Radiolabeled hypoxic cell sensitizers: tracers for assessment of ischemia

FMISO as a Biomarker for Clinical Radiation Oncology

Tumour hypoxia is a well-known negative prognostic marker in almost all solid tumours. [18F]Fluoromisonidazole (FMISO)-positron emission tomography (PET) is a non-invasive method to detect tumour hypoxia. Compared to other methods of hypoxia assessment it possesses some considerable advantages: It is non-invasive, it delivers spatial information on the hypoxia distribution within the entire tumour volume, and it can be repeated during the course of radio(chemo)therapy. This chapter briefly describes different methods of hypoxia evaluation and focuses on hypoxia PET imaging, with the most commonly used tracer being FMISO. The preclinical rationale and clinical studies to use FMISO-PET for patient stratification in radiation therapy are discussed as well as possible agents or radiation-dose modifications to overcome hypoxia.

PET imaging in ischemic cerebrovascular disease: current status and future directions

Cerebrovascular diseases are caused by interruption or significant impairment of the blood supply to the brain, which leads to a cascade of metabolic and molecular alterations resulting in functional disturbance and morphological damage. These pathophysiological changes can be assessed by positron emission tomography (PET), which permits the regional measurement of physiological parameters and imaging of the distribution of molecular markers. PET has broadened our understanding of the flow and metabolic thresholds critical for the maintenance of brain function and morphology: in this application, PET has been essential in the transfer of the concept of the penumbra (tissue with perfusion below the functional threshold but above the threshold for the preservation of morphology) to clinical stroke and thereby has had great impact on developing treatment strategies. Radioligands for receptors can be used as early markers of irreversible neuronal damage and thereby can predict the size of the final infarcts; this is also important for decisions concerning invasive therapy in large ("malignant") infarctions. With PET investigations, the reserve capacity of blood supply to the brain can be tested in obstructive arteriosclerosis of the supplying arteries, and this again is essential for planning interventions. The effect of a stroke on the surrounding and contralateral primarily unaffected tissue can be investigated, and these results help to understand the symptoms caused by disturbances in functional networks. Chronic cerebrovascular disease causes vascular cognitive disorders, including vascular dementia. PET permits the detection of the metabolic disturbances responsible for cognitive impairment and dementia, and can differentiate vascular dementia from degenerative diseases. It may also help to understand the importance of neuroinflammation after stroke and its interaction with amyloid deposition in the development of dementia. Although the clinical application of PET investigations is limited, this technology had and still has a great impact on research into cerebrovascular diseases.

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.

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.

Positron emission tomography imaging of brain tumors

Energy metabolism and amino acid transport and incorporation are important components of the pathophysiology of gliomas, about which molecular imaging is providing regional biologic information that is useful to clinical practice. Imaging hypoxia is straightforward and proliferation imaging with FLT shows significant promise. Neither has been exploited thoroughly enough to allow judgement of their potential benefit to the practice of neuro-oncology. Although cell division is the most distinguishing function of growth in tumors, probing membrane biosynthesis with PET and 1-[11C]acetate or a choline tracer may yield information as helpful as protein or DNA synthesis. Because astrocytic gliomas frequently carry epidermal growth factor receptor mutations at a frequency that is related to grade, a PET tracer that is specific for this mutated receptor could be useful for grading and prognosis [35]. Methods for imaging angiogenesis are being developed; 18F-labeling of a cyclic RGD-containing glycopeptide, cyclo(-Arg-Gly-Asp-D-Phe-Lys(sugar amino acid)-), with 4-nitro-phenyl 2-[18F]fluoropropionate has been reported [136]. 18F-labeled annexin V is being tested as a new PET agent for quantitating tumor cell death and predicting response to therapy. Annexin V binds to surface membranes that have exposed phosphatidyl serine residues resulting from programmed cell destruction. Recently, a Tc-99m-labeled derivative has been shown to accumulate in late stage lung cancer and lymphoma in response to chemotherapy [137]. As molecular pathways leading to and sustaining neoplasia become better understood, so will our capacity improve to measure them in vivo and intervene to the patient's advantage.

