Hydrazinonicotinic acid (HYNIC) – Coordination chemistry and applications in radiopharmaceutical chemistry

A thiourea-bridged 99mTc(CO)3-dipicolylamine-2-nitroimidazole complex for targeting tumor hypoxia: Utilizing metabolizable thiourea-bridge to improve pharmacokinetics

The 2-nitroimidazole based 99mTc-radiopharmaceuticals are widely explored for imaging tumor hypoxia. Radiopharmaceuticals for targeting hypoxia are often lipophilic and therefore, show significant uptake in liver and other vital organs. In this context, lipophilic radiopharmaceuticals with design features enabling faster clearance from liver may be more desirable. A dipicolylamine-NCS bifunctional chelator that could generate a thiourea-bridge up on conjugation to primary amine bearing molecule was used to synthesize a 2-nitroimidazole-dipicolyl amine ligand for radiolabeling with 99mTc(CO)3 core. Corresponding Re(CO)3-analogue was prepared to establish the structure of 2-nitroimidazole-99mTc(CO)3 complex prepared in trace level. The 2-nitroimidazole-99mTc(CO)3 complex showed a hypoxic to normoxic ratio of ~2.5 in CHO cells at 3 h. In vivo, the complex showed accumulation and retention in tumor with high tumor to blood and tumor to muscle ratio. The study demonstrated the utility of metabolizable thiourea-bridge in 2-nitroimidazole-99mTc(CO)3 complex in inducing faster clearance of the radiotracer from liver. The dipicolylamine-NCS bifunctional chelator reported herein can also be used for radiolabeling other class of target specific molecules with 99mTc(CO)3 core.

Enhanced Tumor Targeting of Radiolabeled Mouse/Human Chimeric Anti-Tn Antibody in Losartan-Treated Mice Bearing Tn-Expressing Lung Tumors

Aim: ChiTn, a mouse/human chimeric anti-Tn monoclonal antibody, was radiolabeled with iodine-131 (131I) and technetium-99m (99mTc) to assess its biodistribution and internalization in Tn-expressing (Tn+) and wild-type (Tn-) LL/2 lung cancer cells. Results: Selective accumulation and gradual internalization of ChiTn were observed in Tn+ cells. Biodistribution in mice with both Tn+ or Tn- lung tumors indicated that the uptake of radiolabeled ChiTn within tumors increased over time. Dual-labeling experiments with 99mTc and 131I showed different biodistribution patterns, with 99mTc exhibiting higher values in the liver, spleen, and kidneys, while 131I showed higher uptake in the thyroid and stomach. However, tumor uptake did not significantly differ between Tn+ and Tn- tumors. To improve tumor targeting, Losartan, an antihypertensive drug known to enhance tumor perfusion and drug delivery, was investigated. Biodistribution studies in Losartan-treated mice revealed significantly higher radiolabeled ChiTn uptake in Tn+ tumors. No significant changes were observed in the uptake of the control molecule IgG-HYNIC™99mTc. Conclusions: These findings demonstrate the enhanced tumor targeting of radiolabeled ChiTn in Losartan-treated mice with Tn-expressing lung tumors. They highlight the potential of ChiTn as a theranostic agent for cancer treatment and emphasize the importance of Losartan as an adjunctive treatment to improve tumor perfusion and drug delivery.

[99mTc]Tc-labeled HYNIC conjugated chlorambucil as a tumor targeting Agent: Synthesis, characterization and ex-vivo evaluation

Chlorambucil is an alkylating drug that finds application towards chemotherapy of different types of cancers. In order to explore the possibility of utilization of this drug as an imaging agent for early diagnosis of solid tumors, attempt was made to synthesize a 99mTc complex of chlorambucil and evaluate its potential in tumor bearing small animal model. HYNIC-chlorambucil was synthesized by conjugation of HYNIC with chlorambucil via an ethylenediamine linker. All the intermediates and final product were purified and characterized by standard spectroscopic techniques viz. FT-IR, 1H/13C-NMR as well as by mass spectrometry. HYNIC-chlorambucil conjugate was radiolabeled with [99mTc]Tc and found to be formed with > 95 % radiochemical purity via RP-HPLC studies. The partition coefficient (Log10Po/w) of the synthesized complex was found to be -0.78 ± 0.25 which indicated the moderate hydrophilic nature for the complex. Biological behaviour of [99mTc]Tc-HYNIC-chlorambucil, studied in fibrosarcoma bearing Swiss mice, revealed a tumor uptake of about 4.16 ± 1.52 %IA/g at 30 min post-administration, which declined to 1.91 ± 0.13 % IA/g and 1.42 ± 0.14 %IA/g at 1 h and 2 h post-administration, respectively. A comparison of different [99mTc]Tc-chlorambucil derivatives (reported in the contemporary literature) formulated using different methodologies revealed that tumor uptake and pharmacokinetics exhibited by these agents strongly depend on the lipophilicity/hydrophilicity of such agents, which in turn is dependent on the bifunctional chelators used for formulating the radiolabeled chlorambucils.

Preparation, Characterization, and Radiolabeling of Anti-HER2 scFv With Technetium Tricarbonyl and Stability Studies

Breast cancer is the most common diagnosed cancer, and the second cause of cancer death among women, worldwide. HER2 overexpression occurred in approximately 15% to 20% of breast cancers. Invasive biopsy method has been used for detection of HER2 overexpression. HER2-targeted imaging via an appropriate radionuclide is a promising method for sensitive and accurate identification of HER2+ primary and metastatic lesions. 99mTc-anti-HER2 scFv can specifically target malignancies and be used for diagnosis of the cancer type and metastasis as well as treatment of breast cancer. We radiolabeled anti-HER2 scFv that was expressed in Escherichia coli and purified through Ni-NTA resin under native condition with 99mTc-tricarbonyl formed from boranocarbonate. HER2-based ELISA, BCA, TLC, and HPLC were used in this study. In the current study, anti-HER2 scFv was lyophilized before radiolabeling. It was found that freeze-drying did not change the binding activity of anti-HER2 scFv to HER2. Results demonstrated direct anti-HER2 scFv radiolabeling by 99mTc-tricarbonyl to hexahistidine sequence (His-tag) without any changes in biological activity and radiochemical purity of around 98%. Stability analysis revealed that 99mTc-anti-HER2 scFv is stable for at least 24 h in PBS buffer, normal saline, human plasma proteins, and histidine solution.

