Synthesis of C-functionalized TE1PA and comparison with its analogues. An example of bioconjugation on 9E7.4 mAb for multiple myeloma 64Cu-PET imaging

Relevance of Oxocyclam from Palladium(II) Coordination to Radiopharmaceutical Development

We provide a comprehensive study of the coordination of oxocyclam with palladium(II), including presentation of a novel bifunctional analogue, p-H2N-Bn-oxocyclam, bearing an aniline pendant. The complexation of palladium(II) with oxocyclam was examined by various techniques, including NMR analysis and potentiometric titrations which revealed that the Pd(II) complex can adopt different configurations such as trans-I and trans-III. In addition, oxocyclam forms a thermodynamically stable palladium(II) complex, the stabilization being attributed to the deprotonation of the amide function. The crystal structures of [Pd(H-1oxocyclam)]+ and [Pd(oxocyclam)]2+ were obtained, revealing the structural details previously anticipated, including, in the second case, the presence of the proton on the carbonyl oxygen atom. Additionally, the study explored the redox behavior of the Pd(II)-oxocyclam complex through reduction and oxidation voltammograms at different pH values. Successful 109Pd-labeling of oxocyclam and p-H2N-Bn-oxocyclam at pH 3.5 demonstrated high labeling efficiencies, whatever the species formed. The stability of the radiocomplexes was assessed and moderate transchelation toward EDTA was observed. Overall, oxocyclam displayed favorable properties for Pd(II) coordination and radiolabeling, suggesting its potential as a chelating agent for this metal in palladium-based applications.

Selective synthesis of an elusive C-functional bis-cyclam and study of its inhibition of the CXCR4 chemokine receptor

This article presents the controlled synthesis of a rare example of C,C'-linked bis-cyclam architecture in mild conditions through the "bis-aminal" route previously used for the advantageous synthesis of cyclam, N- and C-functional cyclams and N,N'-bis-cyclams. Two synthetic pathways were explored with the smart design of α,β-unsaturated ketones or alkyl halides bis-cyclizing agents. The first led to the isolation of a key intermediate for the future design of N-functionalized bis-cyclams, whereas the second allowed the preparation of the targeted C,C'-xylylene-bis-cyclam under mild conditions with decent yield. This compound was then studied as a CXCR4 receptor inhibitor, one of the main applications known for bis-macrocyclic compounds, in particular in the context of HIV (human immunodeficiency virus) infection. Although results demonstrated that its potency is lower (i.e. 137-fold higher IC50) than the gold standard AMD3100 against HIV infection, clear evidence of CXCR4 inhibition is presented, confirming the potential of this novel architecture and related compounds in this research field.

Exploring the Limits of Ligand Rigidification in Transition Metal Complexes with Mono-N-Functionalized Pyclen Derivatives

We report the synthesis of a new family of side-bridged pyclen ligands. The incorporation of an ethylene bridge between two adjacent nitrogen atoms was reached from the pyclen-oxalate precursor described previously. Three new side-bridged pyclen macrocycles, Hsb-3-pc1a, sb-3-pc1py, and Hsb-3-pc1pa, were obtained with the aim to assess their coordination properties toward Cu2+ and Zn2+ ions. We also prepared their nonreinforced analogues H3-pc1a, 3-pc1py, and H3-pc1pa as comparative benchmarks. The two series of ligands were characterized and their coordination properties were investigated in detail. The Zn2+ and Cu2+ complexes with the nonside-bridged series H3-pc1a, 3-pc1py, and H3-pc1pa were successfully isolated and their structures were assessed by X-ray diffraction studies. In the case of the side-bridged family, the synthesis of the complexes was far more difficult and, in some cases, unsuccessful. The results of our studies demonstrate that this difficulty is related to the extreme stiffening and basicity of such side-bridged pyclens.

