Comparative studies of Cu-64-ATSM and C-11-acetate in an acute myocardial infarction model: ex vivo imaging of hypoxia in rats

Non-FDG hypoxia tracers

Hypoxia plays a critical role in tumor biology, influencing cancer progression, treatment resistance, and patient prognosis. While 18-Fluorine fluoredeoxyglucose ([18F]F-FDG) PET imaging has been the standard for metabolic assessment, its limitations in accurately depicting hypoxic tumor regions necessitate the exploration of non-FDG hypoxia tracers. This review aims to evaluate emerging non-FDG radiotracers, such as nitroimidazole derivatives, copper-based agents, gallium-based agents and other innovative compounds, highlighting their mechanisms of action, biodistribution, and clinical applications. We will discuss the advantages and challenges associated with hypoxia imaging, as well as recent advancements in imaging techniques that enhance the assessment of tumor hypoxia. By synthesizing current research, this review seeks to provide insights into the potential of non-FDG hypoxia tracers for improving cancer diagnosis, treatment planning, and monitoring, ultimately contributing to more personalized and effective cancer care.

An Overview of In Vitro Assays of 64Cu-, 68Ga-, 125I-, and 99mTc-Labelled Radiopharmaceuticals Using Radiometric Counters in the Era of Radiotheranostics

Radionuclides are unstable isotopes that mainly emit alpha (α), beta (β) or gamma (γ) radiation through radiation decay. Therefore, they are used in the biomedical field to label biomolecules or drugs for diagnostic imaging applications, such as positron emission tomography (PET) and/or single-photon emission computed tomography (SPECT). A growing field of research is the development of new radiopharmaceuticals for use in cancer treatments. Preclinical studies are the gold standard for translational research. Specifically, in vitro radiopharmaceutical studies are based on the use of radiopharmaceuticals directly on cells. To date, radiometric β- and γ-counters are the only tools able to assess a preclinical in vitro assay with the aim of estimating uptake, retention, and release parameters, including time- and dose-dependent cytotoxicity and kinetic parameters. This review has been designed for researchers, such as biologists and biotechnologists, who would like to approach the radiobiology field and conduct in vitro assays for cellular radioactivity evaluations using radiometric counters. To demonstrate the importance of in vitro radiopharmaceutical assays using radiometric counters with a view to radiogenomics, many studies based on ⁶⁴Cu-, ⁶⁸Ga-, ¹²⁵I-, and ^99mTc-labeled radiopharmaceuticals have been revised and summarized in this manuscript.

Cerebral Oxidative Stress in Early Alzheimer's Disease Evaluated by 64Cu-ATSM PET/MRI: A Preliminary Study

Oxidative stress imaging using diacetyl-bis (N4-methylthiosemicarbazone) (Cu-ATSM) was applied to the evaluation of patients with early Alzheimer's disease (eAD). Ten eAD patients (72 ± 9 years) and 10 age-matched healthy controls (HCs) (73 ± 9 years) participated in this study. They underwent dynamic PET/MRI using 11C-PiB and 64Cu-ATSM with multiple MRI sequences. To evaluate cerebral oxidative stress, three parameters of 64Cu-ATSM PET were compared: standardized uptake value (SUV), tracer influx rate (Kin), and a rate constant k3. The input functions were estimated by the image-derived input function method. The relative differences were analyzed by statistical parametric mapping (SPM) using SUV and Kin images. All eAD patients had positive and HC subjects had negative PiB accumulation, and MMSE scores were significantly different between them. The 64Cu-ATSM accumulation tended to be higher in eAD than in HCs for both SUV and Kin. When comparing absolute values, eAD patients had a greater Kin in the posterior cingulate cortex and a greater k3 in the hippocampus compared with lobar cortical values of HCs. In SPM analysis, eAD had an increased left operculum and decreased bilateral hippocampus and anterior cingulate cortex compared to HCs. 64Cu-ATSM PET/MRI and tracer kinetic analysis elucidated cerebral oxidative stress in the eAD patients, particularly in the cingulate cortex and hippocampus.

64Cu-ATSM Predicts Efficacy of Carbon Ion Radiotherapy Associated with Cellular Antioxidant Capacity

Simple Summary

Carbon ion radiotherapy is an emerging cancer treatment modality that has a greater therapeutic window than conventional photon radiotherapy. To maximize the efficacy of this extremely scarce medical resource, it is important to identify predictive biomarkers of higher carbon ion relative biological effectiveness (RBE) over photons. Here we show that the carbon ion RBE in human cancer cells correlates with the cellular uptake of ⁶⁴Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone) (⁶⁴Cu-ATSM), a potential radioligand that reflects an over-reduced intracellular environment. High RBE/⁶⁴Cu-ATSM cells show greater steady-state levels of antioxidant proteins and increased capacity to scavenge reactive oxygen species in response to X-rays than low RBE/⁶⁴Cu-ATSM counterparts. These data suggest that the cellular antioxidant activity is a possible determinant of carbon ion RBE predictable by ⁶⁴Cu-ATSM uptake.

Abstract

Carbon ion radiotherapy is an emerging cancer treatment modality that has a greater therapeutic window than conventional photon radiotherapy. To maximize the efficacy of this extremely scarce medical resource, it is important to identify predictive biomarkers of higher carbon ion relative biological effectiveness (RBE) over photons. We addressed this issue by focusing on cellular antioxidant capacity and investigated ⁶⁴Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone) (⁶⁴Cu-ATSM), a potential radioligand that reflects an over-reduced intracellular environment. We found that the carbon ion RBE correlated with ⁶⁴Cu-ATSM uptake both in vitro and in vivo. High RBE/⁶⁴Cu-ATSM cells showed greater steady-state levels of antioxidant proteins and increased capacity to scavenge reactive oxygen species in response to X-rays than low RBE/⁶⁴Cu-ATSM counterparts; this upregulation of antioxidant systems was associated with downregulation of TCA cycle intermediates. Furthermore, inhibition of nuclear factor erythroid 2-related factor 2 (Nrf2) sensitized high RBE/⁶⁴Cu-ATSM cells to X-rays, thereby reducing RBE values to levels comparable to those in low RBE/⁶⁴Cu-ATSM cells. These data suggest that the cellular activity of Nrf2-driven antioxidant systems is a possible determinant of carbon ion RBE predictable by ⁶⁴Cu-ATSM uptake. These new findings highlight the potential clinical utility of ⁶⁴Cu-ATSM imaging to identify high RBE tumors that will benefit from carbon ion radiotherapy.

