Human brain imaging of nicotinic acetylcholine α4ß2* receptors using [18 F]Nifene: Selectivity, functional activity, toxicity, aging effects, gender effects, and extrathalamic pathways.

Nicotinic acetylcholinergic receptors (nAChR's) have been implicated in several brain disorders, including addiction, Parkinson's disease, Alzheimer's disease and schizophrenia. Here we report in vitro selectivity and functional properties, toxicity in rats, in vivo evaluation in humans, and comparison across species of [18 F]Nifene, a fast acting PET imaging agent for α4ß2* nAChRs. Nifene had subnanomolar affinities for hα2ß2 (0.34 nM), hα3ß2 (0.80 nM) and hα4ß2 (0.83 nM) nAChR but weaker (27-219 nM) for hß4 nAChR subtypes and 169 nM for hα7 nAChR. In functional assays, Nifene (100 µM) exhibited 14% agonist and >50% antagonist characteristics. In 14-day acute toxicity in rats, the maximum tolerated dose (MTD) and the no observed adverse effect level (NOAEL) were estimated to exceed 40 µg/kg/day (278 µg/m2 /day). In human PET studies, [18 F]Nifene (185 MBq; <0.10 µg) was well tolerated with no adverse effects. Distribution volume ratios (DVR) of [18 F]Nifene in white matter thalamic radiations were ∼1.6 (anterior) and ∼1.5 (superior longitudinal fasciculus). Habenula known to contain α3ß2 nAChR exhibited low levels of [18 F]Nifene binding while the red nucleus with α2ß2 nAChR had DVR ∼1.6-1.7. Females had higher [18 F]Nifene binding in all brain regions, with thalamus showing >15% than males. No significant aging effect was observed in [18 F]Nifene binding over 5 decades. In all species (mice, rats, monkeys, and humans) thalamus showed highest [18 F]Nifene binding with reference region ratios >2 compared to extrathalamic regions. Our findings suggest that [18 F]Nifene PET may be used to study α4ß2* nAChRs in various CNS disorders and for translational research.

PET radiotracer development for imaging high-affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine-18 labeled aminotetralins in rodents.

Imaging the high-affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of 18 F-5-OH-FPPAT, and the newer 18 F-5-OH-FHXPAT and 18 F-7-OH-FHXPAT. Syntheses of 18 F-5-OH-FHXPAT and 18 F-7-OH-FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague-Dawley rats were used with the 3 radiotracers (74-111 kBq/cc). Competition with dopamine (1-100 nM) and Gpp(NH)p (10-50 µM) were carried out to demonstrate binding to dopamine D2 and D3 HA-states and binding kinetics of 18 F-5-OH-FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with 18 F-5-OH-FHXPAT to ascertain HA-state binding. PET/CT imaging in rats and wild type (WT) and D2 knock-out mice were carried out using 18 F-7-OH-FHXPAT (2-37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 × 10-2 min-1 for 18 F-5-OH-FPPAT. Treatment with Gpp(NH)p significantly reduced 50-80% striatal binding with faster dissociation rates (5.0 × 10-2 min-1 ), suggesting HA-state binding of 18 F-5-OH-FPPAT and 18 F-5-OH-FHXPAT. Striatal binding of 18 F-5-OH-FHXPAT in ex vivo brain slices were sensitive to Gpp(NH)p, suggesting HA-state binding in vivo. PET binding ratios of 18 F-7-OH-FHXPAT in rat brain were ventral striatum/cerebellum = 2.09 and dorsal striatum/cerebellum = 1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane-bound HA-state of the dopamine receptor.

Synthesis and evaluation of (S)-[(18)F]fesetron in the rat brain as a potential PET imaging agent for serotonin 5-HT3 receptors.

