Brain Penetration of the ROS1/ALK Inhibitor Lorlatinib Confirmed by PET.

The Massachusetts General Hospital Radiochemistry Program, in collaboration with Pfizer, has developed unique 11C and 18F-labeling strategies to synthesize isotopologs of lorlatinib (PF-06463922) which is undergoing phase III clinical trial investigations for treatment of non-small-cell lung cancers with specific molecular alterations. A major goal in cancer therapeutics is to measure the concentrations of this drug in the brain metastases of patients with lung cancer, and penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Our recent publication in Nature Communications employed radiolabeled lorlatinib and positron emission tomography (PET) studies in preclinical models including nonhuman primates (NHPs) that demonstrated high brain permeability of this compound. Our future work with radiolabeled lorlatinib will include advanced PET evaluations in rodent tumor models and normal NHPs with the goal of clinical translation.

Synthesis and preliminary PET imaging of 11C and 18F isotopologues of the ROS1/ALK inhibitor lorlatinib.

Lorlatinib (PF-06463922) is a next-generation small-molecule inhibitor of the orphan receptor tyrosine kinase c-ros oncogene 1 (ROS1), which has a kinase domain that is physiologically related to anaplastic lymphoma kinase (ALK), and is undergoing Phase I/II clinical trial investigations for non-small cell lung cancers. An early goal is to measure the concentrations of this drug in brain tumour lesions of lung cancer patients, as penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Here we prepare both 11C- and 18F-isotopologues of lorlatinib to determine the biodistribution and whole-body dosimetry assessments by positron emission tomography (PET). Non-traditional radiolabelling strategies are employed to enable an automated multistep 11C-labelling process and an iodonium ylide-based radiofluorination. Carbon-11-labelled lorlatinib is routinely prepared with good radiochemical yields and shows reasonable tumour uptake in rodents. PET imaging in non-human primates confirms that this radiotracer has high brain permeability.

First-in-Human Assessment of the Novel PDE2A PET Radiotracer 18F-PF-05270430.

UNLABELLED: This was a first-in-human study of the novel phosphodiesterase-2A (PDE2A) PET ligand (18)F-PF-05270430. The primary goals were to determine the appropriate tracer kinetic model to quantify brain uptake and to examine the within-subject test-retest variability. METHODS: In advance of human studies, radiation dosimetry was determined in nonhuman primates. Six healthy male subjects participated in a test-retest protocol with dynamic scans and metabolite-corrected input functions. Nine brain regions of interest were studied, including the striatum, white matter, neocortical regions, and cerebellum. Multiple modeling methods were applied to calculate volume of distribution (VT) and binding potentials relative to the nondisplaceable tracer in tissue (BPND), concentration of tracer in plasma (BPP), and free tracer in tissue (BPF). The cerebellum was selected as a reference region to calculate binding potentials. RESULTS: The dosimetry study provided an effective dose of less than 0.30 mSv/MBq, with the gallbladder as the critical organ; the human target dose was 185 MBq. There were no adverse events or clinically detectable pharmacologic effects reported. Tracer uptake was highest in the striatum, followed by neocortical regions and white matter, and lowest in the cerebellum. Regional time-activity curves were well fit by multilinear analysis-1, and a 70-min scan duration was sufficient to quantify VT and the binding potentials. BPND, with mean values ranging from 0.3 to 0.8, showed the best intrasubject and intersubject variability and reliability. Test-retest variability in the whole brain (excluding the cerebellum) of VT, BPND, and BPP were 8%, 16%, and 17%, respectively. CONCLUSION: (18)F-PF-05270430 shows promise as a PDE2A PET ligand, albeit with low binding potential values.

Evaluation of Cu-64 and Ga-68 Radiolabeled Glucagon-Like Peptide-1 Receptor Agonists as PET Tracers for Pancreatic ß cell Imaging.

