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

Fluorinated carbohydrates for 18F-positron emission tomography (PET)

Carbohydrate diversity is foundational in the molecular literacy that regulates cellular function and communication. Consequently, delineating and leveraging this structure-function interplay continues to be a core research objective in the development of candidates for biomedical diagnostics. A totemic example is the ubiquity of 2-deoxy-2-[18F]-fluoro-D-glucose (2-[18F]-FDG) as a radiotracer for positron emission tomography (PET), in which metabolic trapping is harnessed. Building on this clinical success, more complex sugars with unique selectivities are gaining momentum in molecular recognition and personalised medicine: this reflects the opportunities that carbohydrate-specific targeting affords in a broader sense. In this Tutorial Review, key milestones in the development of 2-[18F]-FDG and related glycan-based radiotracers for PET are described, with their diagnostic functions, to assist in navigating this rapidly expanding field of interdisciplinary research.

Advances in PET and MRI imaging of tumor hypoxia

Tumor hypoxia is a complex and evolving phenomenon both in time and space. Molecular imaging allows to approach these variations, but the tracers used have their own limitations. PET imaging has the disadvantage of low resolution and must take into account molecular biodistribution, but has the advantage of high targeting accuracy. The relationship between the signal in MRI imaging and oxygen is complex but hopefully it would lead to the detection of truly oxygen-depleted tissue. Different ways of imaging hypoxia are discussed in this review, with nuclear medicine tracers such as [¹⁸F]-FMISO, [¹⁸F]-FAZA, or [⁶⁴Cu]-ATSM but also with MRI techniques such as perfusion imaging, diffusion MRI or oxygen-enhanced MRI. Hypoxia is a pejorative factor regarding aggressiveness, tumor dissemination and resistance to treatments. Therefore, having accurate tools is particularly important.

Nuclear Imaging of Glucose Metabolism: Beyond 18F-FDG

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

Synthesis and Biological Evaluation of Iodoglucoazomycin (I-GAZ), an Azomycin-Glucose Adduct with Putative Applications in Diagnostic Imaging and Radiotherapy of Hypoxic Tumors

Iodoglucoazomycin (I-GAZ; N-(2-iodo-3-(6-O-glucosyl)propyl)-2-nitroimidazole), a non-glycosidic nitroimidazole-6-O-glucose adduct, was synthesized, radioiodinated, and evaluated as a substrate of glucose transporter 1 (GLUT1) for radiotheranostic (therapy+diagnostic) management of hypoxic tumors. Nucleophilic iodination of the nosylate synthon of I-GAZ followed by deprotection afforded I-GAZ in 74 % overall yield. I-GAZ was radioiodinated via 'exchange' labeling using [(123/131) I]iodide (50-70 % RCY) and then purified by Sep-Pak™ (>96 % RCP). [(131) I]I-GAZ was stable in 2 % ethanolic solution in sterile water for 14 days when stored at 5 °C. In cell culture, I-GAZ was found to be nontoxic to EMT-6 cells at concentrations <0.5 mm, and weakly radiosensitizing (SER 1.1 at 10 % survival of EMT-6 cells; 1.2 at 0.1 % survival in MCF-7 cells). The hypoxic/normoxic uptake ratio of [(123) I]I-GAZ in EMT-6 cells was 1.46 at 2 h, and under normoxic conditions the uptake of [(123) I]I-GAZ by EMT-6 cells was unaltered in the presence of 5 mm glucose. The biodistribution of [(131) I]I-GAZ in EMT-6 tumor-bearing Balb/c mice demonstrated rapid clearance from blood and extensive renal and hepatic excretion. Tumor/blood and tumor/muscle ratios reached ∼3 and 8, respectively, at 4 h post-injection. Regression analysis of the first order polynomial plots of the blood and tumor radioactivity concentrations supported a perfusion-excretion model with low hypoxia-dependent binding. [(131) I]I-GAZ was found to be stable in vivo, and did not deiodinate.

The chemistry and radiochemistry of hypoxia-specific, radiohalogenated nitroaromatic imaging probes

Hypoxia is prevalent in many solid tumors. Hypoxic tumors tend to exhibit rapid growth and aberrant vasculature, which lead to oxygen (O2) depletion and impaired drug delivery. The reductive environment in hypoxic tumors alters cellular metabolism, which can trigger transcriptional responses; induce genetic alterations; promote invasion, metastasis, resistance to radiotherapy and chemotherapy, tumor progression, and recurrence; and leads to poor local control and reduced survival rates. Therefore, exploiting the reductive microenvironment in hypoxic tumors by delivering electron-affinic, O2-mimetic radioactive drugs that bioreductively activate selectively in the hypoxic microenvironment offers a logical approach to molecular imaging of focal hypoxia. Because these agents also radiosensitize hypoxic cells, they provide an innovative approach to the therapy management of such tumors. To date, nuclear imaging of hypoxic tumor has proven to be clinically effective, whereas chemical radiosensitization by these compounds has not been helpful. The current review provides an insight into the chemistry, radiochemistry, and purification strategies for selected nitroaromatics that directly exploit the bioreductive environment in hypoxic cells. Both experimental and calculated single-electron reduction potentials of electron-affinic compounds, nitroimidazoles in particular, correlate with in vitro radiosensitizing properties, making them preferred choices for use as radiopharmaceuticals for diagnostic imaging and as sensitizers to enhance the killing effects of low-energy-transfer x-rays (O2-mimetic radiosensitization). Extensive research and careful drug design have led to the development of several potentially useful hypoxia-targeting drugs, for example, [(18)F]FAZA, [(18)F]FMISO, [(18)F]EF5, and [(123)I]IAZA, that accrue selectively in hypoxic cells. These molecular probes are now globally used in clinical hypoxia imaging, including cancer. Future innovative developments must, however, consider hypoxia-selective molecular processes and the physicochemical properties of the drugs that dictate their biodistribution, hypoxia-selective accumulation, pharmacokinetics, clearance, biochemical behavior, and metabolism. This will facilitate their ultimate transformation to effective molecular theranostics, leading to improved multimodal management of cancer.