68Ga-DOTA and analogs: Current status and future perspectives

Stability Estimation of Gallium Complexes of DOTA Derivatives for Radiotheranostic Applications

1,4,7,10-Tetraazacyclododecane-1,4,7-triacetic acid (DO3A) has been used to prepare ⁶⁸Ga-labeled probes for the diagnostic counterpart of radiotheranostic applications. While DO3A provides stable complexes with therapeutic radionuclides such as ⁹⁰Y, ¹⁷⁷Lu, and ²²⁵Ac, further improvement of the in vivo stability of the Ga-DO3A complex is required. Considering the high stability of an intact Ga-DOTA complex, the stability of Ga complexes of DOTA and DO3A derivatives, including benzyl-DOTA (Bn-DOTA), was evaluated to gain fundamental knowledge for developing the next-generation radiotheranostic probes using ⁶⁸Ga as a diagnostic counterpart. Following the complexation reaction to prepare ⁶⁷Ga-labeled DOTA and DO3A derivatives, the stability of the resulting ⁶⁷Ga-labeled compounds was evaluated in murine plasma and apo-transferrin challenge. [⁶⁷Ga]Ga-Bn-DOTA produced two isomers, and one of the isomers exhibited the highest stability among the tested complexes. The X-ray crystallography showed that the less stable isomer of Ga-Bn-DOTA suggested an N₃O₃ coordination geometry, while Ga-DOTA and Ga-Bn-DO3A show N₄O₂ coordination. To further evaluate the stability, a synthetic somatostatin analogue, [Tyr³]octreotide (TOC), was used as a model peptide, and p-COOH-Bn-DOTA and DO3A were conjugated with TOC to prepare DOTA-Bn-TOC and DOTATOC. [⁶⁷Ga]Ga-DOTA-Bn-TOC also yielded two isomers with varying stability, and one isomer exhibited significantly higher stability than [⁶⁷Ga]Ga-DOTATOC both in vitro and in vivo. These findings indicate that para-substituted Bn-DOTA would constitute a suitable chelating agent for developing next-generation radiotheranostic probes, although high-performance liquid chromatography purification is needed. Thus, further chemical modification on the Bn-DOTA molecule is also needed to avoid the formation of a Ga complex with the N₃O₃ configuration.

Synthesis and ex vivo biodistribution of two 68Ga-labeled tetrazine tracers: Comparison of pharmacokinetics

Pretargeted PET imaging allows the use of radiotracers labeled with short-living PET radionuclides for tracing drugs with slow pharmacokinetics. Recently, especially methods based on bioorthogonal chemistry have been under intensive investigation for pretargeted PET imaging. The pharmacokinetics of the radiotracer is one of the factors that determine the success of the pretargeted strategy. Here, we report synthesis and biological evaluation of two ⁶⁸Ga-labeled tetrazine (Tz)-based radiotracers, [⁶⁸Ga]Ga-HBED-CC-PEG₄-Tz ([⁶⁸Ga]4) and [⁶⁸Ga]Ga-DOTA-PEG₄-Tz ([⁶⁸Ga]6), aiming for development of new tracer candidates for pretargeted PET imaging based on the inverse electron demand Diels-Alder (IEDDA) ligation between a tetrazine and a strained alkene, such as trans-cyclooctene (TCO). Excellent radiochemical yield (RCY) was obtained for [⁶⁸Ga]4 (RCY > 96%) and slightly lower for [⁶⁸Ga]6 (RCY > 88%). Radiolabeling of HBED-CC-Tz proved to be faster and more efficient under milder conditions compared to the DOTA analogue. The two tracers exhibited excellent radiolabel stability both in vitro and in vivo. Moreover, [⁶⁸Ga]4 was successfully used for radiolabeling two different TCO-functionalized nanoparticles in vitro: Hepatitis E virus nanoparticles (HEVNPs) and porous silicon nanoparticles (PSiNPs).

Preliminary Development and Testing of C595 Radioimmunoconjugates for Targeting MUC1 Cancer Epitopes in Pancreatic Ductal Adenocarcinoma

