A novel side-bridged hybrid phosphonate/acetate pendant cyclam: synthesis, characterization, and 64Cu small animal PET imaging

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.

New synthesis of phenyl-isothiocyanate C-functionalised cyclams. Bioconjugation and (64)Cu phenotypic PET imaging studies of multiple myeloma with the te2a derivative

Azamacrocyclic bifunctional chelating agents (BCAs) are essential for the development of radiopharmaceuticals in nuclear medicine and we wish to prove that their bioconjugation by a function present on a carbon atom of the macrocyclic skeleton is a solution of choice to maintain their in vivo inertness. Based on our very recent methodology using a bisaminal template and selective N-alkylation approach, a new synthesis of conjugable C-functionalised teta, te2a and cb-te2a has been developed. These chelators have indeed a growing interest in nuclear medicine for positron emission tomography (PET) and radioimmunotherapy (RIT) where they show in several cases better complexation properties than dota or dota-like macrocycles, especially with (64)Cu or (67)Cu radioisotopes. Chelators are bearing an isothiocyanate grafting function introduced by C-alkylation to avoid as much as possible a critical decrease of their chelating properties. The synthesis is very efficient and yields the targeted ligands, teta-Ph-NCS, te2a-Ph-NCS and cb-te2a-Ph-NCS without fastidious work-up and could be easily extended to other cyclam based-BCAs. The newly synthetised te2a-Ph-NCS has been conjugated to an anti mCD138 monoclonal antibody (mAb) to evaluate its in vivo behavior and potentiality as BCA and to explore a first attempt of PET-phenotypic imaging in multiple myeloma (MM). Mass spectrometry analysis of the immunoconjugate showed that up to 4 chelates were conjugated per 9E7.4 mAb. The radiolabeling yield and specific activity post-purification of the bioconjugate 9E7.4-CSN-Ph-te2a were 95 ± 2.8% and 188 ± 27 MBq mg(-1) respectively and the immunoreactivity of (64)Cu-9E7.4-CSN-Ph-te2a was 81 ± 7%. Animal experiments were carried out on 5T33-Luc(+) tumor bearing mice, either in subcutaneous or orthotopic. To achieve PET imaging, mice were injected with (64)Cu-9E7.4-CNS-Ph-te2a and acquisitions were conducted 2 and 20 h post-injection (PI). A millimetric bone uptake was localised in a sacroiliac of a MM orthotopic tumor. Nonspecific uptakes were observed at 2 h PI but, unlike for the tumor, a significant decrease was observed at 20 h PI which improves the contrast of the images.

Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn²⁺, Fe²⁺, and Cu²⁺ cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu²⁺ complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of these complexes were determined by means of solid state magnetic moment, with a low value of μ = 3.10 μ_B for the Fe²⁺ complex suggesting it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu²⁺ complex, while the μ = 5.52 μ_B value for the Mn²⁺ complex suggests it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe²⁺ complex as the oxidation catalysts most worthy of continued development.

Radiometals: towards a new success story in nuclear imaging?

The use of radiometal isotopes in positron emission tomography: a new success story in nuclear imaging?

Monopicolinate-dipicolyl derivative of triazacyclononane for stable complexation of Cu2+ and 64Cu2+

The synthesis and characterization of Hno1pa2py, a new tacn-based ligand, is reported. The complexation process with Cu(2+) was proved to be very fast even in acidic medium. Potentiometric titrations allowed us to establish that Hno1pa2py exhibits an overall low basicity as well as a high selectivity for Cu(2+) over Zn(2+) cations. The copper(II) complex was synthesized and characterized using UV-vis and EPR spectroscopies and density functional theory (DFT) calculations. The studies clearly showed that the [Cu(no1pa2py)](+) complex is present in solution as a mixture of two isomers in which the ligand is coordinated to the metal center using a N5O donor set with the metal center in a distorted octahedral geometry. The very high kinetic inertness of the [Cu(no1pa2py)](+) complex was demonstrated by using acid-assisted dissociation assays as well as cyclic voltammetry. Preliminary investigations of (64)Cu complexation were performed to validate the potential use of such chelating agent for further application in nuclear medicine. The X-ray crystal structures of copper(II) complexes of L1, the ester derivative of Hno1pa2py, have been determined.