Serotonin 5-HT3 receptors are involved in various brain functions including as an emesis target during cancer chemotherapy. We report here the development of (S)-2,3-dimethoxy-5-(3'-[(18)F]fluoropropyl)-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide ([(18)F]fesetron) as a potential PET imaging agent for serotonin 5-HT3 receptors. By radiolabeling((S)-2,3-dimethoxy-5-(3'-tosyloxypropyl)-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide) with fluorine-18, (S)-[(18)F]fesetron was obtained in 5 to 10% decay-corrected yields and with specific activities >74GBq/µmol at the end of radiosynthesis. PET imaging in rats showed low uptake of [(18)F]fesetron in the brain with retention of binding in the striatal and cerebellar regions. Using colliculi as a reference region, ratios were 3.4 for striata and 2.5 for cerebellum. Ex vivo brain PET analysis displayed binding of [(18)F]fesetron in the hippocampus, striatum and cerebellar regions. Cerebellar regions corresponded to area postrema and nucleus tract solitaris known to contain 5-HT3 receptors. Dorsal hippocampus showed the highest uptake with ratio of >17 with respect to colliculi, while area postrema and striata had ratios of >10. Thus, [(18)F]fesetron exhibited a unique binding profile to rat brain regions known to contain significant amounts of serotonin 5-HT3 receptors. However, the very low brain uptake limits its usefulness as a PET radiotracer in this animal model.

Evaluation of [11C]TAZA for amyloid ß plaque imaging in postmortem human Alzheimer's disease brain region and whole body distribution in rodent PET/CT.

OBJECTIVE: Alzheimer's disease (AD) is a neurodegenerative disease characterized by Aß plaques in the brain. The aim of this study was to evaluate the effectiveness of a novel radiotracer, 4-[(11) C]methylamino-4'-N,N-dimethylaminoazobenzene ([(11)C]TAZA), for binding to Aß plaques in postmortem human brain (AD and normal control (NC)). METHODS: Radiosyntheses of [(11)C]TAZA, related [(11)C]Dalene ((11)C-methylamino-4'-dimethylaminostyrylbenzene), and reference [(11)C]PIB were carried out using [(11)C]methyltriflate prepared from [(11) C]CO(2) and purified using HPLC. In vitro binding affinities were carried out in human AD brain homogenate with Aß plaques labeled with [(3) H]PIB. In vitro autoradiography studies with the three radiotracers were performed on hippocampus of AD and NC brains. PET/CT studies were carried out in normal rats to study brain and whole body distribution. RESULTS: The three radiotracers were produced in high radiochemical yields (>40%) and had specific activities >37 GBq/µmol. TAZA had an affinity, K(i) = 0.84 nM and was five times more potent than PIB. [(11)C]TAZA bound specifically to Aß plaques present in AD brains with gray matter to white matter ratios >20. [(11)C]TAZA was displaced by PIB (>90%), suggesting similar binding site for [(11)C]TAZA and [(11)C]PIB. [(11)C]TAZA exhibited slow kinetics of uptake in the rat brain and whole body images showed uptake in interscapular brown adipose tissue (IBAT). Binding in brain and IBAT were affected by preinjection of atomoxetine, a norepinephrine transporter blocker. CONCLUSION: [(11)C]TAZA exhibited high binding to Aß plaques in human AD hippocampus. Rat brain kinetics was slow and peripheral binding to IBAT needs to be further evaluated.

Comparative assessment of (18) F-Mefway as a serotonin 5-HT1A receptor PET imaging agent across species: Rodents, nonhuman primates, and humans.

We have developed (18) F-trans-Mefway ((18) F-Mefway) for positron emission tomography (PET) imaging studies of serotonin 5-HT1A receptors which are implicated in various brain functions. Translation of imaging the 5-HT1A receptor in animal models to humans will facilitate an understanding of the role of the receptor in human brain disorders. We report comparative brain distribution of (18) F-Mefway in normal mice, rats, monkeys, and healthy human volunteers. Mefway was found to be very selective, with subnanomolar affinity for the 5-HT1A receptor. Affinities of >55 nM were found for all other human-cloned receptor subtypes tested. Mefway was found to be a poor substrate (>30 µM) for the multidrug resistance 1 protein, suggesting low likelihood of brain uptake being affected by P-glycoprotein. Cerebellum was used as a reference region in all imaging studies across all species due to the low levels of (18) F-Mefway binding. Consistent binding of (18) F-Mefway in cortical regions, hippocampus, and raphe was observed across all species. (18) F-Mefway in the human brain regions correlated with the known postmortem distribution of 5-HT1A receptors. Quantitation of raphe was affected by the resolution of the PET scanners in rodents, whereas monkeys and humans showed a raphe to cerebellum ratio of approximately 3. (18) F-Mefway appears to be an effective 5-HT1A receptor imaging agent in all models, including humans. (18) F-Mefway therefore may be used to quantify 5-HT1A receptor distribution in brain regions for the study of various CNS disorders. J. Comp. Neurol. 524:1457-1471, 2016. © 2015 Wiley Periodicals, Inc.

