NRM 2021

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Oral Talks 1

Session chair: Vasdev , Neil , (Centre for Addiction and Mental Health, Toronto, Canada); Lillethorup , Thea P. , PhD/MD (Aarhus Universitet, Department of Clinical Medicine, PET-centre, Aarhus N, Denmark); Herscovitch , Peter , Dr. (NIH Clinical Center PET, Bethesda, USA); Coomans , Emma , M.Sc./M.A. (University of Amsterdam, Amsterdam, Netherlands)
 
Shortcut: S-01
Date: Tuesday, 14 December, 2021, 9:30 AM - 12:15 PM
Room: Track
Session type: Oral Talks

Contents

Click on an contribution to preview the abstract content.

9:30 AM S-01-01

The PET radioligands ([11C]NR2B-X; X = Me, R or S; X = SMe, S) are selective for binding to the NR2B subunit of NMDA over the s1 receptor in rat in vivo (#191)

Lisheng Cai1, Jeih-San Liow1, Cheryl Morse1, Sanjay Telu1, Riley Davies1, Emily Feigen1, Robert B. Innis1, Victor W. Pike1

1 NIMH, Molecular Imaging Branch, Bethesda, Maryland, United States of America

Introduction

The NR2B subunit within the NMDA complex and sigma receptors (σ1, σ2) are physically intertwined [1].  In our effort to develop PET radioligands for PET imaging of the NR2B receptor, we found that some ligands have quite high affinities for s-receptors [2].  Here we used our candidate radioligands targetting NR2B ([11C]NR2B; X-Me, R- or S-enantiomer; X = SMe; S-enantiomer) and an established radioligand targetting s1 receptors ([18F]FTC146) [3] with various challenge agents to investigate whether s1 receptors influence the PET imaging of NR2B in rat brain.

Methods

PET imaging of brain was performed after intravenous administration of an [11C]NR2B-X radioligand or [18F]FTC146 to rats at baseline and after intravenous administration of ligands putatively selective for NR2B, such as NR2B-SMe and Ro 25 6981, or σ1, such as FTC146, and BD1047 at a range of doses (each typically at 0.01−3 mg/kg), at 10 min before radioligand injection. For each radioligand, a dose-response curve was established for each blocking agent.

Results

FTC146 gave an ED50 value of 46 nmol/kg body for self-blockade of [18F]FTC146 binding in rat brain, whereas ED50 values were far lower for blockade of the three tested NR2B radioligands depending on radioligand (i.e., 2571, 725, or >1000 nmol/kg).  Similarly, BD1047 gave an ED50 of 169 nmol/kg for blockade of [18F]FTC146, and far lower ED50 values for blockade of the three NR2B radioligands (>1000, 555, or 900 nmol/kg).  NR2B-SMe, gave a weak ED50 of 1064 nmol/kg against [18F]FTC146 and a strong ED50 value of 9.5 nmol/kg against [11C](S)-NR2B-SMe (Figure).

Conclusion

The ED50 values of FTC146, BD1047, and NR2B-SMe depended on the radioligand used in PET imaging of rat brain.  Both FTC146 and BD1047 potently blocked rat brain s1 receptors, affirming the reported high s1 selectivity of [18F]FTC146.  FTC146 was extremely weak at blocking the NR2B radioligands. (S)-NR2B-SMe potently blocked the test NR2B radioligand [11C](S)-NR2B-SMe, but not [18F]FTC146.  Taken together the data indicate that the influence of s1 receptors on the PET imaging of NR2B receptors with the tested [11C]NR2B radioligands is likely negligible.

AcknowledgmentIntramural Research Program of the National Insitutes of Health (NIMH).
References
[1] Pabba M and Sibille E. Mol. Neuropsychiatry, 2015, 1, 47.
[2] Cai L et al., J. Nucl. Med. 2020, doi:10.2967/jnumed.119.235143
[3] James ML et al., J. Med. Chem. 2012, 55, 8272.
Figure
Mutual preblocking of [18F]FTC146 and [11C]NR2B-X (X = Me, SMe, R or S) by various sigma ligands and NR2B ligands in PET imaging of rat brain.
Keywords: NR2B/NMDA receptor, sigma receptor, NR2B-SMe, FTC146
9:45 AM S-01-02

Positron Emission Tomography Imaging of Alpha-Synuclein?In Vitro and In Vivo Evaluation of MODAG-005 (#324)

Ran Sing Saw1, Sabrina Buss1, Laura Kuebler1, Sergey Ryazanov2, 3, Andrei Leonov2, 3, Federica Bonanno1, Felix Schmidt2, Viktoria Ruf4, Daniel Bleher1, Marilena Poxleitner1, Ann-Kathrin Grotegerd1, Bernd J. Pichler1, Gregory D. Bowden1, Andreas Maurer1, Christian Griesinger4, Armin Giese2, 4, Kristina Herfert1

1 Eberhard Karls University of Tübingen, Werner Siemens Imaging Center, Department of Preclinical Imaging & Radiopharmacy, Tübingen, Baden-Württemberg, Germany
2 MODAG GmbH, Wendelsheim, Rhineland-Palatinate, Germany
3 Max Planck Institute for Biophysical Chemistry, Göttingen, Lower Saxony, Germany
4 Ludwig Maximilians University, Center for Neuropathology and Prion Research, Munich, Bavaria, Germany

Introduction

Positron emission tomography (PET) imaging of alpha-synuclein (αSYN) aggregates would be invaluable for the non-invasive diagnosis of synucleinopathies as well as facilitating the development of novel treatment strategies. Here, we report the in vitro and in vivo evaluation of MODAG-005 as a promising αSYN PET tracer.

