Nitroimidazoles and imaging hypoxia

Relationship between activated astrocytes and hypoxic cerebral tissue in a rat model of cerebral ischemia/reperfusion

Following cerebral infarction, hypoxic tissues remains in the ischemic cortex for long periods of time. Glial fibrillary acidic protein (GFAP) is a specific marker of astrocytes, which is thought to be essential for neuronal survival. We aimed to clarify the relationship between hypoxic tissue and astrocytes following cerebral infarction. Rats with middle cerebral artery occlusion were randomly divided into a 1.5-hour ischemia-reperfusion(1.5-hour IR) group and a permanent ischemia (PI) group. Hypoxic tissue and GFAP fluorescence intensity in the ischemic cortex were observed postoperatively on days 1, 3, 7, and 14. Results showed that hypoxic tissue was present from day 1 to 14 in the 1.5-hour IR group and on days 1 and 3 in the PI group. The GFAP fluorescence intensity in the 1.5-hour IR group was stronger than that in the PI group at the same time point of observation. Over time, GFAP expression increased and peaked at 7 days in each group, followed by a decrease in signal. In hypoxic tissue, the GFAP fluorescence intensity was stronger than that in the surrounding tissue at all observation time points. These data indicate that astrocytes were strongly activated in hypoxic tissue induced by temporary ischemia followed by reperfusion. The activation of astrocytes may partially contribute to the survival and repair of hypoxic tissue following brain ischemia.

The concept of the penumbra: can it be translated to stroke management?

The 'penumbra' is a concept coined in animal experiments suggesting that functionally impaired tissue can survive and recover if sufficient reperfusion is re-established within a limited time period, which depends on the level of residual flow. In an ischaemic territory, irreversible damage progresses over time from the centre of the most severe flow reduction to the periphery with less disturbed perfusion. This centrifugal progression of irreversible tissue damage is characterised by a complex cascade of interconnected electrophysiological, molecular, metabolic and perfusion disturbances. Waves of depolarisations, the peri infarct spreading depressions, inducing activation of ion pumps and liberation of excitatory transmitters play an important role in the drastically increased metabolic demand during reduced oxygen supply causing hypoxic tissue changes and lactacidosis, which further damage the tissue. Positron emission tomography allows the quantification of regional cerebral blood flow, the regional metabolic rate for oxygen and the regional oxygen extraction fraction, which can be used to identify regions with a critical reduction in these physiologic variables as indicators of penumbra and irreversible damage within ischaemic territories in animal models and patients with stroke. These positron emission tomography methods require arterial blood sampling and due to the complex logistics involved, are limited for routine application. Therefore, newer tracers were developed for the noninvasive detection of irreversible tissue damage (flumazenil) and of hypoxic tissue changes (fluoromisonidazole). As a widely applicable clinical tool, diffusion/perfusion-weighted magnetic resonance imaging is used; the 'mismatch' between perfusion and diffusion changes serves as a surrogate marker of the penumbra. However, in comparative studies of magnetic resonance imaging and positron emission tomography, diffusion-weighted imaging showed a high false-positive rate of irreversible damage, and the perfusion-weighted-diffusion-weighted mismatch overestimated the penumbra as defined by positron emission tomography. Advanced analytical procedures of magnetic resonance imaging data may improve the reliability of these surrogate markers but should be validated with quantitative procedures.

Hypoxic tissues are associated with microvessel density following brain ischemia-reperfusion

Hypoxic tissue has been observed in the surrounding areas of the ischemic core following cerebral infarction. The underlying mechanisms for this potentially reversible ischemic region remain to be determined. In this study, we generated permanent brain ischemia (PI) and reperfusion after inducing ischemia for 1.5 h (ischemia-reperfusion or IR) in a rat model of middle cerebral artery occlusion. Using immunofluorescence, we observed hypoxic tissue in ischemic brains and assessed microvessel density in and surrounding the hypoxic tissue. We found that the hypoxic tissues were observed at 1 and 3 days in PI rats and at 1, 3, 7, and 14 days in IR rats. The hypoxic tissue gradually decreased over time. The microvessel density increased in a time-dependent manner in focal brain ischemic tissue in PI and IR rats. Furthermore, IR induced a significant increase in microvessel density when compared with PI rats (P < 0.05). Microvessel density surrounding hypoxic tissue was significantly higher when compared with within the hypoxic tissue (P < 0.05). These data demonstrate that hypoxic tissue may exist for a long period (14 days) following brain IR and indicate that hypoxic tissue usually existed with low microvessel density. Furthermore, the duration of hypoxic tissue was partially dependent on the degree of microvessel proliferation.

Imaging of hypoxic lesions in patients with gliomas by using positron emission tomography with 1-(2-[18F] fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole, a new 18F-labeled 2-nitroimidazole analog

OBJECT

Assessment of hypoxic conditions in brain tumors is important for predicting tumor aggressiveness and treatment response. A new hypoxia imaging agent, 1-(2-[(18)F]fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole (FRP-170), with higher image contrast and faster clearance than preexisting hypoxia tracers for PET, was used to visualize hypoxic tissues in 8 patients with glioma.

