In vitro and in vivo evaluations of a hydrophilic 64Cu-bis(thiosemicarbazonato)-glucose conjugate for hypoxia imaging

Preparation and Bioevaluation of a Novel 99mTc-Labeled Glucose Derivative Containing Cyclohexane as a Promising Tumor Imaging Agent

To develop novel tumor imaging agents with high tumor uptake and excellent tumor/non-target ratios, a glucose derivative containing cyclohexane (CNMCHDG) was synthesized and labeled with Tc-99m. [^99mTc]Tc-CNMCHDG was prepared by a kit formulation that was straightforward to operate and fast. Without purification, [^99mTc]Tc-CNMCHDG had a high radiochemical purity of over 95% and great in vitro stability and hydrophilicity (log P = -3.65 ± 0.10). In vitro cellular uptake studies showed that the uptake of [^99mTc]Tc-CNMCHDG was significantly inhibited by pre-treatment with _D-glucose and increased by pre-treatment with insulin. Preliminary cellular studies have demonstrated that the mechanism by which the complex enters into cells may be related to GLUTs. The results of biodistribution and SPECT imaging studies displayed high tumor uptake and good retention of [^99mTc]Tc-CNMCHDG in A549 tumor-bearing mice (4.42 ± 0.36%ID/g at 120 min post-injection). Moreover, [^99mTc]Tc-CNMCHDG exhibited excellent tumor-to-non-target ratios and a clean imaging background and is a potential candidate for clinical transformation.

Imaging of Tumor Hypoxia With Radionuclide-Labeled Tracers for PET

The hypoxic state in a solid tumor refers to the internal hypoxic environment that appears as the tumor volume increases (the maximum radius exceeds 180-200 microns). This state can promote angiogenesis, destroy the balance of the cell’s internal environment, and lead to resistance to radiotherapy and chemotherapy, as well as poor prognostic factors such as metastasis and recurrence. Therefore, accurate quantification, mapping, and monitoring of hypoxia, targeted therapy, and improvement of tumor hypoxia are of great significance for tumor treatment and improving patient survival. Despite many years of development, PET-based hypoxia imaging is still the most widely used evaluation method. This article provides a comprehensive overview of tumor hypoxia imaging using radionuclide-labeled PET tracers. We introduced the mechanism of tumor hypoxia and the reasons leading to the poor prognosis, and more comprehensively included the past, recent and ongoing studies of PET radiotracers for tumor hypoxia imaging. At the same time, the advantages and disadvantages of mainstream methods for detecting tumor hypoxia are summarized.

Synthesis and bioevaluation of radioiodinated nitroimidazole hypoxia imaging agents by one-pot click reaction

Eight radioiodinated 2-nitroimidazole derivatives for use as hypoxia imaging agents were synthesized by one-pot click reaction using four azides, two alkynes, and [¹³¹I]iodide ions and evaluated by hypoxic cellular uptake and biodistribution experiments. The results suggested that radiotracers with suitable partition coefficients (log P: -0.2-1.2) were more likely to have higher hypoxic cellular uptake. Among these eight molecules, [¹³¹I]15 ([¹³¹I]-(5-iodo-1-(2-(2-(2-nitro-1H-imidazol-1-yl)ethoxy)ethyl)-4-((2-nitro-1H-imidazol-1-yl)methyl)-1H-1,2,3-triazole)) had a suitable log P (0.05 ± 0.03) and contained two 2-nitroimidazole groups. The hypoxic/aerobic cellular uptake ratio of [¹³¹I]15 was 4.4 ± 0.5, and the tumor/blood (T/B) and tumor/muscle (T/M) ratios were 2.03 ± 0.45 and 6.82 ± 1.70, respectively. These results suggested that [¹³¹I]15 was a potential hypoxia imaging agent.

