Comparison of (64)Cu-complexing bifunctional chelators for radioimmunoconjugation: labeling efficiency, specific activity, and in vitro/in vivo stability

New Glucocyclic RGD Dimers for Positron Emission Tomography Imaging of Tumor Integrin Receptors

Most studies of radiolabeled arginine-glycine-aspartic acid (RGD) peptides have shown in vitro affinity for integrin ανβ3, allowing for the targeting of receptor-positive tumors in vivo. However, major differences have been found in the pharmacokinetic profiles of different radiolabeled RGD peptide analogs. The purposes of this study were to prepare (64)Cu-DOTA-gluco-E[c(RGDfK)]2 (R8), (64)Cu-NOTA-gluco-E[c(RGDfK)]2 (R9), and (64)Cu-NODAGA-gluco-E[c(RGDfK)]2 (R10) and compare their pharmacokinetics and tumor imaging properties using small-animal positron emission tomography (PET). All three compounds were produced with high specific activity within 10 minutes. The IC50 values were similar for all the substances, and their affinities were greater than that of c(RGDyK). R8, R9, and R10 were stable for 24 hours in human and mouse serums and showed high uptake in U87MG tumors with high tumor-to-blood ratios. Compared to the control, a cyclic RGD peptide dimer without glucosamine, R10, showed low uptake in the liver. Because of their good imaging qualities and improved pharmacokinetics, (64)Cu-labeled dimer RGD conjugates (R8, R9, and R10) may have potential applications as PET radiotracers. R9 (NOTA) with highly in vivo stability consequentially showed an improved PET tumor uptake than R8 (DOTA) or R10 (NODAGA).

Promising bifunctional chelators for copper 64-PET imaging: practical (64)Cu radiolabeling and high in vitro and in vivo complex stability

Positron emission tomography (PET) using copper-64 is a sensitive and non-invasive imaging technique for diagnosis and staging of cancer. A bifunctional chelator that can present rapid radiolabeling kinetics and high complex stability with (64)Cu is a critical component for targeted PET imaging. Bifunctional chelates 3p-C-NE3TA, 3p-C-NOTA, and 3p-C-DE4TA were evaluated for complexation kinetics and stability with (64)Cu in vitro and in vivo. Hexadentate 3p-C-NOTA and heptadentate 3p-C-NE3TA possess a smaller TACN-based macrocyclic backbone, while nonadentate 3p-C-DE4TA is constructed on a larger CYCLEN-based ring. The frequently explored chelates of (64)Cu, octadentate C-DOTA and hexadentate C-NOTA were also comparatively evaluated. Radiolabeling kinetics of bifunctional chelators with (64)Cu was assessed under mild conditions. All bifunctional chelates instantly bound to (64)Cu in excellent radiolabeling efficiency at room temperature. C-DOTA was less efficient in binding (64)Cu than all other chelates. All (64)Cu-radiolabeled bifunctional chelates remained stable in human serum without any loss of (64)Cu for 2 days. When challenged by an excess amount of EDTA, (64)Cu complexes of C-NOTA, 3p-C-NE3TA and 3p-C-NOTA were shown to be more stable than (64)Cu-C-DOTA and (64)Cu-3p-C-DE4TA. (64)Cu complexes of the new chelates 3p-C-NE3TA and 3p-C-NOTA displayed comparable in vitro and in vivo complex stability to (64)Cu-C-NOTA. In vivo biodistribution result indicates that the (64)Cu-radiolabeled complexes of 3p-C-NOTA and 3p-C-NE3TA possess excellent in vivo complex stability, while (64)Cu-3p-C-DE4TA was dissociated as evidenced by high renal and liver retention in mice. The results of in vitro and in vivo studies suggest that the bifunctional chelates 3p-C-NE3TA and 3p-C-NOTA offer excellent chelation chemistry with (64)Cu for potential PET imaging applications.

Radiopharmacological characterization of ⁶⁴Cu-labeled α-MSH analogs for potential use in imaging of malignant melanoma

The melanocortin-1 receptor (MC1R) plays an important role in melanoma growth, angiogenesis and metastasis, and is overexpressed in melanoma cells. α-Melanocyte stimulating hormone (α-MSH) and derivatives are known to bind with high affinity at this receptor that provides the potential for selective targeting of melanoma. In this study, one linear α-MSH-derived peptide Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) without linker and with εAhx-β-Ala linker, and a cyclic α-MSH derivative, [Lys-Glu-His-D-Phe-Arg-Trp-Glu]-Arg-Pro-Val-NH2 (NAP-NS2) with εAhx-β-Ala linker were conjugated with p-SCN-Bn-NOTA and labeled with (64)Cu. Radiochemical and radiopharmacological investigations were performed with regard to transchelation, stability, lipophilicity and in vitro binding assays as well as biodistribution in healthy rats. No transchelation reactions, but high metabolic stability and water solubility were demonstrated. The linear derivatives showed higher affinity than the cyclic one. [(64)Cu]Cu-NOTA-εAhx-β-Ala-NAP-NS1 ([(64)Cu]Cu-2) displayed rapid cellular association and dissociation in murine B16F10 cell homogenate. All [(64)Cu]Cu-labeled conjugates exhibited affinities in the low nanomolar range in B16F10. [(64)Cu]Cu-2 showed also high affinity in human MeWo and TXM13 cell homogenate. In vivo studies suggested that [(64)Cu]Cu-2 was stable, with about 85 % of intact peptide in rat plasma at 2 h p.i. Biodistribution confirmed the renal pathway as the major elimination route. The uptake of [(64)Cu]Cu-2 in the kidney was 5.9 % ID/g at 5 min p.i. and decreased to 2.0 % ID/g at 60 min p.i. Due to the prospective radiochemical and radiopharmacological properties of the linear α-MSH derivative [(64)Cu]Cu-2, this conjugate is a promising candidate for tracer development in human melanoma imaging.