HNODThia: A Promising Chelator for the Development of 64Cu Radiopharmaceuticals

Herein, we present the synthesis and coordination chemistry of copper(II) and zinc(II) complexes of two novel heterocyclic triazacyclononane (tacn)-based chelators (HNODThia and NODThia-AcNHEt). The chelator HNODThia was further derivatized to obtain a novel PSMA-based bioconjugate (NODThia-PSMA) and a bifunctional photoactivatable azamacrocyclic analogue, NODThia-PEG3-ArN3, for the development of copper-64 radiopharmaceuticals. 64Cu radiolabeling experiments were performed on the different metal-binding chelates, whereby quantitative radiochemical conversion (RCC) was obtained in less than 10 min at room temperature. The in vitro stability of NODThia-PSMA in human plasma was assessed by ligand-challenge and copper-exchange experiments. Next, we investigated the viability of the photoactivatable analog (NODThia-PEG3-ArN3) for the light-induced photoradiosynthesis of radiolabeled proteins. One-pot photoconjugation reactions to human serum albumin (HSA) as a model protein and the clinically relevant monoclonal antibody formulation MetMAb were performed. [64Cu]Cu-7-azepin-HSA and [64Cu]Cu-7-azepin-onartuzumab were prepared in less than 15 min by irradiation at 395 nm, with radiochemical purities (RCP) of >95% and radiochemical yields (RCYs) of 42.7 ± 5.3 and 49.6%, respectively. Together, the results obtained here open the way for the development of highly stable 64Cu-radiopharmaceuticals by using aza-heterocyclic tacn-based chelators, and the method can easily be extended to the development of 67Cu radiopharmaceuticals for future applications in molecularly targeted radio(immuno)therapy.

Multistep synthesis of a novel copper complex with potential for Alzheimer's disease diagnosis

Positron emission tomography (PET) imaging of Aβ plaques, is recognized as a tool for the diagnosis of Alzheimer's disease. As a contribution to the development of new strategies for early diagnosis of the disease, using PET medical imaging technique, a new copper complex, the [Cu(TE1PA-ONO)]+ was synthesized in ten steps. The key step of our strategy is the coupling of a monopicolinate-N-alkylated cyclam-based ligand with a moiety capable of recognizing Aβ plaques via a successful and challenging Buchwald-Hartwig coupling reaction. To our knowledge, it is the first time that such a strategy is used to functionalize polyazamacrocyclic derivatives. The thermodynamic stability constants determined in MeOH/H2O solvent indicate that the attachment of this moiety does not weaken the chelating properties of TE1PA-ONO ligand in relation to parent HTE1PA. The novel complex described here is able to recognize amyloid plaques in brain sections from Alzheimer's disease patients and shows low toxicity to human neuronal cells.

Preclinical Evaluation of a 64Cu-Based Theranostic Approach in a Murine Model of Multiple Myeloma

Although the concept of theranostics is neither new nor exclusive to nuclear medicine, it is a particularly promising approach for the future of nuclear oncology. This approach is based on the use of molecules targeting specific biomarkers in the tumour or its microenvironment, associated with optimal radionuclides which, depending on their emission properties, allow the combination of diagnosis by molecular imaging and targeted radionuclide therapy (TRT). Copper-64 has suitable decay properties (both β⁺ and β- decays) for PET imaging and potentially for TRT, making it both an imaging and therapy agent. We developed and evaluated a theranostic approach using a copper-64 radiolabelled anti-CD138 antibody, [⁶⁴Cu]Cu-TE1PA-9E7.4 in a MOPC315.BM mouse model of multiple myeloma. PET imaging using [⁶⁴Cu]Cu-TE1PA-9E7.4 allows for high-resolution PET images. Dosimetric estimation from ex vivo biodistribution data revealed acceptable delivered doses to healthy organs and tissues, and a very encouraging tumour absorbed dose for TRT applications. Therapeutic efficacy resulting in delayed tumour growth and increased survival without inducing major or irreversible toxicity has been observed with 2 doses of 35 MBq administered at a 2-week interval. Repeated injections of [⁶⁴Cu]Cu-TE1PA-9E7.4 are safe and can be effective for TRT application in this syngeneic preclinical model of MM.