Methods

In vitro binding of [3H]MODAG-005 was determined using human recombinant αSYN, amyloid-beta1-42 (Aβ) and tau fibrils. (Micro)autoradiography was performed in postmortem human brain tissues from multiple system atrophy (MSA), Parkinson’s disease (PD), Alzheimer’s disease (AD), progressive supranuclear palsy (PSP), healthy controls, and in the transgenic αSYN(A30P) mouse model. Subsequently, the binding affinity to αSYN in human brain was determined in MSA tissues using autoradiography. In vivo pharmacokinetics and metabolism of [11C]MODAG-005 were studied in healthy mice and rats, and in rats intrastriatally injected with αSYN fibrils. Time-activity curves were generated from dynamic 60-minute PET for the striata (target region) and cerebellum (reference), and the binding potential (BPND) values were calculated using Logan’s reference tissue model.

Results

Saturation binding assays revealed Kd values for [3H]MODAG-005 of 0.2 nM for αSYN, 7 nM for tau and >100 nM for Aβ fibrils. Autoradiography in human brain tissues confirmed the high affinity binding of [3H]MODAG-005 to αSYN with a Kd of 0.25 nM in MSA, while showing a specific binding in the αSYN(A30P) mouse model. [11C]MODAG-005 was obtained at high molar activities of >200 GBq/μmol. The tracer showed an excellent blood-brain barrier penetration and a fast clearance from the brain. We observed one metabolite in the brain, with 96% and 79% of the parent compound remaining at 5 and 15 minutes post-injection. Increased tracer binding was detected in the fibril-injected striatum compared to the sham-injected striatum, whereas no difference was detected in non-injected rats.

Conclusion

Here, we present a novel PET tracer targeting αSYN in the human brain. MODAG-005 possesses a very high affinity to αSYN in MSA tissues, in line with the αSYN fibril binding assay. It also exhibits excellent brain availability, good kinetics, and sufficient signal-to-noise ratio in fibril-injected rats. Despite the lack of full selectivity, MODAG-005 is currently one of the most promising αSYN-targeting PET tracers.

AcknowledgmentWe thank Ramona Stremme, Elena Kimmerle and Johannes Kinzler for the radiosynthesis. We also thank the technical assistants Linda Schramm, Maren Harant, Stacy Huang, Miriam Owczorz, Natalie Hermann and Isabel Sehnke for their experimental support. Additionally, we acknowledge Dr. Julia Mannheim, Dr. Rebecca Rock, Dr. Neele Hübner, Dr. Andreas Dieterich, Hans Jörg Rahm, Dr. Carsten Calaminus and Funda Cay for their administrative support.
Keywords: alpha-synuclein, PET tracer evaluation, autoradiography, multiple system atrophy, Parkinson’s disease
10:00 AM S-01-03

Impact of Cerebral Blood Flow and Amyloid load on SUVR bias (#316)

Fiona Heeman1, Maqsood Yaqub1, Janine Hendriks1, Bart N.M. van Berckel1, Lyduine E. Collij1, Katherine R. Gray2, Richard Manber2, Robin Wolz2, Valentina Garibotto3, 4, Catriona Wimberley5, Craig Ritchie5, Frederik Barkhof1, 6, Juan Domingo Gispert López7, 8, David Vállez García1, Isadora Lopes Alves1, Adriaan A. Lammertsma1

1 Amsterdam UMC, Vrije Universiteit Amsterdam, Radiology & Nuclear Medicine, Amsterdam, Netherlands
2 IXICO Plc, London, United Kingdom
3 Geneva University, NIMTLab, Faculty of Medicine, Geneva, Switzerland
4 Geneva University Hospitals, Division of Nuclear Medicine and Molecular Imaging, Geneva, Switzerland
5 University of Edinburgh, Edinburgh Imaging, Queen's Medical Research Institute, Edinburgh, United Kingdom
6 UCL, Institutes of Neurology and Healthcare Engineering, London, United Kingdom
7 Barcelonaβeta Brain Research Centre, Pasqual Maragall Foundation, Barcelona, Spain
8 Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Madrid, Spain

on behalf of the AMYPAD consortium

Introduction

Despite the widespread use of static amyloid PET scans for clinical and research purposes, it is known that the standardized uptake value ratio (SUVR), calculated from a static scan, may be biased compared with the distribution volume ratio (DVR) derived from a dynamic scan(1). This bias may be partially explained by changes in cerebral blood flow (CBF) and is likely to be also dependent on the severity of the underlying amyloid-β (Aβ) pathology(2,3). To date, most Alzheimer’s disease (AD) studies have compared SUVR and DVR only at a diagnostic group level and not as a function of underlying Aβ pathology. The purpose of the present study was to compare SUVR with DVR and to evaluate the effects of underlying Aβ pathology and CBF on bias in SUVR in a group of mainly cognitively unimpaired participants.