PURPOSE: Copper-64 (Cu-64) and Galium-68 (Ga-68) radiolabeled DO3A and NODA conjugates of exendin-4 were used for preclinical imaging of pancreatic ß cells via targeting of glucagon-like peptide-1 receptor (GLP-1R). PROCEDURES: DO3A-VS- and NODA-VS-tagged Cys(40)exendin-4 (DO3A-VS-Cys(40)-exendin-4 and NODA-VS-Cys(40)-exendin-4, respectively) were labeled with Cu-64 and Ga-68 using standard techniques. Biodistribution and dynamic positron emission tomography (PET) were carried out in normal Sprague-Dawley (SD) rats. Ex vivo autoradiography imaging was conducted with freshly frozen pancreatic thin sections. RESULTS: DO3A-VS- and NODA-VS-Cys(40)-exendin-4 analogues were labeled with Cu-64 and Ga-68 to a specific activity of 518.7 ± 3.7 Ci/mmol (19.19 ± 0.14 TBq/mmol) and radiochemical yield above 98 %. Biodistribution data demonstrated pancreatic uptake of 0.11 ± 0.02 %ID/g for [(64)Cu]DO3A-VS-, 0.14 ± 0.02 %ID/g for [(64)Cu]NODA-VS-, 0.11 ± 0.03 for [(68)Ga]DO3A-VS-, and 0.26 ± 0.03 for [(68)Ga]NODA-VS-Cys(40)-exendin-4. Excess exendin-4 and exendin-(9-39)-amide displaced all four Cu-64 and Ga-68 labeled exendin-4 derivatives in blocking studies. CONCLUSIONS: [(64)Cu]/[(68)Ga]DO3A-VS-Cys(40)- and [(64)Cu]/[(68)Ga]NODA-VS-Cys(40)-exendin-4 can be used as PET imaging agents specific for GLP-1R expressed on ß cells. Here, we report the first evidence of pancreatic uptake visualized with exendin-4 derivative in a rat animal model via in vivo dynamic PET imaging.

The phosphodiesterase 10 positron emission tomography tracer, [18F]MNI-659, as a novel biomarker for early Huntington disease.

IMPORTANCE: In Huntington disease (HD) striatal neuron loss precedes and predicts motor signs or symptoms. Current imaging biomarkers lack adequate sensitivity for assessing the early stages of HD. Developing an imaging biomarker for HD spanning the time of onset of motor signs remains a major unmet research need. Intracellular proteins whose expression is altered by the mutant huntingtin protein may be superior markers for early HD stages. OBJECTIVE: To evaluate whether [18F]MNI-659 (2-(2-(3-(4-(2-[18F]fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione), a novel phosphodiesterase 10 positron emission tomography (PET) ligand, is a sensitive marker for striatal changes in early HD. DESIGN, SETTING, AND PARTICIPANTS: A cohort of individuals with HD, including premanifest (pre-HD) or manifest with motor signs (mHD), underwent clinical assessments, genetic determination, [18F]MNI-659 PET imaging, and brain magnetic resonance imaging. Age-matched healthy volunteers (HVs) also received clinical assessments and PET and magnetic resonance imaging. MAIN OUTCOMES AND MEASURES: Binding potentials (BPnds) were estimated for brain regions of interest, specifically within the basal ganglia, and compared between participants with HD and the HVs and correlated with markers of HD severity and atrophy of basal ganglia nuclei. RESULTS: Eleven participants with HD (8 mHD and 3 pre-HD) and 9 HVs participated. Ten of 11 HD participants had known huntingtin CAG repeat length, allowing determination of a burden of pathology (BOP) score. One individual with HD declined CAG determination. All participants with mHD had relatively early-stage disease (4 with stage 1 and 4 with stage 2) and a Unified Huntington's Disease Rating Scale (UHDRS) total Motor subscale score of less than 50. The HD cohort had significantly lower striatal [18F]MNI-659 uptake than did the HV cohort (mean, -48.4%; P < .001). The HD cohort as a whole had a reduction in the basal ganglia BPnd to approximately 50% of the level in the HVs (mean, -47.6%; P < .001). The 3 pre-HD participants had intermediate basal ganglia BPnds. Striatal [18F]MNI-659 uptake correlated strongly with the severity of disease measured by the clinical scale (UHDRS Motor subscale; R = 0.903; P < .001), the molecular marker (BOP; R = 0.908; P < .001), and regional atrophy (R = 0.667; P < .05). CONCLUSIONS AND RELEVANCE: As a promising striatal imaging biomarker, [18F]MNI-659 is potentially capable of assessing the extent of disease in early mHD. Furthermore, [18F]MNI-659 may identify early changes in medium spiny neurons and serve as a marker to predict conversion to mHD. Additional studies with larger, stratified cohorts of patients with HD and prospective studies of individuals with pre-HD are warranted.