A brief overview of metal complexes as nuclear imaging agents

Metallic radionuclides have been instrumental in the field of nuclear imaging for over half a century. While recent years have played witness to a dramatic rise in the use of radiometals as labels for chelator-bearing biomolecules, imaging agents based solely on coordination compounds of radiometals have long played a critical role in the discipline as well. In this work, we seek to provide a brief overview of metal complex-based radiopharmaceuticals for positron emission tomography (PET) and single photon emission computed tomography (SPECT). More specifically, we have focused on imaging agents in which the metal complex itself rather than a pendant biomolecule or targeting moiety is responsible for the in vivo behavior of the tracer. This family of compounds contains metal complexes based on an array of different nuclides as well as probes that have been used for the imaging of a variety of pathologies, including infection, inflammation, cancer, and heart disease. Indeed, two of the defining traits of transition metal complexes-modularity and redox chemistry-have both been creatively leveraged in the development of imaging agents. In light of our audience, particular attention is paid to structure and mechanism, though clinical data is addressed as well. Ultimately, it is our hope that this review will not only educate readers about some of the seminal work performed in this space over the last 30 years but also spur renewed interest in the creation of radiopharmaceuticals based on small metal complexes.

Nuclear Imaging of Glucose Metabolism: Beyond 18F-FDG

Glucose homeostasis plays a key role in numerous fundamental aspects of life, and its dysregulation is associated with many important diseases such as cancer. The atypical glucose metabolic phenomenon, known as the Warburg effect, has been recognized as a hallmark of cancer and serves as a promising target for tumor specific imaging. At present, 2-deoxy-2-[¹⁸F]fluoro-glucose (¹⁸F-FDG)-based positron emission tomography/computed tomography (PET/CT) represented the state-of-the-art radionuclide imaging technique for this purpose. The powerful impact of ¹⁸F-FDG has prompted intensive research efforts into other glucose-based radiopharmaceuticals for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging. Currently, glucose and its analogues have been labeled with various radionuclides such as ^99mTc, ¹¹¹In, ¹⁸F, ⁶⁸Ga, and ⁶⁴Cu and have been successfully investigated for tumor metabolic imaging in many preclinical studies. Moreover, ^99mTc-ECDG has advanced into its early clinical trials and brings a new era of tumor imaging beyond ¹⁸F-FDG. In this review, preclinical and early clinical development of glucose-based radiopharmaceuticals for tumor metabolic imaging will be summarized.

Modification of Biodistribution and Brain Uptake of Copper Bis(thiosemicarbazonato) Complexes by the Incorporation of Amine and Polyamine Functional Groups

The synthesis of new bis(thiosemicarbazonato)copper(II) complexes featuring polyamine substituents via selective transamination reactions is presented. Polyamines of different lengths, with different ionizable substituent groups, were used to modify and adjust the hydrophilic/lipophilic balance of the copper complexes. The new analogues were radiolabeled with copper-64 and their lipophilicities estimated using distribution coefficients. The cell uptake of the new polyamine complexes was investigated with preliminary in vitro biological studies using a neuroblastoma cancer cell line. The in vivo biodistribution of three of the new analogues was investigated in vivo in mice using positron-emission tomography imaging, and one of the new complexes was compared to [⁶⁴Cu]Cu(atsm) in an A431 squamous cell carcinoma xenograft model. Modification of the copper complexes with various amine-containing functional groups alters the biodistribution of the complexes in mice. One complex, with a pendent ( N, N-dimethylamino)ethane functional group, displayed tumor uptake similar to that of [⁶⁴Cu]Cu(atsm) but higher brain uptake, suggesting that this compound has the potential to be of use in the diagnostic brain imaging of tumors and neurodegenerative diseases.

Pre-clinical Positron Emission Tomography Reconstruction Algorithm Effect on Cu-64 ATSM Lesion Hypoxia

Objective:

Application of distinct positron emission tomography (PET) scan reconstruction algorithms can lead to statistically significant differences in measuring lesion functional properties. We looked at the influence of two-dimensional filtered back projection (2D FBP), two-dimensional ordered subset expectation maximization (2D OSEM), three-dimensional ordered subset expectation maximization (3D OSEM) without 3D maximum a posteriori and with (3D OSEM MAP) on lesion hypoxia tracer uptake using a pre-clinical PET scanner.

Methods:

Reconstructed images of a rodent tumor model bearing P22 carcinosarcoma injected with hypoxia tracer Copper-64-Diacetyl-bis (N4-methylthiosemicarbazone) (i.e. Cu-64 ATSM) were analyzed at 10 minute intervals till 60 minute post injection. Lesion maximum standardized uptake values (SUV_max) and SUV_max/background SUV_mean (T/B) were recorded and investigated after application of multiple algorithm and reconstruction parameters to assess their influence on Cu-64 ATSM measurements and associated trends over time.