Methods

Participants (N=121) were scanned according to a dual-time window protocol(4), with either [18F]flutemetamol (N=90) or [18F]florbetaben (N=31). The validated voxel-based implementation of the two-step simplified reference tissue model was used to derive DVR and R1 and SUVR was calculated for a 90-110 min post-injection uptake window, all with the cerebellar grey matter as reference tissue(5). First, linear regression and Bland-Altman analyses were used to compare SUVR with DVR. Then, Generalized Linear Models were applied to evaluate whether (bias in) SUVR relative to DVR could be explained by R1 for four regions (i.e. global cortical average (GCA), precuneus, posterior cingulate, and orbitofrontal cortex).

Results

Parameter distributions for both tracers are depicted in Figure 1. Despite high correlations (GCA: R2≥0.85), large overestimation and proportional bias of SUVR relative to DVR was observed (regression line of the proportional bias: GCA: [18F]flutemetamol R2=0.69, slope=0.56 and intercept=-0.48; [18F]florbetaben: R2=0.65, slope=0.31 and intercept=-0.20). Figure 2. Negative associations were observed between both SUVR or SUVRbias and R1, albeit non-significant.

Conclusion

The present findings demonstrate that bias in SUVR relative to DVR is primarily due to underlying Aβ pathology. Furthermore, in a cohort consisting mainly of cognitively unimpaired individuals, the effect of CBF on bias in SUVR appears limited.

Acknowledgment

The authors would like to thank all staff of the various centres for skilful acquisition of the scans.
This work has received support from the EU-EFPIA Innovative Medicines Initiatives 2 Joint Undertaking (grant No 115952). This communication reflects the views of the authors and neither IMI nor the European Union and EFPIA are liable for any use that may be made of the information contained herein.

References
[1] Carson RE, Channing MA, Blasberg RG, et al. Comparison of Bolus and Infusion Methods for Receptor Quantitation: Application to [18F]Cyclofoxy and Positron Emission Tomography. Journal of Cerebral Blood Flow & Metabolism. 1993;13:24-42.
[2] Berckel  van BNM, Ossenkoppele R, Tolboom N, et al. Longitudinal Amyloid Imaging Using 11C-PiB: Methodologic Considerations. J Nucl Med. 2013;54:1570-157
[3] Lopes Alves I, Heeman F, Collij LE, et al. Strategies to reduce sample sizes in Alzheimer’s disease primary and secondary prevention trials using longitudinal amyloid PET imaging. Alzheimers Res Ther. 2021;13:82.
[4] Heeman F, Yaqub M, Lopes Alves I, et al. Optimized dual-time-window protocols for quantitative [18F]flutemetamol and [18F]florbetaben PET studies. EJNMMI Research. 2019;9:32.
[5] Heeman F, Yaqub M, Hendriks J, et al. Parametric imaging of dual-time window [18F]flutemetamol and [18F]florbetaben studies. NeuroImage. 2021;234:117953.
Parameter distribution across tracers

Violin plots showing the distribution of (A) DVR, (B) SUVR and (C) R1 for Aβ positive and negative scans. Small boxplots inside the violin plots display median and quartile range of the distribution.

Relationship between DVR and SUVR

Correlation and Bland-Altman plots to assess the relationship between global cortical DVR and SUVR for (A) [18F]flutemetamol and (B) [18F]florbetaben. Dotted lines corresponds to 95% Limits of Agreement. p<0.001, VR: visual read.

Keywords: Alzheimer's disease, Amyloid, cerebral blood flow, quantification, SUVR bias
10:01 AM S-01-04

Validation of PET-compatible chemogenetic tools in squirrel monkeys (#125)

Matthew Boehm1, 2, Hank Jedema1, Jordi Bonaventura1, Omar Gharbawie3, Juan Gomez1, Elliot Stein1, Charles Bradberry1, Michael Michaelides1, 4

1 National Institute on Drug Abuse, NIH Intramural Research Program, Baltimore, Maryland, United States of America
2 Brown University, Dept of Neuroscience, Providence, Rhode Island, United States of America
3 University of Pittsburgh, Dept of Neurobiology, Pittsburgh, Pennsylvania, United States of America
4 Johns Hopkins Medicine, Dept of Psychiatry & Behavioral Sciences, Baltimore, Maryland, United States of America

Introduction

Chemogenetic technologies offer an innovative approach for modulating activity of specific cell types in targeted brain regions. Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), and the recently developed Pharmacologically Selective Actuator Modules (PSAMs), are two types of chemogenetic systems used to modulate neural activity.  Current applications of these tools in primates and humans is limited in part by an inability to track the location and function of chemogenetic receptors in vivo.  Positron emission tomography (PET) is a translational molecular imaging modality uniquely poised to enable in vivo confirmation of expression and function of chemogenetic systems. Recent studies have demonstrated the use of PET-reporter ligands for imaging chemogenetic receptors, namely [18F]-JHU37107 for DREADDs and [18F]-ASEM for PSAMs.  In addition, newly developed actuator ligands JHU37160 and uPSEM817 claim improved selectivity and potency at DREADDs and PSAMs, respectively. Here we employ PET imaging methods to test the function of these chemogenetic ligands in squirrel monkeys (Saimiri sciureus).

