Automated radiosynthesis of mGluR5 PET tracer [18F]FPEB from aryl-chloro precursor and validation for clinical application.

The radioligand [18F]FPEB, used for PET imaging of the brain's metabotropic glutamate receptor subtype 5 (mGluR5), undergoes a thorough validation process to ensure its safety, efficacy, and quality for clinical use. The process starts by optimizing the synthesis of [18F]FPEB to achieve high radiochemical yield and purity. This study focuses on optimizing the radiolabeling process using an aryl-chloro precursor and validating the GMP production for clinical applications. Fully automated radiolabeling was achieved via one-step nucleophilic substitution reaction. [18F]FPEB was produced and isolated in high radioactivity and radiochemical purity. Throughout the validation process, thorough quality control measures are implemented. Radiopharmaceutical batch release criteria are established, including testing for physical appearance, filter integrity, pH, radiochemical purity, molar activity, radiochemical identity, chemical impurity, structural identity, stability, residual solvent, sterility, and endotoxin levels. In conclusion, the validation of [18F]FPEB involved a comprehensive process of synthesis optimization, quality control, which ensure the safety, efficacy, and quality of [18F]FPEB, enabling its reliable use in clinical PET. Here, we successfully radiolabeled and validated [18F]FPEB using aryl-chloro precursor according to GMP production for clinical application.

Characterization of a Novel M4 PAM PET Radioligand [11C]PF06885190 in Nonhuman Primates (NHP).

Muscarinic acetylcholine receptors (mAChR), including M4, draw attention as therapeutic targets for several neurodegenerative diseases including Alzheimer's disease (AD). PET imaging of M4 positive allosteric modulator (PAM) allows qualification of the distribution as well as the expression of this receptor under physiological conditions and thereby helps to assess the receptor occupancy (RO) of a drug candidate. In this study, our aims were (a) to synthesize a novel M4 PAM PET radioligand [11C]PF06885190 (b) to evaluate the brain distribution of [11C]PF06885190 in nonhuman primates (NHP) and (c) to analyze its radiometabolites in the blood plasma of NHP. Radiolabeling of [11C]PF06885190 was accomplished via N-methylation of the precursor. Six PET measurements were performed using two male cynomolgus monkeys, where three PET measurements were at baseline, two after pretreatment with a selective M4 PAM compound CVL-231 and one after pretreatment with donepezil. The total volume of distribution (VT) of [11C]PF06885190 was examined using Logan graphical analysis with arterial input function. Radiometabolites were analyzed in monkey blood plasma using gradient HPLC system. Radiolabeling of [11C]PF06885190 was successfully accomplished and the radioligand was found to be stable in the formulation, with radiochemical purity exceeding 99% 1 h after the end of the synthesis. [11C]PF06885190 was characterized in the cynomolgus monkey brain where a moderate brain uptake was found at the baseline condition. However, it showed fast wash-out as it dropped to half of the peak at around 10 min. Change of VT from baseline was around -10% after pretreatment with a M4 PAM, CVL-231. Radiometabolite studies showed relatively fast metabolism. Although sufficient brain uptake of [11C]PF06885190 was observed, these data suggest that [11C]PF06885190 might have too low specific binding in the NHP brain to be further applied in PET imaging.

Preclinical evaluation of [11C]GW457427 as a tracer for neutrophil elastase.

