PET radiotracers and fluorescent probes for imaging human carbonic anhydrase IX and XII in hypoxic tumors.

Positron emission tomography (PET) and fluorescent imaging play a pivotal role in medical diagnosis, biomedical oncologic research, and drug development process, which include identification of target location, target engagement, but also prove on mechanism of action or pharmacokinetics of new drug candidates. PET estimates physiological changes at the molecular level using specific radiotracers containing a short-lived positron emitting radionuclide such as fluorine-18 or carbon-11, whereas fluorescent imaging techniques use fluorescent probes labeled with suitable drug candidates for detection at the molecular level. The human carbonic anhydrase (hCA) isoforms IX and XII are overexpressed in hypoxic cancer cells, promoting tumor growth by regulating extra/intracellular pH, ferroptosis, and metabolism, being recognized as promising targets for anticancer theranostic agents. In this review, we have focused on PET radiotracers as well as fluorescent probes for diagnosis and treatment of tumors expressing hCA IX and hCA XII.

PET Imaging of the Neuropeptide Y System: A Systematic Review.

Neuropeptide Y (NPY) is a vastly studied biological peptide with numerous physiological functions that activate the NPY receptor family (Y1, Y2, Y4 and Y5). Moreover, these receptors are correlated with the pathophysiology of several diseases such as feeding disorders, anxiety, metabolic diseases, neurodegenerative diseases, some types of cancers and others. In order to deepen the knowledge of NPY receptors' functions and molecular mechanisms, neuroimaging techniques such as positron emission tomography (PET) have been used. The development of new radiotracers for the different NPY receptors and their subsequent PET studies have led to significant insights into molecular mechanisms involving NPY receptors. This article provides a systematic review of the imaging biomarkers that have been developed as PET tracers in order to study the NPY receptor family.

Production of [11C]Carbon Labelled Flumazenil and L-Deprenyl Using the iMiDEV™ Automated Microfluidic Radiosynthesizer.

In the last decade, microfluidic techniques have been explored in radiochemistry, and some of them have been implemented in preclinical production. However, these are not suitable and reliable for preparing different types of radiotracers or dose-on-demand production. A fully automated iMiDEV™ microfluidic radiosynthesizer has been introduced and this study is aimed at using of the iMiDEV™ radiosynthesizer with a microfluidic cassette to produce [11C]flumazenil and [11C]L-deprenyl. These two are known PET radioligands for benzodiazepine receptors and monoamine oxidase-B (MAO-B), respectively. Methods were successfully developed to produce [11C]flumazenil and [11C]L-deprenyl using [11C]methyl iodide and [11C]methyl triflate, respectively. The final products 1644 ± 504 MBq (n = 7) and 533 ± 20 MBq (n = 3) of [11C]flumazenil and [11C]L-deprenyl were produced with radiochemical purities were over 98% and the molar activity for [11C]flumazenil and [11C]L-deprenyl was 1912 ± 552 GBq/µmol, and 1463 ± 439 GBq/µmol, respectively, at the end of synthesis. All the QC tests complied with the European Pharmacopeia. Different parameters, such as solvents, bases, methylating agents, precursor concentration, and different batches of cassettes, were explored to increase the radiochemical yield. Synthesis methods were developed using 3-5 times less precursor than conventional methods. The fully automated iMiDEV™ microfluidic radiosynthesizer was successfully applied to prepare [11C]flumazenil and [11C]L-deprenyl.

The Rise of Synaptic Density PET Imaging.

Many neurological disorders are related to synaptic loss or pathologies. Before the boom of positrons emission tomography (PET) imaging of synapses, synaptic quantification could only be achieved in vitro on brain samples after autopsy or surgical resections. Until the mid-2010s, electron microscopy and immunohistochemical labelling of synaptic proteins were the gold-standard methods for such analyses. Over the last decade, several PET radiotracers for the synaptic vesicle 2A protein have been developed to achieve in vivo synapses visualization and quantification. Different strategies were used, namely radiolabelling with either 11C or 18F, preclinical development in rodent and non-human primates, and binding quantification with different kinetic modelling methods. This review provides an overview of these PET tracers and underlines their perspectives and limitations by focusing on radiochemical aspects, as well as preclinical proof-of-concept and the main clinical outcomes described so far.