Results:

SUV_maxSUV_max or T/B between 2D FBP, exhibited convergence for OSEM reconstructions while ANOVA results showed a significant difference in SUV_max or T/B between 2D FBP, 2D OSEM, 3D OSEM and 3D OSEM MAP reconstructions across all time frames. SUV_max and T/B were greatest in magnitude for 2D OSEM followed by 3D OSEM MAP, 3D OSEM and then 2D FBP at all time frames respectively. Similarly SUV_max and T/B standard deviations (SD) were lowest for 2D OSEM in comparison with other algorithms.

Conclusion:

Significantly higher magnitude lesion SUV_max and T/B combined with lower SD were observed using 2D OSEM reconstruction in comparison with 2D FBP, 3D OSEM and 3D OSEM MAP algorithms at all time frames. Results are SUV_max or T/B between 2D FBP, consistent with other published studies however more specimens are required for full validation.

A dual radiolabelling approach for tracking metal complexes: investigating the speciation of copper bis(thiosemicarbazonates) in vitro and in vivo

Copper(II)bis(thiosemicarbazonato) complexes such as [(64)Cu]Cu-ATSM continue to be investigated for positron emission tomography (PET) imaging of tumour hypoxia. However, the currently proposed mechanisms for the mode of action of these complexes are unable to account fully for their observed biological behaviour. In order to examine the roles of the copper metal and the ligand, we designed a pair of (123)I/(64)Cu-copper bis(thiosemicarbazonates), radiolabelled at either the metal or at the ligand. In vitro cellular retention studies of the orthogonal pair demonstrate for the first time that retention under hypoxia involves dissociation of the copper bis(thiosemicarbazone) complex, consistent with the previously suggested mechanism of reductive trapping of copper. In contrast, in vivo biodistribution and dynamic PET/SPECT imaging of the orthogonally labelled complexes underline our previous findings for [(64)Cu]Cu-ATSM and [(64)Cu]Cu-acetate, providing further support for the important contribution of copper metabolism in the in vivo hypoxia selectivity of Cu-ATSM. This dual radiolabelling approach may find applications for determining the speciation of other metal complexes in vitro and in vivo.

Synthesis of sulfonamide conjugates of Cu(ii), Ga(iii), In(iii), Re(v) and Zn(ii) complexes: carbonic anhydrase inhibition studies and cellular imaging investigations

New sulfonamides and their metal complexes are reported, with a focus on porphyrin derivatives for simultaneous cellular optical imaging, radiolabelling and Carbonic Anhydrase inhibition capabilities.

Positron emission tomography radiotracers for imaging hypoxia

Localized hypoxia, the physiological hallmark of many clinical pathologies, is the consequence of acute or chronic ischemia in the affected region or tissue. The versatility, sensitivity, quantitative nature, and increasing availability of positron emission tomography (PET) make it the preclinical and clinical method of choice for functional imaging of tissue hypoxia at the molecular level. The progress and current status of radiotracers for hypoxia-specific PET imaging are reviewed in this article including references mainly focused on radiochemistry and also relevant to molecular imaging of hypoxia in preclinical and clinical studies.

New frontiers in the design and synthesis of imaging probes for PET oncology: current challenges and future directions

Despite being developed over 30 years ago, 2-deoxy-2-[(18)F]fluoro-D-glucose remains the most frequently used radiotracer in PET oncology. In the last decade, interest in new and more specific radiotracers for imaging biological processes of oncologic interest has increased exponentially. This review summarizes the strategies underlying the development of those probes together with their validation and status of clinical translation; a brief summary of new radiochemistry strategies applicable to PET imaging is also included. The article finishes with a consideration of the challenges imaging scientists must overcome to bring about increased adoption of PET as a diagnostic or pharmacologic tool.

⁹⁹mTc/Re complexes bearing bisnitroimidazole or mononitroimidazole as potential bioreductive markers for tumor: synthesis, physicochemical characterization and biological evaluation

Four monoamine-monoamide dithiol (MAMA) ligands containing two or one nitroimidazole moieties were synthesized and labeled with (99m)Tc (labeling yield > 95%). The proposed structures of (99m)Tc-complexes are identified by comparison with analogous Re-MAMA complexes. (99m)Tc-MAMA complexes show better physicochemical characters than (99m)TcO-(PnAO-1-(2-nitroimidazole)). Reduction potentials of nitro groups of the rhenium complexes are within the range for bioreductive compounds. As expected, biodistribution studies demonstrate that the 2-nitroimidazole complex shows better tumor-to-tissue ratios than 4-nitroimidazole analog for mononitroimidazole complexes, but not for MAMA-bisnitroimidazoles due to higher lipophilicity. Both the bisnitroimidazole compounds show rapider excretion, lower background activity in liver and higher tumor-to-tissue ratios than the mononitroimidazoles. Better biodistribution characteristic makes both the MAMA-bisnitroimidazole complexes, especially (99m)Tc-15, be potential tumor hypoxia marker.

