Quantifying cardiac sympathetic denervation: first studies of 18F-fluorohydroxyphenethylguanidines in cardiomyopathy patients.

PURPOSE: 4-18F-Fluoro-m-hydroxyphenethylguanidine (18F-4F-MHPG) and 3-18F-fluoro-p-hydroxyphenethylguanidine (18F-3F-PHPG) were developed for quantifying regional cardiac sympathetic nerve density using tracer kinetic analysis. The aim of this study was to evaluate their performance in cardiomyopathy patients. METHODS: Eight cardiomyopathy patients were scanned with 18F-4F-MHPG and 18F-3F-PHPG. Also, regional resting perfusion was assessed with 13N-ammonia. 18F-4F-MHPG and 18F-3F-PHPG kinetics were analyzed using the Patlak graphical method to obtain Patlak slopes Kp (mL/min/g) as measures of regional nerve density. Patlak slope polar maps were used to evaluate the pattern and extent of cardiac denervation. For comparison, "retention index" (RI) values (mL blood/min/mL tissue) were also calculated and used to assess denervation. Perfusion polar maps were used to estimate the extent of hypoperfusion. RESULTS: Patlak analysis of 18F-4F-MHPG and 18F-3F-PHPG kinetics was successful in all subjects, demonstrating the robustness of this approach in cardiomyopathy patients. Substantial regional denervation was observed in all subjects, ranging from 25 to 74% of the left ventricle. Denervation zones were equal to or larger than the size of corresponding areas of hypoperfusion. The two tracers provided comparable metrics of regional nerve density and the extent of left ventricular denervation. 18F-4F-MHPG exhibited faster liver clearance than 18F-3F-PHPG, reducing spillover from the liver into the inferior wall. 18F-4F-MHPG was also metabolized more consistently in plasma, which may allow application of population-averaged metabolite corrections. CONCLUSION: The advantages of 18F-4F-MHPG (more rapid liver clearance, more consistent metabolism in plasma) make it the better imaging agent to carry forward into future clinical studies in patients with cardiomyopathy. TRIAL REGISTRATION: Registered at the ClinicalTrials.gov website (NCT02669563). URL: https://clinicaltrials.gov/ct2/show/NCT02669563.

Network mechanisms and dysfunction within an integrated computational model of progression through mitosis in the human cell cycle.

The cellular protein-protein interaction network that governs cellular proliferation (cell cycle) is highly complex. Here, we have developed a novel computational model of human mitotic cell cycle, integrating diverse cellular mechanisms, for the purpose of generating new hypotheses and predicting new experiments designed to help understand complex diseases. The pathogenic state investigated is infection by a human herpesvirus. The model starts at mitotic entry initiated by the activities of Cyclin-dependent kinase 1 (CDK1) and Polo-like kinase 1 (PLK1), transitions through Anaphase-promoting complex (APC/C) bound to Cell division cycle protein 20 (CDC20), and ends upon mitotic exit mediated by APC/C bound to CDC20 homolog 1 (CDH1). It includes syntheses and multiple mechanisms of degradations of the mitotic proteins. Prior to this work, no such comprehensive model of the human mitotic cell cycle existed. The new model is based on a hybrid framework combining Michaelis-Menten and mass action kinetics for the mitotic interacting reactions. It simulates temporal changes in 12 different mitotic proteins and associated protein complexes in multiple states using 15 interacting reactions and 26 ordinary differential equations. We have defined model parameter values using both quantitative and qualitative data and using parameter values from relevant published models, and we have tested the model to reproduce the cardinal features of human mitosis determined experimentally by numerous laboratories. Like cancer, viruses create dysfunction to support infection. By simulating infection of the human herpesvirus, cytomegalovirus, we hypothesize that virus-mediated disruption of APC/C is necessary to establish a unique mitotic collapse with sustained CDK1 activity, consistent with known mechanisms of virus egress. With the rapid discovery of cellular protein-protein interaction networks and regulatory mechanisms, we anticipate that this model will be highly valuable in helping us to understand the network dynamics and identify potential points of therapeutic interventions.

Improved synthesis of 4-[18 F]fluoro-m-hydroxyphenethylguanidine using an iodonium ylide precursor.

