Carbon-11 and Fluorine-18 Labeled Amino Acid Tracers for Positron Emission Tomography Imaging of Tumors

18F-fluoro-ethyl-tyrosine PET co-registered with MRI in patients with persisting acromegaly

OBJECTIVE

To report our experience with 18F-fluoro-ethyl-tyrosine (FET) positron emission tomography-computed tomography (PET-CT) co-registered with magnetic resonance imaging (MRI) (FET-PET/MRICR) in the care trajectory for persistent acromegaly.

DESIGN

Prospective case series.

PATIENTS

Ten patients with insufficiently controlled acromegaly referred to our team to evaluate surgical options.

MEASUREMENTS

FET-PET/MRICR was used to support decision-making if MRI alone and multidisciplinary team evaluation did not provide sufficient clarity to proceed to surgery.

RESULTS

FET-PET/MRICR showed suspicious (para)sellar tracer uptake in all patients. In five patients FET-PET/MRICR was fully concordant with conventional MRI, and in one patient partially concordant. FET-PET/MRICR identified suggestive new foci in four other patients. Surgical re-exploration was performed in nine patients (aimed at total resection (6), debulking (2), diagnosis (1)), and one patient underwent radiation therapy. In 7 of 9 (78%) operated patients FET-PET/MRICR findings were confirmed intraoperatively, and in six (67%) also histologically. IGF-1 decreased significantly in eight patients (89%). All patients showed clinical improvement. Complete biochemical remission was achieved in three patients (50% of procedures in which total resection was anticipated feasible). Biochemistry improved in five and was unchanged in one patient. No permanent complications occurred. At six months, optimal outcome (preoperative intended goal achieved without permanent complications) was achieved in six (67%) patients and an intermediate outcome (goal not achieved, but no complications) in the other three patients.

CONCLUSIONS

In patients with persisting acromegaly without a clear surgical target on MRI, FET-PET/MRICR is a new tracer to provide additional information to aid decision-making by the multidisciplinary pituitary team.

Synthesis and Biological Evaluation of Fluorine-18 and Deuterium Labeled l-Fluoroalanines as Positron Emission Tomography Imaging Agents for Cancer Detection

To fully explore the potential of 18F-labeled l-fluoroalanine for imaging cancer and other chronic diseases, a simple and mild radiosynthesis method has been established to produce optically pure l-3-[18F]fluoroalanine (l-[18F]FAla), using a serine-derivatized, five-membered-ring sulfamidate as the radiofluorination precursor. A deuterated analogue, l-3-[18F]fluoroalanine-d3 (l-[18F]FAla-d3), was also prepared to improve metabolic stability. Both l-[18F]FAla and l-[18F]FAla-d3 were rapidly taken up by 9L/lacZ, MIA PaCa-2, and U87MG cells and were shown to be substrates for the alanine-serine-cysteine (ASC) amino acid transporter. The ability of l-[18F]FAla, l-[18F]FAla-d3, and the d-enantiomer, d-[18F]FAla-d3, to image tumors was evaluated in U87MG tumor-bearing mice. Despite the significant bone uptake was observed for both l-[18F]FAla and l-[18F]FAla-d3, the latter had enhanced tumor uptake compared to l-[18F]FAla, and d-[18F]FAla-d3 was not specifically taken up by the tumors. The enhanced tumor uptake of l-[18F]FAla-d3 compared with its nondeuterated counterpart, l-[18F]FAla, warranted the further biological investigation of this radiotracer as a potential cancer imaging agent.

18F-FACBC and 18F-FDG PET/MRI in the evaluation of 3 patients with primary central nervous system lymphoma: a pilot study

BACKGROUND

This PET/MRI study compared contrast-enhanced MRI, 18F-FACBC-, and 18F-FDG-PET in the detection of primary central nervous system lymphomas (PCNSL) in patients before and after high-dose methotrexate chemotherapy. Three immunocompetent PCNSL patients with diffuse large B-cell lymphoma received dynamic 18F-FACBC- and 18F-FDG-PET/MRI at baseline and response assessment. Lesion detection was defined by clinical evaluation of contrast enhanced T1 MRI (ce-MRI) and visual PET tracer uptake. SUVs and tumor-to-background ratios (TBRs) (for 18F-FACBC and 18F-FDG) and time-activity curves (for 18F-FACBC) were assessed.

RESULTS

At baseline, seven ce-MRI detected lesions were also detected with 18F-FACBC with high SUVs and TBRs (SUVmax:mean, 4.73, TBRmax: mean, 9.32, SUVpeak: mean, 3.21, TBRpeak:mean: 6.30). High TBR values of 18F-FACBC detected lesions were attributed to low SUVbackground. Baseline 18F-FDG detected six lesions with high SUVs (SUVmax: mean, 13.88). In response scans, two lesions were detected with ce-MRI, while only one was detected with 18F-FACBC. The lesion not detected with 18F-FACBC was a small atypical MRI detected lesion, which may indicate no residual disease, as this patient was still in complete remission 12 months after initial diagnosis. No lesions were detected with 18F-FDG in the response scans.

CONCLUSIONS

18F-FACBC provided high tumor contrast, outperforming 18F-FDG in lesion detection at both baseline and in response assessment. 18F-FACBC may be a useful supplement to ce-MRI in PCNSL detection and response assessment, but further studies are required to validate these findings. Trial registration ClinicalTrials.gov. Registered 15th of June 2017 (Identifier: NCT03188354, https://clinicaltrials.gov/study/NCT03188354 ).

Diagnostic accuracy of anti-3-[18F]-FACBC PET/MRI in gliomas

PURPOSE

The primary aim was to evaluate whether anti-3-[18F]FACBC PET combined with conventional MRI correlated better with histomolecular diagnosis (reference standard) than MRI alone in glioma diagnostics. The ability of anti-3-[18F]FACBC to differentiate between molecular and histopathological entities in gliomas was also evaluated.

METHODS

In this prospective study, patients with suspected primary or recurrent gliomas were recruited from two sites in Norway and examined with PET/MRI prior to surgery. Anti-3-[18F]FACBC uptake (TBRpeak) was compared to histomolecular features in 36 patients. PET results were then added to clinical MRI readings (performed by two neuroradiologists, blinded for histomolecular results and PET data) to assess the predicted tumor characteristics with and without PET.