In vitro and in vivo evaluation of [(18)F]F-GAZ, a novel oxygen-mimetic azomycin-glucose conjugate, for imaging hypoxic tumor

Several F-18-labeled 2-nitroimidazole (azomycin) derivatives have been proposed for imaging hypoxia using positron emission tomography (PET). Their cell penetration is based on passive diffusion, which limits their intracellular concentration maxima. The purpose of this study was to investigate the uptake of N-(2-[(18)F]fluoro-3-(6-O-glucosyl)propyl-azomycin ([(18)F]F-GAZ), a new azomycin-glucose conjugate, in vitro and in vivo. [(18)F]F-GAZ was synthesized from its tetraacetyl nosylate precursor by nucleophilic radiofluorination. [(18)F]F-GAZ was evaluated in vivo in EMT-6 tumor-bearing Balb/C mice utilizing the PET and biodistribution analysis. In vitro uptake of [(18)F]FDG by EMT-6 cells was measured in the presence of unlabeled F-GAZ, 2-FDG, and D-glucose. [(18)F]F-GAZ was rapidly cleared from all tissues, including the blood pool and kidneys, with ultimate accumulation in the urinary bladder. Uptake of tracer doses of [(18)F]F-GAZ into EMT-6 tumors was fast, reaching a standardized uptake value of 0.66±0.05 within 5-6 minutes postinjection (p.i.), and decreased to 0.24±0.04 by 60 minutes p.i. (n=6). A tumor-muscle ratio of 1.87±0.18 was observed after 60 minutes. Total uptake of [(18)F]F-GAZ in tumors (60 minutes) amounted to 1.25%±0.15% ID/g versus 0.61%±0.14% ID/g (n=4) in muscle. Similar biodistribution and excretion were observed using carrier-added (100 mg/kg) doses of F-GAZ. In vitro, D-glucose and unlabeled 2-FDG were two orders of magnitude more potent than F-GAZ as competitive inhibitors of [(18)F]FDG uptake into EMT-6 cells. Besides its interaction with glucose transporters, F-GAZ seems to be not transported in the presence of glucose. Furthermore, [(18)F]F-GAZ is unlikely to be effective as a hypoxia imaging agent. The low in vivo toxicity and substantial retention in tumor observed at high doses of F-GAZ do provide rationale for further testing as a radiosensitizer for external beam radiation therapy of radioresistant, hypoxic tumors.

Preparation, characterization of 2-deoxy-D-glucose functionalized dimercaptosuccinic acid-coated maghemite nanoparticles for targeting tumor cells

PURPOSE

To report a modified preparation and to systematically study the structure, magnetic and other properties of γ-Fe(2)O(3)-DMSA-DG NPs (2-deoxy-D-glucose (2-DG) conjugated meso-2,3-dimercaptosuccinic acid coated γ-Fe(2)O(3) nanoparticles) and test its ability to improve Hela tumor cells targeting in vitro compared to the γ-Fe(2)O(3)-DMSA NPs.

METHODS

The conjugation of 2-DG on the surface of γ-Fe(2)O(3)-DMSA NPs was performed by esterification reaction and characterized. Acute toxicity was evaluated using MTT assay. Cellular uptake was investigated by Prussian blue staining and UV colorimetric assay.

RESULTS

DG was successfully functionalized onto the surface of γ-Fe(2)O(3)-DMSA NPs; binding efficiency was ~60%. The mean diameter of single core of γ-Fe(2)O(3)-DMSA-DG NPs was 10 nm. Particle size and polydispersity index of its aggregates were 156.2 nm and 0.162, respectively. 2-DG-conjugated nanoparticles caused little cytotoxic effects on Hela cells at the concentration range of 0-600 μg/mL. When 2-DG-conjuated and non-conjugated nanoparticles were incubated with Hela cells for 4, 8 and 12 h, the 2-DG-conjugated nanoparticle showed significant amount of uptake in cells compared to their non-targeted counterparts.

