Positron emission tomography radiotracers for imaging hypoxia

A thiourea-bridged 99mTc(CO)3-dipicolylamine-2-nitroimidazole complex for targeting tumor hypoxia: Utilizing metabolizable thiourea-bridge to improve pharmacokinetics

The 2-nitroimidazole based 99mTc-radiopharmaceuticals are widely explored for imaging tumor hypoxia. Radiopharmaceuticals for targeting hypoxia are often lipophilic and therefore, show significant uptake in liver and other vital organs. In this context, lipophilic radiopharmaceuticals with design features enabling faster clearance from liver may be more desirable. A dipicolylamine-NCS bifunctional chelator that could generate a thiourea-bridge up on conjugation to primary amine bearing molecule was used to synthesize a 2-nitroimidazole-dipicolyl amine ligand for radiolabeling with 99mTc(CO)3 core. Corresponding Re(CO)3-analogue was prepared to establish the structure of 2-nitroimidazole-99mTc(CO)3 complex prepared in trace level. The 2-nitroimidazole-99mTc(CO)3 complex showed a hypoxic to normoxic ratio of ~2.5 in CHO cells at 3 h. In vivo, the complex showed accumulation and retention in tumor with high tumor to blood and tumor to muscle ratio. The study demonstrated the utility of metabolizable thiourea-bridge in 2-nitroimidazole-99mTc(CO)3 complex in inducing faster clearance of the radiotracer from liver. The dipicolylamine-NCS bifunctional chelator reported herein can also be used for radiolabeling other class of target specific molecules with 99mTc(CO)3 core.

Molecular imaging of tumor hypoxia: Evolution of nitroimidazole radiopharmaceuticals and insights for future development

Though growing evidence has been collected in support of the concept of dose escalation based on the molecular level images indicating hypoxic tumor sub-volumes that could be radio-resistant, validation of the concept is still a work in progress. Molecular imaging of tumor hypoxia using radiopharmaceuticals is expected to provide the required input to plan dose escalation through Image Guided Radiation Therapy (IGRT) to kill/control the radio-resistant hypoxic tumor cells. The success of the IGRT, therefore, is heavily dependent on the quality of images obtained using the radiopharmaceutical and the extent to which the image represents the true hypoxic status of the tumor in spite of the heterogeneous nature of tumor hypoxia. Available literature on radiopharmaceuticals for imaging hypoxia is highly skewed in favor of nitroimidazole as the pharmacophore given their ability to undergo oxygen dependent reduction in hypoxic cells. In this context, present review on nitroimidazole radiopharmaceuticals would be immensely helpful to the researchers to obtain a birds-eye view on what has been achieved so far and what can be tried differently to obtain a better hypoxia imaging agent. The review also covers various methods of radiolabeling that could be utilized for developing radiotracers for hypoxia targeting applications.

Comparison of three 18F-labeled 2-nitroimidazoles for imaging hypoxia in breast cancer xenografts: [18F]FBNA, [18F]FAZA and [18F]FMISO

BACKGROUND

Tumour hypoxia is associated with increased metastasis, invasion, poor therapy response and prognosis. Most PET radiotracers developed and used for clinical hypoxia imaging belong to the 2-nitroimidazole family. Recently we have developed novel 2-nitroimidazole-derived PET radiotracer [18F]FBNA (N-(4-[18F]fluoro-benzyl)-2-(2-nitro-1H-imidazol-1-yl)-acet-amide), an 18F-labeled analogue of antiparasitic drug benznidazole. The present study aimed to analyze its radio-pharmacological properties and systematically compare its PET imaging profiles with [18F]FMISO and [18F]FAZA in preclinical triple-negative (MDA-MB231) and estrogen receptor-positive (MCF-7) breast cancer models.

METHODS

In vitro cellular uptake experiments were carried out in MDA-MB321 and MCF-7 cells under normoxic and hypoxic conditions. Metabolic stability in vivo was determined in BALB/c mice using radio-TLC analysis. Dynamic PET experiments over 3 h post-injection were performed in MDA-MB231 and MCF-7 tumour-bearing mice. Those PET data were used for kinetic modelling analysis utilizing the reversible two-tissue-compartment model. Autoradiography was carried out in tumour tissue slices and compared to HIF-1α immunohistochemistry. Detailed ex vivo biodistribution was accomplished in BALB/c mice, and this biodistribution data were used for dosimetry calculation.

