Radiochemistry with {Al18F}2+: Current status and optimization perspectives for efficient radiofluorination by complexation

Enhancing the radionuclide theranostic concept through the radiohybrid approach

Radionuclide theranostics - a fast-growing emerging field in radiopharmaceutical sciences and nuclear medicine - offers a personalised and precised treatment approach by combining diagnosis with specific and selective targeted endoradiotherapy. This concept is based on the application of the same molecule, labelled with radionuclides possessing complementary imaging and therapeutic properties, respectively. In radionuclide theranostics, radionuclide pairs consisting of the same element, such as 61/64Cu/67Cu, 203Pb/212Pb or 123/124I/131I are of significant interest due to their identical chemical and pharmacological characteristics. However, such "true matched pairs" are seldom, necessitating the use of complementary radionuclides from different elements for diagnostics and endoradiotherapy with similar chemical characteristics, such as 99mTc/186/188Re, 68Ga/177Lu or 68Ga/225Ac. Corresponding combinations of such two radionuclides in one and the same radioconjugate is referred to as a "matched pair". Notably, the pharmacological behavior remains consistent across both diagnostic and therapeutic applications with "true matched pairs", which may differ for "matched pairs". As "true matched pairs" of theranostic radioisotopes are rare and that some relevant radionuclides do not fit with the diagnostic or therapeutic counterpart, the radionuclide theranostic concept can be expanded and improved by the introduction of the radiohybrid approach. Radiohybrid (rh) ligands represent a new class of radiopharmaceutical bearing two different positions for the introduction of a (radio)metal and (radio)halogen in one molecule, which can be then used for both therapeutic and diagnostic purposes. The following review will give an insight into recent developments of this approach.

A novel Al18F-labelled NOTA-modified ubiquicidin 29-41 derivative as a bacterial infection PET imaging agent

The antimicrobial peptide ubiquicidin 29-41 (TGRAKRRMQYNRR) is a potential target for detecting bacterial infection. A novel UBI 29-41 derivative modified with 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) on the amino side of lysine was synthesized and radiolabelled with Al18F, named [18F]AlF-NOTA-UBI 29-41. The novel PET tracer maintained good in vitro stability in saline at room temperature and mouse serum at 37 °C. In vitro bacterial binding experiments indicated that the tracer specifically bound to Staphylococcus aureus. A significant difference in the uptake of [18F]AlF-NOTA-UBI 29-41 between infected muscle and inflamed muscle was observed in biodistribution. A PET imaging study in mouse models with bacterial infection and sterile inflammation showed apparent accumulation at the infection site, suggesting that the complex is a potential PET tracer for distinguishing bacterial infection from sterile inflammation.

A Lanthanide Nanoparticle-Aggregation-Induced Emission Photosensitizer Complex System Drives Coupled Triplet Energy Transfer for Enhanced Radio-Photodynamic Therapy

Cerenkov light (CL), utilized as an internal excitation source for photodynamic therapy (PDT), addresses the limitations of laser penetration and has substantial potential for seamlessly integrating clinical radiotheranostics with phototheranostics. Nevertheless, the effectiveness of CL-mediated PDT is significantly hindered by challenges, such as the low intensity of CL and inadequate energy transfer between the CL donor and photosensitizers (PSs). In this study, a novel approach is introduced for enhanced radionuclide-activated radio-photodynamic therapy utilizing a hybrid nanoparticle system composed of lanthanide nanoparticles and an aggregation-induced emission photosensitizer (AIE PS), designated LnNP-TQ NPs. This system enables lanthanide nanoparticles to optimize the decay energy of radionuclides, effectively sensitizing the AIE PS through triplet energy transfer (TET)-mediated processes with an efficiency approaching 100%. When activated by the clinical radionuclide 18F for positron emission tomography imaging, the LnNP-TQ NPs substantially inhibited tumor growth via effective singlet oxygen (1O2) generation. This strategy, which optimally harnesses radionuclide energy and achieves efficient energy transfer, offers a promising pathway for enhancing radiotherapy-phototherapy efficacy in tumor treatment.

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.