Preparation and Bioevaluation of Novel 99mTc-Labeled Complexes with a 2-Nitroimidazole HYNIC Derivative for Imaging Tumor Hypoxia

To develop novel 99mTc-labeled single-photon emission computed tomography (SPECT) radiotracers for imaging hypoxia, a novel HYNICNM ligand (6-hydrazinonicotinamide (HYNIC) 2-nitroimidazole derivative) was designed and synthesized. It was radiolabeled with technetium-99m using tricine/trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS), tricine/sodium triphenylphosphine-3-monosulfonate (TPPMS) and tricine as co-ligands to obtain [99mTc]Tc-tricine-TPPTS-HYNICNM, [99mTc]Tc-tricine-TPPMS-HYNICNM, and [99mTc]Tc-(tricine)2-HYNICNM, respectively. The three technetium-99m complexes were radiolabeled in one step with a high yield (95%) and had good stability in saline and mouse serum. In vitro cellular uptake results showed that these complexes exhibited good hypoxic selectivity. The partition coefficient indicated that they were good hydrophilic complexes, and [99mTc]Tc-tricine-TPPTS-HYNICNM displayed the highest hydrophilicity (-3.02 ± 0.08). The biodistribution in mice bearing S180 tumors showed that [99mTc]Tc-tricine-TPPTS-HYNICNM exhibited higher tumor uptake (1.05 ± 0.27% IA/g); more rapid clearance from the liver, blood, muscle, and other non-target organs; and a higher tumor/non-target ratio, especially for the tumor/liver ratio (1.95), than [99mTc]Tc-tricine-TPPMS-HYNICNM and [99mTc]Tc-(tricine)2-HYNICNM. The results of single-photon emission computed tomography (SPECT) imaging studies of [99mTc]Tc-tricine-TPPTS-HYNICNM were in accordance with the biodistribution results, which suggested that [99mTc]Tc-tricine-TPPTS-HYNICNM is a promising agent for imaging tumor hypoxia.

Development of 64Cu-loaded Perfluoropentane Nanodroplet: A Potential Tumor Theragnostic Nano-carrier and Dual-Modality PET-Ultrasound Imaging Agents

The purpose of this study was to develop and preliminarily evaluate phospholipid-shelled nanodroplets (NDs) encapsulating perfluoropentane (PFP) and radioactive ⁶⁴Cu as a hybrid positron emission tomography (PET)-ultrasound (US) probe. PFP NDs were fabricated by mixing liquid-phase PFP with a phospholipid solution. The ⁶⁴Cu was encapsulated into the NDs in a size-controlled manner by exploiting the hydrophobicity of ⁶⁴Cu-diacetyl-bis(N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM) using a vial mixer and an extruder. The fabricated ⁶⁴Cu-loaded PFP NDs (⁶⁴Cu-PFP NDs) were evaluated using in vitro/in vivo PET-computed tomography (PET-CT), US imaging and transmission electron microscopy. In the in vitro PET images, the ⁶⁴Cu-PFP NDs were observed as a hot spot in the lower section of the test tube. In the acquired US images, the mean region of interest brightness values of ⁶⁴Cu-PFP NDs were revealed by their strong echo image. In a tumor-bearing mouse animal model, tumor uptake of the ⁶⁴Cu-PFP NDs was low, that is, approximately 65%, compared with that of only free ⁶⁴Cu, as determined by PET-delayed imaging analysis. The dual-function concept of the NDs is expected to contribute to the prognosis and effectiveness of therapy by fusing the science and technology of nuclear medicine and US.

PET Imaging for Oxidative Stress in Neurodegenerative Disorders Associated with Mitochondrial Dysfunction

Oxidative stress based on mitochondrial dysfunction is assumed to be the principal molecular mechanism for the pathogenesis of many neurodegenerative disorders. However, the effects of oxidative stress on the neurodegeneration process in living patients remain to be elucidated. Molecular imaging with positron emission tomography (PET) can directly evaluate subtle biological changes, including the redox status. The present review focuses on recent advances in PET imaging for oxidative stress, in particular the use of the Cu-ATSM radioligand, in neurodegenerative disorders associated with mitochondrial dysfunction. Since reactive oxygen species are mostly generated by leakage of excess electrons from an over-reductive state due to mitochondrial respiratory chain impairment, PET with ⁶²Cu-ATSM, the accumulation of which depends on an over-reductive state, is able to image oxidative stress. ⁶²Cu-ATSM PET studies demonstrated enhanced oxidative stress in the disease-related brain regions of patients with mitochondrial disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, the magnitude of oxidative stress increased with disease severity, indicating that oxidative stress based on mitochondrial dysfunction contributes to promoting neurodegeneration in these diseases. Oxidative stress imaging has improved our insights into the pathological mechanisms of neurodegenerative disorders, and is a promising tool for monitoring further antioxidant therapies.

64Cu-ATSM and 99mTc(CO)3-DCM20 potential in the early detection of rheumatoid arthritis

OBJECTIVES

Molecular imaging constitutes a promising technique for the early detection of rheumatoid arthritis (RA). Macrophages and hypoxia play significant roles in inflamed synovium. In the present study, we evaluated the efficacy of radiopharmaceuticals that target macrophage mannose receptors (^99mTc-labeled mannosylated dextran or ^99mTc(CO)₃-DCM20) and hypoxia (copper(II) diacetyl-di(N⁴-methylthiosemicarbazone) or Cu-ATSM) for the early detection of RA in collagen-induced arthritis (CIA) mice models.

METHODS

CIA model was developed in DBA/1 mice, and the clinical score for arthritis was visually assessed on a regular basis. Two biodistribution studies were performed in a paired-labeled format using 2-deoxy-2-¹⁸F-fluoro-D-glucose (¹⁸F-FDG) as a reference: (1) ^99mTc(CO)₃-DCM20 with ¹⁸F-FDG and (2) ⁶⁷Cu-ATSM with ¹⁸F-FDG.

RESULTS

The accumulation levels of ^99mTc(CO)₃-DCM20 and ⁶⁷Cu-ATSM in forepaws, hindpaws, and knee joints of CIA mice were significantly higher than that of control mice. In contrast, ¹⁸F-FDG uptake in hindpaws and knee joints showed no significant difference between CIA and control mice. The radioactivity levels of ^99mTc(CO)₃-DCM20 and ⁶⁷Cu-ATSM were significantly correlated with the clinical scores for the paws.

CONCLUSION

These results suggest the potential usefulness of ^99mTc(CO)₃-DCM20 and radiolabeled Cu-ATSM for the imaging and early detection of RA.

