18F-FSPG PET/CT Imaging of System xC- Transporter Activity in Patients with Primary and Metastatic Brain Tumors.

Background The PET tracer (4S)-4-(3-[18F]fluoropropyl)-l-glutamate (18F-FSPG) targets the system xC- cotransporter, which is overexpressed in various tumors. Purpose To assess the role of 18F-FSPG PET/CT in intracranial malignancies. Materials and Methods Twenty-six patients (mean age, 54 years ± 12; 17 men; 48 total lesions) with primary brain tumors (n = 17) or brain metastases (n = 9) were enrolled in this prospective, single-center study (ClinicalTrials.gov identifier: NCT02370563) between November 2014 and March 2016. A 30-minute dynamic brain 18F-FSPG PET/CT scan and a static whole-body (WB) 18F-FSPG PET/CT scan at 60-75 minutes were acquired. Moreover, all participants underwent MRI, and four participants underwent fluorine 18 (18F) fluorodeoxyglucose (FDG) PET imaging. PET parameters and their relative changes were obtained for all lesions. Kinetic modeling was used to estimate the 18F-FSPG tumor rate constants using the dynamic and dynamic plus WB PET data. Imaging parameters were correlated to lesion outcomes, as determined with follow-up MRI and/or pathologic examination. The Mann-Whitney U test or Student t test was used for group mean comparisons. Receiver operating characteristic curve analysis was used for performance comparison of different decision measures. Results 18F-FSPG PET/CT helped identify all 48 brain lesions. The mean tumor-to-background ratio (TBR) on the whole-brain PET images at the WB time point was 26.6 ± 24.9 (range: 2.6-150.3). When 18F-FDG PET was performed, 18F-FSPG permitted visualization of non-18F-FDG-avid lesions or allowed better lesion differentiation from surrounding tissues. In participants with primary brain tumors, the predictive accuracy of the relative changes in influx rate constant Ki and maximum standardized uptake value to discriminate between poor and good lesion outcomes were 89% and 81%, respectively. There were significant differences in the 18F-FSPG uptake curves of lesions with good versus poor outcomes in the primary brain tumor group (P < .05) but not in the brain metastases group. Conclusion PET/CT imaging with (4S)-4-(3-[18F]fluoropropyl)-l-glutamate (18F-FSPG) helped detect primary brain tumors and brain metastases with a high tumor-to-background ratio. Relative changes in 18F-FSPG uptake with multi-time-point PET appear to be helpful in predicting lesion outcomes. Clinical trial registration no. NCT02370563 © RSNA, 2022 Online supplemental material is available for this article.

Tau PET imaging with 18F-PI-2620 in Patients with Alzheimer Disease and Healthy Controls: A First-in-Humans Study.

18F-PI-2620 is a PET tracer with high binding affinity for aggregated tau, a key pathologic feature of Alzheimer disease (AD) and other neurodegenerative disorders. Preclinically, 18F-PI-2620 binds to both 3-repeat and 4-repeat tau isoforms. The purpose of this first-in-humans study was to evaluate the ability of 18F-PI-2620 to detect tau pathology in AD patients using PET imaging, as well as to assess the safety and tolerability of this new tau PET tracer. Methods: Participants with a clinical diagnosis of probable AD and healthy controls (HCs) underwent dynamic 18F-PI-2620 PET imaging for 180 min. 18F-PI-2620 binding was assessed visually and quantitatively using distribution volume ratios (DVR) estimated from noninvasive tracer kinetics and SUV ratio (SUVR) measured at different time points after injection, with the cerebellar cortex as the reference region. Time-activity curves and SUVR were assessed in AD and HC subjects, as well as DVR and SUVR correlations and effect size (Cohen's d) over time. Results:18F-PI-2620 showed peak brain uptake around 5 min after injection and fast washout from nontarget regions. In AD subjects, focal asymmetric uptake was evident in temporal and parietal lobes, precuneus, and posterior cingulate cortex. DVR and SUVR in these regions were significantly higher in AD subjects than in HCs. Very low background signal was observed in HCs. 18F-PI-2620 administration was safe and well tolerated. SUVR time-activity curves in most regions and subjects achieved a secular equilibrium after 40 min after injection. A strong correlation (R2 > 0.93) was found between noninvasive DVR and SUVR for all imaging windows starting at more than 30 min after injection. Similar effect sizes between AD and HC groups were obtained across the different imaging windows. 18F-PI-2620 uptake in neocortical regions significantly correlated with the degree of cognitive impairment. Conclusion: Initial clinical data obtained in AD and HC subjects demonstrated a high image quality and excellent signal-to-noise ratio of 18F-PI-2620 PET for imaging tau deposition in AD subjects. Noninvasive quantification using DVR and SUVR for 30-min imaging windows between 30 and 90 min after injection-for example, 45-75 min-provides robust and significant discrimination between AD and HC subjects. 18F-PI-2620 uptake in expected regions correlates strongly with neurocognitive performance.

