The "Carbonyl Story" and Beyond; Experiences, Lessons and Implications

Preparation, Characterization, and Radiolabeling of Anti-HER2 scFv With Technetium Tricarbonyl and Stability Studies

Breast cancer is the most common diagnosed cancer, and the second cause of cancer death among women, worldwide. HER2 overexpression occurred in approximately 15% to 20% of breast cancers. Invasive biopsy method has been used for detection of HER2 overexpression. HER2-targeted imaging via an appropriate radionuclide is a promising method for sensitive and accurate identification of HER2+ primary and metastatic lesions. 99mTc-anti-HER2 scFv can specifically target malignancies and be used for diagnosis of the cancer type and metastasis as well as treatment of breast cancer. We radiolabeled anti-HER2 scFv that was expressed in Escherichia coli and purified through Ni-NTA resin under native condition with 99mTc-tricarbonyl formed from boranocarbonate. HER2-based ELISA, BCA, TLC, and HPLC were used in this study. In the current study, anti-HER2 scFv was lyophilized before radiolabeling. It was found that freeze-drying did not change the binding activity of anti-HER2 scFv to HER2. Results demonstrated direct anti-HER2 scFv radiolabeling by 99mTc-tricarbonyl to hexahistidine sequence (His-tag) without any changes in biological activity and radiochemical purity of around 98%. Stability analysis revealed that 99mTc-anti-HER2 scFv is stable for at least 24 h in PBS buffer, normal saline, human plasma proteins, and histidine solution.

Induced fac-mer rearrangements in {M(CO)3}+ complexes (M = Re, 99(m)Tc) by a PNP ligand

The fac-mer rearrangements in [MX3(CO)3]2- (M = Re, 99Tc) induced by a pincer-type ligand (PNP) and a "halide scavenger" are reported. The reactions of fac-[99mTc(CO)3(OH2)3]+ or [99mTcO4]- in saline both yield mer-[99mTc(PNP)(CO)3]+, the first example of a mer-{99mTc(CO)3}+ type complex. In contrast, reactions with terpyridine (terpy) only gave the facial κ2-terpy complexes with Re and 99Tc.

Role of Pure Technetium Chemistry: Are There Still Links to Applications in Imaging?

The discovery and development of new 99mTc-based radiopharmaceuticals or labeled drugs in general is based on innovative, pure chemistry and subsequent, application-targeted research. This was the case for all currently clinically applied imaging agents. Most of them were market-introduced some 20 years ago, and the few more recent ones are based on even older chemistry, albeit technetium chemistry has made substantial progress over the last 20 years. This progress though is not mirrored by new molecular imaging agents and is even accompanied by a steady decrease in the number of groups active in pure and applied technetium chemistry, a contrast to the trends in most other fields in which d-elements play a central role. The decrease in research with technetium has been partly counterbalanced by a strong increase of research activities with homologous, cold rhenium compounds for therapy, disclosing in the future eventually a quite unique opportunity for theranostics. This Viewpoint analyzes the pathways that led to radiopharmaceuticals in the past and their underlying fundamental contributions. It attempts to tackle the question of why new chemistry still does not lead to new imaging agents, i.e., the question of whether pure technetium chemistry is still needed at all.

Unlocking Air- and Water-Stable Technetium Acetylides and Other Organometallic Complexes

The first technetium complexes containing anionic alkynido ligands in an end-on coordination mode have been prepared by using the nonprotic, cationic precursor mer,trans-[Tc(SMe₂)(CO)₃(PPh₃)₂]⁺. This cation acts as a functional analogue of the highly reactive 16-electron metallo Lewis acid {Tc(CO)₃(PPh₃)₂}⁺ in reactions with alkynes, acetylides, and other organometallic reagents. Such reactions give a variety of organometallic technetium complexes in excellent yields and enable the preparation of [Tc(CH₃)(CO)₃(PPh₃)₂], [Tc(Ph)(CO)₃(PPh₃)₂], [Tc(Cp)(CO)₂(PPh₃)], [Tc(═CCH₂CH₂CH₂O)(CO)₃(PPh₃)₂]⁺, [Tc(═CCH₂CH₂CH₂CH₂O)(CO)₃(PPh₃)₂]⁺, [Tc(C≡C-H)(CO)₃(PPh₃)₂], [Tc(C≡C-Ph)(CO)₃(PPh₃)₂], [Tc(C≡C-^tBu)(CO)₃(PPh₃)₂], [Tc(C≡C-ⁿBu)(CO)₃(PPh₃)₂], [Tc(C≡C-SiMe₃)(CO)₃(PPh₃)₂], and [Tc{C≡C-C₆H₃(CF₃)₂}(CO)₃(PPh₃)₂]. The bonding situation in the alkynyl complexes is compared to that in corresponding alkyl- and arylnitrile and -isonitrile complexes. [Tc(N≡C-Ph)(CO)₃(PPh₃)₂](BF₄), [Tc(C≡N-Ph)(CO)₃(PPh₃)₂](BF₄), [Tc(N≡C-^tBu)(CO)₃(PPh₃)₂](BF₄), and [Tc(C≡N-^tBu)(CO)₃(PPh₃)₂](BF₄) were prepared in high yields by ligand exchange reactions starting from mer,trans-[Tc(OH₂)(CO)₃(PPh₃)₂](BF₄). The novel complexes were characterized by single-crystal X-ray diffraction and spectroscopic methods. In particular, ⁹⁹Tc nuclear magnetic resonance spectroscopy proved to be an invaluable and sensitive tool for the characterization of the complexes. Density functional theory calculations strongly suggest similar bonding situations for the related alkynyl, nitrile, and isonitrile complexes of technetium.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