Dopamine D3 receptor binding of (18)F-fallypride: Evaluation using in vitro and in vivo PET imaging studies.

Identification of dopamine D3 receptors (D3R) in vivo is important to understand several brain functions related to addiction. The goal of this work was to identify D3R binding of the dopamine D2 receptor (D2R)/D3R imaging agent, (18)F-fallypride. Brain slices from male Sprague-Dawley rats (n = 6) and New Zealand White rabbits (n = 6) were incubated with (18)F-fallypride and D3R selective agonist (R)-7-OH-DPAT (98-fold D3R selective). Rat slices were also treated with BP 897 (68-fold D3R selective partial agonist) and NGB 2904 (56-fold D3R selective antagonist). In vivo rat studies (n = 6) were done on Inveon PET using 18-37 MBq (18)F-fallypride and drug-induced displacement by (R)-7-OH-DPAT, BP 897 and NGB 2904. PET/CT imaging of wild type (WT, n = 2) and D2R knock-out (KO, n = 2) mice were carried out with (18)F-fallypride. (R)-7-OH-DPAT displaced binding of (18)F-fallypride, both in vitro and in vivo. In vitro, at 10 nM (R)-7-OH-DPAT, (18)F-fallypride binding in the rat ventral striatum (VST) and dorsal striatum (DST) and rabbit nucleus accumbens were reduced by ∼10-15%. At 10 µM (R)-7-OH-DPAT all regions in rat and rabbit were reduced by ≥85%. In vivo reductions for DST and VST before and after (R)-7-OH-DPAT were: low-dose (0.015 mg kg(-1)) DST -22%, VST -29%; high-dose (1.88 mg kg(-1)) DST -58%, VST -77%, suggesting D3R/D2R displacement. BP 897 and NGB 2904 competed with (18)F-fallypride in vitro, but unlike BP 897, NGB 2904 did not displace (18)F-fallypride in vivo. The D2R KO mice lacked (18)F-fallypride binding in the DST. In summary, our findings suggest that up to 20% of (18)F-fallypride may be bound to D3R sites in vivo.

Enhancement of 18F-fluorodeoxyglucose metabolism in rat brain frontal cortex using a ß3 adrenoceptor agonist.

We report the use of ß3 adrenergic receptor mediated activation of rat brain frontal cortex using mirabegron (a selective ß3 adrenoceptor agonist), measured by (18)F-FDG PET/CT. Another ß3 agonist, CL 316,243, did not have this effect due to impermeability through the blood brain barrier (BBB), while atomoxetine, a norepinephrine transporter blocker, did increase (18)F-FDG uptake in the frontal cortex. Mirabegron exhibited a dose-dependent increase in frontal cortex (18)F-FDG uptake. These findings suggest a possible use of selective ß3 adrenoceptor agonists in reversing regional glucose hypometabolism in the brain.

Spinal cord dopamine D2/D3 receptors: in vivo and ex vivo imaging in the rat using (18)F/(11)C-fallypride.

OBJECTIVES: The spinal cord is known to be innervated with dopaminergic cells with catecholaminergic projections arising from the medulla and pons and dopaminergic transmission in the spinal cord is vital for sensory and motor function. Our goal was to evaluate and compare the imaging capability of dopamine D2/D3 receptors in the rat spinal cord using PET ligands (18)F-fallypride and (11)C-fallypride. METHODS: Male Sprague-Dawley rats were used in all in vitro and in vivo studies. Spinal cord and brain sections were used for in vitro autoradiography and ex vivo autoradiography. For in vivo studies animals received a (18)F-fallypride scan or a (11)C-fallypride PET scan. The spinal cord and the brain were then harvested, flash-frozen and imaged ex vivo. For in vivo analysis Logan plots with cerebellum as a reference was used to evaluate binding potentials (BP). Tissue ratios were used for ex vivo analysis. Drug effects were evaluated using clozapine, haloperidol and dopamine were evaluated on spinal cord sections in vitro. RESULTS: In vitro studies showed (18)F-fallypride binding to superficial dorsal horn (SDH), dorsal horn (DH), ventral horn (VH) and the pars centralis (PC). In the cervical section, the greatest amount of binding appeared to be in the SDH. Ex vivo studies showed approximately 6% of (18)F-fallypride in SDH compared to that observed in the striatum. In vivo analysis of both (18)F-fallypride and (11)C-fallypride in the spinal cord were comparable to that in the extrastriatal regions. Haloperidol and clozapine displaced more than 75% of the (18)F-fallypride in spinal cord sections. CONCLUSIONS: Our studies showed (18)F-fallypride and (11)C-fallypride binding in the spinal cord in vitro and in vivo. The binding pattern correlates well with the known distribution of dopamine D2/D3 receptors in the spinal cord.