Design of a New 99mTc-radiolabeled Cyclo-peptide as Promising Molecular Imaging Agent of CXCR4 Receptor: Molecular Docking, Synthesis, Radiolabeling, and Biological Evaluation

INTRODUCTION

C-X-C Chemokine receptor type 4 (CXCR4) is often overexpressed or overactivated in different types and stages of cancer disease. Therefore, it is considered a promising target for imaging and early detection of primary tumors and metastasis. In the present research, a new cyclo-peptide radiolabelled with 99mTc, 99mTc-Cyclo [D-Phe-D-Tyr-Lys (HYNIC)- D-Arg-2-Nal-Gly-Lys(iPr)], was designed based on the parental LY251029 peptide, as a potential in vivo imaging agent of CXCR4-expressing tumors.

METHODS

The radioligand was successfully prepared using the method of Fmoc solid-phase peptide synthesis and was evaluated in biological assessment. Molecular docking findings revealed high affinity (binding energy of -9.7 kcal/mol) and effective interaction of Cyclo [D-Phe- D-Tyr-Lys (HYNIC)-D-Arg-2-Nal-Gly-Lys(iPr)] in the binding pocket of CXCR4 receptor (PDB code: 3OE0) as well.

RESULT

The synthesized peptide and its purity were assessed by both reversed-phase high-performance liquid chromatography (RP-HPLC) and mass spectroscopy. High stability (95%, n = 3) in human serum and favorable affinity (Kd = 28.70 ± 13.56 nM and Bmax = 1.896 ± 0.123 fmol/mg protein) in the B16-F10 cell line resulted. Biodistribution evaluation findings and planar image interpretation of mice both showed high affinity and selectivity of the radiotracer to the CXCR4 receptors.

CONCLUSION

Therefore, the findings indicate this designed radioligand could be used as a potential SPECT imaging agent in highly proliferated CXCR4 receptor tumors.

Development and Characterization of 99mTc-scFvD2B as a Potential Radiopharmaceutical for SPECT Imaging of Prostate Cancer

Previously, we demonstrated that the 177Lu-labeled single-chain variable fragment of an anti-prostate-specific membrane antigen (PSMA) IgG D2B antibody (scFvD2B) showed higher prostate cancer (PCa) cell uptake and tumor radiation doses compared to 177Lu-labeled Glu-ureide-based PSMA inhibitory peptides. To obtain a 99mTc-/177Lu-scFvD2B theranostic pair, this research aimed to synthesize and biochemically characterize a novel 99mTc-scFvD2B radiotracer. The scFvD2B-Tag and scFvD2B antibody fragments were produced and purified. Then, two HYNIC derivatives, HYNIC-Gly-Gly-Cys-NH2 (HYNIC-GGC) and succinimidyl-HYNIC (S-HYNIC), were used to conjugate the scFvD2B-Tag and scFvD2B isoforms, respectively. Subsequently, chemical characterization, immunoreactivity tests (affinity and specificity), radiochemical purity tests, stability tests in human serum, cellular uptake and internalization in LNCaP(+), PC3-PIP(++) or PC3(-) PCa cells of the resulting unlabeled HYNIC-scFvD2B conjugates (HscFv) and 99mTc-HscFv agents were performed. The results showed that incorporating HYNIC as a chelator did not affect the affinity, specificity or stability of scFvD2B. After purification, the radiochemical purity of 99mTc-HscFv radiotracers was greater than 95%. A two-sample t-test of 99mTc-HscFv1 and 99mTc-HscFv1 uptake in PC3-PIP vs. PC3 showed a p-value < 0.001, indicating that the PSMA receptor interaction of 99mTc-HscFv agents was statistically significantly higher in PSMA-positive cells than in the negative controls. In conclusion, the results of this research warrant further preclinical studies to determine whether the in vivo pharmacokinetics and tumor uptake of 99mTc-HscFv still offer sufficient advantages over HYNIC-conjugated peptides to be considered for SPECT/PSMA imaging.

[99mTc]Tc-PSMA-T4-Novel SPECT Tracer for Metastatic PCa: From Bench to Clinic

Despite significant advances in nuclear medicine for diagnosing and treating prostate cancer (PCa), research into new ligands with increasingly better biological properties is still ongoing. Prostate-specific membrane antigen (PSMA) ligands show great potential as radioisotope carriers for the diagnosis and therapy of patients with metastatic PCa. PSMA is expressed in most types of prostate cancer, and its expression is increased in poorly differentiated, metastatic, and hormone-refractory cancers; therefore, it may be a valuable target for the development of radiopharmaceuticals and radioligands, such as urea PSMA inhibitors, for the precise diagnosis, staging, and treatment of prostate cancer. Four developed PSMA-HYNIC inhibitors for technetium-99m labeling and subsequent diagnosis were subjected to preclinical in vitro and in vivo studies to evaluate and compare their diagnostic properties. Among the studied compounds, the PSMA-T4 (Glu-CO-Lys-L-Trp-4-Amc-HYNIC) inhibitor showed the best biological properties for the diagnosis of PCa metastases. [99mTc]Tc-PSMA-T4 also showed effectiveness in single-photon emission computed tomography (SPECT) studies in humans, and soon, its usefulness will be extensively evaluated in phase 2/3 clinical trials.