Mucin 1 is a transmembrane glycoprotein which overexpresses cancer-specific epitopes (MUC1-CE) on pancreatic ductal adenocarcinoma (PDAC) cells. As PDAC is a low survival and highly aggressive malignancy, developing radioimmunoconjugates capable of targeting MUC1-CE could lead to improvements in PDAC outcomes. The aim of this study was to develop and perform preliminary testing of diagnostic and therapeutic radioimmunoconjugates for PDAC using an anti-MUC1 antibody, C595. Firstly, p-SCN-Bn-DOTA was conjugated to the C595 antibody to form a DOTA-C595 immunoconjugate. The stability and binding affinity of the DOTA-C595 conjugate was evaluated using mass spectrometry and ELISA. DOTA-C595 was radiolabelled to Copper-64, Lutetium-177, Gallium-68 and Technetium-99m to form novel radioimmunoconjugates. Cell binding assays were performed in PANC-1 (strong MUC1-CE expression) and AsPC-1 (weak MUC1-CE expression) cell lines using ⁶⁴Cu-DOTA-C595 and ¹⁷⁷Lu-DOTA-C595. An optimal molar ratio of 4:1 DOTA groups per C595 molecule was obtained from the conjugation process. DOTA-C595 labelled to Copper-64, Lutetium-177, and Technetium-99m with high efficiency, although the Gallium-68 labelling was low. ¹⁷⁷Lu-DOTA-C595 demonstrated high cellular binding to the PANC-1 cell lines which was significantly greater than AsPC-1 binding at concentrations exceeding 100 nM (p < 0.05). ⁶⁴Cu-DOTA-C595 showed similar binding to the PANC-1 and AsPC-1 cells with no significant differences observed between cell lines (p > 0.05). The high cellular binding of ¹⁷⁷Lu-DOTA-C595 to MUC1-CE positive cell lines suggests promise as a therapeutic radioimmunoconjugate against PDAC while further work is required to harness the potential of ⁶⁴Cu-DOTA-C595 as a diagnostic radioimmunoconjugate.

Development of 68Ga-Labeled Hepatitis E Virus Nanoparticles for Targeted Drug Delivery and Diagnostics with PET

Targeted delivery of diagnostics and therapeutics offers essential advantages over nontargeted systemic delivery. These include the reduction of toxicity, the ability to reach sites beyond biological barriers, and the delivery of higher cargo concentrations to diseased sites. Virus-like particles (VLPs) can efficiently be used for targeted delivery purposes. VLPs are derived from the coat proteins of viral capsids. They are self-assembled, biodegradable, and homogeneously distributed. In this study, hepatitis E virus (HEV) VLP derivatives, hepatitis E virus nanoparticles (HEVNPs), were radiolabeled with gallium-68, and consequently, the biodistribution of the labeled [⁶⁸Ga]Ga-DOTA-HEVNPs was studied in mice. The results indicated that [⁶⁸Ga]Ga-DOTA-HEVNPs can be considered as promising theranostic nanocarriers, especially for hepatocyte-targeting therapies.

PET/CT features of a novel gallium-68 labelled hypoxia seeking agent in patients diagnosed with tuberculosis: a proof-of-concept study

INTRODUCTION

Positron emission tomography/computed tomography (PET/CT) in infection and inflammation has yielded promising results across a range of radiopharmaceuticals. In particular, PET/CT imaging of tuberculosis (TB) allows for a better understanding of this complex disease by providing insights into molecular processes within the TB microenvironment. TB lesions are hypoxic with research primarily focussed on cellular processes occurring under hypoxic stress. With the development of hypoxia seeking PET/CT radiopharmaceuticals, that can be labelled in-house using a germanium-68/gallium-68 (68Ge/68Ga) generator, a proof-of-concept for imaging hypoxia in TB is presented.

METHODS

Ten patients diagnosed with TB underwent whole-body PET/CT imaging, 60-90 min after intravenous administration of 74-185 MBq (2-5 mCi) 68Ga-nitroimidazole. No oral or intravenous contrast was administered. Images were visually and semiquantitatively assessed for abnormal 68Ga-uptake in the lungs.

RESULTS

A total of 28 lesions demonstrating hypoxic uptake were identified. Low- to moderate-uptake was seen in nodules, areas of consolidation and cavitation as well as effusions. The mean standard uptake value (SUVmean) of the lesions was 0.47 (IQR, 0.32-0.82) and SUVmax was 0.71 (IQR, 0.41-1.11). The lesion to muscle ratio (median, 1.70; IQR, 1.15-2.31) was higher than both the left ventricular and the aorta lesion to blood ratios.

CONCLUSION

Moving towards the development of unique host-directed therapies (HDT), modulation of oxygen levels may improve therapeutic outcome by reprogramming TB lesions to overcome hypoxia. This proof-of-concept study suggests that hypoxia in TB lesions can be imaged and quantified using 68Ga-nitroimidazole PET/CT. Subsequently, hypoxic load can be estimated to inform personalised treatment plans of patients diagnosed with TB.

Porphyrins as Chelating Agents for Molecular Imaging in Nuclear Medicine

Porphyrin ligands, showing a significant affinity for cancer cells, also have the ability to chelate metallic radioisotopes to form potential diagnostic radiopharmaceuticals. They can be applied in single-photon emission computed tomography (SPECT) and positron emission tomography (PET) to evaluate metabolic changes in the human body for tumor diagnostics. The aim of this paper is to present a short overview of the main metallic radionuclides complexed by porphyrin ligands and used in these techniques. These chelation reactions are discussed in terms of the complexation conditions and kinetics and the complex stability.

