Synthesis of novel PEG-modified nitroimidazole derivatives via “hot-click” reaction and their biological evaluation as potential PET imaging agent for tumors

68Ga/177Lu-Labeled Bivalent Agents for Targeting Hypoxia and PSMA-Binding in Prostate Cancer

Prostate-specific membrane antigen (PSMA) is an excellent target for cancer detection and therapy. Hypoxia is prevalent in solid tumors, and various nitroimidazole (NI) radioligands can be trapped inside hypoxic cells for diagnosis and therapy. To enhance tumor uptake and retention, we designed bivalent agents (compounds 1-8) incorporating a hypoxia-sensitive NI-moiety and a PSMA-targeting group. Ligands 1-8 were successfully prepared and labeled with 68Ga or 177Lu. Among them, [68Ga]Ga-8 ([68Ga]Ga-AAZTA-NI-PSMA-093) demonstrated significantly higher cellular accumulation under hypoxic conditions than under normoxic conditions, suggesting hypoxia-selective trapping by the introduction of NI group. PET/CT imaging at 60 min postinjection of [68Ga]Ga-8 revealed high tumor uptake (SUVmax: 10.68%ID/mL) in the tumor-bearing mice model. SPECT/CT imaging of [177Lu]Lu-8 at 24 and 48 h postinjection demonstrated excellent accumulation and retention. Preliminary studies indicate that [68Ga]Ga/[177Lu]Lu-8 may be promising bivalent agents targeting hypoxia and PSMA binding for diagnosis and radiotherapy.

Hetero-Diels-Alder and CuAAC Click Reactions for Fluorine-18 Labeling of Peptides: Automation and Comparative Study of the Two Methods

Radiolabeled peptides are valuable tools for diagnosis or therapies; they are often radiofluorinated using an indirect approach based on an F-18 prosthetic group. Herein, we are reporting our results on the F-18 radiolabeling of three peptides using two different methods based on click reactions. The first one used the well-known CuAAC reaction, and the second one is based on our recently reported hetero-Diels-Alder (HDA) using a dithioesters (thia-Diels-Alder) reaction. Both methods have been automated, and the 18F-peptides were obtained in similar yields and synthesis time (37-39% decay corrected yields by both methods in 120-140 min). However, to obtain similar yields, the CuAAC needs a large amount of copper along with many additives, while the HDA is a catalyst and metal-free reaction necessitating only an appropriate ratio of water/ethanol. The HDA can therefore be considered as a minimalist method offering easy access to fluorine-18 labeled peptides and making it a valuable additional tool for the indirect and site-specific labeling of peptides or biomolecules.

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.

Improving tumor/muscle and tumor/blood ratios of 99mTc-labeled nitroimidazole propylene amine oxime (PnAO) complexes with ethylene glycol linkers

Three nitroimidazole propylene amine oxime (PnAO) derivatives with different lengths of ethylene glycol chain were synthesized and radiolabeled with ^99mTc. The radiochemical purities of three ^99mTc-labeled complexes, oxo[[6,6,12,12-tetramethyl-1,17-bis(2-nitro-1H-imidazol-1-yl)-3,15-dioxa-7,11-diazaheptadecane-5, 13-dione dioximato] (3-)-N,N',N'',N''']-technetium-99m (^99mTc-2P2O1), oxo[[9,9,15,15-tetramethyl-1,23-bis(2-nitro-1H-imidazol-1-yl)-3,6,18,21-tetraoxa-10, 14-diazatricosane-8,16-dione dioximato] (3-)-N,N',N'',N''']-technetium-99m (^99mTc-2P2O2) and oxo[[15,15,21,21-tetramethyl-1,35-bis(2-nitro-1H-imidazol-1-yl)-3,6,9,12,24,27,30,33-octaoxa-16,20-diazapentatriacontane-14,22-dione dioximato] (3-)-N,N',N'',N''']-technetium-99m (^99mTc-2P2O4), were above 90%, and they were all stable both in vitro and in vivo. The hypoxia/normoxia uptake ratios of the three complexes were 2.92 ± 0.61, 2.63 ± 0.64 and 2.29 ± 0.67 in S180 cellular uptake assay (4 h). All of these complexes presented good hypoxia selectivity. The results of biodistribution studies in S180 tumor-bearing mice revealed that the tumor/muscle (T/M) ratios (7.20 ± 2.37, 7.19 ± 1.75, 5.56 ± 1.10) and tumor/blood (T/B) ratios (1.66 ± 0.34, 1.73 ± 0.25, 2.13 ± 0.19) at 4 h of three complexes were significantly higher than those of ^99mTc-2P2 (3.24 ± 0.65, 0.81 ± 0.34) without the ethylene glycol chains. Among them, ^99mTc-2P2O4 had the best T/B ratio. The new complexes have higher tumor/blood and tumor/muscle ratios by adding suitable length of ethylene glycol chain. It is helpful for the design and optimization of hypoxic imaging agents.