RESULTS

Under hypoxic conditions in vitro cellular uptake was elevated in both cell lines, MCF-7 and MDA-MB231, for all three radiotracers. After intravenous injection, [18F]FBNA formed two radiometabolites, resulting in a final fraction of 65 ± 9 % intact [18F]FBNA after 60 min p.i. After 3 h p.i., [18F]FBNA tumour uptake reached SUV values of 0.78 ± 0.01 in MCF-7 and 0.61 ± 0.04 in MDA-MB231 tumours (both n = 3), representing tumour-to-muscle ratios of 2.19 ± 0.04 and 1.98 ± 0.15, respectively. [18F]FMISO resulted in higher tumour uptakes (SUV 1.36 ± 0.04 in MCF-7 and 1.23 ± 0.08 in MDA-MB231 (both n = 4; p < 0.05) than [18F]FAZA (0.66 ± 0.11 in MCF-7 and 0.63 ± 0.14 in MDA-MB231 (both n = 4; n.s.)), representing tumour-to-muscle ratios of 3.24 ± 0.30 and 3.32 ± 0.50 for [18F]FMISO, and 2.92 ± 0.74 and 3.00 ± 0.42 for [18F]FAZA, respectively. While the fraction per time of radiotracer entering the second compartment (k3) was similar within uncertainties for all three radiotracers in MDA-MB231 tumours, it was different in MCF-7 tumours. The ratios k3/(k3 + k2) and K1*k3/(k3 + k2) in MCF-7 tumours were also significantly different, indicating dissimilar fractions of radiotracer bound and trapped intracellularly: K1*k3/(k2 + k3) [18F]FMISO (0.0088 ± 0.001)/min, n = 4; p < 0.001) > [18F]FAZA (0.0052 ± 0.002)/min, n = 4; p < 0.01) > [18F]FBNA (0.003 ± 0.001)/min, n = 3). In contrast, in MDA-MB231 tumours, only K1 was significantly elevated for [18F]FMISO. However, this did not result in significant differences for K1*k3/(k2 + k3) for all three 2-nitroimidazoles in MDA-MB231 tumours.

CONCLUSION

Novel 2-nitroimidazole PET radiotracer [18F]FBNA showed uptake into hypoxic breast cancer cells and tumour tissue presumably associated with elevated HIF1-α expression. Systematic comparison of PET imaging performance with [18F]FMISO and [18F]FAZA in different types of preclinical breast cancer models revealed a similar tumour uptake profile for [18F]FBNA with [18F]FAZA and, despite its higher lipophilicity, still a slightly higher muscle tissue clearance compared to [18F]FMISO.

In Vivo Imaging of Tumor Hypoxia by Maintaining Green Fluorescence of 9-Aminoanthracene Under Hypoxic Conditions

In this study, 9-aminoanthracene (9AA) was used as a new fluorescence reagent for the in vivo imaging of tumor hypoxia by taking advantage of the maintenance of its green fluorescence under hypoxic conditions. As 9AA is insoluble in water, polyethylene glycol (PEG)-400 was used to dissolve 9AA in saline. Each organ was successfully stained with 9AA, as observed by green fluorescence using in vivo imaging, following intragastric administration of a 9AA PEG-saline solution in mice. Therefore, the intragastric administration of 9AA can be used for in vivo imaging of normal mice. Tumor hypoxia staining using the 9AA fluorescence method was evaluated by in vivo imaging of mice subcutaneously transplanted with Ehrlich ascites carcinoma cells and compared with conventional pimonidazole (PIMO) staining under hypoxic conditions. The tumor sections were stained with green fluorescence derived from 9AA and the same sections corresponded to hypoxic areas upon immunohistochemical staining with PIMO.

Recent developments on the application of molecular probes in multiple myeloma: Beyond [18F]FDG

Multiple myeloma (MM) is a neoplastic plasma cell proliferative disorder characterized by various osteolytic bone destruction as a radiological morphological marker. Functional imaging, particularly nuclear medicine imaging, is a promising method to visualize disease processes before the appearance of structural changes by targeting specific biomarkers related to metabolism ability, tumor microenvironment as well as neoplastic receptors. In addition, by targeting particular antigens with therapeutic antibodies, immuno-PET imaging can support the development of personalized theranostics. At present, various imaging agents have been prepared and evaluated in MM at preclinical and clinical levels. A summary overview of molecular functional imaging in MM is provided, and commonly used radiotracers are characterized.

Synthesis of a 2-nitroimidazole derivative N-(4-[18F]fluorobenzyl)-2-(2-nitro-1H-imidazol-1-yl)-acetamide ([18 F]FBNA) as PET radiotracer for imaging tumor hypoxia

BACKGROUND

Tissue hypoxia is a pathological condition characterized by reducing oxygen supply. Hypoxia is a hallmark of tumor environment and is commonly observed in many solid tumors. Non-invasive imaging techniques like positron emission tomography (PET) are at the forefront of detecting and monitoring tissue hypoxia changes in vivo.