High-throughput radio-TLC analysis

INTRODUCTION

Radio thin layer chromatography (radio-TLC) is commonly used to analyze purity of radiopharmaceuticals or to determine the reaction conversion when optimizing radiosynthesis processes. In applications where there are few radioactive species, radio-TLC is preferred over radio-high-performance liquid chromatography due to its simplicity and relatively quick analysis time. However, with current radio-TLC methods, it remains cumbersome to analyze a large number of samples during reaction optimization. In a couple of studies, Cerenkov luminescence imaging (CLI) has been used for reading radio-TLC plates spotted with a variety of isotopes. We show that this approach can be extended to develop a high-throughput approach for radio-TLC analysis of many samples.

METHODS

The high-throughput radio-TLC analysis was carried out by performing parallel development of multiple radioactive samples spotted on a single TLC plate, followed by simultaneous readout of the separated samples using Cerenkov imaging. Using custom-written MATLAB software, images were processed and regions of interest (ROIs) were drawn to enclose the radioactive regions/spots. For each sample, the proportion of integrated signal in each ROI was computed. Various crude samples of [¹⁸F]fallypride, [¹⁸F]FET and [¹⁷⁷Lu]Lu-PSMA-617 were prepared for demonstration of this new method.

RESULTS

Benefiting from a parallel developing process and high resolution of CLI-based readout, total analysis time for eight [¹⁸F]fallypride samples was 7.5 min (2.5 min for parallel developing, 5 min for parallel readout), which was significantly shorter than the 48 min needed using conventional approaches (24 min for sequential developing, 24 min for sequential readout on a radio-TLC scanner). The greater separation resolution of CLI enabled the discovery of a low-abundance side product from a crude [¹⁸F]FET sample that was not discernable using the radio-TLC scanner. Using the CLI-based readout method, we also observed that high labeling efficiency (99%) of [¹⁷⁷Lu]Lu-PSMA-617 can be achieved in just 10 min, rather than the typical 30 min timeframe used.

CONCLUSIONS

Cerenkov imaging in combination with parallel developing of multiple samples on a single TLC plate proved to be a practical method for rapid, high-throughput radio-TLC analysis.

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.

Preclinical Pharmacokinetic and Safety Studies of Copper-Diacetyl-Bis(N4-Methylthiosemicarbazone) (Cu-ATSM): Translational Studies for Internal Radiotherapy

Hypoxia plays important roles in the prognosis of malignant brain tumors such as glioblastoma because it causes drug delivery deficiencies and the induction of hypoxia-inducible factor-1α in tumor cells. Extensive hypoxic areas are associated with poor prognosis of these fatal diseases. We previously reported that multiple administrations of the hypoxia-targeted internal radiotherapy agent ⁶⁴Cu-diacetyl-bis(N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM), four times at intervals of 1 or 2 weeks, show antitumor effects in glioblastoma without treatment-related adverse events. Before initiating clinical trials, preclinical safety studies using Cu-ATSM composed of stable isotopes and its precursor ATSM were required to understand the potential risks of systemic and repeated chemical exposure of our investigational drug. In this study, the concentrations of Cu-ATSM and ATSM in mouse plasma after intravenous administration were determined by liquid chromatography–tandem mass spectrometry, and the half-lives were estimated to be 21.5 and 22.4 minutes for Cu-ATSM and ATSM, respectively. Based on this result, approach 2 of the current ICH M3 [R2] guideline was adopted, and a 7-day intravenous toxicity study was conducted in mice. Cu-ATSM and ATSM in a ratio of 2:25 mimicking our current investigational drug was used, and no adverse effects were observed when Cu-ATSM and ATSM were administered at 81 μg/kg. These results and those of previous studies suggest that our current investigational drug formulation containing Cu-ATSM and ATSM at a dose of 15 μg can be safely administered to patients once per week for 4 weeks for treatment with ⁶⁴Cu-ATSM.

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Clinical imaging of hypoxia: Current status and future directions

Tissue hypoxia is a key feature of many important causes of morbidity and mortality. In pathologies such as stroke, peripheral vascular disease and ischaemic heart disease, hypoxia is largely a consequence of low blood flow induced ischaemia, hence perfusion imaging is often used as a surrogate for hypoxia to guide clinical diagnosis and treatment. Importantly, ischaemia and hypoxia are not synonymous conditions as it is not universally true that well perfused tissues are normoxic or that poorly perfused tissues are hypoxic. In pathologies such as cancer, for instance, perfusion imaging and oxygen concentration are less well correlated, and oxygen concentration is independently correlated to radiotherapy response and overall treatment outcomes. In addition, the progression of many diseases is intricately related to maladaptive responses to the hypoxia itself. Thus there is potentially great clinical and scientific utility in direct measurements of tissue oxygenation. Despite this, imaging assessment of hypoxia in patients is rarely performed in clinical settings. This review summarises some of the current methods used to clinically evaluate hypoxia, the barriers to the routine use of these methods and the newer agents and techniques being explored for the assessment of hypoxia in pathological processes.

A Perspective - can copper complexes be developed as a novel class of therapeutics?

Although copper-ligand complexes appear to be promising as a new class of therapeutics, other than the family of copper(ii) coordination compounds referred to as casiopeínas these compounds have yet to reach the clinic for human use. The pharmaceutical challenges associated with developing copper-based therapeutics will be presented in this article along with a discussion of the potential for high-throughput chemistry, computer-aided drug design, and nanotechnology to address the development of this important class of drug candidates.

PET Imaging of Cardiac Hypoxia: Hitting Hypoxia Where It Hurts

Purpose of Review

In this review, we outline the potential for hypoxia imaging as a diagnostic and prognostic tool in cardiology. We describe the lead hypoxia PET radiotracers currently in development and propose a rationale for how they should most appropriately be screened and validated.

Recent Findings

While the majority of hypoxia imaging agents has been developed for oncology, the requirements for hypoxia imaging in cardiology are different. Recent work suggests that the bis(thiosemicarbazone) family of compounds may be capable of detecting the subtle degrees of hypoxia associated with cardiovascular syndromes, and that they have the potential to be “tuned” to provide different tracers for different applications.

Summary

New tracers currently in development show significant promise for imaging evolving cardiovascular disease. Fundamental to their exploitation is their careful, considered validation and characterization so that the information they provide delivers the greatest prognostic insight achievable.