Molecular imaging of regional brain tumor biology

Energy metabolism measurements in gliomas in vivo are now performed widely with positron emission tomography (PET). This capability has developed from a large number of basic and clinical science investigations that have cross fertilized one another. This article presents several areas that exemplify questions that have been explored over the last two decades. While the application of PET with [(18)F]-2-fluoro-2-deoxyglucose (FDG-PET) has proven useful for grading and prognosis assessments, this approach is less clinically suitable for assessing response to therapy, even though results to date raise very intriguing biological questions. Integration of metabolic imaging results into glioma therapy protocols is a recent and only preliminarily tapped method that may prove useful in additional trials that target DNA or membrane biosynthesis, or resistance mechanisms such as hypoxia. There are exciting future directions for molecular imaging that will undoubtedly be fruitful to explore, especially apoptosis, angiogenesis and expression of mutations of genes, e.g., epidermal growth factor receptor, that promote or suppress cellular malignant behavior.

Synthesis and evaluation of 4-bromo-1-(3-[18F]fluoropropyl)-2-nitroimidazole with a low energy LUMO orbital designed as brain hypoxia-targeting imaging agent

In order to develop new imaging markers for brain hypoxia, 4-bromo-1-(3-fluoropropyl)-2-nitroimidazole (4-BrFPN) was designed based on molecular orbital calculations, synthesized and labeled with fluorine-18 as a lipophilic nitroimidazole analog with a lower energy LUMO orbital than those for fluoromisonidazole (FMISO) and 1-(3-fluoropropyl)-2-nitroimidazole (FPN). In an in vitro radiosensitization study, the sensitizer enhancement ratio for 4-BrFPN was found to be 1.65 at a I mM concentration, in comparison to 1.81 for FMISO. The preparation of 18F-labeled 4-BrFPN (4-Br18FPN) was achieved by [18F]fluoride ion displacement reaction of the tosylate precursor, in a reasonable radiochemical yield (33%, not corrected for decay). Metabolites in tumor and muscle extracts from methylcholanthrene-induced fibrosarcoma mice, as well as the tissue distribution of 4-Br18FPN in normal rats, were studied. The initial uptake into rat brain of 4-Br18FPN was significantly higher relative to 18F-labeled FMISO (18FMISO), followed by a rapid washout from the brain. The tumor uptake of 4-Br18FPN was somewhat enhanced compared to those obtained with 18FMISO and 18F-labeled FPN (18FPN), but with lower tumor localization than 18FMISO. Analyses of tumor and muscle extracts showed metabolites remaining base line on the radio-TLC plates, and they were produced to a greater extent in tumor than muscle. The use of two drugs which increase hypoxic cell fraction in tumor, hydralazine or nitro-L-arginine, produced a significant increase in tumor levels of 4-Br18FPN, suggestive of a hypoxic mechanism of accumulation. The results imply that lowering of the LUMO energy of a molecule alone is not sufficient to improve its biodistribution properties for better imaging of regions of hypoxia.

[18F]-EF5, a marker for PET detection of hypoxia: synthesis of precursor and a new fluorination procedure

There is a great deal of clinical and experimental interest in determining tissue hypoxia using non-invasive imaging methods. We have developed EF5, 2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide, with both invasive and non-invasive hypoxia detection in mind. EF5 and other 2-nitroimidazoles are used to detect hypoxia, because the rate of their bioreductive metabolism is inversely dependent on oxygen partial pressure. Such metabolism leads to the formation of covalent adducts within the metabolizing cells. Previously, we have described the invasive detection of these adducts by highly specific monoclonal antibodies after tissue biopsy. In this report, we demonstrate the synthesis of 18F-labeled EF5, [18F]-2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide, in greater than 10% yield by direct fluorination of the newly synthesized precursor 2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,3,3-trifluoroallyl)-acetamide by [18F]-F2 in trifluoroacetic acid. Our objective was to optimize the electrophilic fluorination of the fluorinated alkene bond with fluorine gas, a new method of 18F-labeling of polyfluorinated molecules. Previous biodistribution studies in mice have demonstrated uniform access of EF5 to all tissues with bioelimination dominated by renal excretion. When [18F]-EF5 was injected into a rat followed by urine collection and analysis, we found no detectable metabolism to other radioactive compounds. Thus, [18F]-EF5 should be well suited for use as a non-invasive hypoxia marker with detection using positron emission tomography (PET).