Engineered rHDL Nanoparticles as a Suitable Platform for Theranostic Applications

Reconstituted high-density lipoproteins (rHDLs) can transport and specifically release drugs and imaging agents, mediated by the Scavenger Receptor Type B1 (SR-B1) present in a wide variety of tumor cells, providing convenient platforms for developing theranostic systems. Usually, phospholipids or Apo-A1 lipoproteins on the particle surfaces are the motifs used to conjugate molecules for the multifunctional purposes of the rHDL nanoparticles. Cholesterol has been less addressed as a region to bind molecules or functional groups to the rHDL surface. To maximize the efficacy and improve the radiolabeling of rHDL theranostic systems, we synthesized compounds with bifunctional agents covalently linked to cholesterol. This strategy means that the radionuclide was bound to the surface, while the therapeutic agent was encapsulated in the lipophilic core. In this research, HYNIC-S-(CH2)3-S-Cholesterol and DOTA-benzene-p-SC-NH-(CH2)2-NH-Cholesterol derivatives were synthesized to prepare nanoparticles (NPs) of HYNIC-rHDL and DOTA-rHDL, which can subsequently be linked to radionuclides for SPECT/PET imaging or targeted radiotherapy. HYNIC is used to complexing 99mTc and DOTA for labeling molecules with 111, 113mIn, 67, 68Ga, 177Lu, 161Tb, 225Ac, and 64Cu, among others. In vitro studies showed that the NPs of HYNIC-rHDL and DOTA-rHDL maintain specific recognition by SR-B1 and the ability to internalize and release, in the cytosol of cancer cells, the molecules carried in their core. The biodistribution in mice showed a similar behavior between rHDL (without surface modification) and HYNIC-rHDL, while DOTA-rHDL exhibited a different biodistribution pattern due to the significant reduction in the lipophilicity of the modified cholesterol molecule. Both systems demonstrated characteristics for the development of suitable theranostic platforms for personalized cancer treatment.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

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.

CD11b-Based Pre-Targeted SPECT/CT Imaging Allows for the Detection of Inflammation in Aortic Aneurysm

Purpose

To investigate the feasibility of a pre-targeted imaging strategy based on the cycloaddition between 1,2,4,5-terazine (Tz) and trans-cyclooctene (TCO) for evaluating CD11b expression in inflammatory aortic aneurysm (AA) using single photon emission computed tomography/computed tomography (SPECT/CT).

Methods

C57BL/6J mice were fed β-aminopropionitrile (1 g/kg/day) for 4 weeks to establish AA models. Anti-CD11b-TCO was synthesized and ^99mTc-HYNIC-PEG₁₁-Tz was designed for pre-targeted SPECT/CT. The affinity and specificity of the probe for the inflammatory cell line Raw-264.7 were investigated. Then, anti-CD11b-TCO pre-targeted and ^99mTc-HYNIC-PEG₁₁-Tz based SPECT/CT were performed to detect in vivo inflammation in AA. Finally, ex vivo aortic breast-specific gamma imaging (BSGI), Western blot assays, and immunohistochemical CD11b staining were performed to confirm the in vivo findings of SPECT/CT.

Results

In the AA models, 65.22% (15/23) had aortic lesions, including 43.48% (10/23) AA lesions. The anti-CD11b-TCO presented with a high TCO coupling ratio (7.43), and the ^99mTc-HYNIC-PEG₁₁-Tz showed high radio-purity (>95%), good in vitro stability and a rapid clearance rate. Additionally, anti-CD11b-TCO and ^99mTc-HYNIC-PEG₁₁-Tz presented high click rate (~89%). The in vitro clicked compound, ^99mTc-HYNIC-PEG₁₁-Tz/TCO-anti-CD11b, showed high affinity and specificity for Raw-264.7 cells. ^99mTc-HYNIC-PEG₁₁-Tz/TCO-anti-CD11b pre-targeting SPECT/CT successfully demonstrated inflammatory AA with a high AA-to-background ratio in AA mice, compared to AA mice that were injected with ^99mTc-HYNIC-Tz/TCO-IgG (8.13 versus 3.71, P < 0.001) and control mice injected with ^99mTc-HYNIC-Tz/TCO-anti-CD11b (8.13 versus 3.66, P < 0.001). This result was confirmed by ex vivo BSGI performed immediately after SPECT/CT and immunohistochemical CD11b staining.

Conclusion

SPECT/CT imaging using the anti-CD11b-TCO/Tz-PEG₁₁-HYNIC-^99mTc based pre-targeting imaging strategy allows for the detection of inflammation in progressive AA.

One-step, kit-based radiopharmaceuticals for molecular SPECT imaging: a versatile diphosphine chelator for 99mTc radiolabelling of peptides

Radiotracers labelled with technetium-99m (99mTc) enable accessible diagnostic imaging of disease, provided that radiotracer preparation is simple. Whilst 99mTc radiopharmaceuticals for imaging perfusion are routinely prepared from kits, and regularly used in healthcare, there are no 99mTc-labelled receptor-targeted radiopharmaceuticals in widespread clinical use. This is in part due to the multistep radiosyntheses required for the latter. We demonstrate that the diphosphine, 2,3-bis(diphenylphosphino)maleic anhydride (BMA), is an excellent platform for preparation of kit-based, receptor-targeted 99mTc-labelled radiotracers: its conjugates are simple to prepare and can be easily labelled with 99mTc using one-step, kit-based protocols. Here, reaction of BMA with the αvβ3-integrin receptor targeted cyclic peptide, Arg-Gly-Asp-DPhe-Lys (RGD), provided the first diphosphine-peptide conjugate, DP-RGD. DP-RGD was incorporated into a "kit", and addition of a saline solution containing 99mTcO4- to this kit, followed by heating, furnished the radiotracer [99mTcO2(DP-RGD)2]+ in consistently high radiochemical yields (>90%). The analogous [ReO2(DP-RGD)2]+ compound was prepared and characterised, revealing that both [99mTcO2(DP-RGD)2]+ and [ReO2(DP-RGD)2]+ consist of a mixture of cis and trans geometric isomers. Finally, [99mTcO2(DP-RGD)2]+ exhibited high metabolic stability, and selectively targeted αvβ3-integrin receptors, enabling in vivo SPECT imaging of αvβ3-integrin receptor expression in mice.

A Pretargeted Imaging Strategy for Immune Checkpoint Ligand PD-L1 Expression in Tumor Based on Bioorthogonal Diels-Alder Click Chemistry

PURPOSE

The use of antibodies as tracers requires labeling with isotopes with long half-lives due to their slow pharmacokinetics, which creates prohibitively high radiation dose to non-target organs. Pretargeted methodology could avoid the high radiation exposure due to the slow pharmacokinetics of antibodies. In this investigation, we reported the development of a novel pretargeted single photon emission computed tomography (SPECT) imaging strategy (atezolizumab-TCO/[99mTc]HYNIC-PEG11-Tz) for evaluating immune checkpoint ligand PD-L1 expression in tumor based on bioorthogonal Diels-Alder click chemistry.