On the dissociation pathways of copper complexes relevant as PET imaging agents

Several bifunctional chelators have been synthesized in the last years for the development of new ⁶⁴Cu-based PET agents for in vivo imaging. When designing a metal-based PET probe, it is important to achieve high stability and kinetic inertness once the radioisotope is coordinated. Different competitive assays are commonly used to evaluate the possible dissociation mechanisms that may induce Cu(II) release in the body. Among them, acid-assisted dissociation tests or transchelation challenges employing EDTA or SOD are frequently used to evaluate both solution thermodynamics and the kinetic behavior of potential metal-based systems. Despite of this, the Cu(II)/Cu(I) bioreduction pathway that could be promoted by the presence of bioreductants still remains little explored. To fill this gap we present here a detailed spectroscopic study of the kinetic behavior of different macrocyclic Cu(II) complexes. The complexes investigated include the cross-bridge cyclam derivative [Cu(CB-TE1A)]⁺, whose structure was determined using single-crystal X-ray diffraction. The acid-assisted dissociation mechanism was investigated using HClO₄ and HCl to analyse the effect of the counterion on the rate constants. The complexes were selected so that the effects of complex charge and coordination polyhedron could be assessed. Cyclic voltammetry experiments were conducted to investigate whether the reduction to Cu(I) falls within the window of common bioreducing agents. The most striking behavior concerns the [Cu(NO2Th)]²⁺ complex, a 1,4,7-triazacyclononane derivative containing two methylthiazolyl pendant arms. This complex is extremely inert with respect to dissociation following the acid-catalyzed mechanism, but dissociates rather quickly in the presence of a bioreductant like ascorbic acid.

Relevance of Palladium to Radiopharmaceutical Development Considering Enhanced Coordination Properties of TE1PA

The limited use of palladium‐103 and ‐109 radionuclides for molecular radiotherapy is surely due to the lack of appropriate ligands capable of fulfilling all criteria required for application in nuclear medicine. Furthermore, the thermodynamic properties of these complexes in solution remain difficult to establish. The challenge is compounded when considering that radiolabeling of compounds for translation to clinical trials requires fast complexation. Thus, the coordination of Pd(II) and ^103/109Pd‐nuclides is a huge challenge in terms of molecular design and physicochemical characterization. Herein, we report a comprehensive study highlighting TE1PA, a monopicolinate cyclam – already established in nuclear imaging with ⁶⁴Cu‐PET (positron emission tomography) imaging tracers – as a highly relevant chelator for natural Pd and subsequently ¹⁰⁹Pd‐nuclide. The structural, thermodynamic, kinetic and radiolabeling studies of Pd(II) with TE1PA, as well as the comparison of this complex with three structurally related derivatives, support palladium‐TE1PA radiopharmaceuticals as leading candidates for targeted nuclear medicine.

Cyclam with a phosphinate-bis(phosphonate) pendant arm is a bone-targeting carrier of copper radionuclides

