Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc

Selective radiolabelling with 68Ga under mild conditions: a route towards a porphyrin PET/PDT theranostic agent

A theranostic conjugate for use as a positron emission tomography (PET) radiotracer and as a photosensitiser for photodynamic therapy (PDT) has been synthesised. A water-soluble porphyrin was coupled with the bifunctional chelate, H4Dpaa.ga. This conjugate is capable of rapid 68Ga complexation under physiological conditions; with 93% and 80% radiochemical yields achieved, at pH 4.5 and pH 7.4 respectively, in 15 min at 25 °C. Photocytotoxicity was evaluated on HT-29 cells and showed the conjugate was capable of >50% cell death at 50 μM upon irradiation with light, while causing minimal toxicity in the absence of light (>95% cell survival).

N,1,4-Tri(4-alkoxy-2-hydroxybenzyl)-DAZA: efficient one-pot synthesis and labelling with 68Ga for PET liver imaging in ovo

We report the isolation of a new type of 1,4-diazepan-6-amine (DAZA)-based ligand. Condensation of aldehydes with DAZA gives a novel class of 1,5-diazabicyclo[3.2.1]octanes in nearly quantitative yields. Subsequent reductive cleavage of these bicyclic aminal species with sodium borohydride selectively leads to N,1,4-tri(4-alkoxy-2-hydroxybenzyl)-1,4-diazepan-6-amines (alkoxy = Me: TMeOHB-DAZA; alkoxy = Et: TEtOHB-DAZA) via a unique reductive alkylation reaction in which a substituent is added to the DAZA moiety without the presence of an alkylating agent. Mass spectrometry studies of the intermediates suggest that the mechanism involves insertion of in situ released carbonyl species into an aminal bond to form hemiaminal intermediates, and subsequent reduction. TMeOHB-DAZA and TEtOHB-DAZA are hexadentate ligands suitable for effectively coordinating Ga(iii) ions. Chelation of the radionuclide 68Ga was achieved within 5 min at 100 °C. In vitro stability studies in PBS and human serum confirmed the kinetic inertness of the tracers as no 68Ga demetallation was observed over a period of 4 h. Positron emission tomography (PET)/computed tomography (CT) imaging after in ovo administration to incubated ostrich eggs showed a high uptake in the liver, namely 27% (60 min post injection), and subsequent biliary excretion. These results suggest that [68Ga]Ga-TMeOHB-DAZA and [68Ga]Ga-TEtOHB-DAZA have excellent potential as hepatobiliary PET/CT imaging agents.

Recent Advances in Zirconium-89 Chelator Development

The interest in zirconium-89 (⁸⁹Zr) as a positron-emitting radionuclide has grown considerably over the last decade due to its standardized production, long half-life of 78.2 h, favorable decay characteristics for positron emission tomography (PET) imaging and its successful use in a variety of clinical and preclinical applications. However, to be utilized effectively in PET applications it must be stably bound to a targeting ligand, and the most successfully used ⁸⁹Zr chelator is desferrioxamine B (DFO), which is commercially available as the iron chelator Desferal^®. Despite the prevalence of DFO in ⁸⁹Zr-immuno-PET applications, the development of new ligands for this radiometal is an active area of research. This review focuses on recent advances in zirconium-89 chelation chemistry and will highlight the rapidly expanding ligand classes that are under investigation as DFO alternatives.

NOTA Complexes with Copper(II) and Divalent Metal Ions: Kinetic and Thermodynamic Studies