PROCEDURES

The radioligand [^99mTc]HYNIC-PEG₁₁-Tz was achieved by the synthesis of a 6-hydrazinonicotinc acid (HYNIC) modified 1,2,4,5-tetrazine (Tz) and subsequently radiolabeled with technetium-99m (Tc-99m). The stability of [^99mTc]HYNIC-PEG₁₁-Tz was evaluated in vitro, and its blood pharmacokinetic test was performed in vivo. Atezolizumab was modified with trans-cyclooctene (TCO). The [^99mTc]HYNIC-PEG₁₁-Tz and atezolizumab-TCO interaction was tested in vitro. Pretargeted H1975 cell immunoreactivity binding and saturation binding assays were evaluated. Pretargeted biodistribution and SPECT imaging experiments were performed in H1975 and A549 tumor-bearing modal mice to evaluate the PD-L1 expression level.

RESULTS

[^99mTc]HYNIC-PEG₁₁-Tz was successfully radiosynthesized with a specific activity of 9.25 MBq/μg and a radiochemical purity above 95 % as confirmed by reversed-phase HPLC (RP-HPLC). [^99mTc]HYNIC-PEG₁₁-Tz showed favorable stability in NS, PBS, and FBS and rapid blood clearance in mice. The atezolizumab was modified with TCO-NHS ester to produce a conjugate with an average 6.4 TCO moieties as confirmed by liquid chromatograph-mass spectrometer (LC-MS). Size exclusion HPLC revealed almost complete reaction between atezolizumab-TCO and [^99mTc]HYNIC-PEG₁₁-Tz in vitro, with the 1:1 Tz-to-mAb reaction providing a conversion yield of 88.65 ± 1.22 %. Pretargeted cell immunoreactivity binding and saturation binding assays showed high affinity to H1975 cells. After allowing 48 h for accumulation of atezolizumab-TCO in H1975 tumor, pretargeted in vivo biodistribution revealed high uptake of the radiotracer in the tumor with a tumor-to-muscle ratio of 27.51 and tumor-to-blood ratio of 1.91. Pretargeted SPECT imaging delineated the H1975 tumor clearly. Pretargeted biodistribution and SPECT imaging in control groups demonstrated a significantly reduced tracer accumulation in the A549 tumor.

CONCLUSIONS

We have developed a HYNIC-modified Tz derivative, and the HYNIC-PEG₁₁-Tz was labeled with Tc-99m with a high specific activity and radiochemical purity. [^99mTc]HYNIC-PEG₁₁-Tz reacted rapidly and almost completely towards atezolizumab-TCO in vitro with the 1:1 Tz-to-mAb reaction. SPECT imaging using the pretargeted strategy (atezolizumab-TCO/[^99mTc]HYNIC-PEG₁₁-Tz) demonstrated high-contrast images for high PD-L1 expression H1975 tumor and a low background accumulation of the probe. The pretargeted imaging strategy is a powerful tool for evaluating PD-L1 expression in xenograft mice tumor models and a potential candidate for translational clinical application.

The chemical tool-kit for molecular imaging with radionuclides in the age of targeted and immune therapy

Nuclear medicine has evolved over the last half-century from a functional imaging modality using a handful of radiopharmaceuticals, many of unknown structure and mechanism of action, into a modern speciality that can properly be described as molecular imaging, with a very large number of specific radioactive probes of known structure that image specific molecular processes. The advances of cancer treatment in recent decades towards targeted and immune therapies, combined with recognition of heterogeneity of cancer cell phenotype among patients, within patients and even within tumours, has created a growing need for personalised molecular imaging to support treatment decision. This article describes the evolution of the present vast range of radioactive probes – radiopharmaceuticals – leveraging a wide variety of chemical disciplines, over the last half century. These radiochemical innovations have been inspired by the need to support personalised medicine and also by the parallel development in development of new radionuclide imaging technologies – from gamma scintigraphy, through single photon emission tomography (SPECT), through the rise of clinical positron emission tomography (PET) and PET-CT, and perhaps in the future, by the advent of total body PET. Thus, in the interdisciplinary world of nuclear medicine and molecular imaging, as quickly as radiochemistry solutions are developed to meet new needs in cancer imaging, new challenges emerge as developments in one contributing technology drive innovations in the others.

Review on 99mTc radiopharmaceuticals with emphasis on new advancements

Rapid imaging acquisition, high spatial resolution and sensitivity, powered by advancements in solid-state detector technology, are significantly changing the perspective of single photon emission tomography (SPECT). In particular, this evolutionary step is fueling a rediscovery of technetium-99m, a still unique radionuclide within the nuclear medicine scenario because of its ideal nuclear properties and easy preparation of its radiopharmaceuticals that does not require a costly infrastructure and complex procedures. Scope of this review is to show that the arsenal of technetium-99m radiopharmaceuticals is already equipped with imaging agents that may complement and integrate the role played by analogous tracers developed for positron emission tomography (PET). These include, in particular, somatostatin (SST) and prostate-specific membrane antigen (PSMA) receptor targeting agents, and a number of peptide-derived radiopharmaceuticals. Additionally, these recent technological developments, combined with new myocardial perfusion tracers having more favorable biodistribution and pharmacokinetic properties as compared to current commercial agents, may also reinvigorate the prevailing position still hold by technetium-99m radiopharmaceuticals in nuclear cardiology.

Radiolabeled Peptides and Antibodies in Medicine

Radiolabeled peptides are a relatively new, very specific radiotracer group, which is still expanding. This group is very diverse in terms of peptide size. It contains very small structures containing several amino acids and whole antibodies. Moreover, radiolabeled peptides are diverse in terms of the binding aim and therapeutic or diagnostic applications. The majority of this class of radiotracers is utilized in oncology, where the same structure can be used in therapy and diagnostic imaging by varying the radionuclide. In this study, we collected new reports of radiolabeled peptide applications in diagnosis and therapy in oncology and other fields of medicine. Radiolabeled peptides are also increasingly being used in rheumatology, cardiac imaging, or neurology. The studies collected in this review concern new therapeutic and diagnostic procedures in humans and new structures tested on animals. We also performed an analysis of clinical trials, which concerns application of radiolabeled peptides and antibodies that were reported in the clinicaltrials.gov database between 2008 and 2018.