Immuno-PET Imaging and Radioimmunotherapy of 64Cu-/177Lu-Labeled Anti-EGFR Antibody in Esophageal Squamous Cell Carcinoma Model

Immuno-PET provides valuable information about tumor location, phenotype, susceptibility to therapy, and treatment response, especially to targeted radioimmunotherapy. In this study, we prepared antiepidermal growth factor receptor (EGFR) antibody via identical chelator, 3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-1(15),11,13-trience-3,6,9,-triacetic acid (PCTA), labeled with (64)Cu or (177)Lu to evaluate the EGFR expression levels using immuno-PET and the feasibility of radioimmunotherapy in an esophageal squamous cell carcinoma (ESCC) model.

METHODS

Cetuximab was conjugated with p-SCN-Bn-PCTA and radiolabeled with (64)Cu or (177)Lu. In vitro EGFR expression levels were determined and compared using flow cytometry and cell binding assay. In vivo EGFR expression levels were evaluated via immuno-PET imaging of (64)Cu-cetuximab and biodistribution analysis. Micro-SPECT/CT imaging, biodistribution, and radioimmunotherapy studies of (177)Lu-cetuximab were performed in the ESCC model. Therapeutic responses were monitored using (18)F-FDG PET and immunohistochemical staining.

RESULTS

(64)Cu- or (177)Lu-labeled antibodies showed high radiolabeling yield (>98%), stability (>90%), and favorable immunoreactivity. In vitro EGFR status measured by cell binding assay was correlated with the flow cytometry data. Immuno-PET, micro-SPECT/CT, and biodistribution demonstrated specific uptake in ESCC tumors depending on the EGFR expression levels. Tumor accumulation of (64)Cu- and (177)Lu-cetuximab was peaked at 48 and 120 h, respectively. Radioimmunotherapy with (177)Lu-cetuximab showed significant inhibition of tumor growth (P < 0.01) and marked reduction of (18)F-FDG SUV compared with that of control (P < 0.05). Terminal deoxynucleotidyl transferase dUTP nick-end labeling positivity and Ki-67 staining indices increased and decreased, respectively, in the radioimmunotherapy group compared with other groups (P < 0.01).

CONCLUSION

(64)Cu-cetuximab immuno-PET represented EGFR expression levels in ESCC tumors, and (177)Lu-cetuximab radioimmunotherapy effectively inhibited the tumor growth. The diagnostic and therapeutic convergence radiopharmaceutical (64)Cu-/(177)Lu-PCTA-cetuximab may be useful as a diagnostic tool in patient selection and a potent radioimmunotherapy agent in EGFR-positive ESCC tumors.

Evaluation of H2CHXdedpa, H2dedpa- and H2CHXdedpa-N,N′-propyl-2-NI ligands for 64Cu(ii) radiopharmaceuticals

Hexadentate acyclic chelate H₂CHXdedpa and related N,N′-alkylated ligands are radiolabeled with radioactive ⁶⁴Cu(ii) under mild conditions forming kinetically inert complexes.

The interaction of NOTA as a bifunctional chelator with competitive alkali metal ions: a DFT study

This work investigates NOTA–alkali metal (Li⁺, Na⁺ and K⁺ and Rb⁺) complexation using density functional theory.

Bifunctional (64)Cu-labelled macrobicyclic cage amine isothiocyanates for immuno-positron emission tomography

New macrobicyclic cage amine or "sarcophagine" (sar) bifunctional chelators have been synthesised that form copper complexes of exceptional in vivo stability and incorporate isothiocyanate (-NCS) functional groups for conjugation to an antibody. The chelators were synthesised from the methyl-capped complex [Mg(II)(CH3)(NH2)sar](2+). Coordination of Mg(II) within the cavity of the cage amine ligand protects the secondary amine atoms from reacting with the -NCS functional groups. Two different [Mg(II)(NCS-sar)](2+) derivatives were conjugated to the HER2/neu-targeting antibody trastuzumab and the progress of the reaction monitored by electrospray mass spectrometry. The Mg(II) ion was removed from the immunoconjugates under mild conditions (0.1 M citrate buffer, pH 6). Labelling of the (CH3)(p-NCS-Ph)sar-trastuzumab conjugate with (64)Cu(II), a radioisotope suitable for positron emission tomography (PET), was fast (∼5 min) and easily performed at room temperature with high radiochemical purity (>95%). Biodistribution and PET imaging studies in vivo showed that (64)Cu-labelled (CH3)(p-NCS-Ph)sar-trastuzumab maintained high stability under physiological conditions with high and selective uptake in a HER2-positive cancer cell line. The stability of the copper complex and the 12.7 h half-life of the radioisotope allows clear visualisation of tumours out to 48 h.