Methods

Four monkeys underwent baseline scans with the PET-reporter ligands [18F]-JHU37107 and [18F]-ASEM (~1.5mCi, bolus i.v.). Baseline scans were also performed with [18F]-fluorodeoxyglucose (FDG) as a measure of brain activity following saline or pretreatment with JHU37160 or uPSEM817 (0.1mg/kg, i.v.).  Following baseline scans, monkeys were injected with AAV2/5-hSyn-HA-hM3Dq and AAV2/5-Syn1-PSAM4-GlyR in left hand/forelimb area of motor cortex.  PET scans with [18F]-JHU37107, [18F]-ASEM and FDG will be repeated and compared with baseline scans to determine the location of chemogenetic receptors and their effects on brain activity.

Results

In baseline PET-reporter scans, [18F]-JHU37107 signal in the brain peaked at 10min post-injection and remained detectable after 90min, indicating some level of endogenous binding. Similarly, [18F]-ASEM signal peaked at 20min post-injection and remained detectable at 90min (n=4).  FDG-PET scans revealed rapid uptake of FDG in the brain, with over 95% occurring by 5min post-injection. In addition, scans following administration of  JHU37160 or uPSEM817 showed no significant differences compared to baseline saline scans, suggesting these ligands do not produce off-target effects on brain activity (n=4).

Conclusion

These findings support the use of recently developed PET-reporters and actuator ligands for DREADDs and PSAMs.

AcknowledgmentThis research was supported [in part] by the Intramural Research Program of the NIH, NIDA
References
[1] Bonaventura, J., Eldridge, M.A.G., Hu, F. et al. (2019). High-potency ligands for DREADD imaging and activation in rodents and monkeys. Nat Commun 10, 4627 doi.org/10.1038/s41467-019-12236-z
[2] Magnus, CJ et al. (2019) Ultrapotent chemogenetics for research and potential clinical applications. Science 364, eaav5282. doi:10.1126/science.aav5282
Baseline FDG scans show no off-target effects of JHU37160 or uPSEM817 on brain activity.
Comparisons of baseline FDG uptake following JHU37160 or uPSEM817 show no evidence of off-target effects on brain activity (0.1mg/kg i.v., n=4).
Keywords: Chemogenetics, DREADDs, PSAMs, Monkey
10:16 AM S-01-05

Evaluation of [18F]FR - a potential PET tracer for the diagnosis of cerebral amyloid angiopathy (#403)

Daniel Bleher1, Marilena Poxleitner1, Gregory D. Bowden1, 2, Sabrina Buss1, Ann-Kathrin Grotegerd1, Sonja Schembecker1, Bernd J. Pichler1, 2, Andreas Maurer1, 2, Kristina Herfert1

1 Eberhard Karls Universität, Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Tübingen, Baden-Württemberg, Germany
2 Eberhard Karls Universität, Cluster of Excellence iFIT (EXC 2180), Tübingen, Baden-Württemberg, Germany

Introduction

The β-amyloid peptide Aβ1-40 is the main component in cerebral amyloid angiopathy (CAA), a degenerative disorder of the brain vasculature with a prevalence of 50% in elderly people, but its diagnosis is still not possible without brain biopsy. A selective radiotracer would aid the clinical delineation of vascular from parenchymal amyloid depositions (PEA) present in Alzheimer’s Disease (AD) mainly composed of Aβ1-42. Herein, we evaluate the benzoxazinole derivative [18F]FR as PET tracer targeting CAA.

Methods

Specificity and selectivity of [3H]FR was tested in in vitro binding assays using recombinant Aβ1-40 and Aβ1-42 fibrils. Two different AD mouse models, the APP23 model with, and the APPPS1 model without CAA pathology in the brain were used for further evaluation. In vitro autoradiography (AR) and immunofluorescence staining was performed on brain sections. ICTAD-1, a fluorescent dye able to distinguish between Aβ1-40 and Aβ1-42, was used as reference. After automated radiolabeling, purification, and formulation of [18F]FR, the pharmacokinetic profile was determined in mice. In vivo PET with [18F]FR was performed in wild type and transgenic mice.

Results

[3H]FR shows specific binding to both fibril entities with a selectivity towards Aβ1-40 (Kd=5.7 nM) over Aβ1-42 (Kd=122 nM). AR with [18F]FR demonstrated specific binding of the radiotracer towards CAA in mice brain sections and only moderate binding to PEA. Metabolite analysis revealed one radio-metabolite in plasma and brain with 91% of the parent compound remaining in the brain at 5 min. Preliminary in vivo PET data analysis revealed a high binding potential (BPnd) of [18F]FR in the cortex of APP23 mice with CAA (BPnd = 0.23 ± 0.04; n = 5) with negligible binding in APPPS1 mice (BPnd = 0.04 ± 0.04; n = 4) and wild type mice (BPnd = 0.02 ± 0.04; n = 9).

Conclusion

FR demonstrated selectivity towards CAA over PEA in vitro and in vivo. After completed in vivo evaluation of FR, future studies will evaluate the compound in the CAA mouse model APPDutch and in pigs to investigate metabolite formation in higher species.

Acknowledgment

The authors want to thank Johannes Kinzler, Ramona Stremme, Linda Schramm and Christian Köder for support on radiosynthesis and animal handling.

