N-[18F]-FluoropropylJDTic for κ-opioid receptor PET imaging: Radiosynthesis, pre-clinical evaluation, and metabolic investigation in comparison with parent JDTic.

INTRODUCTION: To image kappa opioid receptor (KOR) for preclinical studies, N-fluoropropylJDTic 9 derived from the best-established KOR antagonist JDTic, was labeled with fluorine-18. METHODS: Radiosynthesis of [18F]9 was achieved according to an automated two-step procedure from [18F]-fluoride. Peripheral and cerebral distributions were determined by ex vivo experiments and by PET imaging in mouse. Radiometabolism studies were performed both in vivo in mice and in vitro in mouse and human liver microsomes. Identification of the major metabolic fragmentations was carried out by UPLC-MS analysis of enzymatic cleavage of non-radioactive ligand 9. Microsomal metabolic degradation of parent JDTic was also achieved for comparison. RESULTS: The radiotracer [18F]9 was produced after 140±5min total synthesis time (2.2±0.4% not decay corrected radiochemical yield) with a specific activity of 41-89GBq/µmol (1.1-2.4Ci/µmol). Peripheral and regional brain distributions of [18F]9 were consistent with known KOR locations but no significant specific binding in brain was shown. [18F]9 presented a typical hepatobiliary and renal elimination, and was rapidly metabolized. The in vivo and in vitro radiometabolic profiles of [18F]9 were similar. Piperidine 12 was identified as the major metabolic fragment of the non-radioactive ligand 9. JDTic 7 was found to be much more stable than 9. CONCLUSION: Although the newly proposed radioligand [18F]9 was concluded to be not suitable for KOR PET imaging due to the formation of brain penetrating radiometabolites, our findings highlight the metabolic stability of JDTic and may help in the design of novel JDTic derivatives for in vivo applications.

In Vivo Evaluation of Radiofluorinated Caspase-3/7 Inhibitors as Radiotracers for Apoptosis Imaging and Comparison with [18F]ML-10 in a Stroke Model in the Rat.

PURPOSE: The first biological evaluation of two potent fluorine-18 radiolabelled inhibitors of caspase-3/7 was achieved in a cerebral stroke rat model to visualize apoptosis. PROCEDURES: In vivo characteristics of isatins [(18)F]-2 and [(18)F]-3 were studied and compared by µPET to previously described 1-[4-(2-[(18)F]fluoroethyl)benzyl]-5-(2-methoxymethylpyrrolidin-1-ylsulfonyl)isatin ([(18)F]-1) and to 2-(5-[(18)F]fluoropentyl)-2-methyl-malonic acid ([(18)F]ML-10) used as a reference radiotracer in a rat stroke model. RESULTS: [(18)F]-2 and [(18)F]-3 were radiolabelled with high radiochemical purity and high specific radioactivity. Radioactivity uptakes in ischemic and contralateral brain regions were weak for the three radiolabelled isatins and lower for [(18)F]ML-10. In µPET, time activity curves showed significant uptake differences between both regions of interest for [(18)F]-1 after 45 min. No differences were observed for [(18)F]ML-10. CONCLUSIONS: Radiolabelled isatins are more promising radiotracers to image apoptosis than [(18)F]ML-10 in this stroke animal model without craniectomy. In particular, [(18)F]-1 presented significant uptake in apoptotic area 45 min after administration.

Fully automated radiosynthesis of 2-[18F]fludarabine for PET imaging of low-grade lymphoma.

PURPOSE: An efficient and fully automated radiosynthesis of 2-[(18)F]fluoro-9-ß-D-arabinofuranosyl-adenine (2-[(18)F]fludarabine, [(18)F]-5) based on a GE TRACERlab™ FX-FN module has been developed. PROCEDURES: A 2-nitro purine derivative 3 was developed as precursor for labeling with fluorine-18. The radiosynthesis of [(18)F]-5 was performed in two steps in a single reactor with an intermediary purification on Sep-Pak® silica which involved the addition of a three-way valve on the original module. After hydrolysis, [(18)F]-5 was purified by semi-preparative high-pressure liquid chromatography (HPLC) and a quality control was established. RESULTS: The labeling precursor 3 was obtained in 45% overall yield. Nucleophilic substitution with K(18)F/K2.2.2 afforded protected 2-[(18)F]fludarabine ([(18)F]-4) in 73 ± 4%, radiochemical yield (decay corrected to the end of bombardment (EOB)) and based on the initial [(18)F]F(-) activity. An aqueous ammonia/methanol solution was used for the deprotection reaction and gave the desired [(18)F]-5 in 67 ± 3% yield after 20 min at 70 °C based on HPLC profile. CONCLUSIONS: The process afforded pure 2-[(18)F]fludarabine in 48 ± 3% yield (decay corrected to the EOB) in 85 min, with a specific activity of 310 ± 72 GBq/µmol at the end of synthesis (EOS) and a radiochemical purity up to 99%.

