Ligands for molecular imaging: the synthesis of bis(thiosemicarbazone) ligands

Desulfurization of thiosemicarbazones: the role of metal ions and biological implications

Thiosemicarbazones are biologically active substances whose structural formula is formed by an azomethine, an hydrazine, and a thioamide fragments, to generate a R2C=N-NR-C(=S)-NR2 backbone. These compounds often act as ligands to generate highly stable metal-organic complexes. In certain experimental conditions, however, thiosemicarbazones undergo reactions leading to the cleavage of the chain. Sometimes, the breakage involves desulfurization processes. The present work summarizes the different chemical factors that influence the desulfurization reactions of thiosemicarbazones, such as pH, the presence of oxidant reactants or the establishment of redox processes as those electrochemically induced, the effects of the solvent, the temperature, and the electromagnetic radiation. Many of these reactions require coordination of thiosemicarbazones to metal ions, even those present in the intracellular environment. The nature of the products generated in these reactions, their detection in vivo and in vitro, together with the relevance for the biological activity of these compounds, mainly as antineoplastic agents, is discussed.

Development of an Automated Production Process of [ 64 Cu][Cu (ATSM)] for PET imaging and theranostic Applications

Cyclotron‐produced copper‐64 radioisotope tracers offer the possibility to perform both diagnostic investigation by positron emission tomography (PET) and radiotherapy by a theranostic approach with bifunctional chelators. The versatile chemical properties of copper add to the importance of this isotope in medicinal investigation. [⁶⁴Cu][Cu (ATSM)] has shown to be a viable candidate for imaging of tumor hypoxia; a critical tumor microenvironment characteristic that typically signifies tumor progression and resistance to chemo‐radiotherapy. Various production and radiosynthesis methods of [⁶⁴Cu][Cu (ATSM)] exist in labs, but usually involved non‐standardized equipment with varying production qualities and may not be easily implemented in wider hospital settings. [⁶⁴Cu][Cu (ATSM)] was synthesized on a modified GE TRACERlab FXN automated synthesis module. End‐of‐synthesis (EOS) molar activity of [⁶⁴Cu][Cu (ATSM)] was 2.2–5.5 Ci/μmol (HPLC), 2.2–2.6 Ci/μmol (ATSM‐titration), and 3.0–4.4 Ci/μmol (ICP‐MS). Radiochemical purity was determined to be >99% based on radio‐HPLC. The final product maintained radiochemical purity after 20 h. We demonstrated a simple and feasible process development and quality control protocols for automated cyclotron production and synthesis of [⁶⁴Cu][Cu (ATSM)] based on commercially distributed standardized synthesis modules suitable for PET imaging and theranostic studies.

Building up PtII -Thiosemicarbazone-Lysine-sC18 Conjugates

Three chiral tridentate N^N^S coordinating pyridine-carbaldehyde (S)-N4-(α-methylbenzyl)thiosemicarbazones (HTSCmB) were synthesised along with lysine-modified derivatives. One of them was selected and covalently conjugated to the cell-penetrating peptide sC18 by solid-phase peptide synthesis. The HTSCmB model ligands, the HTSCLp derivatives and the peptide conjugate rapidly and quantitatively form very stable PtII chlorido complexes [Pt(TSC)Cl] when treated with K2 PtCl4 in solution. The Pt(CN) derivatives were obtained from one TSCmB model complex and the peptide conjugate complex through Cl- →CN- exchange. Ligands and complexes were characterised by NMR, IR spectroscopy, HR-ESI-MS and single-crystal XRD. Intriguingly, no decrease in cell viability was observed when testing the biological activity of the lysine-tagged HdpyTSCLp, its sC18 conjugate HdpyTSCL-sC18 or the PtCl and Pt(CN) conjugate complexes in three different cell lines. Thus, given the facile and effective preparation of such Pt-TSC-peptide conjugates, these systems might pave the way for future use in late-stage labelling with Pt radionuclides and application in nuclear medicine.