INTRODUCTION: Neutrophils are part of the innate immune system and function as a first line of defense against invading microorganisms. Overactivity of the immune system may result in a devastating immuno-inflammation with extensive damage to tissue leading to organ damage and/or failure. The literature suggests several human diseases in which neutrophil elastase (NE) is postulated to be important in the pathophysiology including inflammatory bowel disease (IBD), chronic obstructive pulmonary disorder (COPD), abdominal aortic aneurysms (AAA), breast and lung cancer, and recently also in Sars-cov-2 virus infection (Covid-19). In particular, the lungs are affected by the destructive power of the protease neutrophil elastase (NE). In this paper, we report the pre-clinical development of a selective and specific positron emission tomography (PET) tracer, [11C]GW457427, as an in vivo biomarker for the study of NE, now available for human studies. METHODS: [11C]GW457427 was produced by methylation of GW447631 using [11C]methyl triflate and GMP validated production and quality control methods were developed. Chemical purity was high with no traces of the precursor GW611437 or other uv-absorbing compounds. A method for the determination of intact [11C]GW457427 in plasma was developed and the binding characteristics were evaluated in vitro and in vivo. An animal model for lung inflammation was used to investigate the specificity and sensitivity of the [11C]GW457427 tracer for neutrophil elastase (NE) in pulmonary inflammation, verified by blockade using two structurally different elastase inhibitors. RESULTS: [11C]GW457427 was obtained in approximately 45% radiochemical yield and with a radiochemical purity higher than 98%. Molar activity was in the range 130-360 GBq/µmol. Binding to NE was shown to be highly specific both in vitro and in vivo and a significantly higher uptake of tracer was found in a lipopolysaccharide mouse model of pulmonary inflammation compared with control animals. The uptake in lung tissue measured as standardized uptake value (SUV) strongly correlated with tissue NE content as measured by ELISA. In vitro studies also showed specific tracer binding in aortic tissue of patients with abdominal aorta aneurysm (AAA). The rate of metabolism in rats was appropriate considering the critical balance between available tracer for binding and requirement for blood clearance with about 40% and 20% intact [11C]GW457427 in plasma at 5 and 40 min, respectively. Radioactivity was cleared from blood and organs in control animals with mainly hepatobiliary excretion with distribution in the intestines and the urinary bladder; but without retention of the tracer in healthy organs of interests such as the lung, liver, kidneys or in the cardiovascular system. A dosimetry study in rat indicated that the whole-body effective dose was 2.2 µSv/MBq with bone marrow as the limiting organ. It is estimated that up to five PET-CT investigations could be performed in humans without exceeding a total dose of 10 mSv. CONCLUSION: [11C]GW457427 is a promising in vivo PET-biomarker for NE with high specific binding demonstrated both in vitro and in vivo. A GMP validated production method including quality control has been developed and a microdosing toxicity study performed with no adverse signs. [11C]GW457427 is currently being evaluated in a First-In-Man PET study.

PET Imaging of VMAT2 with the Novel Radioligand [18F]FE-DTBZ-d4 in Nonhuman Primates: Comparison with [11C]DTBZ and [18F]FE-DTBZ.

The vesicular monoamine transporter type 2 (VMAT2) is believed to be responsible for the uptake of monoamines into the vesicles of the synaptic terminals. Two VMAT2 radioligands [11C]DTBZ and [18F]FP-DTBZ have been used to assess the degree of nigrostriatal deficit in Parkinson's disease (PD) using positron emission tomography (PET). [18F]FE-DTBZ-d4, the nondeuterated analogue of [18F]FE-DTBZ showed similar imaging properties with better stability against defluorination. Therefore, [18F]FE-DTBZ-d4 draws attention to be investigated as an imaging marker for VMAT2 in the brain. The aim of this study was to investigate the brain kinetics and quantification of [18F]FE-DTBZ-d4 in nonhuman primates (NHPs), with comparison to [11C]DTBZ and [18F]FE-DTBZ. Radiolabeling was successfully achieved either by one-step 11C-methylation or by a two-step fluorine-18 nucleophilic substitution reaction. The stability and radiochemical yield were analyzed with high-performance liquid chromatography (HPLC). Three female cynomolgus monkeys were included in the study and underwent a total of 12 positron emission tomography (PET) measurements. Each monkey was examined with each tracer. In addition, two pretreatment and one displacement PET measurements with tetrabenazine (2.0 mg/kg) were performed for [18F]FE-DTBZ-d4. All PET measurements were conducted using a high-resolution research tomograph (HRRT) system. Radiometabolites were measured in monkey plasma using gradient radio-HPLC. [18F]FE-DTBZ-d4 (SUV: 4.28 ± 1.01) displayed higher brain uptake compared to both [18F]FE-DTBZ (SUV: 3.43 ± 0.54) and [11C]DTBZ (SUV: 3.06 ± 0.32) and faster washout. Binding potential (BPND) values of [18F]FE-DTBZ-d4 in different brain regions (putamen: 5.5 ± 1.4; caudate: 4.4 ± 1.1; midbrain: 1.4 ± 0.4) were higher than those of [11C]DTBZ and [18F]FE-DTBZ. [18F]FE-DTBZ showed faster radiometabolism in plasma compared to [11C]DTBZ and [18F]FE-DTBZ-d4. [18F]FE-DTBZ-d4 is a suitable radioligand for quantification of VMAT2 in the nonhuman primate brain, with better imaging properties than [11C]DTBZ and [18F]FE-DTBZ. A preliminary comparison suggests that [18F]FE-DTBZ-d4 has increased stability against defluorination compared to the nondeuterated analogue.