Multiple Administrations of 64Cu-ATSM as a Novel Therapeutic Option for Glioblastoma: a Translational Study Using Mice with Xenografts

Glioblastoma is the most aggressive malignant brain tumor in humans and is difficult to cure using current treatment options. Hypoxic regions are frequently found in glioblastoma, and increased levels of hypoxia are associated with poor clinical outcomes of glioblastoma patients. Hypoxia plays important roles in the progression and recurrence of glioblastoma because of drug delivery deficiencies and induction of hypoxia-inducible factor-1α in tumor cells, which lead to poor prognosis. We focused on a promising hypoxia-targeted internal radiotherapy agent, ⁶⁴Cu-diacetyl-bis (N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM), to address the need for additional treatment for glioblastoma. This compound can target the overreduced state under hypoxic conditions within tumors. Clinical positron emission tomography studies using radiolabeled Cu-ATSM have shown that Cu-ATSM accumulates in glioblastoma and its uptake is associated with high hypoxia-inducible factor-1α expression. To evaluate the therapeutic potential of this agent for glioblastoma, we examined the efficacy of ⁶⁴Cu-ATSM in mice bearing U87MG glioblastoma tumors. Administration of single dosage (18.5, 37, 74, 111, and 148 MBq) and multiple dosages (37 MBq × 4) of ⁶⁴Cu-ATSM was investigated. Single administration of ⁶⁴Cu-ATSM in high-dose groups dose-dependently inhibited tumor growth and prolonged survival, with slight and reverse signs of adverse events. Multiple dosages of ⁶⁴Cu-ATSM remarkably inhibited tumor growth and prolonged survival. By splitting the dose of ⁶⁴Cu-ATSM, no adverse effects were observed. Our findings indicate that multiple administrations of ⁶⁴Cu-ATSM have effective antitumor effects in glioblastoma without side effects, indicating its potential for treating this fatal disease.

64Cu-ATSM internal radiotherapy to treat tumors with bevacizumab-induced vascular decrease and hypoxia in human colon carcinoma xenografts

Bevacizumab, an anti-vascular endothelial growth factor (VEGF) antibody, is an antiangiogenic agent clinically used for various cancers. However, repeated use of this agent leads to tumor-decreased vascularity and hypoxia with activation of an HIF-1 signaling pathway, which results in drug delivery deficiency and induction of malignant behaviors in tumors. Here, we developed a novel strategy to treat tumors with bevacizumab-induced vascular decrease and hypoxia using ⁶⁴Cu-diacetyl-bis (N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM), a potential theranostic agent, which possesses high tissue permeability and can target over-reduced conditions under hypoxia in tumors, with a human colon carcinoma HT-29 tumor-bearing mouse model. The long-term treatment with bevacizumab caused decreased blood vessel density and activation of an HIF-1 signaling pathway; increased uptake of ⁶⁴Cu-ATSM was also observed despite limited blood vessel density in HT-29 tumors. In vivo high-resolution SPECT/PET/CT imaging confirmed reduced vascularity and increased proportion of ⁶⁴Cu-ATSM uptake areas within the bevacizumab-treated tumors. ⁶⁴Cu-ATSM therapy was effective to inhibit tumor growth and prolong survival of the bevacizumab-treated tumor-bearing mice without major adverse effects. In conclusion, ⁶⁴Cu-ATSM therapy effectively enhanced anti-tumor effects in tumors with bevacizumab-induced vascular decrease and hypoxia. ⁶⁴Cu-ATSM therapy could represent a novel approach as an add-on to antiangiogenic therapy.

Copper-Bispidine Complexes: Synthesis and Complex Stability Study

A new series of dicarboxylic derivatives of bispidines have been synthesized to develop novel copper(II) complexes suitable as imaging agents for positron emission tomography. For characterization purposes, copper complexes of bispidines were synthesized in the pure form and in quantitative yields by neutralization of ligands with malachite. The formation of complexes and their stoichiometries were studied by potentiometric titration, cyclic voltammetry, and spectroscopic methods. The stability constants were found to be fairly suitable for copper cation fixation inside dianionic chelate molecules.

Imaging of hypoxia in mouse atherosclerotic plaques with (64)Cu-ATSM

INTRODUCTION

Cardiovascular disease is the leading cause of death in the United States. The identification of vulnerable plaque at risk of rupture has been a major focus of research. Hypoxia has been identified as a potential factor in the formation of vulnerable plaque, and it is clear that decreased oxygen plays a role in the development of plaque angiogenesis leading to plaque destabilization. The purpose of this study is to demonstrate the feasibility of copper-64 labeled diacetyl-bis (N(4)-methylthiosemicarbazone) ((64)Cu-ATSM), a positron-emitting radiopharmaceutical taken up in low-oxygen-tension cells, for the identification of hypoxic and potentially unstable atherosclerotic plaque in a mouse model.

METHODS

(64)Cu-ATSM PET was performed in 21 atherosclerotic apolipoprotein E knockout (ApoE(-/-)) mice, 6 of which were fed high-fat diet (HFD) while the others received standard-chow diet (SCD), and 13 control wild type mice fed SCD. 4 SCD ApoE(-/-) mice and 4 SCD wild type mice also underwent (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET) imaging one day prior to (64)Cu-ATSM PET.

RESULTS

(64)Cu-ATSM uptake was increased in the aortic arch in SCD ApoE(-/-) mice (average aortic arch/muscle (A/M) standardized uptake value ratio 7.5-30min post injection: (5.66±0.23) compared to control mice (A/M SUV ratio 7.5-30min post injection (3.87±0.22), p<0.0001). HFD ApoE(-/-) mice also showed similarly increased aortic arch uptake on PET imaging in comparison to control mice. Immunohistochemistry in both HFD and SCD ApoE(-/-) mice revealed noticeable hypoxia by pimonidazole stain in atherosclerosis which was co-localized to macrophage by CD68 staining. Autoradiography assessment demonstrated the presence of hypoxia by (64)Cu-ATSM uptake correlated with pimonidazole uptake within the ex vivo atherosclerotic aortic arch specimens. A significant increase in (18)F-FDG uptake in the SCD ApoE(-/-) mice in comparison to controls was also observed at delayed time points.

CONCLUSION

This pre-clinical study suggests that (64)Cu-ATSM is a potential PET tracer for hypoxia imaging in atherosclerosis.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

While studies in humans are necessary for conclusive data, in the long term, a (64)Cu-ATSM PET imaging strategy could help facilitate the study of plaque biology in human patients.

Copper-64 Radiopharmaceuticals for Oncologic Imaging

The positron emitting radionuclide (64)Cu has a radioactive half-life of 12.7 hours. The decay characteristics of (64)Cu allow for PET images that are comparable in quality to those obtained using (18)F. Given the longer radioactive half-life of (64)Cu compared with (18)F and the versatility of copper chemistry, copper is an attractive alternative to the shorter-lived nuclides for PET imaging of peptides, antibodies, and small molecules that may require longer circulation times. This article discusses a number of copper radiopharmaceuticals, such as Cu-ATSM, that have been translated to the clinic and new developments in copper-based radiopharmaceuticals.