Keywords: Amyloid, Cerebral Amyloid Angiopathy, PET imaging, Tracer Development
10:31 AM S-01-06

Break

10:46 AM S-01-07

Examining kinetic spectrum of extracerebral signal and its contributions to reference regions of 18F-MK6240 PET (#280)

Jessie Fanglu Fu1, 2, Cristina Lois2, 3, Justin Sanchez3, Alex Becker2, 3, Zoe Rubinstein3, Emma Thibault3, Andrew Salvatore1, Hasan Sari1, 2, Michelle Farrell2, Marc Normandin2, 3, Nicolas J. Guehl2, 3, Georges El Fakhri2, 3, Keith Johnson2, 3, Julie C. Price1, 2

1 Athinoula A. Martinos Center for Biomedical Research, Massachusetts General Hospital, Charlestown, Massachusetts, United States of America
2 Harvard Medical School, Department of Radiology, Boston, Massachusetts, United States of America
3 Gordon Center for Medical Imaging, Massachusetts General Hospital, Division of Nuclear Medicine and Molecular Imaging, Boston, Massachusetts, United States of America

Introduction

Off-target 18F-MK-6240 binding to melanocytes (extracerebral signal (EC), Fig.1A) (1,2) can contaminate cerebellar gray matter (CerGM) reference uptake and impact the detection of emergent tau signal in Alzheimer’s disease (AD) (3,4) by reference tissue methods. We compared 18F-MK6240 kinetic spectra quantified in EC, CerGM and alternative reference regions to inform reference region selection, across the AD spectrum.

Methods

Thirteen subjects (8 controls (CN)–63±12yrs; 5 mild cognitive impairment (MCI) or AD–75±9yrs) underwent 120 min 18F-MK6240 PET, MRI, and arterial blood sampling. Masks for EC (Fig.1B-1C) and reference regions (CerGM, eroded CerGM 3mm, inferior CerGM (InfCer), cerebral white matter (WM) and pons) were generated (Freesurfer). Region-level spectral analysis (SA)(5) was applied to decompose total 18F-MK6240 uptake into spectral components with frequency band amplitude a. SA kinetic spectra were compared between EC and reference regions (T-tests, p<0.05).

Results

SA indicated 2 reversible components in reference regions (Fig.2A-2B): 1) b ~0.03min-1 (slow, non-displaceable uptake) and 2) b ~0.18min-1 (fast, tracer delivery). Compared to CerGM, EC SUV was 30% higher and constantly increasing in late frames (Fig.1D) and only exhibited a fast component with 90% lower amplitudes (P<10E-5, Figs.2D). A trapping/irreversible component (b =10E-5min-1) was detected in all regions, with amplitudes significantly higher in EC (~3-fold) than other regions (P<0.05) except InfCer (Fig.2E). Compared to CerGM, trapping amplitudes were higher in InfCer (50%), but lower for WM and pons (60%) and eroded CerGM (18%).

Conclusion

SA supports a 2-tissue compartment configuration in reference regions. EC signal uniquely exhibited 1 reversible component and largest trapping amplitudes. The trapping component in reference regions likely reflects EC contamination, which was most evident in InfCer and less prevalent in WM and pons. Further studies will quantify the EC signal longitudinally and examine model-based methods to account for the EC signal as an alternative to partial-volume-correction.

References
[1] Betthauser TJ, Cody KA, Zammit MD, Murali D, Converse AK, Barnhart TE, et al. In Vivo Characterization and Quantification of Neurofibrillary Tau PET Radioligand 18F-MK-6240 in Humans from Alzheimer Disease Dementia to Young Controls. J Nucl Med. 2019;60(1):93–9.
[2] Aguero C, Dhaynaut M, Normandin MD, Amaral AC, Guehl NJ, Neelamegam R, et al. Autoradiography validation of novel tau PET tracer [F-18]-MK-6240 on human postmortem brain tissue. Acta Neuropathol Commun. 2019 Dec 11;7(1):37. 
[3] Pascoal TA, Shin M, Kang MS, Chamoun M, Chartrand D, Mathotaarachchi S, et al. In vivo quantification of neurofibrillary tangles with [18F]MK-6240. Alzheimer’s Res Ther. 2018;10(1):74. 
[4] Guehl NJ, Wooten DW, Yokell DL, Moon SH, Dhaynaut M, Katz S, et al. Evaluation of pharmacokinetic modeling strategies for in-vivo quantification of tau with the radiotracer [18F]MK6240 in human subjects. Eur J Nucl Med Mol Imaging. 2019;46(10):2099–111. 
[5] Veronese M, Rizzo G, Bertoldo A, Turkheimer FE. Spectral Analysis of Dynamic PET Studies: A Review of 20 Years of Method Developments and Applications. Computational and Mathematical Methods in Medicine Hindawi Limited; 2016 p. 7187541. 
Figure 1. Example of [18F]-MK-6240 PET image and EC mask for a CN subject with high EC signal.
Figure 2. Spectral analysis kinetic spectrum for reference regions and EC signal.
Keywords: Alzheimer's disease, Tau PET, MK-6240, extracerebral signal, spectral analysis
11:01 AM S-01-08

Development of voxel-level EC50 Images for use in CNS Drug Development (#364)

Jocelyn Hoye1, Bart de Laat1, Heather Liu1, Evan D. Morris1

1 Yale University, Radiology and Biomedical Imaging, New Haven, Connecticut, United States of America

Introduction

We recently introduced voxel-level images of drug occupancy from PET via our “Lassen Plot Filter”[1]. Occupancy images revealed clear dependence (locally) of 11C-flumazenil displacement on dose of GABAa inhibitor, CVL-865. We hypothesized that regions requiring higher drug concentrations to achieve desired occupancy would have higher EC50 values. We introduce novel “EC50 images” from human data and supporting simulations.