Para-chloro-2-[18F]fluoroethyl-etomidate: A promising new PET radiotracer for adrenocortical imaging.

Introduction: [11C]Metomidate ([11C]MTO), the methyl ester analogue of etomidate, was developed as a positron emission tomography (PET) radiotracer for adrenocortical tumours and has also been suggested for imaging in primary aldosteronism (PA). A disadvantage of [11C]MTO is the rather high non-specific binding in the liver, which impacts both visualization and quantification of the uptake in the right adrenal gland. Furthermore, the short 20-minute half-life of carbon-11 is a logistic challenge in the clinical setting. Objectives: The aim of this study was to further evaluate the previously published fluorine-18 (T1/2=109.5 min) etomidate analogue, para-chloro-2-[18F]fluoroethyl etomidate; [18F]CETO, as an adrenal PET tracer. Methods: In vitro experiments included autoradiography on human and cynomolgus monkey (non-human primate, NHP) tissues and binding studies on adrenal tissue from NHPs. In vivo studies with [18F]CETO in mice, rats and NHP, using PET and CT/MRI, assessed biodistribution and binding specificity in comparison to [11C]MTO. Results: The binding of [18F]CETO in the normal adrenal cortex, as well as in human adrenocortical adenomas and adrenocortical carcinomas, was shown to be specific, both in vitro (in humans) and in vivo (in rats and NHP) with an in vitro Kd of 0.66 nM. Non-specific uptake of [18F]CETO in NHP liver was found to be low compared to that of [11C]MTO. Conclusions: High specificity of [18F]CETO to the adrenal cortex was demonstrated, with in vivo binding properties qualitatively surpassing those of [11C]MTO. Non-specific binding to the liver was significantly lower than that of [11C]MTO. [18F]CETO is a promising new PET tracer for imaging of adrenocortical disease and should be evaluated further in humans.

Synthesis, Biodistribution, and Radiation Dosimetry of a Novel mGluR5 Radioligand: 18F-AZD9272.

The metabotropic glutamate receptor subtype mGluR5 has been proposed as a potential drug target for CNS disorders such as anxiety, depression, Parkinson's disease, and epilepsy. The AstraZeneca compound AZD9272 has previously been labeled with carbon-11 and used as a PET radioligand for mGluR5 receptor binding. The molecular structure of AZD9272 allows one to label the molecule with fluorine-18 without altering the structure. The aim of this study was to develop a fluorine-18 analogue of AZD9272 and to examine its binding distribution in the nonhuman primate brain in vivo as well as to obtain whole body radiation dosimetry. 18F-AZD9272 was successfully synthesized from a nitro precursor. The radioligand was stable, with a radiochemical purity of >99% at 2 h after formulation in a sterile phosphate buffered solution (pH = 7.4). After injection of 18F-AZD9272 in two cynomolgus monkeys, the maximum whole brain radioactivity concentration was 4.9-6.7% of the injected dose (n = 2) and PET images showed a pattern of regional radioactivity consistent with that previously obtained for 11C-AZD9272. The percentage of parent radioligand in plasma was 59 and 64% (n = 2) at 120 min after injection of 18F-AZD9272, consistent with high metabolic stability. Two whole body PET scans were performed in nonhuman primates for a total of 231 min after injection of 18F-AZD9272. Highest uptakes were seen in liver and small intestine, followed by brain and kidney. The estimated effective dose was around 0.017 mSv/MBq. 18F-AZD9272 shows suitable properties as a PET radioligand for in vivo imaging of binding in the primate brain. 18F-labeled AZD9272 offers advantages over 11C-AZD9272 in terms of higher image resolution, combined with a longer half-life. Moreover, based on the distribution and the estimated radiation burden, imaging of 18F-AZD9272 could be used as an improved tool for quantitative assessment and characterization of AZD9272 binding sites in the human brain by using PET.

