Rapid 18F-labeling via Pd-catalyzed S-arylation in aqueous medium

We report radiolabeling of thiol-containing substrates via Pd-catalyzed S-arylation with 2-[¹⁸F]fluoro-5-iodopyridine, which is readily accessible using the "minimalist" radiofluorination method. The practicality of the procedure was confirmed by preparation of a novel PSMA-specific PET-tracer as well as labeling of glutathione, Aβ oligomer-binding RD2 peptide, bovine serum albumin and PSMA I&S.

Towards chronic deep brain stimulation in freely moving hemiparkinsonian rats: Applicability and functionality of a fully implantable stimulation system

Objective This study aimed at investigating a novel fully implantable deep brain stimulation system and its ability to modulate brain metabolism and behavior through subthalamic nucleus stimulation in a hemiparkinsonian rat model. Approach Twelve male rats were unilaterally lesioned with 6-hydroxydopamine in the medial forebrain bundle and received a fully implantable deep brain stimulation system aiming at the ipsilesional subthalamic nucleus. Each rat underwent three cylinder tests to analyze front paw use: A PRE test before any surgical intervention, an OFF test after surgery but before stimulation onset and an ON test under deep brain stimulation. To visualize brain glucose metabolism in the awake animal, two [18F]FDG scans were conducted in the OFF and ON condition. At least four weeks after surgery, an [18F]FDOPA scan was used to check for dopaminergic integrity. Main results In general, STN DBS increased [18F]FDG uptake ipsilesionally and decreased it contralesionally. More specifically, bilateral orbitofrontal cortex, ipsilateral caudate putamen, sensorimotor cortex and nucleus accumbens showed significantly higher tracer uptake in ON compared to OFF condition. Contralateral cingulate and secondary motor cortex, caudate putamen, amygdala, hippocampus, retrosplenial granular cortex, superior colliculus, and parts of the cerebellum exhibited significantly higher [18F]FDG uptake in the OFF condition. On the behavioral level, stimulation was able improve use of the contralesional affected front paw suggesting an effective stimulation produced by the implanted system. Significance The fully implantable stimulation system developed by us and presented here offers the output of arbitrary user-defined waveforms, patterns and stimulation settings and allows tracer accumulation in freely moving animals. It is therefore a suitable device for implementing behavioral PET studies. It contributes immensely to the possibilities to characterize and unveil the effects and mechanisms of deep brain stimulation offering valuable clues for future improvements of this therapy.

Finding New Communities: A Principle of Neuronal Network Reorganization in Alzheimer's Disease

Background: Graph-theoretical analyses have been previously used to investigate changes in the functional connectome in patients with Alzheimer's disease (AD). However, these analyses generally assume static organizational principles, thereby neglecting a fundamental reconfiguration of functional connections in the face of neurodegeneration. Methods: Here, we focus on differences in the community structure of the functional connectome in young and old individuals and patients with AD. Patients with AD, moreover, underwent molecular imaging positron emission tomography by using [18F]AV1451 to measure tau burden, a major hallmark of AD. Results: Although the overall organizational principles of the community structure of the human functional connectome were preserved even in advanced healthy aging, they were considerably changed in AD. We discovered that the communities in AD are re-organized, with nodes changing their allegiance to communities, thus resulting in an overall less efficient re-organized community structure. We further discovered that nodes with a tendency to leave the communities displayed a relatively higher tau pathology burden. Discussion: Together, this study suggests that local tau pathology in AD is associated to fundamental changes in basic organizational principles of the human connectome. Our results shed new light on previous findings obtained by using the graph theory in AD and imply a general principle of the brain in response to neurodegeneration.

Design, synthesis and biological evaluation of Tozadenant analogues as adenosine A2A receptor ligands

With the aim to obtain potent adenosine A_2A receptor (A_2AR) ligands, a series of eighteen derivatives of 4-hydroxy-N-(4-methoxy-7-morpholin-4-yl-1,3-benzo[d]thiazol-2-yl)-4-methylpiperidine-1-carboxamide (SYN-115, Tozadenant) were designed and synthesized. The target compounds were obtained by a chemical building block principle that involved reaction of the appropriate aminobenzothiazole phenyl carbamates with either commercially available or readily synthesized functionalized piperidines. Their affinity and subtype selectivity with regard to human adenosine A₁-and A_2A receptors were determined using radioligand binding assays. K_i values for human A_2AR ranged from 2.4 to 38 nM, with more than 120-fold selectivity over A₁ receptors for all evaluated compounds except 13k which had a K_i of 361 nM and 18-fold selectivity. The most potent fluorine-containing derivatives 13e, 13g and 13l exhibited K_i values of 4.9 nM, 3.6 nM and 2.8 nM for the human A_2AR. Interestingly, the corresponding values for rat A_2AR were found to be four to five times higher. Their binding to A_2AR was further confirmed by radiolabeling with ¹⁸F and in vitro autoradiography in rat brain slices, which showed almost exclusive striatal binding and complete displacement by the A_2AR antagonist ZM 241385. We conclude that these compounds represent potential candidates for the visualization of the A_2A receptor and open pathways to novel therapeutic treatments of neurodegenerative disorders or cancer.

Comparison of [18F]Fluoroethyltyrosine PET and Sodium MRI in Cerebral Gliomas: a Pilot Study

PURPOSE

Positron emission tomography (PET) using O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) improves the diagnostics of cerebral gliomas compared with conventional magnetic resonance imaging (MRI). Sodium MRI is an evolving method to assess tumor metabolism. In this pilot study, we explored the relationship of [¹⁸F]FET-PET and sodium MRI in patients with cerebral gliomas in relation to the mutational status of the enzyme isocitrate dehydrogenase (IDH).

PROCEDURES

Ten patients with untreated cerebral gliomas and one patient with a recurrent glioblastoma (GBM) were investigated by dynamic [¹⁸F]FET-PET and sodium MRI using an enhanced simultaneous single-quantum- and triple-quantum-filtered imaging of ²³Na (SISTINA) sequence to estimate total (NaT), weighted non-restricted (NaNR, mainly extracellular), and restricted (NaR, mainly intracellular) sodium in tumors and normal brain tissue. [¹⁸F]FET uptake and sodium parameters in tumors with a different IDH mutational status were compared. After biopsy or resection, histology and the IDH mutational status were determined neuropathologically.

RESULTS

NaT (p = 0.05), tumor-to-brain ratios (TBR) of NaT (p = 0.02), NaNR (p = 0.003), and the ratio of NaT/NaR (p < 0.001) were significantly higher in IDH-mutated than in IDH-wild-type gliomas (n = 5 patients each) while NaR was significantly lower in IDH-mutated gliomas (p = 0.01). [¹⁸F]FET parameters (TBR, time-to-peak) were not predictive of IDH status in this small cohort of patients. There was no obvious relationship between sodium distribution and [¹⁸F]FET uptake. The patient with a recurrent GBM exhibited an additional radiation injury with strong abnormalities in sodium MRI.

CONCLUSIONS

Sodium MRI appears to be more strongly related to the IDH mutational status than are [¹⁸F]FET-PET parameters. A further evaluation of the combination of the two methods in a larger group of high- and low-grade gliomas seems promising.

Excitatory-inhibitory balance within EEG microstates and resting-state fMRI networks: assessed via simultaneous trimodal PET-MR-EEG imaging

The symbiosis of neuronal activities and glucose energy metabolism is reflected in the generation of functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) signals. However, their association with the balance between neuronal excitation and inhibition (E/I-B), which is closely related to the activities of glutamate and γ-aminobutyric acid (GABA) and the receptor availability (RA) of GABAA and mGluR5, remains unexplored. This research investigates these associations during the resting state (RS) condition using simultaneously recorded PET/MR/EEG (trimodal) data. The trimodal data were acquired from three studies using different radio-tracers such as, [11C]ABP688 (ABP) (N = 9), [11C]Flumazenil (FMZ) (N = 10) and 2-[18F]fluoro-2-deoxy-D-glucose (FDG) (N = 10) targeted to study the mGluR5, GABAA receptors and glucose metabolism respectively. Glucose metabolism and neuroreceptor binding availability (non-displaceable binding potential (BPND)) of GABAA and mGluR5 were found to be significantly higher and closely linked within core resting-state networks (RSNs). The neuronal generators of EEG microstates and the fMRI measures were most tightly associated with the BPND of GABAA relative to mGluR5 BPND and the glucose metabolism, emphasising a predominance of inhibitory processes within in the core RSNs at rest. Changes in the neuroreceptors leading to an altered coupling with glucose metabolism may render the RSNs vulnerable to psychiatric conditions. The paradigm employed here will likely help identify the precise neurobiological mechanisms behind these alterations in fMRI functional connectivity and EEG oscillations, potentially benefitting individualised healthcare treatment measures.

