Imaging Brain Injury in Former National Football League Players

Importance

Pilot studies that involved early imaging of the 18 kDa translocator protein (TSPO) using positron emission tomography (PET) indicated high levels of TSPO in the brains of active or former National Football League (NFL) players. If validated further in larger studies, those findings may have implications for athletes involved in collision sport.

Objective

To test for higher TSPO that marks brain injury and repair in a relatively large, unique cohort of former NFL players compared with former elite, noncollision sport athletes.

Design, Setting, and Participants

This cross-sectional study used carbon 11-labeled N,N-diethyl-2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide positron emission tomography ([11C]DPA-713 PET) data from former NFL players within 12 years of last participation in the NFL and elite noncollision sport athletes from across the US. Participants were enrolled between April 2018 and February 2023.

Main outcomes and measures

Regional [11C]DPA-713 total distribution volume from [11C]DPA-713 PET that is a measure of regional brain TSPO; regional brain volumes on magnetic resonance imaging; neuropsychological performance, including attention, executive function, and memory domains.

Results

This study included 27 former NFL players and 27 former elite, noncollision sport athletes. Regional TSPO levels were higher in former NFL players compared with former elite, noncollision sport athletes (unstandardized β coefficient, 1.08; SE, 0.22; 95% CI, 0.65 to 1.52; P < .001). The magnitude of the group difference depended on region, with largest group differences in TSPO in cingulate and frontal cortices as well as hippocampus. Compared with noncollision sport athletes, former NFL players performed worse in learning (mean difference [MD], -0.70; 95% CI, -1.14 to -0.25; P = .003) and memory (MD, -0.77; 95% CI, -1.24 to -0.30; P = .002), with no correlation between total gray matter TSPO and these cognitive domains.

Conclusions and relevance

In this cross-sectional study using [11C]DPA-713 PET, higher brain TSPO was found in former NFL players compared with noncollision sport athletes. This finding is consistent with neuroimmune activation even after cessation of NFL play. Future longitudinal [11C]DPA-713 PET and neuropsychological testing promises to inform whether neuroimmune-modulating therapy may be warranted.

First-in-human imaging using [11C]MDTC: a radiotracer targeting the cannabinoid receptor type 2

PURPOSE

We report findings from the first-in-human study of [11C]MDTC, a radiotracer developed to image the cannabinoid receptor type 2 (CB2R) with positron emission tomography (PET).

METHODS

Ten healthy adults were imaged according to a 90-min dynamic PET protocol after bolus intravenous injection of [11C]MDTC. Five participants also completed a second [11C]MDTC PET scan to assess test-retest reproducibility of receptor-binding outcomes. The kinetic behavior of [11C]MDTC in human brain was evaluated using tissue compartmental modeling. Four additional healthy adults completed whole-body [11C]MDTC PET/CT to calculate organ doses and the whole-body effective dose.

RESULTS

[11C]MDTC brain PET and [11C]MDTC whole-body PET/CT was well-tolerated. A murine study found evidence of brain-penetrant radiometabolites. The model of choice for fitting the time activity curves (TACs) across brain regions of interest was a three-tissue compartment model that includes a separate input function and compartment for the brain-penetrant metabolites. Regional distribution volume (VT) values were low, indicating low CB2R expression in the brain. Test-retest reliability of VT demonstrated a mean absolute variability of 9.91%. The measured effective dose of [11C]MDTC was 5.29 μSv/MBq.

CONCLUSION

These data demonstrate the safety and pharmacokinetic behavior of [11C]MDTC with PET in healthy human brain. Future studies identifying radiometabolites of [11C]MDTC are recommended before applying [11C]MDTC PET to assess the high expression of the CB2R by activated microglia in human brain.

Molecular imaging of the serotonin transporter availability and occupancy by antidepressant treatment in late-life depression

Patients with late-life depression (LLD) have a more variable response to pharmacotherapy relative to patients with mid-life depression. Degeneration of the serotonergic system and lower occupancy of the initial target for antidepressant medications, the serotonin transporter (5-HTT), may contribute to variability in treatment response. The focus of this study was to test the hypotheses that lower cortical and limbic serotonin transporter (5-HTT) availability in LLD patients relative to controls and less 5-HTT occupancy by antidepressant medications would be associated with less improvement in mood and cognition with treatment in LLD patients. Twenty LLD patients meeting DSM-IV criteria for a current major depressive episode and 20 non-depressed controls underwent clinical and neuropsychological assessments, magnetic resonance imaging to measure gray matter volumes and high-resolution positron emission tomography (PET) scanning to measure 5-HTT before and after 10-12 weeks of treatment with Citalopram or Sertraline (patients only). Prior to treatment, 5-HTT was lower in LLD patients relative to controls in mainly temporal cortical and limbic (amygdala and hippocampus) regions. Gray matter volumes were not significantly different between groups. 5-HTT occupancy was detected throughout cortical, striatal, thalamic and limbic regions. The magnitude of regional 5-HTT occupancy by antidepressants was 70% or greater across cortical and sub-cortical regions, consistent with the magnitude of 5-HTT occupancy observed in mid-life depressed patients. Greater regional 5-HTT occupancy correlated with greater improvement in depressive symptoms and visual-spatial memory performance. These data support the hypothesis that serotonin degeneration and variability in 5-HTT occupancy may contribute to heterogeneity in treatment response in LLD patients.

18F-labeled radiotracers for in vivo imaging of DREADD with positron emission tomography

Designer Receptors Exclusively Activated by Designer Drugs (DREADD) are a preclinical chemogenetic approach with clinical potential for various disorders. In vivo visualization of DREADDs has been achieved with positron emission tomography (PET) using ¹¹C radiotracers. The objective of this study was to develop DREADD radiotracers labeled with ¹⁸F for a longer isotope half-life. A series of non-radioactive fluorinated analogs of clozapine with a wide range of in vitro binding affinities for the hM3Dq and hM4Di DREADD receptors has been synthesized for PET. Compound [¹⁸F]7b was radiolabeled via a modified ¹⁸F-deoxyfluorination protocol with a commercial ruthenium reagent. [¹⁸F]7b demonstrated encouraging PET imaging properties in a DREADD hM3Dq transgenic mouse model, whereas the radiotracer uptake in the wild type mouse brain was low. [¹⁸F]7b is a promising long-lived alternative to the DREADD radiotracers [¹¹C]clozapine ([¹¹C]CLZ) and [¹¹C]deschloroclozapine ([¹¹C]DCZ).

Osteopontin/secreted phosphoprotein-1 behaves as a molecular brake regulating the neuroinflammatory response to chronic viral infection

BACKGROUND

Osteopontin (OPN) as a secreted signaling protein is dramatically induced in response to cellular injury and neurodegeneration. Microglial inflammatory responses in the brain are tightly associated with the neuropathologic hallmarks of neurodegenerative disease, but understanding of the molecular mechanisms remains in several contexts poorly understood.

METHODS

Micro-positron emission tomography (PET) neuroimaging using radioligands to detect increased expression of the translocator protein (TSPO) receptor in the brain is a non-invasive tool used to track neuroinflammation in living mammals.