In vivo assessment and dosimetry of 2 novel PDE10A PET radiotracers in humans: 18F-MNI-659 and 18F-MNI-654.

UNLABELLED: Phosphodiesterase (PDE) 10A is an enzyme involved in the regulation of cyclic adenosine monophosphate and cyclic guanosine monophosphate and is highly expressed in medium-sized spiny neurons of the striatum, making it an attractive target for novel therapies for a variety of neurologic and psychiatric disorders that involve striatal function. Potential ligands for PET imaging of PDE10A have been reported. Here, we report the first-in-human characterization of 2 new PDE10A radioligands, 2-(2-(3-(1-(2-fluoroethyl)-1H-indazol-6-yl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ((18)F-MNI-654) and 2-(2-(3-(4-(2-fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ((18)F-MNI-659), with the goal of selecting the best one for use in future studies interrogating pathophysiologic changes in neuropsychiatric disorders and aiding pharmaceutical development targeting PDE10A. METHODS: Eleven healthy volunteers participated in this study ((18)F-MNI-654 test-retest, 2 men; (18)F-MNI-659 test-retest, 4 men and 1 woman; (18)F-MNI-659 dosimetry, 2 men and 2 women). Brain PET images were acquired over 5.5 h for (18)F-MNI-654 and over 3.5 h for (18)F-MNI-659, and pharmacokinetic modeling with plasma- and reference-region (cerebellar cortex)-based methods was performed. Whole-body PET images were acquired over 6 h for (18)F-MNI-659 and radiation dosimetry estimated with OLINDA. RESULTS: Both radiotracers were similarly metabolized, with about 20% of intact parent remaining at 120 min after injection. PET time-activity data demonstrated that (18)F-MNI-654 kinetics were much slower than (18)F-MNI-659 kinetics. For (18)F-MNI-659, there was good agreement between the Logan and simplified reference tissue models for nondisplaceable binding potential (BPND), supporting noninvasive quantification, with test-retest variability less than 10% and intraclass correlation greater than 0.9. The (18)F-MNI-659 effective dose was estimated at 0.024 mSv/MBq. CONCLUSION: PET imaging in the human brain with 2 novel PDE10A (18)F tracers is being reported. Noninvasive quantification of (18)F-MNI-659 with the simplified reference tissue model using the cerebellum as a reference is possible. In addition, (18)F-MNI-659 kinetics are fast enough for a good estimate of BPND with 90 min of data, with values around 3.0 in the basal ganglia. Finally, (18)F-MNI-659 dosimetry is favorable and consistent with values reported for other PET radiotracers currently used in humans.

18F-FPEB, a PET radiopharmaceutical for quantifying metabotropic glutamate 5 receptors: a first-in-human study of radiochemical safety, biokinetics, and radiation dosimetry.

UNLABELLED: Identification of safe and valid PET radioligands for metabotropic glutamate receptor, type 5 (mGluR5), is essential to measure changes in brain mGluR5 in neuropsychiatric disorders, to confirm central mGluR5 occupancy of drug candidates, and to guide dose selection for obtaining an optimum therapeutic window. Here we present the results of a first-in-human study assessing the safety and effectiveness of a novel PET radiopharmaceutical, (18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB), for quantifying regional brain concentrations of mGluR5. METHODS: Quantification of whole-body biokinetics was conducted in 6 healthy adults (3 men and 3 women). The radiation safety profile was estimated with OLINDA/EXM software. Subsequently, pairs of dynamic brain scans were obtained for 11 healthy men to identify optimal methods for derivation of regional distribution volume and binding potential and to determine the repeatability of measurement. RESULTS: The whole-body effective radiation dose was approximately 17 µSv/MBq (62 mrem/mCi), with the gallbladder receiving the highest dose of 190 µSv/MBq. In brain studies, time-activity curves showed high accumulation in the insula/caudate nucleus, moderate uptake in the thalamus, and the lowest concentration in the cerebellum/pons. The plasma reference graphical analysis method appeared optimal for (18)F-FPEB; it showed acceptable test-retest variability of nondisplaceable binding potential (<10%) and identified the highest nondisplaceable binding potential values (from ∼0.5 in the globus pallidus to ∼3.5 in the insula) for target regions. Safety assessments revealed no clinically meaningful changes in vital signs, electrocardiogram, or laboratory values. CONCLUSION: (18)F-FPEB is safe and well tolerated, and its regional cerebral distribution is consistent with previous reports in the literature for metabotropic glutamate receptors. The repeatability of measurement suggests that (18)F-FPEB is suitable for quantifying mGluR5 in humans.