PET imaging of beta-secretase 1 in the human brain: radiation dosimetry, quantification, and test-retest examination of [18F]PF-06684511.

PURPOSE: Beta-secretase 1 (BACE1) enzyme is implicated in the pathophysiology of Alzheimer's disease. [18F]PF-06684511 is a positron emission tomography (PET) radioligand for imaging BACE1. Despite favorable brain kinetic properties, the effective dose (ED) of [18F]PF-06684511 estimated in non-human primates was relatively high. This study was therefore designed to evaluate the whole-body distribution, dosimetry, quantification, and test-retest reliability of imaging brain BACE1 with [18F]PF-06684511 in healthy volunteers. METHODS: Five subjects were studied for the dosimetry study. Whole-body PET was performed for 366 min with 4 PET-CT sessions. Estimates of the absorbed radiation dose were calculated using the male adult model. Eight subjects participated in the test-retest study. Brain PET measurements were conducted for 123 min with an interval of 5 to 19 days between test and retest conditions. The total distribution volume (VT) was estimated with one-tissue (1T), two-tissue (2T), compartment model (CM), and graphical analysis. Test-retest variability (TRV) and intraclass correlation coefficient (ICC) of VT were calculated as reliability measures. RESULTS: In the dosimetry study, the highest uptake was found in the liver (25.2 ± 2.3 %ID at 0.5 h) and the largest dose was observed in the pancreas (92.9 ± 52.2 µSv/MBq). The calculated ED was 24.7 ± 0.8 µSv/MBq. In the test-retest study, 2TCM described the time-activity curves well. VT (2TCM) was the highest in the anterior cingulate cortex (6.28 ± 1.09 and 6.85 ± 0.81) and the lowest in the cerebellum (4.23 ± 0.88 and 4.20 ± 0.75). Mean TRV and ICC of VT (2TCM) were 16.5% (12.4-20.5%) and 0.496 (0.291-0.644). CONCLUSION: The ED of [18F]PF-06684511 was similar to other 18F radioligands, allowing repeated PET measurements. 2TCM was the most appropriate quantification method. TRV of VT was similar to other radioligands without a reference region, albeit with lower ICC. These data indicated that [18F]PF-06684511 is a suitable radioligand to measure BACE1 level in the human brain. TRIAL REGISTRATION: EudraCT 2016-001110-19 (registered 2016-08-08).

Characterization of [11C]PXT012253 as a PET Radioligand for mGlu4 Allosteric Modulators in Nonhuman Primates.

PURPOSE: Modulation of presynaptic metabotropic glutamate receptor 4 (mGlu4) by an allosteric ligand has been proposed as a promising therapeutic target in Parkinson's disease and levodopa-induced dyskinesia. A positron emission tomography (PET) ligand for an allosteric site of mGlu4 may provide evidence that a clinical drug candidate reaches and binds the target. A carbon-11-labeled PET radioligand binding an allosteric site of mGlu4, [11C]PXT012253, has been recently developed. Here, we describe the detailed characterization of this novel radiolabeled mGlu4 ligand in nonhuman primates. PROCEDURES: [11C]PXT012253 binding in the brain of cynomolgus monkeys, under the baseline and blocking conditions with the structurally different mGlu4 allosteric ligand PXT002331, currently in clinical trials for Parkinson's disease, was quantified with compartment and graphical modeling approaches using a radiometabolite-corrected plasma input function. Whole-body biodistribution of [11C]PXT012253 was then assessed using PET/x-ray computed tomography to estimate the human effective doses of [11C]PXT012253 for further clinical studies. RESULTS: [11C]PXT012253 displayed binding in mGlu4-expressing regions in the brain of cynomolgus monkeys. Brain regional time-activity curves of [11C]PXT012253 were well described in the two-tissue compartment model (2TC). Total distribution volume was stably estimated using Logan plot and multilinear analysis (MA1) although 2TC showed unstable values in some cases. Competition with PXT002331 showed high specific binding in the total distribution volume. Whole-body PET showed high accumulation of [11C]PXT012253 in the liver, kidney, heart, and brain in the initial phase. The radioligand was excreted through both the gastrointestinal and the urinary tracts. Effective dose of [11C]PXT012253 was estimated to be 0.0042 mSv/MBq. CONCLUSIONS: [11C]PXT012253 was shown to be a promising PET radioligand for mGlu4 allosteric modulators in the monkey brain. MA1 would be the choice of quantitative method. Further development of [11C]PXT012253 in human subjects is warranted.