RESULTS

In humanized, chronically HIV-infected female mice in which OPN expression was knocked down with functional aptamers, uptake of TSPO radioligand DPA-713 was markedly upregulated in the cortex, olfactory bulb, basal forebrain, hypothalamus, and central grey matter compared to controls. Microglia immunoreactive for Iba-1 were more abundant in some HIV-infected mice, but overall, the differences were not significant between groups. TSPO+ microglia were readily detected by immunolabeling of post-mortem brain tissue and unexpectedly, two types of neurons also selectively stained positive for TSPO. The reactive cells were the specialized neurons of the cerebellum, Purkinje cells, and a subset of tyrosine hydroxylase-positive neurons of the substantia nigra.

CONCLUSIONS

In female mice with wild-type levels of osteopontin, increased levels of TSPO ligand uptake in the brain was seen in animals with the highest levels of persistent HIV replication. In contrast, in mice with lower levels of osteopontin, the highest levels of TSPO uptake was seen, in mice with relatively low levels of persistent infection. These findings suggest that osteopontin may act as a molecular brake regulating in the brain, the inflammatory response to HIV infection.

Dynamic imaging in patients with tuberculosis reveals heterogeneous drug exposures in pulmonary lesions

Tuberculosis (TB) is the leading cause of death from a single infectious agent, requiring at least 6 months of multidrug treatment to achieve cure¹. However, the lack of reliable data on antimicrobial pharmacokinetics (PK) at infection sites hinders efforts to optimize antimicrobial dosing and shorten TB treatments². In this study, we applied a new tool to perform unbiased, noninvasive and multicompartment measurements of antimicrobial concentration-time profiles in humans³. Newly identified patients with rifampin-susceptible pulmonary TB were enrolled in a first-in-human study⁴ using dynamic [¹¹C]rifampin (administered as a microdose) positron emission tomography (PET) and computed tomography (CT). [¹¹C]rifampin PET-CT was safe and demonstrated spatially compartmentalized rifampin exposures in pathologically distinct TB lesions within the same patients, with low cavity wall rifampin exposures. Repeat PET-CT measurements demonstrated independent temporal evolution of rifampin exposure trajectories in different lesions within the same patients. Similar findings were recapitulated by PET-CT in experimentally infected rabbits with cavitary TB and confirmed using postmortem mass spectrometry. Integrated modeling of the PET-captured concentration-time profiles in hollow-fiber bacterial kill curve experiments provided estimates on the rifampin dosing required to achieve cure in 4 months. These data, capturing the spatial and temporal heterogeneity of intralesional drug PK, have major implications for antimicrobial drug development.

Noninvasive 11C-rifampin positron emission tomography reveals drug biodistribution in tuberculous meningitis

Tuberculous meningitis (TBM) is a devastating form of tuberculosis (TB), and key TB antimicrobials, including rifampin, have restricted brain penetration. A lack of reliable data on intralesional drug biodistribution in infected tissues has limited pharmacokinetic (PK) modeling efforts to optimize TBM treatments. Current methods to measure intralesional drug distribution rely on tissue resection, which is difficult in humans and generally limited to a single time point even in animals. In this study, we developed a multidrug treatment model in rabbits with experimentally induced TBM and performed serial noninvasive dynamic ¹¹C-rifampin positron emission tomography (PET) over 6 weeks. Area under the curve brain/plasma ratios were calculated using PET and correlated with postmortem mass spectrometry. We demonstrate that rifampin penetration into infected brain lesions is limited, spatially heterogeneous, and decreases rapidly as early as 2 weeks into treatment. Moreover, rifampin concentrations in the cerebrospinal fluid did not correlate well with those in the brain lesions. First-in-human ¹¹C-rifampin PET performed in a patient with TBM confirmed these findings. PK modeling predicted that rifampin doses (≥30 mg/kg) were required to achieve adequate intralesional concentrations in young children with TBM. These data demonstrate the proof of concept of PET as a clinically translatable tool to noninvasively measure intralesional antimicrobial distribution in infected tissues.

Effect of STN DBS on vesicular monoamine transporter 2 and glucose metabolism in Parkinson's disease

INTRODUCTION

Deep brain stimulation (DBS) is an established treatment for Parkinson's Disease (PD). Despite the improvement of motor symptoms in most patients by sub-thalamic nucleus (STN) DBS and its widespread use, the neurobiological mechanisms are not completely understood. The objective of the present study was to elucidate the effects of subthalamic nucleus (STN) DBS in PD on the dopamine system and neural circuitry, employing high-resolution positron emission tomography (PET) imaging. The hypotheses tested were that STN DBS would decrease the striatal vesicular monoamine transporter (VMAT2), secondary to an increase in dopamine concentrations, and would decrease striatal cerebral metabolism and increase cortical cerebral metabolism.

METHODS

PET imaging of the vesicular monoamine transporter (VMAT2) and cerebral glucose metabolism was performed prior to DBS surgery and after 4-6 months of STN stimulation in seven PD patients (mean age 67 ± 7).

RESULTS

The patients demonstrated significant improvement in motor and neuropsychiatric symptoms after STN DBS. Decreased VMAT2 was observed in the caudate, putamen and associative striatum and in extra-striatal, cortical and limbic regions. Cerebral glucose metabolism was decreased in striatal sub-regions and increased in temporal and parietal cortices and the cerebellum. Decreased striatal VMAT2 was correlated with decreased striatal and increased cortical and limbic metabolism. Improvement of depressive symptoms was correlated with decreased VMAT2 in striatal and extra-striatal regions and with striatal decreases and cortical increases in metabolism.

CONCLUSIONS

The present results support further investigation of the role of VMAT2, and associated changes in neural circuitry in the improvement of motor and non-motor symptoms with STN DBS in PD.

Imaging glial activation in patients with post-treatment Lyme disease symptoms: a pilot study using [11C]DPA-713 PET

The pathophysiology of post-treatment Lyme disease syndrome (PTLDS) may be linked to overactive immunity including aberrant activity of the brain's resident immune cells, microglia. Here we used [¹¹C]DPA-713 and positron emission tomography to quantify the 18 kDa translocator protein, a marker of activated microglia or reactive astrocytes, in the brains of patients with post-treatment Lyme disease symptoms of any duration compared to healthy controls. Genotyping for the TSPO rs6971 polymorphism was completed, and individuals with the rare, low affinity binding genotype were excluded. Data from eight brain regions demonstrated higher [¹¹C]DPA-713 binding in 12 patients relative to 19 controls. [¹¹C]DPA-713 PET is a promising tool to study cerebral glial activation in PTLDS and its link to cognitive symptoms.

Feasibility Evaluation of Myocardial Cannabinoid Type 1 Receptor Imaging in Obesity: A Translational Approach

OBJECTIVES

The aim of this study was to evaluate the feasibility of targeted imaging of myocardial cannabinoid type 1 receptor (CB1-R) and its potential up-regulation in obese mice with translation to humans using [11C]-OMAR and positron emission tomography (PET)/computed tomography (CT).

BACKGROUND

Activation of myocardial CB1-R by endocannabinoids has been implicated in cardiac dysfunction in diabetic mice. Obesity may lead to an up-regulation of myocardial CB1-R, potentially providing a mechanistic link between obesity and the initiation and/or progression of cardiomyopathy.