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Synthesis of new hypoxia markers EF1 and [18F]-EF1

We report on the preparation of a hypoxia marker 2-(2-nitroimidazol-1[H]-yl)-N-(3-fluoropropyl)acetamide (EF1) and its 18F analog, 2-(2-nitroimidazol-1[H]-yl)-N- (3-[18F]fluoropropyl)acetamide ([18F]-EF1). Two methods for the preparation of 3-fluoropropylamine, the EF1 side chain, are described. [18F]-EF1 was prepared with a radiochemical yield of 2% by nucleophilic substitution of bromine in 2-(2-nitroimidazol-1[H]-yl)-N-(3-bromopropyl)acetamide (EBr1) by carrier-added 18F in DMSO at 120 degrees C. Our results demonstrate the preparation of clinically relevant amounts of [18F]-EF1 for use as a non-invasive hypoxia marker with detection using positron emission tomography (PET).

Utilization of metabolic, transport and receptor-mediated processes to deliver agents for cancer diagnosis

The use of radiopharmaceuticals for the non-invasive diagnosis of cancer has been established in diagnostic radiology over the last few decades. In particular, with the use of sophisticated imaging modalities such as PET and SPECT and a myriad of radioisotopes, advances have been made in the detection and treatment of cancer. This article focuses on three available methods of tumor targeting with radiopharmaceuticals: the utilization of metabolic, transport and receptor-mediated processes to deliver agents for cancer diagnosis. With selected reference to both clinically approved drugs and drugs currently under development, methods of uptake are presented either in terms of flow, metabolic or receptor mediated uptakes. A section of this article is devoted to the monitoring of cancer therapy regimes using radiopharmaceuticals. This review also discusses some mechanistic approaches available in radiopharmaceutical chemistry to be able to effectively diagnose and treat sufferers of cancer in the future.

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

Copper labeled diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) is a promising agent for the imaging of hypoxic tissues. In the present study 64Cu(t1/2 = 12.8 h) labeled Cu-ATSM was used in combination with 11C (t1/2 = 20.3 min) labeled acetate as a regional perfusion marker to visualize hypoxic rat heart tissue in an acute left anterior descending (LAD) coronary artery occluded rat model using an ex vivo tissue slice imaging technique. 64Cu-ATSM was injected intravenously c.a. 10 min after occlusion and rats were sacrificed by cervical dislocation 10 min after injection. Carbon-11-acetate was injected 1 min before sacrifice to obtain a measure of blood flow. The heart was dissected, frozen, and cut into 1-mm thick slices with a gauged slicer, and 11C images were obtained with an electronic autoradiography instrument. After decay of 11C, 64Cu images were obtained in the same manner. In ischemic regions, where there was low 11C accumulation, 64Cu showed high accumulation when compared with normal regions. In rats with a large occlusion, the center of the ischemia did not show any accumulation of either 11C or 64Cu, indicating no blood supply. Cu-ATSM appears to be useful for the detection of hypoxia with contrast being observed at short times (10 min) postinjection.