Opportunities and challenges for metal chemistry in molecular imaging: from gamma camera imaging to PET and multimodality imaging

The development of medical imaging is a highly multidisciplinary endeavor requiring the close cooperation of clinicians, physicists, engineers, biologists and chemists to identify capabilities, conceive challenges and solutions and apply them in the clinic. The chemistry described in this article illustrates how synergistic advances in these areas drive the technology and its applications forward, with each discipline producing innovations that in turn drive innovations in the others. The main thread running through the article is the shift from single photon radionuclide imaging towards PET, and in turn the emerging shift from PET/CT towards PET/MRI and further, combination of these with optical imaging. Chemistry to support these transitions is exemplified by building on a summary of the status quo, and recent developments, in technetium-99m chemistry for SPECT imaging, followed by a report of recent developments to support clinical application of short lived (Ga-68) and long-lived (Zr-89) positron emitting isotopes, copper isotopes for PET imaging, and combined modality imaging agents based on radiolabelled iron oxide based nanoparticles.

Development and characterization of an αvβ6-specific diabody and a disulfide-stabilized αvβ6-specific cys-diabody

INTRODUCTION

This work describes the development and characterization of two antibody fragments that specifically target the α(v)β(6) integrin, a non-covalent diabody and a disulfide-stabilized cys-diabody. The diabodies were analyzed for their ability to bind both immobilized and cell surface-bound α(v)β(6). Radiolabeling was done using non-site-specific and site-specific conjugation approaches with N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]-SFB) and the bifunctional chelator 1,4,7-triazacyclononane-triacetic acid maleimide (NOTA-maleimide) and copper-64 ([(64)Cu]), respectively. The affects of each radiolabeling method on RCY, RCP, and immunoreactivity were analyzed for the [(18)F]-FB-α(v)β(6) diabody, [(18)F]-FB-α(v)β(6) cys-diabody, and the [(64)Cu]-NOTA-α(v)β(6) cys-diabody.

METHODS

Diabodies were constructed from the variable domains of the humanized 6.3G9 anti-α(v)β(6) intact antibody. The anti-α(v(β(6) cys-diabody was engineered with C-terminal cysteines to enable covalent dimerization and site-specific modification. Biochemical characterization included SDS-PAGE, Western blot, and electrospray ionization to confirm MW, and flow cytometry and ELISA experiments were used to determine binding affinity and specificity to α(v)β(6). The diabodies were radiolabeled with [(18)F]-SFB and in addition, the anti-α(v)β(6) cys-diabody was also radiolabeled site-specifically using NOTA-maleimide and [(64)Cu]. Immunoreactivities were confirmed using in vitro cell binding to DX3Puroβ(6) (α(v)β(6)+) and DX3Puro (α(v)β(6)-)cell lines.

RESULTS

The diabodies were purified from cell culture supernatants with purities >98%. Subnanomolar binding affinity towards αvβ6 was confirmed by ELISA (diabody IC(50)=0.8 nM, cys-diabody IC(50)=0.6 nM) and flow cytometry revealed high specificity only to the DX3Puroβ(6) cell line for both diabodies. RCYs were 22.6%±3.6% for the [(18)F]-FB-α(v)β(6) diabody, 8.3%±1.7% for the [(18)F]-FB-α(v)β(6) cys-diabody and 43.5%±5.5% for the [(64)Cu]-NOTA-α(v)β(6) cys-diabody. In vitro cell binding assays revealed excellent specificity and retention of immunoreactivity ([(18)F]-FB-α(v)β(6) diabody=58.7%±6.7%, [(18)F]-FB-α(v)β(6) cys-diabody=80.4%±4.4%, [(64)Cu]-NOTA-α(v)β(6) cys-diabody=59.4%±0.6%) regardless of the radiolabeling method used.

CONCLUSIONS

Two novel diabodies with excellent binding affinity and specificity for the α(v)β(6) integrin in vitro were developed. Radiolabeling of the diabodies with fluorine-18 ([(18)F]) and [(64)Cu] revealed advantages and disadvantages with regards to methodologies and RCYs, however immunoreactivities were well preserved regardless of radiolabeling approach.

Development of [⁶⁴Cu]-DOTA-PR81 radioimmunoconjugate for MUC-1 positive PET imaging

OBJECTIVE

Breast cancer radioimmunoscintigraphy targeting MUC1 expression is a growing field of work in nuclear medicine research. PR81 is a monoclonal antibody that binds with high affinity to MUC1, which is over expressed on breast tumors. In this study, we report production, quality control and preclinical qualifications of a copper-64 labeled PR81 for PET imaging of breast cancer.

METHODS

PR81 was conjugated with DOTA-NHS-ester and purified by molecular filtration followed by chelate:mAb ratio determination by spectrophotometric method. DOTA-PR81 was labeled with (64)Cu followed by radiochemical purity, in vitro stability, in vitro internalization and immunoreactivity determination. The tissue biodistribution of the (64)Cu-DOTA-PR81 and (64)Cu-DOTA-hIgG was evaluated in BALB/c mice with breast carcinoma tumors using tissue counting and imaging.

RESULTS

The radiochemical purity of radioimmunoconjugate was >95±1.9% (ITLC) (specific activity; 4.6 μCi/μg). The average number of chelators per antibody was 3.4±0.3:1. The (64)Cu-DOTA-PR81 showed immunoreactivity towards MUC1 antigen and MCF7 cell line with significant in vitro stability (>89% in PBS and 78±0.5% in human serum) over 48 h. Maximum internalized activity of radiolabeled PR81 in 4-8 h was 81.5%. The biodistribution and scintigraphy studies showed the accumulation of the complex at the site of tumors with high sensitivity and specificity compared to control probes.