Calix[4]arene-Analogous Technetium Supramolecules

Calix[4]arene-analogous technetium supramolecules (1 and 2) were assembled using (NBu₄)[Tc₂(μ-Cl)₃(CO)₆] and neutral flexible bidentate nitrogen-donor ligands (L¹ and L²) consisting of four arene units covalently joined via methylene units. The neutral homoleptic technetium macrocycles adopt a partial cone/cone-shaped conformation in the solid state. These supramolecules are the first example of fac-[Tc(CO)₃]⁺ core-based metallocalix[4]arenes and second example of fac-[Tc(CO)₃]⁺ core-based metallomacrocycles. Structurally similar fac-[Re(CO)₃]⁺ core-based macrocycles (3 and 4) were also prepared using [Re(CO)₅X] (where X = Cl or Br) and L¹ or L². The products were characterized spectroscopically and by X-ray analysis.

Technetium(I) Carbonyl Chemistry with Small Inorganic Ligands

[Tc(OH₂)(CO)₃(PPh₃)₂](BF₄) has been used as a synthon for reactions with small inorganic ligands with relevance for the treatment of nuclear waste solutions such as nitrate, nitrite, pseudohalides, permetalates (M = Mn, Tc, Re), and BH₄^-. The formation of bond isomers and/or a distinct reactivity has been observed for most of the products. [Tc(NCO)(CO)₃(PPh₃)₂], [Tc(NCS)(CO)₃(PPh₃)₂], [Tc(CN)(CO)₃(PPh₃)₂], [Tc(N₃)(CO)₃(PPh₃)₂], [Tc(NCO)(OH₂)(CO)₂(PPh₃)₂], [Tc(η²-OON)(CO)₂(PPh₃)₂], [Tc(η¹-NO₂)(CO)₃(PPh₃)₂], [Tc(η²-OONO)(CO)₂(PPh₃)₂], [Tc(η¹-ONO₂)(CO)₃(PPh₃)₂], [Tc(η²-OO(CCH₃))(CO)₂(PPh₃)₂], [Tc(η²-SSC(SCH₃))(CO)₂(PPh₃)₂], [Tc(η²-SSC(OCH₃))(CO)₂(PPh₃)₂], [Tc(η²-SSC(CH₃))(CO)₂(PPh₃)₂], [Tc(η²-SS(CH))(CO)₂(PPh₃)₂], [Tc(OTcO₃)(acetone)(CO)₂(PPh₃)₂], [Tc(OTcO₃)(CO)₃(PPh₃)₂], and [Tc(η²-HHBH₂)(CO)₂(PPh₃)₂] have been isolated in crystalline form and studied by X-ray crystallography. Additionally, the typical reactivity patterns (isomerization, thermal decomposition, hydrolysis, or decarbonylation) of the products have been studied by spectroscopic methods. ⁹⁹Tc NMR spectroscopy has proved to be a particularly useful tool for the evaluation of such reactions of the diamagnetic technetium(I) compounds in solution.

[Tc(OH2)(CO)3(PPh3)2]+: A Synthon for Tc(I) Complexes and Its Reactions with Neutral Ligands

A scalable synthesis of the novel and highly reactive [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ cation is described. The ligand-exchange chemistry of this compound with neutral ligands coordinating through C, N, O, S, Se, and Te has been explored systematically. The complexes either retain the original mer-trans tricarbonyl core under exclusive exchange of the aqua ligand or form dicarbonyl complexes by thermal decarbonylation. Ligand exchange reactions starting from [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ proceed under mild conditions and are generally almost quantitative. Some of the formed complexes are remarkably stable and inert, while others provide products with one labile ligand for further reactions. The derived complexes of the type [Tc(L)(CO)₃(PPh₃)₂]⁺ and [Tc(L)₂(CO)₂(PPh₃)₂]⁺ represent an interesting opportunity for the development of ^99mTc complexes with potential use in radiopharmacy. The ready displacement of the aqua ligand highlights the synthetic value of [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ as a reactive entry point for further studies in the little explored field of the organometallic chemistry of technetium.