METHODS

Binding specificity of [11C]-OMAR to CB1-R was investigated by blocking studies with rimonabant in mice. The heart was harvested from each mouse, and its radioactivity was determined by γ-counter. Furthermore, [11C]-OMAR dynamic micro-PET/CT was carried out in obese and normal-weight mice. Ex vivo validation was performed by droplet digital polymerase chain reaction (absolute quantification) and RNAscope Technology (an in situ ribonucleic acid analysis platform). Subsequently, myocardial CB1-R expression was probed noninvasively with intravenous injection of CB1-R ligand [11C]-OMAR and PET/CT in humans with advanced obesity and normal-weight human control subjects, respectively.

RESULTS

Rimonabant significantly blocked OMAR uptake in the heart muscle compared with vehicle, signifying specific binding of OMAR to the CB1-R in the myocardium. The myocardial OMAR retention quantified by micro-PET/CT in mice was significantly higher in obese compared with normal-weight mice. Absolute quantification of CB1-R gene expression with droplet digital polymerase chain reaction and in situ hybridization confirmed CB1-R up-regulation in all major myocardial cell types (e.g., cardiomyocytes, endothelium, vascular smooth muscle cells, and fibroblasts) of obese mice. Obese mice also had elevated myocardial levels of endocannabinoids anandamide and 2-arachidonoylglycerol compared with lean mice. Translation to humans revealed higher myocardial OMAR retention in advanced obesity compared with normal-weight subjects.

CONCLUSIONS

Noninvasive imaging of cardiac CB1-R expression in obesity is feasible applying [11C]-OMAR and PET/CT. These results may provide a rationale for further clinical testing of CB1-R-targeted molecular imaging in cardiometabolic diseases.

Chemogenetics revealed: DREADD occupancy and activation via converted clozapine

The chemogenetic technology DREADD (designer receptors exclusively activated by designer drugs) is widely used for remote manipulation of neuronal activity in freely moving animals. DREADD technology posits the use of "designer receptors," which are exclusively activated by the "designer drug" clozapine N-oxide (CNO). Nevertheless, the in vivo mechanism of action of CNO at DREADDs has never been confirmed. CNO does not enter the brain after systemic drug injections and shows low affinity for DREADDs. Clozapine, to which CNO rapidly converts in vivo, shows high DREADD affinity and potency. Upon systemic CNO injections, converted clozapine readily enters the brain and occupies central nervous system-expressed DREADDs, whereas systemic subthreshold clozapine injections induce preferential DREADD-mediated behaviors.

Development of a radioligand for imaging V1a vasopressin receptors with PET

A series of vasopressin receptor V_1a ligands have been synthesized for positron emission tomography (PET) imaging. The lead compound (1S,5R)-1 ((4-(1H-indol-3-yl)-3-methoxyphenyl) ((1S,5R)-1,3,3-trimethyl-6-azabicyclo[3.2.1]octan-6-yl)methanone) and its F-ethyl analog 6c exhibited the best combination of high binding affinity and optimal lipophilicity within the series. (1S,5R)-1 was radiolabeled with ¹¹C for PET studies. [¹¹CH₃](1S,5R)-1 readily entered the mouse (4.7% ID/g tissue) and prairie vole brains (∼2% ID/g tissue) and specifically (30-34%) labeled V_1a receptor. The common animal anesthetic Propofol significantly blocked the brain uptake of [¹¹CH₃](1S,5R)-1 in the mouse brain, whereas anesthetics Ketamine and Saffan increased the uptake variability. Future PET imaging studies with V_1a radiotracers in non-human primates should be performed in awake animals or using anesthetics that do not affect the V_1a receptor.

Dose-dependent, saturable occupancy of the metabotropic glutamate subtype 5 receptor by fenobam as measured with [11 C]ABP688 PET imaging

Fenobam is a negative allosteric modulator of the metabotropic glutamate receptor subtype 5 (mGluR5) with inverse agonist activity and is expected to contribute to the treatment of neuropsychiatric disorders involving dysfunction of mGluR5 including Fragile X syndrome. This study examined whether [¹¹ C]ABP688, an antagonist PET radioligand, competes with fenobam for the same binding site in the nonhuman primate brain and would allow examination of occupancy-plasma concentration relationships in the evaluation of the drug for target disorders in the human brain. Four paired PET studies with [¹¹ C]ABP688 were performed in baboons at a baseline condition and after intravenous treatment with fenobam at different dose levels (0.3-1.33 mg/kg). Total distribution volume (V_T ) and binding potential (BP_ND ) using the cerebellum as a reference region were obtained by the plasma reference graphical method. Then it was examined whether occupancy follows a dose-dependent, saturating pattern that was predicted by a modified first-order Hill equation in individual regions. Baseline regional V_T and BP_ND values agreed with previously published data. Occupancy showed dose-dependent and saturating patterns in individual regions, reaching >90% occupancy at 1.33 mg/kg dose of fenobam in the majority of regions. To our knowledge, this is the first use of PET to characterize the mGluR5 therapeutic drug fenobam. This study demonstrates a proof of principle for determining the in vivo occupancy of fenobam in primates. The results indicate that [¹¹ C]ABP688 and PET may be useful for examination of occupancy of mGluR5 by fenobam, which should prove to be useful for designing future studies and treatment of human disease states. Synapse 68:565-573, 2014. © 2014 Wiley Periodicals, Inc.

Targeting of endothelin receptors in the healthy and infarcted rat heart using the PET tracer 18F-FBzBMS

The endothelin subtype-A receptor (ET-A) is a promising therapeutic target in cardiovascular disease. We sought to determine the feasibility of an (18)F-labeled ligand, (18)F-(N-[[29-[[(4,5-dimethyl-3-isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,19-biphenyl]-2-yl]methyl]-N,4-fluorobenzamide) ((18)F-FBzBMS), for imaging ET-A in the healthy and injured rat heart.

METHODS

Male Wistar rats were used for all experiments. The specificity of cardiac (18)F-FBzBMS uptake was determined in healthy animals (n = 23) using pretreatment with various blocking agents and doses. Myocardial infarction (MI) was induced by permanent left coronary ligation in 32 animals. Autoradiography was conducted to determine regional FBzBMS distribution relative to tissue perfusion at various times after MI. Histology and immunohistochemistry were performed for validation. The feasibility of in vivo detection of the tracer signal was tested using dedicated small-animal PET (n = 6).

RESULTS

At autoradiography, intravenous pretreatment with the selective ET-A blocker BMS-207940 reduced myocardial FBzBMS uptake by 93% ± 0.7%. Oral pretreatment with the clinical blocker bosentan resulted in a dose-dependent partial blockade (5 mg/kg, 48% ± 6%; 50 mg/kg, 61% ± 7%; and 100 mg/kg, 88% ± 0.7%). After MI, FBzBMS uptake was preserved in the infarct region from day 1 to month 6, whereas the perfusion tracer (201)Tl showed a persistent defect (MI-to-remote ratios: (201)Tl, 0.23 ± 0.28, 0.39 ± 0.07, 0.31 ± 0.07, 0.24 ± 0.12, 0.29 ± 0.10, and 0.23 ± 0.09; and FBzBMS, 0.94 ± 0.28, 0.92 ± 0.20, 0.88 ± 0.13, 0.82 ± 0.12, 0.80 ± 0.11, and 0.84 ± 0.08 at day 1, day 3, week 1, month 1, month 2, and month 6, respectively) (P < 0.01 vs. (201)Tl). Ex vivo analysis confirmed ET-A expression in the infarct area, where the signal was partially colocalized with CD31 expression on endothelial cells. In vivo small-animal PET successfully confirmed specific uptake and blockade of FBzBMS in healthy myocardium.

