Room Temperature Al18 F Labeling of 2-Aminomethylpiperidine-Based Chelators for PET Imaging

Tyrosinase-Activated MRI and PET Probes for Selective Melanoma Imaging

Melanoma is one of the most aggressive forms of skin cancer. Accurate and early diagnosis of melanoma is crucial for improving patient outcomes. This study develops two TYR-activatable molecular probes, Mn-TyrEDTA and Al-18F-TyrEDTA, for the selective detection of melanoma in vivo. In vitro studies reveal that Mn-TyrEDTA exhibits TYR activity-dependent relaxivity enhancement, undergoing TYR-mediated oxidative polymerization, resulting in the formation of paramagnetic oligomers. UV-vis analysis supports this mechanism, showing time- and TYR concentration-dependent increases in broad band absorbance in the UV-vis region, specifically around 475 nm, due to the formation of o-quinone intermediates and melanin-like oligomers. HPLC analysis further confirmed the presence of polar oligomeric products in Mn-TyrEDTA solutions incubated with TYR/O2. MRI studies demonstrate Mn-TyrEDTA's selective retention and signal enhancement in TYR-expressing melanoma tissues. Furthermore, PET imaging with Al-18F-TyrEDTA conducted using a dual-xenograft mouse model reveals significantly higher uptake and retention of Al-18F-TyrEDTA in TYR-expressing melanoma compared to TYR-negative tumors. This selective retention could be attributed to a TYR-mediated proximity labeling mechanism, where highly reactive quinones form covalent bonds with nearby tumor proteins. In summary, our findings establish Mn-TyrEDTA and Al-18F-TyrEDTA as promising TYR-activatable imaging probes, offering a novel strategy for the early diagnosis and prognosis of melanoma.

Bifunctional Hexadentate Pyclen-Based Chelating Agent for Mild Radiofluorination in Aqueous Solution at Room Temperature with a Ga-18F Ternary Complex

Positron Emission Tomography (PET) is used in oncology for tumor diagnosis, commonly relying on fluorine-18 (18F) emission detection. The conventional method of 18F incorporation on to probes by covalent bonding is harsh for sensitive biomolecules, which are nonetheless compounds of choice for the development of targeted probes. This study explores gallium-18F (Ga18F) coordination, a milder alternative method occurring in aqueous media at the final stage of radiosyntheses. Pyclen-based chelating agents were proposed to capture (GaF) species at room temperature and pH≥5 making the radiofluorination process compatible with heat- and acid-sensitive biomolecules. Highly promising results were obtained with the PC2A-based chelating agent LH2 derived from the new bifunctional PC2A-OAE-NCS compound. The solid-state structure of GaF(L) was elucidated by X-ray diffraction and revealed an unconventional heptacoordination of Ga(III). A high radiochemical conversion (RCC) of 86 % was achieved at room temperature, in water at pH 5 within 20 minutes. Stability studies showed the robustness of the GaF(L) complex in aqueous media for at least one day and at least one hour for the radiolabeled analog Ga18F(L). These findings demonstrated that PC2A-based compounds are chelating agents of choice for (Ga18F) species, suggesting a real technological breakthrough for PET imaging and precision medicine.

A versatile fluorinated azamacrocyclic chelator enabling 18F PET or 19F MRI: a first step towards new multimodal and smart contrast agents

Macrocyclic chelators play a central role in medical imaging and nuclear medicine owing to their unparalleled metal cation coordination abilities. Their functionalization by fluorinated groups is an attractive design, to combine their properties with those of 18F for Positron Emission Tomography (PET) or natural 19F for Magnetic Resonance Imaging (MRI), and access potential theranostic or smart medical imaging probes. For the first time, a compact fluorinated macrocyclic architecture has been synthesized, based on a cyclen chelator bearing additional pyridine coordinating units and simple methyltrifluoroborate prosthetic groups. This ligand and its corresponding model Zn(ii) complex were investigated to evaluate the 18F-PET or 19F MRI abilities provided by this novel molecular structure. The chelator and the complex were obtained via a simple and high-yielding synthetic route, present excellent solvolytic stability of the trifluoroborate groups at various pH, and provide facile late-stage 18F-radiolabeling (up to 68% radiochemical yield with high activity) as well as a satisfying detection limit for 19F MRI imaging (low mM range).