Nuclear Medicine in Times of COVID-19: How Radiopharmaceuticals Could Help to Fight the Current and Future Pandemics

The emergence and global spread of COVID-19, an infectious disease caused by the novel coronavirus SARS-CoV-2, has resulted in a continuing pandemic threat to global health. Nuclear medicine techniques can be used for functional imaging of (patho)physiological processes at the cellular or molecular level and for treatment approaches based on targeted delivery of therapeutic radionuclides. Ongoing development of radiolabeling methods has significantly improved the accessibility of radiopharmaceuticals for in vivo molecular imaging or targeted radionuclide therapy, but their use for biosafety threats such as SARS-CoV-2 is restricted by the contagious nature of these agents. Here, we highlight several potential uses of nuclear medicine in the context of SARS-CoV-2 and COVID-19, many of which could also be performed in laboratories without dedicated containment measures. In addition, we provide a broad overview of experimental or repurposed SARS-CoV-2-targeting drugs and describe how radiolabeled analogs of these compounds could facilitate antiviral drug development and translation to the clinic, reduce the incidence of late-stage failures and possibly provide the basis for radionuclide-based treatment strategies. Based on the continuing threat by emerging coronaviruses and other pathogens, it is anticipated that these applications of nuclear medicine will become a more important part of future antiviral drug development and treatment.

On the consensus nomenclature rules for radiopharmaceutical chemistry - Reconsideration of radiochemical conversion

Radiochemical conversion is an important term to be included in the "Consensus nomenclature rules for radiopharmaceutical chemistry". Radiochemical conversion should be used to define reaction efficiency by measuring the transformation of components in a crude reaction mixture at a given time, whereas radiochemical yield is better suited to define the efficiency of an entire reaction process including, for example, separation, isolation, filtration, and formulation.

Entorhinal Tau Predicts Hippocampal Activation and Memory Deficits in Alzheimer's Disease

BACKGROUND

To date, it remains unclear how amyloid plaques and neurofibrillary tangles are related to neural activation and, consequently, cognition in Alzheimer's disease (AD). Recent findings indicate that tau accumulation may drive hippocampal hyperactivity in cognitively normal aging, but it remains to be elucidated how tau accumulation is related to neural activation in AD.

OBJECTIVE

To determine whether the association between tau accumulation and hippocampal hyperactivation persists in mild cognitive impairment (MCI) and mild dementia or if the two measures dissociate with disease progression, we investigated the relationship between local tau deposits and memory-related neural activation in MCI and mild dementia due to AD.

METHODS

Fifteen patients with MCI or mild dementia due to AD underwent a neuropsychological assessment and performed an item memory task during functional magnetic resonance imaging. Cerebral tau accumulation was assessed using positron emission tomography and [18F]-AV-1451.

RESULTS

Entorhinal, but not global tau accumulation, was highly correlated with hippocampal activation due to visual item memory encoding and predicted memory loss over time. Neural activation in the posterior cingulate cortex and the fusiform gyrus was not significantly correlated with tau accumulation.

CONCLUSION

These findings extend previous observations in cognitively normal aging, demonstrating that entorhinal tau continues to be closely associated with hippocampal hyperactivity and memory performance in MCI and mild dementia due to AD. Furthermore, data suggest that this association is strongest in medial temporal lobe structures. In summary, our data provide novel insights into the relationship of tau accumulation to neural activation and memory in AD.

Assessment of 18F-PI-2620 as a Biomarker in Progressive Supranuclear Palsy

Importance

Progressive supranuclear palsy (PSP) is a 4-repeat tauopathy. Region-specific tau aggregates establish the neuropathologic diagnosis of definite PSP post mortem. Future interventional trials against tau in PSP would strongly benefit from biomarkers that support diagnosis.

Objective

To investigate the potential of the novel tau radiotracer 18F-PI-2620 as a biomarker in patients with clinically diagnosed PSP.

Design, Setting, and Participants

In this cross-sectional study, participants underwent dynamic 18F-PI-2620 positron emission tomography (PET) from 0 to 60 minutes after injection at 5 different centers (3 in Germany, 1 in the US, and 1 in Australia). Patients with PSP (including those with Richardson syndrome [RS]) according to Movement Disorder Society PSP criteria were examined together with healthy controls and controls with disease. Four additionally referred individuals with PSP-RS and 2 with PSP-non-RS were excluded from final data analysis owing to incomplete dynamic PET scans. Data were collected from December 2016 to October 2019 and were analyzed from December 2018 to December 2019.

Main Outcomes and Measures

Postmortem autoradiography was performed in independent PSP-RS and healthy control samples. By in vivo PET imaging, 18F-PI-2620 distribution volume ratios were obtained in globus pallidus internus and externus, putamen, subthalamic nucleus, substantia nigra, dorsal midbrain, dentate nucleus, dorsolateral, and medial prefrontal cortex. PET data were compared between patients with PSP and control groups and were corrected for center, age, and sex.

Results

Of 60 patients with PSP, 40 (66.7%) had RS (22 men [55.0%]; mean [SD] age, 71 [6] years; mean [SD] PSP rating scale score, 38 [15]; score range, 13-71) and 20 (33.3%) had PSP-non-RS (11 men [55.0%]; mean [SD] age, 71 [9] years; mean [SD] PSP rating scale score, 24 [11]; score range, 11-41). Ten healthy controls (2 men; mean [SD] age, 67 [7] years) and 20 controls with disease (of 10 [50.0%] with Parkinson disease and multiple system atrophy, 7 were men; mean [SD] age, 61 [8] years; of 10 [50.0%] with Alzheimer disease, 5 were men; mean [SD] age, 69 [10] years). Postmortem autoradiography showed blockable 18F-PI-2620 binding in patients with PSP and no binding in healthy controls. The in vivo findings from the first large-scale observational study in PSP with 18F-PI-2620 indicated significant elevation of tracer binding in PSP target regions with strongest differences in PSP vs control groups in the globus pallidus internus (mean [SD] distribution volume ratios: PSP-RS, 1.21 [0.10]; PSP-non-RS, 1.12 [0.11]; healthy controls, 1.00 [0.08]; Parkinson disease/multiple system atrophy, 1.03 [0.05]; Alzheimer disease, 1.08 [0.06]). Sensitivity and specificity for detection of PSP-RS vs any control group were 85% and 77%, respectively, when using classification by at least 1 positive target region.

Conclusions and Relevance

This multicenter evaluation indicates a value of 18F-PI-2620 to differentiate suspected patients with PSP, potentially facilitating more reliable diagnosis of PSP.