CONCLUSION

γ-Fe(2)O(3)-DMSA-DG NPs could be developed as a tumor-targeted probe for cervical cancer imaging and therapy.

Hypoxia imaging using PET and SPECT: the effects of anesthetic and carrier gas on [Cu]-ATSM, [Tc]-HL91 and [F]-FMISO tumor hypoxia accumulation

Background

Preclinical imaging requires anaesthesia to reduce motion-related artefacts. For direct translational relevance, anaesthesia must not significantly alter experimental outcome. This study reports on the effects of both anaesthetic and carrier gas upon the uptake of [⁶⁴Cu]-CuATSM, [^99mTc]-HL91 and [¹⁸F]-FMISO in a preclinical model of tumor hypoxia.

Methodology/Principal Findings

The effect of carrier gas and anaesthetic was studied in 6 groups of CaNT-bearing CBA mice using [⁶⁴Cu]-CuATSM, [^99mTc]-HL91 or [¹⁸F]-FMISO. Mice were anaesthetised with isoflurane in air, isoflurane in pure oxygen, with ketamine/xylazine or hypnorm/hypnovel whilst breathing air, or in the awake state whilst breathing air or pure oxygen. PET or SPECT imaging was performed after which the mice were killed for organ/tumor tracer quantitation. Tumor hypoxia was confirmed. Arterial blood gas analysis was performed for the different anaesthetic regimes. The results demonstrate marked influences on tumor uptake of both carrier gas and anaesthetic, and show differences between [^99mTc]-HL91, [¹⁸F]-FMISO and [⁶⁴Cu]-CuATSM. [^99mTc]-HL91 tumor uptake was only altered significantly by administration of 100% oxygen. The latter was not the case for [¹⁸F]-FMISO and [⁶⁴Cu]-CuATSM. Tumor-to-muscle ratio (TMR) for both compounds was reduced significantly when either oxygen or anaesthetics (isoflurane in air, ketamine/xylazine or hypnorm/hypnovel) were introduced. For [¹⁸F]-FMISO no further decrease was measured when both isoflurane and oxygen were administered, [⁶⁴Cu]-CuATSM did show an additional significant decrease in TMR. When using the same anaesthetic regimes, the extent of TMR reduction was less pronounced for [⁶⁴Cu]-CuATSM than for [¹⁸F]-FMISO (40–60% versus 70% reduction as compared to awake animals breathing air).

Conclusions/Significance

The use of anaesthesia can have profound effects on the experimental outcome. More importantly, all tested anaesthetics reduced tumor-hypoxia uptake. Anaesthesia cannot be avoided in preclinical studies but great care has to be taken in preclinical models of hypoxia as anaesthesia effects cannot be generalised across applications, nor disease states.

The development of copper radiopharmaceuticals for imaging and therapy

The increasing use of positron emission tomography in preclinical and clinical settings has widened the demand for radiopharmaceuticals with high specificity that can image biological phenomena in vivo. While many PET tracers have been developed from small organic molecules labeled with carbon-11 or fluorine-18, the short half-lives of these radionuclides preclude their incorporation into radiotracers, which can be used to image biological processes that are not induced immediately after system perturbation. Additionally, the continuing development of targeted agents, such as antibodies and nanoparticles, which undergo extended circulation, require that radionuclides with half-lives that are complimentary to the biological half-lives of these molecules be developed. Copper radionuclides have received considerable attention since they offer a variety of half-lives and decay energies and because the coordination chemistry of cooper and its role in biology is well understood. However, in addition to the radiometal chelate, a successful copper based radiopharmaceutical depends upon the chemical structure of the entire radiotracer, which may include a biologically important molecule and a chemical linker that can be used to deliver the copper radionuclide to a specific target and modulate its in vivo properties, respectively. This review discusses the development of copper radiopharmaceuticals and the importance of factors such as chemical structure on their pharmacokinetics in vivo.