Synthesis, Characterization, and Computational Studies on Gallium(III) and Iron(III) Complexes with a Pentadentate Macrocyclic bis-Phosphinate Chelator and Their Investigation As Molecular Scaffolds for 18F Binding

With the aim of obtaining improved molecular scaffolds for 18F binding to use in PET imaging, gallium(III) and iron(III) complexes with a macrocyclic bis-phosphinate chelator have been synthesized and their properties, including their fluoride binding ability, investigated. Reaction of Bn-tacn (1-benzyl-1,4,7-triazacyclononane) with paraformaldehyde and PhP(OR)2 (R = Me or Et) in refluxing THF, followed by acid hydrolysis, yields the macrocyclic bis(phosphinic acid) derivative, H2(Bn-NODP) (1-benzyl-4,7-phenylphosphinic acid-1,4,7-triazacyclononane), which is isolated as its protonated form, H2(Bn-NODP)·2HCl·4H2O, at low pH (HClaq), its disodium salt, Na2(Bn-NODP)·5H2O at pH 12 (NaOHaq), or the neutral H2(Bn-NODP) under mildly basic conditions (Et3N). A crystal structure of H2(Bn-NODP)·2HCl·H2O confirmed the ligand's identity. The mononuclear [GaCl(Bn-NODP)] complex was prepared by treatment of either the HCl or sodium salt with Ga(NO3)3·9H2O or GaCl3, while treatment of H2(Bn-NODP)·2HCl·4H2O with FeCl3 in aqueous HCl gives [FeCl(Bn-NODP)]. The addition of 1 mol. equiv of aqueous KF to these chloro complexes readily forms the [MF(Bn-NODP)] analogues. Spectroscopic analysis on these complexes confirms pentadentate coordination of the doubly deprotonated (bis-phosphinate) macrocycle via its N3O2 donor set, with the halide ligand completing a distorted octahedral geometry; this is further confirmed through a crystal structure analysis on [GaF(Bn-NODP)]·4H2O. The complex adopts the geometric isomer in which the phosphinate arms are coordinated unsymmetrically (isomer 1) and with the stereochemistry of the three N atoms of the tacn ring in the RRS configuration, denoted (N)RRS, and the phosphinate groups in the RR stereochemistry, denoted (P)RR, (isomer 1/RR), together with its (N)SSR (P)SS enantiomer. The greater thermodynamic stability of isomer 1/RR over the other possible isomers is also indicated by density functional theory (DFT) calculations. Radiofluorination experiments on the [MCl(Bn-NODP)] complexes in partially aqueous MeCN/NaOAcaq (Ga) or EtOH (Ga or Fe; i.e. without buffer) with 18F- target water at 80 °C/10 min lead to high radiochemical incorporation (radiochemical yields 60-80% at 1 mg/mL, or ∼1.5 μM, concentration of the precursor). While the [Fe18F(n-NODP)] is unstable (loss of 18F-) in both H2O/EtOH and PBS/EtOH (PBS = phosphate buffered saline), the [Ga18F(Bn-NODP)] radioproduct shows excellent stability, RCP = 99% at t = 4 h (RCP = radiochemical purity) when formulated in 90%:10% H2O/EtOH and ca. 95% RCP over 4 h when formulated in 90%:10% PBS/EtOH. This indicates that the new "GaIII(Bn-NODP)" moiety is a considerably superior fluoride binding scaffold than the previously reported [Ga18F(Bn-NODA)] (Bn-NODA = 1-benzyl-4,7-dicarboxylate-1,4,7-triazacyclononane), which undergoes rapid and complete hydrolysis in PBS/EtOH (refer to Chem. Eur. J. 2015, 21, 4688-4694).

Thermodynamic and Kinetic Stabilities of Al(III) Complexes with N2O3 Pentadentate Ligands