Bolus infusion scheme for the adjustment of steady state [11C]Flumazenil levels in the grey matter and in the blood plasma for neuroreceptor imaging

The use of hybrid PET/MR imaging facilitates the simultaneous investigation of challenge-related changes in ligand binding to neuroreceptors using PET, while concurrently measuring neuroactivation or blood flow with MRI. Having attained a steady state of the PET radiotracer using a bolus-infusion protocol, it is possible to observe alterations in ligand neuroreceptor binding through changes in distribution volumes. Here, we present an iterative procedure for establishing an administration scheme to obtain steady state [¹¹C]flumazenil concentrations in grey matter in the human brain. In order to achieve a steady state in the shortest possible time, the bolus infusion ratio from a previous examination was adapted to fit the subsequent examination. 17 male volunteers were included in the study. Boli and infusions with different weightings were given to the subjects and were characterised by kbol values from 74 ​min down to 42 ​min. Metabolite analysis was used to ascertain the value of unmetabolised flumazenil in the plasma, and PET imaging was used to assess its binding in the grey matter. The flumazenil time-activity curves (TACs) in the brain were decomposed into activity contributions from pure grey and white matter and analysed for 12 ​vol of interest (VOIs). The curves highlighted a large variability in metabolic rates between the subjects, with kbol ​= ​54.3 ​min being a reliable value to provide flumazenil equilibrium conditions in the majority of the VOIs and cases. The distribution volume of flumazenil in all 12 VOIs was determined.

Thermochromatographic separation of 45Ti and subsequent radiosynthesis of [45Ti]salan

Due to its favorable decay properties, the non-standard radionuclide 45Ti is a promising PET isotope for tumor imaging. Additionally, titanium complexes are widely used as anti-tumor agents and 45Ti could be used to study their in vivo distribution and metabolic fate. However, although 45Ti can be obtained using the 45Sc(p,n)45Ti nuclear reaction its facile production is offset by the high oxophilicity and hydrolytic instability of Ti4+ ions in aqueous solutions, which complicate recovery from the irradiated Sc matrix. Most available 45Ti recovery procedures rely on ion exchange chromatography or solvent extraction techniques which are time-consuming, produce large final elution volumes, or, in case of solvent extraction, cannot easily be automated. Thus a more widespread application of 45Ti for PET imaging has been hampered. Here, we describe a novel, solvent-free approach for recovery of 45Ti that involves formation of [45Ti]TiCl4 by heating of an irradiated Sc target in a gas stream of chlorine, followed by thermochromatographic separation of the volatile radiometal chloride from co-produced scandium chloride and trapping of [45Ti]TiCl4 in a glass vial at − 78 °C. The recovery of 45Ti amounted to 76 ± 5% (n = 5) and the radionuclidic purity was determined to be > 99%. After trapping, the [45Ti]TiCl4 could be directly used for 45Ti-radiolabeling, as demonstrated by the successful radiosynthesis of [45Ti][Ti(2,4-salan)].

[18F]-JK-PSMA-7 PET/CT Under Androgen Deprivation Therapy in Advanced Prostate Cancer

PURPOSE

PSMA imaging is frequently used for monitoring of androgen deprivation therapy (ADT) in prostate cancer. In a previous study, [¹⁸F]-JK-PSMA-7 exhibited favorable properties for tumor localization after biochemical recurrence. In this retrospective study, we evaluated the performance of [¹⁸F]-JK-PSMA-7 under ADT.

PROCEDURES

We examined the performance of [¹⁸F]-JK-PSMA-7 in 70 patients (first cohort) with increasing or detectable PSA values under ADT (PSA < 2 ng/ml for 21/70 patients). We further analyzed 58 independent patients with PSA levels < 2 ng/ml under ADT, who were imaged with [⁶⁸Ga]PSMA-11 or [¹⁸F]DCFPyL (second cohort). Finally, we compared detection rates between [¹⁸F]-JK-PSMA-7, [⁶⁸Ga]PSMA-11, and [¹⁸F]DCFPyL.

RESULTS

In the first cohort, we detected [¹⁸F]-JK-PSMA-7-positive lesions in 63/70 patients. In patients with PSA levels ≥ 2 ng/ml, the detection rate was 100 % (49/49). In patients with PSA < 2 ng/ml, the detection rate was significantly lower (66.7 %, 14/21, p = 9.7 × 10^-5) and dropped from 85.7 % (12/14, PSA levels between 0.3 and 2.0 ng/ml) to 28.6 % (2/7) for PSA levels < 0.3 ng/ml (p = 1.73 × 10^-2). In the second cohort (PSA < 2 ng/ml), the detection rate was 79.3 % (46/58) for [⁶⁸Ga]PSMA-11 or [¹⁸F]DCFPyL. Again, the detection rate was significantly higher (p = 1.1 × 10^-2) for patients with PSA levels between 0.3 and 2.0 ng/ml (87.0 %, 40/46) relative to those with PSA levels < 0.3 ng/ml (50 %, 6/12). No significant difference was found between [¹⁸F]-JK-PSMA-7 and [⁶⁸Ga]PSMA-11 or [¹⁸F]DCFPyL in patients with PSA levels < 2 ng/ml (p = 0.4295).

CONCLUSION

[¹⁸F]-JK-PSMA-7 PET showed a high detection rate in patients with PSA levels ≥ 0.3 ng/ml under ADT. The lower PSA threshold of 0.3 ng/ml for high detection rates was consistent across the three PSMA ligands. Thus, PSMA imaging is suitable for clinical follow-up of patients with increasing PSA levels under ADT.

11C- and 18F-labelled tryptophans as PET-tracers for imaging of altered tryptophan metabolism in age-associated disorders

The ageing of the world’s population is the result of increased life expectancy observed in almost all countries throughout the world. Consequently, a rising tide of ageing-associated disorders, like cancer and neurodegenerative diseases, represents one of the main global challenges of the 21st century. The ability of mankind to overcome these challenges is directly dependent on the capability to develop novel methods for therapy and diagnosis of age-associated diseases. One hallmark of age-related pathologies is an altered tryptophan metabolism. Numerous pathological processes including neurodegenerative and neurological diseases like epilepsy, Parkinson’s and Alzheimer’s diseases, cancer and diabetes exhibit marked changes in tryptophan metabolism. Visualization of key processes of tryptophan metabolic pathways, especially using positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI, can be exploited to early detect the aforementioned disorders with considerable accuracy, allowing appropriate and timely treatment of patients. Here we review the published 11C- and 18F-labelled tryptophans with respect to the production and also preclinical and clinical evaluation as PET-tracers for visualization of different branches of tryptophan metabolism.

The bibliography includes 159 references.

Dnmt3a2/Dnmt3L Overexpression in the Dopaminergic System of Mice Increases Exercise Behavior through Signaling Changes in the Hypothalamus

Dnmt3a2, a de novo DNA methyltransferase, is induced by neuronal activity and participates in long-term memory formation with the increased expression of synaptic plasticity genes. We wanted to determine if Dnmt3a2 with its partner Dnmt3L may influence motor behavior via the dopaminergic system. To this end, we generated a mouse line, Dnmt3a2/3L^Dat/wt, with dopamine transporter (DAT) promotor driven Dnmt3a2/3L overexpression. The mice were studied with behavioral paradigms (e.g., cylinder test, open field, and treadmill), brain slice patch clamp recordings, ex vivo metabolite analysis, and in vivo positron emission tomography (PET) using the dopaminergic tracer 6-[¹⁸F]FMT. The results showed that spontaneous activity and exercise performance were enhanced in Dnmt3a2/3L^Dat/wt mice compared to Dnmt3a2/3L^wt/wt controls. Dopaminergic substantia nigra pars compacta neurons of Dnmt3a2/3L^Dat/wt animals displayed a higher fire frequency and excitability. However, dopamine concentration was not increased in the striatum, and dopamine metabolite concentration was even significantly decreased. Striatal 6-[¹⁸F]FMT uptake, reflecting aromatic L-amino acid decarboxylase activity, was the same in Dnmt3a2/3L^Dat/wt mice and controls. [¹⁸F]FDG PET showed that hypothalamic metabolic activity was tightly linked to motor behavior in Dnmt3a2/3L^Dat/wt mice. Furthermore, dopamine biosynthesis and motor-related metabolic activity were correlated in the hypothalamus. Our findings suggest that Dnmt3a2/3L, when overexpressed in dopaminergic neurons, modulates motor performance via activation of the nigrostriatal pathway. This does not involve increased dopamine synthesis.