Targeting histone deacetylase in lung cancer for early diagnosis: (18)F-FAHA PET/CT imaging of NNK-treated A/J mice model.

Elevated levels of histone deacetylases (HDACs) have been indicated in the development of some cancers. HDAC has been imaged using (18)F-FAHA and may serve as a marker to study epigenetics. We report evaluation of (18)F-FAHA as a probe in the early diagnosis of lung cancer using (18)F-FAHA PET/CT studies of A/J mice treated with NNK. (18)F-FAHA radiosynthesis was carried out in specific activity of ~2 Ci/µmol. A/J mice were divided into 2 groups: 1. Controls; 2. NNK treatment group with NNK (100 mg/kg, ip, weekly for 4 wks). Mice were injected 100-200 µCi i.v. (18)F-FAHA and then scanned in Inveon PET/CT under anesthesia using 2.0% isoflurane. Midbrain, cerebellum and brainstem uptake of (18)F-FAHA was displaced by the known HDAC inhibitor, suberanilohydroxamic acid (SAHA) with less than 10% activity remaining. CT revealed presence of lung nodules in 8 to 10-month old NNK mice while control mice were free of tumors. Little uptake of (18)F-FAHA was observed in the control mice lungs while significant (18)F-FAHA uptake occurred in the lungs of NNK-treated mice with tumor/nontumor >2.0. Ex vivo scans of the excised NNK and control mice lungs confirmed presence of extensive amounts of lung nodules seen by CT and confirmed by (18)F-FAHA in the NNK mice with tumor/nontumor >6.0. Our preliminary imaging studies with A/J mice lung cancer model suggest (18)F-FAHA PET may allow the study of epigenetic mechanisms involved in NNK-induced tumorigenesis in the lungs.

Imaging pancreas in healthy and diabetic rodent model using [18F]fallypride positron emission tomography/computed tomography.

BACKGROUND: A noninvasive method of monitoring the loss of islet cells can provide an earlier and improved diagnosis for therapeutics development of preclinical phases of diabetes. The use of [(18)F]fallypride, a dopamine D2/D3 receptor radiotracer, has been developed as a surrogate marker to evaluate loss of pancreatic islet cells in a rodent model of type 1 diabetes. MATERIALS AND METHODS: Healthy Sprague-Dawley rats were administered [(18)F]fallypride and imaged for 2 h in a positron emission tomography (PET)/computed tomography (CT) scan. Diabetes was then induced in the same rats by administration of streptozotocin, and a PET/CT scan was performed 4 days after establishing diabetes. Pancreata of a separate set of rats were evaluated by insulin immunostaining for loss of islet cells by streptozotocin. RESULTS: Blood glucose levels of 125 mg/dL and 550 mg/dL were established for those rats without and with diabetes, respectively. [(18)F]Fallypride uptake in the pancreas of both groups of rats was rapid, but the rats with diabetes showed a significantly lower uptake (less than 50%). The specific binding ratio was decreased by 77% in the diabetic rats. CONCLUSIONS: [(18)F]Fallypride can be a useful surrogate marker for monitoring changes in pancreatic islet cells, thus providing a noninvasive method to evaluate efficacy of therapeutics.

Adrenergic pathway activation enhances brown adipose tissue metabolism: a [¹8F]FDG PET/CT study in mice.