METHODS

The FRP-170 was injected and PET imaging was performed 2 hours later in 8 patients, including 3 with glioblastoma multiforme, 2 with oligodendroglioma, and 1 each with diffuse astrocytoma, anaplastic ganglioglioma, and recurrent anaplastic astrocytoma. All 8 patients also underwent MR imaging, and some patients underwent [(11)C]methionine or [(18)F]fluorodeoxyglucose PET, and proton MR spectroscopy for comparison. Tissues obtained at biopsy or radical resection were immunostained with hypoxia-inducible factor-1alpha (HIF-1alpha) antibody for the confirmation of hypoxia, except in the patient with recurrent anaplastic astrocytoma who was treated using Gamma Knife surgery.

RESULTS

The FRP-170 PET images showed marked uptake with upregulation of HIF-1alpha in the 3 glioblastomas multiforme, and moderate uptake in the recurrent anaplastic astrocytoma and one oligodendroglioma, but no uptake in the other tumors. The FRP-170 PET images showed positive correlation with HIF-1alpha immunoreactivity and some correlation with FDG PET and MR imaging enhancement, but no correlation with [(11)C]methionine PET. Imaging with FRP-170 PET seemed to be more sensitive for detecting hypoxia than identifying the lactate peak on proton MR spectroscopy.

CONCLUSIONS

Imaging with FRP-170 PET can visualize hypoxic lesions in patients with glioma, as confirmed by histological examination. This new method can assess tumor hypoxia preoperatively and noninvasively.

PET imaging of hypoxia using [18F]HX4: a phase I trial

BACKGROUND AND PURPOSE

Noninvasive PET imaging of tumour hypoxia could help in the selection of those patients who could benefit from chemotherapy or radiation with specific antihypoxic treatments such as bioreductive drugs or hypoxic radiosensitizers. In this phase I trial, we aimed to determine the toxicity of [(18)F]HX4, a member of the 2-nitroimidazole family, at different dose levels. The secondary aim was to analyse image quality related to the HX4 dose and the timing of imaging.

METHODS

Patients with a histologically proven solid cancer without curative treatment options were eligible for this study. A study design with two dose steps was used in which a single dose of a maximum of 222 MBq (step 1) or 444 MBq (step 2) [(18)F]HX4 was injected. Toxicity was scored on day 0 and on days 3 and 7 after injection, according to the CTCAE 3.0 scoring system. PET/CT images of the largest tumour site were acquired 30, 60 and 120 min after injection.

RESULTS

Six patients with stage IV carcinoma were included, four with non-small-cell lung carcinoma, one with thymus carcinoma, and one with colon carcinoma. No toxicity was observed in any of the patients at either dose level. The median tumour to muscle ratio 120 min after injection was 1.40 (range 0.63-1.98).

CONCLUSION

The findings of this study showed that [(18)F]HX4 PET imaging for the detection of hypoxia is not associated with any toxicity. Imaging was successful; however, future trials are needed to determine the optimal image parameters.

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.

Imaging of hypoxic-ischemic penumbra with (18)F-fluoromisonidazole PET/CT and measurement of related cerebral metabolism in aneurysmal subarachnoid hemorrhage

This study aimed to characterize hypoxic, but salvageable, tissue imaged by (18)F-fluoromisonidazole ((18)F-FMISO), combining with perfusion-computed tomography (PCT) for regional cerebral blood flow (rCBF) measurement and metabolism by microdialysis (MD) in aneurysmal subarachnoidal hemorrhage (SAH) patients. (18)F-FMISO positron-emission tomography (PET)/CT was performed within the period of possible vasospasm (day 6.8+/-3 after SAH) in seven SAH patients. In parallel, rCBF was determined within the MD region of interest (MD-ROI) (n=5). The MD catheter was inserted into the brain parenchyma with highest risk for ischemia; extracellular levels of glutamate and energy metabolites were registered at time of PET and hourly for 10 days. Twelve-month outcome was evaluated. In asymptomatic patients (n=3) no hypoxia was detected and glutamate levels were low (<10 mmol/L), whereas symptomatic patients had higher glutamate concentrations (P<0.001). Increased (18)F-FMISO uptake within the MD-ROI (n=3) was related to higher glutamate levels, while rCBF was above the ischemic range. Hypoxia (increased (18)F-FMISO uptake) was present in symptomatic patients and associated with relevant metabolic derangement of extracellular glutamate levels, whereas energy metabolism and rCBF were preserved. This technique has the potential to improve our understanding of the role of cellular hypoxia in aneurysmal SAH.

PET in Cerebrovascular Disease

Investigation of the interplay between the cerebral circulation and brain cellular function is fundamental to understanding both the pathophysiology and treatment of stroke. Currently, PET is the only technique that provides accurate, quantitative in vivo regional measurements of both cerebral circulation and cellular metabolism in human subjects. We review normal human cerebral blood flow and metabolism and human PET studies of ischemic stroke, carotid artery disease, vascular dementia, intracerebral hemorrhage and aneurysmal subarachnoid hemorrhage and discuss how these studies have added to our understanding of the pathophysiology of human cerebrovascular disease.

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.