Preparation of labeled aromatic amino acids via late-stage 18F-fluorination of chiral nickel and copper complexes

A general protocol for the preparation of 18F-labeled AAAs and α-methyl-AAAs applying alcohol-enhanced Cu-mediated radiofluorination of Bpin-substituted chiral complexes using Ni/Cu-BPX templates as double protecting groups is reported. The chiral auxiliaries are easily accessible from commercially available starting materials in a few synthetic steps. The versatility of the method was demonstrated by the high-yielding preparation of a series of [18F]F-AAAs and the successful implementation of the protocol into automated radiosynthesis modules.

Radiosynthesis and evaluation of 18F-labeled dopamine D4-receptor ligands

INTRODUCTION

The dopamine D₄ receptor (D₄R) has attracted considerable attention as potential target for the treatment of a broad range of central nervous system disorders. Although many efforts have been made to improve the performance of putative radioligand candidates, there is still a lack of D₄R selective tracers suitable for in vivo PET imaging. Thus, the objective of this work was to develop a D₄-selective PET ligand for clinical applications.

METHODS

Four compounds based on previous and new lead structures were prepared and characterized with regard to their D₄R subtype selectivity and predicted lipophilicity. From these, 3-((4-(2-fluorophenyl)piperazin-1-yl)methyl)-1H-pyrrolo[2,3-b]pyridine I and (S)-4-(3-fluoro-4-methoxybenzyl)-2-(phenoxymethyl)morpholine II were selected for labeling with fluorine-18 and subsequent evaluation by in vitro autoradiography to assess their suitability as D₄ radioligand candidates for in vivo imaging.

RESULTS

The radiosynthesis of [¹⁸F]I and [¹⁸F]II was successfully achieved by copper-mediated radiofluorination with radiochemical yields of 7% and 66%, respectively. The radioligand [¹⁸F]II showed specific binding in areas where D₄ expression is expected, whereas [¹⁸F]I did not show any uptake in distinct brain regions and exhibited an unacceptable degree of non-specific binding.

CONCLUSIONS

The compounds studied exhibited high D₄R subtype selectivity and logP values compatible with high brain uptake, but only ligand [¹⁸F]II showed low non-specific binding and is therefore a good candidate for further evaluation.

ADVANCES IN KNOWLEDGE

The discovery of new lead structures for high-affinity D₄ ligands opens up new possibilities for the development of suitable PET-radioligands.

IMPLICATIONS FOR PATIENT

PET-imaging of dopamine D₄-receptors could facilitate understanding, diagnosis and treatment of neuropsychiatric and neurodegenerative diseases.

Accurate determination of production data of the non-standard positron emitter 86Y via the 86Sr(p,n)-reaction

In view of several significant discrepancies in the excitation function of the 86Sr(p,n)86g+xmY reaction which is the method of choice for the production of the non-standard positron emitter 86Y for theranostic application, we carried out a careful measurement of the cross sections of this reaction from its threshold up to 16.2 MeV at Forschungszentrum Jülich (FZJ) and from 14.3 to 24.5 MeV at LBNL. Thin samples of 96.4% enriched 86SrCO3 were prepared by sedimentation and, after irradiation with protons in a stacked-form, the induced radioactivity was measured by high-resolution γ-ray spectrometry. The projectile flux was determined by using the monitor reactions natCu(p,xn)62,63,65Zn and natTi(p,x)48V, and the calculated proton energy for each sample was verified by considering the ratios of two reaction products of different thresholds. The experimental cross section data obtained agreed well with the results of a nuclear model calculation based on the code TALYS. From the cross section data, the integral yield of 86Y was calculated. Over the optimum production energy range Ep = 14 → 7 MeV the yield of 86Y amounts to 291 MBq/μA for 1 h irradiation time. This value is appreciably lower than the previous literature values calculated from measured and evaluated excitation functions. It is, however, more compatible with the experimental yields of 86Y obtained in clinical scale production runs. The levels of the isotopic impurities 87mY, 87gY, and 88Y were also estimated and found to be <2% in sum.

One-Stop Shop: 18F-Flortaucipir PET Differentiates Amyloid-Positive and -Negative Forms of Neurodegenerative Diseases

Tau protein aggregations are a hallmark of amyloid-associated Alzheimer disease and some forms of non-amyloid-associated frontotemporal lobar degeneration. In recent years, several tracers for in vivo tau imaging have been under evaluation. This study investigated the ability of ¹⁸F-flortaucipir PET not only to assess tau positivity but also to differentiate between amyloid-positive and -negative forms of neurodegeneration on the basis of different ¹⁸F-flortaucipir PET signatures. Methods: The ¹⁸F-flortaucipir PET data of 35 patients with amyloid-positive neurodegeneration, 19 patients with amyloid-negative neurodegeneration, and 17 healthy controls were included in a data-driven scaled subprofile model (SSM)/principal-component analysis (PCA) identifying spatial covariance patterns. SSM/PCA pattern expression strengths were tested for their ability to predict amyloid status in a receiver-operating-characteristic analysis and validated with a leave-one-out approach. Results: Pattern expression strengths predicted amyloid status with a sensitivity of 0.94 and a specificity of 0.83. A support vector machine classification based on pattern expression strengths in 2 different SSM/PCA components yielded a prediction accuracy of 98%. Anatomically, prediction performance was driven by parietooccipital gray matter in amyloid-positive patients versus predominant white matter binding in amyloid-negative patients. Conclusion: SSM/PCA-derived binding patterns of ¹⁸F-flortaucipir differentiate between amyloid-positive and -negative neurodegenerative diseases with high accuracy. ¹⁸F-flortaucipir PET alone may convey additional information equivalent to that from amyloid PET. Together with a perfusion-weighted early-phase acquisition (¹⁸F-FDG PET-equivalent), a single scan potentially contains comprehensive information on amyloid (A), tau (T), and neurodegeneration (N) status as required by recent biomarker classification algorithms (A/T/N).

High uptake of 68Ga-PSMA and 18F-DCFPyL in the peritumoral area of rat gliomas due to activated astrocytes

Background

Recent studies reported on high uptake of the PSMA ligands [⁶⁸Ga]HBED-CC (⁶⁸Ga-PSMA) and ¹⁸F-DCFPyL in cerebral gliomas. This study explores the regional uptake and cellular targets of ⁶⁸Ga-PSMA and ¹⁸F-DCFPyL in three different rat glioma models.

Methods

F98, 9 L, or U87 rat gliomas were implanted into the brains of 38 rats. After 13 days of tumor growth, ⁶⁸Ga-PSMA (n = 21) or ¹⁸F-DCFPyL (n = 17) was injected intravenously, and animals were sacrificed 40 min later. Five animals for each tracer and tumor model were additionally investigated by micro-PET at 20–40 min post injection. Cryosections of the tumor bearing brains were analyzed by ex vivo autoradiography and immunofluorescence staining for blood vessels, microglia, astrocytes, and presence of PSMA. Blood-brain barrier (BBB) permeability was tested by coinjection of Evans blue dye (EBD). ⁶⁸Ga-PSMA uptake after restoration of BBB integrity by treatment with dexamethasone (Dex) was evaluated in four animals with U87 gliomas. Competition experiments using the PSMA-receptor inhibitor 2-(phosphonomethyl)pentane-1,5-dioic acid (PMPA) were performed for both tracers in two animals each.

Results

Autoradiography demonstrated a strong ⁶⁸Ga-PSMA and ¹⁸F-DCFPyL binding in the peritumoral area and moderate binding in the center of the tumors. PMPA administration led to complete inhibition of ⁶⁸Ga-PSMA and ¹⁸F-DCFPyL binding in the peritumoral region. Restoration of BBB by Dex treatment reduced EBD extravasation but ⁶⁸Ga-PSMA binding remained unchanged. Expression of activated microglia (CD11b) was low in the intra- and peritumoral area but GFAP staining revealed strong activation of astrocytes in congruency to the tracer binding in the peritumoral area. All tumors were visualized in micro PET, showing a lower tumor/brain contrast with ⁶⁸Ga-PSMA than with ¹⁸F-DCFPyL.