Fluorescent gallium and indium bis(thiosemicarbazonates) and their radiolabelled analogues: synthesis, structures and cellular confocal fluorescence imaging investigations

New fluorescent and biocompatible aromatic Ga(III)- and In(III)-bis(thiosemicarbazonato) complexes for dual mode optical and PET or SPECT molecular imaging have been synthesised via a synthetic method based on transmetallation reactions from Zn(II) precursors. Complexes have been fully characterised in the solid state by single crystal X-ray diffraction and in solution by spectroscopic methods (UV/Vis, fluorescence, (1)H and (13)C{(1)H} NMR). The bis(thiosemicarbazones) radiolabelled rapidly in high yields under mild conditions with (111)In (a gamma and Auger emitter for SPECT imaging and radiotherapy with t(1/2) = 2.8 d) and (68)Ga (a generator-available positron emitter for PET imaging with t(1/2) = 68 min). Cytotoxicity and biolocalisation studies using confocal fluorescence imaging and fluorescence lifetime imaging (FLIM) techniques have been used to study their in vitro activities and stabilities in HeLa and PC-3 cells to ascertain their suitability as synthetic scaffolds for future multimodality molecular imaging in cancer diagnosis and therapy. The observation that the indium complexes show certain nuclear uptake could be of relevance towards developing (111)In therapeutic agents based on Auger electron emission to induce DNA damage.

One to chelate them all: investigation of a versatile, bifunctional chelator for 64Cu, 99mTc, Re and Co

We describe the synthesis of the dip (di-picolyl-carboxylate) bifunctional chelator system, capable of fast coordination of Cu(2+), (64)Cu(2+) and Co(2+), as well as the [M(CO)(3)](+)-core (M = (99m)Tc, Re); it displays a variety of binding modes--tridentate when protected, tetradentate when deprotected. Syntheses of both the benzyl-nitro derivative and the benzyl-amino derivatives are described. The latter was coupled to biotin to show the viability of the system for functionalization with biomolecules. Besides coordination chemistry with stable isotopes, we also present labelling data with (64)Cu and (99m)Tc, as well as in vitro stability studies.

The preparation and biological characterization of a new HL91-derivative for hypoxic imaging on stroke mice

AIM

(99m)Tc-HL91 (Prognox, GE-Healthcare) was the first nonnitro-aryl-based radiotracer for evaluating hypoxic fraction in neoplasm, stroke and myocardium infarction regions. However, the high hydrophilicity of (99m)Tc-HL91 might hamper its penetration into cells. In this study, we prepared a new ligand 4,4,11,11-tetramethyl- 5,10-diazatetradecane- 3,12-dionedioxime (HL91-ET) with higher lipophilicity but structurally similar compared with that of HL91. The chemical and biological characterizations of (99m)Tc-HL91-ET as a scintigraphic probe for hypoxia were performed with a stroke-bearing mouse model.

MATERIALS AND METHODS

HL91-ET was synthesized and formulated with stannous chloride and buffer to afford kits. After mixing with (99m)Tc-pertechnetate, (99m)Tc-HL91-ET can be prepared in high yield and high radiochemical purity (both >96%). The partition coefficient of (99m)Tc-HL91-ET was determined in n-octanol/PBS system. Cellular uptake assays under normoxic and hypoxic conditions were performed in an oxygen-controlled CO(2) incubator. Brain stroke in the mouse model was induced by the electrocautery of the middle cerebral artery. After intravenous injection of (99m)Tc-HL91-ET into the Balb/c mouse suffering brain stroke, small-animal SPECT images were acquired at designated time points and autoradiography of the brain slides was conducted. Parallel studies of (99m)Tc-HL91 were also conducted at the same conditions for comparison.

RESULTS

The higher partition coefficient of (99m)Tc-HL91-ET (0.294+/-0.007) indicated higher lipophlicity compared with that of (99m)Tc-HL91 (0.089+/-0.005). The (99m)Tc-HL91-ET preparation was stable at ambient temperature for 24h. Cellular uptake assay showed that (99m)Tc-HL91-ET was less selectively retained in hypoxic cells than (99m)Tc-HL91. The target-to-normal brain ratios derived from the autoradiograms of the brains of stroke mice were 1.31+/-0.02 and 17.47+/-0.10 (n=3), respectively, at 2h post injection of (99m)Tc-HL91-ET and (99m)Tc-HL91.

CONCLUSIONS

This study revealed that (99m)Tc-HL91-ET, though with higher lipophilicity than (99m)Tc-HL91, did not suggest better specific accumulation in hypoxic cells or tissues than (99m)Tc-HL91. The uptake mechanism of (99m)Tc-HL91 was at least not solely by passive diffusion. Lipophilicity should not be the major consideration in designing HL91-derivatives for hypoxia imaging.