Seesaw conformations of Npl4 in the human p97 complex and the inhibitory mechanism of a disulfiram derivative

p97, also known as valosin-containing protein (VCP) or Cdc48, plays a central role in cellular protein homeostasis. Human p97 mutations are associated with several neurodegenerative diseases. Targeting p97 and its cofactors is a strategy for cancer drug development. Despite significant structural insights into the fungal homolog Cdc48, little is known about how human p97 interacts with its cofactors. Recently, the anti-alcohol abuse drug disulfiram was found to target cancer through Npl4, a cofactor of p97, but the molecular mechanism remains elusive. Here, using single-particle cryo-electron microscopy (cryo-EM), we uncovered three Npl4 conformational states in complex with human p97 before ATP hydrolysis. The motion of Npl4 results from its zinc finger motifs interacting with the N domain of p97, which is essential for the unfolding activity of p97. In vitro and cell-based assays showed that the disulfiram derivative bis-(diethyldithiocarbamate)-copper (CuET) can bypass the copper transporter system and inhibit the function of p97 in the cytoplasm by releasing cupric ions under oxidative conditions, which disrupt the zinc finger motifs of Npl4, locking the essential conformational switch of the complex.

A bis(thiosemicarbazonato)-copper complex, a new catalyst for electro- and photo-reduction of CO2 to methanol

A bis(thiosemicarbazonato)-copper complex, a new catalyst for electro- and photo-reduction of CO₂ to methanol.

Synthesis and Characterisation of Indium(III) Bis-Thiosemicarbazone Complexes: 18F Incorporation for PET Imaging

Several structurally related indium chlorido complexes of bis-thiosemicarbazones were prepared, starting from the appropriately substituted bis-thiosemicarbazones, using sodium methoxide in methanol. Detailed NMR studies were conducted to assign the structure including COSY, HSQC, and HMBC techniques. The structures of all indium complexes were solved using single crystal X-ray diffraction. The chlorido ligand was present at the apex of the square pyramidal coordination sphere in all indium complexes. In some complexes, an intermolecular hydrogen bond was present between the chlorine atom and an NH group. Three different indium chlorido complexes were converted into the corresponding fluorido-derivative by a simple halide exchange method using K18F. These novel complexes, containing the positron emitting isotope 18F, may have potential applications in positron emission tomography (PET).

Transglutaminase-mediated conjugation and nitride-technetium-99m labelling of a bis(thiosemicarbazone) bifunctional chelator

An assessment study involving the use of the transglutaminase (TGase) conjugation method and the nitride-technetium-99m labelling on a bis(thiosemicarbazone) (BTS) bifunctional chelating agent is presented. The previously described chelator diacetyl-2-(N⁴-methyl-3-thiosemicarbazone)-3-(N⁴-amino-3-thiosemicarbazone), H₂ATSM/A, has been functionalized with 6-aminohexanoic acid (ε-Ahx) to generate the bifunctional chelating agent diacetyl-2-(N⁴-methyl-3-thiosemicarbazone)-3-[N⁴-(amino)-(6-aminohexanoic acid)-3-thiosemicarbazone], H₂ATSM/A-ε-Ahx (1), suitable for conjugation to glutamine (Gln) residues of bioactive molecules via TGase. The feasibility of the TGase reaction in the synthesis of a bioconjugate derivative was investigated using Substance P (SP) as model peptide. Compounds 1 and H₂ATSM/A-ε-Ahx-SP (2) were labelled with nitride-technetium-99m, obtaining the complexes [^99mTc][Tc(N)(ATSM/A-ε-Ahx)] (^99mTc1) and [^99mTc][Tc(N)(ATSM/A-ε-Ahx-SP)] (^99mTc2). The chemical identity of ^99mTc1 and ^99mTc2 was confirmed by radio/UV-RP-HPLC combined with ESI-MS analysis on the respective carrier-added products ^99g/99mTc1 and ^99g/99mTc2. The stability of the radiolabelled complexes after incubation in various environments was investigated. All the results were compared with those obtained for the corresponding ⁶⁴Cu-analogues, ⁶⁴Cu1 and ⁶⁴Cu2. The TGase reaction allows the conjugation of 1 with the peptide, but it is not highly efficient due to instability of the chelator in the required conditions. The SP-conjugated complexes are unstable in mouse and human sera. However, indeed the BTS system can be exploited as nitride-technetium-99m chelator for highly efficient technetium labelling, thus making compound 1 worthy of further investigations for new targeted technetium and copper radiopharmaceuticals encompassing Single Photon Emission Computed Tomography and Positron Emission Tomography imaging.