CONCLUSION

Cardiac uptake of the PET tracer (18)F-FBzBMS is specific for ET-A expression in rats, shows infarct-related alterations, and can be imaged noninvasively. Further efforts to establish myocardial ET-A imaging methodology are warranted, with the perspective of determining role, efficacy, and benefit of ET-A targeted drug treatment in cardiovascular disease.

Molecular hybrid positron emission tomography/computed tomography imaging of cardiac angiotensin II type 1 receptors

OBJECTIVES

The goal of this study was to explore the feasibility of targeted imaging of the angiotensin II type 1 receptor (AT1R) in cardiac tissue, using clinical hybrid positron emission tomography/computed tomography (PET/CT).

BACKGROUND

AT1R is an attractive imaging target due to its key role in various cardiac pathologies, including post-infarct left ventricular remodeling.

METHODS

Using the novel AT1R ligand [(11)C]-KR31173, dynamic PET/CT was performed in young farm pigs under healthy conditions (n = 4) and 3 to 4 weeks after experimental myocardial infarction (n = 5). Ex vivo validation was carried out by immunohistochemistry and polymerase chain reaction. First-in-man application was performed in 4 healthy volunteers at baseline and under AT1R blocking.

RESULTS

In healthy pigs, myocardial KR31173 retention was detectable, regionally homogeneous, and specific for AT1R, as confirmed by blocking experiments. Metabolism in plasma was low (85 ± 2% of intact tracer after 60 min). After myocardial infarction, KR31173 retention, corrected for regional perfusion, revealed AT1R up-regulation in the infarct area relative to remote myocardium, whereas retention was elevated in both regions when compared with myocardium of healthy controls (8.7 ± 0.8% and 7.1 ± 0.3%/min vs. 5.8 ± 0.4%/min for infarct and remote, respectively, vs. healthy controls; p < 0.01 each). Postmortem analysis confirmed AT1R up-regulation in remote and infarct tissue. First-in-man application was safe, and showed detectable and specific myocardial KR31173 retention, albeit at a lower level than pigs (left ventricular average retention: 1.2 ± 0.1%/min vs. 4.4 ± 1.2%/min for humans vs. pigs; p = 0.04).

CONCLUSIONS

Noninvasive imaging of cardiac AT1R expression is feasible using clinical PET/CT technology. Results provide a rationale for broader clinical testing of AT1R-targeted molecular imaging.

Molecular imaging of the kidneys

Radionuclide imaging of the kidneys with gamma cameras involves the use of labeled molecules seeking functionally critical molecular mechanisms to detect the pathophysiology of the diseased kidneys and achieve an early, sensitive, and accurate diagnosis. The most recent imaging technology, positron emission tomography, permits quantitative imaging of the kidney at a spatial resolution appropriate for the organ. H(2)(15)O, (82)RbCl, and [(64)Cu] ETS are the most important radiopharmaceuticals for measuring renal blood flow. The renin angiotensin system is the most important regulator of renal blood flow; this role is being interrogated by detecting angiotensin receptor subtype angiotensin subtype 1 receptor by the use of in vivo positron emission tomography. Membrane organic anion transporters are important for the function of the tubular epithelium; therefore, Tc99m MAG3 as well as some novel radiopharmaceuticals, such as copper-64 labeled mono oxo-tetraazamacrocyclic ligands, have been used for molecular renal imaging. In addition, other radioligands that interact with the organic cation transporters or peptide transporters have been developed. Focusing on early detection of kidney injury at the molecular level is an evolving field of great significance. Potential imaging targets are the kidney injury molecule 1, which is highly expressed in kidney injury and renal cancer but not in normal kidneys. Although pelvic clearance, in addition to parenchymal transport, is an important measure in obstructive nephropathy, techniques that focus on up-regulated molecules in response to tissue stress resulting from obstruction will be of great implication. Monocyte chemoattractant protein-1 is a well-suited molecule here. The greatest advances in molecular imaging of the kidneys have been recently achieved in detecting renal cancer. In addition to the ubiquitous [(18)F] fluorodeoxyglucose, other radioligands, such as [(11)C] acetate and anti-1-amino-3-[18F]fluorocyclobutane-1-carboxylic acid, have emerged. Radioimmunoimaging with [(124)I] G250 could lead to radioimmunotherapy for renal cancer. Considering the increasing age of general population, the incidence of kidney diseases, such as atherosclerosis, diabetic nephropathy, and cancer, is expected to increase. Successful management of these diseases offers an opportunity and a challenge for development of novel molecular imaging technologies.

Radioligands for the angiotensin II subtype 1 (AT1) receptor

Over the last years, ligands for angiotensin II subtype 1 receptor (AT1R) have been developed as an alternative to the use of angiotensin-converting enzyme (ACE) inhibitors for controlling high blood pressure. Radiolabeled versions of these ligands have proven vital to the development of more potent and specific drugs for the treatment of hypertension. Imaging studies using radiolabeled AT1R ligands have also elucidated the role these receptors play in angiogenesis as well as in various disease states beyond hypertension.

Positron emission tomographic studies of brain dopamine and serotonin transporters in abstinent (+/-)3,4-methylenedioxymethamphetamine ("ecstasy") users: relationship to cognitive performance

BACKGROUND

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is a recreational drug and brain serotonin (5-HT) neurotoxin. Under certain conditions, MDMA can also damage brain dopamine (DA) neurons, at least in rodents. Human MDMA users have been found to have reduced brain 5-HT transporter (SERT) density and cognitive deficits, although it is not known whether these are related. This study sought to determine whether MDMA users who take closely spaced sequential doses, which engender high plasma MDMA concentrations, develop DA transporter (DAT) deficits, in addition to SERT deficits, and whether there is a relationship between transporter binding and cognitive performance.

MATERIALS AND METHODS

Sixteen abstinent MDMA users with a history of using sequential MDMA doses (two or more doses over a 3- to 12-h period) and 16 age-, gender-, and education-matched controls participated. Subjects underwent positron emission tomography with the DAT and SERT radioligands, [11C]WIN 35,428 and [11C]DASB, respectively. Subjects also underwent formal neuropsychiatric testing.

RESULTS

MDMA users had reductions in SERT binding in multiple brain regions but no reductions in striatal DAT binding. Memory performance in the aggregate subject population was correlated with SERT binding in the dorsolateral prefrontal cortex, orbitofrontal cortex, and parietal cortex, brain regions implicated in memory function. Prior exposure to MDMA significantly diminished the strength of this relationship.

CONCLUSIONS

Use of sequential MDMA doses is associated with lasting decreases in brain SERT, but not DAT. Memory performance is associated with SERT binding in brain regions involved in memory function. Prior MDMA exposure appears to disrupt this relationship. These data are the first to directly relate memory performance to brain SERT density.