PET imaging for lymph node dissection in prostate cancer.

The detection of neoplastic lymph nodal involvement in prostate cancer (PCa) patients has relevant therapeutic and prognostic significance, both in the clinical settings of primary staging and restaging. Lymph nodal dissection (LND) currently represents the gold standard for evaluating the presence of lymph nodal involvement. However, this procedure is invasive, associated with morbidity, and may fail in detecting all potential lymph nodal metastatic regions. Currently the criteria for lymph nodal detection using conventional imaging techniques mainly rely on morphological assessment with unsatisfactory diagnostic accuracy. Positron emission tomography (PET) represents a helpful imaging technique for a proper staging of lymph nodal status. The most investigated PET radiotracer is choline, although many others have been explored as guide for both primary and salvage LND, such as fluorodeoxyglucose, acetate, fluorocyclobutanecarboxylic acid and prostate-specific membrane antigen. In the present review, a comprehensive literature review addressing the role of PET for LND in PCa patients is reported, with the use of the above-mentioned radiotracers.

In vitro evaluation of PET radiotracers reflecting multidimensionality of Alzheimer's disease: building more roadmaps for clinical translation.

In the current issue of American Journal of Nuclear Medicine and Molecular Imaging, Vasdev et al. presented a work entitled "In Vitro Evaluation of PET Radiotracers for Imaging Synaptic Density, the Acetylcholine Transporter, AMPA-tarp-γ8 and Muscarinic M4 receptors in Alzheimer's disease". In which, in vitro autoradiography studies using radioligands were employed as a valuable tool to gain more insights for potential clinical translation. In this invited perspective, we would like to briefly introduce the current state of AD diagnosis, especially PET imaging on synapse, and highlight the advances of PET imaging in pre-clinic and clinic that might assist on precise therapy in the future.

Advances in PET/CT Imaging for Breast Cancer.

One out of eight women will be affected by breast cancer during her lifetime. Imaging plays a key role in breast cancer detection and management, providing physicians with information about tumor location, heterogeneity, and dissemination. In this review, we describe the latest advances in PET/CT imaging of breast cancer, including novel applications of 18F-FDG PET/CT and the development and testing of new agents for primary and metastatic breast tumor imaging and therapy. Ultimately, these radiopharmaceuticals may guide personalized approaches to optimize treatment based on the patient's specific tumor profile, and may become a new standard of care. In addition, they may enhance the assessment of treatment efficacy and lead to improved outcomes for patients with a breast cancer diagnosis.

More than Just Skin-Deep: A Review of Imaging's Role in Guiding CAR T-Cell Therapy for Advanced Melanoma.

Advanced melanoma is one of the deadliest cancers, owing to its invasiveness and its propensity to develop resistance to therapy. Surgery remains the first-line treatment for early-stage tumors but is often not an option for advanced-stage melanoma. Chemotherapy carries a poor prognosis, and despite advances in targeted therapy, the cancer can develop resistance. CAR T-cell therapy has demonstrated great success against hematological cancers, and clinical trials are deploying it against advanced melanoma. Though melanoma remains a challenging disease to treat, radiology will play an increasing role in monitoring both the CAR T-cells and response to therapy. We review the current imaging techniques for advanced melanoma, as well as novel PET tracers and radiomics, in order to guide CAR T-cell therapy and manage potential adverse events.

Recent advancements in 18F-labeled PSMA targeting PET radiopharmaceuticals.

Prostate specific membrane antigen (PSMA) is an attractive target for molecular imaging of prostate cancer and several other solid tumors because of its overexpression in prostate carcinoma and tumor neovasculature, respectively. While currently most commonly used PSMA PET radioligands are 68Ga-labeled compounds, the short half-life and relatively low available radioactivity of gallium-68 have led to a steep increase in the development of 18F-labeled PSMA ligands. Several 18F-PSMA tracers such as [18F]DCFPyL and [18F]PSMA-1007 are already established in clinical practice, but there are still several drawbacks to be considered. Radiofluorination is often a multistep and time-consuming process requiring harsh labeling conditions. The limited sensitivity in the lower PSA ranges raises the need for improving the binding affinity of the ligands. Due to the metallic character of therapeutic radionuclides, there is very limited experience with 18F-PSMA tracers that can be applied for a theranostic approach. However, developments in the past few years have brought forward several improvements in these fields. These include the application of new radiosynthesis pathways for radiofluorination that reduces the process complexity, new approaches for the design of the pharmacophore, improving target interaction and the introduction of radiohybrid ligands, allowing labeling of the ligand with both diagnostic and therapeutic radionuclides. In this review, we will give an overview of these recent advancements of 18F-labeled PSMA PET radioligands.

