18F-Mefway (N-{2-[4-(2′-methoxyphenyl)piperazinyl]ethyl}-N-(2-pyridyl)-N-(4′-18F-fluoro-methylcyclohexane)carboxamide) was developed and evaluated for use as a PET ligand for imaging 5-HT1A receptors. Ongoing studies of 18F-Mefway have shown it to be an effective PET radiotracer. We have synthesized isomers of Mefway by changing the position of the methyl group in attempts to evaluate stability for imaging purposes. 2-Methyl-, 3-methyl-, and 4-methyl-cyclohexane-1-carboxylic acids and 3-carbomethoxy-, 4-carbomethoxycyclohexane-1-carboxylic acids were coupled with WAY-100634 to provide the methylcyclohexyl derivatives (2-, 3-, and 4-methyl). Mefway and 3-Mefway analogs were prepared by reduction of carbomethoxy- derivatives followed by fluorination. In vitro binding affinities for the methylated derivatives in rat brain homogenates were found to be 10.4 nM (2-methyl), 77 nM (3-methyl), and 21.5 nM (4-methyl). Binding affinity of 3-Mefway and 4-Mefway was found to be 17.4 nM and 6.26 nM, respectively. Our results suggest that 3-methyl/3-fluoromethyl substituent has approx. threefold lower affinities compared to the 4-methyl/4-fluoromethyl substituent.

Nicotinic acetylcholine receptors (nAChRs) are downregulated in disease conditions such as Alzheimer’s and substance abuse. Presently, 123I-5-IA-85380 is used in human studies and requires over 6 h 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 123I-5-IA-85380, which may reduce scanning time and help in imaging studies. Binding affinity Ki of niodene for rat brain α4β2 receptors in brain homogenate assays using 3H-cytisine was 0.27 nM. Niodene, 10 nM displaced >95% of 18F-nifene bound to α4β2 receptors in rat brain slices. By using the iododestannylation method, 123I-niodene was obtained in high radiochemical purity (>95%) but with low radiochemical yield (<5%) and low specific activity (∼100 Ci/mmol). Autoradiograms show 123I-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 123I-niodene in higher radiochemical yields and higher specific activity of this potentially useful new SPECT imaging agent.

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 β3-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 β3-adrenoceptor agonist) and measured by 18F-FDG PET/computed tomography [CT].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.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 vivo18F-FDG autoradiography and histology of transverse sections of IBAT confirmed intense uptake in the CL-activated group and activated IBAT visualized by PET.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.

ObjectivesThe 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 18F-fallypride and 11C-fallypride.MethodsMale 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 18F-fallypride scan or a 11C-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.ResultsIn vitro studies showed 18F-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 18F-fallypride in SDH compared to that observed in the striatum. In vivo analysis of both 18F-fallypride and 11C-fallypride in the spinal cord were comparable to that in the extrastriatal regions. Haloperidol and clozapine displaced more than 75% of the 18F-fallypride in spinal cord sections.ConclusionsOur studies showed 18F-fallypride and 11C-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.

ObjectivesA new radiotracer, 124I-epidepride, has been developed for the imaging of dopamine D2/3 receptors (D2/3Rs). 124I-Epidepride (half-life of 124I = 4.2 days) allows imaging over extended periods compared to 18 F-fallypride (half-life of 18 F = 0.076 days) and may maximize visualization of D2/3Rs in the brain and pancreas (allowing clearance from adjacent organs). D2/3Rs are also present in pancreatic islets where they co-localize with insulin to produce granules and may serve as a surrogate marker for imaging diabetes.Methods124I-Epidepride was synthesized using N-[[(2S)-1-ethylpyrrolidin-2-yl]methyl]-5-tributyltin-2,3-dimethoxybenzamide and 124I-iodide under no carrier added condition. Rats were used for in vitro and in vivo imaging. Brain slices were incubated with 124I-epidepride (0.75 μCi/cc) and nonspecific binding measured with 10 μM haloperidol. Autoradiograms were analyzed by OptiQuant. 124I-Epidepride (0.2 to 0.3 mCi, iv) was administered to rats and brain uptake at 3 hours, 24 hours, and 48 hours post injection was evaluated.Results124I-Epidepride was obtained with 50% radiochemical yield and high radiochemical purity (> 95%). 124I-Epidepride localized in the striatum with a striatum to cerebellum ratio of 10. Binding was displaced by dopamine and haloperidol. Brain slices demonstrated localization of 124I-epidepride up until 48 hours in the striatum. However, the extent of binding was reduced significantly.Conclusions124I-Epidepride is a new radiotracer suitable for extended imaging of dopamine D2/3 receptors and may have applications in imaging of receptors in the brain and monitoring pancreatic islet cell grafting.

ObjectivePharmacologic 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.MethodsA β3-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.ResultsActivated 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) (R2 = 0.55, p < 0.001) and between CT HU levels of IBAT and liver (R2 = 0.69, p < 0.006) was observed.ConclusionsThe three pharmacologic approaches reported here enhanced BAT metabolism by targeting different sites in adrenergic system as measured by [18 F]FDG PET/CT.

BackgroundWe have investigated β3-adrenoceptor agonist mediated brown adipose tissue (BAT) activation using 18F-FDG PET/CT in Zucker lean (ZL) and obese (ZF) rats.Methods18F-FDG was injected into ZL and ZF rats pretreated with saline or agonist CL316,243 for scans. 18F-FDG metabolic activity was computed as standard uptake values.ResultsCL316,243 in ZL activated BAT up to 4-fold compared to saline, while ZF BAT was only up by 2 fold. The decreased activation was consistent with lower β3-adrenoceptor levels in ZF rats.ConclusionsThe genetically modified ZL and ZF rats may provide a useful rat model to evaluate the significance of β3-adrenoceptor agonist-induced BAT activation in obesity.