(18)F-MCL-524, an (18)F-Labeled Dopamine D2 and D3 Receptor Agonist Sensitive to Dopamine: A Preliminary PET Study.
PET has been used to examine changes in neurotransmitter concentrations in the living brain. Pioneering PET studies on the dopamine system have used D2 and D3 receptor (D2/D3) antagonists such as (11)C-raclopride. However, more recently developed agonist radioligands have shown enhanced sensitivity to endogenous dopamine. A limitation of available agonist radioligands is that they incorporate the short-lived radioisotope (11)C. In the current study, we developed the (18)F-labeled D2/D3 receptor agonist (R)-(-)-2-(18)F-fluoroethoxy-N-n-propylnorapomorphine ((18)F-MCL-524).


METHODS
In total, 10 PET measurements were conducted on 5 cynomolgus monkeys. Initially, the binding of (18)F-MCL-524 was compared with that of (11)C-MNPA in 3 monkeys. Second, the specificity of (18)F-MCL-524 binding was examined in pretreatment studies using raclopride (1.0 mg/kg) and d-amphetamine (1.0 mg/kg). Third, a preliminary kinetic analysis was performed using the radiometabolite-corrected arterial input function of the baseline studies. Finally, 2 whole-body PET measurements were conducted to evaluate biodistribution and radiation dosimetry after intravenous injection of (18)F-MCL-524.


RESULTS
(18)F-MCL-524 entered the brain and provided striatum-to-cerebellum ratios suitable for reliable quantification of receptor binding using the multilinear reference tissue model. Mean striatal nondisplaceable binding potential (BPND) values were 2.0 after injection of (18)F-MCL-524 and 1.4 after (11)C-MNPA. The ratio of the BPND values of (18)F-MCL-524 and (11)C-MNPA was 1.5 across striatal subregions. After administration of raclopride and d-amphetamine, the (18)F-MCL-524 BPND values were reduced by 89% and 56%, respectively. Preliminary kinetic analysis demonstrated that BPND values obtained with the 1-tissue- and 2-tissue-compartment models were similar to values obtained with the multilinear reference tissue model. Estimated radiation doses were highest for gallbladder (0.27 mSv/MBq), upper large intestine (0.19 mSv/MBq), and small intestine (0.17 mSv/MBq). The estimated effective dose was 0.035 mSv/MBq.


CONCLUSIONS
The (18)F-labeled agonist (18)F-MCL-524 appears suitable for quantification of D2/D3 receptor binding in vivo, and the results encourage extension to human studies. The longer half-life of (18)F makes (18)F-MCL-524 attractive for studies on modulation of the dopamine concentration-for example, in combination with simultaneous measurement of changes in blood-oxygen-level-dependent signal using bimodal PET/functional MRI.
Karolinska Institutet, Department of Clinical Neuroscience, Center for Psychiatric Research, Stockholm, Sweden sjoerd.finnema@ki.se.
Sjoerd J
SJ
Finnema
Karolinska Institutet, Department of Clinical Neuroscience, Center for Psychiatric Research, Stockholm, Sweden.
Vladimir
V
Stepanov
Karolinska Institutet, Department of Clinical Neuroscience, Center for Psychiatric Research, Stockholm, Sweden.
Ryuji
R
Nakao
McLean Hospital, Harvard Medical School, Belmont, Massachusetts.
Anna W
AW
Sromek
McLean Hospital, Harvard Medical School, Belmont, Massachusetts.
Tangzhi
T
Zhang
McLean Hospital, Harvard Medical School, Belmont, Massachusetts.
John L
JL
Neumeyer
University of Toronto, Toronto, Ontario, Canada.
Susan R
SR
George
University of Toronto, Toronto, Ontario, Canada.
Philip
P
Seeman
Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, Tennessee.
Michael G
MG
Stabin
Department of Nuclear Medicine, Karolinska University Hospital, Stockholm, Sweden; and.
Cathrine
C
Jonsson
Karolinska Institutet, Department of Clinical Neuroscience, Center for Psychiatric Research, Stockholm, Sweden AstraZeneca, Translational Science Center at Karolinska Institutet, Stockholm, Sweden.
Lars
L
Farde
Karolinska Institutet, Department of Clinical Neuroscience, Center for Psychiatric Research, Stockholm, Sweden.
Christer
C
Halldin
D000818:Animals; D001058:Apomorphine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology; D004298:Dopamine / Q000378:metabolism*; D007553:Isotope Labeling; D007700:Kinetics; D008252:Macaca fascicularis; D049268:Positron-Emission Tomography / Q000379:methods*; D011854:Radiochemistry; D011874:Radiometry; D017448:Receptors, Dopamine D2 / Q000819:agonists*; D050637:Receptors, Dopamine D3 / Q000819:agonists*
C000599321:2-fluoroethoxy-N-n-propylnorapomorphine; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; C008668:norapomorphine; D001058:Apomorphine; D004298:Dopamine
18F-MCL-524; PET; agonist; dopamine; monkey