Bis(thiosemicarbazones) as bifunctional chelators for the room temperature 64-copper labeling of peptides

A range of new carboxylate functionalised bis(thiosemicarbazone) ligands and their Cu(II) complexes have been prepared, fully characterised and radiolabeled in high yield with both (64)Cu and (99m)Tc. Conjugation to a bombesin derivative was achieved using standard solid phase synthetic methodologies and the (64)Cu-labeled conjugate was shown to have good tumour uptake in mice with xenografted PC-3 tumours.

Macrocyclic diamide ligand systems: potential chelators for 64Cu- and 68Ga-based positron emission tomography imaging agents

The N(4)-macrocyclic ligand 2,10-dioxo-1,4,8,11-tetraazabicyclo[11.4.0]1,12-heptadeca-1(12),14,16-triene H(2)L has been synthesized by the [1 + 1] condensation reaction between N,N'-bis(chloroacetyl)-1,2-phenylenediamine and 1,3-propylenediamine. The coordination chemistry of this ligand has been investigated with the metal ions Cu(II), Ni(II), Zn(II), and Ga(III) (complexes 1, 2, 3 and 4, respectively). H(2)L and its metal complexes have been fully characterized by the use of NMR, UV/vis, electron paramagnetic resonance, and elemental analysis where appropriate. The four metal complexes 1-4 have been structurally characterized by X-ray crystallography which confirmed that in all cases the amide nitrogen atoms are deprotonated and coordinated to the metal center. Complexes 3 and 4 are five-coordinate with a water molecule and chloride ion occupying the apical site, respectively. Cyclic voltammetric measurements on complex 1 show that this complex is oxidized reversibly with a half-wave potential, E(1/2) = 0.47 V, and reduced irreversibly at E(P) = -1.84 V. Density functional theory calculations reproduce the geometries of the four complexes. The one-electron reduction and oxidation potentials for 1 were calculated by using two solvent models, DMF and H(2)O. The calculations indicated that the one electron oxidation of 1 may involve removal of an electron from the ligand as opposed to the metal center, producing a diradical. The diamide macrocyle is of interest for the development of new positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging agents, and a radiolabeled complex has been synthesized with the positron emitting isotope (64)Cu. In vivo biodistribution studies for the (64)Cu labeled complex, (64)Cu-1, in male Lewis rats, showed that the activity is cleared rapidly from the blood within 1-2 h post-administration.

Nitroimidazole conjugates of bis(thiosemicarbazonato)64Cu(II) - Potential combination agents for the PET imaging of hypoxia

Combination agents comprising two different pharmacophores with the same biological target have the potential to show additive or synergistic activity. Bis(thiosemicarbazonato)copper(II) complexes (e.g. (64)Cu-ATSM) and nitroimidazoles (e.g. (18)F-MISO) are classes of tracer used for the delineation of tumor hypoxia by positron emission tomography (PET). Three nitroimidazole-bis(thiosemicarbazonato)copper(II) conjugates were produced in order to investigate their potential as combination hypoxia imaging agents. Two were derived from the known bifunctional bis(thiosemicarbazone) H(2)ATSM/A and the third from the new precursor diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-ethylamino-3-thiosemicarbazone) - H(2)ATSM/en. Oxygen-dependent uptake studies were performed using the (64)Cu radiolabelled complexes in EMT6 carcinoma cells. All the complexes displayed appreciable hypoxia selectivity, with the nitroimidazole conjugates displaying greater selectivity than a simple propyl derivative used as a control. Participation of the nitroimidazole group in the trapping mechanism is indicated by the increased hypoxic uptake of the 2- vs. the 4-substituted (64)Cu-ATSM/A derivatives. The 2-nitroimidazole derivative of (64)Cu-ATSM/en demonstrated superior hypoxia selectivity to (64)Cu-ATSM over the range of oxygen concentrations tested. Biodistribution of the radiolabelled 2-nitroimidazole conjugates was carried out in EMT6 tumor-bearing mice. The complexes showed significantly different uptake trends in comparison to each other and previously studied Cu-ATSM derivatives. Uptake of the Cu-ATSM/en conjugate in non-target organs was considerably lower than for derivatives based on Cu-ATSM/A.