Evaluation of Dosimetry, Quantitative Methods, and Test-Retest Variability of 18F-PI-2620 PET for the Assessment of Tau Deposits in the Human Brain.

18F-PI-2620 is a next-generation tau PET tracer that has demonstrated ability to image the spatial distribution of suspected tau pathology. The objective of this study was to assess the tracer biodistribution, dosimetry, and quantitative methods of 18F-PI-2620 in the human brain. Full kinetic modeling to quantify tau load was investigated. Noninvasive kinetic modeling and semiquantitative methods were evaluated against the full tracer kinetics. Finally, the reproducibility of PET measurements from test and retest scans was assessed. Methods: Three healthy controls (HCs) and 4 Alzheimer disease (AD) subjects underwent 2 dynamic PET scans, including arterial sampling. Distribution volume ratio (DVR) was estimated using full tracer kinetics (reversible 2-tissue-compartment [2TC] model and Logan graphical analysis [LGA]) and noninvasive kinetic models (noninvasive LGA [NI-LGA] and the multilinear reference tissue model [MRTM2]). SUV ratio (SUVR) was determined at different imaging windows after injection. The correlation between DVR and SUVR, effect size (Cohen's d), and test-retest variability (TRV) were evaluated. Additionally, 6 HCs received 1 tracer administration and underwent whole-body PET for dosimetry calculation. Organ doses and the whole-body effective dose were calculated using OLINDA 2.0. Results: A strong correlation was found across different kinetic models (R2 > 0.97) and between DVR(2TC) and SUVR between 30 and 90 min, with an R2 of more than 0.95. Secular equilibrium was reached at around 40 min after injection in most regions and subjects. TRV and effect size for SUVR across different regions were similar at 30-60 min (TRV, 3.8%; Cohen's d, 3.80), 45-75 min (TRV, 4.3%; Cohen's d, 3.77) and 60-90 min (TRV, 4.9%; Cohen's d, 3.73) and increased at later time points. Elimination was via the hepatobiliary and urinary systems. The whole-body effective dose was 33.3 ± 2.1 µSv/MBq for an adult female and 33.1 ± 1.4 µSv/MBq for an adult male, with a 1.5-h urinary bladder voiding interval. Conclusion:18F-PI-2620 exhibits fast kinetics, suitable dosimetry, and low TRV. DVR measured using the 2TC model with arterial sampling correlated strongly with DVR measured by NI-LGA, MRTM2, and SUVR. SUVR can be used for 18F-PI-2620 PET quantification of tau deposits, avoiding arterial blood sampling. Static 18F-PI-2620 PET scans between 45 and 75 min after injection provide excellent quantification accuracy, a large effect size, and low TRV.

Commentary to 18F-GP1, a Novel PET Tracer Designed for High-Sensitivity, Low-Background Detection of Thrombi: Imaging Activated Platelets in Clots-Are We Getting There?

Thrombus formation can lead to heart attacks, stroke and pulmonary embolism, which are major causes of mortality. Current standard diagnostic imaging methods detect anatomic abnormalities such as vascular flow impairment but have limitations. By using a targeted molecular imaging approach critical components of a pathology can be selectively visualized and exploited for an improved diagnosis and patient management. The GPIIb/IIIa receptor is abundantly and specifically exposed on activated platelets and is the key receptor in thrombus formation. This commentary describes the current status of GPIIb/IIIa-based PET imaging approaches with a focus on the recently published preclinical data of the small-molecule PET tracer 18F-GP1. Areas of future research and potential clinical applications are discussed that may lead to an improved detection of critical thromboembolic events and an optimization of available antithrombotic therapies by tracking activated platelets.

18F-GP1, a Novel PET Tracer Designed for High-Sensitivity, Low-Background Detection of Thrombi.

Thromboembolic diseases such as myocardial infarction, stroke, transient ischemic attacks, and pulmonary embolism are major causes of morbidity and mortality worldwide. Glycoprotein IIb/IIIa (GPIIb/IIIa) is the key receptor involved in platelet aggregation and is a validated target for therapeutic approaches and diagnostic imaging. The aim of this study was to develop and characterize a specific small-molecule tracer for PET imaging that binds with high affinity to GPIIb/IIIa receptors and has suitable pharmacokinetic properties to overcome limitations of previous approaches. Methods: Binding of 18F-GP1 to GPIIb/IIIa receptors was investigated in competition binding assays and autoradiography using a fresh cardiac thrombus from an explanted human heart. The clot-to-blood ratio for 18F-GP1 was investigated by an in vitro blood flow model. Biodistribution and thrombus detection was investigated in cynomolgus monkeys after insertion of a roughened catheter into either the vena cava or the aorta. Results:18F-GP1 is an 18F-labeled small molecule for PET imaging of thrombi. The half maximal inhibitory concentration of 18F-GP1 to GPIIb/IIIa was 20 nM. 18F-GP1 bound to thrombi with a mean clot-to-blood ratio of 95. Binding was specific and can be displaced by excess nonradioactive derivative. Binding was not affected by anticoagulants such as aspirin or heparin. 18F-GP1 showed rapid blood clearance and a low background after intravenous injection in cynomolgus monkeys. Small arterial, venous thrombi, thrombotic depositions on damaged endothelial surface, and small cerebral emboli were detected in vivo by PET imaging. Conclusions:18F-GP1 binds specifically with high affinity to the GPIIb/IIIa receptor involved in platelet aggregation. Because of its favorable preclinical characteristics, 18F-GP1 is currently being investigated in a human clinical study.