Conclusions

High uptake of ⁶⁸Ga-PSMA and ¹⁸F-DCFPyL in the peritumoral area of all glioma models is presumably caused by activated astrocytes. This may represent a limitation for the clinical application of PSMA ligands in gliomas.

Minimalist approach meets green chemistry: Synthesis of 18 F- labeled (hetero)aromatics in pure ethanol

The application of toxic solvents and additives is inevitable for most of the described protocols for ¹⁸ F-labeling. Herein, a novel "green" procedure for nucleophilic aromatic radiofluorination of highly activated (hetero)aromatic substrates in pure EtOH is described. Using this method a series of ¹⁸ F-labeled (hetero)arenes have been synthesized in radiochemical yields (RCYs) of up to 97%.

mGluR5 receptor availability is associated with lower levels of negative symptoms and better cognition in male patients with chronic schizophrenia

Consistent findings postulate disturbed glutamatergic function (more specifically a hypofunction of the ionotropic NMDA receptors) as an important pathophysiologic mechanism in schizophrenia. However, the role of the metabotropic glutamatergic receptors type 5 (mGluR5) in this disease remains unclear. In this study, we investigated their significance (using [¹¹C]ABP688) for psychopathology and cognition in male patients with chronic schizophrenia and healthy controls. In the patient group, lower mGluR5 binding potential (BP_ND) values in the left temporal cortex and caudate were associated with higher general symptom levels (negative and depressive symptoms), lower levels of global functioning and worse cognitive performance. At the same time, in both groups, mGluR5 BP_ND were significantly lower in smokers (F_[27,1] = 15.500; p = .001), but without significant differences between the groups. Our findings provide support for the concept that the impaired function of mGluR5 underlies the symptoms of schizophrenia. They further supply a new perspective on the complex relationship between tobacco addiction and schizophrenia by identifying glutamatergic neurotransmission—in particularly mGluR5—as a possible connection to a shared vulnerability.

Prediction of survival in patients with IDH-wildtype astrocytic gliomas using dynamic O-(2-[18F]-fluoroethyl)-L-tyrosine PET

PURPOSE

Integrated histomolecular diagnostics of gliomas according to the World Health Organization (WHO) classification of 2016 has refined diagnostic accuracy and prediction of prognosis. This study aimed at exploring the prognostic value of dynamic O-(2-[¹⁸F]-fluoroethyl)-L-tyrosine (FET) PET in newly diagnosed, histomolecularly classified astrocytic gliomas of WHO grades III or IV.

METHODS

Before initiation of treatment, dynamic FET PET imaging was performed in patients with newly diagnosed glioblastoma (GBM) and anaplastic astrocytoma (AA). Static FET PET parameters such as maximum and mean tumour/brain ratios (TBR_max/mean), the metabolic tumour volume (MTV) as well as the dynamic FET PET parameters time-to-peak (TTP) and slope, were obtained. The predictive ability of FET PET parameters was evaluated concerning the progression-free and overall survival (PFS, OS). Using ROC analyses, threshold values for FET PET parameters were obtained. Subsequently, univariate Kaplan-Meier and multivariate Cox regression survival analyses were performed to assess the predictive power of these parameters for survival.

RESULTS

Sixty patients (45 GBM and 15 AA patients) of two university centres were retrospectively identified. Patients with isocitrate dehydrogenase (IDH)-mutant or O⁶-methylguanine-DNA-methyltransferase (MGMT) promoter-methylated tumours had a significantly longer PFS and OS (both P < 0.001). Furthermore, ROC analysis of IDH-wildtype glioma patients (n = 45) revealed that a TTP > 25 min (AUC, 0.90; sensitivity, 90%; specificity, 87%; P < 0.001) was highly prognostic for longer PFS (13 vs. 7 months; P = 0.005) and OS (29 vs. 12 months; P < 0.001). In contrast, at a lower level of significance, TBR_max, TBR_mean, and MTV were only prognostic for longer OS (P = 0.004, P = 0.038, and P = 0.048, respectively). Besides complete resection and a methylated MGMT promoter, TTP remained significant in multivariate survival analysis (all P ≤ 0.02), indicating an independent predictor for OS.

CONCLUSIONS

Our data suggest that dynamic FET PET allows the identification of patients with longer OS among patients with newly diagnosed IDH-wildtype GBM and AA.

Flare Phenomenon in O-(2-18F-Fluoroethyl)-l-Tyrosine PET After Resection of Gliomas

PET using O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) is useful to detect residual tumor tissue after glioma resection. Recent animal experiments detected reactive changes in ¹⁸F-FET uptake at the rim of the resection cavity within the first 2 wk after resection of gliomas. In the present study, we evaluated pre- and postoperative ¹⁸F-FET PET scans of glioma patients with particular emphasis on the identification of reactive changes after surgery. Methods: Forty-three patients with cerebral gliomas (9 low-grade, 34 high-grade; 9 primary tumors, 34 recurrent tumors) who had preoperative (time before surgery: median, 23 d; range, 6-44 d) and postoperative ¹⁸F-FET PET (time after surgery: median, 14 d; range, 5-28 d) were included. PET scans (20-40 min after injection) were evaluated visually for complete or incomplete resection and compared with MRI. Changes in ¹⁸F-FET uptake were evaluated by tumor-to-brain ratios in residual tumor and by maximum lesion-to-brain ratios near the resection cavity. Results: Visual analysis of ¹⁸F-FET PET scans revealed complete resection in 16 of 43 patients and incomplete resection in the remaining patients. PET results were concordant with MRI in 69% of the patients. The maximum lesion-to-brain ratio for ¹⁸F-FET uptake near the resection cavity was significantly higher than preoperative values (1.59 ± 0.36 vs. 1.14 ± 0.17; n = 43; P < 0.001). In 11 patients (26%), a flare phenomenon was observed, with a considerable increase in ¹⁸F-FET uptake compared with preoperative values in either the residual tumor (n = 5) or areas remote from the tumor on the preoperative PET scan (n = 6) (2.92 ± 1.24 vs. 1.62 ± 0.75; P < 0.001). Further follow-up in 5 patients showed decreasing ¹⁸F-FET uptake in the flare areas in 4 patients and progress in 1 patient. Conclusion: Our study confirmed that ¹⁸F-FET PET provides valuable information for assessing the success of glioma resection. Postoperative reactive changes at the rim of the resection cavity appear to be mild. However, in 23% of the patients, a postoperative flare phenomenon was observed that warrants further investigation.

Image-Derived Input Functions for Quantification of A1 Adenosine Receptors Availability in Mice Brains Using PET and [18F]CPFPX

Purpose

In vivo imaging for the A₁ adenosine receptors (A₁ARs) with positron emission tomography (PET) using 8-cyclopentyl-3-(3-[¹⁸F]fluoropropyl)-1-propylxan- thine ([¹⁸F]CPFPX) has become an important tool for studying physiological processes quantitatively in mice. However, the measurement of arterial input functions (AIFs) on mice is a method with restricted applicability because of the small total blood volume and the related difficulties in withdrawing blood. Therefore, the aim of this study was to extract an appropriate [¹⁸F]CPFPX image-derived input function (IDIF) from dynamic PET images of mice.

Procedures

In this study, five mice were scanned with [¹⁸F]CPFPX for 60 min. Arterial blood samples (n = 7 per animal) were collected from the femoral artery and corrected for metabolites. To generate IDIFs, three different approaches were selected: (A) volume of interest (VOI) placed over the heart (cube, 10 mm); (B) VOI set over abdominal vena cava/aorta region with a cuboid (5 × 5 × 15 mm); and (C) with 1 × 1 × 1 mm voxels on five consecutive slices. A calculated scaling factor (α) was used to correct for partial volume effect; the method of obtaining the total metabolite correction of [¹⁸F]CPFPX for IDIFs was developed. Three IDIFs were validated by comparison with AIF. Validation included the following: visual performance; computing area under the curve (AUC) ratios (IDIF/AIF) of whole-blood curves and parent curves; and the mean distribution volume (V_T) ratios (IDIF/AIF) of A₁ARs calculated by Logan plot and two-tissue compartment model.

