their high interest in biological applications. There are, however, no reviews specifically covering the mitochondria-localized luminescent Ru(II) complexes and their subsequent mitochondria-mediated anticancer activities. Therefore, this review describes the physicochemical basis for the mitochondrial accumulation of ruthenium complexes, their synthetic strategies to localize and monitor the mitochondria in living cells, and their related underlying anticancer results. Finally, we review the related areas from previous works describing the mitochondria-localized ruthenium complexes for the treatment of cancer-related diseases. Along with this, we also deliberate the perspectives and future directions for emerging more bifunctional Ru(II) complexes that can target, image, and kill tumors more efficiently in comparison with the existing mitochondria-targeted cancer therapeutics.

Ruthenium(II) complexes as mitochondrial inhibitors of topoisomerase induced A549 cell apoptosis

Two new ruthenium(II) complexes [Ru(dip)2(PPβC)]PF6 (Ru1, dip = 4,7-diphenyl-1,10-phenanthroline, PPβC = N-(1,10-phenanthrolin-5-yl)-1-phenyl-9H-pyrido[3,4-b]indole-3-carboxamide) and [Ru(phen)2(PPβC)]PF6 (Ru2, phen = 1, 10-phenanthroline) with β-carboline derivative PPβC as the primary ligand, were designed and synthesized. Ru1 and Ru2 displayed higher antiproliferative activity than cisplatin against the test cancer cells, with IC50 values ranging from 0.5 to 3.6 μM. Moreover, Ru1 and Ru2 preferentially accumulated in mitochondria and caused a series of changes in mitochondrial events, including the depolarization of mitochondrial membrane potential, the damage of mitochondrial DNA, the depletion of cellular ATP, and the elevation of intracellular reactive oxygen species levels. Then, it induced caspase-3/7-mediated A549 cell apoptosis. More importantly, both complexes could act as topoisomerase I catalytic inhibitors to inhibit mitochondrial DNA synthesis. Accordingly, the developed Ru(II) complexes hold great potential to be developed as novel therapeutics for cancer treatment.

Phototoxicity of Tridentate Ru(II) Polypyridyl Complex with Expanded Bite Angles toward Mammalian Cells and Multicellular Tumor Spheroids

Tridentate ligand-coordinated ruthenium (II) polypyridyl complexes with large N-Ru-N bite angles have been shown to promote ligand field splitting and reduce singlet-triplet state mixing leading to dramatically extended emission quantum yields and lifetimes under ambient conditions. These effects are anticipated to enhance their photoinduced singlet oxygen production, promoting prospects for such complexes as type II phototherapeutics. In this contribution, we examined this putative effect for [Ru(bqp)(bqpCOOEt)]2+, Ru-bqp-ester, a heteroleptic complex containing bqp = [2,6-bi(quinolin-8-yl)pyridine], a well-established large bite angle tridentate ligand, as well as its peptide conjugates [Ru(bqp)(bqpCONH-ahx-FrFKFrFK(Ac)-CONH2)]5+ (Ru-bqp-MPP) and [Ru(bqp) (bqp)(CONH-ahx-RRRRRRRR-CONH2)]10+ (Ru-bqp-R8) that were prepared in an effort to promote live cell/tissue permeability and targeting of the parent. Membrane permeability of both parent and peptide conjugates were compared across 2D cell monolayers; A549, Chinese hamster ovary, human pancreatic cancer (HPAC), and 3D HPAC multicellular tumor spheroids (MCTS) using confocal microscopy. Both the parent complex and peptide conjugates showed exceptional permeability with rapid uptake in both 2D and 3D cell models but with little distinction in permeability or distribution in cells between the parent or peptide conjugates. Unexpectedly, the uptake was temperature independent and so attributed to passive permeation. Both dark and photo-toxicity of the Ru(II) complexes were assessed across cell types, and the parent showed notably low dark toxicity. In contrast, the parent and conjugates were found to be highly phototoxic, with impressive phototoxic indices (PIs) toward HPAC cell monolayers in particular, with PI values ranging from ∼580 to 760. Overall, our data indicate that the Ru(II) parent complex and its peptide conjugates show promise at both cell monolayers and 3D MCTS as photosensitizers for photodynamic therapy.