Preparation and preliminary biological evaluation of a 177Lu labeled nitroimidazole derivative for possible use in targeted tumor therapy

The preparation and preliminary biological evaluation of a metronidazole-BFCA (bifunctional chelating agent) conjugate labeled with 177Lu is reported. Metronidazole, a well known hypoxia marker has been suitably derivatized and coupled with a polyazamacrocyclic BFCA, namely, para-aminobenzyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (p-amino-benzyl-DOTA). 177Lu, which is presently being considered as one of the pivotal radionuclides for targeted therapy was produced in adequate specific activity (∼185 TBq/g) and high radionuclidic purity (99.99%) by irradiating enriched (60.6% 176Lu) Lu2O3 target at a moderate thermal neutron flux of 3×1013 n/cm2/s. The metronidazole-BFCA conjugate was radiolabeled with 177Lu in high radiochemical purity (97%). Preliminary biodistribution studies carried out in Swiss mice bearing fibrosarcoma tumors revealed good tumor uptake (1.30% ID/g at 30 min post-injection) with rapid renal clearance (94.48% ID at 30 min post-injection) and significant tumor to blood (28.00 at 3 h post-injection) and tumor to muscle (14.00 at 3 h post-injection) ratios.

Significance of nitroimidazole compounds and hypoxia-inducible factor-1 for imaging tumor hypoxia

A tumor-specific microenvironment is characterized by hypoxia, in which oxygen tension is considerably lower than in normal tissues. The hypoxic status of various solid tumors has been attributed as an indicator of adverse prognosis due to tumor progression toward a more malignant phenotype with increased metastatic potential and resistance to treatment. Various exogenous and endogenous markers for hypoxia are currently available and studied in relation to each other, tumor architecture, and tumor microenvironment. Over the last few decades, various methods have been suggested to assess the level of oxygenation in solid tumors. Among them, nitroimidazole compounds have provided promising information on tumor hypoxia. To quantify the extent of hypoxia requires that nitroimidazole binding be primarily dependent on oxygen concentration as well as nitroreductase levels in the tumor cells. Furthermore, recent progress in molecular biology has highlighted a transcription factor, hypoxia-inducible factor (HIF)-1, whose activity is induced by hypoxia. HIF-1 plays a central role in malignant progression by inducing the expression of various genes, whose functions are strongly associated with malignant alteration of the entire tumor. The cellular changes induced by HIF-1 are extremely important therapeutic targets of cancer therapy, particularly in the therapy against refractory cancers. In this review, we will discuss the significance of pimonidazole and HIF-1 as exogenous and endogenous hypoxia markers, respectively, as well as their evaluation and imaging of tumor hypoxia.

Characterisation of the timing of binding of the hypoxia tracer FMISO after stroke

The hypoxia tracer fluorine-18 fluoromisonidazole ([18F]FMISO) and its tritiated counterpart ([(3)H]FMISO) have been used as markers of potentially salvageable brain (ischemic penumbra) after stroke. In experimental models, the dynamics and half-life of [3H]FMISO allow concurrent histology after 24 h. Our aim was to further validate these techniques, by determining the optimum tracer exposure interval to delineate ischemic penumbra, and the effects of prolonged exposure on tracer retention in permanent ischemia. Middle cerebral artery occlusion (MCAO) of varying durations was created in rats using the thread occlusion model. Autoradiography using objective thresholding to define tracer-retention volume was performed to determine the time course of tracer retention in hypoxic tissues and the duration of ongoing retention after bolus administration. An ischemic duration of < or =90 min resulted in a tracer-retention volume underestimating 'tissue at risk' (histological infarction 24 h after permanent occlusion) by >1/2. Two hour ischemia resulted in a volume equal to 'tissue at risk'. Twenty-four hour permanent ischemia resulted in tracer-retaining tissue volumes greater than final infarction. However, the use of more stringent thresholding of autoradiographic signal produced a volume of FMISO retention closely approximating infarct volume. The findings indicate that the timing of imaging is crucial, with an optimal imaging time of 2 h using the current threshold. Earlier imaging is limited by tracer dynamics with this particular agent, however autoradiography with a longer ischemic interval (permanent occlusion) is feasible with modified thresholds. These findings support a role for hypoxia tracers in providing new insight into the ischemic penumbra.

Radiopharmaceuticals in preclinical and clinical development for monitoring of therapy with PET

This review article discusses PET agents, other than (18)F-FDG, with the potential to monitor the response to therapy before, during, or after therapeutic intervention. This review deals primarily with non-(18)F-FDG PET tracers that are in the final stages of preclinical development or in the early stages of clinical application for monitoring the therapeutic response. Four sections related to the nature of the tracers are included: radiotracers of DNA synthesis, such as the 2 most promising agents, the thymidine analogs 3'-(18)F-fluoro-3'-deoxythymidine and (18)F-1-(2'-deoxy-2'-fluoro-beta-d-arabinofuranosyl)thymine; agents for PET imaging of hypoxia within tumors, such as (60/62/64)Cu-labeled diacetyl-bis(N(4)-methylthiosemicarbazone) and (18)F-fluoromisonidazole; amino acids for PET imaging, including the most popular such agent, l-[methyl-(11)C]methionine; and agents for the imaging of tumor expression of androgen and estrogen receptors, such as 16beta-(18)F-fluoro-5alpha-dihydrotestosterone and 16alpha-(18)F-fluoro-17beta-estradiol, respectively.

