A study on nitroimidazole-99mTc(CO)3 complexes as hypoxia marker: Some observations towards possible improvement in in vivo efficacy

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 and Evaluation of 99mTc-Labelled 2-Nitroimidazole Derivatives with Different Linkers for Tumour Hypoxia Imaging

When developing novel radiopharmaceuticals, a linker moiety between the chelator and targeting vector can have a crucial influence on adjusting the affinity of the tracer and its biodistribution in organisms. To develop novel 99mTc-labelled hypoxia imaging radiotracers, in this study, five isocyanide-containing 2-nitroimidazole derivatives with different linkers (L1, L2, L3, L4 and L5) were synthesised and radiolabelled with technetium-99m to obtain five stable 99mTc-complexes ([99mTc]Tc-L1, [99mTc]Tc-L2, [99mTc]Tc-L3, [99mTc]Tc-L4 and [99mTc]Tc-L5). Corresponding rhenium analogues of [99mTc]Tc-L1 were synthesised and suggested the structures of these 99mTc-complexes would be a monovalent cation with a technetium (I) core surrounded by six ligands. [99mTc]Tc-L1 is hydrophilic, while the lipophilicities of [99mTc]Tc-L2, [99mTc]Tc-L3, [99mTc]Tc-L4 and [99mTc]Tc-L5 are close. In vitro cell experiments showed that all five novel 99mTc-complexes had higher uptake in hypoxic cells compared with aerobic cells, which indicates the complexes have good hypoxia selectivity. The biodistribution of the five 99mTc-complexes in S180 tumour-bearing mice showed that they all had certain uptake in the tumours. Among them, [99mTc]Tc-L1 had the highest tumour-to-muscle (4.68 ± 0.44) and tumour-to-blood (3.81 ± 0.46) ratios. The introduction of polyethylene glycol (PEG) chains effectively reduced the lipophilicity and decreased uptake by the liver, intestine and blood but also increased clearance from the tumours. In vivo metabolic studies showed [99mTc]Tc-L1 kept intact and remained stable in tumour, blood and urine at 2 h post-injection. The results of SPECT imaging showed that [99mTc]Tc-L1 had significant tumour uptake at 2 h post-injection, but there was still high uptake in abdominal organs such as the liver and kidney, suggesting that this complex needs to be further optimised before being used for tumour hypoxia imaging.

Recent Developments in PET and SPECT Radiotracers as Radiopharmaceuticals for Hypoxia Tumors

Hypoxia, a deficiency in the levels of oxygen, is a common feature of most solid tumors and induces many characteristics of cancer. Hypoxia is associated with metastases and strong resistance to radio- and chemotherapy, and can decrease the accuracy of cancer prognosis. Non-invasive imaging methods such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using hypoxia-targeting radiopharmaceuticals have been used for the detection and therapy of tumor hypoxia. Nitroimidazoles are bioreducible moieties that can be selectively reduced under hypoxic conditions covalently bind to intracellular macromolecules, and are trapped within hypoxic cells and tissues. Recently, there has been a strong motivation to develop PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazole moieties for the visualization and treatment of hypoxic tumors. In this review, we summarize the development of some novel PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazoles, as well as their physicochemical properties, in vitro cellular uptake values, in vivo biodistribution, and PET/SPECT imaging results.

68Ga-labeled PET tracers for targeting tumor hypoxia: Role of bifunctional chelators on pharmacokinetics

INTRODUCTION

By virtue of their oxygen dependant accumulation in hypoxic cells, radiolabeled nitroimidazole analogues have been widely used for detecting tumor hypoxia. Present study evaluates two 2-nitroimidazole (2-NIM) based ⁶⁸Ga-labeled radiotracers, [⁶⁸Ga]Ga-DOTAGA-2-NIM and [⁶⁸Ga]Ga-NODAGA-2-NIM, for hypoxia targeting applications.

METHODS

Bifunctional chelating agents suitable for radiolabeling with ⁶⁸Ga, viz. 1,4,7,10-tetraazacyclododececane,1-(glutaric acid)-4,7,10-triacetic acid (DOTAGA) and 1,4,7-triazacyclododececane,1-(glutaric acid)-4,7-diacetic acid (NODAGA), were coupled to appropriately modified 2-nitroimidazole to obtain 2-NIM-DOTAGA and 2-NIM-NODAGA, respectively. These ligands were radiolabeled using [⁶⁸Ga]GaCl₃ obtained from a commercial ⁶⁸Ge/⁶⁸Ga-generator to obtain corresponding ⁶⁸Ga-complexes. Both the radiotracers were tested for their hypoxia selectivity in CHO cells under hypoxic and normoxic conditions. Biodistribution studies in fibrosarcoma tumor bearing Swiss mice were carried out to evaluate the radiotracer in vivo.