Roles of DNA Target in Cancer Cell-Selective Cytotoxicity by Dicopper Complexes with DNA Target/Ligand Conjugates

The DNA target/ligand conjugates (HLX, X = Pn and Mn, n = 1-3) were synthesized where various lengths of -CONH(CH2CH2O)nCH2CH2NHCO- linkers with a 9-phenanthrenyl (P) or methyl (M) terminal as DNA targets replace the methyl group of 2,6-di(amide-tether cyclen)-p-cresol ligand (HL). DNA binding, DNA cleavage, cellular uptake, and cytotoxicity of [Cu2(μ-OH)(LX)](ClO4)2 (1X) are examined and compared with those of [Cu2(μ-OH)(L)](ClO4)2 (1) to clarify roles of DNA targets. Upon reaction of 1X with H2O2, μ-1,1-O2H complexes are formed for DNA cleavage. 1P1, 1P2, and 1P3 are 22-, 11-, 3-fold more active for conversion of Form II to III in the cleavage of supercoiled plasmid DNA with H2O2 than 1, where the short P-linker may fix a dicopper moiety within a small number of base pairs to facilitate DNA double-strand breaks (dsb). This enhances the proapoptotic activity of 1P1, 1P2, and 1P3, which are 30-, 12-, and 9.9-fold cytotoxic against HeLa cells than 1. DNA dsb and cytotoxicity are 44% correlated in 1P1-3 but 5% in 1M1-3, suggesting specific DNA binding of P-linkers and nonspecific binding of M-linkers in biological cells. 1P1-3 exert cancer cell-selective cytotoxicity against lung and pancreas cancer and normal cells where the short P-linker enhances the selectivity, but 1M1-3 do not. Intracellular visualization, apoptosis assay, and caspase activity assay clarify mitochondrial apoptosis caused by 1P1-3. The highest cancer cell selectivity of 1P1 may be enabled by the short P-linker promoting dsb of mitochondrial DNA with H2O2 increased by mitochondrial dysfunction in cancer cells.

Imaging of mitochondria/lysosomes in live cells and C. elegans

Two rhodamine-phenothiazine conjugates, RP1 and RP2, were synthesized, and their photophysical properties, subcellular localization, and photocytotoxicity were investigated. We observed robust localization of RP1 in mitochondria and dual localization in mitochondria and lysosomes with RP2 in live cells. Live cell imaging with these probes allowed us to track the dynamics of mitochondria and lysosomes during ROS-induced mitochondrial damage and the subsequent lysosomal digestion of the damaged mitochondria. The fluorophores also demonstrated preferential accumulation in cancer cells compared to normal cells and had strong photo-cytotoxicity. However, no cytotoxicity was observed in the dark. The mitochondrial staining and light-induced ROS production were not limited to mammalian cell lines, but were also observed in the animal model C. elegans. The study demonstrated the potential applications of these probes in visualizing the mitochondria-lysosome cross-talk after ROS production and for photodynamic therapy.

Mitochondria as a target of third row transition metal-based anticancer complexes

In pursuit of better treatment options for malignant tumors, metal-based complexes continue to show promise as attractive chemotherapeutics due to tunability, novel mechanisms, and potency exemplified by platinum agents. The metabolic character of tumors renders the mitochondria and other metabolism pathways fruitful targets for medicinal inorganic chemistry. Cumulative understanding of the role of mitochondria in tumorigenesis has ignited research in mitochondrial targeting metal-based complexes to overcome resistance and inhibit tumor growth with high potency and selectivity. Here, we discuss recent progress made in third row transition metal-based mitochondrial targeting agents with the goal of stimulating an active field of research toward new clinical anticancer agents and the elucidation of novel mechanisms of action.

Ruthenium-based antitumor drugs and delivery systems from monotherapy to combination therapy

Ruthenium complex is an important compound group for antitumor drug research and development. NAMI-A, KP1019, TLD1433 and other ruthenium complexes have entered clinical research. In recent years, the research on ruthenium antitumor drugs has not been limited to single chemotherapy drugs; other applications of ruthenium complexes have emerged such as in combination therapy. During the development of ruthenium complexes, drug delivery forms of ruthenium antitumor drugs have also evolved from single-molecule drugs to nanodrug delivery systems. The review summarizes the following aspects: (1) ruthenium complexes from monotherapy to combination therapy, including the development of single-molecule compounds, carrier nanomedicine, and self-assembly of carrier-free nanomedicine; (2) ruthenium complexes in the process of ADME in terms of absorption, distribution, metabolism and excretion; (3) the applications of ruthenium complexes in combination therapy, including photodynamic therapy (PDT), photothermal therapy (PTT), photoactivated chemotherapy (PACT), immunotherapy, and their combined application; (4) the future prospects of ruthenium-based antitumor drugs.

Photofunctional cyclometallated iridium(III) polypyridine methylsulfone complexes as sulfhydryl-specific reagents for bioconjugation, bioimaging and photocytotoxic applications

We report herein near-infrared (NIR)-emitting cyclometallated iridium(III) complexes bearing a heteroaromatic methylsulfone moiety as sulfhydryl-specific reagents; one of the complexes was conjugated to cysteine and cysteine-containing peptides and proteins for bioimaging and photocytotoxic applications.

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Luminescent and Photofunctional Transition Metal Complexes: From Molecular Design to Diagnostic and Therapeutic Applications

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical applications.