Synthesis of an activated phosphonated bifunctional chelate with potential for PET imaging and radiotherapy

The synthesis of a phosphonated acyclic bifunctional chelate L* for the labeling of biomaterial is described. L* is based on a pyridine backbone, functionalized in ortho positions by aminomethyl-bis-methylphosphonic acids, and, in the para position, by a side chain containing a reactive NHS carbamate function. The stability of L* in aqueous solutions at different pH values was studied by mass spectrometry, showing the activated function to be sensitive to hydrolysis above neutral pH. The reactivity of L* towards amine functions was tested using ethylamine under different conditions of pH and concentrations, and by the labeling of two reference peptides containing both an N-terminal amino function and a ε-amino group of a lysine residue in the backbone, and a supplementary thiol group of a cysteine residue for one of these two peptides. The results showed the coupling to be efficient at pH 8.0, with a total selectivity for the terminal amine function with respect to lysine and cysteine. The labeling was further performed on B28-13, a mouse monoclonal antibody specifically recognizing tenascin-C protein in human cancer. The labeled antibody was characterized by means of mass spectrometry and spectrofluorimetry, unraveling a labeling ratio of one chelate per antibody. Finally, the affinity of the labeled antibody towards its target was controlled by immunofluorescence staining experiments on human colon cancer biopsies, confirming the affinity of the labeled peptide for tenascin-C.

Cyclam-based polymeric copper chelators for gene delivery and potential PET imaging

A series of reducible polycationic copper chelators (RPCs) based on 1,4,8,11-tetraazacyclotetradecane (cyclam) were synthesized by Michael addition. Molecular weight of the polycations was controlled by reaction stoichiometry and reaction conditions, resulting in polymers with molecular weights ranging from 4400 to 13 800. The cyclam moieties in the polycations retained their ability to form complexes with Cu(II). The presence of disulfide bonds in the polycations resulted in substantially lower cytotoxicity than control 25 kDa poly(ethyleneimine). RPC as well as their complexes with Cu(II) exhibited high transfection activity in vitro. The reported polycationic Cu(II) chelates represent promising nucleic acid delivery vectors with potential for future theranostic applications.

Antibody delivery of drugs and radionuclides: factors influencing clinical pharmacology

The therapeutic rationale of antibody conjugates is the selective delivery of a cytotoxin to tumor cells via binding and internalization of the monoclonal antibodies to a specific cell-surface antigen, thereby enhancing the therapeutic index of the cytotoxin. The key structural and functional components of an antibody conjugate are the antibody, the linker and the cytotoxin (chemical or radionuclide) with each component being critical for the successful development of the conjugate. Considerable efforts have been made in understanding the pharmacokinetics, pharmacodynamics, tissue distribution, metabolism and pharmacologic effects of these complex macromolecular entities. The purpose of this article is to discuss the properties and various structural components of antibody conjugates that influence their clinical pharmacology.

Synthesis, Cu(II) complexation, 64Cu-labeling and biological evaluation of cross-bridged cyclam chelators with phosphonate pendant arms

A new class of cross-bridged cyclam-based macrocycles featuring phosphonate pendant groups has been developed. 1,4,8,11-tetraazacyclotetradecane-1,8-di(methanephosphonic acid) (CB-TE2P, 1) and 1,4,8,11-tetraazacyclotetradecane-1-(methanephosphonic acid)-8-(methanecarboxylic acid) (CB-TE1A1P, 2) have been synthesized and have been shown to readily form neutral copper(II) complexes at room temperature as the corresponding dianions. Both complexes showed high kinetic inertness to demetallation and crystal structures confirmed complete encapsulation of copper(II) ion within each macrocycle's cleft-like structure. Unprecedented for cross-bridged cyclam derivatives, both CB-TE2P (1) and CB-TE1A1P (2) can be radiolabeled with (64)Cu at room temperature in less than 1 h with specific activities >1 mCi μg(-1). The in vivo behavior of both (64)Cu-CB-TE2P and (64)Cu-CB-TE1A1P were investigated through biodistribution studies using healthy male Lewis rats. Both new compounds showed rapid clearance with similar or lower accumulation in non-target organs/tissues when compared to other copper chelators including CB-TE2A, NOTA and Diamsar.