RESULTS

Histomolecular analyses revealed two CNS WHO grade 1, nine grade 2, eight grade 3, and 17 grade 4 gliomas. All tumors were visible on MRI FLAIR. The sensitivity of contrast-enhanced MRI and anti-3-[18F]FACBC PET was 61% (95%CI [45, 77]) and 72% (95%CI [58, 87]), respectively, in the detection of gliomas. Median TBRpeak was 7.1 (range: 1.4-19.2) for PET positive tumors. All CNS WHO grade 1 pilocytic astrocytomas/gangliogliomas, grade 3 oligodendrogliomas, and grade 4 glioblastomas/astrocytomas were PET positive, while 25% of grade 2-3 astrocytomas and 56% of grade 2-3 oligodendrogliomas were PET positive. Generally, TBRpeak increased with malignancy grade for diffuse gliomas. A significant difference in PET uptake between CNS WHO grade 2 and 4 gliomas (p < 0.001) and between grade 3 and 4 gliomas (p = 0.002) was observed. Diffuse IDH wildtype gliomas had significantly higher TBRpeak compared to IDH1/2 mutated gliomas (p < 0.001). Adding anti-3-[18F]FACBC PET to MRI improved the accuracy of predicted glioma grades, types, and IDH status, and yielded 13.9 and 16.7 percentage point improvement in the overall diagnoses for both readers, respectively.

CONCLUSION

Anti-3-[18F]FACBC PET demonstrated high uptake in the majority of gliomas, especially in IDH wildtype gliomas, and improved the accuracy of preoperatively predicted glioma diagnoses.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov ID: NCT04111588, URL: https://clinicaltrials.gov/study/NCT04111588.

Evaluation of (2S,4S)-4-[18F]FEBGln as a Positron Emission Tomography Tracer for Tumor Imaging

Glutamine metabolism-related tracers have the potential to visualize numerous tumors because glutamine is the second largest source of energy for tumors. (2S,4S)-4-[18F]FEBGln was designed by introducing [18F]fluoroethoxy benzyl on carbon-4 of glutamine. The aim of this study was to investigate the pharmacokinetic properties and tumor positron emission tomography (PET) imaging characteristics of (2S,4S)-4-[18F]FEBGln in detail. The biodistribution results of nude mice bearing MCF-7 tumor showed that (2S,4S)-4-[18F]FEBGln had high initial tumor uptake, and a fast clearance rate, resulting in a high tumor-to-muscle ratio at 30 min postinjection. There was no obvious defluorination in vivo. The micro-PET-CT imaging results of (2S,4S)-4-[18F]FEBGln orthotopic MCF-7 tumor-bearing nude mice were consistent with the biological distribution results. Compared with (2S,4R)-4-[18F]FGln, (2S,4S)-4-[18F]FEBGln showed poor tumor retention, but its clearance in normal tissues was also fast, so it had better PET image contrast than the former. Unlike poor retention in MCF-7-bearing nude mice, (2S,4S)-4-[18F]FEBGln has good retention in NCI-h1975 and 22Rv1 tumor models. Since (2S,4S)-4-[18F]FEBGln has low uptake in normal lungs and high uptake in the bladder, it is expected to be used in the accurate diagnosis of lung cancer but cannot accurately determine prostate cancer. Consistent with the advantages of radiolabeled amino acids in the application of brain tumors, (2S,4S)-4-[18F]FEBGln accurately diagnoses U87MG glioma with higher contrast than [18F]FET and [18F]FDG, and there is a correlation between (2S,4S)-4-[18F]FEBGln uptake and tumor growth cycle. Further kinetic model analysis showed that (2S,4S)-4-[18F]FEBGln was similar to (2S,4R)-4-[18F]FGln, conforming to the one-compartment model and the Logan graphical model, and was expected to assess the size of the glutamine pool of the tumor. Therefore, (2S,4S)-4-[18F]FEBGln is expected to provide a strong imaging basis for the diagnosis, formulation of personalized plans, and efficacy evaluation of glioma, lung cancer, and breast cancer.

Head-To-Head Comparison of PET and Perfusion Weighted MRI Techniques to Distinguish Treatment Related Abnormalities from Tumor Progression in Glioma

The post-treatment imaging surveillance of gliomas is challenged by distinguishing tumor progression (TP) from treatment-related abnormalities (TRA). Sophisticated imaging techniques, such as perfusion-weighted magnetic resonance imaging (MRI PWI) and positron-emission tomography (PET) with a variety of radiotracers, have been suggested as being more reliable than standard imaging for distinguishing TP from TRA. However, it remains unclear if any technique holds diagnostic superiority. This meta-analysis provides a head-to-head comparison of the diagnostic accuracy of the aforementioned imaging techniques. Systematic literature searches on the use of PWI and PET imaging techniques were carried out in PubMed, Embase, the Cochrane Library, ClinicalTrials.gov and the reference lists of relevant papers. After the extraction of data on imaging technique specifications and diagnostic accuracy, a meta-analysis was carried out. The quality of the included papers was assessed using the QUADAS-2 checklist. Nineteen articles, totaling 697 treated patients with glioma (431 males; mean age ± standard deviation 50.5 ± 5.1 years) were included. The investigated PWI techniques included dynamic susceptibility contrast (DSC), dynamic contrast enhancement (DCE) and arterial spin labeling (ASL). The PET-tracers studied concerned [S-methyl-¹¹C]methionine, 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG), O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) and 6-[¹⁸F]-fluoro-3,4-dihydroxy-L-phenylalanine ([¹⁸F]FDOPA). The meta-analysis of all data showed no diagnostic superior imaging technique. The included literature showed a low risk of bias. As no technique was found to be diagnostically superior, the local level of expertise is hypothesized to be the most important factor for diagnostically accurate results in post-treatment glioma patients regarding the distinction of TRA from TP.

The other immuno-PET: Metabolic tracers in evaluation of immune responses to immune checkpoint inhibitor therapy for solid tumors

Unique patterns of response to immune checkpoint inhibitor therapy, discernable in the earliest clinical trials, demanded a reconsideration of the standard methods of radiological treatment assessment. Immunomonitoring, that characterizes immune responses, offers several significant advantages over the tumor-centric approach currently used in the clinical practice: 1) better understanding of the drugs' mechanism of action and treatment resistance, 2) earlier assessment of response to therapy, 3) patient/therapy selection, 4) evaluation of toxicity and 5) more accurate end-point in clinical trials. PET imaging in combination with the right agent offers non-invasive tracking of immune processes on a whole-body level and thus represents a method uniquely well-suited for immunomonitoring. Small molecule metabolic tracers, largely neglected in the immuno-PET discourse, offer a way to monitor immune responses by assessing cellular metabolism known to be intricately linked with immune cell function. In this review, we highlight the use of small molecule metabolic tracers in imaging immune responses, provide a view of their value in the clinic and discuss the importance of image analysis in the context of tracking a moving target.

Evaluation of sulfone-labeled amino acid derivatives as potential PET agents for cancer imaging

INTRODUCTION

As one of the most important and frequently used molecular imaging techniques in the clinic, positron emission tomography (PET) features high sensitivity and specificity, which generally involves the use of PET contrast agents. Despite the exceptional promise, the availability of novel PET agents could limit its application and there is a clear need to develop new PET agents to improve our understanding of targets of interest and increase the diagnostic specificity.