Pilot Preclinical and Clinical Evaluation of (4S)-4-(3-[18F]Fluoropropyl)-L-Glutamate (18F-FSPG) for PET/CT Imaging of Intracranial Malignancies.

PURPOSE: (S)-4-(3-[18F]Fluoropropyl)-L-glutamic acid (18F-FSPG) is a novel radiopharmaceutical for Positron Emission Tomography (PET) imaging. It is a glutamate analogue that can be used to measure xC- transporter activity. This study was performed to assess the feasibility of 18F-FSPG for imaging orthotopic brain tumors in small animals and the translation of this approach in human subjects with intracranial malignancies. EXPERIMENTAL DESIGN: For the small animal study, GS9L glioblastoma cells were implanted into brains of Fischer rats and studied with 18F-FSPG, the 18F-labeled glucose derivative 18F-FDG and with the 18F-labeled amino acid derivative 18F-FET. For the human study, five subjects with either primary or metastatic brain cancer were recruited (mean age 50.4 years). After injection of 300 MBq of 18F-FSPG, 3 whole-body PET/Computed Tomography (CT) scans were obtained and safety parameters were measured. The three subjects with brain metastases also had an 18F-FDG PET/CT scan. Quantitative and qualitative comparison of the scans was performed to assess kinetics, biodistribution, and relative efficacy of the tracers. RESULTS: In the small animals, the orthotopic brain tumors were visualized well with 18F-FSPG. The high tumor uptake of 18F-FSPG in the GS9L model and the absence of background signal led to good tumor visualization with high contrast (tumor/brain ratio: 32.7). 18F-FDG and 18F-FET showed T/B ratios of 1.7 and 2.8, respectively. In the human pilot study, 18F-FSPG was well tolerated and there was similar distribution in all patients. All malignant lesions were positive with 18F-FSPG except for one low-grade primary brain tumor. In the 18F-FSPG-PET-positive tumors a similar T/B ratio was observed as in the animal model. CONCLUSIONS: 18F-FSPG is a novel PET radiopharmaceutical that demonstrates good uptake in both small animal and human studies of intracranial malignancies. Future studies on larger numbers of subjects and a wider array of brain tumors are planned. TRIAL REGISTRATION: ClinicalTrials.gov NCT01186601.

Exploratory Clinical Investigation of (4S)-4-(3-18F-Fluoropropyl)-L-Glutamate PET of Inflammatory and Infectious Lesions.

UNLABELLED: We explored system [Formula: see text] transporter activity and the detection of inflammatory or infectious lesions using (4S)-4-(3-(18)F-fluoropropyl)-l-glutamate ((18)F-FSPG) PET. METHODS: In 10 patients with various inflammatory or infectious diseases, as many as 5 of the largest lesions were selected as reference lesions. (18)F-FSPG images were assessed visually and quantitatively. Expression levels of xCT, CD44, and surface markers of inflammatory cells were evaluated by immunohistochemistry. RESULTS: (18)F-FSPG PET detected all reference lesions. (18)F-FSPG uptake in sarcoidosis was significantly higher than that in nonsarcoidosis. The lesion-to-blood-pool SUV ratio for (18)F-FSPG was comparable to that for (18)F-FDG in sarcoidosis. In nonsarcoidosis, however, it was significantly lower. In 5 patients with available tissue samples, the SUVmax for (18)F-FSPG and CD163 were negatively correlated (ρ = -0.872, P = 0.054). CONCLUSION: (18)F-FSPG PET may detect inflammatory lesions when activated macrophages or monocytes are present, such as in sarcoidosis.

Aß imaging with 18F-florbetaben in prodromal Alzheimer's disease: a prospective outcome study.