Cyclohexanediamine Triazole (CHDT) Functionalization Enables Labeling of Target Molecules with Al18F/68Ga/111In

The Al18F-labeling approach offers a one-step access to radiofluorinated biomolecules by mimicking the labeling process for radiometals. Although these labeling conditions are considered to be mild compared to classic radiofluorinations, improvements of the chelating units have led to the discovery of (±)-H3RESCA, which allows Al18F-labeling already at ambient temperature. While the suitability of (±)-H3RESCA for functionalization and radiofluorination of proteins is well established, its use for small molecules or peptides is less explored. Herein, we advanced this acyclic pentadentate ligand by introducing an alkyne moiety for the late-stage functionalization of biomolecules via click chemistry. We show that in addition to Al18F-labeling, the cyclohexanediamine triazole (CHDT) moiety allows stable complexation of 68Ga and 111In. Three novel CHDT-functionalized PSMA inhibitors were synthesized and their Al18F-, 68Ga-, and 111In-labeled analogs were subjected to a detailed in vitro radiopharmacological characterization. Stability studies in vitro in human serum revealed among others a high kinetic inertness of all radiometal complexes. Furthermore, the Al18F-labeled PSMA ligands were characterized for their biodistribution in a LNCaP derived tumor xenograft mouse model by PET imaging. One radioligand, Al[18F]F-CHDT-PSMA-1, bearing a small azidoacetyl linker at the glutamate-urea-lysine motif, provided an in vivo performance comparable to that of [18F]PSMA-1007 but with even higher tumor-to-blood and tumor-to-muscle ratios at 120 min p.i. Overall, our results highlight the suitability of the novel CHDT moiety for functionalization and radiolabeling of small molecules or peptides with Al18F, 68Ga, and 111In and the triazole ring seems to entail favorable pharmacokinetic properties for molecular imaging purposes.

Synthesis and properties of metal trifluoride complexes with amide-functionalised tacn macrocycles and radiofluorination of [GaF3(L1)] by 18F/19F isotopic exchange

Three amide-functionalised tacn macrocyclic derivatives (tacn = 1,4,7-triazacyclononane) are reported, two tris-amide derivatives, L1 containing three -CH2C(O)NHPh pendant arms, L2 containing three -CH2CH2C(O)NHiPr pendant arms, and one mono-amide, L3, containing iPr groups on two of the tacn amine functions and a single -CH2C(O)NHPh function on the third. The reactions of these new ligands towards [MF3(dmso)(OH2)2] (M = Al, Ga) and towards FeF3·3H2O in alcoholic solution afford the complexes [MF3(L)] (L = L1-L3) in good yields as powdered solids. These complexes are characterised by IR and multinuclear NMR spectroscopy (diamagnetic species only) and mass spectrometry. [GaF3(L1)], [GaF3(L3)] and [FeF3(L3)] are also characterised by single crystal X-ray analysis. The corresponding reactions involving [InF3(dmso)(OH2)2] yield mixtures of products (along with F-), consistent with the M-F bond strengths decreasing as group 13 is descended. A few crystals of the target complex, [InF3(L2)], were also obtained from one such reaction. All of the complexes adopt fac-octahedral coordination via the amine N-donor atoms and retain the three fluoride ligands both in solution and in the solid state. Extensive intramolecular hydrogen-bonding involving the amide NH pendant groups and the MF3 moieties is evident in the crystal structures. In the isostructural [MF3(L3)] (M = Ga, Fe) complexes the head-to-tail C(O)NH⋯F H-bonded dimers observed in the solid state resemble those found frequently in organic compounds and that form the cornerstone of many supramolecular assemblies. This is consistent with the MF3 fragments being strong H-bond acceptors. Radiofluorination of [GaF3(L1)] by 18F/19F isotopic exchange in MeOH at 3 μmol mL-1 precursor concentration and using aqueous [18F]F- in target water (75% : 25%) with gentle heating (80 °C, 10 min) gave ca. 20% radiochemical yield of [Ga18FF2(L1)]. In contrast, no 18F incorporation occurs with [GaF3(L3)] under any of the conditions explored.

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).

Automated radiosynthesis and preclinical evaluation of two new PSMA-617 derivatives radiolabelled via [18F]AlF2+ method

BACKGROUND

In the last decade the development of new PSMA-ligand based radiopharmaceuticals for the imaging and therapy of prostate cancer has been a highly active and important area of research. The most promising derivative in terms of interaction with the antigen and clinical properties has been found to be "PSMA-617", and its lutetium-177 radiolabelled version has recently been approved by EU and USA regulatory agencies for therapeutic purposes. For the above reasons, the development of new derivatives of PSMA-617 radiolabelled with fluorine-18 may still be of great interest. This paper proposes the comparison of two different PSMA-617 derivatives functionalized with NODA and RESCA chelators, respectively, radiolabelled via [18F]AlF2+ complexation.

RESULTS

The organic synthesis of two PSMA-617 derivatives and their radiolabelling via [18F]AlF2+ complexation resulted to proceed efficiently and successfully. Moreover, stability in solution and in plasma has been evaluated. The whole radiosynthesis procedure has been fully automated, and the final products have been obtained with radiochemical yield and purity potentially suitable for clinical studies. The biodistribution of the two derivatives was performed both in prostate cancer and glioma tumour models. Compared with the reference [18F]F-PSMA-1007 and [18F]F-PSMA-617-RESCA, [18F]F-PSMA-617-NODA derivative showed a higher uptake in both tumors, faster clearance in non-target organs, and lower uptake in salivary glands.