OBJECTIVE: Pharmacologic approaches to study brown adipocyte activation in vivo with a potential of being translational to humans are desired. The aim of this study was to examine pre- and postsynaptic targeting of adrenergic system for enhancing brown adipose tissue (BAT) metabolism quantifiable by [(18)F]fluoro-2-deoxyglucose ([(18)F]FDG) positron emission tomography (PET)/computed tomography (CT) in mice. METHODS: A ß3-adrenoreceptor selective agonist (CL 316243), an adenylyl cyclase enzyme activator (forskolin) and a potent blocker of presynaptic norepinephrine transporter (atomoxetine), were injected through the tail vein of Swiss Webster mice 30 minutes before intravenous (iv) administration of [(18)F]FDG. The mice were placed on the PET/CT bed for 30 min PET acquisition followed by 10 min CT acquisition for attenuation correction and anatomical delineation of PET images. RESULTS: Activated interscapular (IBAT), cervical, periaortic and intercostal BAT were observed in 3-dimentional analysis of [(18)F]FDG PET images. CL 316243 increased the total [(18)F]FDG standard uptake value (SUV) of IBAT 5-fold greater compared to that in placebo-treated mice. It also increased the [(18)F]FDG SUV of white adipose tissue (2.4-fold), and muscle (2.7-fold), as compared to the control. There was no significant difference in heart, brain, spleen and liver uptakes between groups. Forskolin increased [(18)F]FDG SUV of IBAT 1.9-fold greater than that in placebo-treated mice. It also increased the [(18)F]FDG SUV of white adipose tissue (2.2-fold) and heart (5.4-fold) compared to control. There was no significant difference in muscle, brain, spleen, and liver uptakes between groups. Atomoxetine increased [(18)F]FDG SUV of IBAT 1.7-fold greater than that in placebo-treated mice. There were no significant differences in all other organs compared to placebo-treated mice except liver (1.6 fold increase). A positive correlation between SUV levels of IBAT and CT Hounsfield unit (HU) (R(2)=0.55, p<0.001) and between CT HU levels of IBAT and liver (R(2)=0.69, p<0.006) was observed. CONCLUSIONS: The three pharmacologic approaches reported here enhanced BAT metabolism by targeting different sites in adrenergic system as measured by [(18)F]FDG PET/CT.

Evaluation of [18F]Mefway biodistribution and dosimetry based on whole-body PET imaging of mice.

PURPOSE: [(18)F]Mefway is a novel radiotracer specific to the serotonin 5-HT1A receptor class. In preparation for using this tracer in humans, we have performed whole-body PET studies in mice to evaluate the biodistribution and dosimetry of [(18)F]Mefway. METHODS: Six mice (three females and three males) received IV injections of [(18)F]Mefway and were scanned for 2 h in an Inveon-dedicated PET scanner. Each animal also received a high-resolution CT scan using an Inveon CT. The CT images were used to draw volume of interest on the following organs: the brain, large intestine, stomach, heart, kidneys, liver, lungs, pancreas, bone, spleen, testes, thymus, gallbladder, uterus, and urinary bladder. All organ time-activity curves without decay correction were normalized to the injected activity. The area under the normalized curves was then used to compute the residence times in each organ. Data were analyzed using PMOD and Matlab software. The absorbed doses in mouse organs were computed using the RAdiation Dose Assessment Resource animal models for dose assessment. The residence times in mouse organs were converted to human values using scale factors based on differences between organ and body weights. OLINDA/EXM 1.1 software was used to compute the absorbed human doses in multiple organs for both female and male phantoms. RESULTS: The highest mouse residence times were found in the liver, urinary bladder, and kidneys. The largest doses in mice were found in the urinary bladder (critical organ), kidney, and liver for both females and males, indicating primary elimination via urinary system. The projected human effective doses were 1.21E - 02 mSv/MBq for the adult female model and 1.13E - 02 mSv/MBq for the adult male model. The estimated human biodistribution of [(18)F]Mefway was similar to that of [(11)C]WAY 100,635, a 5-HT1A tracer for which dosimetry has been evaluated in humans. CONCLUSIONS: The elimination of radiotracer was primarily via the kidney and urinary bladder with the urinary bladder being the critical organ. Whole-body mouse imaging can be used as a preclinical tool to provide initial estimates of the absorbed doses of [(18)F]Mefway in humans.

Targeting presynaptic norepinephrine transporter in brown adipose tissue: a novel imaging approach and potential treatment for diabetes and obesity.