CONCLUSION

The results showed that (64)Cu-DOTA-PR81 may be considered as a potential PET tracer for diagnosis and follow-up of MUC1 expression in oncology.

Tripodal tris(hydroxypyridinone) ligands for immunoconjugate PET imaging with (89)Zr(4+): comparison with desferrioxamine-B

Due to its long half-life (78 h) and decay properties (77% electron capture, 23% β(+), Emax = 897 keV, Eav = 397 keV, Eγ = 909 keV, Iγ = 100%) (89)Zr is an appealing radionuclide for immunoPET imaging with whole IgG antibodies. Derivatives of the siderophore desferrioxamine-B (H3DFO) are the most widely used bifunctional chelators for coordination of (89)Zr(4+) because the radiolabeling of the resulting immunoconjugates is rapid under mild conditions. (89)Zr-DFO complexes are reportedly stable in vitro but there is evidence that (89)Zr(4+) is released in vivo, and subsequently taken up by the skeleton. We have evaluated a novel tripodal tris(hydroxypyridinone) chelator, H3CP256 and its bifunctional maleimide derivative, H3YM103, for coordination of Zr(4+) and compared the NMR spectra, and the (89)Zr(4+) radiolabeling, antibody conjugation, serum stability and in vivo distribution of radiolabelled immunoconjugates with those of H3DFO and its analogues. H3CP256 coordinates (89)Zr(4+) at carrier-free concentrations forming [(89)Zr(CP256)](+). Both H3DFO and H3CP256 were efficiently radiolabelled using [(89)Zr(C2O4)4](4-) at ambient temperature in quantitative yield at pH 6-7 at millimolar concentrations of chelator. Competition experiments demonstrate that (89)Zr(4+) dissociates from [(89)Zr(DFO)](+) in the presence of one equivalent of H3CP256 (relative to H3DFO) at pH 6-7, resulting largely in [(89)Zr(CP256)](+). To assess the stability of H3DFO and H3YM103 immunoconjugates radiolabelled with (89)Zr, maleimide derivatives of the chelators were conjugated to the monoclonal antibody trastuzumab via reduced cysteine side chains. Both immunoconjugates were labelled with (89)Zr(4+) in >98% yield at high specific activities and the labeled immunoconjugates were stable in serum with respect to dissociation of the radiometal. In vivo studies in mice indicate that (89)Zr(4+) dissociates from YM103-trastuzumab with significant amounts of activity becoming associated with bones and joints (25.88 ± 0.58% ID g(-1) 7 days post-injection). In contrast, <8% ID g(-1) of (89)Zr activity becomes associated with bone in animals administered (89)Zr-DFO-trastuzumab over the course of 7 days. The tris(hydroxypyridinone) chelator, H3CP256, coordinates (89)Zr(4+) rapidly under mild conditions, but the (89)Zr-labelled immunoconjugate, (89)Zr-YM103-trastuzumab was observed to release appreciable amounts of (89)Zr(4+)in vivo, demonstrating inferior stability when compared with (89)Zr-DFO-trastuzumab. The significantly lower in vivo stability is likely to be a result of lower kinetic stability of the Zr(4+) tris(hydroxypyridinone complex) relative to that of DFO and its derivatives.

A nuclear chocolate box: the periodic table of nuclear medicine

Radioisotopes of elements from all parts of the periodic table find both clinical and research applications in radionuclide molecular imaging and therapy (nuclear medicine). This article provides an overview of these applications in relation to both the radiological properties of the radionuclides and the chemical properties of the elements, indicating past successes, current applications and future opportunities and challenges for inorganic chemistry.

The ionic charge of copper-64 complexes conjugated to an engineered antibody affects biodistribution

The development of biomolecules as imaging probes requires radiolabeling methods that do not significantly influence their biodistribution. Sarcophagine (Sar) chelators form extremely stable complexes with copper and are therefore a promising option for labeling proteins with (64)Cu. However, initial studies using the first-generation sarcophagine bifunctional chelator SarAr to label the engineered antibody fragment ch14.18-ΔCH2 (MW 120 kDa) with (64)Cu showed high tracer retention in the kidneys, presumably because the high local positive charge on the Cu(II)-SarAr moiety resulted in increased binding of the labeled protein to the negatively charged basal cells of the glomerulus. To test this hypothesis, ch14.18-ΔCH2 was conjugated with a series of Sar derivatives of decreasing positive charge and three commonly used macrocyclic polyaza polycarboxylate (PAC) bifunctional chelators (BFC). The immunoconjugates were labeled with (64)Cu and injected into mice, and PET/CT images were obtained at 24 and 48 h postinjection (p.i.). At 48 h p.i., ex vivo biodistribution was assessed. In addition, to demonstrate the potential of metastasis detection using (64)Cu-labeled ch14.18-ΔCH2, a preclinical imaging study of intrahepatic neuroblastoma tumors was performed. Reducing the positive charge on the Sar chelators decreased kidney uptake of Cu-labeled ch14.18-ΔCH2 by more than 6-fold, from >45 to <6% ID/g, whereas the uptake in most other tissues, including liver, was relatively unchanged. However, despite this dramatic decrease, the renal uptake of the PAC BFCs was generally lower than that of the Sar derivatives, as was the liver uptake. Uptake of (64)Cu-labeled ch14.18-ΔCH2 in neuroblastoma hepatic metastases was detected using PET.