Ex vivo imaging of pancreatic beta cells using a radiolabeled GLP-1 receptor agonist.

PURPOSE: The purpose of this study was to evaluate the binding specificity of the radiolabeled glucagon-like peptide 1 receptor (GLP-1R) agonist (Lys4°(DOTA)NH2)Exendin-4 in the pancreas using a combination of ex vivo autoradiography and immunohistochemistry. PROCEDURES: Sprague-Dawley rats were administered [64Cu](Lys4°(DOTA)NH2)Exendin-4 i.v. with or without unlabeled Exendin (9-39) to determine binding specificity. Similar experiments were performed using Zucker diabetic fatty (ZDF) and Zucker lean (ZLC) rats. Animals were euthanized and the pancreas was extracted, immediately frozen, and sectioned. The sections were apposed to phosphor imaging plates, scanned, and immunostained for insulin. RESULTS: Co-registration of the autoradiographic and immunohistochemical images revealed that [64Cu] (Lys4°(DOTA)NH2)Exendin-4 specific binding was restricted to islet cells. This binding was blocked by the co-administration of Exendin(9-39) indicating that the radiotracer uptake is mediated by GLP-1R. Uptake of [64Cu](Lys4°(DOTA)NH2)Exendin-4 was greatly decreased in the pancreas of ZDF rats. CONCLUSIONS: Ex vivo autoradiography results using [64Cu](Lys4°(DOTA)NH2)Exendin-4 suggest that GLP-1R agonists based on Exendin-4 are attractive PET ligands for the in vivo determination of ß-cell mass.

Molecular imaging of neurovascular cell death in experimental cerebral stroke by PET.

UNLABELLED: Clinical molecular imaging of apoptosis is a highly desirable yet unmet challenge. Here we provide the first report on (18)F-labeled 5-fluoropentyl-2-methyl-malonic acid ((18)F-ML-10), a small-molecule, (18)F-labeled PET tracer for the imaging of apoptosis in vivo; this report includes descriptions of the synthesis, radiolabeling, and biodistribution of this novel apoptosis marker. We also describe the use of (18)F-ML-10 for small-animal PET of neurovascular cell death in experimental cerebral stroke in mice. METHODS: (18)F-ML-10 was synthesized by nucleophilic substitution from the respective mesylate precursor, and its biodistribution was assessed in healthy rats. Permanent occlusion of the middle cerebral artery (MCA) was induced in mice, and small-animal PET was performed 24 h later. RESULTS: Efficient radiolabeling of ML-10 with (18)F was achieved. Biodistribution studies with (18)F-ML-10 revealed rapid clearance from blood (half-life of 23 min), a lack of binding to healthy tissues, and rapid elimination through the kidneys. No significant tracer metabolism in vivo was observed. Clear images of distinct regions of increased uptake, selectively in the ischemic MCA territory, were obtained in the in vivo small-animal PET studies. Uptake measurements ex vivo revealed 2-fold-higher uptake in the affected hemisphere and 6- to 10-fold-higher uptake in the region of interest of the infarct. The cerebral uptake of (18)F-ML-10 was well correlated with histologic evidence of cell death. The tracer was retained in the stroke area but was cleared from blood and from intact brain areas. CONCLUSION: (18)F-ML-10 is useful for noninvasive PET of neurovascular histopathology in ischemic cerebral stroke in vivo. Such an assessment may assist in characterization of the extent of stroke-related cerebral damage and in the monitoring of disease course and effect of treatment.