Results

Compared with the AIF, the IDIF with VOI over heart showed the best performance among the three IDIFs after scaling by 1.77 (α) in terms of visual analysis, AUC ratios (IDIF/AIF; whole-blood AUC ratio, 1.03 ± 0.06; parent curve AUC ratio, 1.01 ± 0.10) and V_T ratios (IDIF/AIF; Logan V_T ratio, 1.00 ± 0.17; two-tissue compartment model V_T ratio, 1.00 ± 0.13) evaluation. The A₁ARs distribution of average parametric images was in good accordance to autoradiography of the mouse brain.

Conclusion

The proposed study provides evidence that IDIF with VOI over heart can replace AIF effectively for quantification of A₁ARs using PET and [¹⁸F]CPFPX in mice brains.

Alcohol-Supported Cu-Mediated 18F-Fluorination of Iodonium Salts under "Minimalist" Conditions

In the era of personalized precision medicine, positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI gain recognition as indispensable tools of clinical diagnostics. A broader implementation of these imaging modalities in clinical routine is closely dependent on the increased availability of established and emerging PET-tracers, which in turn could be accessible by the development of simple, reliable, and efficient radiolabeling procedures. A further requirement is a cGMP production of imaging probes in automated synthesis modules. Herein, a novel protocol for the efficient preparation of ¹⁸F-labeled aromatics via Cu-mediated radiofluorination of (aryl)(mesityl)iodonium salts without the need of evaporation steps is described. Labeled aromatics were prepared in high radiochemical yields simply by heating of iodonium [¹⁸F]fluorides with the Cu-mediator in methanolic DMF. The iodonium [¹⁸F]fluorides were prepared by direct elution of ¹⁸F^- from an anion exchange resin with solutions of the corresponding precursors in MeOH/DMF. The practicality of the novel method was confirmed by the racemization-free production of radiolabeled fluorophenylalanines, including hitherto unknown 3-[¹⁸F]FPhe, in 22-69% isolated radiochemical yields as well as its direct implementation into a remote-controlled synthesis unit.

Positron-emitting radionuclides for applications, with special emphasis on their production methodologies for medical use

A survey of the positron-emitting radionuclides over the whole mass range of the Periodic Table of Elements was carried out. As already known, positrons are preferably emitted from light mass neutron deficient radionuclides. Their emission from heavier mass nuclides is rather rare. The applications of positron annihilation in three areas, namely materials research, plant physiology and medical diagnosis, are reported. The methods of production of positron emitters are discussed, with emphasis on radionuclides presently attracting more attention in theranostics and multimodal imaging. Some future perspectives of radionuclide development technologies are considered.

MLTI-17. DIFFERENTIATION OF RADIATION INJURY FROM RECURRENT BRAIN METASTASIS USING COMBINED FET PET/MRI RADIOMICS

BACKGROUND: The aim of this study was to investigate the potential of combined radiomics textural feature analysis of contrast-enhanced MRI (CE MRI) and static O-(2-[¹⁸F]fluoroethyl)-L-tyrosine(FET) PET for the differentiation of recurrent brain metastasis from radiation injury. PATIENTS AND METHODS: Fifty-two patients with newly diagnosed or progressive contrast-enhancing brain lesions on MRI after radiotherapy (predominantly radiosurgery, 84% of patients) of brain metastases were additionally investigated using FET PET. Based on histology (n=19) or clinicoradiological follow-up (n=33), local recurrent brain metastases were diagnosed in 21 patients (40%) and radiation injury in 31 patients (60%). Forty-two features (shape-based, first and second order features) were calculated on both unfiltered and filtered CE MRI and summed FET PET images (20–40 min p.i). After feature selection, logistic regression models using a maximum of five features to avoid overfitting were calculated for each imaging modality separately and for the combined FET PET/MRI features. The resulting models were validated using cross-validation. Diagnostic accuracies were calculated for each imaging modality separately as well as for the combined model. RESULTS: For differentiation between radiation injury and brain metastasis recurrence, textural features extracted from CE MRI had a diagnostic accuracy of 81%. FET PET textural features revealed a slightly higher diagnostic accuracy of 83%. However, the highest diagnostic accuracy was obtained when combining CE MRI and FET PET features (accuracy, 89%). CONCLUSION: Our findings suggest that combined FET PET/MRI radiomics using textural feature analysis offers a great potential to contribute significantly to the management of patients with brain metastases. SUPPORT: This work was supported by the Wilhelm-Sander Stiftung, Germany

Spatial distributions of cholinergic impairment and neuronal hypometabolism differ in MCI due to AD

Elucidating the relationship between neuronal metabolism and the integrity of the cholinergic system is prerequisite for a profound understanding of cholinergic dysfunction in Alzheimer's disease. The cholinergic system can be investigated specifically using positron emission tomography (PET) with [¹¹C]N-methyl-4-piperidyl-acetate (MP4A), while neuronal metabolism is often assessed with 2-deoxy-2-[¹⁸F]fluoro-d-glucose-(FDG) PET. We hypothesised a close correlation between MP4A-perfusion and FDG-uptake, permitting inferences about metabolism from MP4A-perfusion, and investigated the patterns of neuronal hypometabolism and cholinergic impairment in non-demented AD patients. MP4A-PET was performed in 18 cognitively normal adults and 19 patients with mild cognitive impairment (MCI) and positive AD biomarkers. In nine patients with additional FDG-PET, the sum images of every combination of consecutive early MP4A-frames were correlated with FDG-scans to determine the optimal time window for assessing MP4A-perfusion. Acetylcholinesterase (AChE) activity was estimated using a 3-compartmental model. Group comparisons of MP4A-perfusion and AChE-activity were performed using the entire sample. The highest correlation between MP4A-perfusion and FDG-uptake across the cerebral cortex was observed 60-450 s after injection (r = 0.867). The patterns of hypometabolism (FDG-PET) and hypoperfusion (MP4A-PET) in MCI covered areas known to be hypometabolic early in AD, while AChE activity was mainly reduced in the lateral temporal cortex and the occipital lobe, sparing posterior midline structures. Data indicate that patterns of cholinergic impairment and neuronal hypometabolism differ significantly at the stage of MCI in AD, implying distinct underlying pathologies, and suggesting potential predictors of the response to cholinergic pharmacotherapy.

Biodistribution and radiation dosimetry of [18F]-JK-PSMA-7 as a novel prostate-specific membrane antigen-specific ligand for PET/CT imaging of prostate cancer

Aim

We investigated the whole-body distribution and the radiation dosimetry of [¹⁸F]-JK-PSMA-7, a novel ¹⁸F-labeled PSMA-ligand for PET/CT imaging of prostate cancer.

Methods

Ten patients with prostate cancer and biochemical recurrence or radiologic evidence of metastatic diseases were examined with 329–384 MBq (mean 359 ± 17 MBq) [¹⁸F]-JK-PSMA-7. Eight sequential positron emission tomography (PET) scans were acquired from 20 min to 3 h after injection with IRB approval. The kidneys, liver, lungs, spleen, and salivary glands were segmented into volumes of interest using the QDOSE dosimetry software suite (ABX-CRO, Germany). Absorbed and effective dose were calculated using the ICRP-endorsed IDAC 1.0 package. The absorbed dose of the salivary glands was determined using the spherical model of OLINDA 1.1. PSMA-positive lesions were evaluated separately. Quantitative assessment of the uptake in suspicious lesions was performed by analysis of maximum (max) and peak SUV values. The gluteus maximus muscle (SUV_mean) served as a reference region for the calculation of tumor-to-background ratios (TBR’s).

