Chelation chemistry of manganese-52 for PET imaging applications

INTRODUCTION

Due to its decay and chemical properties, interest in manganese-52 has increased for development of long-lived PET radiopharmaceuticals. Its long half-life of 5.6 days, low average positron energy (242 keV), and sufficient positron decay branching ratio make it suitable for radiolabeling macromolecules for investigating slow biological processes. This work aims to establish suitable chelators for manganese-52 that can be radiolabeled at mild conditions through the evaluation of commercially available chelators.

METHODS

Manganese-52 was produced through the nuclear reaction NatCr(p,n)52Mn by irradiation of natural chromium targets on a TR24 cyclotron followed by purification through ion exchange chromatography. The radiolabeling efficiencies of chelators: DOTA, DiAmsar, TETA, DO3A, NOTA, 4'-Formylbenzo-15-crown-5, Oxo-DO3A, and DFO, were assessed by investigating the impact of pH, buffer type, and temperature. In vitro stability of [52Mn]Mn(DO3A)-, [52Mn]Mn(Oxo-DO3A)-, and [52Mn]Mn(DOTA)2- were evaluated in mouse serum. The radiocomplexes were also evaluated in vivo in mice. Crystals of [Mn(Oxo-DO3A)]- were synthesized by reacting Oxo-DO3A with MnCl2 and characterized by single crystal X-ray diffraction.

RESULTS

Yields of 185 ± 19 MBq (5.0 ± 0.5 mCi) (n = 4) of manganese-52 were produced at the end of a 4 h, 15 μA, bombardment with 12.5 MeV protons. NOTA, DO3A, DOTA, and Oxo-DO3A chelators were readily radiolabeled with >96 % radiochemical purity at all conditions. Manganese radiocomplexes of Oxo-DO3A, DOTA, and DO3A remained stable in vitro up to 5 days and exhibited different biodistribution profiles compared to [52Mn]MnCl2. The solid-state structure of Mn-Oxo-DO3A complex was determined by single-crystal X-ray diffraction.

CONCLUSIONS

DO3A and Oxo-DO3A are suitable chelators for manganese-52 which are readily radiolabeled at mild conditions with high molar activity, and demonstrate both in vitro and in vivo stability.

Structural Variations in Carboxylated Bispidine Ligands: Influence of Positional Isomerism and Rigidity on the Conformation, Stability, Inertness and Relaxivity of their Mn2+ Complexes

Mn2+ complexes of 2,4-pyridyl-disubstituted bispidine ligands have emerged as more biocompatible alternatives to Gd3+ -based MRI probes. They display relaxivities comparable to that of commercial contrast agents and high kinetic inertness, unprecedented for Mn2+ complexes. The chemical structure, in particular the substituents on the two macrocyclic nitrogens N3 and N7, are decisive for the conformation of the Mn2+ complexes, and this will in turn determine their thermodynamic, kinetic and relaxation properties. We describe the synthesis of four ligands with acetate substituents in positions N3, N7 or both. We evidence that the bispidine conformation is dependent on N3 substitution, with direct impact on the thermodynamic stability, kinetic inertness, hydration state and relaxivity of the Mn2+ complexes. These results unambiguously show that (i) solely a chair-chair conformation allows for favorable inertness and relaxivity, and (ii) in this family such chair-chair conformation is accessible only for ligands without N3-appended carboxylates.

64CuCl2 PET Imaging of 4T1-Related Allograft of Triple-Negative Breast Cancer in Mice