Al(III) complexes have been recently investigated for their potential use in imaging with positron emission tomography (PET) by formation of ternary complexes with the radioisotope fluorine-18 (¹⁸F). Although the derivatives of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) are the most applied chelators for [Al¹⁸F]²⁺ labelling and (pre)clinical PET imaging, non-macrocyclic, semi-rigid pentadentate chelators having two N- and three O-donor atoms such as RESCA1 and AMPDA-HB have been proposed with the aim to allow room temperature labelling of temperature-sensitive biomolecules. The paucity of stability data on Al(III) complexes used for PET imaging instigated a complete thermodynamic and kinetic solution study on Al(III) complexes with aminomethylpiperidine (AMP) derivatives AMPTA and AMPDA-HB and the comparison with a RESCA1-like chelator CD3A-Bn (trans-1,2-diaminocyclohexane-N-benzyl-N,N',N'-triacetic acid). The stability constant of [Al(AMPDA-HB)] is about four orders of magnitude higher than that of [Al(AMPTA)] and [Al(CD3A-Bn)], highlighting the greater affinity of phenolates with respect to acetate O-donors. On the other hand, the kinetic inertness of the complexes, determined by following the Cu²⁺-mediated transmetallation reactions in the 7.5-10.5 pH range, resulted in a spontaneous and hydroxide-assisted dissociation slightly faster for [Al(AMPTA)] than for the other two complexes (t_1/2 = 4.5 h for [Al(AMPTA)], 12.4 h for [Al(AMPDA-HB)], and 24.1 h for [Al(CD3A-Bn)] at pH 7.4 and 25 °C). Finally, the [AlF]²⁺ ternary complexes were prepared and their stability in reconstituted human serum was determined by ¹⁹F NMR experiments.

Detrimental impact of aqueous mobile phases on 18F-labelled radiopharmaceutical analysis via radio-TLC

The list of new positron-emission tomography (PET) tracers has rapidly grown in the past decade, following discoveries of new biological targets and therapeutic strategies, with several compounds garnering recent regulatory approval for clinical use. During the development of synthesis methods and production of new tracers for imaging, analytical methods for radio-high performance liquid chromatography (radio-HPLC) and radio-thin layer chromatography (radio-TLC) separations need to be developed to assess radiochemical compositions. Radio-TLC is often faster, simpler, and sometimes more accurate than radio-HPLC (as there is no underestimation of [18F]fluoride when analyzing 18F-labeled radiopharmaceuticals). Many protocols have been developed for separating 18F-radiopharmaceuticals on silica TLC plates, typically with [18F]fluoride retained at the origin and the radiopharmaceutical (and impurities) migrating along the plate. Interestingly, many reports describe the use of aqueous conditions to mobilize polar species, but it is known that aqueous conditions can modify silica and alter its chromatographic behavior. In this technical note, we explore the effects that aqueous conditions have on the analysis of 18F-radiopharmaceutical mixtures, revealing that with sufficient water, the radionuclide ([18F]fluoride) can migrate away from the origin and can be split into multiple bands. Furthermore, water can hinder the migration of the radiopharmaceutical. These effects can lead to overlapped bands or reversal of the normally expected order of bands, potentially leading to the misinterpretation of results if care is not taken to validate the TLC method carefully.

Dual Probes for Positron Emission Tomography (PET) and Fluorescence Imaging (FI) of Cancer

Dual probes that possess positron emission tomography (PET) and fluorescence imaging (FI) capabilities are precision medicine tools that can be used to improve patient care and outcomes. Detecting tumor lesions using PET, an extremely sensitive technique, coupled with fluorescence-guided surgical resection of said tumor lesions can maximize the removal of cancerous tissue. The development of novel molecular probes is important for targeting different biomarkers as every individual case of cancer has different characteristics. This short review will discuss some aspects of dual PET/FI probes and explore the recently reported examples.

Promise and challenges of clinical non-invasive T-cell tracking in the era of cancer immunotherapy

In the last decades, our understanding of the role of the immune system in cancer has significantly improved and led to the discovery of new immunotherapeutic targets and tools, which boosted the advances in cancer immunotherapy to fight a growing number of malignancies. Approved immunotherapeutic approaches are currently mainly based on immune checkpoint inhibitors, antibody-derived targeted therapies, or cell-based immunotherapies. In essence, these therapies induce or enhance the infiltration and function of tumor-reactive T cells within the tumors, ideally resulting in complete tumor eradication. While the clinical application of immunotherapies has shown great promise, these therapies are often accompanied either by a variety of side effects as well as partial or complete unresponsiveness of a number of patients. Since different stages of disease progression elicit different local and systemic immune responses, the ability to longitudinally interrogate the migration and expansion of immune cells, especially T cells, throughout the whole body might greatly facilitate disease characterization and understanding. Furthermore, it can serve as a tool to guide development as well as selection of appropriate treatment regiments. This review provides an overview about a variety of immune-imaging tools available to characterize and study T-cell responses induced by anti-cancer immunotherapy. Moreover, challenges are discussed that must be taken into account and overcome to use immune-imaging tools as predictive and surrogate markers to enhance assessment and successful application of immunotherapies.