Prospects for the production of radioisotopes and radiobioconjugates for theranostics

The development of diagnostic methods in medicine as well as the progress in the synthesis of biologically active compounds allows the use of selected radioisotopes for the simultaneous diagnosis and treatment of diseases, especially cancerous ones, in patients. This approach is called theranostic. This review article includes chemical and physical characterization of chosen theranostic radioisotopes and their compounds that are or could be useful in nuclear medicine.

Development and characterization of two novel 68 Ga-labelled neuropeptide Y short analogues with potential application in breast cancer imaging

In vivo receptor targeting with radiolabelled peptide-based probes is an attractive approach for the development of novel radiotracers for molecular imaging. This work presents the development and characterization of two novel neuropeptide Y analogues labelled with a positron emitter 68 Ga, for potential use in breast cancer imaging. Both analogues share the same amino acid sequence and were derivatized with NOTA through either a lysine linker (L1) or an acetylated lysine (L2). In both cases, a single product with radiochemical purity higher than 95% was obtained. The two complexes were hydrophilic, showed remarkable in vitro stability, good cellular uptake, binding affinity in the nanomolar range and high cellular internalization rate. Biodistribution studies revealed low blood uptake and elimination through the urinary tract. The addition of an acetyl group in the spacer increased the lipophilicity of C2 and modified the reactivity of the ε-amino group of the lysine which resulted in lower protein binding and lower percentage of injected dose in bladder and urine. The tumour versus muscle ratio was (3.8 ± 0.4) for 68 Ga-L1 and (4.7 ± 0.4) for 68 Ga-L2. These results encourage performing further studies in order to complete the evaluation of both tracers as potential radiopharmaceutical for breast cancer imaging.

Squaric Acid-Based Radiopharmaceuticals for Tumor Imaging and Therapy

Targeting vectors bound to a chelator represent a significant fraction of radiopharmaceuticals used nowadays for diagnostic and therapeutic purposes in nuclear medicine. The use of squaramides as coupling units for chelator and targeting vector helps to circumvent the disadvantages of several common coupling methods. This review gives an overview of the use of squaric acid diesters (SADE) as linking agents. It focuses on the conjugation of cyclic chelators, e.g., DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), as well as hybrid chelators like AAZTA⁵ (6-pentanoic acid-6-amino-1,4-diazepine tetracetic acid) or DATA^5m (6-pentanoic acid-6-amino-1,4-diazapine-triacetate) to different targeting vectors, e.g., prostate-specific membrane antigen inhibitors (KuE; PSMAi), fibroblast activation protein inhibitors (FAPi), and monoclonal antibodies (mAbs). An overview of the synthesis, radiolabeling, and in vitro and in vivo behavior of the described structures is given. The unique properties of SADE enable a fast and simple conjugation of chelators to biomolecules, peptides, and small molecules under mild conditions. Furthermore, SA-containing conjugates could not only display similar in vitro characteristics in terms of binding affinity when compared to reference compounds, but may even induce beneficial effects on the pharmacokinetic properties of these radiopharmaceuticals.

Synthesis, Radiolabeling, and Preliminary in vivo Evaluation of [68Ga] IPCAT-NOTA as an Imaging Agent for Dopamine Transporter

Introduction

Novel radiotracer development for imaging dopamine transporters is a subject of interest because although [^99mTc]TRODAT-1, [¹²³I]β-CIT, and [¹²³I]FP-CIT are commercially available; ⁹⁹Mo/^99mTc generator is in short supply and ¹²³I production is highly dependent on compact cyclotron. Therefore, we designed a novel positron emission tomography (PET) tracer based on a tropane derivative through C-2 modification to conjugate NOTA for chelating ⁶⁸Ga, a radioisotope derived from a ⁶⁸Ge/⁶⁸Ga generator.

Methods

IPCAT-NOTA 22 was synthesized and labeled with [⁶⁸Ga]GaCl₄⁻ at room temperature. Biological studies on serum stability, LogP, and in vitro autoradiography (binding assay and competitive assay) were performed. Furthermore, ex vivo autoradiography, biodistribution, and dynamic PET imaging studies were performed in Sprague Dawley rats.

