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

G-Quadruplex Binding of an NIR Emitting Osmium Polypyridyl Probe Revealed by Solution NMR and Time-Resolved Infrared Studies

G-quadruplexes are emerging targets in cancer research and understanding how diagnostic probes bind to DNA G-quadruplexes in solution is critical to the development of new molecular tools. In this study the binding of an enantiopure NIR emitting [Os(TAP)₂ (dppz)]²⁺ complex to different G-quadruplex structures formed by human telomer (hTel) and cMYC sequences in solution is reported. The combination of NMR and time-resolved infrared spectroscopic techniques reveals the sensitivity of the emission response to subtle changes in the binding environment of the complex. Similar behaviour is also observed for the related complex [Os(TAP)₂ (dppp2)]²⁺ upon quadruplex binding.

Photophysical Properties and DNA Binding of Two Intercalating Osmium Polypyridyl Complexes Showing Light-Switch Effects

The synthesis and photophysical characterization of two osmium(II) polypyridyl complexes, [Os(TAP)₂dppz]²⁺ (1) and [Os(TAP)₂dppp2]²⁺ (2) containing dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppp2 (pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline) intercalating ligands and TAP (1,4,5,8-tetraazaphenanthrene) ancillary ligands, are reported. The complexes exhibit complex electrochemistry with five distinct reductive redox couples, the first of which is assigned to a TAP-based process. The complexes emit in the near-IR (1 at 761 nm and 2 at 740 nm) with lifetimes of >35 ns with a low quantum yield of luminescence in aqueous solution (∼0.25%). The Δ and Λ enantiomers of 1 and 2 are found to bind to natural DNA and with AT and GC oligodeoxynucleotides with high affinities. In the presence of natural DNA, the visible absorption spectra are found to display significant hypochromic shifts, which is strongly evident for the ligand-centered π–π* dppp2 transition at 355 nm, which undergoes 46% hypochromism. The emission of both complexes increases upon DNA binding, which is observed to be sensitive to the Δ or Λ enantiomer and the DNA composition. A striking result is the sensitivity of Λ-2 to the presence of AT DNA, where a 6-fold enhancement of luminescence is observed and reflects the nature of the binding for the enantiomer and the protection from solution. Thermal denaturation studies show that both complexes are found to stabilize natural DNA. Finally, cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Osmium(II) polypyridyl complexes comprising extended dipyrido[3,2-a:2′,3′-c]phenazine (1) and pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (2) intercalating ligands are shown to be effective DNA binders accompanied by enhanced near-IR emission. The emission response to B-DNA is found to be sensitive to the enantiomer and the composition of DNA, with greater emission observed for AT-rich sequences. Thermal denaturation studies show that both complexes stabilize natural DNA. Cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

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.

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.

Physical, spectroscopic, and biological properties of ruthenium and osmium photosensitizers bearing diversely substituted 4,4'-di(styryl)-2,2'-bipyridine ligands

Capitalising on the previous identification of a distyryl coordinated Ru(II) polypyridine complex as a promising photosensitizer for photodynamic therapy, eight new complexes were synthesized by modifications of the ligands or by changing the metal coordinated. We report in this work the effects of these modifications on the physical, spectroscopic, and biological properties of the synthesized complexes. Subtle structural modifications of the distyryl ligand only had a moderate effect on the corresponding complexes' visible light absorption and singlet oxygen quantum yield. These modifications however had a significant effect on the lipophilicity, the cellular uptake and the phototoxicity of the complexes. Although the lipophilicity of the complexes had a somewhat expected effect on their cellular uptake, this last parameter could not be directly correlated to their phototoxicity, revealing other underlying phenomena. Overall, this work allowed identification of two promising ruthenium complexes as photosensitisers for photodynamic therapy and provides some guidance on how to design better photosensitizers.

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.

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.

Triazole-based osmium(ii) complexes displaying red/near-IR luminescence: antimicrobial activity and super-resolution imaging

Cellular uptake, luminescence imaging and antimicrobial activity against clinically relevant methicillin-resistant S. aureus (MRSA) bacteria are reported. The osmium(ii) complexes [Os(N^N)3]2+ (N^N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (1 2+); 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (2 2+); 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (3 2+)) were prepared and isolated as the chloride salts of their meridional and facial isomers. The complexes display prominent spin-forbidden ground state to triplet metal-to-ligand charge transfer (3MLCT) state absorption bands enabling excitation as low as 600 nm for fac/mer-3 2+ and observation of emission in aqueous solution in the deep-red/near-IR regions of the spectrum. Cellular uptake studies within MRSA cells show antimicrobial activity for 1 2+ and 2 2+ with greater toxicity for the meridional isomers in each case and mer-1 2+ showing the greatest potency (32 μg mL-1 in defined minimal media). Super-resolution imaging experiments demonstrate binding of mer- and fac-1 2+ to bacterial DNA with high Pearson's colocalisation coefficients (up to 0.95 using DAPI). Phototoxicity studies showed the complexes exhibited a higher antimicrobial activity upon irradiation with light.