METHODS

Based on the fact that amino acid transport and protein anabolism are increased in tumor tissues, a series of ¹⁸F-labeled amino acid analog was labeled with ¹⁸F by using [¹⁸F]fluoro-4-(vinylsulfonyl)benzene as the radionuclide linker. The obtained probes were subjected to in vitro and in vivo evaluation, including stability, cell line transport channel specificity, PET/CT imaging on tumor and inflammation bearing mice, and biodistribution.

RESULTS

Our data shows that [¹⁸F]2a had moderate decay corrected labeling yield (>42 %) and high radiochemical purity (>99 %). When tested in vivo, the uptake of [¹⁸F]2a was 1.5 ± 0.2%ID/g in NCI-H1975 tumors and 1.1 ± 0.2%ID/g in inflammatory tissues. In contrast, the values for [¹⁸F]FDG were 5.7 ± 0.2%ID/g and 4.8 ± 0.1%ID/g, respectively. The inflammatory lesion-to-muscle contrast is 2.4 for [¹⁸F]2a, which is 3.0 for [¹⁸F]FDG.

CONCLUSION

Clearly, [¹⁸F]2a hold the great potential for cancer imaging. Its application in distinguishing tumor from inflammatory lesion would still need to be investigated further.

An overview of radiolabeled amino acid tracers in oncologic imaging

Molecular imaging has witnessed a great progress in the field of oncology over the past few decades. Radiolabeled amino acid (AA) tracers are particularly helpful in the areas where the utility of 18F-Fluorodeoxyglucose (18F-FDG) positron emission tomography with computed tomography imaging has been limited such as in evaluating brain tumors, neuroendocrine tumors (NETs), and prostate cancer. Radiolabeled AA tracers such as 6-[18F]-L-fluoro-L-3, 4-dihydroxyphenylalanine (18F-FDOPA), 18F-fluoro-ethyl-tyrosine (18F-FET), and 11C-methionine have found wide applications in brain tumors, which, unlike 18F-FDG, concentrate in the tumor tissue to a greater extent than that in normal brain tissue by providing accurate information about tumor volume and boundaries. 18F-FDOPA is also useful in evaluating NETs. Tracers such as 18F-FACBC (Fluciclovine) and anti-1-amino-2-[18F]fluorocyclopentyl-1-carboxylic acid (18F-FACPC) are used in imaging of prostate cancer and provide valuable information of locoregional, recurrent, and metastatic disease. This review highlights AA tracers and their major applications in imaging, viz., in evaluating brain tumors, NETs, and prostate cancer.

Application of Metabolic Reprogramming to Cancer Imaging and Diagnosis

Cellular metabolism governs the signaling that supports physiological mechanisms and homeostasis in an individual, including neuronal transmission, wound healing, and circadian clock manipulation. Various factors have been linked to abnormal metabolic reprogramming, including gene mutations, epigenetic modifications, altered protein epitopes, and their involvement in the development of disease, including cancer. The presence of multiple distinct hallmarks and the resulting cellular reprogramming process have gradually revealed that these metabolism-related molecules may be able to be used to track or prevent the progression of cancer. Consequently, translational medicines have been developed using metabolic substrates, precursors, and other products depending on their biochemical mechanism of action. It is important to note that these metabolic analogs can also be used for imaging and therapeutic purposes in addition to competing for metabolic functions. In particular, due to their isotopic labeling, these compounds may also be used to localize and visualize tumor cells after uptake. In this review, the current development status, applicability, and limitations of compounds targeting metabolic reprogramming are described, as well as the imaging platforms that are most suitable for each compound and the types of cancer to which they are most appropriate.

Direct C(sp3)-H Polyfluoroarylation: Access to Polyfluoroaryl Amino Acids via Rh-Catalyzed Selective C-F Bond Cleavage

A catalytic selective C-F bond alkylation method for polyfluoroarene with glycinates and derivatives in the presence of a DavePhos-ligated Rh catalyst was developed. This method avoids the preactivation of alkylating reagents and provides an efficient and straightforward route to synthesize a series of polyfluoroaryl amino acids via C(sp³)-H functionalization. This reaction proceeds under mild conditions and exhibits high reactivity and excellent chemoselectivities. Meanwhile, the synthetic potential of this method was demonstrated by gram-scale synthesis, and further transformations proved the application value of the products as well.

Comparison of [18F]fluciclovine and [18F]FDG PET/CT in Newly Diagnosed Multiple Myeloma Patients

PURPOSE

[¹⁸F]FDG PET/CT in multiple myeloma (MM) is currently the best technology to demonstrate patchy and extramedullary disease. However, [¹⁸F]FDG PET has some limitations, and imaging with alternative tracers should be explored. In this study, we aimed to evaluate the performance of [¹⁸F]fluciclovine PET compared to [¹⁸F]FDG PET in newly diagnosed MM patients.

PROCEDURES

Thirteen newly diagnosed transplant eligible MM patients were imaged both with [¹⁸F]FDG PET/CT and [¹⁸F]fluciclovine PET/CT within 1 week in a prospective study. The subjects were visually assessed positive or negative for disease. The number of lesions and the SUV_max of selected lesions were measured for both tracers. Furthermore, tracer uptake ratios were obtained by dividing lesion SUV_max by blood or bone marrow SUV_max. Between-group differences and correlations were assessed with paired t-tests and Pearson tests. Bone marrow SUVs were compared to bone marrow plasma cell percentage in biopsy samples.

RESULTS

Nine subjects were assessed positively by [¹⁸F]FDG PET (69%) and 12 positives by [¹⁸F]fluciclovine PET (92%). All positive subjects had [¹⁸F]fluciclovine scans that were qualitatively scored as easier to interpret visually than the [¹⁸F]FDG scans. The number of lesions was also higher; seven of nine subjects with distinct hot spots on [¹⁸F]fluciclovine PET had fewer or no visible lesions on [¹⁸F]FDG PET. The mean lesion SUV_max values were 8.2 and 3.8 for [¹⁸F]fluciclovine and [¹⁸F]FDG, respectively. The mean tumour to blood values were 6.4 and 2.0 for [¹⁸F]fluciclovine and [¹⁸F]FDG, and the mean ratios between tumour and bone marrow were 2.1 and 1.5 for [¹⁸F]fluciclovine and [¹⁸F]FDG. The lesion SUV_max and ratios were significantly higher for [¹⁸F]fluciclovine (all p < 0.01). Local [¹⁸F]fluciclovine SUV_max or SUV_mean values in os ilium and the percentage of plasma cells in bone marrow biopsies were linearly correlated (p = 0.048). There were no significant correlations between [¹⁸F]FDG SUVs and plasma cells (p = 0.82).

CONCLUSIONS

Based on this pilot study, [¹⁸F]fluciclovine is a promising tracer for MM. The visual and semi-quantitative evaluations indicate that [¹⁸F]fluciclovine PET/CT can out-perform [¹⁸F]FDG PET/CT at diagnosis.