Fluorine-18 labeled hydroxyphenethylguanidines were recently developed in our laboratory as a new class of PET radiopharmaceuticals for quantifying regional cardiac sympathetic nerve density in heart disease patients. Studies of 4-[18 F]fluoro-m-hydroxyphenethylguanidine ([18 F]4F-MHPG) and 3-[18 F]fluoro-p-hydroxyphenethylguanidine ([18 F]3F-PHPG) in human subjects have shown that these radiotracers can be used to generate high-resolution maps of regional sympathetic nerve density using the Patlak graphical method. Previously, these compounds were synthesized using iodonium salt precursors, which provided sufficient radiochemical yields for on-site clinical PET studies. However, we were interested in exploring new methods that could offer significantly higher radiochemical yields. Spirocyclic iodonium ylide precursors have recently been established as an attractive new approach to radiofluorination of electron-rich aromatic compounds, offering several advantages over iodonium salt precursors. The goal of this study was to prepare a spirocyclic iodonium ylide precursor for synthesizing [18 F]4F-MHPG and evaluate its efficacy in production of this radiopharmaceutical. Under optimized automated reaction conditions, the iodonium ylide precursor provided radiochemical yields averaging 7.8% ± 1.4% (n = 8, EOS, not decay corrected), around threefold higher than those achieved previously using an iodonium salt precursor. With further optimization and scale-up, this approach could potentially support commercial distribution of [18 F]4F-MHPG to PET centers without on-site radiochemistry facilities.

Constriction model of actomyosin ring for cytokinesis by fission yeast using a two-state sliding filament mechanism.

We developed a model describing the structure and contractile mechanism of the actomyosin ring in fission yeast, Schizosaccharomyces pombe. The proposed ring includes actin, myosin, and α-actinin, and is organized into a structure similar to that of muscle sarcomeres. This structure justifies the use of the sliding-filament mechanism developed by Huxley and Hill, but it is probably less organized relative to that of muscle sarcomeres. Ring contraction tension was generated via the same fundamental mechanism used to generate muscle tension, but some physicochemical parameters were adjusted to be consistent with the proposed ring structure. Simulations allowed an estimate of ring constriction tension that reproduced the observed ring constriction velocity using a physiologically possible, self-consistent set of parameters. Proposed molecular-level properties responsible for the thousand-fold slower constriction velocity of the ring relative to that of muscle sarcomeres include fewer myosin molecules involved, a less organized contractile configuration, a low α-actinin concentration, and a high resistance membrane tension. Ring constriction velocity is demonstrated as an exponential function of time despite a near linear appearance. We proposed a hypothesis to explain why excess myosin heads inhibit constriction velocity rather than enhance it. The model revealed how myosin concentration and elastic resistance tension are balanced during cytokinesis in S. pombe.

Synthesis and bioevaluation of [(18)F]4-fluoro-m-hydroxyphenethylguanidine ([(18)F]4F-MHPG): a novel radiotracer for quantitative PET studies of cardiac sympathetic innervation.

A new cardiac sympathetic nerve imaging agent, [(18)F]4-fluoro-m-hydroxyphenethylguanidine ([(18)F]4F-MHPG), was synthesized and evaluated. The radiosynthetic intermediate [(18)F]4-fluoro-m-tyramine ([(18)F]4F-MTA) was prepared and then sequentially reacted with cyanogen bromide and NH4Br/NH4OH to afford [(18)F]4F-MHPG. Initial bioevaluations of [(18)F]4F-MHPG (biodistribution studies in rats and kinetic studies in the isolated rat heart) were similar to results previously reported for the carbon-11 labeled analog [(11)C]4F-MHPG. The neuronal uptake rate of [(18)F]4F-MHPG into the isolated rat heart was 0.68ml/min/g wet and its retention time in sympathetic neurons was very long (T1/2 >13h). A PET imaging study in a nonhuman primate with [(18)F]4F-MHPG provided high quality images of the heart, with heart-to-blood ratios at 80-90min after injection of 5-to-1. These initial kinetic and imaging studies of [(18)F]4F-MHPG suggest that this radiotracer may allow for more accurate quantification of regional cardiac sympathetic nerve density than is currently possible with existing neuronal imaging agents.

Crosstalk between Plk1, p53, cell cycle, and G2/M DNA damage checkpoint regulation in cancer: computational modeling and analysis.