Results

Physiologic radiotracer accumulation was observed in the salivary and lacrimal glands, liver, spleen, and intestines, in a pattern resembling the distribution known from other PSMA-tracers with excretion via urinary and biliary pathways. The effective dose from [¹⁸F]-JK-PSMA-7 for the whole body was calculated to be 1.09E−02 mGy/MBq. The highest radiation dose was observed in the kidneys (1.76E−01 mGy/MBq), followed by liver (7.61E−02 mGy/MBq), salivary glands (4.68E−02 mGy/MBq), spleen (1.89E−02 mGy/MBq), and lungs (1.10E-2 mGy/MBq). No adverse effects of tracer injection were observed. Six out of ten patients were scored as PSMA-positive. A total of 18 suspicious lesions were analyzed, which included six bone lesions, nine lymph nodes, and three local lesions within the prostate fossa. The values for the SUV_max and SUV_peak in the PSMA-positive lesions increased until 60 min p.i. and remained at this intensity in the PET/CT scans until 140 min. In the period between 170 and 200 min after injection, a further significant increase in SUV_max and SUV_peak within the PSMA-positive lesions was observed.

Conclusions

The highest TBR of [¹⁸F]-JK-PSMA-7 was found 3 h after injection. From the kinetically collected data, it can be concluded that this trend may also continue in the further course. The start of the PET/CT acquisition should be chosen as late as possible. The high uptake in suspicious lesions in terms of absolute SUV_max and relative TBR values indicates potentially high sensitivity of the tracer for detection of prostate cancer manifestations.

Effect of cholinergic treatment depends on cholinergic integrity in early Alzheimer's disease

In early Alzheimer's disease, which initially presents with progressive loss of short-term memory, neurodegeneration especially affects cholinergic neurons of the basal forebrain. Pharmacotherapy of Alzheimer's disease therefore often targets the cholinergic system. In contrast, cholinergic pharmacotherapy of mild cognitive impairment is debated since its efficacy to date remains controversial. We here investigated the relationship between cholinergic treatment effects and the integrity of the cholinergic system in mild cognitive impairment due to Alzheimer's disease. Fourteen patients with high likelihood of mild cognitive impairment due to Alzheimer's disease and 16 age-matched cognitively normal adults performed an episodic memory task during functional magnetic resonance imaging under three conditions: (i) without pharmacotherapy; (ii) with placebo; and (iii) with a single dose of rivastigmine (3 mg). Cortical acetylcholinesterase activity was measured using PET with the tracer 11C-N-methyl-4-piperidyl acetate (MP4A). Cortical acetylcholinesterase activity was significantly decreased in patients relative to controls, especially in the lateral temporal lobes. Without pharmacotherapy, mild cognitive impairment was associated with less memory-related neural activation in the fusiform gyrus and impaired deactivation in the posterior cingulate cortex, relative to controls. These differences were attenuated under cholinergic stimulation with rivastigmine: patients showed increased neural activation in the right fusiform gyrus but enhanced deactivation of the posterior cingulate cortex under rivastigmine, compared to placebo. Conversely, controls showed reduced activation of the fusiform gyrus and reduced deactivation of the posterior cingulate under rivastigmine, compared to placebo. In both groups, the change in neural activation in response to rivastigmine was negatively associated with local acetylcholinesterase activity. At the behavioural level, an analysis of covariance revealed a significant group × treatment interaction in episodic memory performance when accounting for hippocampal grey matter atrophy and function. Our results indicate that rivastigmine differentially affects memory-related neural activity in patients with mild cognitive impairment and cognitively normal, age-matched adults, depending on acetylcholinesterase activity as a marker for the integrity of the cortical cholinergic system. Furthermore, hippocampal integrity showed an independent association with the response of memory performance to acetylcholinesterase inhibition.

Peripheral ganglia in healthy rats as target structures for the evaluation of PSMA imaging agents

Background

The recent implementation of PET with prostate specific membrane antigen (PSMA)-specific radiotracers into the clinical practice has resulted in the significant improvement of accuracy in the detection of prostate carcinoma (PCa). PSMA-expression in ganglia has been regarded as an important pitfall in prostate carcinoma-PET diagnostics but has not found any practical use for diagnosis or therapy.

Methods

We explored this phenomenon and demonstrated the applicability of peripheral ganglia in healthy rats as surrogates for small PSMA positive lesions for the preclinical evaluation of diagnostic PCa PET probes. Healthy rats were measured with PET/CT using the tracers [¹⁸F]DCFPyL, [Al¹⁸F]PSMA-11 and [⁶⁸Ga]PSMA-11. Sections of ganglia were stained with an anti-PSMA antibody. [¹⁸F]DCFPyL uptake in ganglia was compared to that in LNCaP tumor xenografts in mice.

Results

Whereas [¹⁸F]DCFPyL and [⁶⁸Ga]PSMA-11 were stable in vivo and accumulated in peripheral ganglia, [Al¹⁸F]PSMA-11 suffered from fast in vivo deflourination resulting in high bone uptake. Ganglionic PSMA expression was confirmed by immunohistochemistry. [¹⁸F]DCFPyL uptake and signal-to-noise ratio in the superior cervical ganglion was not significantly different from LNCaP xenografts.

Conclusions

Our results demonstrated the non-inferiority of the novel model compared to conventionally used tumor xenografts in immune compromised rodents with regard to reproducibility and stability of the PSMA signal. Furthermore, the model involves less expense and efforts while it is permanently available and avoids tumor-growth associated animal morbidity and distress. To the best of our knowledge, this is the first tumor-free model suitable for the in vivo evaluation of tumor imaging agents.

Level of education mitigates the impact of tau pathology on neuronal function

PURPOSE

Using PET imaging in a group of patients with Alzheimer's disease (AD), we investigated whether level of education, a proxy for resilience, mitigates the harmful impact of tau pathology on neuronal function.

METHODS

We included 38 patients with mild-to-moderate AD (mean age 67 ± 7 years, mean MMSE score 24 ± 4, mean years of education 14 ± 4; 20 men, 18 women) in whom a [¹⁸F]AV-1451 scan (a measure of tau pathology) and an [¹⁸F]FDG scan (a measure of neuronal function) were available. The preprocessed PET scans were z-transformed using templates for [¹⁸F]AV-1451 and [¹⁸F]FDG from healthy controls, and subsequently thresholded at a z-score of ≥3.0, representing an one-tailed p value of 0.001. Next, three volumes were computed in each patient: the tau-specific volume (tau pathology without neuronal dysfunction), the FDG-specific volume (neuronal dysfunction without tau pathology), and the overlap volume (tau pathology and neuronal dysfunction). Mean z-scores and volumes were extracted and used as dependent variables in regression analysis with years of education as predictor, and age and MMSE score as covariates.

RESULTS

Years of education were positively associated with tau-specific volume (β = 0.362, p = 0.022), suggesting a lower impact of tau pathology on neuronal function in patients with higher levels of education. Concomitantly, level of education was positively related to tau burden in the overlap volume (β = 0.303, p = 0.036) implying that with higher levels of education more tau pathology is necessary to induce neuronal dysfunction.

CONCLUSION

In patients with higher levels of education, tau pathology is less paralleled by regional and remote neuronal dysfunction. The data suggest that early life-time factors such as level of education support resilience mechanisms, which ameliorate AD-related effects later in life.

Effects of subthalamic deep brain stimulation on striatal metabolic connectivity in a rat hemiparkinsonian model

Deep brain stimulation (DBS) in the subthalamic nucleus (STN) has been successfully used for the treatment of advanced Parkinson's disease, although the underlying mechanisms are complex and not well understood. There are conflicting results about the effects of STN-DBS on neuronal activity of the striatum, and its impact on functional striatal connectivity is entirely unknown. We therefore investigated how STN-DBS changes cerebral metabolic activity in general and striatal connectivity in particular. We used ipsilesional STN stimulation in a hemiparkinsonian rat model in combination with [¹⁸F]FDOPA-PET, [¹⁸F]FDG-PET and metabolic connectivity analysis. STN-DBS reversed ipsilesional hypometabolism and contralesional hypermetabolism in hemiparkinsonian rats by increasing metabolic activity in the ipsilesional ventrolateral striatum and by decreasing it in the contralesional hippocampus and brainstem. Other STN-DBS effects were subject to the magnitude of dopaminergic lesion severity measured with [¹⁸F]FDOPA-PET, e.g. activation of the infralimbic cortex was negatively correlated to lesion severity. Connectivity analysis revealed that, in healthy control animals, left and right striatum formed a bilateral functional unit connected by shared cortical afferents, which was less pronounced in hemiparkinsonian rats. The healthy striatum was metabolically connected to the ipsilesional substantia nigra in hemiparkinsonian rats only (OFF condition). STN-DBS (ON condition) established a new functional striatal network, in which interhemispheric striatal connectivity was strengthened, and both the dopamine-depleted and the healthy striatum were functionally connected to the healthy substantia nigra. We conclude that both unilateral dopamine depletion and STN-DBS affect the whole brain and alter complex interhemispheric networks.