⁶⁴CuCl₂ is an economic radiotracer for oncologic PET investigations. In the present study, we characterized the uptake of ⁶⁴CuCl₂ in vivo by µPET/CT in an allograft 4T1-related mouse model (BALB/c) of advanced breast cancer. ¹⁸F-FDG was used as a comparator. Twenty-two animals were imaged 7–9 days following 4T1-cell implantation inside mammary glands. Dynamic ⁶⁴CuCl₂ µPET/CT acquisition or iterative static images up to 8 h p.i. were performed. Animal biodistribution and tumor uptake were first evaluated in vivo by µPET analysis and then assessed on tissue specimens. Concerning ¹⁸F-FDG µPET, a static acquisition was performed at 15 min and 60 min p.i. Tumor ⁶⁴CuCl₂ accumulation increased from 5 min to 4 h p.i., reaching a maximum value of 5.0 ± 0.20 %ID/g. Liver, brain, and muscle ⁶⁴CuCl₂ accumulation was stable over time. The tumor-to-muscle ratio remained stable from 1 to 8 h p.i., ranging from 3.0 to 3.7. Ex vivo data were consistent with in vivo estimations. The ¹⁸F-FDG tumor accumulation was 8.82 ± 1.03 %ID/g, and the tumor-to-muscle ratio was 4.54 ± 1.11. ⁶⁴CuCl₂ PET/CT provides good characterization of the 4T1-related breast cancer model and allows for exploration of non-glycolytic cellular pathways potentially of interest for theragnostic strategies.

Experimental Nuclear Medicine Meets Tumor Biology

Personalized treatment of cancer patients demands specific and validated biomarkers for tumor diagnosis and therapy. The development and validation of such require translational preclinical models that recapitulate human diseases as accurately as possible. Moreover, there is a need for convergence of different (pre)clinical disciplines that openly share their knowledge and methodologies. This review sheds light on the differential perception of biomarkers and gives an overview of currently used models in tracer development and approaches for biomarker discovery.

H4HBEDpa: Octadentate Chelate after A. E. Martell

H₄HBEDpa, a new octadentate chelator inspired by the 1960s ligand HBED of Arthur E. Martell, has been investigated for a selection of trivalent metal ions useful in diagnostic and therapeutic applications (Sc³⁺, Fe³⁺, Ga³⁺, In³⁺, and Lu³⁺). Complex formation equilibria were thoroughly investigated using combined potentiometric and UV-vis spectrophotometric titrations which revealed effective chelation and high metal-sequestering capacity, in particular for Fe³⁺, log K_FeL = 36.62, [Fe(HBEDpa)]^-. X-ray diffraction study of single crystals revealed that the ligand is preorganized and forms hexa-coordinated complexes with Fe³⁺ and Ga³⁺ at acidic pH. Density functional theory (DFT) calculations were applied to probe the geometries and energies of all the possible conformers of [M(HBEDpa)]^- (M = Sc³⁺, Fe³⁺, Ga³⁺, In³⁺, and Lu³⁺). DFT calculations confirmed the experimental findings, indicating that [Fe(HBEDpa)]^- is bound tightly in an asymmetric pattern as compared to the symmetrically bound and more open [Ga(HBEDpa)]^-, prone to hydrolysis at higher pH. DFT calculations also showed that a large metal ion such as Lu³⁺ fully coordinates with HBEDpa^4-, forming a binary octadentate complex in its lowest-energy form. Smaller metal ions form six or seven coordinate complexes with HBEDpa^4-.

High denticity oxinate-linear-backbone chelating ligand for diagnostic radiometal ions [111In]In3+ and [89Zr]Zr4+

A potentially decadentate oxinate-containing ligand was synthesized and assessed through solution thermodynamics studies, concentration dependent radiolabeling and serum stability assays with [^nat/111In]In³⁺ and [^nat/89Zr]Zr⁴⁺.

Preparation of Zirconium-89 Solutions for Radiopharmaceutical Purposes: Interrelation Between Formulation, Radiochemical Purity, Stability and Biodistribution