Synthesis and Characterisation of a Boron-Rich Symmetric Triazine Bearing a Hypoxia-Targeting Nitroimidazole Moiety

Boron Neutron Capture Therapy (BNCT) is a binary therapy that promises to be suitable in treating many non-curable cancers. To that, the discovery of new boron compounds able to accumulate selectively in the tumour tissue is still required. Hypoxia, a deficiency of oxygen in tumor tissue, is a great challenge in the conventional treatment of cancer, because hypoxic areas are resistant to conventional anticancer treatments. 2-Nitroimidazole derivatives are known to be hypoxia markers due to their enrichment by bioreduction in hypoxic cells. In the present work, 2-nitroimidazole was chosen as the starting point for the synthesis of a new boron-containing compound based on a 1,3,5-triazine skeleton. Two o-carborane moieties were inserted to achieve a high ratio of boron on the molecular weight, exploiting a short PEG spacer to enhance the polarity of the compound and outdistance the active part from the core. The compound showed no toxicity on normal human primary fibroblasts, while it showed noteworthy toxicity in multiple myeloma cells together with a consistent intracellular boron accumulation.

Radioiodinated estradiol dimer for estrogen receptor targeted breast cancer imaging

The aim of this study was to develop a 1-(2-(2-(2-(1,2,3-triazol)ethoxy)ethoxy)ethyl)-5-[^125/131 I]iodo-1,2,3-triazole-diestradiol ([^125/131 I]ITE2), for estrogen receptor (ER)-expressing breast cancer imaging with single-photon emission computed tomography (SPECT). [^125/131 I]ITE2 was prepared in good radiochemical yield (94.4 ± 0.4%) with high radiochemical purity (>99%). [^125/131 I]ITE2 had good stability in vitro and moderate molar activity (0.3 ± 0.2 GBq/µmol). Higher uptake in ER-positive MCF-7 cells than that of ER-negative MDA-MB-231 cells was observed at all time points. Rats biodistribution showed that [¹³¹ I]ITE2 had high uptake in ER-abundant uterine and ovarian (5.7 ± 0.4 and 10.1 ± 1.4%ID/g at 1 hr postinjection) and could be blocked by co-injection of estradiol (2.7 ± 0.1 and 5.5 ± 0.4%ID/g) obviously. In the SPECT/CT imaging study, [¹²⁵ I]ITE2 showed significant higher uptake in MCF-7 tumor (3.1 ± 0.4%ID/g) than that of MDA-MB-231 (0.9 ± 0.1%ID/g). Furthermore, the specific uptake of [¹²⁵ I]ITE2 in ER-positive MCF-7 tumor could be blocked effectively by preadministration of fulvestrant (1.2 ± 0.4%ID/g). A novel radioiodinated dimeric estrogen was designed and synthesized with promising ER targeting ability and specificity. It is worthy of further investigation to validate the advantages of the dimer in ER-positive breast cancer diagnosis.

