Design and synthesis of site directed maleimide bifunctional chelators for technetium and rhenium

A thiourea-bridged 99mTc(CO)3-dipicolylamine-2-nitroimidazole complex for targeting tumor hypoxia: Utilizing metabolizable thiourea-bridge to improve pharmacokinetics

The 2-nitroimidazole based 99mTc-radiopharmaceuticals are widely explored for imaging tumor hypoxia. Radiopharmaceuticals for targeting hypoxia are often lipophilic and therefore, show significant uptake in liver and other vital organs. In this context, lipophilic radiopharmaceuticals with design features enabling faster clearance from liver may be more desirable. A dipicolylamine-NCS bifunctional chelator that could generate a thiourea-bridge up on conjugation to primary amine bearing molecule was used to synthesize a 2-nitroimidazole-dipicolyl amine ligand for radiolabeling with 99mTc(CO)3 core. Corresponding Re(CO)3-analogue was prepared to establish the structure of 2-nitroimidazole-99mTc(CO)3 complex prepared in trace level. The 2-nitroimidazole-99mTc(CO)3 complex showed a hypoxic to normoxic ratio of ~2.5 in CHO cells at 3 h. In vivo, the complex showed accumulation and retention in tumor with high tumor to blood and tumor to muscle ratio. The study demonstrated the utility of metabolizable thiourea-bridge in 2-nitroimidazole-99mTc(CO)3 complex in inducing faster clearance of the radiotracer from liver. The dipicolylamine-NCS bifunctional chelator reported herein can also be used for radiolabeling other class of target specific molecules with 99mTc(CO)3 core.

Evaluation of oxygen-containing pentadentate ligands with pyridine/quinoline/isoquinoline binding sites via the structural and electrochemical properties of mononuclear copper(II) complexes

Eighteen mononuclear copper(II) complexes with oxygen-containing N4O1 pentadentate ligands were prepared. The ligand library consists of 2-aminoethanol derivatives ((Ar1CH2)(Ar2CH2)NCH2CH2OCH2Ar3) bearing three nitrogen-containing heteroaromatics (Ars) including pyridine, quinoline and isoquinoline via a methylene linker. Systematic replacements of pyridine binding sites with quinolines and isoquinolines reveal the general trends in the perturbation of bond distances and angles, the redox potential and the absorption maximum wavelength of the copper(II) complexes, depending on the position and number of (iso)quinoline heteroaromatics. The small effect on the redox potentials resulting from quinoline substitution at the Ar3 position (near oxygen) of the ligand comes from the steric hindrance of the peri hydrogen atom in the quinoline moiety at this position, which removes the counter anion to enhance the coordination of quinoline nitrogen and ether oxygen atoms to the metal centre. In the absorption spectra of copper(II) complexes in the d-d transition region, the quinoline substitution at this site (Ar3) exhibits an opposite effect to those at the Ar1 and Ar2 sites. The electronic and steric contributions of the heteroaromatic binding sites to the ligand properties are comprehensively discussed.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

Development of 99mTc-Labeled Human Serum Albumin with Prolonged Circulation by Chelate-then-Click Approach: A Potential Blood Pool Imaging Agent

Technetium-99m-labeled human serum albumin (^99mTc-HSA) has been utilized as a blood pool imaging agent in the clinic for several decades. However, ^99mTc-HSA has a short circulation time, which is a critical shortcoming for a blood pool imaging agent. Herein, we developed a novel ^99mTc-labeled HSA with a long circulation time using click chemistry and a chelator, 2,2'-dipicolylamine (DPA), (^99mTc-DPA-HSA). Specifically, we examined the feasibility of copper-free strain-promoted alkyne-azide cycloaddition (SPAAC) for the incorporation of HSA to the [^99mTc (CO)₃(H₂O)₃]⁺ system by adopting a chelate-then-click approach. In this strategy, a potent chelate system, azide-functionalized DPA, was first complexed with [^99mTc (CO)₃(H₂O)₃]⁺, followed by the SPAAC click reaction with azadibenzocyclooctyne-functionalized HSA (ADIBO-HSA) under biocompatible conditions. Radiolabeling efficiency of azide-functionalized DPA (^99mTc-DPA) was >98%. Click conjugation efficiency of ^99mTc-DPA with ADIBO-HSA was between 76 and 99% depending on the number of ADIBO moieties attached to HSA. In whole-body in vivo single photon emission computed tomography images, the blood pool uptakes of ^99mTc-DPA-HSA were significantly enhanced compared to those of ^99mTc-HSA at 10 min, 2, and 6 h after the injection ( P < 0.001, 0.025, and 0.003, respectively). Furthermore, the blood activities of ^99mTc-DPA-HSA were 8 times higher at 30 min and 10 times higher at 3 h after the injection compared to those of conventional ^99mTc-HSA in ex vivo biodistribution experiment. The results exhibit the potential of ^99mTc-DPA-HSA as a blood pool imaging agent and further illustrate the promise of the pre-labeling SPAAC approach for conjugation of heat-sensitive biological targeting vectors with [^99mTc (CO)₃(H₂O)₃]⁺.