Results

[⁶⁸Ga]IPCAT-NOTA 24 obtained had a radiochemical yield of ≥90% and a specific activity of 4.25 MBq/nmol. [⁶⁸Ga]IPCAT-NOTA 24 of 85% radiochemical purity (RCP%) was stable at 37°C for up to 60 minutes in serum with a lipophilicity of 0.88. The specific binding ratio (SBR%) reached 15.8 ± 6.7 at 60 minutes, and the 85% specific uptake could be blocked through co-injection at 100- and 1000-fold of the cold precursor in in vitro binding studies. Tissue regional distribution studies in rats with [⁶⁸Ga]IPCAT-NOTA 24 showed striatal uptake (0.02% at 5 minutes and 0.007% at 60 minutes) with SBR% of 6%, 25%, and 62% at 5–15, 30–40, and 60–70 minutes, respectively, in NanoPET studies. The RCP% of [⁶⁸Ga]IPCAT-NOTA 24 at 30 minutes in vivo remained 67.65%.

Conclusion

Data described here provide new information on the design of PET probe of conjugate/pendent approach for DAT imaging. Another chelator or another direct method of intracranial injection must be used to prove the relation between [⁶⁸Ga]IPCAT-NOTA 24 uptake and transporter localization.

Cyclotron-produced 68Ga from enriched 68Zn foils

The growing need and limited availability of generator produced ⁶⁸Ga (T_1/2 = 68 min) for PET has provided the impetus for alternative, high output, ⁶⁸Ga production routes such as charge particle activation of enriched ⁶⁸Zn using PET cyclotrons. The work presents a rapid production method for clinically useful ⁶⁸Ga for radiolabeling. The focus is also to expand the production capacity of cyclotron solid target-produced ⁶⁸Ga over generator produced and liquid solutions targets by using enriched ⁶⁸Zn-foils that minimizes target preparation.

Evaluation of F-18 DOPA PET/CT in the detection of recurrent or metastatic medullary thyroid carcinoma: comparison with GA-68 DOTA-TATE PET/CT

OBJECTIVE

PET imaging with F-18 DOPA (FDOPA) and Ga-68 DOTATATE (TATE) shows the most promising results to detect medullary thyroid cancer (MTC) recurrence. We performed this comparative study to detect the site of recurrent or metastatic disease in MTC patients with elevated serum calcitonin (Ctn) and/or carcinoembryonic antigen (CEA) levels.

METHODS

We studied 46 MTC patients (25 women, 21 men) with elevated Ctn and/or CEA levels during follow-up who had both FDOPA and TATE PET/CT scans for re-staging purposes.

RESULTS

FDOPA PET imaging yielded an overall sensitivity of 86.8%, specificity of 100%, PPV of 100%, NPV of 61.5%, and accuracy of 89.1%, while TATE PET scan had the same values as 84.2%, 87.5%, 96.9%, 53.8%, and 84.6%, respectively, and there was no statistically significant difference between the two modalities with the exception of the specificity value that was higher for FDOPA imaging. In a subgroup of patients with overt Ctn or CEA elevation, sensitivity of FDOPA increased significantly, whereas TATE sensitivity did not change. FDOPA PET imaging was significantly superior in detecting liver and regional lymph node (LN) metastases, while TATE PET scan was significantly better in the skeletal metastases. Early FDOPA demonstrated 11 invisible lesions on late FDOPA.

CONCLUSION

Both FDOPA and TATE PET/CT imaging are useful to localize recurrences in MTC patients. While TATE imaging is superior to reveal skeletal disease, FDOPA seems better in liver and regional LN metastases; therefore, the two modalities appear complementary in monitoring MTC patients with elevated serum Ctn and/or CEA levels.

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.

Advances in Development of Radiometal Labeled Amino Acid-Based Compounds for Cancer Imaging and Diagnostics

Radiolabeled biomolecules targeted at tumor-specific enzymes, receptors, and transporters in cancer cells represent an intensively investigated and promising class of molecular tools for the cancer diagnosis and therapy. High specificity of such biomolecules is a prerequisite for the treatment with a lower burden to normal cells and for the effective and targeted imaging and diagnosis. Undoubtedly, early detection is a key factor in efficient dealing with many severe tumor types. This review provides an overview and critical evaluation of novel approaches in the designing of target-specific probes labeled with metal radionuclides for the diagnosis of most common death-causing cancers, published mainly within the last three years. Advances are discussed such traditional peptide radiolabeling approaches, and click and nanoparticle chemistry. The progress of radiolabeled peptide based ligands as potential radiopharmaceuticals is illustrated via novel structure and application studies, showing how the molecular modifications reflect their binding selectivity to significant onco-receptors, toxicity, and, by that, practical utilization. The most impressive outputs in categories of newly developed structures, as well as imaging and diagnosis approaches, and the most intensively studied oncological diseases in this context, are emphasized in order to show future perspectives of radiometal labeled amino acid-based compounds in nuclear medicine.