Mitochondria MicroRNA Spatial Imaging via pH-Responsive Exonuclease-Assisted AIE Nanoreporter

Mitochondrial microRNAs (mitomiRs) critically orchestrate mitochondrial functions. Spatial imaging of mitomiRs is essential to understand its clinical value in diagnosis and prognosis. However, the direct monitoring of mitomiRs in living cells remains a key challenge. Herein, we report an AIE nanoreporter strategy for mitomiRs imaging in living cells through pH-controlled exonuclease (Exo)-assisted target cycle signal amplification. The AIE-labeled DNA detection probes are conjugated on Exo III encapsulated polymeric nanoparticles (NPs) via consecutive adenines (polyA). The amplified sensing functions are off during the cytoplasm delivery process, and it can be spatially switched from off to on when in the alkaline mitochondria (about pH 8) after triphenylphosphonium (TPP)-mediated mitochondrial targeting. Where the NPs degraded to release Exo III and cancer-specific mitomiRs hybridize with AIE-labeled DNA detection probes to expose the cleavage site of released Exo III, enabling spatially restricted mitomiRs imaging. The mitomiRs expression fluctuation was also realized. This study contributes to a facile strategy that could easily extend to a broad application for the understanding of mitomiRs-related pathological processes.

Metal Peptide Conjugates in Cell and Tissue Imaging and Biosensing

Metal complex luminophores have seen dramatic expansion in application as imaging probes over the past decade. This has been enabled by growing understanding of methods to promote their cell permeation and intracellular targeting. Amongst the successful approaches that have been applied in this regard is peptide-facilitated delivery. Cell-permeating or signal peptides can be readily conjugated to metal complex luminophores and have shown excellent response in carrying such cargo through the cell membrane. In this article, we describe the rationale behind applying metal complexes as probes and sensors in cell imaging and outline the advantages to be gained by applying peptides as the carrier for complex luminophores. We describe some of the progress that has been made in applying peptides in metal complex peptide-driven conjugates as a strategy for cell permeation and targeting of transition metal luminophores. Finally, we provide key examples of their application and outline areas for future progress.

Enhancement of Cancer-Cell-Selective Cytotoxicity by a Dicopper Complex with Phenanthrene Amide-Tether Ligand Conjugate via Mitochondrial Apoptosis

Dicopper complexes [Cu2(μ-OH)(Ln)](ClO4)2 [n = 1 (1) and 2 (2)] with a novel phenanthrene amide-tether ligand conjugate (HL1) and the original p-cresol-2,6-bis(amidecyclen) (HL2) were synthesized. A phenanthrene unit of 1...

Mitoepigenetics: An intriguing regulatory layer in aging and metabolic-related diseases

As a key organelle in eukaryotic cells, mitochondria play a central role in maintaining normal cellular functions. Mitochondrial dysfunction is reported to be closely related with aging and various diseases. Epigenetic modifications in nuclear genome provide a substantial layer for the modulation of nuclear-encoded gene expression. However, whether mitochondria could also be subjected to such similar epigenetic alterations and the involved mechanisms remain largely obscure and controversial. Recently, accumulating evidence has suggested that mitochondrial epigenetics, also known as mitoepigenetics may serve as an intriguing regulatory layer in mitochondrial DNA (mtDNA)-encoded gene expression. Given the potential regulatory role of mitoepigenetics, mitochondrial dysfunction derived from mitoepigenetics-induced abnormal gene expression could also be closely associated with aging and disease development. In this review, we summarized the recent advances in mitoepigenetics, with a special focus on mtDNA methylation in aging and metabolic-related diseases as well as the new methods and technologies for the study of mitoepigenetics. Uncovering the regulatory role of mitoepigenetics will help to understand the underlying mechanisms of mitochondrial dysfunction and provide novel strategies for delaying aging and preventing metabolic-related diseases.

Ruthenium Complexes Bearing Axially Chiral Bipyridyls: The Mismatched Diastereomer Showed Red Circularly Polarized Phosphorescence

Ruthenium^II complexes bearing three axially chiral bipyridyl ligands were synthesized as a new family of chiral complex dyes, and Δ-(S)- and Λ-(S)-diastereomers were obtained. The X-ray crystal structure analyses, spectroscopy, and DFT calculations suggested that all the bipyridyls maintained chirality in both the ground and excited states, and the Δ-(S)- and Λ-(S)-isomers are the matched (more relaxed) and mismatched (more constrained) pairs, respectively. The mismatched Λ-(S)-isomer exhibited red circularly polarized phosphorescence (CPP) both in solution and in the solid state. The solution state CPP is the most intense of ruthenium complexes, while the solid state CPP is the first example of them. It is supposed that, for the Λ-(S)-isomer, the six cumulative CH/π interactions suppress further distortion in the T₁ state.