Different cancer cell lines can have varying responses to the same perturbations or stressful conditions. Cancer cells that have DNA damage checkpoint-related mutations are often more sensitive to gene perturbations including altered Plk1 and p53 activities than cancer cells without these mutations. The perturbations often induce a cell cycle arrest in the former cancer, whereas they only delay the cell cycle progression in the latter cancer. To study crosstalk between Plk1, p53, and G2/M DNA damage checkpoint leading to differential cell cycle regulations, we developed a computational model by extending our recently developed model of mitotic cell cycle and including these key interactions. We have used the model to analyze the cancer cell cycle progression under various gene perturbations including Plk1-depletion conditions. We also analyzed mutations and perturbations in approximately 1800 different cell lines available in the Cancer Dependency Map and grouped lines by genes that are represented in our model. Our model successfully explained phenotypes of various cancer cell lines under different gene perturbations. Several sensitivity analysis approaches were used to identify the range of key parameter values that lead to the cell cycle arrest in cancer cells. Our resulting model can be used to predict the effect of potential treatments targeting key mitotic and DNA damage checkpoint regulators on cell cycle progression of different types of cancer cells.

A computational study of ion conductance in the KcsA K(+) channel using a Nernst-Planck model with explicit resident ions.

The biophysical mechanisms underlying the relationship between the structure and function of the KcsA K(+) channel are described. Because of the conciseness of electrodiffusion theory and the computational advantages of a continuum approach, the Nernst-Planck (NP) type models, such as the Goldman-Hodgkin-Katz and Poisson-NP (PNP) models, have been used to describe currents in ion channels. However, the standard PNP (SPNP) model is known to be inapplicable to narrow ion channels because it cannot handle discrete ion properties. To overcome this weakness, the explicit resident ions NP (ERINP) model was formulated, which applies a local explicit model where the continuum model fails. Then, the effects of the ERI Coulomb potential, the ERI induced potential, and the ERI dielectric constant for ion conductance were tested in the ERINP model. The current-voltage (I-V) and current-concentration (I-C) relationships determined in the ERINP model provided biologically significant information that the traditional continuum model could not, explicitly taking into account the effects of resident ions inside the KcsA K(+) channel. In addition, a mathematical analysis of the K(+) ion dynamics established a tight structure-function system with a shallow well, a deep well, and two K(+) ions resident in the selectivity filter. Furthermore, the ERINP model not only reproduced the experimental results with a realistic set of parameters, but it also reduced CPU costs.

5-tert-Butyl-2-(4'-[18F]fluoropropynylphenyl)-1,3-dithiane oxides: potential new GABA A receptor radioligands.

As potential new ligands targeting the binding site of gamma-aminobutyric acid (GABA) receptor ionophore, trans-5-tert-butyl-2-(4'-fluoropropynylphenyl)-2-methyl-1,1-dioxo-1,3-dithiane (1) and cis/trans-5-tert-butyl-2-(4'-fluoropropynylphenyl)-2-methyl-1,1,3,3-tetroxo-1,3-dithiane (2) were selected for radiolabeling and initial evaluation as in vivo imaging agents for positron emission tomography (PET). Both compounds exhibited identical high in vitro binding affinities (K(i)=6.5 nM). Appropriate tosylate-substituted ethynyl precursors were prepared by multistep syntheses involving stepwise sulfur oxidation and chromatographic isolation of desired trans isomers. Radiolabeling was accomplished in one step using nucleophilic [(18)F]fluorination. In vivo biodistribution studies with trans-[(18)F]1 and trans-[(18)F]2 showed significant initial uptake into mouse brain and gradual washout, with heterogeneous regional brain distributions and higher retention in the cerebral cortex and cerebellum and lower retention in the striatum and pons-medulla. These regional distributions of the new radioligands correlated with in vitro and ex vivo measures of standard radioligands binding to the ionophore- and benzodiazepine-binding sites of GABA(A) receptor in rodent brain. A comparison of these results with previously prepared radiotracers for other neurochemical targets, including successes and failures as in vivo radioligands, suggests that higher-affinity compounds with increased retention in target brain tissues will likely be needed before a successful radiopharmaceutical for human PET imaging can be identified.

First-in-Human Studies of [18F] Fluorohydroxyphenethylguanidines.