Evaluation of hypoxia with copper-labeled diacetyl-bis(N-methylthiosemicarbazone)

Imaging of hypoxia is important in many diseases states in oncology, cardiology, and neurology. The radiopharmaceutical, copper-labeled diacetyl-bis(N-methylthiosemicarbazone), has been used to assess hypoxia in many studies. In particular, copper-labeled diacetyl-bis(N-methylthiosemicarbazone) has been used in oncologic settings to investigate tumor hypoxia and the role of this parameter in response to therapy and outcome. Some groups have conducted imaging studies assessing the role of hypoxia in cardiovascular and neurologic disorders. Additionally, several groups have made significant progress into understanding the mechanism by which this compound accumulates in cells. Multiple preclinical and clinical studies have been conducted, shedding light on the importance of careful image analysis when using this tracer. This review article focuses on the recent preclinical and clinical studies with this tracer.

Validation of 64Cu-ATSM damaging DNA via high-LET Auger electron emission

Radioactive copper (II) (diacetyl-bis N4-methylthiosemicarbazone) (Cu-ATSM) isotopes were originally developed for the imaging of hypoxia in tumors. Because the decay of a (64)Cu atom is emitting not only positrons but also Auger electrons, this radionuclide has great potential as a theranostic agent. However, the success of (64)Cu-ATSM internal radiation therapy would depend on the contribution of Auger electrons to tumor cell killing. Therefore, we designed a cell culture system to define the contributions to cell death from Auger electrons to support or refute our hypothesis that the majority of cell death from (64)Cu-ATSM is a result of high-LET Auger electrons and not positrons or other low-LET radiation. Chinese hamster ovary (CHO) wild type and DNA repair-deficient xrs5 cells were exposed to (64)Cu-ATSM during hypoxic conditions. Surviving fractions were compared with those surviving gamma-radiation, low-LET hadron radiation, and high-LET heavy ion exposure. The ratio of the D(10) values (doses required to achieve 10% cell survival) between CHO wild type and xrs5 cells suggested that (64)Cu-ATSM toxicity is similar to that of high-LET Carbon ion radiation (70 keV/μm). γH2AX foci assays confirmed DNA double-strand breaks and cluster damage by high-LET Auger electrons from (64)Cu decay, and complex types of chromosomal aberrations typical of high-LET radiation were observed after (64)Cu-ATSM exposure. The majority of cell death was caused by high-LET radiation. This work provides strong evidence that (64)Cu-ATSM damages DNA via high-LET Auger electrons, supporting further study and consideration of (64)Cu-ATSM as a cancer treatment modality for hypoxic tumors.

64Cu-ATSM Hypoxia Positron Emission Tomography for Detection of Conduit Ischemia in an Experimental Rat Esophagectomy Model

Background

We designed a hypoxia-imaging modality to detect ischemia of the gastric conduit after esophagectomy.

Materials and Methods

A rat esophagectomy model was created using 12-16-week-old, 300-350 g male Sprague-Dawley rats. In the operation group (n=6), partial gastric devascularization was performed by ligating the left gastric artery and the short gastric arteries and an esophagogastric anastomosis was performed. In the control group (n=6), the esophageal-gastric junction was incised and suturing was performed without gastric devascularization. Positron emission tomography (PET) images were taken using a microPET rodent model scanner, 24 h after the initial operation, after injection of 200 μCi 64Cu-diacetyl-bis (N4-methylsemicarbazone) (64Cu-ATSM) and pimonidazole 120 mg/kg. After microPET imaging, autoradiography and immunohistochemistry were performed.

Results

The PET image revealed 64Cu-ATSM uptake at the fundus in the operation group 3 h after 64Cu-ATSM injection. The maximum percentage of the injected dose per gram of tissue was higher in the operation group (0.047±0.015 vs. 0.026±0.006, p=0.021). The fundus/liver ratio was also higher in the operation group (0.541±0.126 vs. 0.278±0.049, p=0.002). Upon autoradiography, 64Cu-ATSM uptake was observed in the fundus in the operation group, and was well-correlated to that observed on the PET image. Upon immunohistochemistry, expression of hypoxia-inducible factor 1a and pimonidazole were significantly increased at the fundus and lesser curvature compared to the greater curvature in the operation group.

Conclusion

Hypoxia PET imaging with 64Cu-ATSM can detect ischemia in a rat esophagectomy model. Further clinical studies are needed to verify whether hypoxia imaging may be useful in humans.

64Cu-CTS: A Promising Radiopharmaceutical for the Identification of Low-Grade Cardiac Hypoxia by PET

The subtle hypoxia underlying chronic cardiovascular disease is an attractive target for PET imaging, but the lead hypoxia imaging agents (64)Cu-2,3-butanedione bis(N4-methylthiosemicarbazone) (ATSM) and (18)F-fluoromisonidazole are trapped only at extreme levels of hypoxia and hence are insufficiently sensitive for this purpose. We have therefore sought an analog of (64)Cu-ATSM better suited to identify compromised but salvageable myocardium, and we validated it using parallel biomarkers of cardiac energetics comparable to those observed in chronic cardiac ischemic syndromes.

METHODS

Rat hearts were perfused with aerobic buffer for 20 min, followed by a range of hypoxic buffers (using a computer-controlled gas mixer) for 45 min. Contractility was monitored by intraventricular balloon, energetics by (31)P nuclear MR spectroscopy, lactate and creatine kinase release spectrophotometrically, and hypoxia-inducible factor 1-α by Western blotting.

RESULTS

We identified a key hypoxia threshold at a 30% buffer O2 saturation that induces a stable and potentially survivable functional and energetic compromise: left ventricular developed pressure was depressed by 20%, and cardiac phosphocreatine was depleted by 65.5% ± 14% (P < 0.05 vs. control), but adenosine triphosphate levels were maintained. Lactate release was elevated (0.21 ± 0.067 mmol/L/min vs. 0.056 ± 0.01 mmol/L/min, P < 0.05) but not maximal (0.46 ± 0.117 mmol/L/min), indicating residual oxidative metabolic capacity. Hypoxia-inducible factor 1-α was elevated but not maximal. At this key threshold, (64)Cu-2,3-pentanedione bis(thiosemicarbazone) (CTS) selectively deposited significantly more (64)Cu than any other tracer we examined (61.8% ± 9.6% injected dose vs. 29.4% ± 9.5% for (64)Cu-ATSM, P < 0.05).

CONCLUSION

The hypoxic threshold that induced survivable metabolic and functional compromise was 30% O2. At this threshold, only (64)Cu-CTS delivered a hypoxic-to-normoxic contrast of 3:1, and it therefore warrants in vivo evaluation for imaging chronic cardiac ischemic syndromes.