Cathepsin B-responsive multifunctional peptide conjugated gold nanorods for mitochondrial targeting and precise photothermal cancer therapy

Nanomaterials have shown great potential in cancer therapy, but the phenomenon of poor tumor recognition without cellular organelle accumulation usually leads to reduced therapeutic effects and enhanced side effects. Herein, we resolved this issue by employing a multifunctional peptide coating mainly composed of, from the inside out, a mitochondrial targeting segment, a cathepsin B-responsive segment and a zwitterionic antifouling segment. Then gold nanorods were modified with a peptide via ligand exchange, displaying excellent photothermal property and superior stability both before and after enzyme treatment. The in vitro and in vivo results showed that this nanoplatform possessed good biocompatibility, satisfactory mitochondria targeting ability, prolonged blood circulation lifetime and enhanced cellular uptake in tumors. This nanoplatform promoted effective near-infrared light-triggered subcellular hyperthermia treatment in vitro and exhibited excellent tumor ablation ability in vivo. These findings suggested that this multifunctional nanoplatform could significantly enhance the therapeutic efficiency of photothermal therapy based on activated mitochondrial targeting.

Synthesis and DNA interaction studies of Ru(II) cell penetrating peptide (CPP) bioconjugates

The synthesis of the first bioconjugates of a set of ruthenium(II) dipyridophenazine complexes with two different cell penetrating peptides (CPPs) is described. The CPPs, an arginine rich TAT-9 (RKKRRQRRR) sequence and the Xentry peptide (LCLRPVG), were synthesized using standard SPPS protocols, and the bioconjugates were obtained by the microwave-assisted coupling of the HOBt/TBTU preactivated metal complexes with the respective peptides on Wang resin. The racemic metal complexes were obtained by modified literature procedures. The bioconjugates were cleaved from the resin, purified by semi-preparative HPLC and characterized by analytical HPLC, high resolution mass spectrometry (HR-MS), and NMR spectroscopy. Despite the bioconjugation of the peptides to the dppz ligand, DNA intercalation was observed by CD spectroscopy, viscometry and the characteristic switch-on fluorescence of this class of compounds. Furthermore, the cellular uptake of the Xentry bioconjugates was confirmed by live cell imaging. Like the parent metal complexes, the bioconjugates show low in vitro cytotoxicity (IC50 > 80 μM), which is similar to the respective metal complexes alone.

Ru(ii)/BODIPY core co-encapsulated ratiometric nanotools for intracellular O2 sensing in live cancer cells

Oxygen is a crucial reagent in many biochemical processes within living cells and its concentration can be an effective marker in disease, particularly in cancer where tissue hypoxia has been shown to indicate tumour growth. Probes that can reflect the oxygen concentration and distribution using ratiometric signals can be applied to a range of conventional methods without the need for specialised equipment and are particularly useful. The preparation and in cellulo study of luminescent ratiometric core–shell nanoparticles are presented. Here, a new lipophilic and oxygen-responsive Ru(ii) tris-heteroleptic polypyridyl complex is co-encapsulated with a reference BODIPY dye into the core of poly-l-lysine-coated polystyrene particles. The co-core encapsulation ensures oxygen response but reduces the impact of the environment on both probes. Single wavelength excitation of the particles, suspended in aqueous buffer, at 480 nm, triggers well-resolved dual emission from both dyes with peak maxima at 515 nm and 618 nm. A robust ratiometric oxygen response is observed from water, with a linear dynamic range of 3.6–262 μM which matches well with typical biological ranges. The uptake of RuBDP NPs was found to be cell-line dependent, but in cancerous cell lines, the particles were strongly permeable with late endosomal and partial lysosomal co-staining observed within 3 to 4 hours, eventually leading to extensive staining of the cytoplasm. The co-localisation of the ruthenium and BODIPY emission confirms that the particles remain intact in cellulo with no indication of dye leaching. The ratiometric O₂ sensing response of the particles in cellulo was demonstrated using a plate-based assay and by confocal xyλ scanning of cells exposed to hypoxic conditions.

Uptake and quantitative ratiometric oxygen sensing response of core–shell nanoparticles containing ruthenium probe and BODIPY reference is demonstrated using a plate reader-based assay and by confocal xyλ scanning of live cancer cells under hypoxic conditions.

Research Progress in the Synthesis of Targeting Organelle Carbon Dots and Their Applications in Cancer Diagnosis and Treatment

With increasing knowledge about diseases at the histological, cytological to sub-organelle level, targeting organelle therapy has gradually been envisioned as an approach to overcome the shortcomings of poor specificity and multiple toxic side effects on tissues and cell-level treatments using the currently available therapy. Organelle carbon dots (CDs) are a class of functionalized CDs that can target organelles. CDs can be prepared by a "synchronous in situ synthesis method" and "asynchronous modification method." The superior optical properties and good biocompatibility of CDs can be preserved, and they can be used as targeting particles to carry drugs into cells while reducing leakage during transport. Given the excellent organelle fluorescence imaging properties, targeting organelle CDs can be used to monitor the physiological metabolism of organelles and progression of human diseases, which will provide advanced understanding and accurate diagnosis and targeted treatment of cancers. This study reviews the methods used for preparation of targeting organelle CDs, mechanisms of accurate diagnosis and targeted treatment of cancer, as well as their application in the area of cancer diagnosis and treatment research. Finally, the current difficulties and prospects for targeting organelle CDs are prospected.