RESULTS

We have developed a novel ¹⁸F-labeled radiotracer for hypoxia PET imaging based on cytotoxic agent benznidazole. Radiotracer N-(4-[¹⁸F]fluorobenzyl)-2-(2-nitro-1H-imidazol-1-yl)acetamide ([¹⁸F]FBNA) was synthesized through acylation chemistry with readily available 4-[¹⁸F]fluorobenzyl amine. Radiotracer [¹⁸F]FBNA was obtained in good radiochemical yields (47.4 ± 5.3%) and high radiochemical purity (> 95%). The total synthesis time was 100 min, including HPLC purification and the molar activity was greater than 40 GBq/µmol. Radiotracer [¹⁸F]FBNA was stable in saline and mouse serum for 6 h. [¹⁸F]FBNA partition coefficient (logP = 1.05) was found to be more lipophilic than [¹⁸F]EF-5 (logP = 0.75), [¹⁸F]FMISO (logP = 0.4) and [¹⁸F]FAZA (logP =  - 0.4). In vitro studies showed that [¹⁸F]FBNA accumulates in gastric cancer cell lines AGS and MKN45 under hypoxic conditions.

CONCLUSIONS

Hence, [¹⁸F]FBNA represents a novel and easy-to-prepare PET radioligand for imaging hypoxia.

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

Longitudinal PET imaging of tumor hypoxia during the course of radiotherapy

Hypoxia results from an imbalance between oxygen supply and consumption. It is a common phenomenon in solid malignant tumors such as head and neck cancer. As hypoxic cells are more resistant to therapy, tumor hypoxia is an indicator for poor prognosis. Several techniques have been developed to measure tissue oxygenation. These are the Eppendorf O2 polarographic needle electrode, immunohistochemical analysis of endogenous (e.g., hypoxia-inducible factor-1α (HIF-1a)) and exogenous markers (e.g., pimonidazole) as well as imaging methods such as functional magnetic resonance imaging (e.g., blood oxygen level dependent (BOLD) imaging, T1-weighted imaging) and hypoxia positron emission tomography (PET). Among the imaging modalities, only PET is sufficiently validated to detect hypoxia for clinical use. Hypoxia PET tracers include 18F-fluoromisonidazole (FMISO), the most commonly used hypoxic marker, 18F-flouroazomycin arabinoside (FAZA), 18Ffluoroerythronitroimidazole (FETNIM), 18F-2-nitroimidazolpentafluoropropylacetamide (EF5) and 18F-flortanidazole (HX4). As technical development provides the opportunity to increase the radiation dose to subregions of the tumor, such as hypoxic areas, it has to be ensured that these regions are stable not only from imaging to treatment but also through the course of radiotherapy. The aim of this review is therefore to characterize the behavior of tumor hypoxia during radiotherapy for the whole tumor and for subregions by using hypoxia PET tracers, with focus on head and neck cancer patients.

Assessing Tumor Oxygenation for Predicting Outcome in Radiation Oncology: A Review of Studies Correlating Tumor Hypoxic Status and Outcome in the Preclinical and Clinical Settings

Tumor hypoxia is recognized as a limiting factor for the efficacy of radiotherapy, because it enhances tumor radioresistance. It is strongly suggested that assessing tumor oxygenation could help to predict the outcome of cancer patients undergoing radiation therapy. Strategies have also been developed to alleviate tumor hypoxia in order to radiosensitize tumors. In addition, oxygen mapping is critically needed for intensity modulated radiation therapy (IMRT), in which the most hypoxic regions require higher radiation doses and the most oxygenated regions require lower radiation doses. However, the assessment of tumor oxygenation is not yet included in day-to-day clinical practice. This is due to the lack of a method for the quantitative and non-invasive mapping of tumor oxygenation. To fully integrate tumor hypoxia parameters into effective improvements of the individually tailored radiation therapy protocols in cancer patients, methods allowing non-invasively repeated, safe, and robust mapping of changes in tissue oxygenation are required. In this review, non-invasive methods dedicated to assessing tumor oxygenation with the ultimate goal of predicting outcome in radiation oncology are presented, including positron emission tomography used with nitroimidazole tracers, magnetic resonance methods using endogenous contrasts (R₁ and R2*-based methods), and electron paramagnetic resonance oximetry; the goal is to highlight results of studies establishing correlations between tumor hypoxic status and patients’ outcome in the preclinical and clinical settings.

Recent Advances in the Development of PET/SPECT Probes for Atherosclerosis Imaging

The rupture of vulnerable atherosclerotic plaques and subsequent thrombus formation are the major causes of myocardial and cerebral infarction. Accordingly, the detection of vulnerable plaques is important for risk stratification and to provide appropriate treatment. Inflammation imaging using 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG) has been most extensively studied for detecting vulnerable atherosclerotic plaques. It is of great importance to develop PET/SPECT probes capable of specifically visualizing the biological molecules involved in atherosclerotic plaque formation and/or progression. In this article, we review recent advances in the development of PET/SPECT probes for visualizing atherosclerotic plaques and their application to therapy monitoring, mainly focusing on experimental studies.