BACKGROUND: We assessed the clinical utility of ß-amyloid (Aß) imaging with (18)F-florbetaben (FBB) in mild cognitive impairment (MCI) by evaluating its prognostic accuracy for progression to Alzheimer's disease (AD), comparing semiquantitative with visual scan assessment, and exploring the relationships among Aß, hippocampal volume (HV) and memory over time. METHODS: 45 MCI underwent FBB positron emission tomography, MRI and neuropsychological assessment at baseline and 2 years and clinical follow-up at 4 years. Positive FBB (FBB+), defined by a cortical to cerebellar cortex standardised uptake value ratio (SUVR) ≥ 1.45, was compared with visual assessment by five readers. Amnestic MCI (aMCI) was defined by a composite episodic memory (EM) Z-score of <-1.5. RESULTS: At baseline, 24 (53%) MCI were FBB+. Majority reads agreed with SUVR classification (κ 0.96). In 2 years, 18 (75%) FBB+ progressed to AD compared with 2 (9.5%) FBB-, yielding a predictive accuracy of 83% (95% CI 61% to 94%). Four FBB- developed non-AD dementia. Predictive accuracies of HV (58% (95% CI 42% to 73%)) and aMCI status (73% (95% CI 58% to 81%)) were lower. Combinations did not improve accuracy. By 4 years, 21 (87.5%) FBB+ had AD whereas 5 (24%) FBB- had non-AD dementia yielding a predictive accuracy of 94% (95% CI 74% to 99%). While the strong baseline association between FBB SUVR and EM declined over 2 years, the association between EM and HV became stronger. FBB SUVR increased 2.2%/year in FBB+ with no change in FBB-. CONCLUSIONS: (18)F-florbetaben Aß imaging facilitates accurate detection of prodromal AD. As neurodegeneration progresses, and in contrast with the early stages of the disease, hippocampal atrophy and not Aß, seems to drive memory decline. TRIAL REGISTRATION NUMBER: NCT01138111.

Gene expression levels of matrix metalloproteinases in human atherosclerotic plaques and evaluation of radiolabeled inhibitors as imaging agents for plaque vulnerability.

INTRODUCTION: Atherosclerotic plaque rupture is the primary cause for myocardial infarction and stroke. During plaque progression macrophages and mast cells secrete matrix-degrading proteolytic enzymes, such as matrix metalloproteinases (MMPs). We studied levels of MMPs and tissue inhibitor of metalloproteinases-3 (TIMP-3) in relation to the characteristics of carotid plaques. We evaluated in vitro two radiolabeled probes targeting active MMPs towards non-invasive imaging of rupture-prone plaques. METHODS: Human carotid plaques obtained from endarterectomy were classified into stable and vulnerable by visual and histological analysis. MMP-1, MMP-2, MMP-8, MMP-9, MMP-10, MMP-12, MMP-14, TIMP-3, and CD68 levels were investigated by quantitative polymerase chain reaction. Immunohistochemistry was used to localize MMP-2 and MMP-9 with respect to CD68-expressing macrophages. Western blotting was applied to detect their active forms. A fluorine-18-labeled MMP-2/MMP-9 inhibitor and a tritiated selective MMP-9 inhibitor were evaluated by in vitro autoradiography as potential lead structures for non-invasive imaging. RESULTS: Gene expression levels of all MMPs and CD68 were elevated in plaques. MMP-1, MMP-9, MMP-12 and MMP-14 were significantly higher in vulnerable than stable plaques. TIMP-3 expression was highest in stable and low in vulnerable plaques. Immunohistochemistry revealed intensive staining of MMP-9 in vulnerable plaques. Western blotting confirmed presence of the active form in plaque lysates. In vitro autoradiography showed binding of both inhibitors to stable and vulnerable plaques. CONCLUSIONS: MMPs differed in their expression patterns among plaque phenotypes, providing possible imaging targets. The two tested MMP-2/MMP-9 and MMP-9 inhibitors may be useful to detect atherosclerotic plaques, but not the vulnerable lesions selectively.

Preclinical evaluation of BAY 1075553, a novel (18)F-labelled inhibitor of prostate-specific membrane antigen for PET imaging of prostate cancer.