Effect of Alkyl Chain Length on the Photophysical, Photochemical, and Photobiological Properties of Ruthenium(II) Polypyridyl Complexes for Their Application as DNA-Targeting, Cellular-Imaging, and Light-Activated Therapeutic Agents

A family of six Ru(II) polypyridyl complexes (1-6) which contain phenanthroline-based ligands functionalized with alkyl chains of different lengths (one methyl group, 10 and 21 carbon alkyl chains) and either 1,10-phenanthroline (phen) or 1,4,5,8-tetraazaphenanthrene (TAP) as ancillary ligands have been synthesized and characterized. The influence of the alkyl chain length on their photophysical and photochemical properties as well as in their photobiological applications has been elucidated by monitoring the changes in their MLCT-centered absorption and emission bands. The presence of one methyl group or 10 carbon alkyl chains does not seem to significantly affect the photophysical and photochemical properties of the resulting Ru(II) complexes when compared to the well-known [Ru(phen)₃]²⁺ and [Ru(TAP)₂phen]²⁺. However, an effect on their emission properties and in their ability to photosensitize singlet oxygen is observed for the Ru(II) complexes containing 21 carbon alkyl chains. The binding of these complexes to salmon testes DNA (stDNA) was investigated by observing the changes in the photophysical properties. Complexes 1, 2, 4, and 5 all showed changes in their MLCT bands that could be analyzed using conventional fitting methods, such as the Bard equation. In contrast, complexes 3 and 6, possessing long aliphatic chains, gave rise to nonclassic behavior. In addition to these analyses, both thermal denaturation and circular dichroism studies of 1-6 were carried out in the presence of stDNA which confirmed that these complexes bind to DNA. Confocal microscopy and viability studies in HeLa cervical cancer cells reveal an alkyl chain-length dependence on the cellular uptake and cytotoxicity of the resulting Ru(II) complexes due to an enhancement of their lipophilicity with increasing alkyl chain length. Thus, complexes containing 10 and 21 carbon alkyl chains are rapidly taken up into HeLa cells and, in particular, those with 21 carbon alkyl chains show a significant phototoxicity against the same cell line. Therefore, this study provides further insight into the possible modulation of the photophysical, photochemical, and photobiological properties of Ru(II) polypyridyl complexes by varying the length of the alkyl chains attached to the polypyridyl ligands coordinated to the Ru(II) center and the nature of the auxiliary groups, which we show has a significant effect on photophysical and biological properties.

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.

Iridium(iii) complexes as mitochondrial topoisomerase inhibitors against cisplatin-resistant cancer cells

Herein, we developed the first metal-based mitochondrial topoisomerase inhibitors to achieve an effective therapeutic outcome for the therapy of cisplatin-resistant tumour cells.

Induction and Monitoring of DNA Phase Separation in Living Cells by a Light-Switching Ruthenium Complex

Phase separation of DNA is involved in chromatin packing for the regulation of gene transcription. Visualization and manipulation of DNA phase separation in living cells present great challenges. Herein, we present a Ru(II) complex (Ru1) with high DNA binding affinity and DNA "light-switch" behavior that can induce and monitor DNA phase separation both in vitro and in living cells. Molecular dynamics simulations indicate that the two phen-PPh₃ ligands with positively charged lipophilic triphenylphosphine substituents and flexible long alkyl chains in Ru1 play essential roles in the formation of multivalent binding forces between DNA molecules to induce DNA phase separation. Importantly, the unique environmental sensitive emission property of Ru1 enables direct visualization of the dynamic process of DNA phase separation in living cells by two-photon phosphorescent lifetime imaging. Moreover, Ru1 can change the gene expression pattern by modulating chromatin accessibility as demonstrated by integrating RNA-sequencing and transposase-accessible chromatin with high-throughput sequencing. In all, we present here the first small-molecule-based tracer and modulator of DNA phase separation in living cells and elucidate its impact on the chromatin state and transcriptome.

Os(II)-Bridged Polyarginine Conjugates: The Additive Effects of Peptides in Promoting or Preventing Permeation in Cells and Multicellular Tumor Spheroids

The preparation of two polyarginine conjugates of the complex Os(II) [bis-(4'-(4-carboxyphenyl)-2,2':6',2″-terpyridine)] [Os-(Rn)2]x+ (n = 4 and 8; x = 10 and 18) is reported, to explore whether the R8 peptide sequence that promotes cell uptake requires a contiguous amino acid sequence for membrane permeation or if this can be accomplished in a linearly bridged structure with the additive effect of shorter peptide sequences. The conjugates exhibit NIR emission centered at 754 nm and essentially oxygen-insensitive emission with a lifetime of 89 ns in phosphate-buffered saline. The uptake, distribution, and cytotoxicity of the parent complex and peptide derivatives were compared in 2D cell monolayers and a three-dimensional (3D) multicellular tumor spheroid (MCTS) model. Whereas, the bis-octaarginine sequences were impermeable to cells and spheroids, and the bis-tetraarginine conjugate showed excellent cellular uptake and accumulation in two 2D monolayer cell lines and remarkable in-depth penetration of 3D MCTSs of pancreatic cancer cells. Overall, the data indicates that cell permeability can be promoted via non-contiguous sequences of arginine residues bridged across the metal centre.