An overview of the developments and potential applications of 68Ga-labelled PET/CT hypoxia imaging

Non-invasive imaging of hypoxia plays a role in monitoring the body's adaptive response or the development of pathology under hypoxic conditions. Various techniques to image hypoxia have been investigated with a shift towards the use of molecular imaging using PET/CT. The role of hypoxia-specific radiopharmaceuticals such as radiolabelled nitroimidazoles is well documented particularly in the oncologic setting. With the increasing utilisation of in-house labelling with a PET benchtop generator, such as the ⁶⁸Ge/⁶⁸Ga generator, the use of ⁶⁸Ga-labelled hypoxic radiopharmaceuticals in the clinical setting is developing. Since hypoxia plays a role in various pathologic states including infectious disease such as TB, there is a need to explore the potential application of ⁶⁸Ga-labelled hypoxia seeking radiopharmaceuticals beyond oncology. The purpose of this review is to describe the developments of ⁶⁸Ga-labelled hypoxic radiopharmaceuticals including the various chelators that have been investigated. Further, the role of hypoxia imaging in various pathologies is discussed with particular emphasis on the potential clinical applications of hypoxia PET/CT in TB.

Toward the Discovery and Development of PSMA Targeted Inhibitors for Nuclear Medicine Applications

BACKGROUND

The rising incidence rate of prostate cancer (PCa) has promoted the development of new diagnostic and therapeutic radiopharmaceuticals during the last decades. Promising improvements have been achieved in clinical practice using prostate specific membrane antigen (PSMA) labeled agents, including specific antibodies and small molecular weight inhibitors. Focusing on molecular docking studies, this review aims to highlight the progress in the design of PSMA targeted agents for a potential use in nuclear medicine.

RESULTS

Although the first development of radiopharmaceuticals able to specifically recognize PSMA was exclusively oriented to macromolecule protein structure such as radiolabeled monoclonal antibodies and derivatives, the isolation of the crystal structure of PSMA served as the trigger for the synthesis and the further evaluation of a variety of low molecular weight inhibitors. Among the nuclear imaging probes and radiotherapeutics that have been developed and tested till today, labeled Glutamate-ureido inhibitors are the most prevalent PSMA-targeting agents for nuclear medicine applications.

CONCLUSION

PSMA represents for researchers the most attractive target for the detection and treatment of patients affected by PCa using nuclear medicine modalities. [99mTc]MIP-1404 is considered the tracer of choice for SPECT imaging and [68Ga]PSMA-11 is the leading diagnostic for PET imaging by general consensus. [18F]DCFPyL and [18F]PSMA-1007 are clearly the emerging PET PSMA candidates for their great potential for a widespread commercial distribution. After paving the way with new imaging tools, academic and industrial R&Ds are now focusing on the development of PSMA inhibitors labeled with alpha or beta minus emitters for a theragnostic application.

Increase in negative charge of 68Ga/chelator complex reduces unspecific hepatic uptake but does not improve imaging properties of HER3-targeting affibody molecules

Upregulation of the human epidermal growth factor receptor type 3 (HER3) is a common mechanism to bypass HER-targeted cancer therapy. Affibody-based molecular imaging has the potential for detecting and monitoring HER3 expression during treatment. In this study, we compared the imaging properties of newly generated ⁶⁸Ga-labeled anti-HER3 affibody molecules (HE)₃-Z_HER3-DOTA and (HE)₃-Z_HER3-DOTAGA with previously reported [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA. We hypothesized that increasing the negative charge of the gallium-68/chelator complex would reduce hepatic uptake, which could lead to improved contrast of anti-HER3 affibody-based PET-imaging of HER3 expression. (HE)₃-Z_HER3-X (X = DOTA, DOTAGA) were produced and labeled with gallium-68. Binding of the new conjugates was specific in HER3 expressing BxPC-3 and DU145 human cancer cells. Biodistribution and in vivo specificity was studied in BxPC-3 xenograft bearing Balb/c nu/nu mice 3 h pi. DOTA- and DOTAGA-containing conjugates had significantly higher concentration in blood than [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA. Presence of the negatively charged ⁶⁸Ga-DOTAGA complex reduced the unspecific hepatic uptake, but did not improve overall biodistribution of the conjugate. [⁶⁸Ga]Ga-(HE)₃-Z_HER3-DOTAGA and [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA had similar tumor-to-liver ratios, but [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA had the highest tumor uptake and tumor-to-blood ratio among the tested conjugates. In conclusion, [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA remains the favorable variant for PET imaging of HER3 expression.