Assessing tumor hypoxia in head and neck cancer by PET with ⁶²Cu-diacetyl-bis(N⁴-methylthiosemicarbazone)

PURPOSE

The aim of this study was to investigate the potential of PET imaging with a hypoxia-selective tracer ⁶²Cu-diacetyl-bis(N⁴-methylthiosemicarbazone) (⁶²Cu-ATSM) for evaluating the prognosis of patients with head and neck cancer (HNC).

METHODS

Twenty-five patients with HNC including stage II to IV underwent both ⁶²Cu-ATSM and ¹⁸F-FDG PET before the initiation of treatment. Volumes of interest were placed on the tumor and sternocleidomastoid muscles to obtain SUVmax and to calculate the tumor-to-muscle activity ratio (TMR). The PET results were correlated with clinical follow-up, and the receiver operating characteristic analysis was used to determine the optimal cutoff values. Progression-free survival (PFS) and cause-specific survival (CSS) were statistically analyzed.

RESULTS

Patients were followed up for periods ranging from 4 to 32 months. Twelve patients died from local recurrence or metastasis of a primary cancer, and 2 had recurrence of the 13 remaining patients. Mean (SD) periods of PFS and CSS were 15.5 (12.5) and 18.6 (11.0) months, respectively. Optimal cutoff values for each PET index were as follows: SUVs of ⁶²Cu-ATSM (SUVATSM) and FDG were 3.6 and 7.9; TMRs of ATSM (TMRATSM) and FDG were 3.2 and 5.6. When the cutoff for TMRATSM was set at 3.2, patients with a greater TMRATSM had significantly worse PFS (P = 0.014) and CSS (P = 0.031). Two-year PFS and CSS rates were 73% and 80% for patients with a lower TMRATSM (≤3.2); however, they were 20% and 33% for those with hypoxic tumors (TMRATSM, >3.2), respectively. F-FDG-related indices did not show any significant difference in either PFS or CSS.

CONCLUSIONS

Pretreatment ⁶²Cu-ATSM PET is useful for predicting the prognosis of patients with HNC.

Comparison of intratumoral FDG and Cu-ATSM distributions in cancer tissue originated spheroid (CTOS) xenografts, a tumor model retaining the original tumor properties

INTRODUCTION

The intratumoral distributions of [(18)F]FDG and [(64)Cu]Cu-ATSM have been reported to be similar in adenocarcinomas but different in squamous cell carcinoma (SCC) in clinical studies. In the present study, we compared the intratumoral distributions of these two tracers in cancer tissue originated spheroid (CTOS) xenografts derived from adenocarcinoma and SCC, which retain the histological characteristics of the original tumors, and in cancer cell line xenografts of corresponding origin, to investigate the underlying mechanism of the distinct FDG and Cu-ATSM distribution patterns in adenocarcinoma and SCC.

METHODS

CTOSs derived from colon adenocarcinoma and lung SCC and cell lines established from colon adenocarcinoma and lung SCC, which were used for comparison, were subcutaneously transplanted into immunodeficient mice. One hour after administering [(14)C]FDG and [(64)Cu]Cu-ATSM, the intratumoral distributions were compared in the xenografts by using dual-tracer autoradiography. Adjacent sections were evaluated for necrosis, vasculature anatomy, Ki-67 antigen, and pimonidazole adducts using hematoxylin and eosin and immunohistochemical staining.

RESULTS

There was a higher regional overlap of high FDG and Cu-ATSM accumulations in the adenocarcinoma CTOS xenografts than in the SCC CTOS xenografts, while the overlap in the adenocarcinoma cell line xenograft was lower than that observed in the SCC cell line. High FDG accumulation occurred primarily in proximity to necrotic or pimonidazole adduct positive regions, while high Cu-ATSM accumulation occurred primarily in live cell regions separate from the necrotic regions. The adenocarcinoma CTOS xenograft had the stereotypical glandular structure, resulting in more intricately mixed regions of live and necrotic cells compared to those observed in the SCC CTOS or the cell line xenografts.

CONCLUSION

Tumor morphological characteristics, specifically the spatial distribution of live and necrotic cell regions, appeared to be one of the most critical factors determining the regional overlap of FDG and Cu-ATSM distributions in adenocarcinoma.

The copper radioisotopes: a systematic review with special interest to 64Cu

Copper (Cu) is an important trace element in humans; it plays a role as a cofactor for numerous enzymes and other proteins crucial for respiration, iron transport, metabolism, cell growth, and hemostasis. Natural copper comprises two stable isotopes, (63)Cu and (65)Cu, and 5 principal radioisotopes for molecular imaging applications ((60)Cu, (61)Cu, (62)Cu, and (64)Cu) and in vivo targeted radiation therapy ((64)Cu and (67)Cu). The two potential ways to produce Cu radioisotopes concern the use of the cyclotron or the reactor. A noncopper target is used to produce noncarrier-added Cu thanks to a chemical separation from the target material using ion exchange chromatography achieving a high amount of radioactivity with the lowest possible amount of nonradioactive isotopes. In recent years, Cu isotopes have been linked to antibodies, proteins, peptides, and nanoparticles for preclinical and clinical research; pathological conditions that influence Cu metabolism such as Menkes syndrome, Wilson disease, inflammation, tumor growth, metastasis, angiogenesis, and drug resistance have been studied. We aim to discuss all Cu radioisotopes application focusing on (64)Cu and in particular its form (64)CuCl2 that seems to be the most promising for its half-life, radiation emissions, and stability with chelators, allowing several applications in oncological and nononcological fields.

Synthesis andIn VitroandIn VivoEvaluation of a New68Ga-Semicarbazone Complex: Potential PET Radiopharmaceutical for Tumor Imaging

In an attempt to develop new tumor imaging radiotracers with favorable biochemical properties, we have synthesized new68Ga-2-acetylpyridine semicarbazone (68Ga-[APSC]2) as a potential positron emission tomography (PET) tumor imaging agent using a straightforward and a one-step simple reaction. Radiochemical yield and purity were quantitative without HPLC purification. Biodistribution studies in nude mice model bearing human MDA-MB-231 cell line xenografts displayed significant tumor uptake of68Ga-[APSC]2radiotracer after 2 h postinjection (p.i.). The initial results demonstrate that68Ga-[APSC]2radiotracer may be useful probe for detecting and staging of hypoxic tumor using PET imaging modality.

Different distribution of (62) Cu ATSM and (18)F-FDG in head and neck cancers

PURPOSE

[⁶²Cu]-diacetyl-bis(N4-methlythiosemicarbazone) (Cu-ATSM) was used to delineate hypoxic tissue in head-and-neck cancer, and its distribution was compared with that of ¹⁸F-FDG.

