⁹⁹mTc/Re complexes bearing bisnitroimidazole or mononitroimidazole as potential bioreductive markers for tumor: synthesis, physicochemical characterization and biological evaluation

Radiosynthesis and Bioevaluation of 99mTc-Labeled Isocyanide Ubiquicidin 29-41 Derivatives as Potential Agents for Bacterial Infection Imaging

To develop a novel 99mTc-labeled ubiquicidin 29-41 derivative for bacterial infection single-photon emission computed tomography (SPECT) imaging with improved target-to-nontarget ratio and lower nontarget organ uptake, a series of isocyanide ubiquicidin 29-41 derivatives (CNnUBI 29-41, n = 5-9) with different carbon linkers were designed, synthesized and radiolabeled with the [99mTc]Tc(I)+ core, [99mTc][Tc(I)(CO)3(H2O)3]+ core and [99mTc][Tc(V)N]2+ core. All the complexes are hydrophilic, maintain good stability and specifically bind Staphylococcus aureus in vitro. The biodistribution in mice with bacterial infection and sterile inflammation demonstrated that [99mTc]Tc-CN5UBI 29-41 was able to distinguish bacterial infection from sterile inflammation, which had an improved abscess uptake and a greater target-to-nontarget ratio. SPECT imaging study of [99mTc]Tc-CN5UBI 29-41 in bacterial infection mice showed that there was a clear accumulation in the infection site, suggesting that this radiotracer could be a potential radiotracer for bacterial infection imaging.

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.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

Preparation and Bioevaluation of Novel 99mTc-Labeled Complexes with a 2-Nitroimidazole HYNIC Derivative for Imaging Tumor Hypoxia

To develop novel ^99mTc-labeled single-photon emission computed tomography (SPECT) radiotracers for imaging hypoxia, a novel HYNICNM ligand (6-hydrazinonicotinamide (HYNIC) 2-nitroimidazole derivative) was designed and synthesized. It was radiolabeled with technetium-99m using tricine/trisodium triphenylphosphine-3,3′,3′′-trisulfonate (TPPTS), tricine/sodium triphenylphosphine-3-monosulfonate (TPPMS) and tricine as co-ligands to obtain [^99mTc]Tc-tricine-TPPTS-HYNICNM, [^99mTc]Tc-tricine-TPPMS-HYNICNM, and [^99mTc]Tc-(tricine)₂-HYNICNM, respectively. The three technetium-99m complexes were radiolabeled in one step with a high yield (95%) and had good stability in saline and mouse serum. In vitro cellular uptake results showed that these complexes exhibited good hypoxic selectivity. The partition coefficient indicated that they were good hydrophilic complexes, and [^99mTc]Tc-tricine-TPPTS-HYNICNM displayed the highest hydrophilicity (−3.02 ± 0.08). The biodistribution in mice bearing S180 tumors showed that [^99mTc]Tc-tricine-TPPTS-HYNICNM exhibited higher tumor uptake (1.05 ± 0.27% IA/g); more rapid clearance from the liver, blood, muscle, and other non-target organs; and a higher tumor/non-target ratio, especially for the tumor/liver ratio (1.95), than [^99mTc]Tc-tricine-TPPMS-HYNICNM and [^99mTc]Tc-(tricine)₂-HYNICNM. The results of single-photon emission computed tomography (SPECT) imaging studies of [^99mTc]Tc-tricine-TPPTS-HYNICNM were in accordance with the biodistribution results, which suggested that [^99mTc]Tc-tricine-TPPTS-HYNICNM is a promising agent for imaging tumor hypoxia.

Microenvironment Stimulated Bioresponsive Small Molecule Carriers for Radiopharmaceuticals

The widespread and successful use of radiopharmaceuticals in diagnosis, treatment, and therapeutic monitoring of cancer and other ailments has spawned significant literature. The transition from untargeted to targeted radiopharmaceuticals reflects the various stages of design and development. Targeted radiopharmaceuticals bind to specific biomarkers, get fixed, and highlight the disease site. A new subset of radioprobes, the bioresponsive radiopharmaceuticals, has been developed in recent years. These probes generally benefit from signal enhancement after undergoing molecular changes due to the fluctuations in the environment (pH, redox, or enzymatic activity) at the site of interest. This review presents a comprehensive overview of bioresponsive radioimaging probes covering the basis, application, and scope of development.

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.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

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.

Targeting tumor hypoxia: a third generation 2-nitroimidazole-indocyanine dye-conjugate with improved fluorescent yield

Tumor hypoxia is associated with the rapid proliferation and growth of malignant tumors, and the ability to detect tumor hypoxia is important for predicting tumor response to anti-cancer treatments. We have developed a class of dye-conjugates that are related to indocyanine green (ICG, ) to target tumor hypoxia, based on in vivo infrared fluorescence imaging using nitroimidazole moieties linked to indocyanine fluorescent dyes. We previously reported that linking 2-nitroimidazole to an indocyanine dicarboxylic acid dye derivative () using an ethanolamine linker (ethanolamine-2-nitroimidazole-ICG, ), led to a dye-conjugate that gave promising results for targeting cancer hypoxia in vivo. Structural modification of the dye conjugate replaced the ethanolamine unit with a piperazineacetyl unit and led a second generation dye conjugate, piperzine-2-nitroimidazole-ICG (). This second generation dye-conjugate showed improved targeting of tumor hypoxia when compared with . Based on the hypothesis that molecules with more planar and rigid structures have a higher fluorescence yield, as they could release less absorbed energy through molecular vibration or collision, we have developed a new 2-nitroimidazole ICG conjugate, , with two carbon atoms less in the polyene linker. Dye-conjugate was prepared from our new dye (), and coupled to 2-nitroimidazole using a piperazine linker to produce this third-generation dye-conjugate. Spectral measurements showed that the absorption/emission wavelengths of 657/670 were shifted ∼100 nm from the second-generation hypoxia dye of 755/780 nm. Its fluorescence quantum yield was measured to be 0.467, which is about 5 times higher than that of (0.083). In vivo experiments were conducted with balb/c mice and showed more than twice the average in vivo fluorescence intensity in the tumor beyond two hours post retro-orbital injection as compared with . These initial results suggest that may significantly improve in vivo tumor hypoxia targeting.

