Recent advances on technetium complexes: coordination chemistry and medical applications¶

Ab Initio Speciation of Tc-Gluconate Complexes in Aqueous Systems

Tc-gluconate complexes in aqueous systems were recently reported and characterized by Tc L3-edge X-ray absorption near-edge structure (XANES) measurements [Dardenne, K.; Inorg. Chem. 2021, 60, 12285-12298]. The puzzling result was reported that the Tc L3-edge XANES of the sample containing Tc(IV)-gluconate species differs substantially from that of the Tc(IV)O2(am,hyd) hydrous oxide reference sample, whereas the Tc K-edge XANES spectra did not differ significantly. We studied this observation theoretically and tracked the unknown Tc(IV)-gluconate species in a three-step procedure: (1) developing chemical models, (2) optimizing the equilibrium structures of the models, and (3) simulating the corresponding Tc L3-edge XANES spectra. We identified the [Tc(IV)(Glu-2H)2(H2O)2]2- structure as the most likely Tc(IV)-gluconate species present in our samples and explain the substantial difference between the two Tc L3-edge XANES spectra. Additionally, we revisited the Tc(V)-gluconate species and identified the [Tc(V)O(Glu-H)2]- structure as the most likely Tc(V)-gluconate species in our sample.

99mTc(I)-Labeled His-Tagged Proteins: Impact in the Development of Novel Imaging Probes and in Drug Discovery

Technetium-99 m (99mTc) remains the cornerstone of nuclear medicine for single photon emission computed tomography (SPECT) due to its widespread availability and chemical and physical features. Its multiple oxidation states allow for the design and production of radiopharmaceuticals with versatile properties, namely in terms of pharmacokinetic profile. 99mTc(V) is the most common oxidation state, but 99mTc(I) gained traction after the pioneering work of Alberto and colleagues, which resulted in the introduction of the organometallic core fac-[99mTc(CO)3(H2O)3]+. This core is readily available from [99mTcO4]- and displays three labile water molecules that can be easily swapped for ligands with different denticity and/or donor atoms in aqueous environment. This makes it possible to radiolabel small molecules as well as high molecular weight molecules, such as antibodies or other proteins, while assuring biological activity. Direct radiolabelling of those proteins with fac-[99mTc(CO)3]+ under mild conditions is accomplished through incorporation of a polyhistidine tag (His-tag), a commonly used tag for purification of recombinant proteins. This review aims to address the direct radiolabelling of His-tagged macromolecules with fac-[99mTc(CO)3]+ for development of molecular imaging agents and the impact of this technology in the discovery and development of imaging and/or therapeutic agents towards clinical application.

Synthesis, 99mTc-labeling, in-vivo study and in-silico investigation of 6-amino-5-[(bis-(2-hydroxy-ethyl)-amino]methyl]2-methyl pyrimidin-4-ol as a potential probe for tumor targeting

This study aimed to synthesize a new pyrimidine derivative with a good synthesis yield of 87% to act as a new cancer marker after radiolabeling with Tc-99m in a high radiochemical yield of 92.3%. In-vivo study in tumor-bearing Swiss albino mice model revealed promising data with high uptake in cancer. Docking study showed good binding interactions of the radiosynthesized complex at the binding site. In conclusion, this novel complex could be a potential probe for cancer targeting.

A Combined Study of Tc Redox Speciation in Complex Aqueous Systems: Wet-Chemistry, Tc K-/L3-Edge X-ray Absorption Fine Structure, and Ab Initio Calculations