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.

Live-cell imaging of the nucleolus and mapping mitochondrial viscosity with a dual function fluorescent probe

Visualization of sub-cellular organelles allows the determination of various cellular processes and the underlying mechanisms. Herein, we report a fluorescent probe, bearing push-pull substituents emitting at 600 nm and its application in cellular imaging. The probe shows dual imaging of mitochondria and nucleoli and maps mitochondrial viscosity in live cells under various physiological variations and show minimum cytotoxicity. Nucleolar staining is confirmed by RNAase digestion.

Mitochondria and Endoplastic Reticulum Targeting Strategy for Enhanced Phototherapy

To ensure improved efficacy and minimized toxicity of therapeutic molecules, it is generally accepted that specifically delivering them to the subcellular site of their action will be attractive. Phototherapy has received considerable attention because of its noninvasiveness, high temporal-spatial resolution, and minimal drug resistance. As important functional organelles in cells, mitochondria and endoplasmic reticulum (ER) participate in fundamental cellular processes, which make them much more sensitive to reactive oxygen species (ROS) and hyperthermia. Thus, mitochondria- or ER-targeted phototherapy will be rational strategies for synergetic cancer therapy. In this review, we focus on the latest advances in molecules and nanomaterials currently used for mitochondria- and ER-targeted phototherapy.

A focus on the biological targets for coinage metal-NHCs as potential anticancer complexes

Metal complexes of N-heterocyclic carbene (NHC) ligands are the object of increasing attention for therapeutic purposes. Among the different metal centres, interest on Au-based compounds started with the application as anti-arthritis drugs. On the other hand, Ag(I) antimicrobial properties have been known for a long time. For Au(I)/Au(III)-NHC and Ag(I)-NHC anti-tumour and anti-proliferative properties have been quite recently demonstrated. In addition to these and as for Group 11, copper is a much less investigated metal centre, but a few papers underline its pharmacological potential. This review wants to focus on the different biological targets for these metal-based compounds. It is divided into chapters which are respectively devoted on: i) mitochondria and thiol oxidoreductase systems; ii) other relevant enzymes; iii) nucleic acids. Examples of representative coinage NHCs for each of the targets are provided together with significant references on recent advances on the topic. Moreover, a final comment summarises the aspects enlightened by each chapter and provides some hints to better understand the metal-NHCs mechanistic behaviour based on structure-activity relationships.

Supramolecular nanofibers increase the efficacy of 10-hydroxycamptothecin by enhancing nuclear accumulation and depleting cellular ATP

Poor nuclear delivery and accumulation are the main reasons for the reduced drug efficacy of many anticancer drugs that target DNA or enzymes in the nucleus, and it is a major obstacle to successful cancer therapy. To address this problem, developing practical drug delivery systems for nuclear delivery is urgently needed. Here we develop a supramolecular hydrogel by conjugating the anticancer agent 10-hydroxycamptothecine (HCPT) and macrocyclic polyamine cyclen to a self-assembling peptide. The cyclen fragment possesses nuclear localization and ATP hydrolysis properties, which can provide a synergistic therapeutic effect for cancer treatment. The HCPT-FFFK-cyclen nanofibers showed improved nuclear accumulation and inhibition capacity in cancer cells including drug-resistant cancer cells in vitro. The nanofibers also exhibited favorable ATP consuming ability in vitro. Moreover, the obtained nanomedicine showed enhanced anticancer efficiency and favorable biocompatibility in vivo when administered to mice via tail vein injection. This constructed self-delivery drug system significantly improved the delivery efficiency of the small molecule agents into the nucleus and showed favorable ATP consuming ability, offering new strategies for developing nanomedicines for cancer combination therapy.

Phosphorescent metal complexes as theranostic anticancer agents: combining imaging and therapy in a single molecule

Phosphorescent metal complexes are a new kind of multifunctional antitumor compounds that can integrate imaging and antitumor functions in a single molecule. In this minireview, we summarize the recent research progress in this field, concentrating on the theranostic applications of phosphorescent iridium(iii), ruthenium(ii) and rhenium(i) complexes. The molecular design that affords these complexes with tumour- or subcellular organelle-targeting properties is elucidated. The potential of these complexes to induce and monitor the dynamic behavior of subcellular organelles and the changes in microenvironment during the process of therapy is demonstrated. Moreover, the potential and advantages of applying new technologies, such as super-resolution imaging and phosphorescence lifetime imaging, are also described. Finally, the challenges faced in the development of novel theranostic metallo-anticancer complexes for possible clinical translation are proposed.