RESULTS

The ⁶⁸Ga complexes of 2-NIM-DOTAGA and 2-NIM-NODAGA could be prepared in ~82% and ~90% yield, respectively. In vitro studies of the complexes in CHO cells showed significant accumulation of [⁶⁸Ga]Ga-NODAGA-2-NIM complex under hypoxic conditions with hypoxic to normoxic ratio of 2.88 ± 0.36 at 180 min post incubation. The [⁶⁸Ga]Ga-DOTAGA-2-NIM complex also showed hypoxia selectivity albeit to a lesser extent. Biodistribution studies of the complexes in Swiss mice bearing fibrosarcoma tumor showed significant tumor uptake by both radiolabeled complexes. [⁶⁸Ga]Ga-NODAGA-2-NIM showed a more favorable pharmacokinetics with respect to [⁶⁸Ga]Ga-DOTAGA-2-NIM.

CONCLUSION

The nitroimidazole radiotracer with NODAGA chelator displayed more favorable pharmacokinetics and good hypoxia selectivity, making it a promising candidate for further investigation. The present study also provides an insight into the possible role of bifunctional chelator on overall pharmacokinetics of small molecule radiotracers.

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 and evaluation of 99mTc-analogues of [123/131I]mIBG prepared via [99mTc][Tc(CO)3(H2O)3]+ synthon for targeting norepinephrine transporter

INTRODUCTION

meta-[^123/131I]Iodobenzylguanidine (mIBG) is a clinical agent used for imaging neuroendocrine tumors, where uptake in tumor is via active transport mechanism through norepinephrine transporters (NET). Our group in past have evaluated a ^99mTc-analogue of the above tracer, based on ^99mTc-4 + 1 labeling approach, which exhibited significant affinity for NET but suffered from reduced specific uptake in comparison to reference standard no-carrier-added (n.c.a.) [¹²⁵I]mIBG. The present work attempts to synthesize two new ^99mTc-analogues of the radio-iodinated derivative following [^99mTc]Tc(CO)₃¹⁺ approach with an aim to improve the above specific uptake content.

METHODS

Two different precursors, xylylenediamine and 1,3-bis(chloromethyl)benzene, were synthetically modified to yield meta-functionalized benzylguanidine derivatives bearing iminodiacetate (IDA) and aminoethylglycine (AEG) tridentate chelating moieties, respectively. These ligands were labeled with technetium-99m via [^99mTc][Tc(CO)₃(H₂O)₃]⁺ synthon to form desired radioactive complexes 9 and 10. The radiolabeling yields of the complexes obtained were >90% as confirmed by radio-HPLC. The HPLC purified complexes were used for in vitro and in vivo evaluation to understand the true biological efficacy. Structural characterization of the radiolabeled complexes was carried after synthesizing and characterizing their Re-analogues.

RESULTS

Cell uptake studies with the radiolabeled complexes in SK-N-SH neuroblastoma cell lines revealed reduced uptake in the cells (<1% of incubated radioactivity/10⁶ cells) in comparison to n.c.a. [¹²⁵I]mIBG (~12%). However, limited specificity (~60%) was observed for the complexes as ascertained through desmethylimipramine (DMI) inhibition. Biodistribution studies in normal Wistar rats exhibited desired non-target clearance pharmacokinetics for the complexes but in vivo NET efficacy in myocardium for the neutral complex 10 could not be established.

CONCLUSIONS

Tridentate [^99mTc]Tc(CO)₃¹⁺ chelation approach severely affects biological behavior of the present small bioactive molecule under study to a significant extent in comparison to monodentate ligation in ^99mTc-4 + 1 strategy.

Novel 99mTc labelled complexes with 2-nitroimidazole isocyanide: design, synthesis and evaluation as potential tumor hypoxia imaging agents

Radiolabelled 2-nitroimidazoles have been used for imaging hypoxia. With the aim of developing novel 99mTc radiotracers for imaging hypoxia, four novel 2-nitroimidazole isocyanide derivatives (2a, 2b, 2c, and 2d) were synthesized and radiolabelling was carried out for preparing their corresponding 99mTc complexes. These 99mTc complexes were stable in vitro and could exhibit good hypoxic selectivity. The partition coefficient results indicated that they were hydrophilic, and an evaluation of biodistribution in mice bearing S180 tumors indicated that all of the complexes could accumulate in the tumors. Among them, 99mTc-2c exhibited the highest tumor uptake and tumor/blood and tumor/muscle ratios at 2 h post-injection. Further, single photon emission computed tomography (SPECT) imaging studies indicated clear accumulation in tumors, suggesting that 99mTc-2c was a promising candidate for hypoxia imaging.