Radiopharmaceuticals for renal positron emission tomography imaging

Radiopharmaceuticals for functional renal imaging, including renal blood flow, renal blood volume, glomerular excretion, and metabolism have been available for some time. This review outlines radiopharmaceuticals for functional renal imaging as well as those that target pertinent molecular constituents of renal injury and repair. The angiotensin and endothelin receptors are particularly appealing molecular targets for renal imaging because of their association with renal physiology and pathology. Other targets such as the vascular endothelial growth factor (VEGF) receptor, integrin, or phosphatidylserine have been investigated at length for cancer imaging, but they are just as important constituents of the renal injury/repair process. Various diseases can involve identical mechanisms, such as angiogenesis and apoptosis, and radiopharmaceuticals developed for these processes in other organs can also be used for renal imaging. The sensitivity and spatial resolution of positron emission tomography makes it an ideal tool for molecular and functional kidney imaging. Radiopharmaceutical development for the kidneys must focus on achieving high target selectivity and binding affinity, stability and slow metabolism in vivo, and minimal nonspecific accumulation and urinary excretion.

Positron-Emission Tomography Imaging of the Angiotensin II Subtype 1 Receptor in Swine Renal Artery Stenosis

The angiotensin II subtype 1 receptor (AT 1 R) has been linked to the development and progression of renovascular hypertension. In this study we applied a pig model of renovascular hypertension to investigate the AT 1 R in vivo with positron-emission tomography (PET) and in vitro with quantitative autoradiography. AT 1 R PET measurements were performed with the radioligand [ 11 C]KR31173 in 11 control pigs and in 13 pigs with hemodynamically significant renal artery stenosis; 4 were treated with lisinopril for 2 weeks before PET imaging. The radioligand impulse response function was calculated by deconvolution analysis of the renal time-activity curves. Radioligand binding was quantified by the 80-minute retention of the impulse response function. Median values and interquartile ranges were used to illustrate group statistics. Radioligand retention was significantly increased ( P =0.044) in hypoperfused kidneys of untreated (0.225; range: 0.150 to 0.373) and lisinopril-treated (0.237; range:0.224 to 0.272) animals compared with controls (0.142; range:0.096 to 0.156). Increased binding of [ 11 C]KR31173 documented by PET in vivo was confirmed by in vitro autoradiography. Both in vivo and in vitro binding measurements showed that the effect of renal artery stenosis on the AT 1 R was not abolished by lisinopril treatment. These studies provide insight into kidney biology as the first in vivo/in vitro experimental evidence about AT 1 R regulation in response to reduced perfusion of the kidney. The findings support the concept of introducing AT 1 R PET as a diagnostic biomarker of renovascular disease.

Chronic ACE inhibitor treatment increases angiotensin type 1 receptor binding in vivo in the dog kidney

PURPOSE

PET imaging has been recently introduced for investigating the type 1 angiotensin II receptor (AT(1)R) in vivo. The goal of the present study was to investigate the effects of acute and chronic exposure to angiotensin converting enzyme inhibitors (ACEI) on the AT(1)R in the dog kidney.

METHODS

Animals were imaged at baseline, after acute intravenous ACEI treatment and after a chronic 2-week exposure to an oral ACEI. Control animals were imaged at identical time points in the absence of ACEI treatment.

RESULTS

In vivo AT(1)R binding expressed by K (i) was increased in the renal cortex by chronic ACEI treatment (p < 0.05). In vitro measurements of AT(1)R density (B (max)) also revealed significant increases in AT(1)R in isolated glomeruli (p < 0.05). Plasma renin activity was increased, but angiotensin II (Ang II) and the Ang II/Ang I ratio showed a weak correlation with chronic ACEI treatment, consistent with an Ang II escape phenomenon.

CONCLUSION

This study reveals, for the first time, that chronic ACEI treatment increases AT(1)R binding in vivo in the dog renal cortex.

PET Imaging of the AT1 receptor with [11C]KR31173

AIM

The goal of this study was to investigate the binding characteristics of [(11)C]KR31173 and its applicability for PET studies of the AT(1) receptor (AT(1)R).

METHODS

Ex vivo biodistribution and pharmacology were tested in mice. PET imaging was performed in mice, beagle dogs and a baboon. To assess nonspecific binding, PET imaging was performed both before and after pretreatment with a potent AT(1)R antagonist. In the baboon, PET imaging was also performed with the previously developed radioligand [(11)C]L-159,884 for comparison.

RESULTS

Ex vivo biodistribution studies in mice showed specific binding rates of 80-90% in the adrenals, kidneys, lungs and heart. Specific binding was confirmed in mice using small animal PET. In dogs, renal cortex tissue concentration at 75-95 min postinjection (pi) was 63 nCi/ml per millicurie at a specific binding rate of 95%. In the baboon renal cortex, tissue activity at 55-75 min pi was 345 nCi/ml per millicurie. In the baboon the specific binding of [(11)C]KR31173 was higher (81%) than the specific binding of [(11)C]L-159,884 (34%).

CONCLUSION

[(11)C]KR31173 shows accumulation and significant specific binding to the AT(1)R in the kidneys of mice, dogs and baboon. These findings suggest that this radioligand is suited for imaging the renal cortical AT(1)R in multiple species.

Future Direction of Renal Positron Emission Tomography

Positron emission tomography (PET) is perfectly suited for quantitative imaging of the kidneys, and the recent improvements in detector technology, computer hardware, and image processing software add to its appeal. Multiple positron emitting radioisotopes can be used for renal imaging. Some, including carbon-11, nitrogen-13, and oxygen-15, can be used at institutions with an on-site cyclotron. Other radioisotopes that may be even more useful in a clinical setting are those that either can be obtained from radionuclide generators (rubidium-82, copper-62) or have a sufficiently long half-life for transportation (fluorine-18). The clinical use of functional renal PET studies (blood flow, glomerular filtration rate) has been slow, in part because of the success of concurrent technologies, including single-photon emission computed tomography (SPECT) and planar gamma camera imaging. Renal blood flow studies can be performed with O-15-labeled water, N-13-labeled ammonia, rubidium-82, and copper-labeled PTSM. With these tracers, renal blood flow can be quantified using a modified microsphere kinetic model. Glomerular filtration can be imaged and quantified with gallium-68 EDTA or cobalt-55 EDTA. Measurements of renal blood flow with PET have potential applications in renovascular disease, in transplant rejection or acute tubular necrosis, in drug-induced nephropathies, ureteral obstruction, before and after revascularization, and before and after the placement of ureteral stents. The most important clinical application for imaging glomerular function with PET would be renovascular hypertension. Molecular imaging of the kidneys with PET is rather limited. At present, research is focused on the investigation of metabolism (acetate), membrane transporters (organic cation and anion transporters, pepT1 and pepT2, GLUT, SGLT), enzymes (ACE), and receptors (AT1R). Because many nephrological and urological disorders are initiated at the molecular and organelle levels and may remain localized at their origin for an extended period of time, new disease-specific molecular probes for PET studies of the kidneys need to be developed. Future applications of molecular renal imaging are likely to involve studies of tissue hypoxia and apoptosis in renovascular renal disease, renal cancer, and obstructive nephropathy, monitoring the molecular signatures of atherosclerotic plaques, measuring endothelial dysfunction and response to balloon revascularization and restenosis, molecular assessment of the nephrotoxic effects of cyclosporine, anticancer drugs, and radiation therapy. New radioligands will enhance the staging and follow-up of renal and prostate cancer. Methods will be developed for investigation of the kinetics of drug-delivery systems and delivery and deposition of prodrugs, reporter gene technology, delivery of gene therapy (nuclear and mitochondrial), assessment of the delivery of cellular, viral, and nonviral vectors (liposomes, polycations, fusion proteins, electroporation, hematopoietic stems cells). Of particular importance will be investigations of stem cell kinetics, including local presence, bloodborne migration, activation, seeding, and its role in renal remodeling (psychological, pathological, and therapy induced). Methods also could be established for investigating the role of receptors and oncoproteins in cellular proliferation, apoptosis, tubular atrophy, and interstitial fibrosis; monitoring ras gene targeting in kidney diseases, assessing cell therapy devices (bioartificial filters, renal tubule assist devices, and bioarticial kidneys), and targeting of signal transduction moleculas with growth factors and cytokines. These potential new approaches are, at best, in an experimental stage, and more research will be needed for their implementation.