The combination of wet-chemistry experiments (measurements of pH, E_h, and [Tc]) and advanced spectroscopic techniques (K- and L₃-edge X-ray absorption fine structure spectroscopy) confirms the formation of a very stable Tc(V)-gluconate complex under anoxic conditions. In the presence of gluconate and an excess of Sn(II) (at pe + pH ≈ 2), technetium forms a very stable Tc(IV)-gluconate complex significantly enhancing the solubility defined by TcO₂(s) in hyperalkaline gluconate-free systems. A new setup for "tender" X-ray spectroscopy (spectral range, ∼2-5 keV) in transmission or total fluorescence yield detection mode based on a He flow cell has been developed at the INE Beamline for radionuclide science (KIT light source). This setup allows handling of radioactive specimens with total activities up to one million times the exemption limit. For the first time, Tc L₃-edge measurements (∼2.677 keV) of Tc species in liquid (aqueous) media are reported, clearly outperforming conventional K-edge spectroscopy as a tool to differentiate Tc oxidation states and coordination environments. The coupling of L₃-edge X-ray absorption near-edge spectroscopy measurements and relativistic multireference ab initio methods opens new perspectives in the definition of chemical and thermodynamic models for systems of relevance in the context of nuclear waste disposal, environmental, and pharmaceutical applications.

Technetium Nitrido-Peroxo Complexes: An Unexplored Class of Coordination Compounds

The purpose of this work was to further expand the chemistry of mixed technetium nitrido-peroxo complexes, a still poorly explored class of compounds containing the Tc(VII) moiety, [99gTc][Tc(N)(O2)2]. A number of novel complexes of the formula [99gTc][Tc(N)(O2)2(L)] with bidentate ligands (L) (where L = deprotonated alanine, glycine, proline) were prepared by reacting a solution of nitrido-technetic(VI) acid with L in the presence of a source of H2O2. Alternatively, the complex [99gTc][Tc(N)(O2)2X]− (X = Cl, Br) was used as a precursor for substitution reactions where the halogenide ion was replaced by the bidentate ligand. The new complexes were characterized by elemental analysis and mass spectroscopy. The preparation of the analogous [99mTc][Tc(N)(O2)2] moiety, radiolabeled with the metastable isomer Tc-99m, was also studied at a no-carrier-added level, using S-methyl-N-methyl-dithiocarbazate as the donor of the nitrido nitrogen atoms.

Preparation and evaluation of 99mTc-labeled porphyrin complexes prepared using PNP and HYNIC cores: studying the effects of core selection on pharmacokinetics and tumor uptake in a mouse model

Porphyrins are tetrapyrrolic macrocyclic ligands known for their affinity towards neoplastic tissues and once radiolabeled with a suitable diagnostic radioisotope could potentially be used for the imaging of tumorous lesions. In the present study, an unsymmetrically substituted porphyrin derivative namely 5-(p-amino-propyloxyphenyl)-10,15,20-tris(carboxymethyleneoxyphenyl)-porphyrin was synthesized and modified further to enable radiolabeling with ^99mTc using two different ^99mTc-cores viz. ^99mTc-HYNIC (hydrazino nicotinic acid) and ^99mTc(N)PNP2 (PNP2 = bis-[(2-dimethylphosphino)ethyl]-methoxy-ethylamine) in order to study the effect of employing different ^99mTc-cores on tumor affinity and pharmacokinetic behavior of the resultant ^99mTc-labeled porphyrin complexes. ^99mTc-Porphyrin complexes were characterized by reversed phase HPLC studies and could be prepared with >95% radiochemical purity under optimized radiolabeling conditions. Both ^99mTc-complexes were found to be adequately stable in human blood serum till 3 h post-preparation. Bio-distribution studies, carried out in Swiss mice bearing fibrosarcoma tumors, revealed relatively higher tumor uptake for the ^99mTc-HYNIC-porphyrin complex (3.95 ± 1.42 and 3.28 ± 0.27% IA per g) compared to that exhibited by the ^99mTc(N)PNP-DTC-porphyrin complex (1.52 ± 0.53 and 1.56 ± 0.10% IA per g) at 1.5 and 3 h post-administration, although the former complex exhibited comparatively lower lipophilicity in the octanol-water system. Higher uptake and longer retention in the blood were observed for the ^99mTc-HYNIC-porphyrin complex (6.63 ± 0.75 and 4.36 ± 0.25% IA per g) compared to that exhibited by the ^99mTc(N)PNP-DTC-porphyrin complex (2.41 ± 0.54 and 2.30 ± 0.16% IA per g) at both 1.5 and 3 h post-administration. However, relatively lower liver uptake was observed for the former complex (19.26 ± 3.48 and 18.45 ± 1.05% IA per g) than that exhibited by the latter one (39.37 ± 3.88 and 34.15 ± 8.25% IA per g) at both 1.5 and 3 h post-administration. This study indicates that the in vivo behavior exhibited by the ^99mTc-labeled porphyrins not only depends on their lipophilicity/hydrophilicity but is also governed by the Tc-cores employed for radiolabeling.