PURPOSE: Prostate-specific membrane antigen (PSMA) is a transmembrane protein overexpressed in prostate cancer and is therefore being explored as a biomarker for diagnosing and staging of the disease. Here we report preclinical data on BAY 1075553 (a 9:1 mixture of (2S,4S)- and (2R,4S)-2-[(18)F]fluoro-4-phosphonomethyl-pentanedioic acid), a novel (18)F-labelled small molecule inhibitor of PSMA enzymatic activity, which can be efficiently synthesized from a direct radiolabelling precursor. METHODS: The (18)F-radiolabelled stereoisomers of 2-[(18)F]fluoro-4-(phosphonomethyl)-pentanedioic acid were synthesized from their respective isomerically pure precursors dimethyl 2-{[bis(benzyloxy)phosphoryl]methyl}-4-(tosyloxy)pentanedioate. In vivo positron emission tomography (PET) imaging and biodistribution studies were conducted in mice bearing LNCaP, 22Rv1 and PC-3 tumours. Pharmacokinetic parameters and dosimetry estimates were calculated based on biodistribution studies in rodents. For non-clinical safety assessment (safety pharmacology, toxicology) to support a single-dose human microdose study, off-target effects in vitro, effects on vital organ functions (cardiovascular in dogs, nervous system in rats), mutagenicity screens and an extended single-dose study in rats were conducted with the non-radioactive racemic analogue of BAY 1075553. RESULTS: BAY 1075553 showed high tumour accumulation specific to PSMA-positive tumour-bearing mice and was superior to other stereoisomers tested. Fast clearance of BAY 1075553 resulted overall in low background signals in other organs except for high uptake into kidney and bladder which was mainly caused by renal elimination of BAY 1075553. A modest uptake into bone was observed which decreased over time indicating organ-specific uptake as opposed to defluorination of BAY 1075553 in vivo. Biodistribution studies found highest organ doses for kidneys and the urinary bladder wall resulting in a projected effective dose (ED) in humans of 0.0219 mSv/MBq. Non-clinical safety studies did not show off-target activity, effects on vital organs function or dose-dependent adverse effects. CONCLUSION: BAY 1075553 was identified as a promising PET tracer for PSMA-positive prostate tumours in preclinical studies. BAY 1075553 can be produced using a robust, direct radiosynthesis procedure. Pharmacokinetic, toxicology and safety pharmacology studies support the application of BAY 1075553 in a first-in-man microdose study with single i.v. administration.

Studies toward the development of new silicon-containing building blocks for the direct (18)F-labeling of peptides.

Silicon-containing prosthetic groups have been conjugated to peptides to allow for a single-step labeling with (18)F radioisotope. The fairly lipophilic di-tert-butylphenylsilane building block contributes unfavorably to the pharmacokinetic profile of bombesin conjugates. In this article, theoretical and experimental studies toward the development of more hydrophilic silicon-based building blocks are presented. Density functional theory calculations were used to predict the hydrolytic stability of di-tert-butylfluorosilanes 2-23 with the aim to improve the in vivo properties of (18)F-labeled silicon-containing biomolecules. As a further step toward improving the pharmacokinetic profile, hydrophilic linkers were introduced between the lipophilic di-tert-butylphenylsilane building block and the bombesin congeners. Increased tumor uptake was shown with two of these peptides in xenograft-bearing mice using positron emission tomography and biodistribution studies. The introduction of a hydrophilic linker is thus a viable approach to improve the tumor uptake of (18)F-labeled silicon-bombesin conjugates.

Design, synthesis, and initial evaluation of a high affinity positron emission tomography probe for imaging matrix metalloproteinases 2 and 9.

The activity of matrix metalloproteinases (MMPs) is elevated locally under many pathological conditions. Gelatinases MMP2 and MMP9 are of particular interest because of their implication in angiogenesis, cancer cell proliferation and metastasis, and atherosclerotic plaque rupture. The aim of this study was to identify and develop a selective gelatinase inhibitor for imaging active MMP2/MMP9 in vivo. We synthesized a series of N-sulfonylamino acid derivatives with low to high nanomolar inhibitory potencies. (R)-2-(4-(4-Fluorobenzamido)phenylsulfonamido)-3-(1H-indol-3-yl)propanoic acid (7) exhibited the best in vitro binding properties: MMP2 IC50 = 1.8 nM, MMP9 IC50 = 7.2 nM. Radiolabeling of 7 with no carrier added (18)F-radioisotope was accomplished starting from iodonium salts as precursors. The radiochemical yield strongly depended on the iodonium counteranion (ClO4(-) > Br(-) > TFA(-) > tosylate). (18)F-7 was obtained in up to 20% radiochemical yield (decay corrected), high radiochemical purity, and >90 GBq/µmol specific radioactivity. The radiolabeled compound showed excellent stability in vitro and in mice in vivo.

(S)-4-(3-18F-fluoropropyl)-L-glutamic acid: an 18F-labeled tumor-specific probe for PET/CT imaging--dosimetry.

