Luminescent lanthanide-based molecular materials: applications in photodynamic therapy

Luminescent lanthanide molecular compounds have recently attracted attention as potential photosensitizers (PSs) for photodynamic therapy (PDT) against malignant cancer tumours because of their predictable systemic toxicity, temporospatial specificity, and minimal invasiveness. A photosensitizer exhibits no toxicity by itself, but in the presence of light and oxygen molecules, it can generate reactive oxygen species (ROS) to cause damage to proteins, nucleic acids, lipids, membranes, and organelles, which can induce cell apoptosis. This review focuses on the latest developments in luminescent lanthanide-based molecular materials as photosensitizers and their applications in photodynamic therapy. These molecular materials include lanthanide coordination complexes, nanoscale lanthanide coordination polymers, and lanthanide-based nanoscale metal-organic frameworks. In the end, the future challenges in the development of robust luminescent lanthanide molecular materials-based photosensitisers are outlined and emphasized to inspire the design of a new generation of phototheranostic agents.

Anticancer Properties of Rhenium(I) Tricarbonyl Complexes of N-Heterocyclic Carbene Ligands

A family of eight rhenium(I) tricarbonyl complexes bearing pyridyl-imidazolylidene or bis-imidazolylidene ligand in combination with a series of N-acetyl amino acids ligands (glycine, isoleucine, proline) and acetate have been synthesised...

Charge neutral rhenium tricarbonyl complexes of tridentate N-heterocyclic carbene ligands that bind to amyloid plaques of Alzheimer's disease

Two tridentate ligand systems bearing N-heterocyclic carbene (NHC), amine and carboxylate donor groups coupled to benzothiazole- or stilbene-based amyloid binding moieties were synthesised. Reaction of the imidazolium salt containing pro-ligands with Re(CO)5Cl yielded the corresponding rhenium metal complexes which were characterised by NMR, and X-ray crystallography. These ligands are of interest for the potential preparation of technetium-99m imaging agents for Alzheimer's disease and the capacity of these rhenium complexes bind to amyloid fibrils composed of amyloid-β peptide and amyloid plaques in human frontal cortex brain tissue was evaluated using fluorescence microscopy. These studies show that the complexes bound efficiently to amyloid-β fibrils and some evidence of binding to amyloid-β plaques.

A luminescent lipid mimetic iridium(III) N-heterocyclic carbene complex for membrane labelling

Labelling phospholipid membranes with luminophores without altering the biophysical characteristics of the system is particularly challenging due to the small size of the phospholipid molecules and the sensitivity of membrane properties to the presence of fused heterocyclic molecules. Here the design and synthesis of a luminescent lipid mimetic Ir(III) N-heterocyclic carbene complex of the form [Ir(ppy)₂(C^N)] (where ppy = 2-(phenyl)-pyridine and C^N is a N-heterocyclic carbene ligand) conjugated to stearic acid is described. This complex was synthesised by the reaction of an acetate functionalised Ir(III) precursor complex with tert-butyl N-(2-aminoethyl)carbamate (mono-BOC protected ethylene diamine) and after deprotection of the amine group this complex was coupled to stearic acid using the peptide coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). The photophysical properties of the synthesised complexes were evaluated and they showed blue-green luminescence in the range of 514-520 nm. Fluorescence microscopy studies showed that the lipid mimetic complex successfully incorporated into liposomes composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), while dynamic light scattering (DLS) and differential scanning calorimetry (DSC) studies showed that the complex had negligible influence on the biophysical properties of the liposomes.

Thiosemicarbazonate complexes with affinity for amyloid-β fibers: synthesis, characterization and biological studies

Aim: Obtain radioimages of amyloid-β fibers using 99mTc-complexes. Methodology: Tridentate thiosemicarbazone and thiocarbonohydrazone ligands containing fragments (stilbene, azobenzene, benzothiazole or benzoxazole) with affinity for amyloid-ß fibers and its Re(I) complexes have been prepared. The molecular structures of several ligands and complexes were determined by x-ray diffraction. Binding affinity studies toward Aß1-42 fibers were performed for the ligands and Re(I) complexes. The ability of formation of some 99mTc(I) complexes, their biodistribution and in vivo stability have been established. Results & conclusion: Complexes of stilbene and benzothiazole thiosemicarbazonates show similar affinity for amyloid-β fibers to the free ligand. These 99mTc complexes present a reasonable in vivo stability and a low capability to cross the blood–brain barrier although not sufficient to brain amyloid imaging.

Luminescent iridium(iii) complexes of N-heterocyclic carbene ligands prepared using the ‘click reaction’

Luminescent and electrochemiluminescent N-heterocyclic carbene-combined 1,2,3-triazole and 1,2,3-triazolylidene Ir(iii) complexes have been prepared and their potential as luminescent probes in cell imaging has been evaluated.

A catalytic antioxidant for limiting amyloid-beta peptide aggregation and reactive oxygen species generation

Alzheimer's disease (AD) is a multifaceted disease that is characterized by increased oxidative stress, metal-ion dysregulation, and the formation of intracellular neurofibrillary tangles and extracellular amyloid-β (Aβ) aggregates. In this work we report the large affinity binding of the iron(iii) 2,17-bis-sulfonato-5,10,15-tris(pentafluorophenyl)corrole complex FeL1 to the Aβ peptide (K _d ∼ 10^-7) and the ability of the bound FeL1 to act as a catalytic antioxidant in both the presence and absence of Cu(ii) ions. Specific findings are that: (a) an Aβ histidine residue binds axially to FeL1; (b) that the resulting adduct is an efficient catalase; (c) this interaction restricts the formation of high molecular weight peptide aggregates. UV-Vis and electron paramagnetic resonance (EPR) studies show that although the binding of FeL1 does not influence the Aβ-Cu(ii) interaction (K _d ∼ 10^-10), bound FeL1 still acts as an antioxidant thereby significantly limiting reactive oxygen species (ROS) generation from Aβ-Cu. Overall, FeL1 is shown to bind to the Aβ peptide, and modulate peptide aggregation. In addition, FeL1 forms a ternary species with Aβ-Cu(ii) and impedes ROS generation, thus showing the promise of discrete metal complexes to limit the toxicity pathways of the Aβ peptide.