Zirconium-89 is a promising radionuclide for nuclear medicine. The aim of the present work was to find a suitable method for obtaining zirconium-89 solutions for radiopharmaceutical purposes. For this purpose, the ion exchange behavior of zirconium-89 solutions was studied. Radio-TLC (thin layer chromatography) and biodistribution studies were carried out to understand speciation of zirconium-89 complexes and their role in the development of new radiopharmaceuticals. Three methods of zirconium-89 isolation were studied using ZR (hydroxamate) and Chelex-100 resins. It was found that ZR-resin alone is not enough to obtain stable zirconium-89 formulations. An easy and effective method of reconstitution of [⁸⁹Zr]Zr-oxalate to [⁸⁹Zr]Zr-citrate using Chelex-100 resin was developed. Developed procedures allow obtaining [⁸⁹Zr]Zr-oxalate (in 0.1 M sodium oxalate solution) and [⁸⁹Zr]Zr-citrate (in 0.1–1.0 M sodium citrate solution). These solutions are perfectly suitable and convenient for radiopharmaceutical purposes. Our results prove [⁸⁹Zr]Zr-citrate to be advantageous over [⁸⁹Zr]Zr-oxalate. During evaluation of speciation of zirconium-89 complexes, a new TLC method was developed, since it was proved that there is no comprehensive method for analysis or zirconium-89 preparations. The new method provides valuable insights about the content of “active” ionic form of zirconium-89. The interrelation of the chromatographic behavior of zirconium-89 preparations and their biodistribution was studied.

Development of a novel 68Ga-dextran carboxylate derivative for blood pool imaging

To develop a possible PET blood pool imaging agent, a series 68Ga-dextran carboxylate derivatives were prepared. Dextran carboxylates with different degree of oxidations (DO) were prepared through stepwise dextran oxidation using NaIO4 and CH3COOOH. The products were characterized by FT-IR and GPC, followed by solubility and toxicity tests on Hella cells (viability=98.6, 97.4 and 95.6% for 3 dextran carboxylates with DOs: 8.3, 24.6 and 39.8%, respectively. The products were labeled with 68Ga (radiochemical purity>98%; ITLC) followed by stability tests in final solution as well as in presence of cycteine and human serum. Two stable tracers (DOs; 24.6 and 39.8%) were adminstered intravenously into wild type rat tail vein separately demonstrating suitable retention in circulation as expected from blood pool imaging agents. Liver and spleen also contained activities. The major excretion was through urinary pathway esp. for derivative with DO. 39.8%. Unlike 68Ga-dextran, lungs showed lower uptake. The dextran carboxylate with the highest 39.8% showed the best characteristics for a blood pool agent, though more studies including PET imaging in larger mammals are required.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The difference in 2-proton's acidity between L_β and L_G led to dramatically different results of their reactions with [^99mTc][Tc(CO)₃(OH₂)₃]⁺.

Crystal structure of aqua-(2-{[2-({2-[bis-(carboxyl-ato-κO-meth-yl)amino-κN]eth-yl}(carboxyl-ato-κO-meth-yl)amino-κN)eth-yl](carb-oxy-meth-yl)aza-niumyl}acetato)-gallium(III) trihydrate

In the title GaIII complex compound with pentetic acid, [Ga(C14H20N3O10)(H2O)]·3H2O, the GaIII centre is bound in a slightly distorted octa-hedral coordination sphere by two amine N atoms, three carboxyl-ate O atoms and one water O atom. The complex mol-ecule exists as a zwitterion. In the crystal, the complexes are linked to each other via O-H⋯O and C-H⋯O hydrogen bonds, forming layers parallel to (001). Three uncoordinating water mol-ecules link the complex layers via O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds, forming a three-dimensional network.

Liquid–liquid extraction in flow of the radioisotope titanium-45 for positron emission tomography applications

The continuous liquid–liquid extraction of the PET radioisotope ⁴⁵Ti using a membrane-based separator allows for efficient ⁴⁵Ti recovery and radiolabeling.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

89Zr-Immuno-Positron Emission Tomography in Oncology: State-of-the-Art 89Zr Radiochemistry

