Synthesis and preliminary biological evaluation of 99mTc(CO)3-labeled pegylated 2-nitroimidazoles

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.

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, 99m Tc-labeling, and preliminary biological evaluation of DTPA-melphalan conjugates

Melphalan (MFL) is a typical nitrogen mustard for the treatment of many types of cancer. For the purpose to develop novel ^99m Tc-labeled tumor imaging agents with SPECT, MFL was directly labeled by ^99m Tc using diethylene triamine pentacetate acid (DTPA) as bifunctional chelating agent. The novel ligands were successfully synthesized by conjugation of DTPA to MFL to get monosubstituted DTPA-MFL and bis-substituted DTPA-2MFL. Radiolabeling was performed in high yield to get ^99m Tc-DTPA-MFL and ^99m Tc-DTPA-2MFL, respectively, which were hydrophilic and stable at room temperature. The high initial tumor uptake with retention, good tumor/muscle ratios, and satisfactory scintigraphic images suggested the potential of ^99m Tc-DTPA-MFL and ^99m Tc-DTPA-2MFL for tumor imaging. However, the slow normal tissue clearance would be a great obstacle. Further modification on the linker and/or ^99m Tc-chelate to improve the tumor targeting efficacy and in vivo kinetic profiles is currently in progress.

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.