Positron emission tomographic imaging of copper 64- and gallium 68-labeled chelator conjugates of the somatostatin agonist tyr3-octreotate

The bifunctional chelator and radiometal have been shown to have a direct effect on the pharmacokinetics of somatostatin receptor (SSTR)-targeted imaging agents. We evaluated three Y3-TATE analogues conjugated to NOTA-based chelators for radiolabeling with 64Cu and 68Ga for small-animal positron emission tomographic/computed tomographic (PET/CT) imaging. Two commercially available NOTA analogues, p-SCN-Bn-NOTA and NODAGA, were evaluated. The p-SCN-Bn-NOTA analogues were conjugated to Y3-TATE through β-Ala and PEG8 linkages. The NODAGA chelator was directly conjugated to Y3-TATE. The analogues labeled with 64Cu or 68Ga were analyzed in vitro for binding affinity and internalization and in vivo by PET/CT imaging, biodistribution, and Cerenkov imaging (68Ga analogues). We evaluated the effects of the radiometals, chelators, and linkers on the performance of the SSTR subtype 2--targeted imaging agents and also compared them to a previously reported agent, 64Cu-CB-TE2A-Y3-TATE. We found that the method of conjugation, particularly the length of the linkage between the chelator and the peptide, significantly impacted tumor and nontarget tissue uptake and clearance. Among the 64Cu- and 68Ga-labeled NOTA analogues, NODAGA-Y3-TATE had the most optimal in vivo behavior and was comparable to 64Cu-CB-TE2A-Y3-TATE. An advantage of NODAGA-Y3-TATE is that it allows labeling with 64Cu and 68Ga, providing a versatile PET probe for imaging SSTr subtype 2-positive tumors.

Development and Validation of an Immuno-PET Tracer as a Companion Diagnostic Agent for Antibody-Drug Conjugate Therapy to Target the CA6 Epitope

PURPOSE

To develop and compare three copper 64 ((64)Cu)-labeled antibody fragments derived from a CA6-targeting antibody (huDS6) as immuno-positron emission tomography (immuno-PET)-based companion diagnostic agents for an antibody-drug conjugate by using huDS6.

MATERIALS AND METHODS

Three antibody fragments derived from huDS6 were produced, purified, conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and evaluated in the following ways: (a) the affinity of the fragments and the DOTA conjugates was measured via flow cytometry, (b) the stability of the labeled fragments was determined ex vivo in human serum over 24 hours, and (c) comparison of the in vivo imaging potential of the fragments was evaluated in mice bearing subcutaneous CA6-positive and CA6-negative xenografts by using serial PET imaging and biodistribution. Isotype controls with antilysozyme and anti-DM4 B-Fabs and blocking experiments with an excess of either B-Fab or huDS6 were used to determine the extent of the antibody fragment (64)Cu-DOTA-B-Fab binding specificity. Immunoreactivity and tracer kinetics were evaluated by using cellular uptake and 48-hour imaging experiments, respectively. Statistical analyses were performed by using t tests, one-way analysis of variance, and Wilcoxon and Mann-Whitney tests.

RESULTS

The antibody fragment (64)Cu-DOTA-B-Fab was more than 95% stable after 24 hours in human serum, had an immunoreactivity of more than 70%, and allowed differentiation between CA6-positive and CA6-negative tumors in vivo as early as 6 hours after injection, with a 1.7-fold uptake ratio between tumors. Isotype and blocking studies experiments showed tracer-specific uptake in antigen-positive tumors, despite some nonspecific uptake in both tumor models.

CONCLUSION

Three antibody fragments were produced and examined as potential companion diagnostic agents. (64)Cu-DOTA-B-Fab is a stable and effective immuno-PET tracer for CA6 imaging in vivo.

Recent advances in biological applications of cage metal complexes

This review highlights advances in biochemical and medical applications of cage metal complexes (clathrochelates) and related polyhedral compounds.

Picolinic acid based acyclic bifunctional chelating agent and its methionine conjugate as potential SPECT imaging agents: syntheses and preclinical evaluation

Syntheses and preclinical evaluation of picolinic acid based acyclic bifunctional chelating agent and its methionine conjugate as SPECT imaging agents.

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?

Imaging in drug development

Imaging has played an important part in the diagnosis of disease and development of the understanding of the underlying disease mechanisms and is now poised to make an impact in the development of new pharmaceuticals. This chapter discusses the underlying technologies that make the field ready for this challenge. In particular, the potentials of magnetic resonance imaging and functional magnetic resonance imaging are outlined, including the new methods developed to provide additional information from the scans carried out. The field of nuclear medicine has seen a rapid increase in interest as advances in radiochemistry have enabled a wide range of new radiotracers to be synthesised.