Bisphosphonates as radionuclide carriers for imaging or systemic therapy

Bisphosphonates (BP's), biologically stable analogs of naturally occurring pyrophosphates, became the treatment of choice for pathologic conditions characterized by increased osteoclast-mediated bone resorption, namely Paget's disease, osteoporosis and tumor bone disease. Moreover, the clinical success of BP's is also associated with their use in (99m)Tc-based radiopharmaceuticals for bone imaging. In addition to the successful delivery of (99m)Tc (γ-emitter) to bone, BP's have also been used to deliver β(-)-particle emitting radiometals (e.g.(153)Sm, (186/188)Re) for bone-pain palliation. The main goal of this Review is to update the most recent research efforts toward the synthesis, characterization and biological evaluation of novel BP-containing radiometal complexes and radiohalogenated compounds for diagnostic or therapeutic purposes. The structure and in vivo properties of those compounds will be discussed and compared to the clinically available ones, namely in terms of image quality and therapeutic effect. We will also mention briefly the use of BP's as carriers of multimodal nuclear and optical imaging probes.

Monoclonal antibodies for copper-64 PET dosimetry and radioimmunotherapy

BACKGROUND

We previously described a two-antibody model of (64)Cu radioimmunotherapy to evaluate low-dose, solid-tumor response. This model was designed to test the hypothesis that cellular internalization is critical in causing tumor cell death by mechanisms in addition to radiation damage. The purpose of the present study was to estimate radiation dosimetry for both antibodies (mAbs) using positron emission tomography (PET) imaging, and evaluate the effect of internalization on tumor growth.

RESULTS

Dosimetry was similar between therapy groups. Median time to tumor progression to 1 g ranged from 7 to 12 days for control groups and was 32 days for both treatment groups (p < 0.0001). No statistically significant difference existed between any control group or between the treatment groups.

MATERIAL AND METHODS

In female nude mice bearing LS174T colon carcinoma xenografts, tumor dosimetry was calculated using serial PET images of three mice in each group of either internalizing (64)Cu-labeled DOTA-cBR96 (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or non-internalizing (64)Cu-labeled DOTA-cT84.66 from 3 to 48 h. For the therapy study, controls (n = 10) received saline, DOTA-cBR96, or DOTA-cT84.66. Treatment animals (n = 9) received 0.890 mCi of (64)Cu-labeled DOTA-cBR96 or 0.710 mCi of (64)Cu-labeled DOTA-cT84.66. Tumors were measured daily.

CONCLUSIONS

PET imaging allows the use of (64)Cu for pre-therapy calculation of tumor dosimetry. In spite of highly similar tumor dosimetry, an internalizing antibody did not improve the outcome of (64)Cu radioimmunotherapy. Radioresistance of this tumor cell line and copper efflux may have confounded the study. Further investigations of the 2 therapeutic efficacy of (64)Cu-labeled mAbs will focus on interaction between (64)Cu and tumor suppressor genes and copper chaperones.

Noninvasive visualization and quantification of tumor αVβ3 integrin expression using a novel positron emission tomography probe, 64Cu-cyclam-RAFT-c(-RGDfK-)4

INTRODUCTION

The α(V)β(3) integrin is a well-known transmembrane receptor involved in tumor invasion, angiogenesis and metastasis. Our aim was to evaluate a novel positron emission tomography (PET) probe, (64)Cu-cyclam-RAFT-c(-RGDfK-)(4), for noninvasive visualization and quantification of α(V)β(3) integrin expression.