Molecular Imaging in Pediatric Brain Tumors

In the last decade, several radiopharmaceuticals have been developed and investigated for imaging in vivo of pediatric brain tumors with the aim of exploring peculiar metabolic processes as glucose consumption, amino-acid metabolism, and protein synthesis with nuclear medicine techniques. Although the clinical shreds of evidence are limited, preliminary results are encouraging. In this review, we performed web-based and desktop research summarizing the most relevant findings of the literature published to date on this topic. Particular attention was given to the wide spectrum of nuclear medicine advances and trends in pediatric neurooncology and neurosurgery. Furthermore, the role of somatostatin receptor imaging through single-photon emission computed tomography (SPECT) and positron emission tomography (PET) probes, with reference to their potential therapeutic implications, was examined in the peculiar context. Preliminary results show that functional imaging in pediatric brain tumors might lead to significant improvements in terms of diagnostic accuracy and it could be of help in the management of the disease.

Feasibility and Evaluation of Automated Methods for Radiolabeling of Radiopharmaceutical Kits with Gallium-68

Objective:

Recent gallium-68 labeled peptides are of increasing interest in PET imaging in nuclear medicine. Somakit TOC® is a radiopharmaceutical kit registered in the European Union for the preparation of [68Ga]Ga-DOTA-TOC used for the diagnosis of neuroendocrine tumors. Development of a labeling process using a synthesizer is particularly interesting for the quality and reproducibility of the final product although only manual processes are described in the Summary of Product (SmPC) of the registered product. The aim of the present study was therefore to evaluate the feasibility and value of using an automated synthesizer for the preparation of [68Ga]Ga-DOTA-TOC according to the SmPC of the Somakit TOC®.

Methods:

Three methods of preparation were compared; each followed the SmPC of the Somakit TOC®. Over time, overheads, and overexposure were evaluated for each method.

Results:

Mean±SD preparation time was 26.2±0.3 minutes for the manual method, 28±0.5 minutes for the semi-automated, and 40.3±0.2 minutes for the automated method. Overcost of the semi-automated method is 0.25€ per preparation for consumables and from 0.58€ to 0.92€ for personnel costs according to the operator (respectively, technician or pharmacist). For the automated method, overcost is 70€ for consumables and from 4.06€ to 6.44€ for personnel. For the manual method, extremity exposure was 0.425mSv for the right finger, and 0.350mSv for the left finger; for both the semi-automated and automated method extremity exposure were below the limit of quantification.

Conclusion:

The present study reports for the first time both the feasibility of using a [68Ga]- radiopharmaceutical kit with a synthesizer and the limits for the development of a fully automated process.

68Ga-DOTANOC and 18F-FDG PET/CT in metastatic medullary thyroid carcinoma: novel correlations with tumoral biomarkers

OBJECTIVE

Metastatic disease is common in medullary thyroid carcinoma (MTC) and it is usually detected by raising calcitonin and carcinoembryonic antigen (CEA) levels. Nuclear medicine imaging has an important role in lesion identification/characterisation. We aim to compare ⁶⁸Ga-DOTANOC PET/CT and ¹⁸F-FDG PET/CT performance and to explore the correlations between tumoral markers and functional imaging.

METHODS

This a retrospective cross-sectional study including 13 patients with MTC and high calcitonin/CEA levels that underwent both ⁶⁸Ga-DOTANOC PET/CT and ¹⁸F-FDG PET/CT.

RESULTS

⁶⁸Ga-DOTANOC PET/CT identified MTC metastases in 2twopatients that were ¹⁸F-FDG-negative (sensitivity of 69.2% vs. 53.9%, respectively). ⁶⁸Ga-DOTANOC PET/CT also detected a higher number of lesions than ¹⁸F-FDG PET/CT in seven patients, with only one patient showing the opposite pattern. Both differences lacked statistical significance (p = 0.50 and p = 0.86, respectively) but ⁶⁸Ga-DOTANOC PET/CT better performance allowed changes in patients' management. ⁶⁸Ga-positive/¹⁸F-FDG-negative patients were the ones with the lowest calcitonin doubling time and presented a CEA doubling time >24 months, while the patient with more ¹⁸F-FDG-positive lesions was the one with the highest CEA/calcitonin ratio. The number of lesions found in ⁶⁸Ga-DOTANOC PET/CT were correlated with calcitonin levels (r = 0.73; p < 0.01) but not with CEA ones (r = 0.42; p = 0.15). The number of ¹⁸F-FDG hypermetabolic focus were correlated with CEA levels (r = 0.60; p < 0.05) but not with calcitonin (r = 0.48; p = 0.09).

CONCLUSIONS

This is the first study to describe a positive correlation between ⁶⁸Ga-positive lesions and calcitonin levels and between ¹⁸F-FDG-positivity and CEA levels. Tumoral markers pattern in metastatic MTC could help clinicians to decide which exam to perform first.