Automated radiosynthesis of GMP quality [18F]HX4 for PET imaging of hypoxia

INTRODUCTION

[(18)F]HX4 is a 2-nitroimidazole based investigational radiotracer for imaging hypoxia.

METHODS

A two-step, one-pot synthetic procedure was developed on a GE Tracerlab MX-FDG with a disposable cassette. Nucleophilic substitution of a nosylate group with [(18)F]fluoride was followed by solvent evaporation and acidic removal of the acetyl protecting group. HPLC purification in a bio-compatible solvent mixture was developed.

RESULTS AND CONCLUSIONS

Using starting activities of 80-110 GBq [(18)F]fluoride, GMP compliant [(18)F]HX4 was produced in non-decay corrected radiochemical yields of 12 ± 3% (n = 9) in 55 min including HPLC purification. No reformulation steps were required. The mean specific activity of the final product was 2450 GBq/μmol. Modifications to the process and final formulation were included to prevent decomposition of the product, and these changes resulted in an improved stability of the formulated [(18)F]HX4, with a shelf-life of at least 8h post-synthesis. The product consistently passed all required quality control tests to determine that the [(18)F]HX4 was suitable for clinical use. Using a 90 minute target bombardment, and 80-110 GBq starting [(18)F]fluoride, the method produced multiple patient doses.

Detecting functional changes with [(18)F]FAZA in a renal cell carcinoma mouse model following sunitinib therapy

BACKGROUND

The multitargeting tyrosine kinase inhibitor (TKI) sunitinib is currently the first-line drug therapy for metastasizing renal cell carcinoma (RCC). TKIs have profound effects on tumor angiogenesis, leading to modifications of the tumor microenvironment. The goal of this study was to determine whether these treatment-induced changes can be detected with [(18)F]FAZA.

METHODS

The present study utilized positron emission tomography (PET) to analyze tumor oxygenation status during and after sunitinib therapy in the murine Caki-1 RCC tumor model. Dynamic and static scans were performed, as well as ex vivo biodistributions at 3 h post injection (p.i.). Immunohistochemical analysis of tumor tissue was carried out for the quantification of pimonidazole binding and the hypoxia-associated factors CD-31, Ki-67, and Von Willebrand factor (VWF). In addition, in vitro cellular uptake studies were done to analyze the direct effects of sunitinib on the Caki-1 cells.

RESULTS

During therapy with sunitinib (40 mg/kg/day), uptake of [(18)F]FAZA into Caki-1 mice decreased by 46 ± 5% (n = 4; 5 days) at 3 h post injection (p.i.) during the first study and 22 ± 5% (n = 8; 9 days) during the long-term study, indicating a decrease in the tumor's hypoxia level. However, when drug therapy was stopped, this effect was reversed completely, and the tumor [(18)F]FAZA uptake increased to 126 ± 6% (n = 6) of the control tumor uptake, indicative of an even higher level of tumor hypoxia compared to the therapy starting point. Sunitinib had no direct effect on [(18)F]FAZA uptake into Caki-1 cells in vitro.

CONCLUSION

[(18)F]FAZA PET could be used to monitor drug response during sunitinib therapy in RCC and may guide combination therapies based on the tumor's hypoxia status.

Production of iodine-124 and its applications in nuclear medicine

Until recently, iodine-124 was not considered to be an attractive isotope for medical applications owing to its complex radioactive decay scheme, which includes several high-energy gamma rays. However, its unique chemical properties, and convenient half-life of 4.2 days indicated it would be only a matter of time for its frequent application to become a reality. The development of new medical imaging techniques, especially improvements in the technology of positron emission tomography (PET), such as the development of new detectors and signal processing electronics, has opened up new prospects for its application. With the increasing use of PET in medical oncology, pharmacokinetics, and drug metabolism, (124)I-labeled radiopharmaceuticals are now becoming one of the most useful tools for PET imaging, and owing to the convenient half-life of I-124, they can be used in PET scanners far away from the radionuclide production site. Thus far, the limited availability of this radionuclide has been an impediment to its wider application in clinical use. For example, sodium [(124)I]-iodide is potentially useful for diagnosis and dosimetry in thyroid disease and [(124)I]-M-iodobenzylguanidine ([(124)I]-MIBG) has enormous potential for use in cardiovascular imaging, diagnosis, and dosimetry of malignant diseases such as neuroblastoma, paraganglioma, pheochromocytoma, and carcinoids. However, despite that potential, both are still not widely used. This is a typical scenario of a rising new star among the new PET tracers.