Fluorine-18: Radiochemistry and Target-Specific PET Molecular Probes Design.

The positron emission tomography (PET) molecular imaging technology has gained universal value as a critical tool for assessing biological and biochemical processes in living subjects. The favorable chemical, physical, and nuclear characteristics of fluorine-18 (97% ß+ decay, 109.8 min half-life, 635 keV positron energy) make it an attractive nuclide for labeling and molecular imaging. It stands that 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) is the most popular PET tracer. Besides that, a significantly abundant proportion of PET probes in clinical use or under development contain a fluorine or fluoroalkyl substituent group. For the reasons given above, 18F-labeled radiotracer design has become a hot topic in radiochemistry and radiopharmaceutics. Over the past decades, we have witnessed a rapid growth in 18F-labeling methods owing to the development of new reagents and catalysts. This review aims to provide an overview of strategies in radiosynthesis of [18F]fluorine-containing moieties with nucleophilic [18F]fluorides since 2015.

PET Imaging as a Tool for Assessing COVID-19 Brain Changes.

A substantial fraction of coronavirus disease 2019 (COVID-19) patients experience neurological manifestations. Nevertheless, brain changes caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remain largely unknown. Here, we provide a brief overview of positron emission tomography (PET) applications that could advance current understanding of CNS pathophysiological alterations associated with SARS-CoV-2 infection.

Role of Nuclear Imaging in Cardiac Amyloidosis Management: Clinical Evidence and Review of Literature.

Cardiac amyloidosis (CA) is an infiltrative disease characterized by the extracellular deposition of fibrils, amyloid, in the heart. The vast majority of patients with CA has one of two types between transthyretin amyloid (ATTR) and immunoglobulin light chain associated amyloid (AL), that have different prognosis and therapeutic options. CA is often underdiagnosed. The histological analysis of endomyocardial tissue is the gold standard for the diagnosis, although it has its limitations due to its invasive nature. Nuclear medicine now plays a key role in the early and accurate diagnosis of this disease, and in the ability to distinguish between the two forms. Recent several studies support the potential advantage of bone-seeking radionuclides as a screening technique for the most common types of amyloidosis, in particular ATTR form. This review presents noninvasive modalities to diagnose CA and focuses on the radionuclide imaging techniques (bone-seeking agents scintigraphy, cardiac sympathetic innervation and positron emission tomography studies) available to visualize myocardial amyloid involvement. Furthermore, we report the case of an 83-year old male with a history of prostate cancer, carcinoma of the cecum and kidney cancer, submitted to bone scan to detect bone metastasis, that revealed a myocardial uptake of 99mTC-HMPD suggestive of ATTR CA. An accurate and early diagnosis of CA able to distinguish beyween AL and ATTR CA combined to the improving therapies could improve the survival of patients with this disease.

Selectivity of probes for PET imaging of dopamine D3 receptors.

Dopamine D3 receptors have key roles in behavioral reward, addiction, Parkinson's disease, and schizophrenia, and there is interest in studying their role in these disorders using PET. However, current PET radiotracers for studying D3 receptors in humans all bind to both D2 and D3 due to similarities between the two receptors. Selective D2 and D3 radioligands would aid investigation of the differences between D2 and D3 circuitry in the central nervous system. While there are currently in vitro measures of ligand D3/D2 selectivity, there is a need for an in vivo PET measure of D3/D2 selectivity. This review discusses current PET imaging of dopamine D2/D3 receptors and proposes methodology for quantitating in vivo selectivity of probes for PET imaging of dopamine D3 receptors.

