Comparison of molecular markers of hypoxia and imaging with (60)Cu-ATSM in cancer of the uterine cervix

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.

Radiobiological effects of hypoxia-dependent uptake of 64Cu-ATSM: enhanced DNA damage and cytotoxicity in hypoxic cells

PURPOSE

Hypoxia occurs frequently in cancers and can lead to therapeutic resistance due to poor perfusion and loss of the oxygen enhancement effect. (64)Cu-ATSM has shown promise as a hypoxia diagnostic agent due to its selective uptake and retention in hypoxic cells and its emission of positrons for PET imaging. (64)Cu also emits radiotoxic Auger electrons and beta(-) particles and may therefore exhibit therapeutic potential when concentrated in hypoxic tissue.

METHODS

MCF-7 cells were treated with 0-10 MBq/ml (64)Cu-ATSM under differing oxygen conditions ranging from normoxia to severe hypoxia. Intracellular response to hypoxia was measured using Western blotting for expression of HIF-1alpha, while cellular accumulation of (64)Cu was measured by gamma counting. DNA damage and cytotoxicity were measured with, respectively, the Comet assay and clonogenic survival.

RESULTS

(64)Cu-ATSM uptake in MCF-7 cells increased as atmospheric oxygen decreased (up to 5.6 Bq/cell at 20.9% oxygen, 10.4 Bq/cell at 0.1% oxygen and 26.0 Bq/cell at anoxia). Toxicity of (64)Cu-ATSM in MCF-7 cells also increased as atmospheric oxygen decreased, with survival of 9.8, 1.5 and 0% in cells exposed to 10 MBq/ml at 20.9, 0.1 and 0% oxygen. The Comet assay revealed a statistically significant increase in (64)Cu-ATSM-induced DNA damage under hypoxic conditions.

CONCLUSION

The results support a model in which hypoxia-enhanced uptake of radiotoxic (64)Cu induces sufficient DNA damage and toxicity to overcome the documented radioresistance in hypoxic MCF-7 cells. This suggests that (64)Cu-ATSM and related complexes have potential for targeted radionuclide therapy of hypoxic tumours.

Molecular imaging of hypoxia with radiolabelled agents

Tissue hypoxia results from an inadequate supply of oxygen (O₂) that compromises biological functions. Structural and functional abnormalities of the tumour vasculature together with altered diffusion conditions inside the tumour seem to be the main causes of tumour hypoxia. Evidence from experimental and clinical studies points to a role for tumour hypoxia in tumour propagation, resistance to therapy and malignant progression. This has led to the development of assays for the detection of hypoxia in patients in order to predict outcome and identify patients with a worse prognosis and/or patients that would benefit from appropriate treatments. A variety of invasive and non-invasive approaches have been developed to measure tumour oxygenation including oxygen-sensitive electrodes and hypoxia marker techniques using various labels that can be detected by different methods such as positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), autoradiography and immunohistochemistry. This review aims to give a detailed overview of non-invasive molecular imaging modalities with radiolabelled PET and SPECT tracers that are available to measure tumour hypoxia.

Noninvasive 64Cu-ATSM and PET/CT assessment of hypoxia in rat skeletal muscles and tendons during muscle contractions

The purpose of the present study was to investigate exercise-related changes in oxygenation in rat skeletal muscles and tendons noninvasively with PET/CT and the hypoxia-selective tracer (64)Cu-diacetyl bis(N(4)-methylthiosemicarbazone) (ATSM) and to quantitatively study concomitant changes in gene expression of 2 hypoxia-related genes, hypoxia-inducible factor 1alpha (HIF1alpha) and carbonic anhydrase III (CAIII).

METHODS

Two groups of Wistar rats performed 1-leg contractions of the calf muscle by electrostimulation of the sciatic nerve. After 10 min of muscle contractions, (64)Cu-ATSM was injected and contractions were continued for 20 min. PET/CT of both hind limbs was performed immediately and 1 h after the contractions. The exercise group (n = 8) performed only muscle contractions as described, whereas the other group, exercise plus cuff (n = 8), in addition underwent cuff-induced hypoxia during the first PET/CT scan. Standardized uptake values (SUVs) were calculated for the Achilles tendons and triceps surae muscles and were correlated to gene expression of HIF1alpha and CAIII using real-time polymerase chain reaction.