Methods

Five healthy subjects were scanned with the nonselective GABAa tracer, 11C-flumazenil, before and twice after administration of CVL-865. We created ten occupancy images (Figure 1) and applied an Emax model at the voxel-level to all images, combined, to create one EC50 image. We performed simulations (Figure 2) to confirm our observations of regional variation in EC50 and to assess the effects of plasma concentration sampling on EC50 variability. EC50 images were simulated using plasma concentrations from the human study (3-279 ng/mL) and a noise model consistent with observed concentration-response data. In a second simulation study the range of plasma concentrations was extended (3-1957 ng/mL), to allow all simulated EC50 regions to reach a minimum of 97% occupancy.

Results

As expected, the EC50 image revealed spatial variation in ‘apparent drug affinity’ (differential displacement for a given dose). High EC50 was found in areas of low occupancy, for a given drug dose (Figure 1). Simulations showed that it is possible to distinguish between regional differences in EC50 estimates (Figure 2). Simulations also demonstrated that sampling from a wider, more complete range of plasma drug concentrations could improve and regularize EC50 precision, spatially (Figure 2).

Conclusion

Our results argue for (a) confidence in the ability of the EC50 images to identify regional variation and (b) a need to tailor the range of drug doses and post-drug sampling times to ensure uniform precision of the EC50. The EC50 image could add value to early phase drug development by identifying regional variation in affinity that might impact therapy or safety, and by guiding dose selection for late-phase trials.

References
[1] de Laat, Bart, Morris, Evan D., 2020, ‘A local-neighborhood Lassen plot filter for creating occupancy and non-displaceable binding images’, J Cereb Blood Flow Metab, Vol 41(6), 1379-1389.
Figure 1
(A) Ten occupancy images at the voxel level demonstrate spatial variation in occupancy. White text shows plasma concentration of CVL-865 (ng/mL). A clear dose-dependency is present between occupancy and CVL-865 dose/concentration. At the same CVL-865 plasma concentration, striatal regions show lower occupancy, unless at extreme plasma concentrations. (B) A human EC50 Image at the voxel level shows spatial variation.
Figure 2
EC50 images (top row, ng/mL) and corresponding CV images (bottom, unitless) generated with the 2-parameter model. Left column: ‘Empirical design’, middle column: ‘Extended design’, right column: ‘Ground Truth’. Color bars are clipped at a maximum value of 50 ng/mL (EC50) and 1 (CV).
Keywords: EC50, Lassen Plot, Occupancy, Brain PET, Simulations
11:16 AM S-01-09

Simultaneous optogenetic [18F]FDG-fPET/fMRI to study brain circuits in rats (#369)

Sabrina Buss1, Tudor M. Ionescu1, Laura Kuebler1, Gerald Reischl1, 2, Bernd J. Pichler1, 2, Kristina Herfert1

1 Eberhard Karls University Tuebingen, Preclinical Imgaing and Radiopharmacy, Tuebingen, Baden-Württemberg, Germany
2 Eberhard Karls University Tuebingen, Cluster of Excellence iFIT (EXC 2180), Tuebingen, Baden-Württemberg, Germany

Introduction

Optogenetic functional magnetic resonance imaging (ofMRI) combines a precise neuronal stimulation technique, with fMRI as indirect readout of neuronal activation. This enables a cell-type specific mapping of the whole brain dynamic response to the activation or inhibition of neuronal circuits. In addition, functional positron emission tomography (fPET) enables the detection of metabolic changes with good temporal resolution on single subject level via a continuous [18F]FDG infusion. In this study, a simultaneous [18F]FDG-fPET/BOLD-fMRI protocol was applied in rats subjected to optogenetic stimulation.

Methods

Male rats were stereotactically injected with 2 µL of an adeno-associated virus vector overexpressing channelrhodopsin-2 (ChR2) (n=17) or GFP (control) (n=10) into the right substantia nigra compacta (rSNc). Eleven weeks post-surgery, an optical fiber connected to a 473 nm laser was implanted into the rSNc. [18F]FDG-fPET/BOLD-fMRI scans were performed on a 7T small-animal MRI equipped with an inhouse-built PET insert under a constant infusion of α-chloralose and pancuronium bromide. [18F]FDG (142±8 MBq) was infused using a bolus plus constant infusion protocol and fMRI data were simultaneously acquired using an EPI-BOLD sequence (TR: 2 s, TE: 18 ms) during a 20 Hz laser light stimulation. Data were preprocessed and analyzed using SPM12.