Molecular imaging of cancer: evaluating characters of individual cancer by PET/SPECT imaging

The present status of cancer molecular imaging (MI) with nuclear medicine techniques is reviewed, highlighting the Japanese activities in this field. With the progress in MI research, including significant contributions from Japanese studies, it has become possible to noninvasively evaluate various important characters of cancer in clinical patients, such as metabolism, cellular proliferation, tumor hypoxia, and receptor expression. Tumor metabolic information is used for tumor characterization, treatment response evaluation, and prognosis prediction. Hypoxia imaging is used for treatment planning and predicting treatment response. Receptor imaging can be used for the selection of the candidate for receptor-targeted treatment. Various novel probes that can target cancer-associated antigens, various cellular growth factor receptors, tumor angiogenesis, and so on, are under development, aiming for clinical evaluation. Application of radiolabeled ligands for treatment (targeted internal radiation therapy) is another important field in which MI technique can play a critical role. MI, which can deliver the outcome of basic oncological research to the bedside, is essential translational research for improved individualized patient management, and further advances in MI studies are eagerly awaited.

Positron Emission Tomography Imaging of Hypoxia

Hypoxia imaging has applications in functional recovery in ischemic events such as stroke and myocardial ischemia, but especially in tumors in which hypoxia can be predictive of treatment response and overall prognosis. Recently there has been development of imaging agents utilizing positron emission tomography for non-invasive imaging of hypoxia. Many of these PET agents have come to the forefront of hypoxia imaging. Halogenated PET nitroimidazole imaging agents labeled with (18)F (t(1/2) = 110 m) and (124)I (t(1/2) = 110 m) have been under investigation for the last 25 years, with radiometal agents ((64)Cu-ATSM) being developed more recently. This review focuses on these positron emission tomography imaging agents for hypoxia.

Autoradiographic and small-animal PET comparisons between (18)F-FMISO, (18)F-FDG, (18)F-FLT and the hypoxic selective (64)Cu-ATSM in a rodent model of cancer

INTRODUCTION

Copper(II)-diacetyl-bis(N(4)-methylthiosemicarbazone), or Cu-ATSM, a hypoxia imaging agent, has been shown to be predictive of response to traditional cancer therapies in patients with a wide range of tumors. It is known that the environment of the tumor results in a myriad of physiological consequences, including hypoxia, alterations in metabolism and proliferation. In an effort to better characterize the relationships between Cu-ATSM and other prominent radiopharmaceuticals, this current study was undertaken to compare the regional distribution of (64)Cu-ATSM with [(18)F]fluoromisonidazole ((18)F-FMISO), [(18)F]fluoro-2-deoxy-d-glucose ((18)F-FDG) and [(18)F]fluorothymidine ((18)F-FLT) in 9L tumors.

METHODS

Taking advantage of the different half-life of (18)F (t(1/2)=110 min) in comparison to (64)Cu (t(1/2)=12.7 h), we undertook a dual-tracer autoradiography study in 9L tumors. Four groups were examined: (a) (18)F-FMISO, 2 h postinjection (p.i.) and (64)Cu-ATSM, 10 min p.i.; (b) (18)F-FMISO, 2 h p.i. and (64)Cu-ATSM, 24 h p.i.; (c) (18)F-FDG, 1 h p.i. and (64)Cu-ATSM, 10 min p.i.; and (d) (18)F-FLT, 1 h p.i. and (64)Cu-ATSM, 10 min p.i. Small-animal PET imaging was performed in 9L tumor-bearing rats with imaging on concurrent days comparing (64)Cu-ATSM with (18)F-FMISO and (18)F-FLT.

RESULTS

It was shown that the regional distribution of (18)F-FMISO and (64)Cu-ATSM showed an excellent correlation when the (64)Cu-ATSM had been allowed to distribute for either 10 min (R(2)=.84) or 24 h (R(2)=.86). The regional comparisons between (64)Cu-ATSM (10 min) and (18)F-FDG (1 h) resulted in a very poor correlation (R(2)=.08) between the regional uptake of the two agents. The comparison between (18)F-FLT and (64)Cu-ATSM showed a strong relationship (R(2)=.83) between the two tracers. The small-animal PET images for the distribution comparisons between (64)Cu-ATSM and (18)F-FMISO and (18)F-FLT were in agreement with the data generated from the autoradiography studies.

CONCLUSIONS

The data show that it is important to remember that a number of different metabolic situations can exist when considering the relationship between regions of high glucose uptake, proliferation and hypoxia.

1,2-Bis[2-(2-nitro-1H-imidazol-1-yl)ethoxy]ethane

In the crystal structure, the title compound, C₁₂H₁₆N₆O₆, lies on an inversion centre. The mol­ecule has an anti­periplanar conformation with respect to the C—C bond of the central ethane unit and the two imidazole rings are parallel to each other. The dihedral angle between the imidazole ring and the mean plane of the C and O atoms of the bis­(eth­oxy)ethane group is 76.04 (6)°. The mol­ecules are stacked along the c axis through a weak C—H⋯O inter­action and a π⋯π inter­action between the imidazole rings with a centroid–centroid distance of 3.5162 (6) Å. An intramolecular C—H⋯O hydrogen bond is also present.