Supramolecular structures of rhenium(I) complexes mediated by ligand planarity via the interplay of substituents

The crystal and molecular structures of two Re^I tricarbonyl complexes, namely fac-tricarbonylchlorido[1-(4-fluorocinnamoyl)-3-(pyridin-2-yl-κN)pyrazole-κN²]rhenium(I), [ReCl(C₁₇H₁₂FN₃O)(CO)₃], (I), and fac-tricarbonylchlorido[1-(4-nitrocinnamoyl)-3-(pyridin-2-yl-κN)pyrazole-κN²]rhenium(I) acetone monosolvate, [ReCl(C₁₇H₁₂ClN₄O₃)(CO)₃]·C₃H₆O, (II), are reported. The complexes form centrosymmetric dimers that are linked into one-dimensional columns by C-H...Cl and N-O...H interactions in (I) and (II), respectively. C-H...Cl interactions in (II) generate two R₂¹(7) loops that merge into a single R₂¹(10) loop. These interactions involve the alkene, pyrazole and benzene rings, hence restricting the ligand rotation and giving rise to a planar conformation. Unlike (II), complex (I) exhibits a twisted conformation of the ligand and a pair of molecules forms a centrosymmetric dimer with an R₂²(10) loop via C-H...O interactions. The unique supramolecular structures of (I) and (II) are determined by their planarity and weak interactions. The planar conformation of (II) provides a base for appreciable π-π stacking interactions compared to (I). In addition, an N-O...π interaction stabilizes the supramolecular structure of (II). We report herein the first n→π* interactions of Re^I tricarbonyl complexes, which account for 0.33 kJ mol^-1. Intermolecular C-H...Cl and C-H...O interactions are present in both complexes, with (II) showing a greater preference for these interactions compared to (I), with cumulative contributions of 48.7 and 41.5%, respectively. The influence of inductive (fluoro) and/or resonance (nitro) effects on the π-stacking ability was further supported by LOLIPOP (localized orbital locator-integrated π over plane) analysis. The benzene ring of (II) demonstrated a higher π-stacking ability compared to that of (I), which is supported by the intrinsic planar geometry. The HOMA (harmonic oscillator model of aromaticity) index of (I) revealed more aromaticity with respect to (II), suggesting that NO₂ greatly perturbed the aromaticity. The Hirshfeld fingerprint (FP) plots revealed the preference of (II) over (I) for π-π contacts, with contributions of 6.8 and 4.4%, respectively.

Rhenium and technetium complexes of thioamide derivatives of pyridylhydrazine that bind to amyloid-β plaques

Age-associated deposition of amyloid-β in cerebral blood vessels, a condition referred to as cerebral amyloid angiopathy, can contribute to stroke and dementia. This research aimed to design new radioactive technetium-99 m complexes that bind to amyloid-β plaques that have the potential to assist in diagnosis of cerebral amyloid angiopathy using single-photon-emitted computed tomography (SPECT) imaging. Six new pyridylthiosemicarbazide ligands containing either benzofuran or styrylpyridyl functional groups that are known to selectively bind to amyloid plaques were prepared. Non-radioactive isotopes of technetium are not available so rhenium was used as a surrogate for exploratory chemistry. The new ligands were used to prepare well-defined [Re-oxo]³⁺ complexes where two pyridylthiosemicarbazide ligands were coordinated to a single metal ion to give bivalent complexes with two amyloid-β targeting functional groups. The interaction of the [Re-oxo]³⁺ complexes with synthetic amyloid-β_1-42 and with amyloid plaques in human brain tissue was investigated. Two ligands were selected to develop methods to prepare their [^99mTc-oxo]³⁺ complexes at the tracer level. These technetium-99 m complexes are likely to be isostructural to their rhenium-oxo analogues.

Properties and prospects for rhenium(i) tricarbonyl N-heterocyclic carbene complexes

Rhenium tricarbonyl complexes bound to N-heterocyclic carbene ligands are emerging as a new class of complexes with promising applications in a wide variety of areas.

Ferrocene-based anilides: synthesis, structural characterization and inhibition of butyrylcholinesterase

Twenty-three compounds in two series of ferrocene-based anilides, with the general formula C₅H₅-Fe-C₅H₄-C₆H₄-NH-CO-C₆H₄-R (where R = H, F, Cl, CH₃ and OCH₃), have been synthesized and found to inhibit butyrylcholinesterase.

Recent progress in the development of metal complexes as β-amyloid imaging probes in the brain

In this review, we have focused on the recent progress in metal complexes that are able to bind to β-amyloid (Aβ) species. We have discussed various radioactive complexes of 99mTc, 68Ga, 64Cu, 89Zr, and 111In, which were designed as Aβ imaging agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, non-radioactive Re and Ru complexes as Aβ sensors using luminescence methods, and Gd3+ complexes as contrast agents for magnetic resonance imaging (MRI).