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

Targeting Mitochondrial Guanine Quadruplexes for Photoactivatable Chemotherapy in Hypoxic Environments

A first example of a mitochondrial G-quadruplex (mitoG4s) targeted Ru(II) photooxidant complex is reported. The complex, Ru-TAP-PDC3 induces photodamage toward guanine quadruplexes (G4s) located in the mitochondrial genome under hypoxic and normoxic conditions. Ru-TAP-PDC3 shows high affinity for mitoG4s and localises within mitochondria of live HeLa cells. Immunolabelling with anti-G4 antibody, BG4, confirms Ru-TAP-PDC3 associates with G4s within the mitochondria of fixed cells. The complex induces depletion of mtDNA in live cells under irradiation at 405 nm, confirmed by loss of PicoGreen signal from mitochondria. Biochemical studies confirm this process induces apoptosis. The complex shows low dark toxicity and an impressive phototoxicity index (PI) of >89 was determined in Hela under very low intensity irradiation, 5 J/cm2. The phototoxicity is thought to operate through both Type II singlet oxygen and Type III pathways depending on normoxic or hypoxic conditions, from live cell assays and plasmid DNA cleavage. Overall, we demonstrate targeting mitoG4s and mtDNA with a photooxidant is a potent route to achieving apoptosis under hypoxic conditions that can be extended to phototherapy.

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.

Advances in peptide-based drug delivery systems

Drug delivery systems (DDSs) are designed to deliver drugs to their specific targets to minimize their toxic effects and improve their susceptibility to clearance during targeted transport. Peptides have high affinity, low immunogenicity, simple amino acid composition, and adjustable molecular size; therefore, most peptides can be coupled to drugs via linkers to form peptide-drug conjugates (PDCs) and act as active pro-drugs. PDCs are widely thought to be promising DDSs, given their ability to improve drug bio-compatibility and physiological stability. Peptide-based DDSs are often used to deliver therapeutic substances such as anti-cancer drugs and nucleic acid-based drugs, which not only slow the degradation rate of drugs in vivo but also ensure the drug concentration at the targeted site and prolong the half-life of drugs in vivo. This article provides an profile of the advancements and future development in functional peptide-based DDSs both domestically and internationally in recent years, in the expectation of achieving targeted drug delivery incorporating functional peptides and taking full advantage of synergistic effects.

On-Resin Conjugation of the Ruthenium Anticancer Agent Plecstatin-1 to Peptide Vectors

Ruthenium piano-stool complexes have been explored for their anticancer activity and some promising compounds have been reported. Herein, we conjugated a derivative of plecstatin-1 to peptides in order to increase their cancer cell targeting ability. For this purpose, plecstatin-1 was modified at the arene ligand to introduce a functional amine handle (3), which resulted in a compound that showed similar activity in an in vitro anticancer activity assay. The cell-penetrating peptide TAT48-60, tumor-targeting neurotensin8-13, and plectin-targeting peptide were functionalized with succinyl or β-Ala-succinyl linkers under standard solid-phase peptide synthesis (SPPS) conditions to spatially separate the peptide backbones from the bioactive metal complexes. These modifications allowed for conjugating precursor 3 to the peptides on resin yielding the desired metal-peptide conjugates (MPCs), as confirmed by high-performance liquid chromatography (HPLC), NMR spectroscopy, and mass spectrometry (MS). The MPCs were studied for their behavior in aqueous solution and under acidic conditions and resembled that of the parent compound plecstatin-1. In in vitro anticancer activity studies in a small panel of cancer cell lines, the TAT-based MPCs showed the highest activity, while the other MPCs were virtually inactive. However, the MPCs were significantly less active than the small molecules plecstatin-1 and 3, which can be explained by the reduced cell uptake as determined by inductively coupled plasma MS (ICP-MS). Although the MPCs did not display potent anticancer activities, the developed conjugation strategy can be extended toward other metal complexes, which may be able to utilize the targeting properties of peptides.

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.

Selective, Disruptive Luminescent Ru(II) Polypyridyl Probes of G-Quadruplex

Sensors capable of transducing G-quadruplex DNA binding are important both in solution and for imaging and interrogation in cellulo. Ru(II)-based light switches incorporating dipyridylphenazine (dppz) ligands are effective probes for recognition and imaging of DNA and its polymorphs including G-quadruplex, although selectivity is a limitation. While the majority of Ru(II)-based light switches reported to date, stabilize the quadruplex, imaging/theranostic probes that can disrupt G4s are of potentially enormous value in study and therapy for a range of disease states. We report here, on a Ru(II) complex (Ru-PDC3) that assembles the light switch capability of a Ru(II) dipyridylphenazine complex with the well-known G4-selective ligand Phen-DC3, into a single structure. The complex shows the anticipated light switch effect and strong affinity for G4 structures. Affinity depended on the G4 topology and sequence, but across all structures bar one, it was roughly an order of magnitude greater than for duplex or single-stranded DNA. Moreover, photophysical and Raman spectral data showed clear discrimination between duplex DNA and G4-bound structures offering the prospect of discrimination in imaging as well as in solution. Crucially, unlike the constituent components of the probe, Ru-PDC3 is a powerful G4 disrupter. From circular dichroism (CD), a reduction of ellipticity of the G4 between 70 and 95% was observed depending on topology and in many cases was accompanied by an induced CD signal for the metal complex. The extent of change in ellipticity is amongst the largest reported for small-molecule ligand G4 binding. While a promising G4 probe, without modification, the complex is fully water-soluble and readily permeable to live cells.

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 for Bioassays in the Near-Infrared (NIR) Region

Near-infrared (NIR, 700-1700 nm) luminescent imaging is an emerging bioimaging technology with low photon scattering, minimal autofluorescence, deep tissue penetration, and high spatiotemporal resolution that has shown fascinating promise for NIR imaging-guided theranostics. In recent progress, NIR luminescent metal complexes have attracted substantially increased research attention owing to their intrinsic merits, including small size, anti-photobleaching, long lifetime, and metal-centered NIR emission. In the past decade, scientists have contributed to the advancement of NIR metal complexes involving efforts to improve photophysical properties, biocompatibility, specificity, pharmacokinetics, in vivo visualization, and attempts to exploit new ligand platforms. Herein, we summarize recent progress and provide future perspectives for NIR metal complexes, including d-block transition metals and f-block lanthanides (Ln) as NIR optical molecular probes for bioassays.

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.

A Dinuclear Osmium(II) Complex Near-Infrared Nanoscopy Probe for Nuclear DNA

With the aim of developing photostable near-infrared cell imaging probes, a convenient route to the synthesis of heteroleptic Os^II complexes containing the Os(TAP)₂ fragment is reported. This method was used to synthesize the dinuclear Os^II complex, [{Os(TAP)₂}₂tpphz]⁴⁺ (where tpphz = tetrapyrido[3,2-a:2',3'-c:3″,2''-h:2‴,3'''-j]phenazine and TAP = 1,4,5,8- tetraazaphenanthrene). Using a combination of resonance Raman and time-resolved absorption spectroscopy, as well as computational studies, the excited state dynamics of the new complex were dissected. These studies revealed that, although the complex has several close lying excited states, its near-infrared, NIR, emission (λ_max = 780 nm) is due to a low-lying Os → TAP based ³MCLT state. Cell-based studies revealed that unlike its Ru^II analogue, the new complex is neither cytotoxic nor photocytotoxic. However, as it is highly photostable as well as live-cell permeant and displays NIR luminescence within the biological optical window, its properties make it an ideal probe for optical microscopy, demonstrated by its use as a super-resolution NIR STED probe for nuclear DNA.