[18F]EF3 is not superior to [18F]FMISO for PET-based hypoxia evaluation as measured in a rat rhabdomyosarcoma tumour model

PURPOSE

The aim of this investigation was to quantitatively compare the novel positron emission tomography (PET) hypoxia marker 2-(2-nitroimidazol-1-yl)-N-(3[(18)F],3,3-trifluoropropyl)acetamide ([(18)F]EF3) with the reference hypoxia tracer [(18)F]fluoromisonidazole ([(18)F]FMISO).

METHODS

[(18)F]EF3 or [(18)F]FMISO was injected every 2 days into two separate groups of rats bearing syngeneic rhabdomyosarcoma tumours. In vivo PET analysis was done by drawing regions of interest on the images of selected tissues. The resulting activity data were quantified by the percentage of injected radioactivity per gram tissue (%ID/g) and tumour to blood (T/B) ratio. The spatial distribution of radioactivity was defined by autoradiography on frozen tumour sections.

RESULTS

The blood clearance of [(18)F]EF3 was faster than that of [(18)F]FMISO. The clearance of both tracers was slower in tumour tissue compared with other tissues. This results in increasing T/B ratios as a function of time post tracer injection (p.i.). The maximal [(18)F]EF3 tumour uptake, compared to the maximum [(18)F]FMISO uptake, was significantly lower at 2 h p.i. but reached similar levels at 4 h p.i. The tumour uptake for both tracers was independent of the tumour volume for all investigated time points. Both tracers showed heterogeneous intra-tumoural distribution.

CONCLUSIONS

[(18)F]EF3 tumour uptake reached similar levels at 4 h p.i. compared with tumour retention observed after injection of [(18)F]FMISO at 2 h p.i. Although [(18)F]EF3 is a promising non-invasive tracer, it is not superior over [(18)F]FMISO for the visualisation of tumour hypoxia. No significant differences between [(18)F]EF3 and [(18)F]FMISO were observed with regard to the intra-tumoural distribution and the extra-tumoural tissue retention.

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.

Examining the relationship between Cu-ATSM hypoxia selectivity and fatty acid synthase expression in human prostate cancer cell lines

INTRODUCTION

Positron emission tomography (PET) imaging with copper (II)-diacetyl-bis(N4-Methylthiosemicarbazone)(Cu-ATSM) for delineating hypoxia has provided valuable clinical information, but investigations in animal models of prostate cancer have shown some inconsistencies. As a defense mechanism in prostate cancer cells, the fatty acid synthesis pathway harnesses its oxidizing power for improving the redox balance despite conditions of extreme hypoxia, potentially altering Cu-ATSM hypoxia selectivity.

METHODS

Human prostate tumor-cultured cell lines (PC-3, 22Rv1, LNCaP and LAPC-4), were treated with a fatty acid synthase (FAS) inhibitor (C75, 100 microM) under anoxia. The 64Cu-ATSM uptake in these treated cells and nontreated anoxic cells was then examined. Fatty acid synthase expression level in each cell line was subsequently quantified by ELISA. An additional study was performed in PC-3 cells to examine the relationship between the restoration of 64Cu-ATSM hypoxia selectivity and the concentration of C75 (100, 20, 4 or 0.8 microM) administered to the cells.

RESULTS

Inhibition of fatty acid synthesis with C75 resulted in a significant increase in 64Cu-ATSM retention in prostate tumor cells in vitro under anoxia over 60 min. Inhibition studies demonstrated higher uptake values of 20.9+/-3.27%, 103.0+/-32.6%, 144.2+/-32.3% and 200.1+/-79.3% at 15 min over control values for LAPC-4, PC-3, LNCaP and 22Rv1 cells, respectively. A correlation was seen (R2=.911) with FAS expression plotted against percentage change in 64Cu-ATSM uptake with C75 treatment.

CONCLUSIONS

Although Cu-ATSM has clinical relevance in the PET imaging of hypoxia in many tumor types, its translation to the imaging of prostate cancer may be limited by the overexpression of FAS associated with prostatic malignancies.

Applications of nitroimidazole in vivo hypoxia imaging in ischemic stroke

BACKGROUND AND PURPOSE

Nitroimidazole imaging is a promising contender for noninvasive in vivo mapping of brain hypoxia after stroke. However, there is a dearth of knowledge about the behavior of these compounds in the various pathophysiologic situations encountered in ischemic stroke. In this article we report the findings from a systematic review of the literature on the use of the nitroimidazoles to map hypoxia after stroke.

SUMMARY OF REVIEW

We describe the characteristics of nitroimidazoles as imaging tracers, their pharmacology, and results of both animal and clinical studies during and after focal cerebral ischemia. Findings in brain tumors are also presented to the extent that they enlighten results in stroke. Early results from application of kinetic modeling for quantitative measurement of tracer binding are briefly discussed.