Graftable SCoMPIs enable the labeling and X-ray fluorescence imaging of proteins

Bio-imaging techniques alternative to fluorescence microscopy are gaining increasing interest as complementary tools to visualize and analyze biological systems. Among them, X-ray fluorescence microspectroscopy provides information on the local content and distribution of heavy elements (Z ≥ 14) in cells or biological samples. In this context, similar tools to those developed for fluorescence microscopy are desired, including chemical probes or tags. In this work, we study rhenium complexes as a convenient and sensitive probe for X-ray fluorescence microspectroscopy. We demonstrate their ability to label and sense exogenously incubated or endogenous proteins inside cells.

Thiourea derivatives as chelating agents for bioconjugation of rhenium and technetium

A^99mTc complex with a tetradentate thiocarbamoylbenzamidine group was used for the conjugation of angiotensin-II. The resulting bioconjugate is stablein vivoandin vitro.

Isothiocyanate-Functionalized Bifunctional Chelates and fac-[M(I)(CO)3](+) (M = Re, (99m)Tc) Complexes for Targeting uPAR in Prostate Cancer

Developing new strategies to rapidly incorporate the fac-[M(I)(CO)3](+) (M = Re, (99m)Tc) core into biological targeting vectors in radiopharmaceuticals continues to expand as molecules become more complex and as efforts to minimize nonspecific binding increase. This work examines a novel isothiocyanate-functionalized bifunctional chelate based on 2,2'-dipicolylamine (DPA) specifically designed for complexing the fac-[M(I)(CO)3](+) core. Two strategies (postlabeling and prelabeling) were explored using the isothiocyanate-functionalized DPA to determine the effectiveness of assembly on the overall yield and purity of the complex with amine containing biomolecules. A model amino acid (lysine) examined (1) amine conjugation of isothiocyanate-functionalized DPA followed by complexation with fac-[M(I)(CO)3](+) (postlabeling) and (2) complexation of fac-[M(I)(CO)3](+) with isothiocyanate-functionalized DPA followed by amine conjugation (prelabeling). Conducted with stable Re and radioactive (99m)Tc analogs, both strategies formed the product in good to excellent yields under macroscopic and radiotracer concentrations. A synthetic peptide (AE105) which targets an emerging biomarker in CaP prognosis, urokinase-type plasminogen activator receptor (uPAR), was also explored using the isothiocyanate-functionalized DPA strategy. In vitro PC-3 (uPAR+) cell uptake assays with the (99m)Tc-labeled peptide (8a) showed 4.2 ± 0.5% uptake at 4 h. In a murine model bearing PC-3 tumor xenografts, in vivo biodistribution of 8a led to favorable tumor uptake (3.7 ± 0.7% ID/g) at 4 h p.i. with relatively low accumulation (<2% ID/g) in normal organs not associated with normal peptide excretion. These results illustrate the promise of the isothiocyanate-functionalized approach for labeling amine containing biological targeting vectors with fac-[M(I)(CO)3](+).

Synthesis of a new HYNIC-DAPI derivative for labelling with 99mTechnetium and its in vitro evaluation in an FRTL5 cell line

A new multifunctional compound that includes the fluorescent dye 4′,6-diamidine-2-phenylindole (DAPI) and the chelator 6-hydrazinonicotinic acid (HYNIC) was developed and radiolabelled with ^99mTc for in vitro evaluation in an FRTL5 cell line.

Underestimated potential of organometallic rhenium complexes as anticancer agents

In the recent years, organometallic compounds have become recognized as promising anti-cancer drug candidates. While radioactive (186/188)Re compounds are already used in clinics for cancer treatment, cold Re organometallic compounds have mostly been explored as luminescent probes for cell imaging and photosensitizers in photocatalysis. However, a growing number of studies have recently revealed the potential of Re organometallic complexes as anti-cancer agents. Several compounds have displayed cytotoxicity equaling or exceeding that of the well-established anti-cancer drug cisplatin. In this review, we present the currently known Re organometallic complexes that have shown anti-proliferative activity on cancer cell lines. A particular emphasis is placed on their cellular uptake and localization as well as their potential mechanism of action.