Synthesis and in vivo evaluation of a PET radioligand for imaging the endothelin-A receptor

The endothelin-A receptor ligand Atrasentan (ABT-627) was radiolabeled by (11)C-methylaton of the desmethyl precursor in phenolate form. In mice, the highest uptake of [(11)C]ABT-627 was in the liver, kidneys and lungs. No significant binding was observed in mouse brain or heart. PET studies in a baboon, however, showed accumulation in the myocardium and lungs with a tissue/blood equilibrium reached at 40 min postinjection. Between 35 and 75 min, the heart/blood and lung/blood ratios were 1.72 and 1.31, respectively. Pretreatment with a 0.39 mg/kg dose of unlabeled ABT-627 inhibited the uptake of the tracer by 53-54% in both the myocardium and lungs at 65 min.

Future direction of renal positron emission tomography

Positron emission tomography (PET) is perfectly suited for quantitative imaging of the kidneys, and the recent improvements in detector technology, computer hardware, and image processing software add to its appeal. Multiple positron emitting radioisotopes can be used for renal imaging. Some, including carbon-11, nitrogen-13, and oxygen-15, can be used at institutions with an on-site cyclotron. Other radioisotopes that may be even more useful in a clinical setting are those that either can be obtained from radionuclide generators (rubidium-82, copper-62) or have a sufficiently long half-life for transportation (fluorine-18). The clinical use of functional renal PET studies (blood flow, glomerular filtration rate) has been slow, in part because of the success of concurrent technologies, including single-photon emission computed tomography (SPECT) and planar gamma camera imaging. Renal blood flow studies can be performed with O-15-labeled water, N-13-labeled ammonia, rubidium-82, and copper-labeled PTSM. With these tracers, renal blood flow can be quantified using a modified microsphere kinetic model. Glomerular filtration can be imaged and quantified with gallium-68 EDTA or cobalt-55 EDTA. Measurements of renal blood flow with PET have potential applications in renovascular disease, in transplant rejection or acute tubular necrosis, in drug-induced nephropathies, ureteral obstruction, before and after revascularization, and before and after the placement of ureteral stents. The most important clinical application for imaging glomerular function with PET would be renovascular hypertension. Molecular imaging of the kidneys with PET is rather limited. At present, research is focused on the investigation of metabolism (acetate), membrane transporters (organic cation and anion transporters, pepT1 and pepT2, GLUT, SGLT), enzymes (ACE), and receptors (AT1R). Because many nephrological and urological disorders are initiated at the molecular and organelle levels and may remain localized at their origin for an extended period of time, new disease-specific molecular probes for PET studies of the kidneys need to be developed. Future applications of molecular renal imaging are likely to involve studies of tissue hypoxia and apoptosis in renovascular renal disease, renal cancer, and obstructive nephropathy, monitoring the molecular signatures of atherosclerotic plaques, measuring endothelial dysfunction and response to balloon revascularization and restenosis, molecular assessment of the nephrotoxic effects of cyclosporine, anticancer drugs, and radiation therapy. New radioligands will enhance the staging and follow-up of renal and prostate cancer. Methods will be developed for investigation of the kinetics of drug-delivery systems and delivery and deposition of prodrugs, reporter gene technology, delivery of gene therapy (nuclear and mitochondrial), assessment of the delivery of cellular, viral, and nonviral vectors (liposomes, polycations, fusion proteins, electroporation, hematopoietic stems cells). Of particular importance will be investigations of stem cell kinetics, including local presence, bloodborne migration, activation, seeding, and its role in renal remodeling (psychological, pathological, and therapy induced). Methods also could be established for investigating the role of receptors and oncoproteins in cellular proliferation, apoptosis, tubular atrophy, and interstitial fibrosis; monitoring ras gene targeting in kidney diseases, assessing cell therapy devices (bioartificial filters, renal tubule assist devices, and bioarticial kidneys), and targeting of signal transduction moleculas with growth factors and cytokines. These potential new approaches are, at best, in an experimental stage, and more research will be needed for their implementation.

Synthesis and biodistribution of (11)C-GW7845, a positron-emitting agonist for peroxisome proliferator-activated receptor-{gamma}

The goal of this study was to synthesize and evaluate in vivo the peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist (11)C-GW7845 ((S)-2-(1-carboxy-2-{4-[2-(5-methyl-2-phenyloxazol-4-yl)ethoxy]phenyl}ethylamino)benzoic acid methyl ester) ((11)C-compound 1). PPARgamma is a member of a family of nuclear receptors that plays a central role in the control of lipid and glucose metabolism. Compound 1 is an analog of tyrosine (inhibitor constant, 3.7 nmol/L), which is an inhibitor of experimental mammary carcinogenesis.

METHODS

Protection of the carboxylic acid moiety of compound 1 was effected by treatment with N,N-dimethylformamide di-tert-butyl acetal to provide compound 2. Hydrolysis of the carbomethoxy group of compound 2 provided the benzoic acid (compound 3) that served as an immediate precursor to radiolabeling. Compound 3 underwent treatment with (11)C-methyl iodide followed by high-performance liquid chromatography to produce a radioactive peak sample that coeluted with a standard sample of compound 1. Analysis of biodistribution was undertaken by injecting male CD-1 mice via the tail vein with 6.03 MBq (163 microCi, 2.55 microg/kg) of (11)C-compound 1. To determine the tumor uptake of the radiotracer, 6 female SCID mice bearing MCF-7 xenografts were injected via the tail vein with 10.5 MBq (283 microCi, 0.235 microg/kg) of (11)C-compound 1.

RESULTS

(11)C-Compound 1 was synthesized at an 8% radiochemical yield in 29 min with an average specific radioactivity of 1,222 GBq/micromol (33,024 mCi/micromol; n = 6) at the end of synthesis. Spleen (target)-to-muscle uptake and tumor-to-muscle uptake ratios were 3.1 and 1.5, respectively, but this uptake could not be blocked with unlabeled compound 1 at 2 mg/kg.

CONCLUSION

Further structural modification, perhaps to generate a less lipophilic tyrosine analog, will be necessary to enable receptor-mediated PPARgamma imaging by this class of agents.

Synthesis and biodistribution of [11C]adenosine 5'-monophosphate ([11C]AMP)

PURPOSE

Imaging purine receptors and adenylate biodistribution in vivo may be of clinical importance not only for the investigation of normal adenylate metabolism but also in pathological conditions where adenylate uptake and/or release from certain tissues and organs may be altered, such as some types of cancer. In order to develop a tracer for positron emission tomography (PET) that would not be subject to loss of its radioisotope, adenosine 5'-monophosphate (AMP) was intrinsically labeled at the C-8 position with carbon-11.