METHODS

RAFT-c(-RGDfK-)(4), a tetrameric cyclic Arg-Gly-Asp (RGD)-based peptide, was conjugated with a bifunctional chelator, 1,4,8,11-tetraazacyclotetradecane (cyclam), radiolabeled with the positron emitter (64)Cu and evaluated in vitro by cell binding and competitive inhibition assays and in vivo by biodistribution and receptor blocking studies, and PET imaging. The following cell lines, human embryonic kidney HEK293(β(1)) [α(V)β(3)-negative] and HEK293(β(3)) [α(V)β(3)-overexpressing] and human glioblastoma U87MG [naturally expressing α(V)β(3)], together with their subcutaneous xenografts in athymic nude mice, were used for the present study. The expression levels of α(V)β(3) on these cell lines and tumor xenografts were analyzed by flow cytometry and sodium dodecyl sulfate-polyacrylamide gel electrophoresis/autoradiography, respectively.

RESULTS

(64)Cu-cyclam-RAFT-c(-RGDfK-)(4) demonstrated the in vitro and in vivo specificity for the α(V)β(3) integrin and displayed rapid blood clearance, predominantly renal excretion and low uptake in nontumor tissues. Tumor uptake of (64)Cu-cyclam-RAFT-c(-RGDfK-)(4) (3 h postinjection) in HEK293(β(3)) (high levels of α(V)β(3)), U87MG (moderate levels of α(V)β(3)) and HEK293(β(1)) (undetectable levels of α(V)β(3)) tumors was 9.35%±1.19%, 3.46%±0.45% and 1.18%±0.30% injected dose per gram, respectively, with a strong and positive correlation with the tumor α(V)β(3) expression levels (correlation coefficient=0.967; P<.0001). Positron emission tomographic images showed that α(V)β(3)-positive tumors were clearly visualized with high tumor-to-background contrast, and agreed well with the biodistribution results.

CONCLUSION

(64)Cu-cyclam-RAFT-c(-RGDfK-)(4) exhibits potential for noninvasively quantifying α(V)β(3) expression.

Copper-64 radiopharmaceuticals for PET imaging of cancer: advances in preclinical and clinical research

Copper-64 (T(1/2) = 12.7 hours; beta(+), 0.653 MeV [17.8 %]; beta(-), 0.579 MeV [38.4 %]) has decay characteristics that allow for positron emission tomography (PET) imaging and targeted radiotherapy of cancer. The well-established coordination chemistry of copper allows for its reaction with a wide variety of chelator systems that can potentially be linked to peptides and other biologically relevant small molecules, antibodies, proteins, and nanoparticles. The 12.7-hours half-life of 64Cu provides the flexibility to image both smaller molecules and larger, slower clearing proteins and nanoparticles. In a practical sense, the radionuclide or the 64Cu-radiopharmaceuticals can be easily shipped for PET imaging studies at sites remote to the production facility. Due to the versatility of 64Cu, there has been an abundance of novel research in this area over the past 20 years, primarily in the area of PET imaging, but also for the targeted radiotherapy of cancer. The biologic activity of the hypoxia imaging agent, 60/64Cu-ATSM, has been described in great detail in animal models and in clinical PET studies. An investigational new drug application for 64Cu-ATSM was recently approved by the U.S. Food and Drug Administration (FDA) in the United States, paving the way for a multicenter trial to validate the utility of this agent, with the hopeful result being FDA approval for routine clinical use. This article discusses state-of-the-art cancer imaging with 64Cu radiopharmaceuticals, including 64Cu-ATSM for imaging hypoxia, 64Cu-labeled peptides for tumor-receptor targeting, (64)Cu-labeled monoclonal antibodies for targeting tumor antigens, and 64Cu-labeled nanoparticles for cancer targeting. The emphasis of this article will be on the new scientific discoveries involving (64)Cu radiopharmaceuticals, as well as the translation of these into human studies.