Brown adipose tissue (BAT) plays a significant role in metabolism. In this study, we report the use of atomoxetine (a clinically applicable norepinephrine reuptake inhibitor) for (18)F-FDG PET imaging of BAT and its effects on heat production and blood glucose concentration. Fasted-male Sprague-Dawley rats were administered with intravenous (18)F-FDG. The same rats were treated with atomoxetine (0.1 mg/kg, i.v.) 30 min before (18)F-FDG administration. To confirm the ß-adrenergic effects, propranolol (ß-adrenergic inhibitor) 5 mg/kg was given intraperitoneally 30 min prior to atomoxetine administration. The effect of atomoxetine on BAT metabolism was assessed in fasted and non-fasted rats and on BAT temperature and blood glucose in fasted rats. In (18)F-FDG PET/CT images, interscapular BAT (IBAT) and other areas of BAT were clearly visualized. When rats were fasted, atomoxetine (0.1 mg/kg) increased the (18)F-FDG uptake of IBAT by factor of 24 within 30 min. Propranolol reduced the average (18)F-FDG uptake of IBAT significantly. Autoradiography of IBAT and white adipose tissue confirmed the data obtained by PET. When rats were not fasted, atomoxetine-induced increase of (18)F-FDG uptake in IBAT was delayed and occurred in 120 min. For comparison, direct stimulation of ß(3)-adrenreceptors in non-fasted rats with CL-316, 243 occurred within 30 min. Atomoxetine-induced IBAT activation was associated with higher IBAT temperature and lower blood glucose. This was mediated by inhibition of norepinephrine reuptake transporters in IBAT leading to increased norepinephrine concentration in the synapse. Increased synaptic norepinephrine activates ß(3)-adrenreceptors resulting in BAT hypermetabolism that is visible and quantifiable by (18)F-FDG PET/CT.

Evaluation of [18F]Nifene biodistribution and dosimetry based on whole-body PET imaging of mice.

INTRODUCTION: [(18)F]Nifene is a novel radiotracer specific to the nicotinic acetylcholine α4ß2 receptor class. In preparation for using this tracer in humans we have performed whole-body PET studies in mice to evaluate the in vivo biodistribution and dosimetry of [(18)F]Nifene. METHODS: Seven BALB/c mice (3 males, 4 females) received IV tail injections of [(18)F]Nifene and were scanned for 2 h in an Inveon dedicated PET scanner. Each animal also received a high resolution CT scan using an Inveon CT. The CT images were used to draw volume of interest (VOI) on the following organs: brain, large intestine, small intestine, stomach, heart, kidneys, liver, lungs, pancreas, bone, spleen, testes, thymus, uterus and urinary bladder. All organ time activity curves had the decay correction reversed and were normalized to the injected activity. The area under the normalized curves was then used to compute the residence times in each organ. The absorbed doses in mouse organs were computed using the RAdiation Dose Assessment Resource (RADAR) animal models for dose assessment. The residence times in mouse organs were converted to human values using scale factors based on differences between organ and body weights. OLINDA 1.1 software was used to compute the absorbed human doses in multiple organs for both female and male phantoms. RESULTS: The highest mouse residence times were found in urinary bladder, liver, bone, small intestine and kidneys. The largest doses in mice were found in urinary bladder and kidneys for both females and males. The elimination of radiotracer was primarily via kidney and urinary bladder with the urinary bladder being the limiting organ. The projected human effective doses were 1.51E-02 mSv/MBq for the adult male phantom and 1.65E-02 mSv/MBq for the adult female model phantom. CONCLUSION: This study indicates that the whole-body mouse imaging can be used as a preclinical tool for initial estimation of the absorbed doses of [(18)F]Nifene in humans.

Synthesis and evaluation of 3-¹²³I-iodo-5-[2-(S)-3-pyrrolinylmethoxy]-pyridine (niodene) as a potential nicotinic α4ß2 receptor imaging agent.

Nicotinic acetylcholine receptors (nAChRs) are downregulated in disease conditions such as Alzheimer's and substance abuse. Presently, (123)I-5-IA-85380 is used in human studies and requires over 6h of scanning time, thus increases patient discomfort. We have designed and synthesized 3-iodo-5-[2-(S)-3-pyrrolinylmethoxy]pyridine (niodene) with the aim to have faster binding kinetics compared to (123)I-5-IA-85380, which may reduce scanning time and help in imaging studies. Binding affinity K(i) of niodene for rat brain α4ß2 receptors in brain homogenate assays using (3)H-cytisine was 0.27 nM. Niodene, 10nM displaced >95% of (18)F-nifene bound to α4ß2 receptors in rat brain slices. By using the iododestannylation method, (123)I-niodene was obtained in high radiochemical purity (>95%) but with low radiochemical yield (<5%) and low specific activity (∼100 Ci/mmol). Autoradiograms show (123)I-niodene localized in the thalamus and cortex, which was displaced by nicotine (thalamus to cerebellum ratio=4; cortex to cerebellum ratio=1.6). Methods of radioiodination need to be further evaluated in order to obtain (123)I-niodene in higher radiochemical yields and higher specific activity of this potentially useful new SPECT imaging agent.