Radiotracers for Nuclear Imaging of Reactive Oxygen Species: Advances Made So Far

Reactive oxygen species (ROS) are a cluster of highly reactive and short-lived oxygen-containing molecules that lead to metabolic disorders where production exceeds catabolism in an organism. Many specific radiotracers for positron/single-photon emission tomography have been developed to reveal the discrepancy of ROS levels in normal and damaged tissues and further clarify the relationship between ROS and diseases. This review summarizes the advances achieved for the development of ROS radiotracers to date. The structure design, radiosynthesis, and imaging performance of existing radiotracers are discussed with the individual ROS-response mechanisms highlighted.

Advanced Imaging Techniques for Newly Diagnosed and Recurrent Gliomas

Management of gliomas following initial diagnosis requires thoughtful presurgical planning followed by regular imaging to monitor treatment response and survey for new tumor growth. Traditional MR imaging modalities such as T1 post-contrast and T2-weighted sequences have long been a staple of tumor diagnosis, surgical planning, and post-treatment surveillance. While these sequences remain integral in the management of gliomas, advances in imaging techniques have allowed for a more detailed characterization of tumor characteristics. Advanced MR sequences such as perfusion, diffusion, and susceptibility weighted imaging, as well as PET scans have emerged as valuable tools to inform clinical decision making and provide a non-invasive way to help distinguish between tumor recurrence and pseudoprogression. Furthermore, these advances in imaging have extended to the operating room and assist in making surgical resections safer. Nevertheless, surgery, chemotherapy, and radiation treatment continue to make the interpretation of MR changes difficult for glioma patients. As analytics and machine learning techniques improve, radiomics offers the potential to be more quantitative and personalized in the interpretation of imaging data for gliomas. In this review, we describe the role of these newer imaging modalities during the different stages of management for patients with gliomas, focusing on the pre-operative, post-operative, and surveillance periods. Finally, we discuss radiomics as a means of promoting personalized patient care in the future.

Bioimaging agents based on redox-active transition metal complexes

Detecting the fluctuation and distribution of various bioactive species in biological systems is of great importance in determining diseases at their early stages. Metal complex-based probes have attracted considerable attention in bioimaging applications owing to their unique advantages, such as high luminescence, good photostability, large Stokes shifts, low toxicity, and good biocompatibility. In this review, we summarized the development of redox-active transition metal complex-based probes in recent five years with the metal ions of iron, manganese, and copper, which play essential roles in life and can avoid the introduction of exogenous metals into biological systems. The designing principles that afford these complexes with optical or magnetic resonance (MR) imaging properties are elucidated. The applications of the complexes for bioimaging applications of different bioactive species are demonstrated. The current challenges and potential future directions of these probes for applications in biological systems are also discussed.

This review summarizes transition metal complexes as bioimaging agents in optical and magnetic resonance imaging.

First-in-Human PET Imaging and Estimated Radiation Dosimetry of l-[5-11C]-Glutamine in Patients with Metastatic Colorectal Cancer

Altered metabolism is a hallmark of cancer. In addition to glucose, glutamine is an important nutrient for cellular growth and proliferation. Noninvasive imaging via PET may help facilitate precision treatment of cancer through patient selection and monitoring of treatment response. l-[5-11C]-glutamine (11C-glutamine) is a PET tracer designed to study glutamine uptake and metabolism. The aim of this first-in-human study was to evaluate the radiologic safety and biodistribution of 11C-glutamine for oncologic PET imaging. Methods: Nine patients with confirmed metastatic colorectal cancer underwent PET/CT imaging. Patients received 337.97 ± 44.08 MBq of 11C-glutamine. Dynamic PET acquisitions that were centered over the abdomen or thorax were initiated simultaneously with intravenous tracer administration. After the dynamic acquisition, a whole-body PET/CT scan was acquired. Volume-of-interest analyses were performed to obtain estimates of organ-based absorbed doses of radiation. Results:11C-glutamine was well tolerated in all patients, with no observed safety concerns. The organs with the highest radiation exposure included the bladder, pancreas, and liver. The estimated effective dose was 4.46E-03 ± 7.67E-04 mSv/MBq. Accumulation of 11C-glutamine was elevated and visualized in lung, brain, bone, and liver metastases, suggesting utility for cancer imaging. Conclusion: PET using 11C-glutamine appears safe for human use and allows noninvasive visualization of metastatic colon cancer lesions in multiple organs. Further studies are needed to elucidate its potential for other cancers and for monitoring response to treatment.

Comparison of: (2S,4R)-4-[18F]Fluoroglutamine, [11C]Methionine, and 2-Deoxy-2-[18F]Fluoro-D-Glucose and Two Small-Animal PET/CT Systems Imaging Rat Gliomas

Purpose

The three positron emission tomography (PET) imaging compounds: (2S,4R)-4-[¹⁸F]Fluoroglutamine ([¹⁸F]FGln), L-[methyl-¹¹C]Methionine ([¹¹C]Met), and 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) were investigated to contrast their ability to image orthotopic BT4C gliomas in BDIX rats. Two separate small animal imaging systems were compared for their tumor detection potential. Dynamic acquisition of [¹⁸F]FGln was evaluated with multiple pharmacokinetic models for future quantitative comparison.

Procedures

Up to four imaging studies were performed on each orthotopically grafted BT4C glioma-bearing BDIX rat subject (n = 16) on four consecutive days. First, a DOTAREM^® contrast enhanced MRI followed by attenuation correction CT and dynamic PET imaging with each radiopharmaceutical (20 min [¹¹C]Met, 60 min [¹⁸F]FDG, and 60 min [¹⁸F]FGln with either the Molecubes PET/CT (n = 5) or Inveon PET/CT cameras (n = 11). Ex vivo brain autoradiography was completed for each radiopharmaceutical and [¹⁸F]FGln pharmacokinetics were studied by injecting 40 MBq into healthy BDIX rats (n = 10) and collecting blood samples between 5 and 60 min. Erythrocyte uptake, plasma protein binding and plasma parent-fraction were combined to estimate the total blood bioavailability of [¹⁸F]FGln over time. The corrected PET-image blood data was then applied to multiple pharmacokinetic models.

Results

Average BT4C tumor-to-healthy brain tissue uptake ratios (TBR) for PET images reached maxima of: [¹⁸F]FGln TBR: 1.99 ± 0.19 (n = 13), [¹⁸F]FDG TBR: 1.41 ± 0.11 (n = 6), and [¹¹C]Met TBR: 1.08 ± 0.08, (n = 12) for the dynamic PET images. Pharmacokinetic modeling in dynamic [¹⁸F]FGln studies suggested both reversible and irreversible uptake play a similar role. Imaging with Inveon and Molecubes yielded similar end-result ratios with insignificant differences (p > 0.25).