The development of a GPR44 targeting radioligand [11C]AZ12204657 for in vivo assessment of beta cell mass.

BACKGROUND: The G-protein-coupled receptor 44 (GPR44) is a beta cell-restricted target that may serve as a marker for beta cell mass (BCM) given the development of a suitable PET ligand. METHODS: The binding characteristics of the selected candidate, AZ12204657, at human GPR44 were determined using in vitro ligand binding assays. AZ12204657 was radiolabeled using 11C- or 3H-labeled methyl iodide ([11C/3H]CH3I) in one step, and the conversion of [11C/3H]CH3I to the radiolabeled product [11C/3H]AZ12204657 was quantitative. The specificity of radioligand binding to GPR44 and the selectivity for beta cells were evaluated by in vitro binding studies on pancreatic sections from human and non-human primates as well as on homogenates from endocrine and exocrine pancreatic compartments. RESULTS: The radiochemical purity of the resulting radioligand [11C]AZ12204657 was > 98%, with high molar activity (MA), 1351 ± 575 GBq/µmol (n = 18). The radiochemical purity of [3H]AZ12204657 was > 99% with MA of 2 GBq/µmol. Pancreatic binding of [11C/3H]AZ12204657 was co-localized with insulin-positive islets of Langerhans in non-diabetic individuals and individuals with type 2 diabetes (T2D). The binding of [11C]AZ12204657 to GPR44 was > 10 times higher in islet homogenates compared to exocrine homogenates. In human islets of Langerhans GPR44 was co-expressed with insulin, but not glucagon as assessed by co-staining and confocal microscopy. CONCLUSION: We radiolabeled [11C]AZ12204657, a potential PET radioligand for the beta cell-restricted protein GPR44. In vitro evaluation demonstrated that [3H]AZ12204657 and [11C]AZ12204657 selectively target pancreatic beta cells. [11C]AZ12204657 has promising properties as a marker for human BCM.

In Vivo Visualization of ß-Cells by Targeting of GPR44.

GPR44 expression has recently been described as highly ß-cell selective in the human pancreas and constitutes a tentative surrogate imaging biomarker in diabetes. A radiolabeled small-molecule GPR44 antagonist, [11C]AZ12204657, was evaluated for visualization of ß-cells in pigs and nonhuman primates by positron emission tomography as well as in immunodeficient mice transplanted with human islets under the kidney capsule. In vitro autoradiography of human and animal pancreatic sections from subjects without and with diabetes, in combination with insulin staining, was performed to assess ß-cell selectivity of the radiotracer. Proof of principle of in vivo targeting of human islets by [11C]AZ12204657 was shown in the immunodeficient mouse transplantation model. Furthermore, [11C]AZ12204657 bound by a GPR44-mediated mechanism in pancreatic sections from humans and pigs without diabetes, but not those with diabetes. In vivo [11C]AZ12204657 bound specifically to GPR44 in pancreas and spleen and could be competed away dose-dependently in nondiabetic pigs and nonhuman primates. [11C]AZ12204657 is a first-in-class surrogate imaging biomarker for pancreatic ß-cells by targeting the protein GPR44.