Towards cancer cell-specific phototoxic organometallic rhenium(I) complexes

Over the recent years, several Re(I) organometallic compounds have been shown to be toxic to various cancer cell lines. However, these compounds lacked sufficient selectivity towards cancer tissues to be used as novel chemotherapeutic agents. In this study, we probe the potential of two known N,N-bis(quinolinoyl) Re(I) tricarbonyl complex derivatives, namely Re(I) tricarbonyl [N,N-bis(quinolin-2-ylmethyl)amino]-4-butane-1-amine (Re-NH₂) and Re(I) tricarbonyl [N,N-bis(quinolin-2-ylmethyl)amino]-5-valeric acid (Re-COOH), as photodynamic therapy (PDT) photosensitizers. Re-NH₂ and Re-COOH proved to be excellent singlet oxygen generators in a lipophilic environment with quantum yields of about 75%. Furthermore, we envisaged to improve the selectivity of Re-COOH via conjugation to two types of peptides, namely a nuclear localization signal (NLS) and a derivative of the neuropeptide bombesin, to form Re-NLS and Re-Bombesin, respectively. Fluorescent microscopy on cervical cancer cells (HeLa) showed that the conjugation of Re-COOH to NLS significantly enhanced the compound's accumulation into the cell nucleus and more specifically into its nucleoli. Importantly, in view of PDT applications, the cytotoxicity of the Re complexes and their bioconjugates increased significantly upon light irradiation. In particular, Re-Bombesin was found to be at least 20-fold more toxic after light irradiation. DNA photo-cleavage studies demonstrated that all compounds damaged DNA via singlet oxygen and, to a minor extent, superoxide production.

Clickable, hydrophilic ligand for fac-[M(I)(CO)3](+) (M = Re/(99m)Tc) applied in an S-functionalized α-MSH peptide

The copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction was used to incorporate alkyne-functionalized dipicolylamine (DPA) ligands (1 and 3) for fac-[M^I(CO)₃]⁺ (M = Re/^99mTc) complexation into an α-melanocyte stimulating hormone (α-MSH) peptide analogue. A novel DPA ligand with carboxylate substitutions on the pyridyl rings (3) was designed to increase the hydrophilicity and to decrease in vivo hepatobiliary retention of fac-[^99mTc^I(CO)₃]⁺ complexes used in single photon emission computed tomography (SPECT) imaging studies with targeting biomolecules. The fac-[Re^I(CO)₃(3)] complex (4) was used for chemical characterization and X-ray crystal analysis prior to radiolabeling studies between 3 and fac-[^99mTc^I(OH₂)₃(CO)₃]⁺. The corresponding ^99mTc complex (4a) was obtained in high radiochemical yields, was stable in vitro for 24 h during amino acid challenge and serum stability assays, and showed increased hydrophilicity by log P analysis compared to an analogous complex with nonfunctionalized pyridine rings (2a). An α-MSH peptide functionalized with an azide was labeled with fac-[M^I(CO)₃]⁺ using both click, then chelate (CuAAC reaction with 1 or 3 followed by metal complexation) and chelate, then click (metal complexation of 1 and 3 followed by CuAAC with the peptide) strategies to assess the effects of CuAAC conditions on fac-[M^I(CO)₃]⁺ complexation within a peptide framework. The peptides from the click, then chelate strategy had different HPLC t_R’s and in vitro stabilities compared to those from the chelate, then click strategy, suggesting nonspecific coordination of fac-[M^I(CO)₃]⁺ using this synthetic route. The fac-[M^I(CO)₃]⁺-complexed peptides from the chelate, then click strategy showed >90% stability during in vitro challenge conditions for 6 h, demonstrated high affinity and specificity for the melanocortin 1 receptor (MC1R) in IC₅₀ analyses, and led to moderately high uptake in B16F10 melanoma cells. Log P analysis of the ^99mTc-labeled peptides confirmed the enhanced hydrophilicity of the peptide bearing the novel, carboxylate-functionalized DPA chelate (10a′) compared to the peptide with the unmodified DPA chelate (9a′). In vivo biodistribution analysis of 9a′ and 10a′ showed moderate tumor uptake in a B16F10 melanoma xenograft mouse model with enhanced renal uptake and surprising intestinal uptake for 10a′ compared to predominantly hepatic accumulation for 9a′. These results, coupled with the versatility of CuAAC, suggests this novel, hydrophilic chelate can be incorporated into numerous biomolecules containing azides for generating targeted fac-[M^I(CO)₃]⁺ complexes in future studies.