Conclusions

In orthotopic BT4C gliomas, [¹⁸F]FGln may offer improved imaging versus [¹¹C]Met and [¹⁸F]FDG. No significant difference in normalized end-result data was found between the Inveon and Molecubes camera systems. Kinetic modelling of [¹⁸F]FGln uptake suggests that both reversible and irreversible uptake play an important role in BDIX rat pharmacokinetics.

Automated Radiosynthesis of cis- and trans-4-[18F]Fluoro-l-proline Using [18F]Fluoride

The positron emission tomography imaging agents cis- and trans-4-[18F]fluoro-l-proline are used for the detection of numerous diseases such as pulmonary fibrosis and various carcinomas. These imaging agents are typically prepared by nucleophilic fluorination of 4-hydroxy-l-proline derivatives, with [18F]fluoride, followed by deprotection. Although effective radiofluorination reactions have been developed, the overall radiosynthesis process is suboptimal due to deprotection methods that are performed manually, require multiple steps, or involve harsh conditions. Here we describe the development of two synthetic routes that allow access to precursors, which undergo highly selective radiofluorination reactions and rapid deprotection, under mild acidic conditions. These methods were found to be compatible with automation, avoiding manual handling of radioactive intermediates.

Enantiomeric purity deviations of radiolabelled amino acids obtained from chiral columns

Enantiomeric purity (EP) is an important value which denotes the relative percentage of the L-isomer with respect to the D-isomer. For 11C and 18F-labelled amino acid (AA) radiopharmaceutical (RP) production, EP represents a quality control parameter specified in European and national monographs for particular RPs. In most instances, EP value of greater then 90 or 95% (depending on AA type) is required as part of the quality control (QC) value of a RP following radiosynthesis. In common practice, two chromatographic columns are used for the EP determination of RPs: Crownpak CR(+) (Daicel), which contains a crown ether stationary phase or Chirobiotic T (Astec), which contains silica-bound glycoproteins as the stationary phase. The application of column Crownpak CR(+) requires that only perchloric acid solution (with pH 1–2) may be used, as the retention capability of the stationary phase is greatly reduced using organic solvents. This work intends to identify which chromatographic system is more accurate and reliable for EP determination as part of QC. We performed a series of parallel injections of the same batch of the widely used AA RPs [11C]MET and [18F]FET on the two aforementioned columns. The EP determination using column Crownpak CR(+) consistently provided a lower EP value compared to the Chirobiotic T column; the EP deviation between the respective columns was found to range from 2.4–4.0% for the same RP sample. Furthermore, the EP value was influenced by a sample’s dilution factor, e.g. the EP was observed to increase up to 1.5% when the radioactive sample had a fivefold dilution factor. This phenomenon was consistent for both Crownpak CR(+) and Chirobiotic T columns. Finally, a series of standard solutions of non-radioactive methionine with various ratios of L-and D-isomers was analyzed. The data obtained for non-radioactive methionine confirmed that column Crownpak CR(+) incorrectly provided a higher D-enantiomer concentration, whereas Chirobiotic T was found to provide a lower D-enantiomer concentration of the same sample. The deviation from the theoretical EP value was between 0.67 and 1.92%.

Production of a broad palette of positron emitting radioisotopes using a low-energy cyclotron: Towards a new success story in cancer imaging?

Over the last several years, positron emission tomography (PET) has matured as an indispensable component of cancer diagnostics. Owing to the large variability observed among the cancer patients and the need to personalize individual patient's diagnosis and treatment, the need for new positron emitting radioisotopes has continued to grow. This mini review opens with a brief introduction to the criteria for radioisotope selection for PET imaging. Subsequently, positron emitting radioisotopes are categorized as: established, emerging and futuristic, based on the stages of their advancement. The production methodologies and the radiochemical separation procedures for obtaining the important radioisotopes in a form suitable for preparation of radiopharmaceuticals for PET imaging are briefly discussed.

Synthesis of L-[5-11 C]Leucine and L-α-[5-11 C]Methylleucine via Pd0 -mediated 11 C-Methylation and Microfluidic Hydrogenation: Potentiality of Leucine PET Probes for Tumor Imaging

The efficient synthesis of L-[5-¹¹ C]leucine and L-α-[5-¹¹ C]methylleucine has been investigated using a continuous two-step sequence of rapid reactions consisting of Pd⁰ -mediated ¹¹ C-methylation and microfluidic hydrogenation. The synthesis of L-[5-¹¹ C]leucine and L-α-[5-¹¹ C]methylleucine was accomplished within 40 min with a decay-corrected radiochemical yield of 15-38 % based on [¹¹ C]CH₃ I, radiochemical purity of 95-99 %, and chemical purity of 95-99 %. The Pd impurities in the injectable solution measured using inductively coupled plasma mass spectrometry met the international criteria for human use. Positron emission tomography scanning after an intravenous injection of L-[5-¹¹ C]leucine or L-α-[5-¹¹ C]methyl leucine in A431 tumor-bearing mice was performed. As a result, L-α-[5-¹¹ C]methylleucine was found to be a potentially useful probe for visualizing the tumor. Tissue distribution analysis showed that the accumulation value of L-α-[5-¹¹ C]methylleucine in tumor tissue was high [12±3% injected dose/g tissue (%ID/g)].

18F-FACBC PET/MRI in the evaluation of human brain metastases: a case report

Background

Patients with metastatic cancer to the brain have a poor prognosis. In clinical practice, MRI is used to delineate, diagnose and plan treatment of brain metastases. However, MRI alone is limited in detecting micro-metastases, delineating lesions and discriminating progression from pseudo-progression. Combined PET/MRI utilises superior soft tissue images from MRI and metabolic data from PET to evaluate tumour structure and function. The amino acid PET tracer ¹⁸F-FACBC has shown promising results in discriminating high- and low-grade gliomas, but there are currently no reports on its use on brain metastases. This is the first study to evaluate the use of ¹⁸F-FACBC on brain metastases.

Case presentation

A middle-aged female patient with brain metastases was evaluated using hybrid PET/MRI with ¹⁸F-FACBC before and after stereotactic radiotherapy, and at suspicion of recurrence. Static/dynamic PET and contrast-enhanced T1 MRI data were acquired and analysed. This case report includes the analysis of four ¹⁸F-FACBC PET/MRI examinations, investigating their utility in evaluating functional and structural metastasis properties.

Conclusion

Analysis showed high tumour-to-background ratios in brain metastases compared to other amino acid PET tracers, including high uptake in a very small cerebellar metastasis, suggesting that ¹⁸F-FACBC PET can provide early detection of otherwise overlooked metastases. Further studies to determine a threshold for ¹⁸F-FACBC brain tumour boundaries and explore its utility in clinical practice should be performed.