UNLABELLED: The glutamic acid derivative (S)-4-(3-(18)F-Fluoropropyl)-l-glutamic acid ((18)F-FSPG, alias BAY 94-9392), a new PET tracer for the detection of malignant diseases, displayed promising results in non-small cell lung cancer patients. The aim of this study was to provide dosimetry estimates for (18)F-FSPG based on human whole-body PET/CT measurements. METHODS: (18)F-FSPG was prepared by a fully automated 2-step procedure and purified by a solid-phase extraction method. PET/CT scans were obtained for 5 healthy volunteers (mean age, 59 y; age range, 51-64 y; 2 men, 3 women). Human subjects were imaged for up to 240 min using a PET/CT scanner after intravenous injection of 299 ± 22.5 MBq of (18)F-FSPG. Image quantification, time-activity data modeling, estimation of normalized number of disintegrations, and production of dosimetry estimates were performed using the RADAR (RAdiation Dose Assessment Resource) method for internal dosimetry and in general concordance with the methodology and principles as presented in the MIRD 16 document. RESULTS: Because of the renal excretion of the tracer, the absorbed dose was highest in the urinary bladder wall and kidneys, followed by the pancreas and uterus. The individual organ doses (mSv/MBq) were 0.40 ± 0.058 for the urinary bladder wall, 0.11 ± 0.011 for the kidneys, 0.077 ± 0.020 for the pancreas, and 0.030 ± 0.0034 for the uterus. The calculated effective dose was 0.032 ± 0.0034 mSv/MBq. Absorbed dose to the bladder and the effective dose can be reduced significantly by frequent bladder-voiding intervals. For a 0.75-h voiding interval, the bladder dose was reduced to 0.10 ± 0.012 mSv/MBq, and the effective dose was reduced to 0.015 ± 0.0010 mSv/MBq. CONCLUSION: On the basis of the distribution and biokinetic data, the determined radiation dose for (18)F-FSPG was calculated to be 9.5 ± 1.0 mSv at a patient dose of 300 MBq, which is of similar magnitude to that of (18)F-FDG (5.7 mSv). The effective dose can be reduced to 4.5 ± 0.30 mSv (at 300 MBq), with a bladder-voiding interval of 0.75 h.

Investigations into the synthesis, radiofluorination and conjugation of a new [¹8F]fluorocyclobutyl prosthetic group and its in vitro stability using a tyrosine model system.

The [(18)F]fluorocyclobutyl group has the potential to be a metabolically stable prosthetic group for PET tracers. The synthesis of the radiolabeling precursor cis-cyclobutane-1,3-diyl bis(toluene-4-sulfonate) 8 was obtained from epibromohydrin in 7 steps (2% overall yield). The radiolabeling of this precursor 8 and its conjugation to L-tyrosine as a model system was successfully achieved to give the new non-natural amino acid 3-[(18)F]fluorocyclobutyl-L-tyrosine (L-3-[(18)F]FCBT) [(18)F]17 in 8% decay-corrected yield from the non-carrier-added [(18)F]fluoride. L-3-[(18)F]FCBT was investigated in vitro in different cancer cell lines to determine the uptake and stability. The tracer [(18)F]17 showed a time dependent uptake into different tumor cell lines (A549, NCI-H460, DU145) with the best uptake of 5.8% injected dose per 5×10(5) cells after 30min in human lung carcinoma cells A549. The stability of L-3-[(18)F]FCBT in human and rat plasma and the stability of the non-radioactive L-3-FCBT in rat hepatocytes were both found to be excellent. These results show that the non-natural amino acid L-3-[(18)F]FCBT is a promising metabolically stable radiotracer for positron emission tomography.

(4S)-4-(3-18F-fluoropropyl)-L-glutamate for imaging of xC transporter activity in hepatocellular carcinoma using PET: preclinical and exploratory clinical studies.

UNLABELLED: (4S)-4-(3-(18)F-fluoropropyl)-l-glutamate ((18)F-FSPG, or BAY 94-9392) is a new tracer to assess system x(C)(¯) transporter activity with PET. The aim of this study was to explore the tumor detection rate of (18)F-FSPG, compared with that of (18)F-FDG, in patients with hepatocellular carcinoma (HCC). METHODS: Preclinically, in vivo HCC models of orthotopically implanted Huh7 and MH3924a cancer cells were studied with (18)F-FSPG in Naval Medical Research Institute nude mice (n = 3) and August-Copenhagen Irish rats (n = 4), respectively. Clinically, 5 patients with HCC who had hyper- or isometabolic lesions on (18)F-FDG PET were enrolled for evaluation of the tracer. Dynamic whole-body PET images with (18)F-FSPG were acquired for up to 120 min after injection of approximately 300 MBq of (18)F-FSPG. Immunohistochemical expression levels of the xCT subunit of the system x(C)(¯) and CD44 of HCC were studied in 4 patients with HCC. RESULTS: Strong tumor uptake and low background from nontarget tissue allowed excellent tumor visualization in animal models with orthotopically implanted liver tumors. (18)F-FSPG PET procedures were well tolerated in all patients. (18)F-FSPG PET and (18)F-FDG detected lesions in 5 of 5 and 3 of 5 patients, respectively. The maximal standardized uptake values (SUV) were comparable ((18)F-FSPG, 4.7 ± 3.2; (18)F-FDG, 6.1 ± 2.9). The ratios of maximal SUV of the tumor to mean SUV of normal liver were also comparable ((18)F-FSPG, 3.6 ± 2.2; (18)F-FDG, 2.7 ± 1.3), but the mean SUV of normal liver of (18)F-FSPG was significantly lower than that of (18)F-FDG (P < 0.05). Two patients with HCC who showed both xCT and CD44 expression had moderate or intense accumulation of (18)F-FSPG, but the remaining 2 patients with negative CD44 expression showed mild uptake. CONCLUSION: (18)F-FSPG was successfully translated from preclinical evaluation into patients with HCC. (18)F-FSPG may be a promising tumor PET agent with a high cancer detection rate in patients with HCC.

