Person:

Hooker, Jacob

The unnatural isotope fluorine-18 ((^{18}F)) is used as a positron emitter in molecular imaging. Currently, many potentially useful (^{18}F)-labeled probe molecules are inaccessible for imaging because no fluorination chemistry is available to make them. The 110-minute half-life of (^{18}F) requires rapid syntheses for which ([^{18}F])fluoride is the preferred source of fluorine because of its practical access and suitable isotope enrichment. However, conventional ([^{18}F])fluoride chemistry has been limited to nucleophilic fluorination reactions. We report the development of a palladium-based electrophilic fluorination reagent derived from fluoride and its application to the synthesis of aromatic (^{18}F)-labeled molecules via late-stage fluorination. Late-stage fluorination enables the synthesis of conventionally unavailable positron emission tomography (PET) tracers for anticipated applications in pharmaceutical development as well as preclinical and clinical PET imaging.

Local prefrontal dopamine signaling supports working memory by tuning pyramidal neurons to task-relevant stimuli. Enabled by simultaneous positron emission tomography–magnetic resonance imaging (PET-MRI), we determined whether neuromodulatory effects of dopamine scale to the level of cortical networks and coordinate their interplay during working memory. Among network territories, mean cortical D1 receptor densities differed substantially but were strongly interrelated, suggesting cross-network regulation. Indeed, mean cortical D1 density predicted working memory–emergent decoupling of the frontoparietal and default networks, which respectively manage task-related and internal stimuli. In contrast, striatal D1 predicted opposing effects within these two networks but no between-network effects. These findings specifically link cortical dopamine signaling to network crosstalk that redirects cognitive resources to working memory, echoing neuromodulatory effects of D1 signaling on the level of cortical microcircuits.

Fatty acid amide hydrolase (FAAH) regulates endocannabinoid signaling. [11C]CURB, an irreversibly binding FAAH inhibitor, has been developed for clinical research imaging with PET. However, no fluorine-18 labeled radiotracer for FAAH has yet advanced to human studies. [18F]DOPP ([18F]3-(4,5-dihydrooxazol-2-yl)phenyl (5-fluoropentyl)carbamate) has been identified as a promising 18F-labeled analogue based on rodent studies. The goal of this work is to evaluate [18F]DOPP in nonhuman primates to support its clinical translation. High specific activity [18F]DOPP (5–6 Ci·μmol–1) was administered intravenously (iv) to three baboons (2M/1F, 3–4 years old). The distribution and pharmacokinetics were quantified following a 2 h dynamic imaging session using a simultaneous PET/MR scanner. Pretreatment with the FAAH-selective inhibitor, URB597, was carried out at 200 or 300 μg/kg iv, 10 min prior to [18F]DOPP administration. Rapid arterial blood sampling for the first 3 min was followed by interval sampling with metabolite analysis to provide a parent radiotracer plasma input function that indicated ∼95% baseline metabolism at 60 min and a reduced rate of metabolism after pretreatment with URB597. Regional distribution data were analyzed with 1-, 2-, and 3-tissue compartment models (TCMs), with and without irreversible trapping since [18F]DOPP covalently links to the active site of FAAH. Consistent with previous findings for [11C]CURB, the 2TCM with irreversible binding was found to provide the best fit for modeling the data in all regions. The composite parameter λk3 was therefore used to evaluate whole brain (WB) and regional binding of [18F]DOPP. Pretreatment studies showed inhibition of λk3 across all brain regions (WB baseline: 0.112 mL/cm3/min; 300 μg/kg URB597: 0.058 mL/cm3/min), suggesting that [18F]DOPP binding is specific for FAAH, consistent with previous rodent data.