Immuno-PET of epithelial ovarian cancer: harnessing the potential of CA125 for non-invasive imaging

Background

Epithelial ovarian cancer (EOC) is characterized by the overexpression of cancer antigen 125 (CA125), a mucinous glycoprotein that serves as a tumor biomarker. Early diagnosis of EOC is plagued by its asymptomatic nature of progression and the limitations of currently used immunoassay techniques that detect CA125 as a shed antigen in serum samples. Presently, there is no technique available for the in vivo evaluation of CA125 expression in malignant tissues. Moreover, there could be an unexplored pathophysiological time window for the detection of CA125 in EOC, during which it is expressed on tumor cells prior to being shed into the bloodstream. A method for the in vivo evaluation of CA125 expression on ovarian neoplasms earlier along disease progression and/or recurrence can potentially contribute to better disease management. To this end, the present work utilizes an anti-CA125 monoclonal antibody (MAb) and a single-chain variable fragment (scFv) labeled with the positron-emitting radionuclide ⁶⁴Cu for preclinical molecular imaging of CA125 expression in vivo.

Methods

Anti-CA125 MAb and scFv were prepared and functionally characterized for target binding prior to being tested as radiotracers in a preclinical setting.

Results

Immunoblotting, immunofluorescence, and flow cytometry revealed specific binding of CA125-targeting vectors to NIH:OVCAR-3 cells and no binding to antigen-negative SKOV3 cells. ⁶⁴Cu-labeled anti-CA125 MAb and scFv were obtained in specific activities of 296 and 122 MBq/mg, respectively. Both radioimmunoconjugate vectors demonstrated highly selective binding to NIH:OVCAR-3 cells and virtually no binding to SKOV3 cells. In vivo radiopharmacological evaluation using xenograft mouse models injected with ⁶⁴Cu-labeled anti-CA125 MAb provided a standardized uptake value (SUV) of 5.76 (29.70 %ID/g) in OVCAR3 tumors 24 h post-injection (p.i.) versus 1.80 (5.91 %ID/g) in SKOV3 tumors. ⁶⁴Cu-labeled anti-CA125 scFv provided an SUV of 0.64 (3.21 %ID/g) in OVCAR3 tumors 24 h p.i. versus 0.25 (1.49 %ID/g) in SKOV3 tumors. Results from small-animal PET imaging were confirmed by ex vivo autoradiography and immunohistochemistry.

Conclusions

Radiolabeling of anti-CA125 MAb and scFv with ⁶⁴Cu did not compromise their immunoreactivity. Both radioimmunoconjugates presented specific tumor uptake and expected biological clearance profiles. This renders them as potential immuno-PET probes for targeted in vivo molecular imaging of CA125 in EOC.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-014-0060-4) contains supplementary material, which is available to authorized users.

PET and SPECT imaging of a radiolabeled minigastrin analogue conjugated with DOTA, NOTA, and NODAGA and labeled with (64)Cu, (68)Ga, and (111)In

Cholecystokinin-2 (CCK-2) receptors, overexpressed in cancer types such as small cell lung cancers (SCLC) and medullary thyroid carcinomas (MTC), may serve as targets for peptide receptor radionuclide imaging. A variety of CCK and gastrin analogues has been developed, but a major drawback is metabolic instability or high kidney uptake. The minigastrin analogue PP-F11 has previously been shown to be a promising peptide for imaging of CCK-2 receptor positive tumors and was therefore further evaluated. The peptide was conjugated with one of the macrocyclic chelators DOTA, NOTA, or NODAGA. The peptide conjugates were then radiolabeled with either (68)Ga, (64)Cu, or (111)In. All (radio)labeled compounds were evaluated in vitro (IC50) and in vivo (biodistribution and PET/CT and SPECT/CT imaging). IC50 values were in the low nanomolar range for all compounds (0.79-1.51 nM). In the biodistribution studies, (68)Ga- and (111)In-labeled peptides showed higher tumor-to-background ratios than the (64)Cu-labeled compounds. All tested radiolabeled compounds clearly visualized the CCK2 receptor positive tumor in PET or SPECT imaging. The chelator did not seem to affect in vivo behavior of the peptide for (111)In- and (68)Ga-labeled peptides. In contrast, the biodistribution of the (64)Cu-labeled peptides showed high uptake in the liver and in other organs, most likely caused by high blood levels, probably due to dissociation of (64)Cu from the chelator and subsequent transchelation to proteins. Based on the present study, (68)Ga-DOTA-PP-F11 might be a promising radiopharmaceutical for PET/CT imaging of CCK2 receptor expressing tumors such as MTC and SCLC. Clinical studies are warranted to investigate the potential of this tracer.

What a difference a carbon makes: H₄octapa vs H₄C3octapa, ligands for In-111 and Lu-177 radiochemistry