CONCLUSIONS

Based on this literature review, nitroimidazole hypoxia imaging agents are of considerable interest in stroke because they appear, both in animal models and in humans, to specifically detect the severely hypoxic viable tissue, but not the reperfused nor the necrotic tissue. To fully realize this potential in stroke, however, formal validation by concurrent measurement of tissue oxygen tension, together with development of novel ligands with faster distribution kinetics, faster clearance from normal tissue, and well-defined trapping mechanisms, are important goals for future investigations.

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

PURPOSE

To determine if hypoxia-related molecular markers are associated with (60)Cu labeled diacetyl-bis (N4 -methylthiosemicarbazone); ((60)Cu-ATSM) imaging of tumor hypoxia in cervical cancer.

PROCEDURES

Fifteen patients were enrolled in a prospective study and underwent evaluation of tumor hypoxia with positron emission tomography (PET) using (60)Cu-ATSM. (60)Cu-ATSM-PET imaging was compared with the expression of tissue molecular markers, which included vascular endothelial growth factor (VEGF), cyclo-oxygenase-2 (COX-2), epidermal growth factor receptor (EGFR), carbonic anyhdrase IX (CA-9), and apoptotic index.

RESULTS

Six patients had hypoxic tumors determined by (60)Cu-ATSM, and nine had non-hypoxic tumors. The 4-year overall survival estimates were 75% for patients with non-hypoxic tumors and 33% for those with hypoxic tumors (p = 0.04). Overexpression of VEGF (p = 0.13), EGFR (p = 0.05), CA-9 (p = 0.02), COX-2 (p = 0.08), and the presence of apoptosis (p = 0.005) occurred in patients with hypoxic tumors. Cox proportional hazards modeling demonstrated hypoxia as determined by (60)Cu-ATSM to be a significant independent predictor of tumor recurrence (p = 0.0287).

CONCLUSIONS

(60)Cu-ATSM hypoxia was correlated with overexpression of VEGF, EGFR, COX-2, CA-9, an increase in apoptosis, and a poor outcome.

Cu-ATSM: a radiopharmaceutical for the PET imaging of hypoxia

Copper(II)-diacetyl-bis(N(4)-methylthiosemicarbazone), Cu-ATSM, labeled with a positron emitting isotope of copper ((60)Cu, (61)Cu, (62)Cu or (64)Cu) has been shown, in vitro and in vivo, to be selective for hypoxic tissue. In silico studies have explored the mechanism of its hypoxia selectivity, and clinical studies with this agent have shown non-invasive imaging data that is predictive of a cancer patients' response to conventional therapy. This Perspective discusses the evolution of Cu-ATSM, how its selectivity can be improved upon, and where this metal-ligand platform could be taken in the future.

pO polarography, contrast enhanced color duplex sonography (CDS), [18F] fluoromisonidazole and [18F] fluorodeoxyglucose positron emission tomography: validated methods for the evaluation of therapy-relevant tumor oxygenation or only bricks in the puzzle of tumor hypoxia?

Background

The present study was conducted to analyze the value of ([¹⁸F] fluoromisonidazole (FMISO) and [¹⁸F]-2-fluoro-2'-deoxyglucose (FDG) PET as well as color pixel density (CPD) and tumor perfusion (TP) assessed by color duplex sonography (CDS) for determination of therapeutic relevant hypoxia. As a standard for measuring tissue oxygenation in human tumors, the invasive, computerized polarographic needle electrode system (pO₂ histography) was used for comparing the different non invasive measurements.

Methods

Until now a total of 38 Patients with malignancies of the head and neck were examined. Tumor tissue pO₂ was measured using a pO₂-histograph. The needle electrode was placed CT-controlled in the tumor without general or local anesthesia. To assess the biological and clinical relevance of oxygenation measurement, the relative frequency of pO₂ readings, with values ≤ 2.5, ≤ 5.0 and ≤ 10.0 mmHg, as well as mean and median pO₂ were stated. FMISO PET consisted of one static scan of the relevant region, performed 120 min after intravenous administration. FMISO tumor to muscle ratios (FMISO_T/M) and tumor to blood ratios (FMISO_T/B) were calculated. FDG PET of the lymph node metastases was performed 71 ± 17 min after intravenous administration. To visualize as many vessels as possible by CDS, a contrast enhancer (Levovist^®, Schering Corp., Germany) was administered. Color pixel density (CPD) was defined as the ratio of colored to grey pixels in a region of interest. From CDS signals two parameters were extracted: color hue – defining velocity (v) and color area – defining perfused area (A). Signal intensity as a measure of tissue perfusion (TP) was quantified as follows: TP = v_mean × A_mean.

Results

In order to investigate the degree of linear association, we calculated the Pearson correlation coefficient. Slight (|r| > 0.4) to moderate (|r| > 0.6) correlation was found between the parameters of pO₂ polarography (pO₂ readings with values ≤ 2.5, ≤ 5.0 and ≤ 10.0 mmHg, as well as median pO₂), CPD and FMISO_T/M. Only a slight correlation between TP and the fraction of pO₂ values ≤ 10.0 mmHg, median and mean pO₂ could be detected. After exclusion of four outliers the absolute values of the Pearson correlation coefficients increased clearly. There was no relevant association between mean or maximum FDG uptake and the different polarographic- as well as the CDS parameters.