Photofunctional transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics

This critical review summarises the recent biological applications of transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics.

Visualizing, quantifying, and manipulating mitochondrial DNA in vivo

Mitochondrial DNA (mtDNA) encodes proteins and RNAs that support the functions of mitochondria and thereby numerous physiological processes. Mutations of mtDNA can cause mitochondrial diseases and are implicated in aging. The mtDNA within cells is organized into nucleoids within the mitochondrial matrix, but how mtDNA nucleoids are formed and regulated within cells remains incompletely resolved. Visualization of mtDNA within cells is a powerful means by which mechanistic insight can be gained. Manipulation of the amount and sequence of mtDNA within cells is important experimentally and for developing therapeutic interventions to treat mitochondrial disease. This review details recent developments and opportunities for improvements in the experimental tools and techniques that can be used to visualize, quantify, and manipulate the properties of mtDNA within cells.

Control of Mitochondrial Localization Using Thermoresponsive Sulfobetaine Polymer

Fast intracellular migration and controlled localization of molecules represent significant challenges for future applications of drug discovery and related fields. In this study, thermoresponsive sulfobetaine polymers with pyridinium cations are evaluated as biocompatible and mitochondria-localizing agents. Among the polymers, poly(3-(4-(2-methacrylamido)ethyl pyridinio-1-yl)propane-1-sulfonate), P(E-PySMAAm)14k (M_n  = 14 000 g mol^-1 ) exhibit thermoresponsiveness with an upper critical solution temperature like behavior in cell culture medium containing serum with minimal cytotoxicity. Upon the addition of P(E-PySMAAm)14k to HeLa cells at temperatures above the clearing point at 37 °C, effective localization is observed in mitochondria. However, increased intensity but nonspecific localization is observed below the clearing point at 4 °C. Doxorubicin is conjugated to the P(E-PySMAAm) and achieves effective mitochondrial delivery while maintaining drug efficacy. Such sulfobetaine polymers represent promising tools for intracellular delivery of molecules.

The Artemisinin-Derived Autofluorescent Compound BG95 Exerts Strong Anticytomegaloviral Activity Based on a Mitochondrial Targeting Mechanism

Human cytomegalovirus (HCMV) is a major human pathogen associated with severe pathology. Current options of antiviral therapy only partly satisfy the needs of a well-tolerated long-term treatment/prophylaxis free from drug-induced viral resistance. Recently, we reported the strong antiviral properties in vitro and in vivo of the broad-spectrum anti-infective drug artesunate and its optimized derivatives. NF-κB signaling was described as a targeting mechanism and additional target proteins have recently been identified. Here, we analyzed the autofluorescent hybrid compound BG95, which could be utilized for intracellular visualization by confocal imaging and a tracking analysis in virus-infected primary human fibroblasts. As an important finding, BG95 accumulated in mitochondria visualized by anti-prohibitin and MitoTracker staining, and induced statistically significant changes of mitochondrial morphology, distinct from those induced by HCMV infection. Notably, mitochondrial membrane potential was found substantially reduced by BG95, an effect apparently counteracting efficient HCMV replication, which requires active mitochondria and upregulated energy levels. This finding was consistent with binding properties of artesunate-like compounds to mitochondrial proteins and thereby suggested a new mechanistic aspect. Combined, the present study underlines an important role of mitochondria in the multifaceted, host-directed antiviral mechanism of this drug class, postulating a new mitochondria-specific mode of protein targeting.

In Situ Monitoring of Mitochondria Regulating Cell Viability by the RNA-Specific Fluorescent Photosensitizer

Cell viability is greatly affected by external stimulus eliciting correlated dynamical physiological processes for cells to choose survival or death. A few fluorescent probes have been designed to detect whether the cell is in survival state or apoptotic state, but monitoring the regulation process of the cell undergoing survival to death remains a long-standing challenge. Herein, we highlight the in situ monitor of mitochondria regulating the cell viability by the RNA-specific fluorescent photosensitizer L. At normal conditions, L anchored mitochondria and interacted with mito-RNA to light up the mitochondria with red fluorescence. With external light stimulus, L generated reactive oxide species (ROS) and cause damage to mitochondria, which activated mitochondrial autophagy to prevent death, during which the red fluorescence of L witnessed dynamical distribution in accordance with the evolution of vacuole structures containing damaged mitochondria into autophagosomes. However, with ROS continuously increasing, the mitochondrial apoptosis was eventually commenced and L with red fluorescent was gradually accumulated in the nucleoli, indicating the programmed cell death. This work demonstrated how the delicate balance between survival and death are regulated by mitochondria.