The acyclic ligands H₄C3octapa and p-SCN-Bn-H₄C3octapa were synthesized for the first time, using nosyl protection chemistry. These new ligands were compared to the previously studied ligands H₄octapa and p-SCN-Bn-H₄octapa to determine the extent to which the addition of a single carbon atom to the backbone of the ligand would affect metal coordination, complex stability, and, ultimately, utility for in vivo radiopharmaceutical applications. Although only a single carbon atom was added to H₄C3octapa and the metal donor atoms and denticity were not changed, the solution chemistry and radiochemistry properties were drastically altered, highlighting the importance of careful ligand design and radiometal–ligand matching. It was found that [In(C3octapa)]⁻ and [Lu(C3octapa)]⁻ were substantially different from the analogous H₄octapa complexes, exhibiting fluxional isomerization and a higher number of isomers, as observed by ¹H NMR, VT-NMR, and 2D COSY/HSQC-NMR experiments. Past evaluation of the DFT structures of [In(octapa)]⁻ and [Lu(octapa)]⁻ revealed very symmetric complexes; in contrast, the [In(C3octapa)]⁻ and [Lu(C3octapa)]⁻ complexes were much less symmetric, suggesting lower symmetry and less rigidity than that of the analogous H₄octapa complexes. Potentiometric titrations revealed the formation constants (log K_ML, pM) were ∼2 units lower for the In³⁺ and Lu³⁺ complexes of H₄C3octapa when compared to that of the more favorable H₄octapa ligand (∼2 orders of magnitude less thermodynamically stable). The bifunctional ligands p-SCN-Bn-H₄C3octapa and p-SCN-Bn-H₄octapa were conjugated to the antibody trastuzumab and radiolabeled with ¹¹¹In and ¹⁷⁷Lu. Over a 5 day stability challenge experiment in blood serum, ¹¹¹In-octapa– and ¹¹¹In-C3octapa–trastuzumab immunoconjugates were determined to be ∼91 and ∼24% stable, respectively, and ¹⁷⁷Lu-octapa– and ¹⁷⁷Lu-C3octapa–trastuzumab, ∼89% and ∼4% stable, respectively. This work suggests that 5-membered chelate rings are superior to 6-membered chelate rings for large metal ions like In³⁺ and Lu³⁺, which is a crucial consideration for the design of bifunctional chelates for bioconjugation to targeting vectors for in vivo work.

New ligands H₄C3octapa and p-SCN-Bn-H₄C3octapa were synthesized and compared to the previously studied ligands H₄octapa and p-SCN-Bn-H₄octapa to determine the extent to which the addition of a single carbon atom to the backbone of the ligand would affect metal coordination, complex stability, and, ultimately, utility for in vivo radiopharmaceutical applications. It was found that [In(C3octapa)]⁻ and [Lu(C3octapa)]⁻ were substantially different from the analogous H₄octapa complexes.

High-denticity ligands based on picolinic acid for 111In radiochemistry

Four new acyclic ligands, Bn-H3nonapa (3), H3nonapa (4), p-NO2-Bn-H3nonapa (10), and Bn-H3trenpa (7), were synthesized and studied with nonradioactive In3+ and with radioactive 111In3+. The coordination of these ligands to In3+ was confirmed by high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy. Radiolabeling experiments were performed with 111In3+; these demonstrated H3nonapa (4) to be the best indium ligand of those studied herein, achieving radiochemical yields of ∼97% in 10 min at ambient temperature, and stability to transchelation in mouse serum of 44.5% ± 25.9% after 24 h. Although the radiolabeling kinetics of H3nonapa (4) were excellent, serum stability results were inferior to the previously studied ligands DOTA, DTPA, and H4octapa, suggesting that the presented ligands may find their optimum radiopharmaceutical applications with isotopes other than 111In. Owing to the high denticity of these ligands (9–10 coordinate), they may realize their potential with large ion isotopes such as 177Lu, 86/90Y, and 225Ac.

Single-chain antibody conjugated to a cage amine chelator and labeled with positron-emitting copper-64 for diagnostic imaging of activated platelets

Imaging of activated platelets using an activation specific anti-GPIIb/IIIa integrin single-chain antibody (scFvanti-LIBS) conjugated to a positron emitting copper-64 complex of a cage amine sarcophagine chelator (MeCOSar) is reported. This tracer was compared in vitro to a (64)Cu(II) complex of the scFv conjugated to another commonly used macrocycle, DOTA. The scFvanti-LIBS-MeCOSar conjugate was radiolabeled with (64)Cu(II) rapidly under mild conditions and with higher specific activity than scFvanti-LIBS-DOTA. The utility of scFvanti-LIBS-MeCOSar as a diagnostic agent was assessed in vivo in a mouse model of acute thrombosis. The uptake of scFvanti-LIBS-(64)CuMeCOSar in the injured vessel was significantly higher than the noninjured vessel. Positron emission tomography (PET) was used to show accumulation of scFvanti-LIBS-(64)CuMeCOSar with high and specific uptake in the injured vessel. ScFvanti-LIBS-(64)CuMeCOSar is an excellent tool for highly sensitive in vivo detection of activated platelets in PET and has the potential to be used for early diagnosis of acute thrombotic events.

An EGFR targeted PET imaging probe for the detection of colonic adenocarcinomas in the setting of colitis

Colorectal cancer is a serious complication associated with inflammatory bowel disease, often indistinguishable by screening with conventional FDG PET probes. We have developed an alternative EGFR-targeted PET imaging probe that may be used to overcome this difficulty, and successfully assessed its utility for neoplastic lesion detection in preclinical models. Cetuximab F(ab′)2 fragments were enzymatically generated, purified, and DOTA-conjugated. Radiolabeling was performed with ⁶⁷Ga for cell based studies and ⁶⁴Cu for in vivo imaging. Competitive binding studies were performed on CT26 cells to assess affinity (K_D) and receptors per cell (B_max). In vivo imaging using the EGFR targeted PET probe and ¹⁸F FDG was performed on CT26 tumor bearing mice in both control and dextran sodium sulfate (DSS) induced colitis settings. Spontaneous adenomas in genetically engineered mouse (GEM) models of colon cancer were additionally imaged. The EGFR imaging agent was generated with high purity (> 98%), with a labeling efficiency of 60 ± 5% and ≥99% radiochemical purity. The K_D was 6.6 ± 0.7 nM and the B_max for CT26 cells was 3.3 ± 0.1 × 10⁶ receptors/cell. Target to background ratios (TBR) for CT26 tumors compared to colonic uptake demonstrated high values for both ¹⁸F-FDG (3.95 ± 0.13) and the developed ⁶⁴Cu-DOTA-cetuximab-F(ab′)2 probe (4.42 ± 0.11) in control mice. The TBR for the EGFR targeted probe remained high (3.78 ± 0.06) in the setting of colitis, while for ¹⁸F FDG, this was markedly reduced (1.54 ± 0.08). Assessment of the EGFR targeted probe in the GEM models demonstrated a correlation between radiotracer uptake in spontaneous colonic lesions and the EGFR staining level ex vivo. A clinically translatable PET imaging probe was successfully developed to assess EGFR. The imaging agent can detect colonic tumors with a high TBR for detection of in situ lesions in the setting of colitis, and opens the possibility for a new approach for screening high-risk patients.