Ligands combining a bis(phosphonate) group with a macrocycle function as metal isotope carriers for radionuclide-based imaging and for treating bone metastases associated with several cancers. However, bis(phosphonate) pendant arms often slow down complex formation and decrease radiochemical yields. Nevertheless, their negative effect on complexation rates may be mitigated by using a suitable spacer between bis(phosphonate) and the macrocycle. To demonstrate the potential of bis(phosphonate) bearing macrocyclic ligands as a copper radioisotope carrier, we report the synthesis of a new cyclam derivative bearing a phosphinate-bis(phosphonate) pendant (H₅te1P^BP). The ligand showed a high selectivity to Cu^II over Zn^II and Ni^II ions, and the bis(phosphonate) group was not coordinated in the Cu^II complex, strongly interacting with other metal ions in solution. The Cu^II complex formed quickly, in 1 s, at pH 5 and at a millimolar scale. The complexation rates significantly differed under a ligand or metal ion excess due to the formation of reaction intermediates differing in their metal-to-ligand ratio and protonation state, respectively. The Cu^II-te1P^BP complex also showed a high resistance to acid-assisted hydrolysis (t_1/2 2.7 h; 1 M HClO₄, 25 °C) and was effectively adsorbed on the hydroxyapatite surface. H₅te1P^BP radiolabeling with [⁶⁴Cu]CuCl₂ was fast and efficient, with specific activities of approximately 30 GBq ⁶⁴Cu per 1 μmol of ligand (pH 5.5, room temperature, 30 min). In a pilot experiment, we further demonstrated the excellent suitability of [⁶⁴Cu]Cu^II-te1P^BP for imaging active bone compartments by dedicated small animal PET/CT in healthy mice and subsequently in a rat femoral defect model, in direct comparison with [¹⁸F]fluoride. Moreover, [⁶⁴Cu]Cu^II-te1P^BP showed a higher uptake in critical bone defect regions. Therefore, our study highlights the potential of [⁶⁴Cu]Cu^II-te1P^BP as a PET radiotracer for evaluating bone healing in preclinical and clinical settings with a diagnostic value similar to that of [¹⁸F]fluoride, albeit with a longer half-life (12.7 h) than ¹⁸F (1.8 h), thereby enabling extended observation times.

AAZTA-Derived Chelators for the Design of Innovative Radiopharmaceuticals with Theranostic Applications

With the development of 68Ga and 177Lu radiochemistry, theranostic approaches in modern nuclear medicine enabling patient-centered personalized medicine applications have been growing in the last decade. In conjunction with the search for new relevant molecular targets, the design of innovative chelating agents to easily form stable complexes with various radiometals for theranostic applications has gained evident momentum. Initially conceived for magnetic resonance imaging applications, the chelating agent AAZTA features a mesocyclic seven-membered diazepane ring, conferring some of the properties of both acyclic and macrocyclic chelating agents. Described in the early 2000s, AAZTA and its derivatives exhibited interesting properties once complexed with metals and radiometals, combining a fast kinetic of formation with a slow kinetic of dissociation. Importantly, the extremely short coordination reaction times allowed by AAZTA derivatives were particularly suitable for short half-life radioelements (i.e., 68Ga). In view of these particular characteristics, the scope of this review is to provide a survey on the design, synthesis, and applications in the nuclear medicine/radiopharmacy field of AAZTA-derived chelators.

Copper(II) and Zinc(II) Complexation with N Ethylene hydroxycyclams and Consequences on the Macrocyclic Backbone Configuration

We report a series of four cyclams and cross-bridged cyclams, N-functionalized by one hydroxyethyl arm, which may incorporate additional methyl(s) group(s). The Cu(II) and Zn(II) complexes of these ligands were...

Complexation of C-Functionalized Cyclams with Copper(II) and Zinc(II): Similarities and Changes When Compared to Parent Cyclam Analogues

Herein, we report a comprehensive coordination study of the previously reported ligands cyclam, CB-cyclam, TMC, DMC, and CB-DMC and of their C-functional analogues, cyclam-E, CB-cyclam-E, TMC-E, DMC-E, and CB-DMC-E. This group of ligands includes cyclam, cross-bridged cyclams, their di- or tetramethylated derivatives, and the analogues bearing an additional hydroxyethyl group on one β-N position of the ring. The Cu(II) and Zn(II) complexes of these macrocycles have been highlighted previously for the biological interest, but the details of their structures in the solid state and in solution remained largely unexplored. In particular, we analyzed the impact that adding noncoordinating N-methyl and C-hydroxyethyl functionalities has in the structures of the complexes. All the Cu(II) and Zn(II) complexes were synthesized and investigated using single crystal X-ray diffraction and NMR, electronic absorption, and EPR spectroscopies, along with DFT studies. Dissociation kinetics experiments in acidic conditions and electrochemical studies were also performed. Special attention was paid to analyze the different configurations present in solution and in the solid state, as well as the impact of the C-appended hydroxyethyl group on the coordination behavior. Various ratios of the trans-I, trans-III, and cis-V configurations have been observed depending on the degree of N-methylation and the presence of the ethylene cross-bridge.