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Research progress of 18F labeled small molecule positron emission tomography (PET) imaging agents

With the development of positron emission tomography (PET) technology, a variety of PET imaging agents labeled with radionuclide ¹⁸F have been developed and widely used in the diagnosis and treatment of various clinical diseases in recent years. For example, they have showed a great value of study in the field of tumor detection, tumor treatment and evaluation of tumor therapy in a non-invasive, qualitative and quantitative way. In this review, we highlight the recent development in chemical synthesis, structure and characterization, imaging characterization, and potential applications of these ¹⁸F labeled small molecule PET imaging agents for the past five years. The development and application of ¹⁸F labeled small molecules will expand our knowledge of the function and distribution of diseases-related molecular targets and shed light on the diagnosis and treatment of various diseases including tumors.

Overview on Multienzymatic Cascades for the Production of Non-canonical α-Amino Acids

The 22 genetically encoded amino acids (AAs) present in proteins (the 20 standard AAs together with selenocysteine and pyrrolysine), are commonly referred as proteinogenic AAs in the literature due to their appearance in ribosome-synthetized polypeptides. Beyond the borders of this key set of compounds, the rest of AAs are generally named imprecisely as non-proteinogenic AAs, even when they can also appear in polypeptide chains as a result of post-transductional machinery. Besides their importance as metabolites in life, many of D-α- and L-α-"non-canonical" amino acids (NcAAs) are of interest in the biotechnological and biomedical fields. They have found numerous applications in the discovery of new medicines and antibiotics, drug synthesis, cosmetic, and nutritional compounds, or in the improvement of protein and peptide pharmaceuticals. In addition to the numerous studies dealing with the asymmetric synthesis of NcAAs, many different enzymatic pathways have been reported in the literature allowing for the biosynthesis of NcAAs. Due to the huge heterogeneity of this group of molecules, this review is devoted to provide an overview on different established multienzymatic cascades for the production of non-canonical D-α- and L-α-AAs, supplying neophyte and experienced professionals in this field with different illustrative examples in the literature. Whereas the discovery of new or newly designed enzymes is of great interest, dusting off previous enzymatic methodologies by a "back and to the future" strategy might accelerate the implementation of new or improved multienzymatic cascades.

Nuclear medicine and molecular imaging advances in the 21st century

Currently, Nuclear Medicine has a clearly defined role in clinical practice due to its usefulness in many medical disciplines. It provides relevant diagnostic and therapeutic options leading to patients' healthcare and quality of life improvement. During the first two decades of the 21stt century, the number of Nuclear Medicine procedures increased considerably.Clinical and research advances in Nuclear Medicine and Molecular Imaging have been based on developments in radiopharmaceuticals and equipment, namely, the introduction of multimodality imaging. In addition, new therapeutic applications of radiopharmaceuticals, mainly in oncology, are underway.This review will focus on radiopharmaceuticals for positron emission tomography (PET), in particular, those labeled with Fluorine-18 and Gallium-68. Multimodality as a key player in clinical practice led to the development of new detector technology and combined efforts to improve resolution. The concept of dual probe (a single molecule labeled with a radionuclide for single photon emission computed tomography)/positron emission tomography and a light emitter for optical imaging) is gaining increasing acceptance, especially in minimally invasive radioguided surgery. The expansion of theranostics, using the same molecule for diagnosis (γ or positron emitter) and therapy (β minus or α emitter) is reshaping personalized medicine.Upcoming research and development efforts will lead to an even wider array of indications for Nuclear Medicine both in diagnosis and treatment.

Fluorinated phenylalanines: synthesis and pharmaceutical applications

Recent advances in the chemistry of peptides containing fluorinated phenylalanines (Phe) represents a hot topic in drug research over the last few decades. ᴅ- or ʟ-fluorinated phenylalanines have had considerable industrial and pharmaceutical applications and they have been expanded also to play an important role as potential enzyme inhibitors as well as therapeutic agents and topography imaging of tumor ecosystems using PET. Incorporation of fluorinated aromatic amino acids into proteins increases their catabolic stability especially in therapeutic proteins and peptide-based vaccines. This review seeks to summarize the different synthetic approaches in the literature to prepare ᴅ- or ʟ-fluorinated phenylalanines and their pharmaceutical applications with a focus on published synthetic methods that introduce fluorine into the phenyl, the β-carbon or the α-carbon of ᴅ-or ʟ-phenylalanines.

Radiosynthesis, in vitro and preliminary in vivo evaluation of the novel glutamine derived PET tracers [18F]fluorophenylglutamine and [18F]fluorobiphenylglutamine

INTRODUCTION

Glucose has been deemed the driving force of tumor growth for decades. However, research has shown that several tumors metabolically shift towards glutaminolysis. The development of radiolabeled glutamine derivatives could be a useful molecular imaging tool for visualizing these tumors. We elaborated on the glutamine-derived PET tracers by developing two novel probes, namely [¹⁸F]fluorophenylglutamine and [¹⁸F]fluorobiphenylglutamine.

MATERIALS AND METHODS

Both tracers were labelled with fluorine-18 using our recently reported ruthenium-based direct aromatic fluorination method. Their affinity was evaluated with a [³H]glutamine inhibition experiment in a human PC-3 and a rat F98 cell line. The imaging potential of [¹⁸F]fluorophenylglutamine and [¹⁸F]fluorobiphenylglutamine was tested using a mouse PC-3 and a rat F98 tumor model.

RESULTS

The radiosynthesis of both tracers was successful with overall non-decay corrected yields of 18.46 ± 4.18% (n = 10) ([¹⁸F]fluorophenylglutamine) and 8.05 ± 3.25% (n = 5) ([¹⁸F]fluorobiphenylglutamine). In vitro inhibition experiments showed a moderate and low affinity of fluorophenylglutamine and fluorobiphenylglutamine, respectively, towards the human ASCT-2 transporter. Both compounds had a low affinity towards the rat ASCT-2 transporter. These results were endorsed by the in vivo experiments with low uptake of both tracers in the F98 rat xenograft, low uptake of [¹⁸F]FBPG in the mice PC-3 xenograft and a moderate uptake of [¹⁸F]FPG in the PC-3 tumors.

CONCLUSION

We investigated the imaging potential of two novel PET radiotracers [¹⁸F]FPG and [¹⁸F]FBPG. [¹⁸F]FPG is the first example of a glutamine radiotracer derivatized with a phenyl group which enables the exploration of further derivatization of the phenyl group to increase the affinity and imaging qualities. We hypothesize that increasing the affinity of [¹⁸F]FPG by optimizing the substituents of the arene ring can result in a high-quality glutamine-based PET radiotracer. Advances in Knowledge and Implications for patient care: We hereby report novel glutamine-based PET-tracers. These tracers are tagged on the arene group with fluorine-18, hereby preventing in vivo defluorination, which can occur with alkyl labelled tracers (e.g. (2S,4R)4-[¹⁸F]fluoroglutamine). [¹⁸F]FPG shows clear tumor uptake in vivo, has no in vivo defluorination and has a straightforward production. We believe this tracer is a good starting point for the development of a high-quality tracer which is useful for the clinical visualization of the glutamine transport.