Immuno-positron emission tomography (immunoPET) with ⁸⁹Zr-labeled antibodies has shown great potential in cancer imaging. It can provide important information about the pharmacokinetics and tumor-targeting properties of monoclonal antibodies and may help in anticipating on toxicity. Furthermore, it allows accurate dose planning for individualized radioimmunotherapy and may aid in patient selection and early-response monitoring for targeted therapies. The most commonly used chelator for ⁸⁹Zr is desferrioxamine (DFO). Preclinical studies have shown that DFO is not an ideal chelator because the ⁸⁹Zr–DFO complex is partly unstable in vivo, which results in the release of ⁸⁹Zr from the chelator and the subsequent accumulation of ⁸⁹Zr in bone. This bone accumulation interferes with accurate interpretation and quantification of bone uptake on PET images. Therefore, there is a need for novel chelators that allow more stable complexation of ⁸⁹Zr. In this Review, we will describe the most recent developments in ⁸⁹Zr radiochemistry, including novel chelators and site-specific conjugation methods.

Direct 11CN-Labeling of Unprotected Peptides via Palladium-Mediated Sequential Cross-Coupling Reactions

A practical procedure for ¹¹CN-labeling of unprotected peptides has been developed. The method was shown to be highly chemoselective for cysteine over other potentially nucleophilic residues, and the radiolabeled products were synthesized and purified in less than 15 min. Appropriate for biomedical applications, the method could be used on an extremely small scale (20 nmol) with a high radiochemical yield. The success of the protocol stems from the use of a Pd-reagent based on a dihaloarene, which enables direct "nucleophile-nucleophile" coupling of the peptide and [¹¹C]cyanide by temporal separation of nucleophile addition.

Comparative preclinical evaluation of 68Ga-NODAGA and 68Ga-HBED-CC conjugated procainamide in melanoma imaging

Malignant melanoma is the most aggressive form of skin cancer. The early detection of primary melanoma tumors and metastases using non-invasive PET imaging determines the outcome of this disease. Previous studies have shown that benzamide derivatives (e.g. procainamide) conjugated with PET radionuclides specifically bind to melanin pigment of melanoma tumors. ⁶⁸Ga chelating agents can have high influence on physiological properties of ⁶⁸Ga labeled bioactive molecules, as was experienced during the application of HBED-CC on PSMA ligand. The aim of this study was to assess this concept in the case of the melanin specific procaindamide (PCA) and to compare the melanin specificity of ⁶⁸Ga-labeled PCA using HBED-CC and NODAGA chelators under in vitro and in vivo conditions. Procainamide (PCA) was conjugated with HBED-CC and NODAGA chelators and was labeled with Ga-68. The melanin specificity of ⁶⁸Ga-HBED-CC-PCA and ⁶⁸Ga-NODAGA-PCA was investigated in vitro and in vivo using amelanotic (MELUR and A375) and melanin containing (B16-F10) melanoma cell lines. Tumor-bearing mice were prepared by subcutaneous injection of B16-F10, MELUR and A375 melanoma cells into C57BL/6 and SCID mice. 21±2days after tumor cell inoculation and 90min after intravenous injection of the ⁶⁸Ga-labelledlabeled radiopharmacons whole body PET/MRI scans were performed. ⁶⁸Ga-NODAGA-PCA and ⁶⁸Ga-HBED-CC-PCA were produced with excellent radiochemical purity (98%). In vitro experiments demonstrated that after 30 and 90min incubation time ⁶⁸Ga-NODAGA-PCA uptake of B16-F10 cells was significantly (p≤0.01) higher than the ⁶⁸Ga-HBED-CC-conjugated PCA accumulation in the same cell line. Furthermore, significant difference (p≤0.01 and 0.05) was found between the uptake of melanin negative and positive cell lines using ⁶⁸Ga-NODAGA-PCA and ⁶⁸Ga-HBED-CC-PCA. In vivo PET/MRI studies using tumor models revealed significantly (p≤0.01) higher ⁶⁸Ga-NODAGA-PCA uptake (SUVmean: 0.46±0.05, SUVmax: 1.96±0.25,T/M ratio: 40.7±4.23) in B16-F10 tumors in contrast to ⁶⁸Ga-HBED-CC-PCA where the SUVmean, SUVmax and T/M ratio were 0.13±0.01, 0.56±0.11 and 11.43±1.24, respectively. Melanin specific PCA conjugated with NODAGA chelator showed higher specific binding properties than conjugated with HBED-CC. The chemical properties of the bifunctional chelators used for ⁶⁸Ga-labeling of PCA determine the biological behaviour of the probes. Due to the high specificity and sensitivity ⁶⁸Ga-labeled PCA molecules are promising radiotracers in melanoma imaging.