Study of the 99m Tc-labeling conditions of 6-hydrazinonicotinamide-conjugated peptides from a new perspective: Introduction to the term radio-stoichiometry

Specific tumor uptake of peptide radiopharmaceuticals depends on tumor binding affinity and their radiochemical purity. Several important parameters that influence the 99m Tc-labeling and consequently the radiochemical purity of 6-hydrazinonicotinamide (HYNIC)-conjugated peptide are radionuclide activity, the amount of peptide, the amount of coligands, and the amount of reducing agents (stannous ion). In this review article, we have attempted studying these parameters in the HYNIC-conjugated peptides (somatostatin, cholecystokinin/gastrin, bombesin, and RGD analogs) from a new perspective to obtain most used and optimized radio-stoichiometric relationships. One of the most important results in this review is that for 99m Tc-labeling of HYNIC-conjugated peptides, it is better to consider the most calculated mole ratio between technetium-99m and the peptide (mole ratio of technetium-99m to the peptide 1:200-400). The statistical results also show that among these 99m Tc-labeled peptides, the most used and favorable coligand is tricine/EDDA with two to one (2:1) mole ratio. These optimized radio-stoichiometric relationships, favorable coligand mole ratio, and applicable radiolabeling points can greatly improve the labeling process of the HYNIC-conjugated peptides, by reducing trial and error, increasing specific activity, and saving materials.

Bioconjugates of Chelators with Peptides and Proteins in Nuclear Medicine: Historical Importance, Current Innovations, and Future Challenges

Molecular radiopharmaceuticals based on bioconjugates of chelators with peptides and proteins have had significant clinical impact in the diagnosis and treatment of several types of cancers. In the 1990s, indium-111 and yttrium-90 labeled chelator-peptide/protein conjugates established the clinical utility of these radiopharmaceuticals for receptor-targeted γ-scintigraphy imaging and systemic radiotherapy. Second-generation bioconjugates based on peptides targeting the somatostatin II receptor and the prostate-specific membrane antigen are now widely used for management of neuroendocrine and prostate cancer, respectively. These bioconjugates are typically radiolabeled with gallium-68 for imaging of target receptor expression with positron emission tomography, and the β--emitter, lutetium-177, for targeted radiotherapy. Innovations in radioisotope technology and biomolecular therapies are likely to drive the future clinical development of radiopharmaceuticals based on radiometals. New chelator-peptide and chelator-protein bioconjugates will underpin nuclear medicine advances in molecular imaging and radiotherapy.

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.

99mTc-HYNIC-(Ser)3-LTVPWY peptide bearing tricine as co-ligand for targeting and imaging of HER2 overexpression tumor

Human epidermal growth factor receptor 2 (HER2) is overexpressed in several cancers. Today’s tumor targeting is receiving more attention due to its specificity to target receptor-dependent cancers. The aim of this study was to evaluate the 99mTc-HYNIC-(tricine)-(Ser)3-LTVPWY peptide for tumor targeting and imaging with overexpression of HER2. HYNIC-(Ser)3-LTVPWY peptide was labeled with 99mTc using tricine as a co-ligand at room temperature. Specific binding of this radiolabeled peptide was assessed on four cancer cell lines with different levels of HER2 receptor expression. Also the affinity of 99mTc-HYNIC-(tricine)-(Ser)3-LTVPWY peptide to the HER2 receptor was evaluated in the SKOV-3 cell line. Biodistribution study of this radiolabeled peptide was performed in SKOV-3 tumor bearing nude mice. The HYNIC conjugated peptide was simply labeled with 99mTc radionuclide with high labeling efficiency about 98±1% showing favorable stability in normal saline and human serum. In the presence of unlabeled peptide as competitor, the HER2 binding capacity of the radiolabeled peptide reduced (approximately five-fold). The KD and Bmax values were found 2.6±0.5 nM and (2.6±0.1)×106, respectively. The tumor/muscle ratios for this radiotracer were determined 1.17±0.77, 1.15±0.32 and 2.65±0.32 at 1, 2 and 4 h after injection, respectively. Presaturation of HER2 receptors in SKOV-3 xenografted nude mice showed a reduction in the tumor/muscle ratio confirming in vivo specificity of the peptide. According to SPECT imaging, the tumor was visualized in mouse after 4 h postinjection of radiolabeled peptide. 99mTc-HYNIC-(tricine)-(Ser)3-LTVPWY peptide exhibited overexpressed HER2 tumor targeting.

Synthesis of hydrophilic HYNIC-[1,2,4,5]tetrazine conjugates and their use in antibody pretargeting with99mTc

Pretargeted imaging is shown to be an attractive strategy to overcome disadvantages associated with traditional radioimmunoconjugates.

Preparation of Tc99m-Labeled Pseudomonas Bacteriophage without Adversely Impacting Infectivity or Biodistribution

Bacteriophages (phages) are ubiquitous viruses which have adapted to infect and replicate within target bacteria, their only known hosts, in a strain specific fashion with minimal cross infectivity. The recent steep rise in antibiotic resistance throughout the world has renewed interest in adapting phages for the imaging and treatment of bacterial infection in humans. In this article, we describe the current limitations surrounding the radiolabeling of phage for the imaging and treatment of bacterial infection and methods to overcome these difficulties. Specifically, we examined the effects of hydrazinonicotinamide conjugation and removal of bacterial DNA on the infectivity, biodistribution, and radionuclide imaging of a phage lytic for a clinically relevant strain of Pseudomonas aeruginosa, a common Gram-negative bacterial pathogen often resistant to multiple antibiotics. We found that all but the briefest reaction of concentrated phage with hydrazinonicotinamide (≤3 min) resulted in nearly complete loss of infectivity. Furthermore, we determined that digestion and removal of bacterial DNA was needed to avoid high nonspecific uptake of hydrazinonicotinamide-labeled phage within the liver and spleen as well as prolonged circulation in the blood. We also demonstrate the surprisingly wide soft tissue and organ biodistribution and rapid pharmacokinetics of ^99mTc-hydrazinonicotinamide-labeled phage in normal mice as well as its imaging characteristics and efficacy in wounded mice infected with bioluminescent Pseudomonas aeruginosa. In conclusion, the preservation of phage infectivity and removal of all bacterial containments including DNA are critical methodologic considerations in the labeling of phages for imaging and therapy.

