Recent Progress in Synthesis of 99mTc-labeled Complexes with Nitroimidazoles as SPECT Probes for Targeting Tumor Hypoxia

The majority of solid tumors have hypoxia, or low oxygen levels, which is one of the hallmarks of cancer. Hypoxia was found to relate to cancer metastases and resistance to therapies, therefore, detection of hypoxia plays an important role in the process of cancer prognosis and treatment. Single-photon emission computed tomography (SPECT) is a non-invasive imaging technique using gamma-emitting radiopharmaceuticals to visualize biological activities within the body. SPECT is also applied for the detection of tumor hypoxia with the development of hypoxia-targeting radiopharmaceuticals. Radiopharmaceuticals containing nitroimidazole moieties have received increasing attention due to their bio-reducible characteristics which make the radiopharmaceuticals accumulate in the hypoxia regions. This review summarizes the recent development of 99mTc-labeled radiopharmaceuticals bearing nitroimidazoles for SPECT imaging of tumor hypoxia including the synthetic methods and results of animal studies.

Preparation and bioevaluation of a novel 99mTc-labelled propylene amine oxime (PnAO) containing two 4-methyl-2-nitroimidazole groups as a promising tumor hypoxia imaging agent

Hypoxia is a common phenomenon in solid tumors, and its presence inhibits the efficacy of tumor chemotherapy and radiotherapy. Accurate measurement of hypoxia before tumor treatment is essential. Three propylene amine oxime (PnAO) derivatives with different substituents attached to 2-nitroimidazole were synthesized in the work, they are 3,3,9,9-tetramethyl-1,11-bis(4-bromo-2-nitro-1H-imidazol-1-yl)-4,8-diazaundecane-2,10-dione dioxime (Br2P2), 3,3,9,9-tetramethyl-1,11-bis(4-methyl-2-nitro-1H-imidazol-1-yl)-4,8-diazaundecane-2,10-dione dioxime (Me2P2) and 3,3,9,9-tetramethyl-1,11-bis(4,5-dimethyl-2-nitro-1H-imidazol-1-yl)-4,8-diazaundecane-2,10-dione dioxime (2Me2P2). The three compounds were radiolabeled with 99mTc to give three complexes([99mTc]Tc-Br2P2, [99mTc]Tc-Me2P2 and [99mTc]Tc-2Me2P2) with good in vitro stability. [99mTc]Tc-Me2P2 with a more suitable reduction potential had the highest hypoxic cellular uptake, compared with [99mTc]Tc-2P2 that have been previously reported, [99mTc]Tc-Br2P2 and [99mTc]Tc-2Me2P2. Biodistribution results in S180 tumor-bearing mice demonstrated that [99mTc]Tc-Me2P2 had the highest tumor-to-muscle (T/M) ratio (12.37 ± 1.16) at 2 h in the four complexes. Autoradiography and immunohistochemical staining results revealed that [99mTc]Tc-Me2P2 specifically targeted tumor hypoxic regions. The SPECT/CT imaging results showed that [99mTc]Tc-Me2P2 could target the tumor site. [99mTc]Tc-Me2P2 may become a potential hypoxia imaging agent.

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.

Synthesis, 99mTc-labeling, in-vivo study and in-silico investigation of 6-amino-5-[(bis-(2-hydroxy-ethyl)-amino]methyl]2-methyl pyrimidin-4-ol as a potential probe for tumor targeting

This study aimed to synthesize a new pyrimidine derivative with a good synthesis yield of 87% to act as a new cancer marker after radiolabeling with Tc-99m in a high radiochemical yield of 92.3%. In-vivo study in tumor-bearing Swiss albino mice model revealed promising data with high uptake in cancer. Docking study showed good binding interactions of the radiosynthesized complex at the binding site. In conclusion, this novel complex could be a potential probe for cancer targeting.

99mTc Labelling Strategies for the Development of Potential Nitroimidazolic Hypoxia Imaging Agents

Technetium-99m has a rich coordination chemistry that offers many possibilities in terms of oxidation states and donor atom sets. Modifications in the structure of the technetium complexes could be very useful for fine tuning the physicochemical and biological properties of potential 99mTc radiopharmaceuticals. However, systematic study of the influence of the labelling strategy on the “in vitro” and “in vivo” behaviour is necessary for a rational design of radiopharmaceuticals. Herein we present a review of the influence of the Tc complexes’ molecular structure on the biodistribution and the interaction with the biological target of potential nitroimidazolic hypoxia imaging radiopharmaceuticals presented in the literature from 2010 to the present. Comparison with the gold standard [18F]Fluoromisonidazole (FMISO) is also presented.