Automated radiosynthesis of [(18)F]ML-10, a PET radiotracer dedicated to apoptosis imaging, on a TRACERLab FX-FN module.

PURPOSE: [(18)F]ML-10 is the most advanced radiopharmaceutical for the clinical imaging of the apoptosis phenomenon by PET. The preparation of this radiopharmaceutical on a commercial radiosynthesis module and the requested quality controls for its release are presented herein. PROCEDURES: ML-10 as reference and its mesyloxy derivative as precursor for labelling with fluorine-18 were prepared. [(18)F]ML-10 was synthesized via a [(18)F]fluorine-de-mesyloxy aliphatic nucleophilic substitution via a GE TRACERLab® FX-FN module. Quality controls were performed. RESULTS: The labelling precursor was obtained in a four step synthesis in 28 % overall yield affording ML-10 in two steps (88 % yield). Pure [(18)F]ML-10 was obtained with a decay corrected yield of 39.8 % ± 8.4 % (n = 7) in 70 min and a specific activity of 235 ± 85 GBq/µmol at the end of synthesis. CONCLUSIONS: [(18)F]ML-10 was prepared on a widely available automated module and passed the quality control. A LC/MS method was developed to measure specific radioactivity.

Detection of glioblastoma response to temozolomide combined with bevacizumab based on µMRI and µPET imaging reveals [18F]-fluoro-L-thymidine as an early and robust predictive marker for treatment efficacy.

The individualized care of glioma patients ought to benefit from imaging biomarkers as precocious predictors of therapeutic efficacy. Contrast enhanced MRI and [(18)F]-fluorodeoxyglucose (FDG)-PET are routinely used in clinical settings; their ability to forecast the therapeutic response is controversial. The objectives of our preclinical study were to analyze sensitive µMRI and/or µPET imaging biomarkers to predict the efficacy of anti-angiogenic and/or chemotherapeutic regimens. Human U87 and U251 orthotopic glioma models were implanted in nude rats. Temozolomide and/or bevacizumab were administered. µMRI (anatomical, diffusion, and microrheological parameters) and µPET ([(18)F]-FDG and [(18)F]-fluoro-l-thymidine [FLT]-PET) studies were undertaken soon (t(1)) after treatment initiation compared with late anatomical µMRI evaluation of tumor volume (t(2)) and overall survival. In both models, FDG and FLT uptakes were attenuated at t(1) in response to temozolomide alone or with bevacizumab. The distribution of FLT, reflecting intratumoral heterogeneity, was also modified. FDG was less predictive for treatment efficacy than was FLT (also highly correlated with outcome, P < .001 for both models). Cerebral blood volume was significantly decreased by temozolomide + bevacizumab and was correlated with survival for rats with U87 implants. While FLT was highly predictive of treatment efficacy, a combination of imaging biomarkers was superior to any one alone (P < .0001 in both tumors with outcome). Our results indicate that FLT is a sensitive predictor of treatment efficacy and that predictability is enhanced by a combination of imaging biomarkers. These findings may translate clinically in that individualized glioma treatments could be decided in given patients after PET/MRI examinations.

PET imaging with [18F]AV-45 in an APP/PS1-21 murine model of amyloid plaque deposition.

Alzheimer's disease (AD), the most common age-related neurodegenerative disorder, is characterized by the accumulation of ß-amyloid peptide. In man, [18F]AV-45 with positron emission tomography (PET) is currently studied and used to track in vivo amyloid accumulation. Here, [18F]-AV45-PET was used to visualize amyloid deposition in a transgenic murine model of amyloidosis (APP/PS1-21). Studies were performed ex vivo by autoradiography and in vivo by microPET. Autoradiograms of the brain sections highlighted an increased uptake of [18F]AV-45 in APP/PS1-21 mice compared with age-matched control mice. From 8 months, an intense labeling was observed in cortex, hippocampus, and striatum. The marked accumulation of radiotracer was found in close association with thioflavin S-positive amyloid plaques. The longitudinal microPET assessment, performed from 3 to 12 months of age, demonstrated an increased [18F]AV-45 uptake in APP/PS1-21 compared with control mice. The elevated tracer uptake was increased in association with age. This study opens the possibility of [18F]AV-45, coupled with microPET, to visualize and quantitatively measure amyloid deposits in the brains of living APP/PS1 mice.