H₃nota derivatives are among the most studied macrocyclic ligands and are widely used for metal ion binding in biology and medicine. Despite more than 40 years of chemical research on H₃nota, the comprehensive study of its solution chemistry has been overlooked. Thus, the coordination behavior of H₃nota with several divalent metal ions was studied in detail with respect to its application as a chelator for copper radioisotopes in medical imaging and therapy. In the solid-state structure of the free ligand in zwitterionic form, one proton is bound in the macrocyclic cavity through a strong intramolecular hydrogen-bond system supporting the high basicity of the ring amine groups (log K_a = 13.17). The high stability of the [Cu(nota)]^- complex (log K_ML = 23.33) results in quantitative complex formation, even at pH <1.5. The ligand is moderately selective for Cu(II) over other metal ions (e.g., log K_ML(Zn) = 22.32 and log K_ML(Ni) = 19.24). This ligand forms a more stable complex with Mg(II) than with Ca(II) and forms surprisingly stable complexes with alkali-metal ions (stability order Li(I) > Na(I) > K(I)). Thus, H₃nota shows high selectivity for small metal ions. The [Cu(nota)]^- complex is hexacoordinated at neutral pH, and the equatorial N₂O₂ interaction is strengthened by complex protonation. Detailed kinetic studies showed that the Cu(II) complex is formed quickly (millisecond time scale at c_Cu ≈ 0.1 mM) through an out-of-cage intermediate. Conversely, conductivity measurements revealed that the Zn(II) complex is formed much more slowly than the Cu(II) complex. The Cu(II) complex has medium kinetic inertness (τ_1/2 46 s; pH 0, 25 °C) and is less resistant to acid-assisted decomplexation than Cu(II) complexes with H₄dota and H₄teta. Surprisingly, [Cu(nota)]^- decomplexation is decelerated in the presence of Zn(II) ions due to the formation of a stable dinuclear complex. In conclusion, H₃nota is a good carrier of copper radionuclides because the [Cu(nota)]^- complex is predominantly formed over complexes with common impurities in radiochemical formulations, Zn(II) and Ni(II), for thermodynamic and, primarily, for kinetic reasons. Furthermore, the in vivo stability of the [Cu(nota)]^- complex may be increased due to the formation of dinuclear complexes when it interacts with biometals.

Advances in the Chemical Biology of Desferrioxamine B

Desferrioxamine B (DFOB) was discovered in the late 1950s as a hydroxamic acid metabolite of the soil bacterium Streptomyces pilosus. The exquisite affinity of DFOB for Fe(III) identified its potential for removing excess iron from patients with transfusion-dependent hemoglobin disorders. Many studies have used semisynthetic chemistry to produce DFOB adducts with new properties and broad-ranging functions. More recent approaches in chemical biology have revealed some nuances of DFOB biosynthesis and discovered new DFOB-derived drugs and radiometal imaging agents. The current and potential applications of DFOB continue to inspire a rich body of chemical biology research focused on this bacterial metabolite.

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.

Optimization of the selectivity and rate of copper radioisotope complexation: formation and dissociation kinetic studies of 1,4,8-trimethylcyclam-based ligands with different coordinating pendant arms

Influence of coordinating pendant arm character on selectivity and rate of copper(ii) complexation was investigated to optimize ligands for radiomedicinal use.

PET and SPECT imaging of melanoma: the state of the art

Melanoma represents the most aggressive form of skin cancer, and its incidence continues to rise worldwide. 18F-FDG PET imaging has transformed diagnostic nuclear medicine and has become an essential component in the management of melanoma, but still has its drawbacks. With the rapid growth in the field of nuclear medicine and molecular imaging, a variety of promising probes that enable early diagnosis and detection of melanoma have been developed. The substantial preclinical success of melanin- and peptide-based probes has recently resulted in the translation of several radiotracers to clinical settings for noninvasive imaging and treatment of melanoma in humans. In this review, we focus on the latest developments in radiolabeled molecular imaging probes for melanoma in preclinical and clinical settings, and discuss the challenges and opportunities for future development.

Re-thinking the role of radiometal isotopes: Towards a future concept for theranostic radiopharmaceuticals

The potential and future role of certain metal radionuclides, for example, ⁴⁴ Sc, ⁸⁹ Zr, ⁸⁶ Y, ⁶⁴ Cu, ⁶⁸ Ga, ¹⁷⁷ Lu, ²²⁵ Ac, and ²¹³ Bi, and several terbium isotopes has been controversially discussed in the past decades. Furthermore, the possible benefits of "matched pairs" of isotopes for tandem applications of diagnostics and therapeutics (theranostics) have been emphasized, while such approaches still have not made their way into routine clinical practice. Analysis of bibliographical data illustrates how popularity of certain nuclides has been promoted by cycles of availability and applications. We furthermore discuss the different practical requirements for diagnostic and therapeutic radiopharmaceuticals and the resulting consequences for efficient development of clinically useful pairs of radionuclide theranostics, with particular emphasis on the underlying economical factors. Based on an exemplary assessment of overall production costs for ⁶⁸ Ga and ¹⁸ F radiopharmaceuticals, we venture a look into the future of theranostics and predict that high-throughput PET applications, that is, diagnosis of frequent conditions, will ultimately rely on ¹⁸ F tracers. PET radiometals will occupy a niche in the clinical low-throughput sector (diagnosis of rare diseases), but above all, dominate preclinical research and clinical translation. Matched isotope pairs will be of lesser relevance for theranostics but may become important for future PET-based therapeutic dosimetry.