PROCEDURES

[11C]AMP was synthesized by reacting 5-amino-1-beta-D-ribofuranosylimidazole-4-carboxamidine-5'-phosphate with [11C]formaldehyde. The metabolism of [11C]AMP in human blood was determined in vitro both in the presence and absence of dipyridamole. The ex vivo biodistribution of [11C]AMP and its in vivo dosimetry were determined in normal mice. The effect of dipyridamole on the distribution of [11C]AMP in mice was also determined.

RESULTS

[11C]AMP was reliably synthesized in 34 minutes (n = 7) with an average radiochemical yield of 2.4% and an average specific activity of 90.10 GBq/micromol (2435 mCi/micromol) at end of synthesis. In normal mice, the highest uptake of [11C]AMP was in the lungs, blood, and heart. The ex vivo mouse experiments showed that the uptake of 11C radiotracer in the lungs at 60 minutes postinjection was significantly lower for dipyridamole-treated animals than controls. Dosimetry showed that the critical organs for radiation dose burden are kidneys and bladder.

CONCLUSIONS

Treatment with dipyridamole blocked the red blood cell uptake of extracellular adenosine and therefore its subsequent intracellular conversion to ATP. The biodistribution studies indicate that the tracer has substantial accumulation in the kidneys, lungs, heart, and blood. [11C]AMP is promising as a PET-imaging agent to trace adenylate biology in vivo.

Quantitative PET studies of the serotonin transporter in MDMA users and controls using [11C]McN5652 and [11C]DASB

(+/-)3,4-Methylenedioxymethamphetamine (MDMA, 'Ecstasy') is a widely used illicit drug that produces toxic effects on brain serotonin axons and axon terminals in animals. The results of clinical studies addressing MDMA's serotonin neurotoxic potential in humans have been inconclusive. In the present study, 23 abstinent MDMA users and 19 non-MDMA controls underwent quantitative positron emission tomography (PET) studies using [11C]McN5652 and [11C]DASB, first- and second-generation serotonin transporter (SERT) ligands previously validated in baboons for detecting MDMA-induced brain serotonin neurotoxicity. Global and regional distribution volumes (DVs) and two additional SERT-binding parameters (DV(spec) and DVR) were compared in the two subject populations using parametric statistical analyses. Data from PET studies revealed excellent correlations between the various binding parameters of [11C]McN5652 and [11C]DASB, both in individual brain regions and individual subjects. Global SERT reductions were found in MDMA users with both PET ligands, using all three of the above-mentioned SERT-binding parameters. Preplanned comparisons in 15 regions of interest demonstrated reductions in selected cortical and subcortical structures. Exploratory correlational analyses suggested that SERT measures recover with time, and that loss of the SERT is directly associated with MDMA use intensity. These quantitative PET data, obtained using validated first- and second-generation SERT PET ligands, provide strong evidence of reduced SERT density in some recreational MDMA users.

A novel radioligand for imaging the AT1 angiotensin receptor with PET

2-Butyl-5-methoxymethyl-6-(1-oxopyridin-2-yl)-3-[[2'-(1H-tetrazol-5-yl)biphenyl-4-yl]methyl]-3H-imidazo[4,5-b]pyridine (KR31173) was radiolabeled by coupling a tetrazole-protected hydroxy precursor with [(11)C] methyl iodide and removing the protecting group by acid hydrolysis. In mice, the highest uptake of [(11)C] KR31173 was in the adrenal glands, kidneys, and liver. Tissue to blood ratios were generally greater than 10:1. Uptake of the tracer in the adrenal glands, kidneys, lungs, and heart was blocked with a 1 mg/kg dose of KR31173 or MK-996.

Positron emission tomography imaging of the serotonin transporter in subjects with a history of alcoholism

BACKGROUND

Our purpose was to investigate the serotonin transporter (SERT) in various brain regions of alcoholics using positron emission tomography and C-11 McN5652.

METHOD

Thirty-two adult subjects were involved, 17 social drinkers as control subjects and 15 subjects who were abstinent or recovering alcoholics. Concomitant psychiatric diseases were ruled out based on DSM-IV criteria. The majority of subjects were men. Radioligand binding in 11 brain areas was expressed as the total distribution volume (DV), distribution volume of specific binding (DV(spec)), and distribution volume ratio (DVR). The cerebellum was used as reference tissue for calculation of DV(spec) and DVR.

RESULTS

In subjects with a history of alcoholism, DV was lower in all brain regions, with significant differences in the midbrain, thalamus, amygdala, pons, cingulate gyrus, frontal cortex, and cerebellum. Additionally, DV(spec) was lower in all brain regions, but differences were only significant in the midbrain; DVR was lower in nine regions but the differences did not reach statistical significance.

CONCLUSIONS

These studies demonstrate lower binding of [(11)C](+)McN5652 to the SERT in the brain of abstinent or recovering alcoholics compared with control subjects. Differences in the radioligand distribution volumes are more significant before than after correction for nonspecific binding of the radioligand.

In vivo investigation of estrogen regulation of adrenal and renal angiotensin (AT1) receptor expression by PET

The renin angiotensin system (RAS) has been implicated as one mediator of the cardiovascular effects of estrogen. Since changes in angiotensin type 1 (AT(1)) receptor expression are central to modulation of the RAS, we used the noninvasive PET imaging technique to study for the in vivo effects of estrogen on membrane and intracellular AT(1) receptors.

METHODS

Dynamic PET measurements of canine AT(1) (cAT(1)) receptors using the radiolabeled AT(1) receptor antagonist, (11)C-L-159,884, were performed during 2-wk consecutive periods of estrogen deprivation induced by ovariectomy and 17beta-estradiol (E(2)) replacement.

RESULTS

Kinetic modeling of time-activity curves in the kidney and adrenal showed lower receptor expression in the estrogen replete state (21% and 30% decrease in Gjedde-Patlak slope, influx constant, respectively). These in vivo findings correlated with in vitro radioligand-binding assays with (125)I-[Sar(1),Ile(8)]angiotensin II showing reduced AT(1) receptor number in the adrenal (35%), glomeruli (30%), myocardium (35%), and liver (21%) in the estrogen-replenished compared with estrogen-depleted animals.

CONCLUSION

Although other endogenous systems are known to regulate AT(1) receptors and could compete with estrogenic actions, these PET studies reveal that estrogen attenuates AT(1) receptor expression in vivo. Thus, estrogen modulation of AT(1) receptors may contribute to the cardiovascular protective effects associated with estrogen.

Positron emission tomography of striatal serotonin transporters in Parkinson disease

BACKGROUND

Little is known about serotonin neurons in Parkinson disease (PD).

OBJECTIVE

To study the serotonin system in PD with positron emission tomography, using the serotonin transporter radioligand [11C](+)McN5652.

DESIGN AND PATIENTS

We measured the density of the serotonin transporter and the density of [11C]WIN35,428-labeled dopamine transporters in the striatum of 13 adults with PD and 13 age- and sex-matched controls. To assess the effects of possible differences in blood flow or brain atrophy, we also measured regional cerebral blood flow and the size of the regions of interest for the caudate nucleus and putamen.

RESULTS

Patients with PD showed reductions in the specific distribution volumes of [11C](+)McN5652 in the caudate (P<.01) and putamen (P<.01), along with the expected reductions in striatal [11C]WIN35,428 binding (P<.01). There were no reductions in regional cerebral blood flow or the sizes of the regions of interest, mitigating against potential confounding effects of blood flow, brain atrophy, or partial volume effects. Reductions in serotonin transporter binding correlated with ratings of disease staging.