64Cu-ATSM/64Cu-Cl2 and their relationship to hypoxia in glioblastoma: a preclinical study.

BACKGROUND: Diacetyl-bis(N4-methylthiosemicarbazone), labeled with 64Cu (64Cu-ATSM) has been suggested as a promising tracer for imaging hypoxia. However, various controversial studies highlighted potential pitfalls that may disable its use as a selective hypoxic marker. They also highlighted that the results may be tumor location dependent. Here, we first analyzed uptake of Cu-ATSM and its less lipophilic counterpart Cu-Cl2 in the tumor over time in an orthotopic glioblastoma model. An in vitro study was also conducted to investigate the hypoxia-dependent copper uptake in tumor cells. We then further performed a comprehensive ex vivo study to compare 64Cu uptake to hypoxic markers, specific cellular reactions, and also transporter expression. METHODS: µPET was performed 14 days (18F-FMISO), 15 days (64Cu-ATSM and 64Cu-Cl2), and 16 days (64Cu-ATSM and 64Cu-Cl2) after C6 cell inoculation. Thereafter, the brains were withdrawn for further autoradiography and immunohistochemistry. C6 cells were also grown in hypoxic workstation to analyze cellular uptake of Cu complexes in different oxygen levels. RESULTS: In vivo results showed that Cu-ASTM and Cu-Cl2 accumulated in hypoxic areas of the tumors. Cu-ATSM also stained, to a lesser extent, non-hypoxic regions, such as regions of astrogliosis, with high expression of copper transporters and in particular DMT-1 and CTR1, and also characterized by the expression of elevated astrogliosis. In vitro results show that 64Cu-ATSM showed an increase in the uptake only in severe hypoxia at 0.5 and 0.2% of oxygen while for 64Cu-Cl2, the cell retention was significantly increased at 5% and 1% of oxygen with no significant rise at lower oxygen percentages. CONCLUSION: In the present study, we show that Cu-complexes undoubtedly accumulate in hypoxic areas of the tumors. This uptake may be the reflection of a direct dependency to a redox metabolism and also a reflection of hypoxic-induced overexpression of transporters. We also show that Cu-ATSM also stained non-hypoxic regions such as astrogliosis.

Dosimetry and Toxicity Studies of the Novel Sulfonamide Derivative of Sulforhodamine 101([18F]SRF101) at a Preclinical Level.

BACKGROUND: The SR101 N-(3-[18F]Fluoropropyl) sulfonamide ([18F]SRF101) is a Sulforhodamine 101 derivative that was previously synthesised by our group. The fluorescent dye SR101 has been reported as a marker of astroglia in the neocortex of rodents in vivo. OBJECTIVE: The aim of this study was to perform a toxicological evaluation of [18F]SRF101 and to estimate human radiation dosimetry based on preclinical studies. METHODS: Radiation dosimetry studies were conducted based on biokinetic data obtained from a mouse model. A single-dose toxicity study was carried out. The toxicological limit chosen was <100 µg, and allometric scaling with a safety factor of 100 for unlabelled SRF101 was selected. RESULTS: The absorbed and effective dose estimated using OLINDA/EXM V2.0 for male and female dosimetric models presented the same tendency. The highest total absorbed dose values were for different sections of the intestines. The mean effective dose was 4.03 x10-3 mSv/MBq and 5.08 x10-3 mSv/MBq for the male and female dosimetric models, respectively, using tissue-weighting factors from ICRP-89. The toxicity study detected no changes in the organ or whole-body weight, food consumption, haematologic or clinical chemistry parameters. Moreover, lesions or abnormalities were not found during the histopathological examination. CONCLUSION: The toxicological evaluation of SRF101 verified the biosafety of the radiotracer for human administration. The dosimetry calculations revealed that the radiation-associated risk of [18F]SRF101 would be of the same order as other 18F radiopharmaceuticals used in clinical applications. These study findings confirm that the novel radiotracer would be safe for use in human PET imaging.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012-2017).

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

Uptake of Ga-curcumin derivatives in different cancer cell lines: Toward the development of new potential 68Ga-labelled curcuminoids-based radiotracers for tumour imaging.