99m Tc-labeled oligomeric nanoparticles as potential agents for folate receptor-positive tumor targeting

One type of biocompatible nanoparticles functionalized with folate and ^99m Tc was successfully synthesized. Maleimide-folic acid (Mal-FA) was selected to covalently conjugate with -SH of the nanoparticles (NPs) to prepare NPs-FA for targeting. ^99m Tc was selected to conjugate with -NH₂ and -SH groups of cysteine residues on the surface of NPs to prepare NPs-FA-^99m Tc for radioactive counting. The ability to target folate receptors of NPs-FA-^99m Tc was assessed in uptake studies with folate-receptor-positive human HepG2 cells. The results showed that the as-prepared NPs can selectively uptake by folate receptor-overexpressing HepG2 tumor cells in vitro. The oligomeric hybrid NPs radiolabeled with ^99m Tc may develop to be SPECT/CT imaging biomaterials with high selectivity.

Fluorine- and rhenium-containing geldanamycin derivatives as leads for the development of molecular probes for imaging Hsp90

Heat shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone responsible for protein quality control in cells. Hsp90 has been shown to be overexpressed in many human cancers. This has prompted extensive research on Hsp90 inhibitors as novel anticancer agents and, more recently, the development of molecular probes for imaging Hsp90 expression in vivo. This work describes the development of various fluorine-containing and rhenium-containing geldanamycin derivatives as leads for the development of corresponding (18)F-labeled and (99m)Tc-labeled PET and SPECT probes for molecular imaging of Hsp90 expression. All compounds were evaluated in an in vitro ATPase activity assay using Hsp90 isoform Hsp82p. Fluorobenzoylated geldanamycin derivative 5 displayed comparable inhibitory potency like parent compound geldanamycin.

Hypoxia-induced redox alterations and their correlation with 99mTc-MIBI and 99mTc-HL-91 uptake in colon cancer cells

Colorectal cancer is one of the most common malignancies in the Western world and is an example of a solid tumour in which hypoxia is a common feature and develops because of the inability of the vascular system to supply adequate amounts of oxygen to growing tumours. Hypoxia effects on tumour cell biology can be detected and characterized using different methods. The use of imaging with gamma-emitting radionuclides to detect hypoxic tissue was first suggested by Chapman in 1979 [N Engl J Med 301 (1979) 1429-1432]. (99m)Tc-4,9-diaza-3,3,10,10-tetramethyldodecan-2,11-dione dioxime, also known as (99m)Tc-HL-91, has been among the most studied hypoxia markers. The objective of this study was to correlate the uptake of (99m)Tc-HL-91 and (99m)Tc-MIBI in colon cancer cells under normoxic and hypoxic conditions and to compare this information with some parameters such as oxidative stress and mitochondrial dysfunction of the cells analyzed by flow cytometry. Our results show that the in vitro (99m)Tc-HL-91 uptake is higher in hypoxic conditions, which is confirmed by the decreased uptake of (99m)Tc-MIBI. Flow cytometry results demonstrate that hypoxic conditions used are not enough to induce cellular death, but are responsible for the alterations in the intracellular redox environment, namely, increase of ROS production, proteic pimonidazol-derived adduct formation and alteration in the mitochondrial membrane permeability. Therefore, these results confirm that (99m)Tc-HL-91 is a radiopharmaceutical with favourable characteristics for detecting hypoxia.

99mTc(CO)3-DTMA bombesin conjugates having high affinity for the GRP receptor

INTRODUCTION

Targeted diagnosis of specific human cancer types continues to be of significant interest in nuclear medicine. 99mTc is ideally suited as a diagnostic radiometal for in vivo tumor targeting due to its ideal physical characteristics and diverse labeling chemistries in numerous oxidation states.