tert-ButylN-{[5-(5-oxohexanamido)pyridin-2-yl]amino}carbamate

In the crystal structure of the title compound, C₁₆H₂₄N₄O₄, mol­ecules are linked by N—H⋯O hydrogen bonds between the carbonyl groups of the carbamoyl and amido functional groups and the amino groups, and by N—H⋯N hydrogen bonds between the amino group and the pyridine ring, forming two-dimensional networks parallel to the ab plane.

fac-Re(CO)3 complexes of 2,6-bis(4-substituted-1,2,3-triazol-1-ylmethyl)pyridine "click" ligands: synthesis, characterisation and photophysical properties

The syntheses of the 4-n-propyl and 4-phenyl substituted fac-Re(CO)(3) complexes of the tridentate "click" ligand (2,6-bis(4-substituted-1,2,3-triazol-1-ylmethyl)pyridine) are described. The complexes were obtained by refluxing methanol solutions of [Re(CO)(5)Cl], AgPF(6) and either the 4-propyl or 4-phenyl substituted ligand for 16 h. The ligands and the two rhenium(I) complexes were characterised by elemental analysis, HR-ESMS, ATR-IR, (1)H and (13)C NMR spectroscopy and the molecular structures of both complexes were confirmed by X-ray crystallography. The electronic structure of the fac-Re(CO)(3) "click" complexes was probed using UV-Vis, Raman and emission spectroscopy, cyclic voltammetry and DFT calculations. Altering the electronic nature of the ligand's substituent, from aromatic to alkyl, had little effect on the absorption/emission maxima and electrochemical properties of the complexes indicating that the 1,2,3-triazole unit may insulate the metal centre from the electronic modification at the ligands' periphery. Both Re(I) complexes were found to be weakly emitting with short excited state lifetimes. The electrochemistry of the complexes is defined by quasi-reversible Re oxidation and irreversible triazole-based ligand reduction processes.

Photo-initiated thiol-ene click reactions as a potential strategy for incorporation of [M(I)(CO)3]+ (M = Re, (99m)Tc) complexes

Click reactions offer a rapid technique to covalently assemble two molecules. In radiopharmaceutical construction, these reactions can be utilized to combine a radioactive metal complex with a biological targeting molecule to yield a potent tool for imaging or therapy applications. The photo-initiated radical thiol-ene click reaction between a thiol and an alkene was examined for the incorporation of [M(I)(CO)3](+) (M = Re, (99m)Tc) systems for conjugating biologically active targeting molecules containing a thiol. In this strategy, a potent chelate system, 2,2'-dipicolylamine (DPA), for [M(I)(CO)3](+) was functionalized at the central amine with a terminal alkene linker that was explored with two synthetic approaches, click then chelate and chelate then click, to determine the flexibility and applicability of the thiol-ene click reaction to specifically incorporate ligand systems and metal complexes with a thiol containing molecule. In the click then chelate approach, the thiol-ene click reaction was carried out with the DPA chelate followed by complexation with [M(I)(CO)3](+). In the chelate then click approach, the alkene functionalized DPA chelate was first complexed with [M(I)(CO)3](+) followed by the conduction of the thiol-ene click reaction. Initial studies utilized benzyl mercaptan as a model thiol for both strategies to generate the identical product from either route to provide information on reactivity and product formation. DPA ligands functionalized with two unique linker systems (allyl and propyl allyl ether) were prepared to examine the effect of the proximity of the chelate or complex on the thiol-ene click reaction. Both the thiol-ene click and coordination reactions with Re, (99m)Tc were performed in moderate to high yields demonstrating the potential of the thiol-ene click reaction for [M(I)(CO)3](+) incorporation into thiol containing biomolecules.

Multimodal radio- (PET/SPECT) and fluorescence imaging agents based on metallo-radioisotopes: current applications and prospects for development of new agents

This perspective focuses on complexes of radioactive metal ions applied in multimodal radio- and optical imaging. The application of metal ions in radioimaging techniques such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) has advantages over lighter nuclei in terms of half-life, but there are particular issues related to their speciation (particularly leaching from complexes) and with the combination of certain ions with fluorescent systems. The basic coordination chemistry of the ions involved and issues relating to biological conditions and their compatibility with optical imaging techniques are reviewed, the current literature presented in context, and the prospect of exploiting the intrinsic luminescence of certain metal-ligand complexes is discussed.