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.

Investigation of the complexation of natZr(iv) and 89Zr(iv) by hydroxypyridinones for the development of chelators for PET imaging applications

Three hydroxypyridinone (HOPO) positional isomers - 1,2-HOPO (L1H) and its water soluble analogue (L1'H), 3,2-HOPO (L2H) and 3,4-HOPO (L3H) have been investigated for the complexation of Zr(iv). Potentiometric and UV-Vis spectrometric studies show a higher thermodynamic stability for the formation of Zr(L1')4 in comparison with Zr(L2)4 and Zr(L3)4 as well as a higher kinetic inertness in competition studies with EDTA or Fe3+ at a radiotracer concentration with 89Zr. Besides the low pKa of L1H or L1'H (pKa = 5.01) in comparison with L2H and L3H (pKa = 8.83 and 9.55, respectively), the higher stability of Zr(L1')4 can be attributed in part to the presence of the amide group next to the chelating oxygen that induces intramolecular H-bond and amide/π interactions that were observed by X-ray crystallography and confirmed by quantum chemical calculations. The data presented here indicate that the 1,2-HOPO L1' exhibits the best characteristics for Zr(iv) complexation. However, 3,2-HOPO and 3,4-HOPO patterns, if appropriately tuned, for instance with the addition of an amide group as in the 1,2-HOPO ligand, may also become interesting alternatives for the design of Zr(iv) chelators with improved characteristics for applications in nuclear imaging with 89Zr.

Hydroxypyridinone Chelators: From Iron Scavenging to Radiopharmaceuticals for PET Imaging with Gallium-68

Derivatives of 3,4-hydroxypyridinones have been extensively studied for in vivo Fe3+ sequestration. Deferiprone, a 1,2-dimethyl-3,4-hydroxypyridinone, is now routinely used for clinical treatment of iron overload disease. Hexadentate tris(3,4-hydroxypyridinone) ligands (THP) complex Fe3+ at very low iron concentrations, and their high affinities for oxophilic trivalent metal ions have led to their development for new applications as bifunctional chelators for the positron emitting radiometal, 68Ga3+, which is clinically used for molecular imaging in positron emission tomography (PET). THP-peptide bioconjugates rapidly and quantitatively complex 68Ga3+ at ambient temperature, neutral pH and micromolar concentrations of ligand, making them amenable to kit-based radiosynthesis of 68Ga PET radiopharmaceuticals. 68Ga-labelled THP-peptides accumulate at target tissue in vivo, and are excreted largely via a renal pathway, providing high quality PET images.

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.

(11)C[double bond, length as m-dash]O bonds made easily for positron emission tomography radiopharmaceuticals

The positron-emitting radionuclide carbon-11 ((11)C, t1/2 = 20.3 min) possesses the unique potential for radiolabeling of any biological, naturally occurring, or synthetic organic molecule for in vivo positron emission tomography (PET) imaging. Carbon-11 is most often incorporated into small molecules by methylation of alcohol, thiol, amine or carboxylic acid precursors using [(11)C]methyl iodide or [(11)C]methyl triflate (generated from [(11)C]carbon dioxide or [(11)C]methane). Consequently, small molecules that lack an easily substituted (11)C-methyl group are often considered to have non-obvious strategies for radiolabeling and require a more customized approach. [(11)C]Carbon dioxide itself, [(11)C]carbon monoxide, [(11)C]cyanide, and [(11)C]phosgene represent alternative reactants to enable (11)C-carbonylation. Methodologies developed for preparation of (11)C-carbonyl groups have had a tremendous impact on the development of novel PET tracers and provided key tools for clinical research. (11)C-Carbonyl radiopharmaceuticals based on labeled carboxylic acids, amides, carbamates and ureas now account for a substantial number of important imaging agents that have seen translation to higher species and clinical research of previously inaccessible targets, which is a testament to the creativity, utility and practicality of the underlying radiochemistry.