Evaluation of F-nifene binding to α4ß2 nicotinic receptors in the rat brain using microPET imaging.

MicroPET imaging studies using (18)F-nifene, a new positron emission tomography (PET) radiotracer for nicotinic acetylcholinergic receptors (nAChR) α4ß2 receptors in rats, have been carried out. Rats were imaged for 90 min after intravenous injection of (18)F-nifene (0.8 to 1 mCi), and binding potential (BP(ND)) was measured. (18)F-Nifene binding to thalamic and extrathalamic brain regions was consistent with the α4ß2 nAChR distribution in the rat brain. Using the cerebellum as a reference, the values for the thalamus varied less than 5% (BP(ND) = 1.30, n = 3), confirming reproducibility of (18)F-nifene binding. (18)F-Nifene microPET imaging was also used to evaluate effects of nicotine in a group of Sprague-Dawley rats under isoflurane anesthesia. Nicotine challenge postadministration of (18)F-nifene demonstrated reversibility of (18)F-nifene binding in vivo. For α4ß2 nAChR receptor occupancy (nAChR(OCC)), various doses of nicotine (0, 0.02, 0.1, 0.25, and 0.50 mg/kg nicotine free base) 15 min prior to (18)F-nifene were administered. Low-dose nicotine (0.02 mg) reached > 80% nAChR(OCC) while at higher doses (0.25 mg) > 90% nAChR(OCC) was measured. The small amount of (18)F-nifene binding with reference to the cerebellum affects an accurate evaluation of nAChR(OCC). Efforts are underway to identify alternate reference regions for (18)F-nifene microPET studies in rodents.

18F-Fallypride PET of pancreatic islets: in vitro and in vivo rodent studies.

UNLABELLED: Islet cell loss in the pancreas results in diabetes. A noninvasive method that measures islet cell loss and also tracks the fate of transplanted islets would facilitate the development of novel therapeutics and improve the management of diabetes. We describe a novel dopamine D(2)/D(3) receptor (D(2)/D(3)R)-based PET method to study islet cells in the rat pancreas and in islet cell transplantation. METHODS: (18)F-fallypride binding to isolated rat islets and pancreas was evaluated in the absence and presence of the D(2)/D(3)R inhibitor haloperidol. After intravenous (18)F-fallypride (28-37 MBq) administration, normal rats and rats pretreated with haloperidol were imaged in a PET/CT scanner and subsequently studied ex vivo for (18)F-fallypride localization in the pancreas. A streptozotocin-treated diabetic rat model was used to study localization of (18)F-fallypride in the pancreas, in vitro and ex vivo. Rat islet cells were transplanted into the spleen and visualized using (18)F-fallypride PET. RESULTS: (18)F-fallypride bound to isolated islet cells and pancreatic sections with an endocrine or exocrine selectivity of approximately 4; selectivity was reduced by haloperidol, suggesting that binding was D(2)/D(3)R-specific. Chemical destruction of islets by streptozotocin decreased (18)F-fallypride binding in pancreas by greater than 50%, paralleling the decrease in insulin immunostaining. Uptake of (18)F-fallypride in the pancreas was confirmed by radiochromatography and was 0.05% injected dose/cm(3) as measured by PET/CT. The ratio of (18)F-fallypride uptake in the pancreas to reference tissue (erector spinae muscle) was 5.5. Rat islets transplanted into the spleen were visualized in vivo by (18)F-fallypride and confirmed by immunostaining. The ratio of spleen-transplanted islets to erector spinae muscle was greater than 5, compared with a ratio of 2.8 in untransplanted rats. CONCLUSION: These studies demonstrate the potential utility of (18)F-fallypride as a PET agent for islet cells.

Striatal and extrastriatal microPET imaging of D2/D3 dopamine receptors in rat brain with [¹8F]fallypride and [¹8F]desmethoxyfallypride.