Exploratory clinical trial of (4S)-4-(3-[18F]fluoropropyl)-L-glutamate for imaging xC- transporter using positron emission tomography in patients with non-small cell lung or breast cancer.

PURPOSE: (4S)-4-(3-[(18)F]fluoropropyl)-l-glutamate (BAY 94-9392, alias [(18)F]FSPG) is a new tracer to image x(C)(-) transporter activity with positron emission tomography (PET). We aimed to explore the tumor detection rate of [(18)F]FSPG in patients relative to 2-[(18)F]fluoro-2-deoxyglucose ([(18)F]FDG). The correlation of [(18)F]FSPG uptake with immunohistochemical expression of x(C)(-) transporter and CD44, which stabilizes the xCT subunit of system x(C)(-), was also analyzed. EXPERIMENTAL DESIGN: Patients with non-small cell lung cancer (NSCLC, n = 10) or breast cancer (n = 5) who had a positive [(18)F]FDG uptake were included in this exploratory study. PET images were acquired following injection of approximately 300 MBq [(18)F]FSPG. Immunohistochemistry was done using xCT- and CD44-specific antibody. RESULTS: [(18)F]FSPG PET showed high uptake in the kidney and pancreas with rapid blood clearance. [(18)F]FSPG identified all 10 NSCLC and three of the five breast cancer lesions that were confirmed by pathology. [(18)F]FSPG detected 59 of 67 (88%) [(18)F]FDG lesions in NSCLC, and 30 of 73 (41%) in breast cancer. Seven lesions were additionally detected only on [(18)F]FSPG in NSCLC. The tumor-to-blood pool standardized uptake value (SUV) ratio was not significantly different from that of [(18)F]FDG in NSCLC; however, in breast cancer, it was significantly lower (P < 0.05). The maximum SUV of [(18)F]FSPG correlated significantly with the intensity of immunohistochemical staining of x(C)(-) transporter and CD44 (P < 0.01). CONCLUSIONS: [(18)F]FSPG seems to be a promising tracer with a relatively high cancer detection rate in patients with NSCLC. [(18)F]FSPG PET may assess x(C)(-) transporter activity in patients with cancer.

Specific PET imaging of xC- transporter activity using a ¹8F-labeled glutamate derivative reveals a dominant pathway in tumor metabolism.

PURPOSE: (18)F-labeled small molecules targeting adaptations of tumor metabolism possess the potential for early tumor detection with high sensitivity and specificity by positron emission tomography (PET) imaging. Compounds tracing deranged pathways other than glycolysis may have advantages in situations where 2-[¹8F]fluoro-2-deoxy-d-glucose (FDG) has limitations. The aim of this study was the generation of a metabolically stable ¹8F-labeled glutamate analogue for PET imaging of tumors. EXPERIMENTAL DESIGN: Derivatives of l-glutamate were investigated in cell competition assays to characterize the responsible transporter. An automated radiosynthesis was established for the most promising candidate. The resulting ¹8F-labeled PET tracer was characterized in a panel of in vitro and in vivo tumor models. Tumor specificity was investigated in the turpentine oil-induced inflammation model in rats. RESULTS: A fluoropropyl substituted glutamate derivative showed strong inhibition in cell uptake assays. The radiosynthesis was established for (4S)-4-(3-[¹8F]fluoropropyl)-l-glutamate (BAY 94-9392). Tracer uptake studies and analysis of knockdown cells showed specific transport of BAY 94-9392 via the cystine/glutamate exchanger designated as system x(C)(-). No metabolites were observed in mouse blood and tumor cells. PET imaging with excellent tumor visualization and high tumor to background ratios was achieved in preclinical tumor models. In addition, BAY 94-9392 did not accumulate in inflammatory lesions in contrast to FDG. CONCLUSIONS: BAY 94-9392 is a new tumor-specific PET tracer which could be useful to examine system x(C)(-) activity in vivo as a possible hallmark of tumor oxidative stress. Both preclinical and clinical studies are in progress for further characterization.

4-[18F]fluoroglutamic acid (BAY 85-8050), a new amino acid radiotracer for PET imaging of tumors: synthesis and in vitro characterization.

There is a high demand for tumor specific PET tracers in oncology imaging. Besides glucose, certain amino acids also serve as energy sources and anabolic precursors for tumors. Therefore, (18)F-labeled amino acids are interesting probes for tumor specific PET imaging. As glutamine and glutamate play a key role in the adapted intermediary metabolism of tumors, the radiosynthesis of 4-[(18)F]fluoro l-glutamic acid (BAY 85-8050) as a new specific PET tracer was established. Cell-uptake studies revealed specific tumor cell accumulation.