(R)-N-Methyl-3-(3-(125)I-pyridin-2-yloxy)-3-phenylpropan-1-amine: a novel probe for norepinephrine transporters.

Alterations in serotonin and norepinephrine neuronal functions have been observed in patients with major depression. Several antidepressants bind to both serotonin transporters and norepinephrine transporters (NET). The ability to image NET in the human brain would be a useful step toward understanding how alterations in NET relate to disease. In this study, we report the synthesis and characterization of a new series of derivatives of iodonisoxetine, a known radioiodinated probe. The most promising, (R)-N-methyl-3-(3-iodopyridin-2-yloxy)-3-phenylpropylamine (PYINXT), displayed a high and saturable binding to NET, with a K(d) value of 0.53+/-0.03 nM. Biodistribution studies of (R)-N-methyl-3-(3-(125)I-pyridin-2-yloxy)-3-phenylpropan-1-amine in rats showed moderate initial brain uptake (0.54% dose/organ at 2 min) with a relatively fast washout from the brain (0.16% dose/organ at 2 h) as compared to [(125)I]INXT. The hypothalamus (a NET-rich region)-to-striatum (a region devoid of NET) ratio was found to be 2.14 at 4 h after intravenous injection. Preliminary results suggest that this improved iodinated ligand, when labeled with (123)I, may be useful for mapping NET-binding sites with single photon emission computed tomography in the living human brain.

In vitro and in vivo binding of neuroactive steroids to the sigma-1 receptor as measured with the positron emission tomography radioligand [18F]FPS.

Sigma-1 receptors are widely expressed in the mammalian brain and also in organs of the immune, endocrine and reproductive systems. Based on behavioral and pharmacological assessments, sigma-1 receptors are important in memory and cognitive processes, and are thought to be involved in specific psychiatric illnesses, including schizophrenia, depression, and drug addiction. It is thought that specific neuroactive steroids are endogenous ligands for these sites. In addition, several sigma-1 receptor binding steroids including progesterone, dihydroepiandrosterone (DHEA), and testosterone are being examined clinically for specific therapeutic purposes; however, their mechanisms of action have not been clearly defined. We previously described the high affinity sigma-1 receptor selective PET tracer [(18)F]FPS. This study examines the effect of neuroactive steroids on [(18)F]FPS binding in vitro and in vivo. Inhibition constants were determined in vitro for progesterone, testosterone, DHEA, estradiol, and estriol binding to the [(18)F]FPS labeled receptor. The affinity order (K(i) values) for these steroids ranged from 36 nM for progesterone to >10,000 nM for estrodiol and estriol. Biodistribution studies revealed that i.v. coadministration of progesterone (10 mg/kg), testosterone (20 mg/kg), or DHEA (20 mg/kg) significantly decreased [(18)F]FPS uptake (%ID/g) by up to 50% in nearly all of eight brain regions examined. [(18)F]FPS uptake in several peripheral organs that express sigma-1 receptors (heart, spleen, muscle, lung) was also reduced (54-85%). These studies clearly demonstrate that exogenously administered steroids can occupy sigma-1 receptors in vivo, and that [(18)F]FPS may provide an effective tool for monitoring sigma-1 receptor occupancy of specific therapeutic steroids during clinical trials.

Imaging sigma receptors: applications in drug development.

Sigma receptors have been implicated in a myriad of cellular functions, biological processes and diseases. While the precise biological functions of sigma receptors have not been elucidated, recent work has shed some light on to these enigmatic systems. Sigma receptors have recently been a target of drug development related to psychiatric and neurological disorders. Sigma ligands have also been shown to modulate endothelial cell proliferation and can control angiogenesis which makes them a promising target for oncology applications. Other areas currently being investigated include treatment of gastrointestinal, cardiovascular, endocrine and immune system disorders. Of interest is that the human sigma-1 receptor gene contains a steroid binding component, and several gonadal steroids, including progesterone, testosterone and dehydroepiandrosterone (DHEA), interact with sigma-1 receptors. Of the steroids examined thus far, progesterone binds with the highest affinity to human sigma-1 receptors, with a reported affinity (Ki) as high as 30 nM while the other steroids exhibit lower affinity. For this and other reasons, sigma-1 receptors have been proposed as a link between the central nervous system and the endocrine and reproductive systems. Taken together, the above information highlights an important yet largely unexplored but promising area of research to examine the biological function and therapeutic potential of sigma receptors. This review provides an overview of the current knowledge of these sites with a focus on specific areas where in vivo sigma receptor imaging is currently being investigated.