Salvia divinorum, a mint plant originally used by the Mazatecs of Oaxaca, Mexico in spiritual rituals has gained popularity, in smoked form, as a legal hallucinogen in the United States and Europe. Abuse results in rapid onset and short-lasting effects that include visual hallucinations and motor-function impairment. Salvinorin A, the psychoactive component of S. divinorum, is a uniquely potent agonist at κ-opioid receptors, targets for new therapeutic drugs. We labeled salvinorin A with C-11 by acylation of salvinorin B with [11C]-acetyl chloride to study whether its kinetic behavior in the brain parallels its uniquely fast, yet brief physiological effects. Positron emission tomography (PET) studies performed in 6 adult female baboons indicated extremely rapid brain uptake reaching a peak accounting for 3.3% of the total administered dose in 40 s and clearing with a half-life of 8 min. [11C]-salvinorin A was distributed throughout the brain with the highest concentration in the cerebellum and a notable concentration in the visual cortex, perhaps accounting for its physiological effects when smoked. Naloxone administration did not reduce the overall concentration of [11C]-salvinorin A significantly nor did it change its regional distribution. Peripheral organ kinetics suggested at least two modes of metabolism and excretion occur: through the renal and biliary systems. Our findings have revealed that the exceptionally rapid uptake and brief duration of salvinorin A in the brain match the time-course of visual hallucinations for S. divinorum when smoked. The effects of salvinorin A may occur at < 10 μg in the human brain, emphasizing its remarkable potency.

Carbon-11 labelled carbon dioxide is the cyclotron-generated feedstock reagent for most positron emission tomography (PET) tracers using this radionuclide. Most carbon-11 labels, however, are installed using derivative reagents generated from [11C]CO2. In recent years, [11C]CO2 has seen a revival in applications for the direct incorporation of carbon-11 into functional groups such as ureas, carbamates, oxazolidinones, carboxylic acids, esters, and amides. This review summarizes classical [11C]CO2 fixation strategies using organometallic reagents and then focuses on newly developed methods that employ strong organic bases to reversibly capture [11C]CO2 into solution, thereby enabling highly functionalized labelled compounds to be prepared. Labelled compounds and radiopharmaceuticals that have been translated to the clinic are highlighted.

A PET project: A powerful reagent for the synthesis of positron-emitting imaging molecules—[11C]formaldehyde—is accessible from [11C]methyl iodide and trimethylamine N-oxide (TMAO) in high yields and under mild conditions. Easy access to [11C]formaldehyde expands the scope of the carbon-11 toolbox and will lead to new reaction methodology and imaging compounds.

A wide range of central nervous system (CNS) disorders, particularly those related to sleep, are associated with the abnormal function of orexin (OX) receptors. Several orexin receptor antagonists have been reported in recent years, but currently there are no imaging tools to probe the density and function of orexin receptors in vivo. To date there are no published data on the pharmacokinetics (PK) and accumulation of some lead orexin receptor antagonists. Evaluation of CNS pharmacokinetics in the pursuit of positron emission tomography (PET) radiotracer development could be used to elucidate the association of orexin receptors with diseases and to facilitate the drug discovery and development. To this end, we designed and evaluated carbon-11 labeled compounds based on diazepane orexin receptor antagonists previously described. One of the synthesized compounds, [11C]CW4, showed high brain uptake in rats and further evaluated in non-human primate (NHP) using PET-MR imaging. PET scans performed in a baboon showed appropriate early brain uptake for consideration as a radiotracer. However, [11C]CW4 exhibited fast kinetics and high nonspecific binding, as determined after co-administration of [11C]CW4 and unlabeled CW4. These properties indicate that [11C]CW4 has excellent brain penetrance and could be used as a lead compound for developing new CNS-penetrant PET imaging probes of orexin receptors.