Rhenium and Technetium-oxo Complexes with Thioamide Derivatives of Pyridylhydrazine Bifunctional Chelators Conjugated to the Tumour Targeting Peptides Octreotate and Cyclic-RGDfK

This research aimed to develop new tumor targeted theranostic agents taking advantage of the similarities in coordination chemistry between technetium and rhenium. A γ-emitting radioactive isotope of technetium is commonly used in diagnostic imaging, and there are two β- emitting radioactive isotopes of rhenium that have the potential to be of use in radiotherapy. Variants of the 6-hydrazinonicotinamide (HYNIC) bifunctional ligands have been prepared by appending thioamide functional groups to 6-hydrazinonicotinamide to form pyridylthiosemicarbazide ligands (SHYNIC). The new bidentate ligands were conjugated to the tumor targeting peptides Tyr3-octreotate and cyclic-RGD. The new ligands and conjugates were used to prepare well-defined {M═O}3+ complexes (where M = 99mTc or natRe or 188Re) that feature two targeting peptides attached to the single metal ion. These new SHYNIC ligands are capable of forming well-defined rhenium and technetium complexes and offer the possibility of using the 99mTc imaging and 188/186Re therapeutic matched pairs.

Preparation and biological evaluation of 99mTc-HYNIC-(Ser)3-D4 peptide for targeting and imaging of non-small-cell lung cancer

Aim: In this study, radiolabeled D4 peptide conjugate was studied as a radiotracer for imaging of non-small-cell lung cancer with overexpression of EGFR. Methods: HYNIC-(Ser)3-D4 peptide was labeled with 99mTc using tricine as a co-ligand. Cellular specific binding and internalization as well as in vivo tumor targeting were assessed. Results: The in vitro experiments showed good cellular specific binding. Tumor uptake values as %ID/g were 7.55 and 6.82% at 1 and 4 h after injection, respectively. The presaturation of EGFR in xenografted nude mice reduced 36% tumor uptake of radioactivity at 1 h after injection that confirmed in vivo specificity. Conclusion: Findings showed this radiolabeled peptide is a promising candidate for tumor targeting and molecular imaging of non-small-cell lung cancer.

Thiourea derivatives as chelating agents for bioconjugation of rhenium and technetium

A^99mTc complex with a tetradentate thiocarbamoylbenzamidine group was used for the conjugation of angiotensin-II. The resulting bioconjugate is stablein vivoandin vitro.

Radiolabeled Rhein as Small-Molecule Necrosis Avid Agents for Imaging of Necrotic Myocardium

A rapid and accurate identification of necrotic myocardium is of great importance for diagnosis, risk stratification, clinical decision-making, and prognosis evaluation of myocardial infarction. Here, we explored technetium-99m labeled rhein derivatives for rapid imaging of the necrotic myocardium. Three hydrazinonicotinic acid-linker-rhein (HYNIC-linker-rhein) derivatives were synthesized, and then, these synthetic compounds were labeled with technetium-99m using ethylenediaminediacetic acid (EDDA) and tricine as coligands [^99mTc(EDDA)-HYNIC-linker-rhein]. The necrosis avidity of the three ^99mTc-labeled rhein derivatives was tested in a mouse model of ethanol-induced muscular necrosis by gamma counting, histochemical staining, and autoradiography. A lead tracer for visualization of necrotic myocardium was assessed by single photon emission computed tomography/computed tomography (SPECT/CT) imaging in a rat model with reperfused myocardial infarction. The necrosis avidity mechanism of the tracer was explored by DNA binding studies in vitro and blocking experiments in vivo. Results showed that the uptake in necrotic muscles of the three ^99mTc-compounds was higher than that in viable muscles (P < 0.001). Autoradiography and histochemical staining results were consistent with selective uptake of the radiotracer in the necrotic regions. Among the these tracers, ^99mTc(EDDA)-HYNIC-ethylenediamine-rhein [^99mTc(EDDA)-HYNIC-2C-rhein] displayed the best distribution profiles for imaging. The necrotic myocardium lesions were clearly visualized by SPECT/CT using ^99mTc(EDDA)-HYNIC-2C-rhein at 1 h after injection. The necrotic-to-viable myocardium and necrotic myocardium-to-blood uptake ratios of ^99mTc(EDDA)-HYNIC-2C-rhein were 4.79 and 3.02 at 1 h after injection. DNA binding studies suggested HYNIC-linker-rhein bound to DNA through intercalation. The uptake of ^99mTc(EDDA)-HYNIC-2C-rhein in necrotic muscle was significantly blocked by excessive unlabeled rhein, with 77.61% decline at 1 h after coinjection. These findings suggested ^99mTc(EDDA)-HYNIC-2C-rhein emerged as a "hot spot" imaging probe that has a potential for rapid imaging of necrotic myocardium. The necrosis avidity mechanism of ^99mTc(EDDA)-HYNIC-linker-rhein may be due to its interaction with exposed DNA in necrotic tissues.