In vivo tomographic imaging studies of neurodegeneration and neuroprotection: a review.

Noninvasive tomographic imaging methods including positron emission tomography (PET) and single photon emission computed tomography (SPECT) are extremely sensitive and are capable of measuring biochemical processes that occur at concentrations in the nanomolar range. Inherent to neurodegenerative processes is neuronal loss. Thus, PET or SPECT monitoring of biochemical processes altered by neuronal loss (changes in neurotransmitter turnover, alterations in receptor, transporter or enzyme concentrations) can provide unique information not attainable by other methods. Such imaging techniques can also be used to longtitudinally monitor the effects of neuroprotective treatments. This review highlights current imaging probes used to evaluate patients with specific neurodegenerative disorders (e.g., Alzheimer's Disease, Parkinson's Disease, Huntington's Chorea), including those that image receptors of the dopaminergic, cholinergic and glutamatergic systems. Areas of future research focus are also defined. It is clear that monitoring the progression of neurodegenerative disorders and the impact of neuroprotective treatments are two different but related goals for which noninvasive imaging via PET and SPECT methods plays a powerful and unique role.

Preclinical acute toxicity studies and dosimetry estimates of the novel sigma-1 receptor radiotracer, [18F]SFE.

[(18)F]1-(2-Fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) is a novel, selective, high-affinity sigma-1 receptor radioligand that has been preclinically well characterized in rodents. To support an investigational new drug (IND) application for the first evaluation of [(18)F]SFE in humans, single-organ and whole-body radiation adsorbed doses associated with [(18)F]SFE injection were estimated from rat distribution data. In addition, single- and multiple-dose toxicity studies were conducted in rabbits and in dogs. Multiple-dose toxicity studies in rabbits and single-dose toxicity studies in beagles suggest at least a 100-fold safety margin for humans studies at a mass dose limit of 4.0 mug per intravenous injection, based on the combined no observable adverse effect levels (NOAEL, mg/m(2)) measured in these species. Radiation dosimetry estimates obtained from rat biodistribution analyses of [(18)F]SFE suggest that most tissues would receive about 0.010-0.020 mGy/MBq, while the adrenal glands, brain, bone, liver, lungs, and spleen would receive slightly higher doses (0.024-0.044 mGy/MBq). The adrenal glands were identified as the critical organ, because they received the highest adsorbed radiation dose. The total exposure resulting from a 5 mCi administration of [(18)F]SFE is well below the FDA-defined limits for yearly cumulative and per-study exposures to research participants. These combined results support the expectation that [(18)F]SFE will be safe for use in human positron emission tomography (PET) imaging studies with the administration of 5 mCi and a mass dose equal to or less than 4.0 mug SFE per injection.

[123I]TPCNE--a novel SPET tracer for the sigma-1 receptor: first human studies and in vivo haloperidol challenge.

[123I]TPCNE (1(trans-[123I]iodopropen-2-yl)-4-[(4-cyanophenoxy)methyl] piperidine; Ki = 0.67 nM; log P = 3.36) is a novel sigma-1 receptor SPET ligand. In this study, we developed an optimized labeling method for [123I]TPCNE and investigated the kinetics, binding characteristics, and whole-body distribution of this tracer for the first time in humans. We also performed a challenge with the sigma-1 receptor antagonist haloperidol against [123I]TPCNE. Seven healthy volunteers were recruited. Dynamic brain SPET scans were performed following i.v. administration of 185 MBq [123I]TPCNE in all seven subjects. Three of the subjects were given oral haloperidol (2.5 mg) approximately 1 h before the scan. The dynamic data were analyzed with both reversible and irreversible compartmental models.[123I]TPCNE showed high uptake in brain and liver. All non-haloperidol-treated subjects showed a high whole-brain uptake (average: 8.7% of injected activity). No significant clearance of the tracer was seen up to 30 h post injection. In the haloperidol-treated subjects, the time-activity curves clearly demonstrated clearance of the tracer from the brain. Regional radioactivity concentrations were reduced by haloperidol from 42% in the cerebellum to 73% in the thalamus.[(123)I]TPCNE demonstrated high brain uptake, with highest binding found in the posterior cingulate. A region in which binding was unaffected by haloperidol pretreatment could not be identified, and the time-activity data were best described by an irreversible model.

In vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine as a potential radiotracer for PET assessment of CNS sigma-1 receptors.

INTRODUCTION: Sigma-1 receptors are expressed throughout the mammalian central nervous system (CNS) and are implicated in several psychiatric disorders, including schizophrenia and depression. We have recently evaluated the high-affinity (K(D)=0.5+/-0.2 nM, log P=2.9) sigma-1 receptor radiotracer [(18)F]1-(3-fluoropropyl)-4-(4-cyanophenoxymethyl)piperidine, [(18)F]FPS, in humans. In contrast to appropriate kinetics exhibited in baboon brain, in the human CNS, [(18)F]FPS does not reach pseudoequilibrium by 4 h, supporting the development of a lower-affinity tracer [Waterhouse RN, Nobler MS, Chang RC, Zhou Y, Morales O, Kuwabara H, et al. First evaluation of the sigma-1 receptor radioligand [(18)F]1-3-fluoropropyl-4-((4-cyanophenoxy)-methyl)piperidine ([(18)F]FPS) in healthy humans. Neuroreceptor Mapping 2004, July 15-18th, Vancouver, BC Canada 2004]. We describe herein the in vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) (K(D)=5 nM, log P=2.4), a structurally similar, lower-affinity sigma-1 receptor radioligand. METHODS: [(18)F]SFE was synthesized (n=4) as previously described in good yield (54+/-6% EOB), high specific activity (2.1+/-0.6 Ci/micromol EOS) and radiochemical purity (98+/-1%) and evaluated in awake adult male rats. RESULTS: Similar to [(18)F]FPS, regional brain radioactivity concentrations [percentage of injected dose per gram of tissue (%ID/g), 15 min] for [(18)F]SFE were highest in occipital cortex (1.86+/-0.06 %ID/g) and frontal cortex (1.76+/-0.38 %ID/g), and lowest in the hippocampus (1.01+/-0.02%ID/g). Unlike [(18)F]FPS, [(18)F]SFE cleared from the brain with approximately 40% reduction in peak activity over a 90-min period. Metabolite analysis (1 h) revealed that [(18)F]SFE was largely intact in the brain. Blocking studies showed a large degree (>80%) of saturable binding for [(18)F]SFE in discrete brain regions. CONCLUSIONS: We conclude that [(18)F]SFE exhibits excellent characteristics in vivo and may provide a superior PET radiotracer for human studies due to its faster CNS clearance compared to [(18)F]FPS.

Modulation of amphetamine-induced dopamine release by group II metabotropic glutamate receptor agonist LY354740 in non-human primates studied with positron emission tomography.

Pharmacological evidence suggests that schizophrenia is associated with increased stimulation of dopamine (DA) D2 receptors. Recently, several groups have demonstrated that amphetamine-induced DA release is increased in schizophrenia, providing direct evidence for dysregulation of DA systems in this condition. In healthy volunteers, pretreatment with the noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine increases amphetamine-induced DA release to levels similar to those observed in patients with schizophrenia. Therefore, the dysregulation of DA function observed in schizophrenia might be secondary to NMDA hypofunction. In this study, the regulation of this response by glutamate (GLU) transmission was further characterized by using a metabotropic glutamate (mGlu) receptor group II agonist to inhibit GLU transmission. The amphetamine- (0.5 mg/kg intravenously (i.v.)) induced decrease in [11C]raclopride equilibrium-specific binding (V3'') was measured under control conditions and following pretreatment with the mGlu2/3 receptor agonist LY354740 (20 mg/kg i.v.) in four baboons. Amphetamine reduced [11C]raclopride V3'' by 28+/-7% under control conditions. Following LY354740 pretreatment, amphetamine-induced reduction in [11C]raclopride V3'' was significantly enhanced (35+/-7%, p=0.002). The enhancement of the amphetamine-induced reduction in [11C]raclopride V3'' by LY354740 was not a simple additive effect, as LY354740 alone did not reduce [11C]raclopride V3''. In conclusion, the results of this study further document the involvement of GLU transmission in regulating the effect of amphetamine-induced DA release, and provide additional support to the hypothesis that the dysregulation of DA function revealed by the amphetamine challenge in schizophrenia might stem from a deficit in GLU transmission.