Fluorine-18 labeling of three novel D-peptides by conjugation with N-succinimidyl-4-[18F]fluorobenzoate and preliminary examination by postmortem whole-hemisphere human brain autoradiography.

INTRODUCTION: ß-Amyloid (Aß) plaques and neurofibrillary tangles are the main characteristics of Alzheimer's disease (AD). Positron emission tomography (PET), a high-resolution, sensitive, and noninvasive imaging technique, has been widely utilized in visualizing the localization of plaques and tangles and thereby distinguishing between AD and healthy controls. A small 12-mer D-enantiomeric peptide (amino acid sequence=QSHYRHISPAQV), denoted as D1, has high binding affinity to Aß in vitro in the sub-micromolar range, and consequently, its radiolabeled analogues have a potential as radioligands for visualizing amyloid plaques in vivo by PET. AIM: The aims of the present work were to develop three different potent D1 derivative peptides labeled with fluorine-18 and to examine them in the AD and control postmortem human brain by autoradiography (ARG). METHODS: Three different D1 derivative peptides were radiolabeled with fluorine-18 ([(18)F]ACI-87, [(18)F]ACI-88, [(18)F]ACI-89) using the prosthetic group N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) and purified by high performance liquid chromatography (HPLC). Preliminary ARG measurements were performed in AD and control brains. RESULTS: The three fluorine-18-labeled d-peptides were obtained in a total synthesis time of 140 min with radiochemical purity higher than 98%. The specific radioactivities of the three different D1 derivative peptides were between 9 and 113 GBq/µmol. ARG demonstrated a higher radioligand uptake in the cortical gray matter and the hippocampus in the AD brain as compared to age-matched control brain. CONCLUSIONS: Fluorine-18 labeling of the three novel D1 derivative peptides using [(18)F]SFB was successfully accomplished. Higher contrast between AD and control brain slices demonstrates their potential applicability for further use in vivo by PET.

Distribution and binding of 18F-labeled and 125I-labeled analogues of ACI-80, a prospective molecular imaging biomarker of disease: a whole hemisphere post mortem autoradiography study in human brains obtained from Alzheimer's disease patients.

One of the major pathological landmarks of Alzheimer's disease and other neurodegenerative diseases is the presence of amyloid deposits in the brain. The early non-invasive visualization of amyloid is a major objective of recent diagnostic neuroimaging approaches, including positron emission tomography (PET), with an eye on follow-up of disease progression and/or therapy efficacy. The development of molecular imaging biomarkers with binding affinity to amyloid in the brain is therefore in the forefront of imaging biomarker and radiochemistry research. Recently, a dodecamer peptide (amino acid sequence=QSHYRHISPAQV; denominated D1 or ACI-80) was identified as a prospective ligand candidate, binding with high ex vivo affinity to L-Aß-amyloid (K(d): 0.4 µM). In order to assess the ligand's capacity to visualize amyloid in Alzheimer's disease (AD), two (125)I labeled and three (18)F labeled analogues of the peptide were synthesized and tested in post mortem human autoradiography experiments using whole hemisphere brain slices obtained from deceased AD patients and age matched control subjects. The (18)F-labeled radioligands showed more promising visualization capacity of amyloid that the (125)I-labeled radioligands. In the case of each (18)F radioligands the grey matter uptake in the AD brains was significantly higher than that in control brains. Furthermore, the grey matter: white matter uptake ratio was over ~2, the difference being significant for each (18)F-radioligands. The regional distribution of the uptake of the various radioligands systematically shows a congruent pattern between the high uptake regions and spots in the autoradiographic images and the disease specific signals obtained in adjacent or identical brain slices labeled with histological, immunohistochemical or autoradiographic stains for amyloid deposits or activated astrocytes. The present data, using post mortem human brain autoradiography in whole hemisphere human brains obtained from deceased AD patients and age matched control subjects, support the visualization capacity of the radiolabeled ACI-80 analogues of amyloid deposits in the human brain. Further studies are warranted to explore the usefulness of the (18)F-labeled analogues as in vivo molecular imaging biomarkers in diagnostic PET studies.