Thanks to the ability to suppress the proliferation and to kill tumour cells, several studies have shown the anti-cancer effects of curcumin (CUR) and its derivatives, i.e. diacetylcurcumin (DAC) and bis-dehydroxycurcumin (bDHC). This study is focused onto the development of curcuminoid complexes with gallium-68 employed as potential new radio-labelled probes to detect neoplastic tissues through imaging techniques such as positron emission tomography. To this purpose, the uptake of three Ga-curcuminoid complexes, namely Ga(CUR)2+, Ga(DAC)2+, Ga(bDHC)2+, by various tumour cell lines was compared with the uptake of the same compounds by normal human lymphocytes by flow cytometry using the intrinsic fluorescence of the curcuminoids. Ga(CUR)2+, and particularly Ga(DAC)2+, showed a higher uptake by colorectal carcinoma (HT29) and lymphoma (K562) cell lines than lymphocytes, while the uptake of Ga(bDHC)2+ was higher in lymphocytes than in all the other cell lines. Based on the fluorescence data, Gallium-68 labelled complexes were then tested in HT29 cell line. 68Ga(DAC)2+ showed the highest uptake by HT29 cells (higher internalization with a lower externalization) and the highest affinity. The obtained results are promising and the findings foster further investigation on the development of curcumin-metal-based radiopharmaceuticals.

PET/MR Imaging: New Frontier in Alzheimer's Disease and Other Dementias.

Alzheimer's disease (AD) is the most common form of dementia; a progressive neurodegenerative disease that currently lacks an effective treatment option. Early and accurate diagnosis, in addition to quick elimination of differential diagnosis, allows us to provide timely treatments that delay the progression of AD. Imaging plays an important role for the early diagnosis of AD. The newly emerging PET/MR imaging strategies integrate the advantages of PET and MR to diagnose and monitor AD. This review introduces the development of PET/MR imaging systems, technical considerations of PET/MR imaging, special considerations of PET/MR in AD, and the system's potential clinical applications and future perspectives in AD.

Automated One-pot Radiosynthesis of [11C]S-adenosyl Methionine.

BACKGROUND: Glycine N-methyltransferase is an enzyme overexpressed in some neoplastic tissues. It catalyses the methylation of glycine using S-adenosyl methionine (SAM or AdoMet) as substrate. SAM is involved in a great variety of biochemical processes, including transmethylation reactions. Thus, [11C]SAM could be used to evaluate transmethylation activity in tumours. The only method reported for [11C]SAM synthesis is an enzymatic process with several limitations. We propose a new chemical method to obtain [11C]SAM, through a one-pot synthesis. METHOD: The optimization of [11C]SAM synthesis was carried out in the automated TRACERlab® FX C Pro module. Different labelling conditions were performed varying methylating agent, precursor amount, temperature and reaction time. The compound was purified using a semipreparative HPLC. Radiochemical stability, lipophilicity and plasma protein binding were evaluated. RESULTS: The optimum labelling conditions were [11C]CH3OTf as the methylating agent, 5 mg of precursor dissolved in formic acid at 60 °C for 1 minute. [11C]SAM was obtained as a diastereomeric mixture. Three batches were produced and quality control was performed according to specifications. [11C]SAM was stable in final formulation and in plasma. Log POCT obtained for [11C]SAM was (-2,01 ± 0,07) (n=4), and its value for plasma protein binding was low. CONCLUSION: A new chemical method to produce [11C]SAM was optimized. The radiotracer was obtained as a diastereomeric mixture with a 53:47 [(R,S)-isomer: (S,S)-isomer] ratio. The compound was within the quality control specifications. In vitro stability was verified. This compound is suitable to perform preclinical and clinical evaluations.

Fluorine-18 patents (2009-2015). Part 2: new radiochemistry.

Fluorine-18 ((18)F) is one of the most common positron-emitting radionuclides used in the synthesis of positron emission tomography radiotracers due to its ready availability, convenient half-life and outstanding imaging properties. In Part 1 of this review, we presented the first analysis of patents issued for novel radiotracers labeled with fluorine-18. In Part 2, we follow-up with a focus on patents issued for new radiochemistry methodology using fluorine-18 issued between January 2009 and December 2015.