Rapid screening of nine unradiolabeled candidate compounds as PET brain imaging agents using cassette-wave microdosing and LC-MS/MS

The R&D of PET imaging agents is a complex system engineering, simplifying screening steps and increasing screening efficiency have become popular issues. The purpose of this study is to develop a new screening procedure using cassette-wave microdosing and LC-MS/MS to enhance the screening throughput of unradiolabeled candidate compounds as PET imaging agents. Nine compounds were divided into 3 sets and made into 3 cassettes. Fifteen rats were randomly divided into 3 groups, and every animal received three intravenous bolus injections at three different time points; the doses were at microdose levels. This dosing approach takes advantage of temporal and spatial differences and is likened to an input wave; therefore, this approach was named cassette-wave microdosing. The samples of different brain regions such as the hypothalamus, striatum, hippocampus, cortex, cerebellum and the remainder of the brain were detected by LC-MS/MS analysis. The research potential of the compounds as PET imaging agents is evaluated in terms of brain biodistribution data. The screening method is rapid, highly efficient, reliable and reduces animal usage. Additionally, it can shorten the evaluation process of radiopharmaceuticals and enhance the screening throughput of PET radiopharmaceuticals without the use of radioactive agents.

Ring opening of epoxides with [18F]FeF species to produce [18F]fluorohydrin PET imaging agents

A simple technique for the preparation of [18F]HF has been developed and applied to the generation of an [18F]FeF species for opening sterically hindered epoxides. This method has been successfully employed to prepare four drug-like molecules, including 5-[18F]fluoro-6-hydroxy-cholesterol, a potential adrenal/endocrine PET imaging agent. This easily automated one-pot procedure produces sterically hindered fluorohydrin PET imaging agents in good yields and high molar activities.

In Vivo Ester Hydrolysis as a New Approach in Development of Positron Emission Tomography Tracers for Imaging Hypoxia

Hypoxia is an important biochemical and physiological condition associated with uncontrolled growth of tumor. Measurement of hypoxia in tumor tissue may be useful in characterization of tumor progression and monitoring drug treatment. [¹⁸F]FMISO is the most widely employed radiotracer for imaging of hypoxic tissue with positron emission tomography (PET). However, it showed relatively low uptake in hypoxic tissues, which led to low target-to-background contrast in PET images. To overcome these shortcomings, two novel 2-fluoroproprioic acid esters, nitroimidazole derivatives 2-fluoropropionic acid 2-(2-nitro-imidazol-1-yl)-ethyl ester (FNPFT, [¹⁹F]5) and 2-fluoropropionic acid 2-(2-methyl-5-nitro-imidazol-1-yl)-ethyl ester (FMNPFT, [¹⁹F]8), were prepared and tested. Radiolabeling of [¹⁸F]5 and [¹⁸F]8 was accomplished in 45 min (radiochemical purity >95%, the decay-corrected radiochemical yield of [¹⁸F]5 was 11 ± 2%, and that of [¹⁸F]8 was 13 ± 2%, n = 5). In vitro cell uptake studies using EMT-6 tumor cells showed that both radiotracers [¹⁸F]5 and [¹⁸F]8 displayed significantly higher uptake in hypoxic cells than those under normoxic condition, while 2-[¹⁸F]fluoropropionic acid (2-[¹⁸F]FPA) displayed no difference. Biodistribution studies in mice bearing EMT-6 tumor showed that [¹⁸F]5, [¹⁸F]8, and 2-[¹⁸F]FPA displayed similar tumor and major organ uptakes. Tumor uptake values for all three agents were higher than those of [¹⁸F]FMISO, respectively ( P < 0.05). This is likely due to a rapid in vivo hydrolysis of [¹⁸F]5 and [¹⁸F]8 to their metabolite, 2-[¹⁸F]FPA. Micro PET imaging studies in the same EMT-6 implanted mice tumor model also demonstrated that both [¹⁸F]5 and [¹⁸F]8 displayed similar tumor uptake comparable to that of 2-[¹⁸F]FPA. In conclusion, two new fluorine-18 labeled nitroimidazole derivatives, [¹⁸F]5 and [¹⁸F]8, showed good tumor uptakes in mice bearing EMT-6 tumor. However, in vivo biodistribution results suggested that they were more likely reflect the predominance of in vivo produced metabolite, 2-[¹⁸F]FPA, which may not be related to tumor hypoxic condition.