A method for the assessment of hypoxia in the arterial wall, with potential application in vivo

According to the anoxemia theory of atherosclerosis, an imbalance between the demand for and supply of oxygen in the arterial wall is a key factor in the development of atherosclerotic lesions. Direct in vitro and in situ measurements have shown that PO2 is decreased in the inner part of the media, but the degree of hypoxia in vivo or the distribution of hypoxia along the arterial tree is not known. We applied a hypoxia marker, 7-(4'-(2-nitroimidazol-1-yl)-butyl)-theophylline (NITP), to develop a method for the detection of hypoxia in the arterial wall. Immunoperoxidase and immunofluorescence were used to detect the marker, and a clearly PO2-dependent staining was observed in the media of rabbit and swine aorta in vitro. The cutoff PO2 level was probably around 2 to 3 mm Hg. In experimental atherosclerotic lesions in the rabbit the marker seemed to bind to foam cells that were already at a higher surrounding PO2, which might reflect a higher local oxygen consumption. The binding of the marker to endothelial cells was not PO2 dependent. One explanation for this finding could be that the marker was metabolized via a non-oxygen-dependent pathway in these cells. We propose that this method may be used to assess arterial wall hypoxia in vivo. Furthermore, the spatial resolution allows the detection of local variations within the arterial tree.

The experimental development of bioreductive drugs and their role in cancer therapy

Bioreductive drugs undergo metabolic reduction to generate cytotoxic metabolites. This process is facilitated by bioreductive enzymes and the lower oxygen conditions present in solid tumours compared to normal tissues. Because of this specificity, bioreductive drugs have enormous potential to contribute to modern cancer therapy. Examples undergoing clinical trials include N-oxides such as tirapazamine, aziridinylnitroimidazoles RSU 1069/RBU 6145 and quinones such as indoloquinone EO9. Other novel structures are also under study. Here we review the experimental development of bioreductive drugs and their role in cancer therapy.

Fluoromisonidazole. A metabolic marker of myocyte hypoxia

Fluoromisonidazole is metabolically trapped in viable hypoxic cells in vitro. This property is the basis for the hypothesis that [18F]fluoromisonidazole can be used to detect hypoxic tissues noninvasively using positron emission tomography. To assess the potential usefulness of this compound as a marker for hypoxic myocardium, we measured the accumulation of [3H]fluoromisonidazole in isolated adult rat myocytes under normoxic, hypoxic (5,000 ppm O2), and anoxic conditions. Both anoxia and hypoxia caused a marked increase in [3H]fluoromisonidazole accumulation. Relative to uptake during normoxia, uptake during anoxia was increased by 8.4-fold at 60 minutes and 26.5-fold at 180 minutes (p less than 0.001). During hypoxia, uptake was increased by 4.4-fold at 60 minutes and by 15.3-fold at 180 minutes (p less than 0.0001) and occurred in the absence of significant cell injury as measured by release of creatine kinase and changes in cell morphology. Additional studies demonstrated a slow oxygen-insensitive efflux of fluoromisonidazole or labeled metabolite(s) from myocytes reincubated in drug-free medium. We conclude that fluoromisonidazole is avidly retained in hypoxic myocytes and thus may be suitable for noninvasive detection of hypoxic myocardium using positron emission tomography.

No-carrier-added synthesis of 3-[18F]fluoro-1-(2-nitro-1-imidazolyl)-2-propanol. A potential PET agent for detecting hypoxic but viable tissues

Four different approaches towards the synthesis of [18F]FMISO have been studied. The first approach was based on the reaction of epoxide 4 and [18F]fluoride. Both specific activity and radiochemical yield (less than 1%) for [18F]FMISO were low. Two new approaches, starting with compounds 8 and 9, have failed to give [18F]FMISO. The fourth approach, based on the reaction of [18F]epifluorohydrin 10, prepared from Tosylate 13 and [18F]KF/Kryptofix 222, has provided a reliable, no-carrier added synthesis of [18F]FMISO. The product was obtained in a radiochemical yield of 7-12% at end-of-synthesis (based on [18F]fluoride) with a specific activity of greater than 400 Ci/mmol and a synthesis time of 1.5 h. Preliminary PET studies suggest that [18F]FMISO may be a promising tracer for delineation of ischemic but viable myocardium.