Conclusion

CDS and FMISO PET represent different approaches for estimation of therapy relevant tumor hypoxia. Each of these approaches is methodologically limited, making evaluation of clinical potential in prospective studies necessary.

Initial evaluation of dynamic human imaging using 18F-FRP170 as a new PET tracer for imaging hypoxia

18F-FRP170, 1-(2-fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole, is a new hypoxia imaging agent for positron emission tomography. This compound was synthesized by 18F-labeling of RP170, which was developed as a new hydrophilic 2-nitroimidazole analog. In the present study, we analyzed dynamic whole-body imaging in healthy volunteers and dynamic tumor imaging in three patients with lung cancer.

METHODS

Four healthy male volunteers and three lung cancer patients were enrolled in this study. Volunteers underwent dynamic whole-body scans just after injection of 18F-FRP170 for about 90 min, while the lung cancer patients underwent dynamic tumor imaging for about 60 or 120 min. Data are expressed as standardized uptake values (SUV). Regions of interest were placed over images of each organ or tumor to generate time-SUV curves.

RESULTS

The series of dynamic whole-body scans showed rapid elimination of 18F-FRP170 from the kidneys following elimination from the liver. Very low physiological uptake was observed above the diaphragm. 18F-FRP170 uptake in the lung cancer lesion could be visualized clearly from early after injection. The changes of tumor SUV, tumor/blood ratio, or tumor/muscle ratio about 30 min after injection or later were small.

CONCLUSIONS

Dynamic imaging using 18F-FRP170 demonstrated rapid elimination from the kidney, suggesting the high hydrophilicity of this imaging agent. The background activity above the diaphragm was very low, and patients with lung cancer showed clear tumor uptake of 18F-FRP170 early after injection.

Tumour hypoxia imaging with [18F]FAZA PET in head and neck cancer patients: a pilot study

PURPOSE

Hypoxia is an important negative prognostic factor for radiation treatment of head and neck cancer. This study was performed to evaluate the feasibility of use of (18)F-labelled fluoroazomycin arabinoside ([(18)F]FAZA) for clinical PET imaging of tumour hypoxia.

METHODS

Eleven patients (age 59.6 +/- 9 years) with untreated advanced head and neck cancer were included. After injection of approximately 300 MBq of [(18)F]FAZA, a dynamic sequence up to 60 min was acquired on an ECAT HR+ PET scanner. In addition, approximately 2 and 4 h p.i., static whole-body PET (n = 5) or PET/CT (n = 6) imaging was performed. PET data were reconstructed iteratively (OSEM) and fused with CT images (either an external CT or the CT of integrated PET/CT). Standardised uptake values (SUVs) and tumour-to-muscle (T/M) ratios were calculated in tumour and normal tissues. Also, the tumour volume displaying a T/M ratio >1.5 was determined.

RESULTS

Within the first 60 min of the dynamic sequence, the T/M ratio generally decreased, while generally increasing at later time points. At 2 h p.i., the tumour SUV(max) and SUV(mean) were found to be 2.3 +/- 0.5 (range 1.5-3.4) and 1.4 +/- 0.3 (range 1.0-2.1), respectively. The mean T/M ratio at 2 h p.i. was 2.0 +/- 0.3 (range 1.6-2.4). The tumour volume displaying a T/M ratio above 1.5 was highly variable. At 2 h p.i., [(18)F]FAZA organ distribution was determined as follows: kidney > gallbladder > liver > tumour > muscle > bone > brain > lung.

CONCLUSION

[(18)F]FAZA PET imaging appears feasible in head and neck cancer patients, and the achieved image quality is adequate for clinical purposes. Based on our initial results, [(18)F]FAZA warrants further evaluation as a hypoxia PET tracer for imaging of cancer.

Imaging of brain hypoxia in permanent and temporary middle cerebral artery occlusion in the rat using 18F-fluoromisonidazole and positron emission tomography: a pilot study

In acute stroke, the target of therapy is the severely hypoxic but salvageable tissue. Previous human studies using 18F-fluoromisonidazole and positron emission tomography (18F-FMISO PET) have shown high tracer retention indicative of tissue hypoxia, which had normalized at repeat scan >48 h later. In the only validation study of 18F-FMISO, using ex vivo autoradiography in thread middle cerebral artery occluded (MCAo) rats, there was unexpected high uptake as late as 22 h after reperfusion, raising questions about the use of 18F-FMISO as a hypoxia tracer. Here we report a pilot study of 18F-FMISO PET in experimental stroke. Spontaneous hypertensive rats were subjected to distal clip MCAo. Three-hour dynamic PET was performed in 7 rats: 3 normals, 1 with permanent MCAo (two sessions: 30 mins and 48 h after clip), and 3 with temporary MCAo (45 mins, n=1; 120 mins, n=2; scanning started 30 mins after clip removal). Experiments were terminated by perfusion-fixation for standard histopathology. Late tracer retention was assessed by both compartmental modelling and simple side-to-side ratios. In the initial PET session of the permanent MCAo rat, striking trapping of 18F-FMISO was observed in the affected cortex, which had normalized 48 h later; histopathology revealed pannecrosis. In contrast, there was no demonstrable tracer retention in either temporary MCAo models, and histopathology showed ischemic changes only. These results document elevated 18F-FMISO uptake in the stroke area only in the early phase of MCAo, but not after early reperfusion nor when tissue necrosis has developed. These findings strongly support the validity of 18F-FMISO as a marker of viable hypoxic tissue/penumbra after stroke.