Copper complexes with dissymmetrically substituted bis(thiosemicarbazone) ligands as a basis for PET radiopharmaceuticals: control of redox potential and lipophilicity

Copper(ii) bis(thiosemicarbazone) derivatives have been used extensively in positron emission tomography (PET) to image hypoxia and blood flow and to radiolabel cells for cell tracking. These applications depend on control of redox potentials and lipophilicity of the bis(thiosemicarbazone) complexes, which can be adjusted by altering peripheral ligand substituents. This paper reports the synthesis of a library of new dissymmetrically substituted bis(thiosemicarbazone) ligands by controlling the condensation reactions between dicarbonyl compounds and 4-substituted-3-thiosemicarbazides or using acetal protection. Copper complexes of the new ligands have been prepared by reaction with copper acetate or via transmetallation of the corresponding zinc complexes, which are convenient precursors for the rapid synthesis of radio-copper complexes. Well-defined structure-activity relationships linking ligand alkylation patterns with redox potential and lipophilicity of the complexes are reported.

1-((E)-{2-[4-(2-{(1E)-[(carbamothioylamino)imino]methyl}phenoxy)butoxy]benzylidene}amino)thiourea dimethyl sulfoxide disolvate

The title compound, C20H24N6O2S2·2C2H6OS, has crystallographically imposed centrosymmetry. The packing is assisted by N—H...O, C—H...O and N—H...S interactions with the lattice solvent molecules, forming a two-dimensional network parallel to (1-10). The lattice dimethyl sulfoxide molecules (except for the S atoms) were modelled over two sites with refined occupancies of 0.831 (3):0.169 (3).

Applications of "Hot" and "Cold" Bis(thiosemicarbazonato) Metal Complexes in Multimodal Imaging

The applications of coordination chemistry to molecular imaging has become a matter of intense research over the past 10 years. In particular, the applications of bis(thiosemicarbazonato) metal complexes in molecular imaging have mainly been focused on compounds with aliphatic backbones due to the in vivo imaging success of hypoxic tumors with PET (positron emission tomography) using (64) CuATSM [copper (diacetyl-bis(N4-methylthiosemicarbazone))]. This compound entered clinical trials in the US and the UK during the first decade of the 21(st) century for imaging hypoxia in head and neck tumors. The replacement of the ligand backbone to aromatic groups, coupled with the exocyclic N's functionalization during the synthesis of bis(thiosemicarbazones) opens the possibility to use the corresponding metal complexes as multimodal imaging agents of use, both in vitro for optical detection, and in vivo when radiolabeled with several different metallic species. The greater kinetic stability of acenaphthenequinone bis(thiosemicarbazonato) metal complexes, with respect to that of the corresponding aliphatic ATSM complexes, allows the stabilization of a number of imaging probes, with special interest in "cold" and "hot" Cu(II) and Ga(III) derivatives for PET applications and (111) In(III) derivatives for SPECT (single-photon emission computed tomography) applications, whilst Zn(II) derivatives display optical imaging properties in cells, with enhanced fluorescence emission and lifetime with respect to the free ligands. Preliminary studies have shown that gallium-based acenaphthenequinone bis(thiosemicarbazonato) complexes are also hypoxia selective in vitro, thus increasing the interest in them as new generation imaging agents for in vitro and in vivo applications.

Nickel(ii) and copper(ii) complexes constructed with N2S2 hybrid benzamidine–thiosemicarbazone ligand: synthesis, X-ray crystal structure, DFT, kinetico-catalytic and in vitro biological applications

We report nickel(ii) and copper(ii) complexes containing the benzamidine–thiosemicarbazone ligand together with DFT, enzyme kinetics and in vitro biological applications such as DNA/BSA affinities and anticancer properties.

Synthesis and biological evaluation of copper(ii) pyrenethiosemicarbazone

A fluorescent Cu(ii) pyrenethiosemicarbazone complex exhibits enhanced DNA-cleavage and cytotoxicity on photoexcitation.