Synthesis and Evaluation of (99m)Tc-Labeled Dimeric Folic Acid for FR-Targeting

The folate receptor (FR) is overexpressed in a wide variety of human tumors. In our study, the multimeric concept was used to synthesize a dimeric folate derivative via a click reaction. The novel folate derivative (HYNIC-D₁-FA₂) was radiolabeled with ^99mTc using tricine and trisodium triphenylphosphine-3,3′,3″-trisulfonate (TPPTS) as coligands (^99mTc-HYNIC-D₁-FA₂) and its in vitro physicochemical properties, ex vivo biodistribution and in vivo micro-SPECT/CT imaging as a potential FR targeted agent were evaluated. It is a hydrophilic compound (log P = −2.52 ± 0.13) with high binding affinity (IC₅₀ = 19.06 nM). Biodistribution in KB tumor-bearing mice showed that ^99mTc-HYNIC-D₁-FA₂ had high uptake in FR overexpressed tumor and kidney at all time-points, and both of them could obviously be inhibited when blocking with free FA in the blocking studies. From the in vivo micro-SPECT/CT imaging results, good tumor uptake of ^99mTc-HYNIC-D₁-FA₂ was observed in KB tumor-bearing mice and it could be blocked obviously. Based on the results, this new radiolabeled dimeric FA tracer might be a promising candidate for FR-targeting imaging with high affinity and selectivity.

Synthesis and antimicrobial evaluation of two peptide LyeTx I derivatives modified with the chelating agent HYNIC for radiolabeling with technetium-99m

Background

Current diagnostic methods and imaging techniques are not able to differentiate septic and aseptic inflammation. Thus, reliable methods are sought to provide this distinction and scintigraphic imaging is an interesting option, since it is based on physiological changes. In this context, radiolabeled antimicrobial peptides have been investigated as they accumulate in infectious sites instead of aseptic inflammation. The peptide LyeTx I, from the venom of Lycosa erythrognatha, has potent antimicrobial activity. Therefore, this study aimed to synthesize LyeTx I derivatives with the chelating compound HYNIC, to evaluate their antimicrobial activity and to radiolabel them with ^99mTc.

Methods

Two LyeTx I derivatives, HYNIC-LyeTx I (N-terminal modification) and LyeTx I-K-HYNIC (C-terminal modification), were synthesized by Fmoc strategy and purified by RP-HPLC. The purified products were assessed by RP-HPLC and MALDI-ToF-MS analysis. Microbiological assays were performed against S. aureus (ATCC® 6538) and E. coli (ATCC® 10536) in liquid medium to calculate the MIC. The radiolabeling procedure of LyeTx I-K-HYNIC with ^99mTc was performed in the presence of co-ligands (tricine and EDDA) and reducing agent (SnCl₂^.2H₂O), and standardized taking into account the amount of peptide, reducing agent, pH and heating. Radiochemical purity analysis was performed by thin-layer chromatography on silica gel strips and the radiolabeled compound was assessed by RP-HPLC and radioactivity measurement of the collected fractions. Data were analyzed by ANOVA, followed by Tukey test (p-values < 0.05).

Results

Both LyeTx I derivatives were suitably synthesized and purified, as shown by RP-HPLC and MALDI-ToF-MS analysis. The microbiological test showed that HYNIC-LyeTx I (N-terminal modification) did not inhibit bacterial growth, whereas LyeTx I-K-HYNIC (C-terminal modification) showed a MIC of 5.05 μmol^.L⁻¹ (S. aureus) and 10.10 μmol^.L⁻¹ (E. coli). Thus, only the latter was radiolabeled with ^99mTc. The radiochemical purity analysis of LyeTx I-K-HYNIC-^99mTc showed that the optimal radiolabeling conditions (10 μg of LyeTx I-K-HYNIC; 250 μg of SnCl₂^.2H₂O; pH = 7; heating for 15 min) yielded a radiochemical purity of 87 ± 1 % (n = 3). However, RP-HPLC data suggested ^99mTc transchelation from LyeTx I-K-HYNIC to the co-ligands (tricine and EDDA).

Conclusions

The binding of HYNIC to the N-terminal portion of LyeTx I seems to affect its activity against bacteria. Nevertheless, the radiolabeling of the C-terminal derivative, LyeTx I-K-HYNIC, must be better investigated to optimize the radiolabeled compound, in order to use it as a specific imaging agent to distinguish septic and aseptic inflammation.

(99m)Tc-bioorthogonal click chemistry reagent for in vivo pretargeted imaging

Metal-free click chemistry has become an important tool for pretargeted approaches in the molecular imaging field. The application of bioorthogonal click chemistry between a pretargeted trans-cyclooctene (TCO) derivatized monoclonal antibody (mAb) and a (99m)Tc-modified 1,2,4,5-tetrazine for tumor imaging was examined in vitro and in vivo. The HYNIC tetrazine compound was synthesized and structurally characterized, confirming its identity. Radiolabeling studies demonstrated that the HYNIC tetrazine was labeled with (99m)Tc at an efficiency of >95% and was radiochemically stable. (99m)Tc-HYNIC tetrazine reacted with the TCO-CC49 mAb in vitro demonstrating its selective reactivity. In vivo biodistribution studies revealed non-specific liver and GI uptake due to the hydrophobic property of the compound, however pretargeted SPECT imaging studies demonstrated tumor visualization confirming the success of the cycloaddition reaction in vivo. These results demonstrated the potential of (99m)Tc-HYNIC-tetrazine for tumor imaging with pretargeted mAbs.

Opportunities and challenges for metal chemistry in molecular imaging: from gamma camera imaging to PET and multimodality imaging

The development of medical imaging is a highly multidisciplinary endeavor requiring the close cooperation of clinicians, physicists, engineers, biologists and chemists to identify capabilities, conceive challenges and solutions and apply them in the clinic. The chemistry described in this article illustrates how synergistic advances in these areas drive the technology and its applications forward, with each discipline producing innovations that in turn drive innovations in the others. The main thread running through the article is the shift from single photon radionuclide imaging towards PET, and in turn the emerging shift from PET/CT towards PET/MRI and further, combination of these with optical imaging. Chemistry to support these transitions is exemplified by building on a summary of the status quo, and recent developments, in technetium-99m chemistry for SPECT imaging, followed by a report of recent developments to support clinical application of short lived (Ga-68) and long-lived (Zr-89) positron emitting isotopes, copper isotopes for PET imaging, and combined modality imaging agents based on radiolabelled iron oxide based nanoparticles.

A nuclear chocolate box: the periodic table of nuclear medicine

Radioisotopes of elements from all parts of the periodic table find both clinical and research applications in radionuclide molecular imaging and therapy (nuclear medicine). This article provides an overview of these applications in relation to both the radiological properties of the radionuclides and the chemical properties of the elements, indicating past successes, current applications and future opportunities and challenges for inorganic chemistry.