BACKGROUND: Disease-induced damage to cardiac autonomic nerve populations is associated with an increased risk of sudden cardiac death. The extent of cardiac sympathetic denervation, assessed using planar scintigraphy or positron emission tomography, has been shown to predict the risk of arrhythmic events in heart failure patients staged for implantable cardioverter defibrillator therapy. The goal of this study was to perform first-in-human evaluations of 4-[18F]fluoro-meta-hydroxyphenethylguanidine and 3-[18F]fluoro-para-hydroxyphenethylguanidine, 2 new positron emission tomography radiotracers developed for quantifying regional cardiac sympathetic nerve density. METHODS AND RESULTS: Cardiac positron emission tomography studies with 4-[18F]fluoro-meta-hydroxyphenethylguanidine and 3-[18F]fluoro-para-hydroxyphenethylguanidine were performed in normal subjects (n=4 each) to assess their imaging properties and organ kinetics. Patlak graphical analysis of their myocardial kinetics was evaluated as a technique for generating nerve density metrics. Whole-body biodistribution studies (n=4 each) were acquired and used to calculate human radiation dosimetry estimates. Patlak analysis proved to be an effective approach for quantifying regional nerve density. Using 960 left ventricular volumes of interest, across-subject Patlak slopes averaged 0.107±0.010 mL/min per gram for 4-[18F]fluoro-meta-hydroxyphenethylguanidine and 0.116±0.010 mL/min per gram for 3-[18F]fluoro-para-hydroxyphenethylguanidine. Tracer uptake was highest in heart, liver, kidneys, and salivary glands. Urinary excretion was the main elimination pathway. CONCLUSIONS: 4-[18F]fluoro-meta-hydroxyphenethylguanidine and 3-[18F]fluoro-para-hydroxyphenethylguanidine each produce high-quality positron emission tomography images of the distribution of sympathetic nerves in human heart. Patlak analysis provides reproducible measurements of regional cardiac sympathetic nerve density at high spatial resolution. Further studies of these tracers in heart failure patients will be performed to identify the best agent for clinical development. CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov . Unique identifier: NCT02385877.

Radiolabeled phenethylguanidines: novel imaging agents for cardiac sympathetic neurons and adrenergic tumors.

The norepinephrine transporter (NET) substrates [123I]-m-iodobenzylguanidine (MIBG) and [11C]-m-hydroxyephedrine (HED) are used as markers of cardiac sympathetic neurons and adrenergic tumors (pheochromocytoma, neuroblastoma). However, their rapid NET transport rates limit their ability to provide accurate measurements of cardiac nerve density. [11C]Phenethylguanidine ([11C]1a) and 12 analogues ([11C]1b-m) were synthesized and evaluated as radiotracers with improved kinetics for quantifying cardiac nerve density. In isolated rat hearts, neuronal uptake rates of [11C]1a-m ranged from 0.24 to 1.96 mL min-1 (g wet wt)-1, and six compounds had extremely long neuronal retention times (clearance T1/2 > 20 h) due to efficient vesicular storage. Positron emission tomography (PET) studies in nonhuman primates with [11C]1e, N-[11C]guanyl-m-octopamine, which has a slow NET transport rate, showed improved myocardial kinetics compared to HED. Compound [11C]1c, [11C]-p-hydroxyphenethylguanidine, which has a rapid NET transport rate, avidly accumulated into rat pheochromocytoma xenograft tumors in mice. These encouraging findings demonstrate that radiolabeled phenethylguanidines deserve further investigation as radiotracers of cardiac sympathetic innervation and adrenergic tumors.

[18F]Fluoro-Hydroxyphenethylguanidines: Efficient Synthesis and Comparison of Two Structural Isomers as Radiotracers of Cardiac Sympathetic Innervation.

Fluorine-18 labeled phenethylguanidines are currently under development in our laboratory as radiotracers for quantifying regional cardiac sympathetic nerve density using PET imaging techniques. In this study, we report an efficient synthesis of 18F-hydroxyphenethylguanidines consisting of nucleophilic aromatic [18F]fluorination of a protected diaryliodonium salt precursor followed by a single deprotection step to afford the desired radiolabeled compound. This approach has been shown to reliably produce 4-[18F]fluoro-m-hydroxyphenethylguanidine ([18F]4F-MHPG, [18F]1) and its structural isomer 3-[18F]fluoro-p-hydroxyphenethylguanidine ([18F]3F-PHPG, [18F]2) with good radiochemical yields. Preclinical evaluations of [18F]2 in nonhuman primates were performed to compare its imaging properties, metabolism, and myocardial kinetics with those obtained previously with [18F]1. The results of these studies have demonstrated that [18F]2 exhibits imaging properties comparable to those of [18F]1. Myocardial tracer kinetic analysis of each tracer provides quantitative metrics of cardiac sympathetic nerve density. Based on these findings, first-in-human PET studies with [18F]1 and [18F]2 are currently in progress to assess their ability to accurately measure regional cardiac sympathetic denervation in patients with heart disease, with the ultimate goal of selecting a lead compound for further clinical development.