RESULTS

Immediately after the contractions, uptake of (64)Cu-ATSM was significantly increased, by approximately 1.5-fold in muscles and 1.3-fold in tendons, compared with resting conditions. The significant increase was maintained in late PET scans in stimulated muscles and tendons independently of cuff application. In muscles, SUV correlated significantly with gene expression of HIF1alpha and CAIII, whereas this coherence was not found in tendons.

CONCLUSION

We found enhanced uptake of (64)Cu-ATSM in both early and late PET scans, thereby supporting the possibility that (64)Cu-ATSM registers exercise-induced transient hypoxia in both skeletal muscles and force-transmitting tendons. The fact that skeletal muscles but not tendons showed upregulation of HIF1alpha and CAIII could indicate that healthy tendons are less responsive than skeletal muscles to low levels of oxygen.

Non-natural amino acids for tumor imaging using positron emission tomography and single photon emission computed tomography

Amino acids are required nutrients for proliferating tumor cells, and amino acid transport is upregulated in many tumor types. Studies of radiolabeled amino acids in animals and humans demonstrate that amino acid based tracers have advantageous characteristics relative to 2-[(18)F]fluoro-2-deoxyglucose in certain tumors, particularly brain gliomas. Non-natural amino acids for tumor imaging generally have greater metabolic stability and can be labeled with longer-lived radionuclides for positron emission tomography and single photon emission computed tomography such as fluorine-18 and iodine-123. Amino acids enter cells via amino acid transport with varying selectivity based on their chemical structure. This review focuses on the rationale, biological basis, current status and future prospects of radiolabeled non-natural amino acids for tumor imaging and discusses various classes of these compounds including aromatic, alicyclic and alpha,alpha-dialkyl amino acids.

Clinical biomarkers for hypoxia targeting

Tumor hypoxia or a reduction of the tissue oxygen tension is a key microenvironmental factor for tumor progression and treatment resistance in solid tumors. Because hypoxic tumor cells have been demonstrated to be more resistant to ionizing radiation, hypoxia has been a focus of laboratory and clinical research in radiation therapy for many decades. It is believed that proper detection of hypoxic regions would guide treatment options and ultimately improve tumor response. To date, most clinical efforts in targeting tumor hypoxia have yielded equivocal results due to the lack of appropriate patient selection. However, with improved understanding of the molecular pathways regulated by hypoxia and the discovery of novel hypoxia markers, the prospect of targeting hypoxia has become more tangible. This chapter will focus on the development of clinical biomarkers for hypoxia targeting.

Spectroelectrochemical and computational studies on the mechanism of hypoxia selectivity of copper radiopharmaceuticals

Detailed chemical, spectroelectrochemical and computational studies have been used to investigate the mechanism of hypoxia selectivity of a range of copper radiopharmaceuticals. A revised mechanism involving a delicate balance between cellular uptake, intracellular reduction, reoxidation, protonation and ligand dissociation is proposed. This mechanism accounts for observed differences in the reported cellular uptake and washout of related copper bis(thiosemicarbazonato) complexes. Three copper and zinc complexes have been characterised by X-ray crystallography and the redox chemistry of a series of copper complexes has been investigated by using electronic absorption and EPR spectroelectrochemistry. Time-dependent density functional theory (TD-DFT) calculations have also been used to probe the electronic structures of intermediate species and assign the electronic absorption spectra. DFT calculations also show that one-electron oxidation is ligand-based, leading to the formation of cationic triplet species. In the absence of protons, metal-centred one-electron reduction gives the reduced anionic copper(I) species, [CuIATSM](-), and for the first time it is shown that molecular oxygen can reoxidise this anion to give the neutral, lipophilic parent complexes, which can wash out of cells. The electrochemistry is pH dependent and in the presence of stronger acids both chemical and electrochemical reduction leads to quantitative and rapid dissociation of copper(I) ions from the mono- or diprotonated complexes, [CuIATSMH] and [Cu(I)ATSMH2]+. In addition, a range of protonated intermediate species have been identified at lower acid concentrations. The one-electron reduction potential, rate of reoxidation of the copper(I) anionic species and ease of protonation are dependent on the structure of the ligand, which also governs their observed behaviour in vivo.

Regional hypoxia in glioblastoma multiforme quantified with [18F]fluoromisonidazole positron emission tomography before radiotherapy: correlation with time to progression and survival

PURPOSE

Hypoxia is associated with resistance to radiotherapy and chemotherapy and activates transcription factors that support cell survival and migration. We measured the volume of hypoxic tumor and the maximum level of hypoxia in glioblastoma multiforme before radiotherapy with [(18)F]fluoromisonidazole positron emission tomography to assess their impact on time to progression (TTP) or survival.