Complementary information from magnetic resonance imaging and (18)F-fluoromisonidazole positron emission tomography in the assessment of the response to an antiangiogenic treatment in a rat brain tumor model.

INTRODUCTION: No direct proof has been brought to light in a link between hypoxic changes in glioma models and the effects of antiangiogenic treatments. Here, we assessed the sensitivity of the detection of hypoxia through the use of (18)F-fluoromisonidazole positron emission tomography ([(18)F]-FMISO PET) in response to the evolution of the tumor and its vasculature. METHODS: Orthotopic glioma tumors were induced in rats after implantation of C6 or 9L cells. Sunitinib was administered from day (D) 17 to D24. At D17 and D24, multiparametric magnetic resonance imaging was performed to characterize tumor growth and vasculature. Hypoxia was assessed by [(18)F]-FMISO PET. RESULTS: We showed that brain hypoxic volumes are related to glioma volume and its vasculature and that an antiangiogenic treatment, leading to an increase in cerebral blood volume and a decrease in vessel permeability, is accompanied by a decrease in the degree of hypoxia. CONCLUSIONS: We propose that [(18)F]-FMISO PET and multiparametric magnetic resonance imaging are pertinent complementary tools in the evaluation of the effects of an antiangiogenic treatment in glioma.

[11C]-MeJDTic: a novel radioligand for kappa-opioid receptor positron emission tomography imaging.

INTRODUCTION: Radiopharmaceuticals that can bind selectively the kappa-opioid receptor may present opportunities for staging clinical brain disorders and evaluating the efficiency of new therapies related to stroke, neurodegenerative diseases or opiate addiction. The N-methylated derivative of JDTic (named MeJDTic), which has been recently described as a potent and selective antagonist of kappa-opioid receptor in vitro, was labeled with carbon-11 and evaluated for in vivo imaging the kappa-opioid receptor in mice. METHODS: [(11)C]-MeJDTic was prepared by methylation of JDTic with [(11)C]-methyl triflate. The binding of [(11)C]-MeJDTic to kappa-opioid receptor was investigated ex vivo by biodistribution and competition studies using nonfasted male CD1 mice. RESULTS: [(11)C]-MeJDTic exhibited a high and rapid distribution in peripheral organs. The uptake was maximal in lung where the kappa receptor is largely expressed. [(11)C]-MeJDTic rapidly crossed the blood-brain barrier and accumulated in the brain regions of interest (hypothalamus). The parent ligand remained the major radioactive compound in brain during the experiment. Chase studies with U50,488 (a kappa referring agonist), morphine (a mu agonist) and naltrindole (a delta antagonist) demonstrated that this uptake was the result of specific binding to the kappa-opioid receptor. CONCLUSION: These findings suggested that [(11)C]-MeJDTic appeared to be a promising selective "lead" radioligand for kappa-opioid receptor PET imaging.

Synthesis and biological evaluation of N-substituted quinolinimides, as potential ligands for in vivo imaging studies of delta-opioid receptors.

We report here the syntheses of N-substituted quinolinimide derivatives displaying sufficient affinity and high selectivity for delta-opioid receptors. Among 9-subsituted derivatives, one showed much higher selectivity for the delta receptor in binding assays than the delta antagonist methylnaltrindole (6: Ki = 42 nM; micro/delta and kappa/delta > 238 on rat brain membranes) and antagonist properties. This compound was labeled with carbon-11 (t1/2 = 20.4 min) as a potential radioligand for the noninvasive assessment of delta opioid receptors in vivo with positron emission tomography (PET). A high yielding radiosynthesis of [11C]-6, based on the [11C]methyl introduction on the pyridine moiety by a Stille reaction, was described (radiochemical yield = 60 +/- 10%, specific activities = 0.8 to 1.5 Ci/micromol). The in vivo pharmacological profile in rats indicated that the radiotracer crossed the blood-brain barrier but was not stable and underwent rapid degradation in both plasma and brain. No specific binding was consequently revealed.