Enzyme-mediated site-specific bioconjugation of metal complexes to proteins: sortase-mediated coupling of copper-64 to a single-chain antibody

The enzyme-mediated site-specific bioconjugation of a radioactive metal complex to a single-chain antibody using the transpeptidase sortase A is reported. Cage amine sarcophagine ligands that were designed to function as substrates for the sortase A mediated bioconjugation to antibodies were synthesized and enzymatically conjugated to a single-chain variable fragment. The antibody fragment scFv(anti-LIBS) targets ligand-induced binding sites (LIBS) on the glycoprotein receptor GPIIb/IIIa, which is present on activated platelets. The immunoconjugates were radiolabeled with the positron-emitting isotope (64)Cu. The new radiolabeled conjugates were shown to bind selectively to activated platelets. The diagnostic potential of the most promising conjugate was demonstrated in an in vivo model of carotid artery thrombosis using positron emission tomography. This approach gives homogeneous products through site-specific enzyme-mediated conjugation and should be broadly applicable to other metal complexes and proteins.

Development of multi-functional chelators based on sarcophagine cages

A new class of multifunctionalized sarcophagine derivatives was synthesized for ⁶⁴Cu chelation. The platform developed in this study could have broad applications in ⁶⁴Cu-radiopharmaceuticals.

Chelators for copper radionuclides in positron emission tomography radiopharmaceuticals

The development of chelating agents for copper radionuclides in positron emission tomography radiopharmaceuticals has been a highly active and important area of study in recent years. The rapid evolution of chelators has resulted in highly specific copper chelators that can be readily conjugated to biomolecules and efficiently radiolabeled to form stable complexes in vivo. Chelators are not only designed for conjugation to monovalent biomolecules but also for incorporation into multivalent targeting ligands such as theranostic nanoparticles. These advancements have strengthened the role of copper radionuclides in the fields of nuclear medicine and molecular imaging. This review emphasizes developments of new copper chelators that have most greatly advanced the field of copper-based radiopharmaceuticals over the past 5 years.

⁶⁴Cu-labeled inhibitors of prostate-specific membrane antigen for PET imaging of prostate cancer

Prostate-specific membrane antigen (PSMA) is a well-recognized target for identification and therapy of a variety of cancers. Here we report five (64)Cu-labeled inhibitors of PSMA, [(64)Cu]3-7, which are based on the lysine-glutamate urea scaffold and utilize a variety of macrocyclic chelators, namely NOTA(3), PCTA(4), Oxo-DO3A(5), CB-TE2A(6), and DOTA(7), in an effort to determine which provides the most suitable pharmacokinetics for in vivo PET imaging. [(64)Cu]3-7 were prepared in high radiochemical yield (60-90%) and purity (>95%). Positron emission tomography (PET) imaging studies of [(64)Cu]3-7 revealed specific accumulation in PSMA-expressing xenografts (PSMA+ PC3 PIP) relative to isogenic control tumor (PSMA- PC3 flu) and background tissue. The favorable kinetics and high image contrast provided by CB-TE2A chelated [(64)Cu]6 suggest it as the most promising among the candidates tested. That could be due to the higher stability of [(64)Cu]CB-TE2A as compared with [(64)Cu]NOTA, [(64)Cu]PCTA, [(64)Cu]Oxo-DO3A, and [(64)Cu]DOTA chelates in vivo.

Noninvasive positron emission tomography and fluorescence imaging of CD133+ tumor stem cells

A technology that visualizes tumor stem cells with clinically relevant tracers could have a broad impact on cancer diagnosis and treatment. The AC133 epitope of CD133 currently is one of the best-characterized tumor stem cell markers for many intra- and extracranial tumor entities. Here we demonstrate the successful noninvasive detection of AC133(+) tumor stem cells by PET and near-infrared fluorescence molecular tomography in subcutaneous and orthotopic glioma xenografts using antibody-based tracers. Particularly, microPET with (64)Cu-NOTA-AC133 mAb yielded high-quality images with outstanding tumor-to-background contrast, clearly delineating subcutaneous tumor stem cell-derived xenografts from surrounding tissues. Intracerebral tumors as small as 2-3 mm also were clearly discernible, and the microPET images reflected the invasive growth pattern of orthotopic cancer stem cell-derived tumors with low density of AC133(+) cells. These data provide a basis for further preclinical and clinical use of the developed tracers for high-sensitivity and high-resolution monitoring of AC133(+) tumor stem cells.