Summary: Deep brain stimulation in the subthalamic nucleus in rats with a unilateral dopaminergic lesion established a new functional interhemispheric striatal network.

Relevance of In Vitro Metabolism Models to PET Radiotracer Development: Prediction of In Vivo Clearance in Rats from Microsomal Stability Data

The prediction of in vivo clearance from in vitro metabolism models such as liver microsomes is an established procedure in drug discovery. The potentials and limitations of this approach have been extensively evaluated in the pharmaceutical sector; however, this is not the case for the field of positron emission tomography (PET) radiotracer development. The application of PET radiotracers and classical drugs differs greatly with regard to the amount of substance administered. In typical PET imaging sessions, subnanomolar quantities of the radiotracer are injected, resulting in body concentrations that cannot be readily simulated in analytical assays. This raises concerns regarding the predictability of radiotracer clearance from in vitro data. We assessed the accuracy of clearance prediction for three prototypical PET radiotracers developed for imaging the A₁ adenosine receptor (A₁AR). Using the half-life (t_1/2) approach and physiologically based scaling, in vivo clearance in the rat model was predicted from microsomal stability data. Actual clearance could be accurately predicted with an average fold error (AFE) of 0.78 and a root mean square error (RMSE) of 1.6. The observed slight underprediction (1.3-fold) is in accordance with the prediction accuracy reported for classical drugs. This result indicates that the prediction of radiotracer clearance is possible despite concentration differences of more than three orders of magnitude between in vitro and in vivo conditions. Consequently, in vitro metabolism models represent a valuable tool for PET radiotracer development.

2-[18F]Fluorophenylalanine: Synthesis by Nucleophilic 18F-Fluorination and Preliminary Biological Evaluation

2-[18F]Fluorophenylalanine (2-[18F]FPhe), a promising PET tracer for imaging of cerebral infarction and tumors, was efficiently prepared from an easily accessible iodonium salt precursor using Cu-mediated radiofluorination under ‘low base’ or ‘minimalist’ conditions. Whereas significant racemization was initially observed if the ‘minimalist’ protocol was applied for radiolabeling, it was completely suppressed by the careful adjustment of 18F– preprocessing. The initial biological study revealed a higher uptake of 2-[18F]FPhe in different tumor cells in comparison to that of [18F]FET. In contrast to 4-[18F]FPhe, which suffered from rapid defluorination in vivo, 2-[18F]FPhe demonstrated a sufficient in vivo stability. Conclusively, 2-[18F]FPhe is a promising PET probe that is now readily available using Cu-mediated radiofluorination under ‘minimalist’ or ‘low base’ conditions. The simplicity of the translation of the proposed procedures to automated synthesis modules allows a broad biological evaluation of 2-[18F]FPhe. Notably, a novel protocol for the preparation of N-Boc protected amino acids from the respective Ni-Schiff base complexes was developed that avoided application of strongly acidic conditions.

52g/55Mn-Labelled CDTA-based trimeric complexes as novel bimodal PET/MR probes with high relaxivity

Multimeric trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA) derivatives labelled with a mixture of paramagnetic ⁵⁵Mn(ii) and β⁺-emitting ^52gMn(ii) offer the access to bimodal Positron Emission Tomography/Magnetic Resonance (PET/MR) tracers.

Discovery of 18F-JK-PSMA-7, a PET Probe for the Detection of Small PSMA-Positive Lesions

Prostate-specific membrane antigen (PSMA), expressed by most prostate carcinomas (PCa), is a promising target for PCa imaging. The application of PSMA-specific ¹⁸F-labeled PET probes such as ¹⁸F-DCFPyL and ¹⁸F-PSMA-1007 considerably improved the accuracy of PCa tumor detection. However, there remains a need for further improvements in sensitivity and specificity. The aim of this study was the development of highly selective and specific PSMA probes with enhanced imaging properties, in comparison with ¹⁸F-DCFPyL, ¹⁸F-PSMA-1007, and ⁶⁸Ga-PSMA-11. Methods: Eight novel ¹⁸F-labeled PSMA ligands were prepared. Their cellular uptake in PSMA-positive LNCaP C4-2 and PSMA-negative PC-3 cells was compared with that of ¹⁸F-DCFPyL. The most promising candidates were additionally evaluated by small-animal PET in healthy rats using PSMA-positive peripheral ganglia as a model for small PCa lesions. PET images of the ligand with the best outcome, ¹⁸F-JK-PSMA-7, were compared with those of ¹⁸F-DCFPyL, ¹⁸F-PSMA-1007, and ⁶⁸Ga-PSMA-11 with respect to key image-quality parameters for the time frame 60-120 min. Results: Compared with ¹⁸F-DCFPyL, ¹⁸F-JK-PSMA-7 demonstrated increased PSMA-specific cellular uptake. Although target-to-background ratios of ¹⁸F-DCFPyL and ¹⁸F-PSMA-1007 were comparable, this parameter was higher for ¹⁸F-JK-PSMA-7 and lower for ⁶⁸Ga-PSMA-11. Image acutance was significantly higher for ¹⁸F-JK-PSMA-7 and ¹⁸F-PSMA-1007 than for ¹⁸F-DCFPyL and ⁶⁸Ga-PSMA-11. Image resolution was similar for all 4 tracers. ¹⁸F-PSMA-1007 demonstrated significantly higher blood protein binding and bone uptake than the other tracers. Conclusion: ¹⁸F-JK-PSMA-7 is a promising candidate for high-quality visualization of small PSMA-positive lesions. Excellent preclinical imaging properties justify further preclinical and clinical studies of this tracer.

FET PET reveals considerable spatial differences in tumour burden compared to conventional MRI in newly diagnosed glioblastoma

PURPOSE

Areas of contrast enhancement (CE) on MRI are usually the target for resection or radiotherapy target volume definition in glioblastomas. However, the solid tumour mass may extend beyond areas of CE. Amino acid PET can detect parts of the tumour that show no CE. We systematically investigated tumour volumes delineated by amino acid PET and MRI in patients with newly diagnosed, untreated glioblastoma.

METHODS

Preoperatively, 50 patients with neuropathologically confirmed glioblastoma underwent O-(2-[¹⁸F]-fluoroethyl)-L-tyrosine (FET) PET, and fluid-attenuated inversion recovery (FLAIR) and contrast-enhanced MRI. Areas of CE were manually segmented. FET PET tumour volumes were segmented using a tumour-to-brain ratio of ≥1.6. The percentage overlap volumes, and Dice and Jaccard spatial similarity coefficients (DSC, JSC) were calculated. FLAIR images were evaluated visually.

RESULTS

In 43 patients (86%), the FET tumour volume was significantly larger than the CE volume (21.5 ± 14.3 mL vs. 9.4 ± 11.3 mL; P < 0.001). Forty patients (80%) showed both increased uptake of FET and CE. In these 40 patients, the spatial similarity between FET uptake and CE was low (mean DSC 0.39 ± 0.21, mean JSC 0.26 ± 0.16). Ten patients (20%) showed no CE, and one of these patients showed no FET uptake. In five patients (10%), increased FET uptake was present outside areas of FLAIR hyperintensity.

CONCLUSION

Our results show that the metabolically active tumour volume delineated by FET PET is significantly larger than tumour volume delineated by CE. Furthermore, the results strongly suggest that the information derived from both imaging modalities should be integrated into the management of patients with newly diagnosed glioblastoma.