18F-FET PET for Diagnosis of Pseudoprogression of Brain Metastases in Patients With Non-Small Cell Lung Cancer

PURPOSE

To evaluate whether F-fluoroethyltyrosine (FET) PET can discriminate progression from pseudoprogression of brain metastases in patients with non-small cell lung cancer undergoing immunotherapy and radiotherapy to the brain.

METHODS

Retrospective analysis of F-FET PET scans in cases with documented progression of brain metastases on MRI in a cohort of 53 patients with non-small cell lung cancer receiving immune-checkpoint inhibitors and radiotherapy of brain metastases at the University Hospital of Zürich from June 2015 until January 2019. Response to radiotherapy was assessed by MRI. In case of equivocal findings and/or radiological progression in clinically asymptomatic patients, further assessment with F-FET PET was performed.

RESULTS

From the cohort of 53 patients, the restaging MRI showed in 30 patients (56.6%) progression of at least 1 treated metastasis. Thereof, F-FET PET was performed in 11 patients, based on the absence of neurological symptoms or presence of systemic response and physicians' decision. F-FET PET correctly identified pseudoprogression in 9 of 11 patients (81.8%). In patients who did not undergo F-FET PET, 5 of 19 (26.3%) were diagnosed with pseudoprogression.

CONCLUSIONS

Pseudoprogression of brain metastases occurred in 50% of patients diagnosed with progression on MRI. F-FET PET may help differentiate pseudoprogression from real progression in order to avoid discontinuation of effective therapy or unneeded interventions.

Holding-Oriented versus Gating-Oriented Live-Cell Distinction: Highlighting the Role of Transporters in Cell Imaging Probe Development

Small molecule imaging probes are powerful tools to understand complex biological systems. The mainstreams of imaging probe developments have been focused on the target holding of the probes; the holding targets are often cell-type-specific biomarkers. This type of the probe mechanism can be designated as holding-oriented live-cell distinction (HOLD). Our group has worked on the development of cell-type-selective probes using a diversity-oriented fluorescence library approach (DOFLA), where unbiased phenotypic screening is employed using fluorescent library compounds. Through the conventional target identification methods such as an affinity-based analysis, we elucidated that some of the probe mechanisms are HOLD. However, we also realized that sometimes there is no specific holding target for probes or the holding targets are ubiquitous. The observation led us to test an alternative mechanism of cell-type-specific probes as gating-oriented live-cell distinction (GOLD). We started to examine the gating mechanism of probes, which is mainly based on transporters but which does not necessarily require probe holding to cellular targets. Transporters can control the in and out movement of various nutrients and chemicals. Different expression levels of transporters in various cell types could provide the molecular mechanism of differential staining of cells by regulating the intracellular accumulation of a certain specific probe. A number of GOLD probes have been developed by modifying or mimicking endogenous substrates of transporters such as inorganic ions, glucose, amino acids, or neurotransmitters, utilizing broad substrate specificity of transporters. The radiolabeled or fluorophore-conjugated substrate mimetics have been widely used for live cell distinction and various applications such as disease-related cell or tissue imaging. In humans, there are about 400 solute carrier (SLC) transporters and 50 ATP-binding cassette (ABC) transporters. Since some transporters have broad substrate specificity, they can transport not only derivatives of endogenous natural substrates but also totally synthetic diverse imaging probes, such as DOFLA probes. Without preconsidering the structure of endogenous substrates, we recently demonstrated a series of live-cell imaging probes and elucidated their molecular mechanism as a gating one, either by SLC or ABC transporters. Transporter inhibitor panel and CRISPR-based transporter libraries could provide a systematic gating target elucidation platform. Considering the generality of DOFLA and the CRISPR-based genomic tool for transporter systems (>450 in humans), the GOLD approach will offer new insight and promise for unprecedented levels of novel cell imaging probe development.

Side Chain Optimization Remarkably Enhances the in Vivo Stability of 18F-Labeled Glutamine for Tumor Imaging

Similar to glycolysis, glutaminolysis acts as a vital energy source in tumor cells, providing building blocks for the metabolic needs of tumor cells. To capture glutaminolysis in tumors, ¹⁸F-(2S,4R)4-fluoroglutamine ([¹⁸F]FGln) and ¹⁸F-fluoroboronoglutamine ([¹⁸F]FBQ) have been successfully developed for positron emission tomography (PET) imaging, but these two molecules lack stability, resulting in undesired yet significant bone uptake. In this study, we found that [¹⁸F]FBQ-C₂ is a stable Gln PET tracer by adding two more methylene groups to the side chain of [¹⁸F]FBQ. [¹⁸F]FBQ-C₂ was synthesized with a good radiochemical yield of 35% and over 98% radiochemical purity. [¹⁸F]FBQ-C₂ showed extreme stability in vitro, and no defluorination was observed after 2 h in phosphate buffered saline at 37 °C. The competitive inhibition assay results indicated that [¹⁸F]FBQ-C₂ enters cells via the system ASC and N, similar to natural glutamine, and can be transported by tumor-overexpressed ASCT2. PET imaging and biodistribution results indicated that [¹⁸F]FBQ-C₂ is stable in vivo with low bone uptake (0.81 ± 0.20% ID/g) and can be cleared rapidly from most tissues. Dynamic scan and pharmacokinetic studies using BGC823-xenograft-bearing mice revealed that [¹⁸F]FBQ-C₂ accumulates specifically in tumors, with a longer half-life (101.18 ± 6.50 min) in tumor tissues than in other tissues (52.70 ± 12.44 min in muscle). Biodistribution exhibits a high tumor-to-normal tissue ratio (4.8 ± 1.7 for the muscle, 2.5 ± 1.0 for the stomach, 2.2 ± 0.9 for the liver, and 17.8 ± 8.4 for the brain). In conclusion, [¹⁸F]FBQ-C₂ can be used to perform high-contrast Gln imaging of tumors and can serve as a PET tracer for clinical research.

Applications of fluorine-containing amino acids for drug design

Fluorine-containing amino acids are becoming increasingly prominent in new drugs due to two general trends in the modern pharmaceutical industry. Firstly, the growing acceptance of peptides and modified peptides as drugs; and secondly, fluorine editing has become a prevalent protocol in drug-candidate optimization. Accordingly, fluorine-containing amino acids represent one of the more promising and rapidly developing areas of research in organic, bio-organic and medicinal chemistry. The goal of this Review article is to highlight the current state-of-the-art in this area by profiling 42 selected compounds that combine fluorine and amino acid structural elements. The compounds under discussion represent pharmaceutical drugs currently on the market, or in clinical trials as well as examples of drug-candidates that although withdrawn from development had a significant impact on the progress of medicinal chemistry and/or provided a deeper understanding of the nature and mechanism of biological action. For each compound, we present features of biological activity, a brief history of the design principles and the development of the synthetic approach, focusing on the source of tailor-made amino acid structures and fluorination methods. General aspects of the medicinal chemistry of fluorine-containing amino acids and synthetic methodology are briefly discussed.