EXPERIMENTAL DESIGN

Twenty-two patients were studied before biopsy or between resection and starting radiotherapy. Each had a 20-minute emission scan 2 hours after i.v. injection of 7 mCi of [(18)F]fluoromisonidazole. Venous blood samples taken during imaging were used to create tissue to blood concentration (T/B) ratios. The volume of tumor with T/B values above 1.2 defined the hypoxic volume (HV). Maximum T/B values (T/B(max)) were determined from the pixel with the highest uptake.

RESULTS

Kaplan-Meier plots showed shorter TTP and survival in patients whose tumors contained HVs or tumor T/B(max) ratios greater than the median (P < or = 0.001). In univariate analyses, greater HV or tumor T/B(max) were associated with shorter TTP or survival (P < 0.002). Multivariate analyses for survival and TTP against the covariates HV (or T/B(max)), magnetic resonance imaging (MRI) T1Gd volume, age, and Karnovsky performance score reached significance only for HV (or T/B(max); P < 0.03).

CONCLUSIONS

The volume and intensity of hypoxia in glioblastoma multiforme before radiotherapy are strongly associated with poorer TTP and survival. This type of imaging could be integrated into new treatment strategies to target hypoxia more aggressively in glioblastoma multiforme and could be applied to assess the treatment outcomes.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

The prognostic value of PET and PET/CT in cervical cancer

Cervical cancer ranks among the top three cancer diagnoses in women worldwide. In the United States, the SEER Cancer Statistics Review identified cervical cancer as the third leading cause (following childhood cancers and testicular cancer) of average years of life lost per person dying of cancer for all races and both genders. Approximately one-third of cervical cancer patients develop disease recurrence and the majority of these recurrences occur within the first 2 years after completion of therapy. Predictors of disease recurrence include stage and lymph node status at the time of initial diagnosis. The initial diagnosis and staging of cervical cancer has traditionally been achieved by history and physical examination and by use of selected imaging studies. Accurate staging is important both for selecting appropriate therapy and for prognosis. Computed tomography (CT) has been the most widely used imaging method for assessment of nodal involvement and detection of distant metastatic disease. Positron emission tomography (PET) has become an established imaging tool for cervical cancer. The functional information about regional glucose metabolism provided by fluorodeoxyglucose (FDG)-PET provides for greater sensitivity and specificity in most cancer imaging applications by comparison with CT and other anatomic imaging methods. PET is superior to conventional imaging modalities for evaluating patients with cervical cancer.

Intensity-modulated x-ray (IMXT) versus proton (IMPT) therapy for theragnostic hypoxia-based dose painting

In this work the abilities of intensity-modulated x-ray therapy (IMXT) and intensity-modulated proton therapy (IMPT) to deliver boosts based on theragnostic imaging were assessed. Theragnostic imaging is the use of functional or molecular imaging data for prescribing radiation dose distributions. Distal gradient tracking, an IMPT method designed for the delivery of non-uniform dose distributions, was assessed. Dose prescriptions for a hypoxic region in a head and neck squamous cell carcinoma patient were designed to either uniformly boost the region or redistribute the dose based on positron emission tomography (PET) images of the (61)Cu(II)-diacetyl-bis(N(4)-methylthiosemicarbazone) ((61)Cu-ATSM) hypoxia surrogate. Treatment plans for the prescriptions were created for four different delivery methods: IMXT delivered with step-and-shoot and with helical tomotherapy, and IMPT delivered with spot scanning and distal gradient tracking. IMXT and IMPT delivered comparable dose distributions within the boost region for both uniform and redistributed theragnostic boosts. Normal tissue integral dose was lower by a factor of up to 3 for IMPT relative to the IMXT. For all delivery methods, the mean dose to the nearby organs at risk changed by less than 2 Gy for redistributed versus uniform boosts. The distal gradient tracking method resulted in comparable plans to the spot scanning method while reducing the number of proton beam spots by a factor of over 3.

Image-guided adaptive brachytherapy for cervix carcinoma

Sectional imaging has played an important role in the management of cervical cancer, in particular for staging, and is now considered for image-guided treatment planning, in particular for brachytherapy. Image-based three-dimensional brachytherapy is a relatively new approach that is increasingly replacing the traditional two-dimensional X-ray-based method. This overview focuses on the potential and some practicalities of this approach. The pros and cons of computed tomography- and magnetic resonance imaging-based brachytherapy are considered and an overview of the current state of research is given. The first encouraging clinical results are presented and future perspectives are discussed.