Targeting of HER2-expressing tumors with a site-specifically 99mTc-labeled recombinant affibody molecule, ZHER2:2395, with C-terminally engineered cysteine.

UNLABELLED: The detection of human epidermal growth factor receptor type 2 (HER2) expression in malignant tumors provides important information influencing patient management. Radionuclide in vivo imaging of HER2 may permit the detection of HER2 in both primary tumors and metastases by a single noninvasive procedure. Small (7 kDa) high-affinity anti-HER2 Affibody molecules may be suitable tracers for SPECT visualization of HER2-expressing tumors. The use of generator-produced (99m)Tc as a label would facilitate the prompt translation of anti-HER2 Affibody molecules into use in clinics. METHODS: A C-terminal cysteine was introduced into the Affibody molecule Z(HER2:342) to enable site-specific labeling with (99m)Tc. Two recombinant variants, His(6)-Z(HER2:342)-Cys (dissociation constant [K(D)], 29 pM) and Z(HER2:2395)-Cys, lacking a His tag (K(D), 27 pM), were labeled with (99m)Tc in yields exceeding 90%. The binding specificity and the cellular processing of Affibody molecules were studied in vitro. Biodistribution and gamma-camera imaging studies were performed in mice bearing HER2-expressing xenografts. RESULTS: (99m)Tc-His(6)-Z(HER2:342)-Cys was capable of targeting HER2-expressing SKOV-3 xenografts in SCID mice, but the liver radioactivity uptake was high. A series of comparative biodistribution experiments indicated that the presence of the His tag caused elevated accumulation in the liver. (99m)Tc-Z(HER2:2395)-Cys, not containing a His tag, showed low uptake in the liver and high and specific uptake in HER2-expressing xenografts. Four hours after injection, the radioactivity uptake values (percentage of injected activity per gram of tissue [%IA/g]) were 6.9 +/- 2.5 (mean +/- SD) %IA/g in LS174T xenografts (moderate level of HER2 expression) and 15 +/- 3 %IA/g in SKOV-3 xenografts (high level of HER2 expression). The corresponding tumor-to-blood ratios were 88 +/- 24 and 121 +/- 24, respectively. Both LS174T and SKOV-3 xenografts were clearly visualized with a clinical gamma-camera 1 h after injection of (99m)Tc-Z(HER2:2395)-Cys. CONCLUSION: The Affibody molecule (99m)Tc-Z(HER2:2395)-Cys is a promising tracer for SPECT visualization of HER2-expressing tumors.

(124)I-L19-SIP for immuno-PET imaging of tumour vasculature and guidance of (131)I-L19-SIP radioimmunotherapy.

PURPOSE: The human monoclonal antibody (MAb) fragment L19-SIP is directed against extra domain B (ED-B) of fibronectin, a marker of tumour angiogenesis. A clinical radioimmunotherapy (RIT) trial with (131)I-L19-SIP was recently started. In the present study, after GMP production of (124)I and efficient production of (124)I-L19-SIP, we aimed to demonstrate the suitability of (124)I-L19-SIP immuno-PET for imaging of angiogenesis at early-stage tumour development and as a scouting procedure prior to clinical (131)I-L19-SIP RIT. METHODS: (124)I was produced in a GMP compliant way via (124)Te(p,n)(124)I reaction and using a TERIMO module for radioiodine separation. L19-SIP was radioiodinated by using a modified version of the IODO-GEN method. The biodistribution of coinjected (124)I- and (131)I-L19-SIP was compared in FaDu xenograft-bearing nude mice, while (124)I PET images were obtained from mice with tumours of <50 to approximately 700 mm(3). RESULTS: (124)I was produced highly pure with an average yield of 15.4 +/- 0.5 MBq/microAh, while separation yield was approximately 90% efficient with <0.5% loss of TeO(2). Overall labelling efficiency, radiochemical purity and immunoreactive fraction were for (124)I-L19-SIP: approximately 80 , 99.9 and >90%, respectively. Tumour uptake was 7.3 +/- 2.1, 10.8 +/- 1.5, 7.8 +/- 1.4, 5.3 +/- 0.6 and 3.1 +/- 0.4%ID/g at 3, 6, 24, 48 and 72 h p.i., resulting in increased tumour to blood ratios ranging from 6.0 at 24 h to 45.9 at 72 h p.i.. Fully concordant labelling and biodistribution results were obtained with (124)I- and (131)I-L19-SIP. Immuno-PET with (124)I-L19-SIP using a high-resolution research tomograph PET scanner revealed clear delineation of the tumours as small as 50 mm(3) and no adverse uptake in other organs. CONCLUSIONS: (124)I-MAb conjugates for clinical immuno-PET can be efficiently produced. Immuno-PET with (124)I-L19-SIP appeared qualified for sensitive imaging of tumour neovasculature and for predicting (131)I-L19-SIP biodistribution.