MATERIALS AND METHODS

Thirty patients with head-and-neck cancer underwent Cu-ATSM and FDG PET within a 1 week interval. Accumulation of tracer for each PET image was converted to SUV. After coregisteration of PET images with individual anatomic images, multiple small ROIs were drawn on the tumor mass and applied to both PET images. SUV values were obtained for all ROIs (SUV(roi)), and the SUV(roi) regression lines between Cu-ATSM and FDG of each tumor were determined.

RESULTS

The SUV mean of Cu-ATSM was lower than that of FDG for both squamous cell carcinoma (SCC) and adenocarcinoma (P < 0.05). In 27 patients with SCC, Cu-ATSM accumulated higher in the peripheral region than in the center of the tumor, and FDG showed the other tendency. Thus, the relationship of the SUV(roi) for Cu-ATSM and FDG showed a negative correlation in SCC. However, 3 adenocarcinoma cases showed similar and homogenous accumulation in the tumor mass with a positive SUV(roi) correlation for the 2 tracers. The regression slope means were -0.12 ± 0.08 for SCC (n = 27) and 0.28 ± 0.12 (n = 3) for adenocarcinoma.

CONCLUSION

In patients with head-and-neck cancer, intratumoral distribution of Cu-ATSM and FDG showed a negative correlation in SCC and a positive correlation in adenocarcinoma. The 2 tracers represented different pathophysiological microenvironments in different tumors, suggesting that noninvasive hypoxic tissue imaging with Cu-ATSM would be beneficial in the pretreatment evaluation of head-and-neck cancer.

Production of 61Cu using natural cobalt target and its separation using ascorbic acid and common anion exchange resin

(61)Cu was produced by (nat)Co(α, xn)(61)Cu reaction. (61)Cu production yield was 89.5 MBq/μAh (2.42 mCi/μAh) at the end of irradiation (EOI). A simple radiochemical separation method using anion exchange resin and ascorbic acid has been employed to separate the product radionuclide from inactive target material and co-produced non-isotopic impurities. The radiochemical separation yield was about 90%. Radiochemical purity of (61)Cu was >99% 1 h after EOI. Final product was suitable for making complex with N(2)S(2) type of ligands.

PET imaging of cardiac hypoxia: opportunities and challenges

Myocardial hypoxia is a major factor in the pathology of cardiac ischemia and myocardial infarction. Hypoxia also occurs in microvascular disease and cardiac hypertrophy, and is thought to be a prime determinant of the progression to heart failure, as well as the driving force for compensatory angiogenesis. The non-invasive delineation and quantification of hypoxia in cardiac tissue therefore has the potential to be an invaluable experimental, diagnostic and prognostic biomarker for applications in cardiology. However, at this time there are no validated methodologies sufficiently sensitive or reliable for clinical use. PET imaging provides real-time spatial information on the biodistribution of injected radiolabeled tracer molecules. Its inherent high sensitivity allows quantitative imaging of these tracers, even when injected at sub-pharmacological (≥pM) concentrations, allowing the non-invasive investigation of biological systems without perturbing them. PET is therefore an attractive approach for the delineation and quantification of cardiac hypoxia and ischemia. In this review we discuss the key concepts which must be considered when imaging hypoxia in the heart. We summarize the PET tracers which are currently available, and we look forward to the next generation of hypoxia-specific PET imaging agents currently being developed. We describe their potential advantages and shortcomings compared to existing imaging approaches, and what is needed in terms of validation and characterization before these agents can be exploited clinically.

Dual-isotope PET using positron-gamma emitters

Positron emission tomography (PET) is widely recognized as a highly effective functional imaging modality. Unfortunately, standard PET cannot be used for dual-isotope imaging (which would allow for simultaneous investigation of two different biological processes), because positron-electron annihilation products from different tracers are indistinguishable in terms of energy. Methods that have been proposed for dual-isotope PET rely on differences in half-lives of the participating isotopes; these approaches, however, require making assumptions concerning kinetic behavior of the tracers and may not lead to optimal results. In this paper we propose a novel approach for dual-isotope PET and investigate its performance using GATE simulations. Our method requires one of the two radioactive isotopes to be a pure positron emitter and the second isotope to emit an additional high-energy gamma in a cascade simultaneously with positron emission. Detection of this auxiliary prompt gamma in coincidence with the annihilation event allows us to identify the corresponding 511 keV photon pair as originating from the same isotope. Two list-mode datasets are created: a primary dataset that contains all detected 511 keV photon pairs from both isotopes, and a second, tagged (much smaller) dataset that contains only those PET events for which a coincident prompt gamma has also been detected. An image reconstructed from the tagged dataset reflects the distribution of the second positron-gamma radiotracer and serves as a prior for the reconstruction of the primary dataset. Our preliminary simulation study with partially overlapping (18)F/(22)Na and (18)F/(60)Cu radiotracer distributions showed that in these two cases the dual-isotope PET method allowed for separation of the two activity distributions and recovered total activities with relative errors of about 5%.

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.

Improving the separation of Cu(II) from Zn(II) based on an anion exchanger for the preparation a 62Zn/62Cu generator

A separation procedure of (64,67)Cu(II) from (65)Zn(II) was studied for serving (62)Zn/(62)Cu generator preparation. The distribution coefficients of (65)Zn(II) and (64,67)Cu(II) ions from homogeneous medium of hydrochloric acid and acetone onto De-Acidite FF anion exchanger were investigated. Both hydrochloric acid and acetone concentrations are important factors that affect the separation process. Effects of decreasing the HCl concentration from 0.2 to 0.04 M, increasing the acetone concentration from 5% to 80% and different acetone concentrations (5-97%) in 0.2M HCl were also studied on the separation of (64,67)Cu(II) from (65)Zn(II). The exchange capacity of Zn(II) on the De-Acidite FF matrix was obtained as 3.8 meq/g and the(64,67)Cu(II) elution efficiency was 92.5% using 20 ml of 0.2M HCl-60% acetone with flow rate of 2 ml/min.The radionuclidic purity and radiochemical purity of the eluted (64,67)Cu(II) were examined.