CONCLUSIONS

These results suggest that the density of serotonin transporters, like that of dopamine transporters, is reduced in the striatum of patients with PD and that these changes are related to disease stage.

Carbon-11 labeled radioligands for imaging brain cannabinoid receptors

Two radioligands, [(11)C] SR149080 and its morpholino analog [(11)C] SR149568, were synthesized by reaction of the respective phenolic precursors with [(11)C] methyl iodide. Both radioligands had appropriate regional brain distribution for cannabinoid receptors in mice with peak target to non-target ratios of 2.2 for [(11)C] SR149080 and 1.6 for [(11)C] SR149568 at 90 and 30 minutes post-injection respectively. The uptake of both tracers was blocked with a 1 mg/kg dose of SR141716A.

Radiosynthesis and mouse brain distribution studies of [11C] CP-126,998: a PET ligand for in vivo study of acetylcholinesterase

The selective, reversible acetylcholinesterase inhibitor 5,7-Dihydro-7-methyl-3- [2-[1-(phenylmethyl]-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol3-6-one (CP-126,998) was labeled with C-11 iodomethane via base-promoted alkylation of the lactam nitrogen. [11C] CP-126,998 was synthesized in good radiochemical yield (13-29% non-decay corrected) and high specific radioactivity (177-418 GBq/micromol). In vivo mouse biodistribution studies reveal [11C] CP-126,998 to localize preferentially in striatal tissue, a region known to be rich in acetylcholinesterase. Competitive blocking studies using a variety of acetylcholinesterase inhibitors (diisopropylfluorophosphate, tacrine, CP-118,954) verified the specificity of the PET radiotracer for brain acetylcholinesterase.

Comparison of (+)-(11)C-McN5652 and (11)C-DASB as serotonin transporter radioligands under various experimental conditions

There has been considerable interest in the development of a PET radioligand selective for the serotonin (5-hydroxytryptamine [5-HT]) transporter (SERT) that can be used to image 5-HT neurons in the living human brain. The most widely used SERT radiotracer to date, trans-1,2,3,5,6,10-beta-hexahydro-6-[4-(methylthio)phenyl[pyrrolo-[2,1-a]isoquinoline ((+)-(11)C-McN5652), has been successful in this regard but may have some limitations. Recently, another promising SERT radiotracer, 3-(11)C-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile ((11)C-DASB), has been described. The purpose of this study was to compare and contrast (+)-(11)C-McN5652 and (11)C-DASB under various experimental conditions.

METHODS

Radioligand comparisons were performed in a control baboon, a baboon with reduced SERT density ((+/-)-3,4-methylenedioxymethamphetamine [MDMA] lesion), and a baboon with reduced SERT availability (paroxetine pretreatment). Under each of these experimental conditions, repeated (triplicate) PET studies were performed with each ligand.

RESULTS

Both radiotracers bound preferentially in brain regions known to contain high SERT density. For both ligands, there was a high correlation between the amount of regional brain ligand binding and the known regional brain concentration of SERT. Binding of both ligands was decreased after MDMA neurotoxicity (reduced SERT density), and (+)-(11)C-McN5652 and (11)C-DASB were comparably effective in detecting reduced SERT density after MDMA-induced 5-HT neurotoxicity. Pretreatment with paroxetine dramatically altered the metabolism and kinetics of both tracers and appeared to displace both ligands primarily from regions with high SERT density. Compared with (+)-(11)C-McN5652, (11)C-DASB had higher brain activity and a faster washout rate and provided greater contrast between subcortical and cortical brain regions.

CONCLUSION

(11)C-DASB and (+)-(11)C-McN5652 are suitable as PET ligands of the SERT and for detecting MDMA-induced 5-HT neurotoxicity. (11)C-DASB may offer some advantages. Additional studies are needed to further characterize the properties and capabilities of both ligands in health and disease.

[(11)C]-GR89696, a potent kappa opiate receptor radioligand; in vivo binding of the R and S enantiomers

The R and S enantiomers of [(11)C]GR89696, [(11)C]-methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl)methyl]-1-piperazinecarboxylate, were synthesized from their appropriate chiral precursors and [(11)C]methyl chloroformate. The [(11)C]-labeled R enantiomer of GR89696, also known as GR103545, demonstrated high affinity in mouse brain with region to cerebellar ratios at 90 minutes of 11.4 and 8.7 for the hypothalamus and olfactory tubercle, respectively. The [(11)C]-labeled S enantiomer showed low affinity and region to cerebellar ratios of 1 for all brain regions. The [(11)C]-labeled GR103545 exhibited a selective and saturable binding for the kappa opioid receptor.

In vivo labeling of endothelin receptors with [(11)C]L-753,037: studies in mice and a dog

Endothelin (ET) is a potent mammalian vasoconstrictive peptide and a pressor agent. Its 3 isoforms, ET-1, ET-2, and ET-3, mediate several physiologic actions in several organ systems, binding to 2 major receptor subtypes: ET(A) and ET(B). This study was undertaken to evaluate [(11)C]L-753,037 [(+)-(5S,6R,7R)-2-butyl-7-[2-((2S)-2-carboxy-propyl)-4-methoxyphenyl]-5-(3,4-methylenedioxyphenyl)cyclopenteno [1,2-beta]pyridine-6-carboxylate), a new mixed ET receptor A and B antagonist, as a tracer for in vivo labeling of ET receptors in mice and a dog.

METHODS

[(11)C]L-753,037 was synthesized, purified, and formulated from a normethyl precursor, L-843,974, and [(11)C]H(3)I. The tracer was studied for its in vivo kinetics, biodistribution, and ET receptor binding characteristics in mice. In the dog, PET imaging was performed to evaluate binding of [(11)C]L-753,037 to ET receptors in the heart. Specificity of binding was studied in the heart with the selective ET(A) antagonist L-753,164.

RESULTS

Kinetic studies in mice showed highest tracer uptake at 5 min after injection in liver (25.0 percentage injected dose per gram [%ID/g]), kidneys (18.7 %ID/g), lungs (15.2 %ID/g), and heart (5.6 %ID/g). Initial high uptake in liver, lungs, and kidneys was followed by rapid washout during the next 10 min and a very slow clearance during the time of observation (2 h after injection). By contrast, the radioactivity in the heart remained constant over 2 h. Administration of both ET(A) (L-753,164) and mixed ET(A)/ET(B) (L-753,137) receptor antagonists resulted in dose-dependent inhibition of [(11)C]L-753,037 binding in mouse heart, lungs, and kidneys but not in the liver. Radioactivity in the brain was very low, indicating that the tracer does not cross the blood-brain barrier. In the dog, a dynamic PET study of the heart showed high tracer accumulation at 55-95 min after injection. Injection of L-753,164 at 30 min before [(11)C]L-753,037 administration led to a significant reduction in tracer binding. [(11)C]methyl triphenyl phosphonium was used as a tracer for reference images of the dog heart muscle.

CONCLUSION

The results suggest that [(11)C]L-753,037 binds to ET receptors in vivo and is, therefore, a promising candidate for investigation of these receptors and their occupancy by ET receptor antagonists using PET.