Synthesis, quality control, and bio-evaluation of 99m Tc-cyclophosphamide

Cancer is found to be the leading cause of death worldwide characterized by uncontrolled cell division. Nuclear medicines imaging using radiopharmaceuticals have pronounced potential for the diagnosis and treatment of cancers. Cyclophosphamide (CPH) is an antineoplastic drug which targets selectively cancer cells. In the present work, labeling of CPH with Tc-99m is performed for diagnostic purpose, which gave labeling yield as high as 99% using 20 μg SnCl₂ ·2H₂ O, 200 μg of ligand at pH 7 for 10 min reaction time at room temperature. The characterization of the prepared complex was performed using ITLC, electrophoresis, and HPLC. In vitro stability was analyzed in the presence of human serum at 37°C which has maximum value of 94 ± 0.5. The biodistribution studies of ^99m Tc-CPH were performed in normal and tumor bearing Swiss Webster mice. The high accumulation of ^99m Tc-CPH was observed in liver and tumours respectively at 4 hr after injection. Biodistribution results revealed that ^99m Tc-CPH may be a potential tumour diagnostic agent simultaneously with chemotherapy.

Toward hypoxia-selective rhenium and technetium tricarbonyl complexes

With the aim of preparing hypoxia-selective imaging and therapeutic agents, technetium(I) and rhenium(I) tricarbonyl complexes with pyridylhydrazone, dipyridylamine, and pyridylaminocarboxylate ligands containing nitrobenzyl or nitroimidazole functional groups have been prepared. The rhenium tricarbonyl complexes were synthesized with short reaction times using microwave irradiation. Rhenium tricarbonyl complexes with deprotonated p-nitrophenyl pyridylhydrazone ligands are luminescent, and this has been used to track their uptake in HeLa cells using confocal fluorescent microscopy. Selected rhenium tricarbonyl complexes displayed higher uptake in hypoxic cells when compared to normoxic cells. A (99m)Tc tricarbonyl complex with a dipyridylamine ligand bearing a nitroimidazole functional group is stable in human serum and was shown to localize in a human renal cell carcinoma (RCC; SK-RC-52) tumor in a mouse.

Metal complexes containing natural and and artificial radioactive elements and their applications

Recent advances (during the 2007–2014 period) in the coordination and organometallic chemistry of compounds containing natural and artificially prepared radionuclides (actinides and technetium), are reviewed. Radioactive isotopes of naturally stable elements are not included for discussion in this work. Actinide and technetium complexes with O-, N-, N,O, N,S-, P-containing ligands, as well π-organometallics are discussed from the view point of their synthesis, properties, and main applications. On the basis of their properties, several mono-, bi-, tri-, tetra- or polydentate ligands have been designed for specific recognition of some particular radionuclides, and can be used in the processes of nuclear waste remediation, i.e., recycling of nuclear fuel and the separation of actinides and fission products from waste solutions or for analytical determination of actinides in solutions; actinide metal complexes are also usefulas catalysts forcoupling gaseous carbon monoxide, as well as antimicrobial and anti-fungi agents due to their biological activity. Radioactive labeling based on the short-lived metastable nuclide technetium-99m (^99mTc) for biomedical use as heart, lung, kidney, bone, brain, liver or cancer imaging agents is also discussed. Finally, the promising applications of technetium labeling of nanomaterials, with potential applications as drug transport and delivery vehicles, radiotherapeutic agents or radiotracers for monitoring metabolic pathways, are also described.

On the structural modification of 2-nitroimidazole-(99m)Tc(CO)(3) complex, a hypoxia marker, for improving in vivo pharmacokinetics

INTRODUCTION

A 2-nitroimidazole-(99m)Tc(CO)(3) complex reported earlier showed promise with respect to its uptake and retention in hypoxic tumor. However, significant uptake and slow clearance from liver imposed severe limitations towards advocating its possible practical utility. In an attempt to improving its liver clearance, an ether linkage, which is known to help in liver clearance, was introduced in the molecule.

METHODS

The 2-nitroimidazole iminodiacetic acid (IDA) derivative containing an ether linkage was synthesized in a five step procedure from 2-nitroimidazole. This derivative was radiolabeled using [(99m)Tc(CO)(3)(H(2)O)(3)](+) precursor complex. The corresponding Re(CO)(3) analogue was also synthesized in the macroscopic level for structural characterization. The (99m)Tc(CO)(3) complex was evaluated in an animal model bearing fibrosarcoma tumor.

RESULTS

The in vivo evaluation of the complex indicated that, as envisaged, introduction of the ether linkage has improved clearance from the liver. The complex also showed higher retention in tumor compared to the 2-nitroimidazole-IDA-(99m)Tc(CO)(3) complex reported earlier. Though the tumor to muscle ratio improved with time, the tumor to blood ratio did not show any significant improvement. Despite improved liver clearance, there was significant liver activity present even at 3h p.i. attributable to gradual accumulation of activity cleared from muscle and blood.

CONCLUSIONS

Though the introduction of ether linkage improved liver clearance of the modified 2-nitroimidazole complex, it was found that a single ether linkage was not sufficient to achieve the desirable level of clearance. Probably, a linker with multiple ether groups, such as a di- or tri-ethylene glycol spacer, may be a possible solution to this issue.