State of the Art in Radiolabeling of Antibodies with Common and Uncommon Radiometals for Preclinical and Clinical Immuno-PET

Inert and stable radiolabeling of monoclonal antibodies (mAb), antibody fragments, or antibody mimetics with radiometals is a prerequisite for immuno-PET. While radiolabeling is preferably fast, mild, efficient, and reproducible, especially when applied for human use in a current Good Manufacturing Practice compliant way, it is crucial that the obtained radioimmunoconjugate is stable and shows preserved immunoreactivity and in vivo behavior. Radiometals and chelators have extensively been evaluated to come to the most ideal radiometal–chelator pair for each type of antibody derivative. Although PET imaging of antibodies is a relatively recent tool, applications with ⁸⁹Zr, ⁶⁴Cu, and ⁶⁸Ga have greatly increased in recent years, especially in the clinical setting, while other less common radionuclides such as ⁵²Mn, ⁸⁶Y, ⁶⁶Ga, and ⁴⁴Sc, but also ¹⁸F as in [¹⁸F]AlF are emerging promising candidates for the radiolabeling of antibodies. This review presents a state of the art overview of the practical aspects of radiolabeling of antibodies, ranging from fast kinetic affibodies and nanobodies to slow kinetic intact mAbs. Herein, we focus on the most common approach which consists of first modification of the antibody with a chelator, and after eventual storage of the premodified molecule, radiolabeling as a second step. Other approaches are possible but have been excluded from this review. The review includes recent and representative examples from the literature highlighting which radiometal–chelator–antibody combinations are the most successful for in vivo application.

Cyclam-Based Chelators Bearing Phosphonated Pyridine Pendants for 64Cu-PET Imaging: Synthesis, Physicochemical Studies, Radiolabeling, and Bioimaging

Herein we present the preparation of two novel cyclam-based macrocycles (te1pyp and cb-te1pyp), bearing phosphonate-appended pyridine side arms for the coordination of copper(II) ions in the context of ⁶⁴Cu PET imaging. The two ligands have been prepared through conventional protection-alkylation sequences on cyclam, and their coordination properties have been thoroughly investigated. The corresponding copper complexes have been fully characterized in the solid state (X-ray diffraction analysis) and in solution (EPR and UV-vis spectroscopies). Potentiometric studies combined with spectrometry have also allowed us to determine their thermodynamic stability constants, confirming their high affinity for copper(II) cations. The kinetic inertness of the complexes has been verified by acid-assisted dissociation experiments, enabling their use in ⁶⁴Cu-PET imaging in mice for the first time. Indeed, the two ligands could be quantitatively radiolabeled under mild conditions, and the resulting ⁶⁴Cu complexes have demonstrated excellent stability in serum. PET imaging demonstrated a set of features emerging from the combination of picolinates and phosphonate units: high stability in vivo, fast clearance from the body via renal elimination, and most interestingly, very low fixation in the liver. This is in contrast with what was observed for monopicolinate cyclam (te1pa), which had a non-negligible accumulation in the liver, owing probably to its different charge and lipophilicity. These results thus pave the way for the use of such phosphonated pyridine chelators for in vivo ⁶⁴Cu-PET imaging.