Results

Optogenetic stimulation of rSNc resulted in BOLD signal changes of 2.4% in the right caudate putamen (CPuR) of ChR2 injected rats, while no changes were detected on the contralateral side and in GFP control rats. BOLD signal changes highly correlated to the stimulation paradigm and mean t-scores of 4.9±1.5 were observed in the dorsal, medial CPuR. A 7% higher [18F]FDG uptake in the CPuR of ChR2 injected rats was observed compared to the contralateral side and control rats in the last 10-minute time-interval of the PET acquisition (ChR2>GFP CPuR mean t-scores: 3.6±1.6). In contrast to fMRI, metabolic activation was more pronounced in the ventral, medial part of the CPuR.

Conclusion

We present for the first time simultaneous, dynamic [18F]FDG-fPET/BOLD-fMRI data during optogenetic stimulation in rats. Our data show a clear spatial and temporal mismatch between activity patterns observed with BOLD-fMRI and [18F]FDG-fPET, which may be related to the complementary readouts of both.

Keywords: simultaneous fPET/fMRI, optogenetics
11:31 AM S-01-10

MRI quantification of brain oxygenation and relationship with cerebrovascular reactivity in Moyamoya disease using simultaneous [15O]-water PET/MRI (#242)

Audrey P. Fan1, 2, David Y.-T. Chen3, 1, David D. Shin4, Moss Y. Zhao1, Jun-Hyung Park1, Bin Shen1, Mohammad M. Khalighi1, Dawn Holley1, Kim Halbert1, Gary K. Steinberg1, Greg Zaharchuk1

1 Stanford University, Department of Radiology, Stanford, California, United States of America
2 University of California, Davis, Department of Biomedical Engineering and Neurology, Davis, California, United States of America
3 Taipei Medical University, Department of Radiology, Taipei, Taiwan
4 GE Healthcare, Menlo Park, California, United States of America

Introduction

We compared two MRI approaches based on tissue R2’ relaxation and vein magnetic susceptibility, respectively, to measure oxygen extraction fraction (OEF) abnormalities in Moyamoya disease1, a steno-occlusive disorder of intracranial arteries. MRI-derived OEF was compared for different stenosis severities, and correlated to [15O]-water PET of cerebral blood flow (CBF) using hybrid PET/MRI.

Methods

Simultaneous 3T PET/MRI was acquired in 15 Moyamoya patients (ages 32-62 years, 9 female). PET perfusion imaging with [15O]-water (550-925 MBq) was performed at baseline and after vasodilation with acetazolamide to observe the perfusion change (ΔCBF)2. For baseline MRI of oxygenation, we calculated multi-parametric maps of R2’, which are proportional to tissue OEF3,4. These scans included fast spin echo with 8 echoes (28-122ms) and multi-echo gradient echo with 10 echoes (11.1-42.4ms). We also acquired gradient echo scans (0.41x0.41x0.70mm3) with flow compensation for quantitative susceptibility mapping (QSM) in veins5. CBF, ΔCBF, and R2’ were assessed within 10 vascular territories per hemisphere; QSM values were also evaluated in cortical veins draining these territories.

Results

Figure1a shows PET/MRI in a 32-y.o. male Moyamoya patient with right middle cerebral artery (MCA) occlusion and impaired PET cerebrovascular reactivity after vasodilation (green arrows). Higher vein density and susceptibility on QSM maps (indicating elevated OEF), and higher white matter R2’ were observed in the right hemisphere. In contrast, the 33-y.o. male patient in Figure1b had healthy baseline PET perfusion and preserved reactivity. The corresponding QSM maps were normal, with symmetric vessel caliber and susceptibility, as were the R2’ oxygenation maps. Across patients, [15O]-water PET revealed 22.5% reduction in baseline CBF and 55.6% reduction in ΔCBF for MCA areas with severe stenosis compared to normal territories (Figure2). Relative OEF assessed by vessel susceptibility was 20.4% higher for severe stenosis compared to normal areas (P=0.007); and inversely correlated with PET baseline CBF and ΔCBF (P<10-4). In contrast, R2’ showed smaller abnormality (9.6%) in stenosed areas, and weaker correlations to PET perfusion.

Conclusion

Advanced MRI revealed abnormal, elevated OEF in Moyamoya disease, which inversely correlated with CBF and reactivity measured by simultaneous PET. Susceptibility MRI detected more robust OEF increases in areas of pathology than R2’, with stronger correlation to perfusion deficits.

Acknowledgment

This study is funded by NIH grants 5K99NS102884-02 and R01EB025220-02.