In vivo imaging of HIF-active tumors by an oxygen-dependent degradation protein probe with an interchangeable labeling system

Hypoxia-inducible factor (HIF) functions as a master transcriptional regulator for adaptation to hypoxia by inducing adaptive changes in gene expression for regulation of proliferation, angiogenesis, apoptosis and energy metabolism. Cancers with high expression of the alpha subunit of HIF (HIFα) are often malignant and treatment-resistant. Therefore, the development of a molecular probe that can detect HIF activity has great potential value for monitoring tumor hypoxia. HIF prolyl hydroxylases (HPHDs) act as oxygen sensors that regulate the fate of HIFα protein through its oxygen-dependent degradation (ODD) domain. We constructed a recombinant protein PTD-ODD-HaloTag (POH) that is under the same ODD regulation as HIFα and contains protein transduction domain (PTD) and an interchangeable labeling system. Administration of near-infrared fluorescently labeled POH (POH-N) to mouse models of cancers allowed successful monitoring of HIF-active regions. Immunohistochemical analysis for intratumoral localization of POH probe revealed its specificity to HIF-active cells. Furthermore, lack of the PTD domain or a point mutation in the ODD domain abrogated the specificity of POH-N to HIF-active cells. Overall results indicate that POH is a practical probe specific to HIF-active cell in cancers and suggest its large potential for imaging and targeting of HIF-related diseases.

SAR of Cu (II) thiosemicarbazone complexes as hypoxic imaging agents: MM3 analysis and prediction of biologic properties

Copper(II) bis(thiosemicarbazone) are very useful for blood flow and hypoxic imaging. The aim of this study was to identify structure-activity relationships (SARs) within a series of analogues with different substitution patterns in the ligands, in order to design improved hypoxia imaging agents and elucidate hypoxia selectivity mechanisms. Genetic algorithms (GAs) were used to develop specific copper metal-ligand force field parameters for the MM3 force-field calculations. These new parameters produced results in good agreement with experiment and previously reported copper metal-ligand parameters. A successful quantitative SAR (QSAR) for predicting the several classes of Cu(II)-chelating ligands was built using a training set of 21 Cu(II) complexes. The QSAR exhibited a correlation between the predicted and experimental test set. The QSAR preformed with great accuracy; r(2) = 0.95 and q(2) = 0.90 utilizing a leave-one-out cross-validation with multiple linear regression analysis to find correlation between different calculated molecular descriptors of these complexes. The final QSAR mathematical models were found as the following: Log P = {3.01698 (+/-0.0590)} - LUMO {0.1248 (+/-0.068)} + MR {0.3219 (+/-0.086)} n = 21 |r| = 0.972 s = 0.188 F = 98.102 The resulting models could act as an efficient strategy for estimating the hypoxic conditions through imaging and provide some insights into the structural features related to the biological activity of these compounds.

Copper-64-diacetyl-bis (N4-methylthiosemicarbazone) accumulates in rich regions of CD133+ highly tumorigenic cells in mouse colon carcinoma

INTRODUCTION

(64)Cu-diacetyl-bis (N(4)-methylthiosemicarbazone) ((64)Cu-ATSM) is a potential imaging agent of hypoxic tumor for use with PET. Recent literature demonstrated that cancer cells expressing CD133, which is a frequently used marker for so-called cancer stem cells or cancer stem cell-like cells (collectively referred to here as CSCs), contribute to tumor's therapeutic resistance and metastasis ability. Culturing under hypoxia is also reported to enlarge the proportion of CD133(+) cells, which would indicate survival advantage of CD133(+) cells under hypoxia. Here, we investigated the relationships between (64)Cu-ATSM accumulation and existence of CD133(+) cells using mouse colon carcinoma (colon-26) tumor.

METHODS

Intratumor distribution of (64)Cu-ATSM and (18)F-fluorodeoxyglucose ((18)FDG) was compared with immunohistochemical staining for CD133 with a colon-26 model. In vitro characterization of CD133(+) colon-26 cells was also performed.

RESULTS

In colon-26 tumors, (64)Cu-ATSM localized preferentially in regions with a high density of CD133(+) cells. The percentage of CD133(+) cells was 11-fold higher in (64)Cu-ATSM high-uptake regions compared with (18)FDG high- (but (64)Cu-ATSM low-) uptake regions. CD133(+) colon-26 cells showed characteristics previously linked with CSCs in other cancer cell lines, such as high colony-forming ability, high tumor-initiating ability and enrichment under hypoxic cultivation. The proportion of CD133(+) cells was enlarged by culturing under glucose starvation as well as hypoxia, and (64)Cu-ATSM uptake was increased under such conditions.

CONCLUSIONS

Our findings showed that, in colon-26 tumors, (64)Cu-ATSM accumulates in rich regions of CD133(+) cells with characteristics of CSCs. Therefore (64)Cu-ATSM could be a potential imaging agent for rich regions of CD133(+) cells, associated with CSCs, within tumors.

Pathophysiologic correlation between 62Cu-ATSM and 18F-FDG in lung cancer

The purpose of this study was to delineate the differences in intratumoral uptake and tracer distribution of (62)Cu-diacetyl-bis(N(4)-methylthiosemicarbazone) ((62)Cu-ATSM), a well-known hypoxic imaging tracer, and (18)F-FDG in patients with lung cancer of pathohistologically different types.

METHODS

Eight patients with squamous cell carcinoma (SCC) and 5 with adenocarcinoma underwent (62)Cu-ATSM and (18)F-FDG PET within a 1-wk interval. For (62)Cu-ATSM PET, 10-min static data acquisition was started at 10 min after a 370- to 740-MBq tracer injection. After image reconstruction, (62)Cu-ATSM and (18)F-FDG images were coregistered, and multiple small regions of interest were drawn on tumor lesions of the 2 images to obtain standardized uptake values (SUVs). The regression lines were determined between SUVs for (62)Cu-ATSM and (18)F-FDG in each tumor. The slope values were compared between SCC and adenocarcinoma to observe pathohistologic differences in intratumoral distribution of the tracers.

RESULTS

SUVs for (62)Cu-ATSM were lower than those for (18)F-FDG in both SCC and adenocarcinoma. SCC tumors showed high (62)Cu-ATSM and low (18)F-FDG uptakes in the peripheral region of tumors but low (62)Cu-ATSM and high (18)F-FDG uptakes toward the center (spatial mismatching). The relationship of SUVs for the 2 tracers was negatively correlated with a mean regression slope of -0.07 +/- 0.05. On the other hand, adenocarcinoma tumors had a spatially similar distribution of (62)Cu-ATSM and (18)F-FDG, with positive regression slopes averaging 0.24 +/- 0.13. The regression slopes for (62)Cu-ATSM and (18)F-FDG differed significantly between SCC and adenocarcinoma (P < 0.001).

CONCLUSION

The intratumoral distribution patterns of (62)Cu-ATSM and (18)F-FDG were different between SCC and adenocarcinoma in lung cancers, indicating that intratumoral regions of high glucose metabolism and hypoxia could differ with the pathohistologic type of lung cancer. The identification of regional biologic characteristics in tumors such as hypoxia, energy metabolism, and proliferation could play a significant role in the clinical diagnosis and therapy planning for non-small cell lung cancer patients.