METHODS

In this study, we report a synthetic approach toward design of a new tridentate amine ligand for the organometallic aqua-ion [99mTc(H2O)3(CO)3]+. The new chelating ligand framework, 2-(N,N'-Bis(tert-butoxycarbonyl)diethylenetriamine) acetic acid (DTMA), was synthesized from a diethylenetriamine precursor and fully characterized by mass spectrometry and nuclear magnetic resonance spectroscopy (1H and 13C). DTMA was conjugated to H2N-(X)-BBN(7-14)NH2, where X=an amino acid or aliphatic pharmacokinetic modifier and BBN=bombesin peptide, by means of solid phase peptide synthesis. DTMA-(X)-BBN(7-14)NH2 conjugates were purified by reversed-phase high-performance chromatography and characterized by electrospray-ionization mass spectrometry.

RESULTS

The new conjugates were radiolabeled with [99mTc(H2O)3(CO)3]+ produced via Isolink radiolabeling kits to produce [99mTc(CO)3-DTMA-(X)-BBN(7-14)NH2]. Radiolabeled conjugates were purified by reversed-phase high-performance chromatography. Effective receptor binding behavior was evaluated in vitro and in vivo.

CONCLUSIONS

[99mTc(CO)3-DTMA-(X)-BBN(7-14)NH2] conjugates displayed very high affinity for the gastrin releasing peptide receptor in vitro and in vivo. Therefore, these conjugates hold some propensity to be investigated as molecular imaging agents that specifically target human cancers uniquely expressing the gastrin releasing peptide receptor subtypes.

Density-functional computation of (99)Tc NMR chemical shifts

99Tc chemical shifts of TcO4(-), TcH9(2-), TcOF5, TcO2F4(-), TcOCl4(-), Tc2(CO)10, and Tc(CO)3L3(+) (L = CO, MeCN, H2O) are computed using geometries optimized with the gradient-corrected BP86 and hybrid B3P86 density functionals, at the gauge-including atomic orbitals (GIAO), -BPW91 and -B3LYP levels. For this set of compounds, substituent effects on delta(99Tc) are better described with the pure BPW91 functional than with B3LYP, in contrast to most other transition-metal chemical shifts studied so far. A rough, qualitative correlation is found between computed electric-field gradients at the Tc nuclei and the corresponding 99Tc NMR line widths. Thermal and solvation effects on magnetic shielding constants of aqueous TcO4(-), as assessed by averaging these properties over trajectories from Car-Parrinello molecular dynamics simulations, are indicated to be small and comparable to those of MnO4(-). Complexation to aqueous uranyl, UO2(2+), is predicted to affect delta(99Tc) of TcO4(-) only slightly; somewhat larger complexation shifts are obtained for the oxygen nuclei of pertechnetate, suggesting that 17O NMR could be a useful probe for the extent of association between both radionuclides in solution.

Caution to HPLC analysis of tricarbonyl technetium radiopharmaceuticals: An example of changing constitution of complexes in column

Radio-HPLC is a powerful tool for analyzing radioactive species in radiopharmaceutical chemistry. In this paper, we found an example that the commonly used eluting solvent, acetonitrile, could coordinate with the popular radiopharmaceutical nuclides, technetium-99m, during chromatography. [(99m)Tc(CO)(3)(H(2)O)(3)](+) and [Re(CO)(3)(H(2)O)(3)](+) showed quite different retention time when they were eluted using acetonitrile/water as mobile phase. However, they almost demonstrated the same retention time when they were eluted using methanol/water as mobile phase. Further analysis showed that both [(99m)Tc(CO)(3)(H(2)O)(3)](+) and [Re(CO)(3)(H(2)O)(3)](+) could be changed into [(99m)Tc(CO)(3)(CH(3)CN)(3)](+) and [Re(CO)(3)(CH(3)CN)(x)(H(2)O)(3-x)](+) during the separation, respectively. Some former works mistook the [(99m)Tc(CO)(3)(CH(3)CN)(3)](+) for [(99m)Tc(CO)(3)(H(2)O)(3)](+) when using acetonitrile and water in analysis. Quality control of the radiopharmaceuticals containing metal complex should be careful since HPLC solvent could replace some liable ligand molecules.