Synthesis and Biological Evaluation of a New Nitroimidazole-99mTc-Complex for Imaging of Hypoxia in Mice Model

BACKGROUND This study was specifically designed to develop a new 99mTc compound with 3-amino-4-[2-(2-methyl-5-nitro-1H-imidazol)-ethylamino]-4-oxo-butyrate (5-ntm-asp) and to verify whether this compound is feasible to be a radiopharmaceutical for hypoxic tumors. MATERIAL AND METHODS Metronidazole derivative 5-ntm-asp was synthesized and then radio-labeled by Na [99mTcO4], forming 99mTc-5-ntm-asp. Another two complexes of 99mTc-2- and 99mTc-5-nitroimidazole-iminodiacetic acid (99mTc-2-ntm-IDA and 99mTc-5-ntm-IDA) were also synthesized based on previous studies. Physicochemical properties (stability, lipophilicity, protein binding) of the compounds were compared, and we also assessed the accumulation status of the compounds within A549 cells under both hypoxic and aerobic conditions. Distribution of the complex was also studied in vivo using BALB/c nude mice that were injected with A549 cells. RESULTS Compared with 99mTc-2-ntm-IDA and 99mTc-5-ntm-IDA, 99mTc-5-ntm-asp was more stable in both phosphate-buffered saline (PBS) buffer and human plasma (P<0.05). Besides that, 99mTc-5-ntm-asp offered lower lipophilicity and protein-binding rate than the two complexes (P<0.05). During assessment of hypoxic uptake status and high hypoxic/aerobic ratio in mice injected with A549 cells, 99mTc-5-ntm-asp exhibited a more favorable profile than 9mTc-2-ntm-IDA and 99mTc-5-ntm-IDA, including uptake ratio of tumor/blood and uptake ratio of tumor/muscle. CONCLUSIONS With overall consideration of physicochemical properties and biological uptake behavior, it is feasible to use 99mTc-5-ntm-asp as an imaging agent for tumor hypoxia.

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.

[Preparation of (99m)Tc-EDTA-MN and Its Bioimaging in Mouse]

BACKGROUND AND OBJECTIVE

Hypoxia is an important biological characteristics of solid tumor, it is not sensitive to radiotherapy and chemotherapy for which is the presence of hypoxic cell, thus increasing their resistance to conventional radiotherapy and chemotherapy, therefore, the detection of hypoxia degree of tumor tissue is of great significance. The hypoxia imaging of nuclear medicine can reflect the degree of tissue hypoxia, which can selectively retained on the hypoxic cells or tissues, including nitroimidazole and non nitroimidazole; the nitroimidazole is widely and deeply researched as hypoxic celles developer in China and abroad at present. The research about application of radionuclide labelled technique has clinical application value to develop the hypoxia imaging agent EDTA-MN complexes which was labeled. To study the feasibility of (99m)Tc by direct labeling method, the radiochemical properties evaluation of (99m)Tc-EDTA-MN, and observe the distribution characteristics of (99m)Tc radiolabeled EDTA-MN in the xenograft lung cancer nude mice bearing non-small cell lung cancer cell (A549), and provide experimental evidence for its further research and application.

METHODS

The radiolabeling of EDTA-MN with (99m)Tc was performed with direct labeling method, respectively, on the reaction dosage (10 mg, 5 mg, 2 mg), stannous chloride dosage (8 mg/mL, 4 mg/mL, 2 mg/mL), mark system pH (2, 4, 5, 6) one by one test, using orthogonal design analysis, to find the optimal labeling conditions. Labelling rate, radiochemical purity, lipid-water partition coefficient and in vitro stability in normal saline (NS) were determined by TLC and HPLC, and the preliminary study on the distribution of (99m)Tc-EDTA-MN in nude mice.

RESULTS

The labeling rate of 99mTc-EDTA-MN with the best labeling conditions was (84.11±2.83)%, and the radiochemical purity was higher than 90% by HPLC purification, without any notable decomposition at room temperature over a period of 12 h. The partition coefficient was lgP=-3.05, indicated that this complex was hydrophilic. At 3 h post-injection, the imaging of (99m)Tc-EDTA-MN in nude mice bearing non-small cell lung cancer cell showed that more radioactive gathered in bladder at 0.5 h, the transplanted tumor was clearly imaged at 1 h post-injection, during whole imaging radioactive in other tissues and organs was low. The radioactivity of tumor uptake by using of ROI technology were (88.14±11.59), (123.17±9.06), (98.08±14.40) and (79.87±10.57) at 0.5, 1, 2, 3 h post-injection, and the ratio of T/NT of tumor and liver area were (1.95±0.19), (3.58±0.78), (3.95±0.39) and (5.01±0.28), respectively. (99m)Tc-EDTA-MN could be quickly cleared from the blood in mice primarily through the kidneys, and the radioactivity in other tissues and organs remained low.

CONCLUSIONS

(99m)Tc-EDTA-MN can be easily prepared and labeled compound with high labeling rate and stability, it appears to be suitable for further experiments requirement in vivo and in vitro application.

Synthesis and biodistribution of novel 99mTc labeled 4-nitroimidazole dithiocarbamate complexes as potential agents to target tumor hypoxia

^99mTcO-N4IPDTC was prepared from a kit without the need for purification and would be a promising hypoxia imaging agent.