In Vivo Albumin-Binding of a C-Functionalized Cyclam Platform for 64 Cu-PET/CT Imaging in Breast Cancer Model

An improved glucose-chelator-albumin bioconjugate (GluCAB) derivative, GluCAB-2^Mal , has been synthesized and studied for in vivo ⁶⁴ Cu-PET/CT imaging in breast cancer mice models together with its first-generation analogue GluCAB-1^Mal . The radioligand works on the principle of tumor targeting through the enhanced permeability and retention (EPR) effect with a supportive role played by glucose metabolism. [⁶⁴ Cu]Cu-GluCAB-2^Mal (99 % RCP) exhibited high serum stability with immediate binding to serum proteins. In vivo experiments for comparison between tumor targeting of [⁶⁴ Cu]Cu-GluCAB-2^Mal and previous-generation [⁶⁴ Cu]Cu-GluCAB-1^Mal encompassed microPET/CT imaging and biodistribution analysis in an allograft E0771 breast cancer mouse model. Tumor uptake of [⁶⁴ Cu]Cu-GluCAB-2^Mal was clearly evident with twice as much accumulation as compared to its predecessor and a tumor/muscle ratio of up to 5 after 24 h. Further comparison indicated a decrease in liver accumulation for [⁶⁴ Cu]Cu-Glu-CAB-2^Mal .

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer. Thus, the advantages of AE emitters for cancer therapy are most likely to be realized by their selective delivery into the nucleus of the malignant cells. To achieve this goal, delivery systems must combine a challenging complex of properties that not only provide cancer cell preferential recognition but also cell entry followed by transport into the cell nucleus. A promising strategy for achieving this is the recruitment of natural cell transport processes of macromolecules, involved in each of the aforementioned steps. To date, a number of constructs exploiting intracellular transport systems have been proposed for AE emitter delivery to the nucleus of a targeted cell. An example of such a multifunctional vehicle that provides smart step-by-step delivery is the so-called modular nanotransporter, which accomplishes selective recognition, binding, internalization, and endosomal escape followed by nuclear import of the delivered radionuclide. The current review will focus on delivery systems utilizing various intracellular transport pathways and their combinations in order to provide efficient targeting of AE to the cancer cell nucleus.

TE1PA as Innovating Chelator for 64Cu Immuno-TEP Imaging: A Comparative in Vivo Study with DOTA/NOTA by Conjugation on 9E7.4 mAb in a Syngeneic Multiple Myeloma Model

Following the successful synthesis of a C-functionalized version of the TE1PA ligand, a monopicolinate cyclam, we looked to demonstrate its in vivo properties versus DOTA and NOTA, after conjugation on the 9E7.4 rat antibody, an IgG_2a against CD138 murine, which has relevant properties for multiple myeloma targeting. For each ligand, different conjugation approaches had been considered to select the most appropriate for the comparative study. The p-SCN-Bn-TE1PA, NHS-DOTA, and p-SCN-Bn-NOTA were finally chosen for conjugation and radiolabeling tests. For in vivo comparison, we used a model of subcutaneous grafted mice with 5T33 tumor cells. In vitro tests and immuno-PET study highlighted ⁶⁴Cu-9E7.4-p-SCN-Bn-NOTA as the least attractive. Further competitive biodistribution and hepatic metabolic studies at 2, 24, and 48 h post-injection (100 μg radiolabeled with 10 MBq of ⁶⁴Cu) were then performed with the ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA and ⁶⁴Cu-9E7.4-NHS-DOTA. Results show a better in vivo resistance of ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA to transchelation compared to ⁶⁴Cu-9E7.4-NHS-DOTA, especially at later times. This was confirmed with ⁶⁴Cu-9E7.4-p-SCN-Bn-NOTA at 48 h PI. ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA also demonstrated an excellent hepatic clearance. ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA displayed an overall superiority compared to ⁶⁴Cu-9E7.4-NHS-DOTA and ⁶⁴Cu-9E7.4-p-SCN-Bn-NOTA in terms of in vivo stability, reinforcing the usefulness of the p-SCN-Bn-TE1PA ligand for ⁶⁴Cu immuno-PET imaging.