Multiwalled Carbon Nanotubes for Combination Therapy: a Biodistribution and Efficacy Pilot Study

A drug delivery system (DDS) for combined therapy, based on a short oxidized multiwalled carbon nanotube, is reported. It was prepared exploiting a synthetic approach which allowed loading of two drugs, doxorubicin and metformin, the targeting agent biotin and a radiolabeling tag, to enable labeling with Ga-68 or Cu-64 in order to perform an extensive biodistribution study by PET/CT. The DDS biodistribution profile changes with different administration methods. Once administered at therapeutic doses, the DDS showed a marginal beneficial effect on 4T1 tumor bearing mice, a syngeneic and orthotopic model of triple negative breast cancer, with survival extended by 1 week and 2 days in 20% of the mice. This is encouraging given the aggressiveness of the 4T1 tumor. Furthermore our DDS was well tolerated, ruling out concerns regarding the toxicity of carbon nanotubes.

Radionuclide generators: the prospect of availing PET radiotracers to meet current clinical needs and future research demands

Targeted molecular imaging with positron emission tomography (PET) constitutes a successful technique for detecting and diagnosing disease conditions promptly and accurately, and for effectively prognosticating outcomes and treating patients with a tailored and more individualized intervention. In order to expand the success of PET in nuclear medicine, it is important to assure access to radiotracers of desired quantities and qualities. In this context, the benefit of accessing PET radiotracers through a radionuclide generator (RNG) cannot be overstated, as generators offer the potential of enriching the PET radiotracer arsenal at the medical centers both with and without onsite cyclotrons. While RNG technology to avail PET tracers is in its infancy, their use is expected to revitalize current PET practices and seems poised to broaden the palette of PET in nuclear medicine in the foreseeable future. In this review, we discuss the principles of RNGs, assess major parent/daughter pairs of interest for PET, RNGs currently in use in clinical PET, and identify the potentially useful RNGs which have made substantial progress or are likely to be used in daily clinical practices in the near future. Availability of the parent radionuclides required for PET RNGs is an important criterion and hence their production will also be reviewed. This overview outlines a critical assessment of RNGs to avail PET tracers, the contemporary status of RNGs, and key challenges and apertures to the near future.

Antibody Fragment and Affibody ImmunoPET Imaging Agents: Radiolabelling Strategies and Applications

Antibodies have long been recognised as potent vectors for carrying diagnostic medical radionuclides, contrast agents and optical probes to diseased tissue for imaging. The area of ImmunoPET combines the use of positron emission tomography (PET) imaging with antibodies to improve the diagnosis, staging and monitoring of diseases. Recent developments in antibody engineering and PET radiochemistry have led to a new wave of experimental ImmunoPET imaging agents that are based on a range of antibody fragments and affibodies. In contrast to full antibodies, engineered affibody proteins and antibody fragments such as minibodies, diabodies, single-chain variable region fragments (scFvs), and nanobodies are much smaller but retain the essential specificities and affinities of full antibodies in addition to more desirable pharmacokinetics for imaging. Herein, recent key developments in the PET radiolabelling strategies of antibody fragments and related affibody molecules are highlighted, along with the main PET imaging applications of overexpressed antigen-associated tumours and immune cells.

Pediatric Musculoskeletal Imaging: The Indications for and Applications of PET/Computed Tomography

The use of PET/computed tomography (CT) for the evaluation and management of children, adolescents, and young adults continues to expand. The principal tracer used is 18F-fluorodeoxyglucose and the principal indication is oncology, particularly musculoskeletal neoplasms. The purpose of this article is to review the common applications of PET/CT for imaging of musculoskeletal issues in pediatrics and to introduce the use of PET/CT for nononcologic issues, such as infectious/inflammatory disorders, and review the use of 18F-sodium fluoride in trauma and sports-related injuries.

Radiolabeling and Preclinical Evaluation of a New S-Alkylated Cysteine Derivative Conjugated to C-Substituted Macrocycle for Positron Emission Tomography

