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.

Replacement of the phosphodiester backbone between canonical nucleosides with a dirhenium carbonyl "click" linker-a new class of luminescent organometallic dinucleoside phosphate mimics

The first-in-class luminescent dinucleoside phosphate analogs with a [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] "click" linker as a replacement for the natural phosphate group are reported together with the synthesis of luminescent adenosine and thymidine derivatives having the [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] entity attached to positions 5' and 3', respectively. These compounds were synthesized by applying inverse-electron-demand Diels-Alder and copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reactions in three or four steps. The obtained compounds exhibited orange emission (λ_PL ≈ 600 nm, Φ_PL ≈ 0.10, and τ = 0.33-0.61 μs) and no toxicity (except for one nucleoside) to human HeLa cervical epithelioid and Ishikawa endometrial adenocarcinoma cancer cells in vitro. Furthermore, the compounds' ability to inhibit the growth of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacterial strains was moderate and only observed at a high concentration of 100 μM. Confocal microscopy imaging revealed that the "dirhenium carbonyl" dinucleosides and nucleosides localized mainly in the membranous structures of HeLa cells and uniformly inside S. aureus and E. coli bacterial cells. An interesting finding was that some of the tested compounds were also found in the nuclei of HeLa cells.

Fluorescent sterol probes for intracellular transport, imaging, and therapeutics

Sterols play a significant role in many physiological processes affecting membrane organization, transport, permeability, and signal transduction. The development of fluorescent sterol analogs that have immediate functional relevance to the natural biomolecules is one approach to understanding the sterol-driven physiological processes. Visualizing cellular compartments with tailor-made fluorescent molecules through specific labeling methods enables organelle targeting and reveals dynamic information. In this review, we focus on the recent literature published between 2020 and 2022, with particular emphasis on extrinsic fluorophores and their investigations of sterol-driven biological processes involving sterol transport, biomolecular interactions, and biological imaging.

Divergent Reactivity of Phosphorylated and Related Allenes: [4 + 2] Cycloaddition with 3,6-Diphenyltetrazine, Self-Addition Leading to Dimers and [Pd]-Complex Formation

Phosphorus-based naphthalenes are formed by self-dimerization-cum-cyclization of α-aryl allenylphosphonates or allenylphosphine oxides using catalytic Pd(OAc)₂in the presence of PPh₃ and Et₃N . This reaction involves [4 + 2]-cycloaddition with the (β,γ) double bond of one allene as the dienophile; the double bonds at the α-aryl-(β',γ') group and (α,β)-carbons of the second allene act as the diene part. A subsequent proton shift also takes place. Upon treating allenylphosphine oxides with Pd(OAc)₂ [stoichiometry 2:1] in the presence of PPh₃/Ag₂CO₃, a [Pd]-complex is isolated and structurally characterized. This complex can be used as a catalyst for C-C bond-forming reactions of phosphorus-based allenes with 2-iodophenol. Densely substituted 3,6-diphenylpyridazines are conveniently obtained in excellent yields by a thermally induced regioselective Inverse Electron Demand Diels-Alder (IEDDA) reaction of allenes with 3,6-diphenyltetrazine, followed by a [1,3]-H shift.

Design of Rhenium Compounds in Targeted Anticancer Therapeutics

Background:

Many rhenium (Re) complexes with potential anticancer properties have been synthesized in the recent years with the aim to overcome the clinical limitations of platinum agents. Re(I) tricarbonyl complexes are the most common but Re compounds with higher oxidation states have also been investigated, as well as hetero-metallic complexes and Re-loaded self-assembling devices. Many of these compounds display promising cytotoxic and phototoxic properties against malignant cells but all Re compounds are still at the stage of preclinical studies.

Methods:

The present review focused on the rhenium based cancer drugs that were in preclinical and clinical trials were examined critically. The detailed targeted interactions and experimental evidences of Re compounds reported by the patentable and non-patentable research findings used to write this review.

Results:

In the present review, we described the most recent and promising rhenium compounds focusing on their potential mechanism of action including, phototoxicity, DNA binding, mitochondrial effects, oxidative stress regulation or enzyme inhibition. Many ligands have been described that modulating the lipophilicity, the luminescent properties, the cellular uptake, the biodistribution, and the cytotoxicity, the pharmacological and toxicological profile.

Conclusion:

Re-based anticancer drugs can also be used in targeted therapies by coupling to a variety of biologically relevant targeting molecules. On the other hand, combination with conventional cytotoxic molecules, such as doxorubicin, allowed to take into profit the targeting properties of Re for example toward mitochondria. Through the example of the diseleno-Re complex, we showed that the main target could be the oxidative status, with a down-stream regulation of signaling pathways, and further on selective cell death of cancer cells versus normal cells.