Cyclometalated Ir(III)-8-oxychinolin complexes acting as red-colored probes for specific mitochondrial imaging and anticancer drugs

A new class of luminescent Ir^III antitumor agents, namely, [Ir(CP1)(PY1)₂] (Ir-1), [Ir(CP1)(PY2)₂] (Ir-2), [Ir(CP1)(PY4)₂] (Ir-3), [Ir(CP2)(PY1)₂] (Ir-4), [Ir(CP2)(PY4)₂] (Ir-5), [Ir(CP3)(PY1)₂]⋅CH₃OH (Ir-6), [Ir(CP4)(PY4)₂]⋅CH₃OH (Ir-7), [Ir(CP5)(PY2)₂] (Ir-8), [Ir(CP5)(PY4)₂]⋅CH₃OH (Ir-9), [Ir(CP6)(PY1)₂] (Ir-10), [Ir(CP6)(PY2)₂]⋅CH₃OH (Ir-11), [Ir(CP6)(PY3)₂] (Ir-12), [Ir(CP6)(PY41)₂] (Ir-13), and [Ir(CP7)(PY1)₂] (Ir-14), supported by 8-oxychinolin derivatives and 1-phenylpyrazole ligands was prepared. Compared with SK-OV-3/DDP and HL-7702 cells, the Ir-1-Ir-14 compounds exhibited half maximal inhibitory concentration (IC₅₀) values within the high nanomolar range (50 nM-10.99 μM) in HeLa cells. In addition, Ir-1 and Ir-3 accumulated and stained the mitochondrial inner membrane of HeLa cells with high selectivity and exhibited a high antineoplastic activity in the entire cervical HeLa cells, with IC₅₀ values of 1.22 ± 0.36 μM and 0.05 ± 0.04 μM, respectively. This phenomenon induced mitochondrial dysfunction, suggesting that these cyclometalated Ir^III complexes can be potentially used in biomedical imaging and Ir(III)-based anticancer drugs. Furthermore, the high cytotoxicity activity of Ir-3 is correlated with the 1-phenylpyrazole (H-PY4) secondary ligands in the luminescent Ir^III antitumor complex.

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.

Complexes of oxoplatin with rhein and ferulic acid ligands as platinum(iv) prodrugs with high anti-tumor activity

We herein designed two new Pt^IV prodrugs of oxoplatin (cis,cis,cis-[PtCl₂(NH₃)₂(OH)₂]), [Pt^IVCl₂(NH₃)₂(O₂C-FA)₂] (Pt-2) and [Pt^IVCl₂(NH₃)₂(O₂C-RH)₂] (Pt-3), by conjugating with ferulic acid (FA-COOH) and rhein (RH-COOH) which have well-known biological activities. Three other Pt(iv) complexes of [Pt^IVCl₂(NH₃)₂(O₂C-BA)₂] (Pt-1), [Pt^IVCl₂(NH₃)₂(O₂C-CA)₂] (Pt-4) and [Pt^IVCl₂(NH₃)₂(O₂C-TCA)₂] (Pt-5) (where BA-COOH = benzoic acid, CA-COOH = crotonic acid and TCA-COOH = trans-cinnamic acid) were also prepared for the comparative study. Like most Pt^IV prodrug complexes, the cytotoxicity of Pt-3 containing the biologically active rhein (RH-COOH) ligand against lung carcinoma (A549 and A549/DDP) cells was higher than those of Pt-1, Pt-2, Pt-4, cisplatin and Pt-5. Moreover, the cytotoxicity of Pt-3 in HL-7702 normal cells was lower than those of Pt^IV derivatives bearing BA-COOH, FA-COOH, TCA-COOH and CA-COOH ligands. The highly efficacious Pt-2 and Pt-3 were found to accumulate strongly in the A549/DDP cells, with the prodrug Pt-3 showing highest levels of penetration into the mitochondria. The prodrug Pt-3 effectively entered the A549/DDP cells and caused mitochondrial damage, significantly greater than Pt-2. In addition, the prodrug Pt-3 exhibited higher antitumor efficacy (inhibition rates (IR) = 67.45%) than Pt-2 (28.12%) and cisplatin (33.05%) in the A549/DDP xenograft mouse model. Thus, the prodrug Pt-3 containing the rhein (RH-COOH) ligand is a promising candidate drug targeting the mitochondria.

Highly cytotoxic, cyclometalated iridium(iii)-5-fluoro-8-quinolinol complexes as cancer cell mitochondriotropic agents

Ir-3 and Ir-4 kill HeLa cells and trigger caspase-mediated mitochondrial dysfunction apoptosis pathways.