A new S-alkylated cysteine-derivatized tumor targeting agent, 2,2'-(12-(2-((2-acetamido-2-carboxyethyl)thio)acetamido)-11,13-dioxo-1,4,7,10-tetraazacyclotridecane-4,7-diyl)diacetic acid was developed for positron emission tomography (PET) imaging. N-Acetyl cysteine (NAC) was conjugated to ATRIDAT as a specific targeting agent toward L-type and ASC amino acid transporter systems in the oncogenic cells. NAC was attached via S-alkylation to prevent its incorporation at undesired recognition sites affecting the signal-to-noise ratio. NAC-ATRIDAT was subjected to gallium-68 complexation with >75% radiolabeling yield. The radiocomplex was purified through the tc18 cartridge to obtain 99.89% radiochemical yield. IC-50 of the NAC-ATRIDAT conjugate was 0.8 mM in A549 cells as evaluated through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazonium bromide assay. Binding affinity experiments on A549 cells showed noteworthy binding with K_D in the nanomolar range. A time course study showed a K_m value of 0.19 μM and V_max value of 0.49 pmol/μg protein/min showing reasonable tumor kinetics. Efflux studies showed that the synthesized radioligand is transported majorly by LAT followed by the ASC system. Clearance was found to be renal with 7.67 ± 1.48% ID/g uptake at 30 min which substantially declined to 0.52 ± 0.% ID/g at 4 h. A significant uptake of 10.06 ± 1.056% ID/g was observed at the tumor site in mice at 1 h. μPET images revealed a high contrast with a tumor-to-kidney ratio of 4.8 and a tumor-to-liver ratio of 35.85 at 1 h after injection. These preclinical in vitro and in vivo evaluation supports its potential on the way of becoming a successful ⁶⁸Ga-radiolabeled amino acid-based PET imaging agent.

The potential of radiolabeled chemotherapeutics in tumor diagnosis: Preliminary investigations with 68 Ga-gemcitabine

Preclinical Research & Development Gemcitabine, a nucleoside analog, is a well-known chemotherapeutic drug that is used either alone or with other agents to treat a wide variety of cancers. The aim of the present work was to evaluate the potential of ⁶⁸ Ga-labeled gemcitabine for its application in positron emission tomography (PET) imaging of tumorous lesions. Gemcitabine was coupled with p-NCS-benzyl-DOTA in order to facilitate radiolabeling with ⁶⁸ Ga. The gemcitabine-p-NCS-benzyl-DOTA was radiolabeled with ⁶⁸ Ga, obtained from a ⁶⁸ Ge/⁶⁸ Ga radionuclide generator. The radiolabeled product was characterized by high performance liquid chromatography (HPLC) and its tumor specificity was evaluated by biodistribution studies in Swiss mice bearing fibrosarcoma tumors. Preliminary bioevaluation study showed good tumor uptake within 1 hr post-administration [2.5% Injected Activity (IA) per g of tumor] with rapid renal clearance (>90% IA) and a high tumor to muscle ratio. ⁶⁸ Ga-gemcitabine may have potential as a PET agent for tumor imaging.

Site-Specific 64Cu Labeling of the Serine Protease, Active Site Inhibited Factor Seven Azide (FVIIai-N3), Using Copper Free Click Chemistry

A method for site-specific radiolabeling of the serine protease active site inhibited factor seven (FVIIai) with ⁶⁴Cu has been applied using a biorthogonal click reaction. FVIIai binds to tissue factor (TF), a trans-membrane protein involved in hemostasis, angiogenesis, proliferation, cell migration, and survival of cancer cells. First a single azide moiety was introduced in the active site of this 50 kDa protease. Then a NOTA moiety was introduced via a strain promoted azide-alkyne reaction and the corresponding conjugate was labeled with ⁶⁴Cu. Binding to TF and the stability was evaluated in vitro. TF targeting capability of the radiolabeled conjugate was tested in vivo by positron emission tomography (PET) imaging in pancreatic human xenograft cancer mouse models with various TF expressions. The conjugate showed good stability (>91% at 16 h), an immunoreactivity of 93.5%, and a mean tumor uptake of 2.1 ± 0.2%ID/g at 15 h post injection. In conclusion, FVIIai was radiolabeled with ⁶⁴Cu in single well-defined position of the protein. This method can be utilized to prepare conjugates from serine proteases with the label at a specific position.

Gallium-68 PET: A Powerful Generator-based Alternative to Infection and Inflammation Imaging

The process of inflammation (with or without infection) forms part of essentially every major debilitating disease. Early detection and accurate distinction of inflammation from infection are important to optimize and individualize therapy. Nuclear medicine is ideally suited for the detection of pathologic changes early on and is able to target a magnitude of role players involved in the aforementioned processes. Hybrid modalities such as PET/CT and PET/MRI offer high spatial resolution that combines morphologic and pathophysiological changes and add various quantification possibilities that are preferable in these settings. It follows then that the development of PET radiopharmaceuticals is imperative to make use of these latest advances. Gallium-68 (Ga-68)-based tracers are exceptionally well suited to these indications, considering the year-round availability from a single generator, the relative cost-effectiveness, and relative ease of labeling. Over the past few years, the development of Ga-68-based tracers has understandably exploded with a recent growing interest in infection and inflammation imaging. This review aims to highlight some of the most important and interesting advances made with Ga-68-based PET/CT in the field of infection and inflammation imaging.