Syntheses and biological evaluation of99mTc-HYNIC-fatty acid complexes for myocardial imaging

The aim of the present study is to identify a^99mTc-labeled fatty acid tracer which could be a possible substitute of the widely used¹²³I-labeled fatty acids in studying myocardial metabolism and in detection of myocardial abnormalities.

Evaluation of K(HYNIC)(2) as a bifunctional chelator for (99m)Tc-labeling of small biomolecules

This study sought to evaluate K(HYNIC)(2) (K = lysine and HYNIC = 6-hydrazinonicotinyl) as a bifunctional chelator for (99m)Tc-labeling of biomolecule. In this study, four K(HYNIC)(2)-conjugated cyclic RGD peptides, K(HYNIC)(2)-RGD(2) (RGD(2) = E[c(RGDfK)](2)), K(HYNIC)(2)-3G-RGD(2) (3G-RGD(2) = Gly-Gly-Gly-E[Gly-Gly-Gly-c(RGDfK)](2)), K(HYNIC)(2)-2P-RGD(2) (2P-RGD(2) = E[PEG4-c(RGDfK)](2), and PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid), and K(HYNIC)(2)-3P-RGD(2) (3P-RGD(2) = PEG4-E[PEG4-c(RGDfK)]2) were prepared, and evaluated for their integrin αvβ3 binding affinity. IC(50) values were determined to be 47 ± 2, 35 ± 2, 37 ± 2, 85 ± 2, and 422 ± 15 nM for K(HYNIC)(2)-2P-RGD(2), K(HYNIC)(2)-3P-RGD(2), K(HYNIC)(2)-3G-RGD(2), K(HYNIC)(2)-RGD(2), and c(RGDyK), respectively, against (125)I-echistatin bound to U87MG cells. Macrocyclic complexes [(99m)Tc(K(HYNIC)(2)-RGD(2))(tricine)] (1), [(99m)Tc(K(HYNIC)(2)-3G-RGD(2))(tricine)] (2), [(99m)Tc(K(HYNIC)(2)-2P-RGD(2))(tricine)] (3), and [(99m)Tc(K(HYNIC)(2)-3P-RGD(2))(tricine)] (4) were prepared, and evaluated in athymic nude mice bearing U87MG glioma xenografts for their tumor targeting capability and biodistribution. It was found that 1-4 all had high solution stability and more than two isomers, as evidenced by the presence of multiple radiometric peaks in their radio-HPLC chromatograms. The tumor uptake of 1-4 was 3.78 ± 0.81, 7.46 ± 1.68, 9.74 ± 1.65, and 8.59 ± 1.52%ID/g, respectively, which was completely consistent with trend of integrin α(v)β(3) binding affinity for cyclic RGD peptides. Replacing [(99m)Tc(HYNIC)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3"-trisulfonate) with [(99m)Tc(K(HYNIC)(2))(tricine)] had little impact on radiotracer tumor uptake; but it had significant effect on the uptake of radiotracer in kidneys, lungs, and spleen. The tumor was clearly visualized by SPECT/CT with excellent contrast in a glioma-bearing mouse administered with 4. K(HYNIC)(2) would be particularly useful for (99m)Tc-labeling of small biomolecules with one or more disulfide linkages.

The hydrazide/hydrazone click reaction as a biomolecule labeling strategy for M(CO)3 (M = Re, (99m)Tc) radiopharmaceuticals

Facile reactivity of hydrazides and aldehydes was explored as potential coupling partners for incorporation into M(CO)(3) (M = Re, (99m)Tc) based radiopharmaceuticals. Both 'click, then chelate' and 'prelabel, then click' synthetic routes produced identical products in high yields and lacked metal-hydrazide/-hydrazone interactions, highlighting the potential of this click strategy.

Synthesis and preliminary evaluation of radiolabeled bis(zinc(II)-dipicolylamine) coordination complexes as cell death imaging agents

The aim of this study was the development of (⁹⁹m)Tc labeled bis(zinc(II)-dipicolylamine) (Zn²⁺-DPA) coordination complexes, and the in vivo evaluation of their usefulness as radiotracers for the detection of cell death. DPA ligand 1 was labeled with (⁹⁹m)Tc via the (⁹⁹m)Tc-tricarbonyl core ([(⁹⁹m)Tc(CO)₃-1]³⁺) or via HYNIC ((⁹⁹m)Tc-HYNIC-1) in good radiochemical yields. Highest in vitro stabilities were demonstrated for [(⁹⁹m)Tc(CO)₃-1]³⁺. A mouse model of hepatic apoptosis (anti-Fas mAb) was used to demonstrate binding to apoptotic cells. (⁹⁹m)Tc-HYNIC-1 showed the best targeting of apoptotic hepatic tissue with a 2.2 times higher liver uptake in anti-Fas treated mice as compared to healthy animals. A rat model of ischemia-reperfusion injury was used to further explore the ability of the (⁹⁹m)Tc-labeled Zn²⁺-DPA coordination complexes to target cell death. Selective accumulation could be detected for both tracers in the area at risk, correlating with histological proof of cell death. Area at risk to normal tissue uptake ratios were 3.82 for [(⁹⁹m)Tc(CO)₃-1]³⁺ and 5.45 for (⁹⁹m)Tc-HYNIC-1.

The role of coordination chemistry in the development of copper and rhenium radiopharmaceuticals

There are several isotopes of copper and rhenium that are of interest in the development of new molecular imaging or radiotherapeutic agents. This perspective article highlights the role of coordination chemistry in the design of copper and rhenium radiopharmaceuticals engineered to selectively target tissue of interest such as cancer cells or pathological features associated with Alzheimer's disease. The coordination chemistry of copper bis(thiosemicarbazone) derivatives and copper macrocyclic complexes is discussed in terms of their potential application as targeted positron emission tomography tracers for non-invasive diagnostic imaging. A range of rhenium complexes with different ligands with rhenium in different oxidation states are introduced and their potential to be translated to new radiotherapeutic agents discussed.