Dependence of cardiac 11C-meta-hydroxyephedrine retention on norepinephrine transporter density.

UNLABELLED: The norepinephrine analog (11)C-meta-hydroxyephedrine (HED) is used with PET to map the regional distribution of cardiac sympathetic neurons. HED is rapidly transported into sympathetic neurons by the norepinephrine transporter (NET) and stored in vesicles. Although much is known about the neuronal mechanisms of HED uptake and retention, there is little information about the functional relationship between HED retention and cardiac sympathetic nerve density. The goal of this study was to characterize the dependence of HED retention on nerve density in rats with graded levels of cardiac denervation induced chemically with the neurotoxin 6-hydroxydopamine (6-OHDA). METHODS: Thirty male Sprague-Dawley rats were divided into 6 groups, and each group was administered a different dose of 6-OHDA: 0 (controls), 7, 11, 15, 22, and 100 mg/kg intraperitoneally. One day after 6-OHDA injection, HED (3.7-8.3 MBq) was injected intravenously into each animal and HED concentrations in heart and blood at 30 min after injection were determined. Heart tissues were frozen and later processed by tissue homogenization and differential centrifugation into a membrane preparation for in vitro measurement of cardiac NET density. A saturation binding assay using (3)H-mazindol as the radioligand was used to measure NET density (maximum number of binding sites [B(max)], fmol/mg protein) for each heart. RESULTS: In control animals, NET B(max) was 388 +/- 23 fmol/mg protein and HED heart uptake (HU) at 30 min was 2.89% +/- 0.35 %ID/g (%ID/g is percentage injected dose per gram tissue). The highest 6-OHDA dose of 100 mg/kg caused severe cardiac denervation, decreasing both NET B(max) and HED HU to 8% of their control values. Comparing values for all doses of 6-OHDA, HED retention had a strong linear correlation with NET density: HU = 0.0077B(max) -0.028, r(2) = 0.95. CONCLUSION: HED retention is linearly dependent on NET density in rat hearts that have been chemically denervated with 6-OHDA, suggesting that HED retention is a good surrogate measure of NET density in the rat heart. This finding is discussed in relation to clinical observations of the dependence of HED retention on cardiac nerve density in human subjects using PET.

Radiotracers for cardiac sympathetic innervation: transport kinetics and binding affinities for the human norepinephrine transporter.

INTRODUCTION: Most radiotracers for imaging of cardiac sympathetic innervation are substrates of the norepinephrine transporter (NET). The goal of this study was to characterize the NET transport kinetics and binding affinities of several sympathetic nerve radiotracers, including [(11)C]-(-)-meta-hydroxyephedrine, [(11)C]-(-)-epinephrine, and a series of [(11)C]-labeled phenethylguanidines under development in our laboratory. For comparison, the NET transport kinetics and binding affinities of some [(3)H]-labeled biogenic amines were also determined. METHODS: Transport kinetics studies were performed using rat C6 glioma cells stably transfected with the human norepinephrine transporter (C6-hNET cells). For each radiolabeled NET substrate, saturation transport assays with C6-hNET cells measured the Michaelis-Menten transport constants Km and Vmax for NET transport. Competitive inhibition binding assays with homogenized C6-hNET cells and [(3)H]mazindol provided estimates of binding affinities (KI) for NET. RESULTS: Km, Vmax and KI values were determined for each NET substrate with a high degree of reproducibility. Interestingly, C6-hNET transport rates for 'tracer concentrations' of substrate, given by the ratio Vmax/Km, were found to be highly correlated with neuronal transport rates measured previously in isolated rat hearts (r(2)=0.96). This suggests that the transport constants Km and Vmax measured using the C6-hNET cells accurately reflect in vivo transport kinetics. CONCLUSION: The results of these studies show how structural changes in NET substrates influence NET binding and transport constants, providing valuable insights that can be used in the design of new tracers with more optimal kinetics for quantifying regional sympathetic nerve density.

4-[18F]Fluoro-m-hydroxyphenethylguanidine: a radiopharmaceutical for quantifying regional cardiac sympathetic nerve density with positron emission tomography.