Evaluation of 99mTc(CO)3 complex of 2-methyl-5-nitroimidazole as an agent for targeting tumor hypoxia

An iminodiacetic acid (IDA) derivative of 2-methyl-5-nitroimidazole was synthesized as a carrier molecule for radiolabeling with the gamma emitting radioisotope, 99mTc, for imaging hypoxic regions of tumors. The ligand was synthesized in excellent yield and labeled using freshly prepared [99mTc(CO)3(H2O)3]+ intermediate. A complexation yield of over 95% could be achieved under mild conditions using a ligand concentration of 1 mg/mL [ approximately 3 x 10(-3)M]. The complex was characterized by HPLC and its stability in human serum was studied. Biodistribution studies performed in Swiss mice bearing fibrosarcoma tumor showed maximum accumulation in the tumor to the extent of approximately 0.52 % ID/g at 30 min post-injection (pi). The major clearance of the complex was through the hepatobiliary route. The complex showed tumor/muscle ratio of 1.75 at 30 min pi, which significantly increased to 17 at 180 min pi. However, the tumor/blood ratio was below one throughout the period of study, which could be due to slow clearance of the complex from blood.

Temporal and topographic profiles of tissue hypoxia following transient focal cerebral ischemia in rats

Intravascular accumulation of blood cells after brain ischemia-reperfusion can cause obstruction of cerebral blood flow and tissue hypoxia/ischemia as a consequence. In the present study, we examined temporal and topographic changes of tissue hypoxia/ischemia after occlusion of the middle cerebral artery (MCA) for 60 min in rats with immunohistochemical staining for hypoxia (2-nitroimidazole hypoxia marker: hypoxyprobe-1 adducts). Our results showed that tissue hypoxia expressed as positive staining for hypoxyprobe-1 adducts preceded neuronal degeneration. Platelets and granulocytes were detected close to the hypoxyprobe-1 adducts positive area. These results suggested that the hypoxic environment could persist even after reperfusion of MCA, because of vascular obstruction with accumulation of platelets and granulocytes.

The three-photon yield from e+annihilation in various fluids

Positronium in the triplet state decays by the emission of three photons and it has been proposed that their simultaneous detection can be used for medical imaging. The three-photon yield has been observed to be enhanced in low O(2) levels in some fluids but has never been measured in biologically relevant liquids. In this study, the delayed three-photon decay yield, at both high and low O(2) levels, has been extracted by fitting the time dependence of the two-photon yield to a set of coupled differential equations. The differential equations, in a simple yet seemingly satisfactory fashion, account for the e(+) capture to form positronium, its decay and the interconversion of the two spin configurations. Our results indicate that the delayed three-photon fraction is 0.25% in water (or blood-like) samples and exhibits no (or exceedingly small) dependence on the dissolved oxygen content. If one assumes that the direct component contributes a fraction expected by annihilation on free electrons (1/372), then the total three-photon fraction is 0.52% in the samples of biological relevance.

Characterization of Fluoromisonidazole Binding in Stroke

BACKGROUND AND PURPOSE

[18F]fluoromisonidazole (FMISO) positron emission tomography has been used to image hypoxia early after human stroke. To further study the role of hypoxia in stroke and the binding characteristics of FMISO, we aimed to develop [3H]FMISO autoradiography in an animal stroke model. We hypothesized that [3H]FMISO binding is prolonged, allowing correlation with 24-hour histology, and that there is no FMISO binding after effective reperfusion.

METHODS

Temporary middle cerebral artery (MCA) occlusion was performed in rats, followed by [3H]FMISO administration. Tissue preparation for autoradiography and histology (from the same sections) was performed 2.5 hours after MCA occlusion (MCAo; replicating [18F]FMISO studies). Then, otherwise identical cohorts with tissue preparation at 2.5 or 24 hours were prepared. For reperfusion studies, animals had 1-hour MCAo, with [3H]FMISO administered 1 hour after reperfusion.

RESULTS

[3H]FMISO autoradiography provided a high-resolution image of hypoxia throughout the ischemic territory. Delaying animal death from 2.5 to 24 hours allowed histological changes of stroke to develop, without significantly altering either relative intensity (1.88+/-0.06 and 2.02+/-0.11, respectively) or volume (25+/-6 mm3 and 28+/-5 mm3, respectively) of hypoxic binding. [3H]FMISO binding did not occur after effective reperfusion, despite histological injury from the preceding MCAo.

CONCLUSIONS

[3H]FMISO autoradiography of hypoxia in experimental stroke offers several advantages. Bound FMISO is retained in tissues long term, enabling direct correlation with 24-hour histology. It is not bound after effective reperfusion. Therefore, positive [18F]FMISO positron emission tomography studies in stroke patients are indicative of ongoing tissue hypoxia, not merely recent tissue injury.