Synthesis and structures of 5-nitro-salicylaldehyde thiosemicarb-azonates of copper(ii): molecular spectroscopy, ESI-mass studies, antimicrobial activity and cytotoxicity

Salicylaldehyde thiosemicarbazonates of copper(ii) have shown significant growth inhibitory activity againstS. aureus, MRSA,K. pneumonia,S. flexneri,P. aeruginosaandC. albicansand are bactericidal in nature with low cytotoxicity.

Synthesis, spectral and electrochemical studies of binuclear Ru(III) complexes containing dithiosemicarbazone ligand

Synthesis of several new octahedral binuclear ruthenium(III) complexes of the general composition [(EPh3)2(X)Ru-L-Ru(X)(EPh3)2] containing benzene dithiosemicarbazone ligands (where E=P or As; X=Cl or Br; L=binucleating ligands) is presented. All the complexes have been fully characterized by elemental analysis, FT-IR, UV-vis and EPR spectroscopy together with magnetic susceptibility measurements. IR study shows that the dithiosemicarbazone ligands behave as dianionic tridentate ligands coordinating through the oxygen atom of the deprotonated phenolic group, nitrogen atom of the azomethine group and thiolate sulphur. In DMF solution, all the complexes exhibit intense d-d transition and ligand-to-metal charge transfer (LMCT) transition in the visible region. The magnetic moment values of the complexes are in the range 1.78-1.82 BM, which reveals the presence of one unpaired electron on each metal ion. The EPR spectra of the liquid samples at LNT show the presence of three different 'g' values (gx≠gy≠gz) indicate a rhombic distortion around the ruthenium ion. All the complexes exhibit two quasi-reversible one electron oxidation responses (Ru(III)-Ru(III)/Ru(III)-Ru(IV); Ru(III)-Ru(IV)/Ru(IV)-Ru(IV)) within the E1/2 range of 0.61-0.74 V and 0.93-0.98 V respectively, versus Ag/AgCl.

The development of copper radiopharmaceuticals for imaging and therapy

The increasing use of positron emission tomography in preclinical and clinical settings has widened the demand for radiopharmaceuticals with high specificity that can image biological phenomena in vivo. While many PET tracers have been developed from small organic molecules labeled with carbon-11 or fluorine-18, the short half-lives of these radionuclides preclude their incorporation into radiotracers, which can be used to image biological processes that are not induced immediately after system perturbation. Additionally, the continuing development of targeted agents, such as antibodies and nanoparticles, which undergo extended circulation, require that radionuclides with half-lives that are complimentary to the biological half-lives of these molecules be developed. Copper radionuclides have received considerable attention since they offer a variety of half-lives and decay energies and because the coordination chemistry of cooper and its role in biology is well understood. However, in addition to the radiometal chelate, a successful copper based radiopharmaceutical depends upon the chemical structure of the entire radiotracer, which may include a biologically important molecule and a chemical linker that can be used to deliver the copper radionuclide to a specific target and modulate its in vivo properties, respectively. This review discusses the development of copper radiopharmaceuticals and the importance of factors such as chemical structure on their pharmacokinetics in vivo.

Copper complexes of bis(thiosemicarbazones): from chemotherapeutics to diagnostic and therapeutic radiopharmaceuticals

The molecules known as bis(thiosemicarbazones) derived from 1,2-diones can act as tetradentate ligands for Cu(II), forming stable, neutral complexes. As a family, these complexes possess fascinating biological activity. This critical review presents an historical perspective of their progression from potential chemotherapeutics through to more recent applications in nuclear medicine. Methods of synthesis are presented followed by studies focusing on their potential application as anti-cancer agents and more recent investigations into their potential as therapeutics for Alzheimer's disease. The Cu(II) complexes are of sufficient stability to be used to coordinate copper radioisotopes for application in diagnostic and therapeutic radiopharmaceuticals. Detailed understanding of the coordination chemistry has allowed careful manipulation of the metal based properties to engineer specific biological activities. Perhaps the most promising complex radiolabelled with copper radioisotopes to date is Cu(II)(atsm), which has progressed to clinical trials in humans (162 references).