Novel "ruthenium cyclopentadienyl"-peptide conjugate complexes against human FGFR(+) breast cancer

In this work we explored the possibility of improving the selectivity of a cytotoxic Ru complex [RuCp(PPh₃)(2,2'-bipy)][CF₃SO₃] (where Cp = η⁵-cyclopentadienyl) TM34 towards FGFR(+) breast cancer cells. Molecular dynamics (MD) simulations of TM34 in a phosphatidylcholine membrane model pinpointed the cyclopentadienyl group as a favorable derivatization position for the peptide conjugation approach. Three new Ru(ii) complexes presenting a functionalized η⁵-cyclopentadienyl were synthesized, namely [Ru(η⁵-C₅H₄COOH)(2,2'-bipy)(PPh₃)][CF₃SO₃] (TM281) and its precursors, [Ru(η⁵-C₅H₄COOCH₂CH₃)(η²-2,2'-bipy)(PPh₃)][CF₃SO₃] (3) and [Ru(η⁵-C₅H₄COOCH₂CH₃)(PPh₃)₂Cl] (2). Complex TM281 was prepared by the hydrolysis of the ethyl ester group appended to the η⁵-cyclopentadienyl ligand of complex 3 with K₂CO₃ in water/acetonitrile, followed by mild protonation using an ion exchange resin. The newly synthesized complexes were fully characterized by NMR, FTIR and UV-vis spectroscopic techniques. Also, electrochemical studies were carried out by means of cyclic voltammetry in order to evaluate the stability of the compounds. Single crystal X-ray diffraction studies were carried out for compounds 3 and TM281 which crystallized in the monoclinic system, space group P21/n. The unprecedented synthesis and characterization of three half-sandwich ruthenium(ii)-cyclopentadienyl peptide conjugates and their preliminary biological evaluation against human FGFR(+) and FGFR(-) breast cancer cells are also reported.

Mitochondrial targeted osmium polypyridyl probe shows concentration dependent uptake, localisation and mechanism of cell death

A symmetric osmium(ii) [bis-(4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine)] was prepared and conjugated to two mitochondrial-targeting peptide sequences; FrFKFrFK (r = d-arginine). The parent and conjugate complexes showed strong near infra-red emission centred at λmax 745 nm that was modestly oxygen dependent in the case of the parent and oxygen independent in the case of the conjugate, attributed in the latter case, surprisingly, to a shorter emission lifetime of the conjugate compared to the parent. Confocal fluorescence imaging of sub-live HeLa and MCF 7 cells showed the parent complex was cell impermeable whereas the conjugate was rapidly internalised into the cell and distributed in a concentration dependent manner. At concentrations below approximately 30 μmol, the conjugate localised to the mitochondria of both cell types where it was observed to trigger apoptosis induced by the collapse of the mitochondrial membrane potential (MPP). At concentrations exceeding 30 μmol the conjugate was similarly internalised rapidly but distributed throughout the cell, including to the nucleus and nucleolus. At these concentrations, it was observed to precipitate a caspase-dependent apoptotic pathway. The combination of concentration dependent organelle targeting, NIR emission coincident with the biological window, and distribution dependent cytotoxicity offers an interesting approach to theranostics with the possibility of eliciting site dependent therapeutic effect whilst monitoring the therapeutic effect with luminescence imaging.

Photostable NIR emitting ruthenium(II) conjugates; uptake and biological activity in live cells

A photostable Ru(2,2-biquinoline)₂(3-(2-pyridyl)-5-(4-carboxyphenyl)-1,2,4-triazolate) (Ru(biq)₂(trzbenzCOOH)) complex that exhibits near-infrared (NIR) emission centred at 786 nm is reported. The parent complex was conjugated via amide coupling to a cell-penetrating peptide sequence octa-arginine (R8), and two signal peptide sequences; the nuclear localizing sequence (NLS) VQRKRQKLMP and the mitochondria penetrating peptide (MPP) FrFKFrFK(Ac) (r = D isomer of arginine, Ac = terminal lysine amine acetyl blocked). Notably, none of the peptide conjugates were cell-permeable as chloride salts but efficient and rapid membrane permeation was observed post ion exchange with perchlorate counterion. Also, surprisingly, all three peptide conjugates exhibited potent dark cytotoxicity in both CHO and HeLa cell lines. The peptide conjugates induce cell death through a caspase dependent apoptotic pathway. At the minimum concentration of dye (approx. 15 μM) required for cell imaging, only 20% of the cells were viable after a 24 h incubation period. To overcome cytotoxicity, the parent complex was PEGylated; this dramatically decreased cytotoxicity, where 50% of cells were viable even at 150 μM concentration after 24 h. Confocal luminescence microscopy indicated that all four bioconjugates, peptides in perchlorate form and polyethylene glycol (PEG) in chloride form, were rapidly internalized within the cell. However, interestingly the precise localisation by the signal peptides observed in related complexes was not observed here and the peptide conjugates were unsuitable as luminescent probes for cell microscopy due to their high cell toxicity. The poor targeting of signal peptides in this instance is attributed to the high lipophilicity of the metal centre.