In vivo evaluation of [123I]-4-iodo-N-(4-(4-(2-methoxyphenyl)-piperazin-1-yl)butyl)-benzamide: a potential sigma receptor ligand for SPECT studies.

In this study, in vivo evaluation in mice and rabbits of [123I]-4-iodo-N-(4-(4-(2-methoxyphenyl)-piperazin-1-yl)butyl)-benzamide ([123I]-BPB), a potential radioligand for visualisation of the sigma receptor by single photon emission computed tomography (SPECT), is reported. The compound possesses appropriate lipophilicity (log P=2.2) and binds sigma-1 and sigma-2 receptors (pKi=6.51 and 6.79, respectively). In mice, this new radioiodinated tracer exhibited high brain uptake (4.99% ID/g tissue at 10 min postinjection) and saturable binding (3.06% ID/g tissue at 10 min postinjection) as determined by pretreatment with unlabeled [123I]-BPB. A metabolite study demonstrated no (less than 5%) labeled metabolites in the brain. In rabbits, regional brain distribution was investigated and the tracer displayed high, homogeneous central nervous system uptake. Selectivity was assessed by competition experiments with known sigma ligands. Metabolite analysis showed no (less than 8%) labeled metabolites in the rabbit brain. In conclusion, our findings indicate that [123I]-BPB is not a suitable tracer for visualisation of D3 receptors while its potential for sigma receptor imaging is severely hampered by its affinity for dopamine receptors.

In vivo evaluation of [11C]N-(2-chloro-5-thiomethylphenyl)-N'-(3-methoxy-phenyl)-N'-methylguanidine ([11C]GMOM) as a potential PET radiotracer for the PCP/NMDA receptor.

The development of imaging methods to measure changes in NMDA ion channel activation would provide a powerful means to probe the mechanisms of drugs and device based treatments (e.g., ECT) thought to alter glutamate neurotransmission. To provide a potential NMDA/PCP receptor PET tracer, we synthesized the radioligand [11C]GMOM (ki = 5.2 +/-0.3 nM; log P = 2.34) and evaluated this ligand in vivo in awake male rats and isoflurane anesthetized baboons. In rats, the regional brain uptake of [11C]GMOM ranged from 0.75+/-0.13% ID/g in the medulla and pons to 1.15+/-0.17% ID/g in the occipital cortex. MK801 (1 mg/kg i.v.) significantly reduced (24-28%) [11C]GMOM uptake in all regions. D-serine (10 mg/kg i.v.) increased [11C]GMOM %ID/g values in all regions (10-24%) reaching significance in the frontal cortex and cerebellum only. The NR2B ligand RO 25-6981 (10 mg/kg i.v.) reduced [11C]GMOM uptake significantly (24-38%) in all regions except for the cerebellum and striatum. Blood activity was 0.11+/-0.03 %ID/g in the controls group and did not vary significantly across groups. PET imaging in isoflurane-anesthetized baboons with high specific activity [11C]GMOM provided fairly uniform regional brain distribution volume (VT) values (12.8-17.1 ml g(-1)). MK801 (0.5 mg/kg, i.v., n = 1, and 1.0 mg/kg, i.v., n = 1) did not significantly alter regional VT values, indicating a lack of saturable binding. However, the potential confounding effects associated with ketamine induction of anesthesia along with isoflurane maintenance must be considered because both agents are known to reduce NMDA ion channel activation. Future and carefully designed studies, presumably utilizing an optimized NMDA/PCP site tracer, will be carried out to further explore these hypotheses. We conclude that, even though [11C]GMOM is not an optimized PCP site radiotracer, its binding is altered in vivo in awake rats as expected by modulation of NMDA ion channel activity by MK801, D-serine or RO 25-6981. The development of higher affinity NMDA/PCP site radioligands is in progress.