Semi-Rigid (Aminomethyl) Piperidine-Based Pentadentate Ligands for Mn(II) Complexation

Two pentadentate ligands built on the 2-aminomethylpiperidine structure and bearing two tertiary amino and three oxygen donors (three carboxylates in the case of AMPTA and two carboxylates and one phenolate for AMPDA-HB) were developed for Mn(II) complexation. Equilibrium studies on the ligands and the Mn(II) complexes were carried out using pH potentiometry, ¹H-NMR spectroscopy and UV-vis spectrophotometry. The Mn complexes that were formed by the two ligands were more stable than the Mn complexes of other pentadentate ligands but with a lower pMn than Mn(EDTA) and Mn(CDTA) (pMn for Mn(AMPTA) = 7.89 and for Mn(AMPDA-HB) = 7.07). ¹H and ¹⁷O-NMR relaxometric studies showed that the two Mn-complexes were q = 1 with a relaxivity value of 3.3 mM⁻¹ s⁻¹ for Mn(AMPTA) and 3.4 mM⁻¹ s⁻¹ for Mn(AMPDA-HB) at 20 MHz and 298 K. Finally, the geometries of the two complexes were optimized at the DFT level, finding an octahedral coordination environment around the Mn²⁺ ion, and MD simulations were performed to monitor the distance between the Mn²⁺ ion and the oxygen of the coordinated water molecule to estimate its residence time, which was in good agreement with that determined using the ¹⁷O NMR data.

The aluminium-[18F]fluoride revolution: simple radiochemistry with a big impact for radiolabelled biomolecules

The aluminium-[¹⁸F]fluoride ([¹⁸F]AlF) radiolabelling method combines the favourable decay characteristics of fluorine-18 with the convenience and familiarity of metal-based radiochemistry and has been used to parallel gallium-68 radiopharmaceutical developments. As such, the [¹⁸F]AlF method is popular and widely implemented in the development of radiopharmaceuticals for the clinic. In this review, we capture the current status of [¹⁸F]AlF-based technology and reflect upon its impact on nuclear medicine, as well as offering our perspective on what the future holds for this unique radiolabelling method.

Reaction of [18F]Fluoride at Heteroatoms and Metals for Imaging of Peptides and Proteins by Positron Emission Tomography

The ability to radiolabel proteins with [18F]fluoride enables the use of positron emission tomography (PET) for the early detection, staging and diagnosis of disease. The direct fluorination of native proteins through C-F bond formation is, however, a difficult task. The aqueous environments required by proteins severely hampers fluorination yields while the dry, organic solvents that promote nucleophilic fluorination can denature proteins. To circumvent these issues, indirect fluorination methods making use of prosthetic groups that are first fluorinated and then conjugated to a protein have become commonplace. But, when it comes to the radiofluorination of proteins, these indirect methods are not always suited to the short half-life of the fluorine-18 radionuclide (110 min). This review explores radiofluorination through bond formation with fluoride at boron, metal complexes, silicon, phosphorus and sulfur. The potential for these techniques to be used for the direct, aqueous radiolabeling of proteins with [18F]fluoride is discussed.

Selective functionalization of 6-amino-6-methyl-1,4-perhydrodiazepine for the synthesis of a library of polydentate chelators

Polydentate chelators are an important part of an imaging probe, which consists of an agent that usually produces signals for imaging purposes connected to a targeting moiety. The goal of this study was to set up a generic protocol to prepare a library of polydentate ligands having a 6-amino-6-methyl-1,4-perhydrodiazepine (AMPED) core and able to chelate metal ions of interest for various diagnostic imaging techniques, including Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT). These ions, among which we can include Mn(ii), Cu(ii), Al(iii) or Ga(iii), require penta- or hexa-dentate chelators for this purpose, and the AMPED scaffold has considerable potential to support various pendant arms for coordination of such ions. AMPED already has three amino nitrogen donors; thus, only two or three additional arms should be introduced to obtain penta- or hexa-dentate systems. This condition implies that symmetrical or asymmetrical structures have to be developed, depending on the functionalization of cyclic and exocyclic amines. Starting from easily available materials, we have designed a convenient protocol for the preparation of multiple AMPED-based ligands endowed with different characteristics, several of which were synthesized as examples.