Development of (68)Ga-labeled multivalent nitroimidazole derivatives for hypoxia imaging

Radiolabeled nitroimidazole (NI) derivatives have been extensively studied for imaging hypoxia. To increase the hypoxic tissue uptake, we developed (68)Ga-labeled agents based on mono-, bis-, and trisnitroimidazole conjugates with the chelating agent 1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid] (TRAP). All the three agents showed high radiolabeling yields (>96%) and were found to be stable up to 4h in prepared medium at room temperature and in human serum at 37°C. The trivalent agent showed a significant increase in hypoxic to normoxic uptake ratio (p <0.005) according to the in vitro cell uptake experiments. Immunohistochemical analysis confirmed the presence of hypoxia in xenografted CT26 tumor tissue. The trivalent derivative ((68)Ga-3: 0.17±0.04, (68)Ga-4: 0.33±0.04, (68)Ga-5: 0.45±0.09, and (68)Ga-6: 0.47±0.05% ID/g) showed the highest uptake by tumor cells according to the biodistribution studies in CT-26 xenografted mice. All the nitroimidazole derivatives showed significantly higher uptake by tumor cells than the control agent (p <0.05) at 1h post-injection. The trivalent derivative ((68)Ga-3: 0.10±0.06; (68)Ga-4: 0.20±0.06; (68)Ga-5: 0.33±0.08; (68)Ga-6: 0.59±0.09) also showed the highest standard uptake value for tumor cells at 1h post-injection in animal PET studies using CT-26 xenografted mice. In conclusion, we successfully synthesized multivalent (68)Ga-labeled NI derivatives for imaging hypoxia. Among them, the trivalent agent showed the highest tumor uptake in biodistribution and animal PET studies.

In vivo click reaction between Tc-99m-labeled azadibenzocyclooctyne-MAMA and 2-nitroimidazole-azide for tumor hypoxia targeting

The bioactivity of nitroimidazole in Tc-99m-labeled 2-nitroimidazole, a traditional solid tumor hypoxia-imaging agent for single photon emission computed tomography (SPECT), is reduced by the presence of large ligand and metallic radionuclide, exhibiting lower tumor-to-nontumor ratios. In an effort to solve this general problem, a pretargeting strategy based on click chemistry (strain-promoted cyclooctyne-azide cycloaddition) was applied. The functional click synthons were synthesized as pretargeting components: an azide group linked to 2-nitroimidazole (2NIM-Az) serves for tumor hypoxia-targeting and azadibenzocyclooctyne conjugated with monoamine monoamide dithiol ligand (AM) functions as radiolabeling and binding group to azides in vivo. 2NIM-triazole-MAMA was obtained from in vitro click reaction with a reaction rate constant of 0.98M(-1)s(-1). AM and 2NIM-triazole-MAMA were radiolabeled with Tc-99m. The hypoxia-pretargeting biodistribution was studied in Kunming mice bearing S180 tumor; (99m)Tc-AM and (99m)Tc-triazole-2NIM were used as blank control and conventional control. Compared to the control groups, the pretargeting experiment exhibits the best radio-uptake and retention in tumor, with higher tumor-to-muscle and tumor-to-blood ratios (up to 8.55 and 1.44 at 8h post-(99m)Tc-complex-injection, respectively). To some extent, the pretargeting strategy protects the bioactivity of nitroimidazole and therefore provides an innovative approach for the development of tumor hypoxia-SPECT imaging agents.

Design and synthesis of calcium responsive magnetic resonance imaging agent: Its relaxation and luminescence studies

Calcium concentration modulation both inside and outside cell is of considerable interest for nervous system function in normal and pathological conditions. MRI has potential for very high spatial resolution at molecular/cellular level. Design, synthesis and evaluation of Gd-DO3A-AME-NPHE, a calcium responsive MRI contrast agent is presented. The probe is comprised of a Gd(3+)-DO3A core coupled to iminoacetate coordinating groups for calcium induced relaxivity switching. In the absence of Ca(2+) ions, inner sphere water binding to the Gd-DO3A-AME-NPHE is restricted with longitudinal relaxivity, r1 = 4.37 mM(-1) s(-1) at 4.7 T. However, addition of Ca(2+) triggers a marked enhancement in r1 = 6.99 mM(-1) s(-1) at 4.7 T (60% increase). The construct is highly selective for Ca(2+) over competitive metal ions at extracellular concentration. The r1 is modulated by changes in the hydration number (0.2 to 1.05), which was confirmed by luminescence emission lifetimes of the analogous Eu(3+) complex. T1 phantom images establish the capability of complex of visualizing changes in [Ca(2+)] by MRI.