Bifunctional bispidine derivatives for copper-64 labelling and positron emission tomography

A bispidine cage coordinates ⁶⁴Cu²⁺ rapidly and quantitatively at room temperature, and biotin and maleimide functions allow for targeted PET imaging.

Exploiting the biosynthetic machinery of Streptomyces pilosus to engineer a water-soluble zirconium(iv) chelator

A combined microbiology-chemistry approach has been used to generate a water-soluble chain-extended octadentate hydroxamic acid designed as a high affinity and selective Zr(iv) ligand.

Amino acid based gallium-68 chelators capable of radiolabeling at neutral pH

Herein we show a flexible synthesis for bifunctional chelators based on amino acids that rapidly complex ⁶⁸Ga under physiological conditions.

MANOTA: a promising bifunctional chelating agent for copper-64 immunoPET

A comparison of four bifunctional chelating agents showed superior behaviour of a new NOTA derivative for ⁶⁴Cu labelling of antibody fragments.

Novel bifunctional DATA chelator for quick access to site-directed PET 68Ga-radiotracers: preclinical proof-of-principle with [Tyr3]octreotide

Comparison of ⁶⁸Ga-DATA-TOC vs. ⁶⁸Ga-DOTA-TOC in sst₂-positive cells and tumor-bearing mice revealed the suitability of DATA for easy access to ⁶⁸Ga-labeled vectors.

67Ga-metalloprobes: monitoring the impact of geometrical isomers on accumulation profiles in rat cardiomyoblasts and human breast carcinoma cells

Geometrically similar monocationic gallium(iii) complexes and their radiolabeled SPECT counterparts were obtained from Schiff base precursor ligands using ligand exchange reactions to evaluate the impact of cis and trans-isomers on their cellular accumulation profiles in rat cardiomyoblasts (H9c2(2-1)) and human breast carcinoma (MCF-7neo) cells. ⁶⁷Ga-metalloprobes comprising trans-phenolates showing an overall octahedral geometry and exhibiting uniform spatial distribution of positive charges on their molecular surface show steady-state accumulation in H9c2(2-1) and MCF-7neo cells, and localize in the mitochondria of the cells. Importantly, the surrogate geometrically similar and monocationic metalloprobe counterparts possessing the cis arrangement of phenolates do not show cellular uptake in H9c2(2-1) and MCF-7neo cells. Exploiting their modest fluorescent traits, live cell imaging indicates that trans-isomers of metalloprobes localize within the mitochondria of cells following their penetration, thereby indicating the excellent correlation of radiotracer data and live-cell microscopy results. Overall, these results indicate that the cell uptake profiles of metalloprobes within this class are mediated by the spatial distribution of charges over their molecular surface and hydrophobicity.

Cyclotron Production of High-Specific Activity 55Co and In Vivo Evaluation of the Stability of 55Co Metal-Chelate-Peptide Complexes

This work describes the production of high-specific activity 55Co and the evaluation of the stability of 55Co-metal-chelate-peptide complexes in vivo. 55Co was produced via the 58Ni(p,α)55Co reaction and purified using anion exchange chromatography with an average recovery of 92% and an average specific activity of 1.96 GBq/μmol. 55Co-DO3A and 55Co-NO2A peptide complexes were radiolabeled at 3.7 MBq/μg and injected into HCT-116 tumor xenografted mice. Positron emission tomography (PET) and biodistribution studies were performed at 24 and 48 hours postinjection and compared to those of 55CoCl2. Both 55Co-metal-chelate complexes demonstrated good in vivo stability by reducing the radiotracers' uptake in the liver by sixfold at 24 hours with ~ 1% ID/g and at 48 hours with ~ 0.5% ID/g and reducing uptake in the heart by fourfold at 24 hours with ~ 0.7% ID/g and sevenfold at 48 hours with ~ 0.35% ID/g. These results support the use of 55Co as a promising new radiotracer for PET imaging of cancer and other diseases.