Decreased defluorination using the novel beta-cell imaging agent [18F]FE-DTBZ-d4 in pigs examined by PET.

BACKGROUND: Fluorine-18 dihydrotetrabenazine [DTBZ] analogues, which selectively target the vesicular monoamine transporter 2 [VMAT2], have been extensively studied for in vivo quantification of beta cell mass by positron-emission tomography [PET]. This study describes a novel deuterated radioligand [18F]fluoroethyl [FE]-DTBZ-d4, aimed to increase the stability against in vivo defluorination previously observed for [18F]FE-DTBZ. METHODS: [18F]FE-DTBZ-d4 was synthesized by alkylation of 9-O-desmethyl-(+)-DTBZ precursor with deuterated [18F]FE bromide ([18F]FCD2CD2Br). Radioligand binding potential [BP] was assessed by an in vitro saturation homogenate binding assay using human endocrine and exocrine pancreatic tissues. In vivo pharmacokinetics and pharmacodynamics [PK/PD] was studied in a porcine model by PET/computed tomography, and the rate of defluorination was quantified by compartmental modeling. RESULTS: [18F]FE-DTBZ-d4 was produced in reproducible good radiochemical yield in 100 ± 20 min. Radiochemical purity of the formulated product was > 98% for up to 5 h with specific radioactivities that ranged from 192 to 529 GBq/µmol at the end of the synthesis. The in vitro BP for VMAT2 in the islet tissue was 27.0 ± 8.8, and for the exocrine tissue, 1.7 ± 1.0. The rate of in vivo defluorination was decreased significantly (kdefluorination = 0.0016 ± 0.0007 min-1) compared to the non-deuterated analogue (kdefluorination = 0.012 ± 0.002 min-1), resulting in a six fold increase in half-life stability. CONCLUSIONS: [18F]FE-DTBZ-d4 has similar PK and PD properties for VMAT2 imaging as its non-deuterated analogue [18F]FE-DTBZ in addition to gaining significantly increased stability against defluorination. [18F]FE-DTBZ-d4 is a prime candidate for future preclinical and clinical studies on focal clusters of beta cells, such as in intramuscular islet grafts.

In vivo and in vitro characterization of [18F]-FE-(+)-DTBZ as a tracer for beta-cell mass.

INTRODUCTION: The positron emission tomography (PET) tracer 9-[(18)F]fluoroethyl-(+)-dihydrotetrabenazine ([(18)F]-FE-(+)-DTBZ) is a potential candidate for quantifying beta-cell mass in vivo. The purpose was to investigate in vitro and in vivo utility of this tracer for the assessment of beta-cell mass. METHODS: Three pigs were intravenously administered [(18)F]-FE-(+)-DTBZ and examined by PET/computed tomography. Binding parameters were estimated by kinetic modeling. In vitro k(D) and B(max) were determined by saturation binding studies of endocrine and exocrine human tissue homogenates. In vitro pancreatic uptake was determined by tissue autoradiography with pancreases from patients with types 1 (T1DM) and 2 diabetes mellitus (T2DM) and healthy controls. RESULTS: [(18)F]-FE-(+)-DTBZ had a k(D) of 3.5+/-1.0 nM, a B(max) of 382+/-108 fmol/mg protein and a specificity of 89+/-1.8% in islet homogenates. The total exocrine uptake was lower and 65% was nondisplaceable. No uptake difference was observed in pancreatic tissue slices from patients with T1DM, T2DM or healthy controls. The in vivo porcine pancreatic uptake reached a peak of standardized uptake value (SUV) of 2.8 with a low distribution volume ratio in all animals. Moderate to high tracer uptake was identified in the bile system and in bone. CONCLUSIONS: [(18)F]-FE-(+)-DTBZ binds to vesicular monoamine transporter 2 (VMAT2) with high specificity in pure islet tissue in vitro. However, there is high nondisplaceable binding to exocrine tissue. In addition, in vivo tracer metabolism and dehalogenation result in severe underestimation of porcine pancreatic VMAT2 expression and BCM. The results do not support [(18)F]-FE-(+)-DTBZ as a suitable tracer for in vivo beta-cell imaging.