[3H]CI-994, a radioactive isotopologue of the benzamide CI-994, a class I histone deacetylase inhibitor (HDACi), was evaluated as an autoradiography probe for ex vivo labeling and localizing of class I HDAC (isoforms 1–3) in the rodent brain. After protocol optimization, up to 80% of total binding was attributed to specific binding. Notably, like other benzamide HDACi, [3H]CI-994 exhibits slow binding kinetics when measured in vitro with isolated enzymes and ex vivo when used for autoradiographic mapping of HDAC1–3 density. The regional distribution and density of HDAC1–3 was determined through a series of saturation and kinetics experiments. The binding properties of [3H]CI-994 to HDAC1–3 were characterized and the data were used to determine the regional Bmax of the target proteins. Kd values, determined from slice autoradiography, were between 9.17 and 15.6 nM. The HDAC1–3 density (Bmax), averaged over whole brain sections, was of 12.9 picomol · mg−1 protein. The highest HDAC1–3 density was found in the cerebellum, followed by hippocampus and cortex. Moderate to low receptor density was found in striatum, hypothalamus and thalamus. These data were correlated with semi-quantitative measures of each HDAC isoform using western blot analysis and it was determined that autoradiographic images most likely represent the sum of HDAC1, HDAC2, and HDAC3 protein density. In competition experiments, [3H]CI-994 binding can be dose-dependently blocked with other HDAC inhibitors, including suberoylanilide hydroxamic acid (SAHA). In summary, we have developed the first known autoradiography tool for imaging class I HDAC enzymes. Although validated in the CNS, [3H]CI-994 will be applicable and beneficial to other target tissues and can be used to evaluate HDAC inhibition in tissues for novel therapies being developed. [3H]CI-994 is now an enabling imaging tool to study the relationship between diseases and epigenetic regulation.

This study employed simultaneous neuroimaging with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to demonstrate the relationship between changes in receptor occupancy measured by PET and changes in brain activity inferred by fMRI. By administering the D2/D3 dopamine receptor antagonist [11C]raclopride at varying specific activities to anesthetized nonhuman primates, we mapped associations between changes in receptor occupancy and hemodynamics [cerebral blood volume (CBV)] in the domains of space, time, and dose. Mass doses of raclopride above tracer levels caused increases in CBV and reductions in binding potential that were localized to the dopamine-rich striatum. Moreover, similar temporal profiles were observed for specific binding estimates and changes in CBV. Injection of graded raclopride mass doses revealed a monotonic coupling between neurovascular responses and receptor occupancies. The distinct CBV magnitudes between putamen and caudate at matched occupancies approximately matched literature differences in basal dopamine levels, suggesting that the relative fMRI measurements reflect basal D2/D3 dopamine receptor occupancy. These results can provide a basis for models that relate dopaminergic occupancies to hemodynamic changes in the basal ganglia. Overall, these data demonstrate the utility of simultaneous PET/fMRI for investigations of neurovascular coupling that correlate neurochemistry with hemodynamic changes in vivo for any receptor system with an available PET tracer.

This report describes a multi-receptor physiological model of the fMRI temporal response and signal magnitude evoked by drugs that elevate synaptic dopamine in basal ganglia. The model is formulated as a summation of dopamine's effects at D1-like and D2-like receptor families, which produce functional excitation and inhibition, respectively, as measured by molecular indicators like adenylate cyclase or neuroimaging techniques like fMRI. Functional effects within the model are described in terms of relative changes in receptor occupancies scaled by receptor densities and neuro-vascular coupling constants. Using literature parameters, the model reconciles many discrepant observations and interpretations of pre-clinical data. Additionally, we present data showing that amphetamine stimulation produces fMRI inhibition at low doses and a biphasic response at higher doses in the basal ganglia of non-human primates (NHP), in agreement with model predictions based upon the respective levels of evoked dopamine. Because information about dopamine release is required to inform the fMRI model, we simultaneously acquired PET 11C-raclopride data in several studies to evaluate the relationship between raclopride displacement and assumptions about dopamine release. At high levels of dopamine release, results suggest that refinements of the model will be required to consistently describe the PET and fMRI data. Overall, the remarkable success of the model in describing a wide range of preclinical fMRI data indicate that this approach will be useful for guiding the design and analysis of basic science and clinical investigations and for interpreting the functional consequences of dopaminergic stimulation in normal subjects and in populations with dopaminergic neuroadaptations.