4-[(18)F]Fluoro-m-hydroxyphenethylguanidine ([(18)F]4F-MHPG, [(18)F]1) is a new cardiac sympathetic nerve radiotracer with kinetic properties favorable for quantifying regional nerve density with PET and tracer kinetic analysis. An automated synthesis of [(18)F]1 was developed in which the intermediate 4-[(18)F]fluoro-m-tyramine ([(18)F]16) was prepared using a diaryliodonium salt precursor for nucleophilic aromatic [(18)F]fluorination. In PET imaging studies in rhesus macaque monkeys, [(18)F]1 demonstrated high quality cardiac images with low uptake in lungs and the liver. Compartmental modeling of [(18)F]1 kinetics provided net uptake rate constants Ki (mL/min/g wet), and Patlak graphical analysis of [(18)F]1 kinetics provided Patlak slopes Kp (mL/min/g). In pharmacological blocking studies with the norepinephrine transporter inhibitor desipramine (DMI), each of these quantitative measures declined in a dose-dependent manner with increasing DMI doses. These initial results strongly suggest that [(18)F]1 can provide quantitative measures of regional cardiac sympathetic nerve density in human hearts using PET.

Quantification of cardiac sympathetic nerve density with N-11C-guanyl-meta-octopamine and tracer kinetic analysis.

UNLABELLED: Most cardiac sympathetic nerve radiotracers are substrates of the norepinephrine transporter (NET). Existing tracers such as (123)I-metaiodobenzylguanidine ((123)I-MIBG) and (11)C-(-)-meta-hydroxyephedrine ((11)C-HED) are flow-limited tracers because of their rapid NET transport rates. This prevents successful application of kinetic analysis techniques and causes semiquantitative measures of tracer retention to be insensitive to mild-to-moderate nerve losses. N-(11)C-guanyl-(-)-meta-octopamine ((11)C-GMO) has a much slower NET transport rate and is trapped in storage vesicles. The goal of this study was to determine whether analyses of (11)C-GMO kinetics could provide robust and sensitive measures of regional cardiac sympathetic nerve densities. METHODS: PET studies were performed in a rhesus macaque monkey under control conditions or after intravenous infusion of the NET inhibitor desipramine (DMI). Five desipramine dose levels were used to establish a range of available cardiac NET levels. Compartmental modeling of (11)C-GMO kinetics yielded estimates of the rate constants K1 (mL/min/g), k2 (min(-1)), and k3 (min(-1)). These values were used to calculate a net uptake rate constant K(i) (mL/min/g) = (K1k3)/(k2 + k3). In addition, Patlak graphical analyses of (11)C-GMO kinetics yielded Patlak slopes K(p) (mL/min/g), which represent alternative measurements of the net uptake rate constant K(i). (11)C-GMO kinetics in isolated rat hearts were also measured for comparison with other tracers. RESULTS: In isolated rat hearts, the neuronal uptake rate of (11)C-GMO was 8 times slower than (11)C-HED and 12 times slower than (11)C-MIBG. (11)C-GMO also had a long neuronal retention time (>200 h). Compartmental modeling of (11)C-GMO kinetics in the monkey heart proved stable under all conditions. Calculated net uptake rate constants K(i) tracked desipramine-induced reductions of available NET in a dose-dependent manner, with a half maximal inhibitory concentration (IC50) of 0.087 ± 0.012 mg of desipramine per kilogram. Patlak analysis provided highly linear Patlak plots, and the Patlak slopes Kp also declined in a dose-dependent manner (IC50 = 0.068 ± 0.010 mg of desipramine per kilogram). CONCLUSION: Compartmental modeling and Patlak analysis of (11)C-GMO kinetics each provided quantitative parameters that accurately tracked changes in cardiac NET levels. These results strongly suggest that PET studies with (11)C-GMO can provide robust and sensitive quantitative measures of regional cardiac sympathetic nerve densities in human hearts.

Synthesis and structure-activity investigation of iodinated arylhydantoins and arylthiohydantoins for development as androgen receptor radioligands.

A series of side-chain derivatives of the arylhydantoin RU 58841 and the arylthiohydantoin RU 59063, wherein the aromatic trifluoromethyl group was replaced with iodine, was synthesized for possible development as radioiodinated androgen receptor (AR) ligands. Derivatives containing the cyanomethyl, methoxyethyl and propenyl side-chains displayed moderately high affinity (K(i)=20-59nM) towards the rat AR. Side-chains containing bulky lipophilic groups such as, benzyl and phenylpropyl, were poorly tolerated (K(i)>219nM). Superior AR binding affinities (0.71nM