Incorporation of gallium-68 into the crystal structure of Prussian blue to form K(68)GaxFe1-x[Fe(CN)6] nanoparticles: toward a novel bimodal PET/MRI imaging agent

Similarity between the Ga(+) ion and the Fe(3+) ion allows for partial replacement of Fe(3+) ions with Ga(3+) ions in the Fe(iii) crystallographic positions in Prussian blue (PB) to form various solid solutions KGaxFe1-x[Fe(CN)6] (0 < x < 1). Such solid solutions possess very high thermodynamic stability as expected from the parent PB structure. Consequently, a simple one-step (68)Ga-labeling method was developed for preparing a single-phase nanoparticulate bimodal PET/MRI imaging agent based on the PB structural platform. Unlike the typical (68)Ga-labelling reaction based on metal complexation, this novel chelator-free (68)Ga-labeling reaction was shown to be kinetically fast under the acidic conditions. The Ga(3+) ion does not hydrolyze, and affords the (68)Ga-labelled PB nanoparticles, which are easy to purify and have extremely high stability against radionuclidic leaching in aqueous solution.

Improving the stability and inertness of Cu(ii) and Cu(i) complexes with methylthiazolyl ligands by tuning the macrocyclic structure

A tacn based ligand bearing two methylthiazolyl arms (no2th) was synthesized with the aim to find ligands forming very stable and inert complexes with Cu(ii) and Cu(i) in aqueous medium for radiopharmaceutical applications. The no2th ligand was efficiently prepared following the orthoamide intermediate synthesis. The complexes with Cu(2+) and Zn(2+) were obtained and analyzed by X-ray diffraction. The [Cu(no2th)](2+) complex presents a pentacoordinated distorted square pyramidal coordination geometry, while the metal ion in [Zn(no2th)](2+) adopts a hexacoordinated distorted trigonal prismatic geometry involving the coordination of a perchlorate counter ion. The acid-base properties of no2th have been studied using potentiometric titrations, and the stability constants of Cu(2+) and Zn(2+) complexes were determined by potentiometric and UV-vis titrations using H4edta as a competitor ligand. The stability constant determined for the Cu(2+) complex is rather high (log KCuL = 20.77 and pCu = 17.15), and moreover no2th exhibits a high selectivity for copper(ii) in relation to zinc(ii). The kinetics of the copper(ii) complexation process is very fast even in acidic medium. In addition, the [Cu(no2th)](2+) complex was found to be inert under rather harsh conditions (up to 2 M HCl and 60 °C), displaying a very high half-life time of about 15 days in 2 M HCl at 90 °C. The electrochemical reduction of the copper(ii) complex in water leads to the reversible formation of a stable copper(i) species. Spectroscopic studies performed by NMR, UV-vis and EPR, assisted by theoretical calculations, show that the [Cu(no2th)](2+) complex presents a structure in solution similar to that observed in the solid state. When compared to its cyclam di-N-methylthiazolyl counterpart, the results reported in this paper unambiguously show that replacing the cyclam unit by a tacn moiety improves the stability and inertness of its Cu(ii) and Cu(i) complexes.

C-Functionalized chiral dioxocyclam and cyclam derivatives with 1,2,3-triazole units: synthesis, complexation properties and crystal structures of copper(ii) complexes

NewC-functionalized dioxocyclam and cyclam derivatives with 1,2,3-triazoles attached to carbon atoms within the skeleton were designed as valuable bifunctional chelators for molecular imaging.

A [two-step/one week] synthesis of C-functionalized homocyclens and cyclams. Application to the preparation of conjugable BCAs without chelating properties alteration

An efficient synthesis ofC-functionalized azamacrocycles is presented together with a coordination study showing the week influence of the added appended group.