Preparation, 99mTc-labeling and biodistribution studies of a PNA oligomer containing a new ligand derivative of 2,2'-dipicolylamine

A new azido derivative of 2,2'-dipicolylamine (Dpa), 2-azido-N,N-bis((pyridin-2-yl)methyl)ethanamine, (Dpa-N(3)) was readily prepared from the known 2-(bis(pyridin-2-ylmethyl)amino)ethanol (Dpa-OH). It was demonstrated that Dpa-N(3) could be efficiently labeled with both [Re(CO)(3)(H(2)O)(3)]Br and [(99m)Tc(H(2)O)(3)(CO)(3)](+) to give [Re(CO)(3)(Dpa-N(3))]Br and [(99m)Tc(CO)(3)(Dpa-N(3))](+), respectively. Furthermore, Dpa-N(3) was successfully coupled, on the solid phase, to a Peptide Nucleic Acid (PNA) oligomer (H-4-pentynoic acid-spacer-spacer-tgca-tgca-tgca-Lys-NH(2); spacer= -NH-(CH(2))(2)-O-(CH(2))(2)-O-CH(2)-CO-) using the Cu(I)-catalyzed [2+3] azide/alkyne cycloaddition (Cu-AAC, often referred to as the prototypical "click" reaction) to give the Dpa-PNA oligomer. Subsequent labeling of Dpa-PNA with [(99m)Tc(H(2)O)(3)(CO)(3)](+) afforded [(99m)Tc(CO)(3)(Dpa-PNA)] in radiochemical yields >90%. Partitioning experiments in a 1-octanol/water system were carried out to get more insight on the lipophilicity of [(99m)Tc(CO)(3)(Dpa-N(3))](+) and [(99m)Tc(CO)(3)(Dpa-PNA)]. Both compounds were found rather hydrophilic (log D(o/w) values at pH=7.4 are -0.50: [(99m)Tc(CO)(3)(Dpa-N(3))](+) and -0.85: [(99m)Tc(CO)(3)(Dpa-PNA)]. Biodistribution studies of [(99m)Tc(CO)(3)(Dpa-PNA)] in Wistar rats showed a very fast blood clearance (0.26 ± 0.1 SUV, 1h p.i.) and modest accumulation in the kidneys (5.45 ± 0.45 SUV, 1h p.i.). There was no significant activity in the thyroid and the stomach, demonstrating a high in vivo stability of the (99m)Tc-labeled Dpa-PNA conjugate.

99m-Technetium carbohydrate conjugates as potential agents in molecular imaging

This feature article covers recent reports of work towards the development of (99m)Tc-carbohydrate based agents for use in SPECT imaging, particularly of cancerous tissue. An outline of some of the key biological functions and coordination chemistry of carbohydrates is given, along with an introduction to bioconjugation and molecular imaging. Technetium coordination chemistry and the subset of this involving bioconjugates are discussed before moving into the focus of the article: glycoconjugates of (99m)Tc(v) and the more recently developed [(99m)Tc(I)(CO)(3)](+). Significant work in the last decade has featured the very attractive [(99m)Tc(CO)(3)](+) core, and the ligand sets designed for this core are discussed in detail.

Cysteine-Specific, Covalent Anchoring of Transition Organometallic Complexes to the Protein Papain fromCarica papaya

Site-directed and covalent introduction of various transition metal-organic entities to the active site of the cysteine endoproteinase, papain, was achieved by treatment of this enzyme with a series of organometallic maleimide derivatives specially designed for the purpose. Kinetic studies made it clear that time-dependent irreversible inactivation of papain occurred in the presence of these organometallic maleimides as a result of Michael addition of the sulfhydryl of Cys25. The rate and mechanism of inactivation were highly dependent on the structure of the organometallic entity attached to the maleimide group. Combined ESI-MS and IR analysis indicated that all the resulting papain adducts contained one organometallic moiety per protein molecule. This confirmed that chemospecific introduction of the metal complexes was indeed achieved. Thus, three novel reagents for heavy-atom derivatization of protein crystals, which include ruthenium, rhenium and tungsten, are now available for the introduction of electron-dense scatterers for phasing of X-ray crystallographic data.

One-pot syntheses, coordination, and characterization of application-specific biodegradable ligand-polymers

Syntheses and chelation of tailored biodegradable polymers to rhenium for medicinal applications are described. A group of bifunctional ligand-initiators consisting of a chelating end for metal complexation and a hydroxyl end suitable to initiate polymerization was utilized in the ring-opening polymerization of l-lactide. The resulting biodegradable ligand-polymers were equipped with a tridentate donor set to coordinate specific metal ions. All synthesized compounds were characterized by IR spectroscopy, 1D/2D NMR spectroscopy and MALDI-TOF mass spectrometry, confirming successful polymerization and coordination to the [Re(CO)(3)](+) core. The pliability of designing application-specific polymers with respect to the nature of the metal ion facilitates extending the application of these biodegradable polymers to early detection of diseases (imaging) and radiotherapy of cancers.