Direct arene C-H fluorination with 18F- via organic photoredox catalysis

Positron emission tomography (PET) plays key roles in drug discovery and development, as well as medical imaging. However, there is a dearth of efficient and simple radiolabeling methods for aromatic C-H bonds, which limits advancements in PET radiotracer development. Here, we disclose a mild method for the fluorine-18 (18F)-fluorination of aromatic C-H bonds by an [18F]F- salt via organic photoredox catalysis under blue light illumination. This strategy was applied to the synthesis of a wide range of 18F-labeled arenes and heteroaromatics, including pharmaceutical compounds. These products can serve as diagnostic agents or provide key information about the in vivo fate of the labeled substrates, as showcased in preliminary tracer studies in mice.

Can Metabolic Pathways Be Therapeutic Targets in Rheumatoid Arthritis?

The metabolic rewiring of tumor cells and immune cells has been viewed as a promising source of novel drug targets. Many of the molecular pathways implicated in rheumatoid arthritis (RA) directly modify synovium metabolism and transform the resident cells, such as the fibroblast-like synoviocytes (FLS), and the synovial tissue macrophages (STM), toward an overproduction of enzymes, which degrade cartilage and bone, and cytokines, which promote immune cell infiltration. Recent studies have shown metabolic changes in stromal and immune cells from RA patients. Metabolic disruption in the synovium provide the opportunity to use in vivo metabolism-based imaging techniques for patient stratification and to monitor treatment response. In addition, these metabolic changes may be therapeutically targetable. Thus, resetting metabolism of the synovial membrane offers additional opportunities for disease modulation and restoration of homeostasis in RA. In fact, rheumatologists already use the antimetabolite methotrexate, a chemotherapy agent, for the treatment of patients with inflammatory arthritis. Metabolic targets that do not compromise systemic homeostasis or corresponding metabolic functions in normal cells could increase the drug armamentarium in rheumatic diseases for combination therapy independent of systemic immunosuppression. This article summarizes what is known about metabolism in synovial tissue cells and highlights chemotherapies that target metabolism as potential future therapeutic strategies for RA.

Automated synthesis of [11C]L-glutamine on Synthra HCN plus synthesis module

Background

L-Glutamine (L-Gln) is the most abundant amino acid present in the human body and is involved in numerous metabolic pathways. Glutaminolysis is the metabolic process deployed by many aggressive cancers such as triple negative breast cancer (TNBC). Imaging the metabolic pathways of L-glutamine could provide more insights into tumor biology. Reliable and reproducible automated synthesis of [¹¹C]L-glutamine PET (Positron Emission Tomography) radiotracer is critical for these studies.

Results

[¹¹C]L-Glutamine ([¹¹C]L-Gln) was reliably and reproducibly synthesized. The automated process involves cleaning and drying of the synthesis module, azeotropic drying of crown ether and cesium bicarbonate, conversion of [¹¹C]CO₂ to [¹¹C] CsCN, incorporation of [¹¹C] CN into the starting material, and hydrolysis and deprotection of the corresponding [¹¹C] nitrile to yield [¹¹C]L-glutamine. Starting with approximately 1 Ci of [¹¹C] cesium cyanide ([¹¹C]CsCN), 47–77 mCi (n = 4) of the final product, [¹¹C]L-Gln, was obtained after sterile filtration. The radiochemical purity of the final product was > 90% with almost exclusively L-glutamine isomer. The yield of [¹¹C]L-Gln was 43–52% (n = 4), decay corrected to end of [¹¹C] CsCN trapping in the reaction vessel.

Conclusions

All the steps including drying of the mixture of base and crown ether, preparation of [¹¹C] cyanide, radiochemical synthesis and formulation were accomplished on a single synthesis unit. [¹¹C]L-Gln has been successfully adapted and optimized on an automated synthesis module, Synthra HCN Plus. This process can be readily adapted for clinical research use.

New fluoroethyl phenylalanine analogues as potential LAT1-targeting PET tracers for glioblastoma

The use of O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) as a positron emission tomography (PET) tracer for brain tumor imaging might have some limitations because of the relatively low affinity for the L-type amino acid transporter 1 (LAT1). To assess the stereospecificity and evaluate the influence of aromatic ring modification of phenylalanine LAT1 targeting tracers, six different fluoroalkylated phenylalanine analogues were synthesized. After in vitro Ki determination, the most promising compound, 2-[18F]-2-fluoroethyl-L-phenylalanine (2-[18F]FELP), was selected for further evaluation and in vitro comparison with [18F]FET. Subsequently, 2-[18F]FELP was assessed in vivo and compared with [18F]FET and [18F]FDG in a F98 glioblastoma rat model. 2-[18F]FELP showed improved in vitro characteristics over [18F]FET, especially when the affinity and specificity for system L is concerned. Based on our results, 2-[18F]FELP is a promising new PET tracer for brain tumor imaging.

Synthesis of enantiopure 18F-trifluoromethyl cysteine as a structure-mimetic amino acid tracer for glioma imaging

Although 11C-labelled sulfur-containing amino acids (SAAs) including L-methyl-[11C]methionine and S-[11C]-methyl-L-cysteine, are attractive tracers for glioma positron emission tomography (PET) imaging, their applications are limited by the short half-life of the radionuclide 11C (t1/2 = 20.4 min). However, development of 18F-labelled SAAs (18F, t1/2 = 109.8 min) without significant structural changes or relying on prosthetic groups remains to be a great challenge due to the absence of adequate space for chemical modification. Methods: We herein present 18F-trifluoromethylated D- and L-cysteines which were designed by replacing the methyl group with 18F-trifluoromethyl group using a structure-based bioisosterism strategy. These two enantiomers were synthesized stereoselectively from serine-derived cyclic sulfamidates via a nucleophilic 18F-trifluoromethylthiolation reaction followed by a deprotection reaction. Furthermore, we conducted preliminary in vitro and in vivo studies to investigate the feasibility of using 18F-trifluoromethylated cysteines as PET tracers for glioma imaging. Results: The two-step radiosynthesis provided the desired products in excellent enantiopurity (ee > 99%) with 14% ± 3% of radiochemical yield. In vitro cell study demonstrated that both enantiomers were taken up efficiently by C6 tumor cells and were mainly transported by systems L and ASC. Among them, the D-enantiomer exhibited relatively good stability and high tumor-specific accumulation in the animal studies. Conclusion: Our findings indicate that 18F-trifluoromethylated D-cysteine, a new SAA tracer, may be a potential candidate for glioma imaging. Taken together, our study represents a first step toward developing 18F-trifluoromethylated cysteines as structure-mimetic tracers for PET tumor imaging.