In this study, we compared two different D(2/3) receptor ligands, [¹8F]fallypride and [¹8F]desmethoxyfallypride ([¹8F]DMFP) with respect to the duration of the scan, visualization of extrastriatal receptors, and binding potentials (BP(ND) ) in the rat brain. In addition, we studied the feasibility of using these tracers following a period of awake tracer uptake, during which the animal may perform a behavioral task. Male Sprague-Dawley rats were imaged with [¹8F]fallypride and with [¹8F]DMFP in four different studies using microPET. All scans were performed under isoflurane anesthesia. The first (test) and second (retest) study were 150-min baseline scans. No retest scans were performed with [¹8F]DMFP. A third study was a 60-min awake uptake of radiotracer followed by a 90-min scan. A fourth study was a 150-min competition scan with haloperidol (0.2 mg/kg) administered via tail vein at 90-min post-[¹8F]fallypride injection and 60-min post-[¹8F]DMFP. For the test-retest studies, BP(ND) was measured using both Logan noninvasive (LNI) method and the interval ratios (ITR) method. Cerebellum was used as a reference region. For the third study, the binding was measured only with the ITR method, and the results were compared to the baseline results. Studies showed that the average transient equilibrium time in the dorsal striatum (DSTR) was at 90 min for [¹8F]fallypride and 30 min for [¹8F]DMFP. The average BP(ND) for [¹8F]fallypride was 14.4 in DSTR, 6.8 in ventral striatum (VSTR), 1.3 in substantia nigra/ventral tegmental area (SN/VTA), 1.4 in colliculi (COL), and 1.5 in central gray area. In the case of [¹8F]DMFP, the average BP(ND) values were 2.2 in DSTR, 2.7 in VSTR, and 0.8 in SN/VTA. The haloperidol blockade showed detectable decrease in binding of both tracers in striatal regions with a faster displacement of [¹8F]DMFP. No significant changes in BP(ND) of [¹8F]fallypride due to the initial awake state of the animal were found, whereas BP(ND) of [¹8F]DMFP was significantly higher in the awake state compared to baseline. We were able to demonstrate that dynamic PET using MicroPET Inveon allows quantification of both striatal and extrastriatal [¹8F]fallypride binding in rats in vivo. Quantification of the striatal regions could be achieved with [¹8F]DMFP.

Quantitative assessment of brown adipose tissue metabolic activity and volume using 18F-FDG PET/CT and β3-adrenergic receptor activation.

BACKGROUND: Brown adipose tissue [BAT] metabolism in vivo is vital for the development of novel strategies in combating obesity and diabetes. Currently, BAT is activated at low temperatures and measured using 2-deoxy-2-18F-fluoro-D-glucose [18F-FDG] positron-emission tomography [PET]. We report the use of ß3-adrenergic receptor-mediated activation of BAT at ambient temperatures using (R, R)-5-[2-[2,3-(3-chlorphenyl)-2-hydroxyethyl-amino]propyl]-1,3-benzodioxole-2,2-dicarboxylate, disodium salt [CL316,243] (a selective ß3-adrenoceptor agonist) and measured by 18F-FDG PET/computed tomography [CT]. METHODS: Control and CL316,243-treated (2 mg/kg) male Sprague-Dawley rats were administered with 18F-FDG for PET/CT studies and were compared to animals at cold temperatures. Receptor-blocking experiments were carried out using propranolol (5 mg/kg). Dose effects of CL316,243 were studied by injecting 0.1 to 1 mg/kg 30 min prior to 18F-FDG administration. Imaging results were confirmed by autoradiography, and histology was done to confirm BAT activation. RESULTS: CL316,243-activated interscapular BAT [IBAT], cervical, periaortic, and intercostal BATs were clearly visualized by PET. 18F-FDG uptake of IBAT was increased 12-fold by CL316,243 vs. 1.1-fold by cold exposure when compared to controls. 18F-FDG uptake of the CL-activated IBAT was reduced by 96.0% using intraperitoneal administration of propranolol. Average 18F-FDG uptake of IBAT increased 3.6-, 3.5-, and 7.6-fold by doses of 0.1, 0.5, and 1 mg/kg CL, respectively. Ex vivo 18F-FDG autoradiography and histology of transverse sections of IBAT confirmed intense uptake in the CL-activated group and activated IBAT visualized by PET. CONCLUSION: Our study indicated that BAT metabolic activity could be evaluated by 18F-FDG PET using CL316,243 at ambient temperature in the rodent model. This provides a feasible and reliable method to study BAT metabolism.