References
[1] Ni WW, Christen T, Rosenberg J, Zun Z, Moseley ME, Zaharchuk G. Imaging of cerebrovascular reserve and oxygenation in Moyamoya disease. J Cereb Blood Flow Metab (2017): 37(4), 1213-1222.
[2] Ishii Y, Thamm T, Guo J, Khalighi MM, Wardak M, Holley D, Gandhi H, Park JH, Shen B, Steinberg GK, Chin FT, Zaharchuk G, Fan AP. Simultaneous phase‐contrast MRI and PET for noninvasive quantification of cerebral blood flow and reactivity in healthy subjects and patients with cerebrovascular disease. J Magn Reson Imag (2019): https://doi.org/10.1002/jmri.26773.
[3] Stone AJ, Blockley NP. A streamlined acquisition for mapping baseline brain oxygenation using quantitative BOLD. NeuroImage (2017): 147, 79-88.
[4] Kaczmarz S, Göttler J, Zimmer C, Hyder F, Preibisch C."Characterizing white matter fiber orientation effects on multi-parametric quantitative BOLD assessment of oxygen extraction fraction." J Cereb Blood Flow Metab (2019): 0271678X19839502.
[5] Fan AP, Khalil AA, Fiebach JB, Zaharchuk G, Villringer A, Villringer K, Gauthier CJ. Elevated brain oxygen extraction fraction measured by MRI susceptibility relates to perfusion status in acute ischemic stroke. J Cereb Blood Flow Metab (2019): 0271678X19827944.
Figure 1.
(a) PET/MRI in a 32-y.o. male Moyamoya patient with severe stenosis and impaired CBF reactivity on [15O]-water PET in the right middle cerebral artery (green arrows). Susceptibility mapping shows dense cortical veins and higher susceptibility in the affected hemisphere (dotted), indicating elevated oxygen extraction fraction (OEF). Asymmetric, large right lenticulostriate veins
are seen next to elevated white matter OEF on the R2' map (black arrow).
(b) PET/MRI in a 33-y.o. male patient shows preserved CBF and reactivity on PET. The susceptibility map and R2' maps have normal, symmetric values.
Figure 2
(Top) Baseline CBF, flow augmentation (ΔCBF), and oxygenation across patients in the middle cerebral artery territory. Regions with severe and moderate stenosis exhibited impaired baseline CBF and ΔCBF compared to normal areas. Oxygen extraction fraction (OEF) in veins (from susceptibility mapping) was 20.4% higher in severely stenosed versus healthy areas (P=0.007). (Bottom) Correlation between PET perfusion and oxygenation MRI across patients. Relative OEF assessed by vein susceptibility inversely correlated with CBF and ΔCBF; similar but weaker relationships were seen with R2' oxygenation.
Keywords: Cerebrovascular disease, PET/MRI, perfusion, oxygenation, cerebrovascular reactivity
11:46 AM S-01-11

Quantification and Kinetic Analysis of [11C]Deschloroclozapine Positron Emission Tomography Imaging for Designer Receptors Exclusively Activated by Designer Drugs in Monkey Brain (#108)

Xuefeng Yan1, Rachel Dick1, Sanjay Telu1, Mark Eldridge2, Jeih-San Liow1, Paolo Zanotti-Fregonara1, Cheryl Morse1, Lester Manly1, Robert Gladding1, Yuji Nagai3, Takafumi Minamimoto3, Sami S. Zoghbi1, Barry J. Richmond2, Victor W. Pike1, Robert B. Innis1

1 National Institutes of Health, Molecular Imaging Branch/National Institute of Mental Health, Bethesda, Maryland, United States of America
2 National Institutes of Health, Laboratory of Neuropsychology/National Institute of Mental Health, Bethesda, Maryland, United States of America
3 National Institutes for Quantum and Radiological Science and Technology, Department of Functional Brain Imaging/National Institute of Radiological Sciences, Chiba, Japan

Introduction

Designer Receptors Exclusively Activated by Designer Drugs (DREADD) is a powerful chemogenetic tool for manipulation of neuronal activity. Positron emission tomography (PET) imaging with a selective radioligand allows non-invasive visualization of DREADD. Although studies have shown that [11C]deschloroclozapine ([11C]DCZ) is a novel high-potency ligand for DREADD, no full quantitative analysis has been reported. Here we aimed to establish the gold standard for [11C]DCZ kinetic analysis and explore the suitability of reference tissue-based quantification methods.

Methods

Dynamic [11C]DCZ PET scans with arterial input function were acquired for 120 minutes at baseline and after intravenous administration of either clozapine-N-oxide (CNO, 10 mg/kg) or DCZ (1 or 10 mg/kg) in one monkey that received injection of virus expressing human M4 muscarinic DREADD (hM4Di) into the right amygdala. Goodness-of-fit for 1TCM and 2TCM were compared with F-tests, Akaike and MSC values. Regional binding potentials (BPND) derived from the total distribution volume (VT) were compared with those obtained from reference tissue models, with the cerebellum as the reference region.

Results

The baseline scans showed a high [11C]DCZ uptake in the hM4Di-DREADD region, which could be blocked by CNO and DCZ. Cerebellum uptake was lowest, but still almost 10% of the activity could be displaced by the blockers. 2TCM fitted the data better than 1TCM. Compared to 2TCM, BPND-hM4Di estimated with reference tissue models was about 50 % lower, while the underestimation was only about 20 % in all the other regions.

Conclusion

[11C]DCZ PET images can be quantified with a 2TCM. However, reference tissue methods show underestimation, which is more important in the regions with high BPND, such as the hM4Di region.

AcknowledgmentWe would like to thank NIH veterinary staff for all of their help with this project.
Representative Logan-derived VT parametric images
Regional plot of BPND difference between reference tissue models and 2TCM
Compared to 2TCM, BPND-hM4Di estimated with reference tissue models was about 50 % lower, while the underestimation was only about 20 % in all the other regions.
Keywords: designer receptor exclusively activated by designer drugs (DREADD), Deschloroclozapine (DCZ), kinetic models