CONCLUSION

PSMA-617 NODA and RESCA derivatives were radiolabelled successfully via [18F]AlF2+ chelation, the former being more stable in solution and human plasma. Moreover, preclinical biodistribution studies showed that [18F]F-PSMA-617-NODA might be of potential interest for clinical applications.

Investigating the Mechanism of Aluminum Fluoride Chelation

Aluminum fluoride (AlF) complexes have been used over the past decade to incorporate [18F]fluoride into large biomolecules in a highly selective fashion by using relatively facile conditions. However, despite their widespread usage, there are a large number of variations in the reaction conditions, without a definitive discussion provided on the mechanism to understand how these changes would alter the end result. Herein, we report a detailed mechanistic investigation of the reaction, using a mixture of theoretical studies, fluorine-19 and fluorine-18 chemistry, and the consequences it has on the efficient clinical translation of AlF-containing imaging agents.

18 F-Fluorination of a supported 2-(aryl-di-tert-butylsilyl)-N-methyl-imidazole for indirect 18 F-labeling of a VH H single-variable domain

Anchoring an imidazole-di-tert-butyl-arylsilane possessing an azido group to a polystyrene resin provided a heterogeneous precursor that was radiolabeled easily using aqueous [18 F]fluoride. After optimizing the conditions (i.e., using DMSO as solvent and heating at 160°C for 15 min), the desired [18 F]fluorosilane was obtained in 24% radiochemical yield (RCY) and 78% radiochemical purity (RCP) using solid-phase extraction as sole purification. Then, this compound was conjugated by strain-promoted alkyne-azide cycloaddition to a model single-variable domain possessing a cyclooctyne tag, yielding to the desired 18 F-labeled bioconjugate in 2% RCY and >95% RCP after purification by a size exclusion chromatography.

Optimization of a 1,4,7-Triazacyclononane-1,4-diacetic acid (NODA) Derivative Radiofluorination by Al18 F Complexation Using a Design of Experiments Approach

Fluorine-18 (18 F) is the most favorable positron emitter for radiolabeling Positron Emission Tomography (PET) probes. However, conventional 18 F labeling through covalent C-F bond formation is challenging, involving multiple steps and stringent conditions unsuitable for sensitive biomolecular probes whose integrity may be altered. Over the past decade, an elegant new approach has been developed involving the coordination of an aluminum fluoride {Al18 F} species in aqueous media at a late-stage of the synthetic process. The objective of this study was to implement this method and to optimize radiolabeling efficiency using a Design of Experiments (DoE). To assess the impact of various experimental parameters on {Al18 F} incorporation, a pentadentate chelating agent NODA-MP-C4 was prepared as a model compound. This model carried a thiourea function present in the final conjugates resulting from the grafting of the chelating agent onto the probe. The formation of the radioactive complex Al18 F-NODA-MP-C4 was studied to achieve the highest radiochemical conversion. A complementary "cold" series study using the natural isotope 19 F was also conducted to guide the radiochemical operating conditions. Ultimately, Al18 F-NODA-MP-C4 was obtained with a reproducible and satisfactory radiochemical conversion of 79±3.5 % (n=5).

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).

Design and synthesis of a new bifunctional chelating agent: Application for Al 18F/177Lu complexation

Theranostic and personalized medicine are blooming strategies to improve oncologic patients' health care and facilitate early treatment. While 18F-radiochemistry for theranostic application is attractive due to its imaging properties, combining diagnosis by positron emission tomography (PET) via aluminum-fluoride-18 and β- therapy with lutetium-177 is relevant. Nevertheless, it requires the use of two different chelating agents, which are NOTA and DOTA for aluminum-fluoride-18 and lutetium-177 radiolabeling, respectively. To overcome this issue, we propose herein the synthesis of a new hybrid chelating agent named NO2A-AHM, which can be labeled with different types of emitters (β+, β- and γ) using the mismatched Al18F/177Lu pair. NO2A-AHM, is based on a hydrazine moiety functionalized by a NOTA cycle, a chelating arm, and a linker with a maleimide function. This design is chosen to increase the flexibility and allow the formation of 5 up to 7 coordination bonds with metal ions. Moreover, this agent can be coupled to targeting moieties containing a thiol function, such as peptides, to increase selectivity towards specific cancer cells. Experimental complexation and computational chemistry studies are performed to confirm the capacity of our chelating agent to label both aluminum-fluoride and lutetium using molecular modeling approaches at Density Functional Theory (DFT) level. The proof of concept of the ability of NO2A-AHM to complex both aluminum-fluoride-18, for PET imaging applications, and lutetium-177 for radiotherapy has shown encouraging results which is prominent for the development of a fully consistent theranostic approach.

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.