Synthesis and evaluation in rats of homologous series of [18F]-labeled dopamine D2/3 receptor agonists based on the 2-aminomethylchroman scaffold as potential PET tracers

EJNMMI Research, Jul 2015

Background Agonist positron emission tomography (PET) tracers for dopamine D2/3 receptors (D2/3Rs) offer greater sensitivity to changes in endogenous dopamine levels than D2/3R antagonist tracers. D2/3R agonist tracers currently available for clinical research are labeled with the short-lived isotope carbon-11, which limits their use. We aimed to develop high-affinity D2R agonists amenable for labeling with the longer-living fluorine-18. Here, we report the evaluation as potential PET tracers of two homologous series of [18F]fluorinated tracers based on the 2-aminomethylchroman-7-ol (AMC) scaffold: (R)-2-((4-(2-fluoroalkoxy)benzylamino)methyl)chroman-7-ols (AMC13 homologues) and (R)-2-((2-(4-(4-(fluoroalkoxy)phenyl)piperazin-1-yl)ethylamino)methyl)chroman-7-ols (AMC15 homologues). We varied the length of the 18F-fluoroalkyl chain in these structures to balance brain penetration and non-specific binding of the radioligands by adjusting their lipophilicity. Methods The tracers were evaluated in brain slices of Sprague-Dawley rats by in vitro autoradiography and in living rats by microPET imaging and ex vivo autoradiography. PET data were analyzed with one- and two-tissue compartmental models (1TCM/2TCM), simplified reference tissue model (SRTM), and Logan graphical analysis. Specificity of binding was tested by blocking D2/3R with raclopride. Results Homologues with a shorter fluoroalkyl chain consistently showed greater D2/3R-specific-to-total binding ratios in the striatum than those with longer chains. The fluoroethoxy homologue of AMC13 ([18F]FEt-AMC13) demonstrated the highest degree of D2/3R-specific binding among the evaluated tracers: mean striatum-to-cerebellum uptake ratio reached 4.4 in vitro and 2.1/2.8 in vivo/ex vivo (PET/autoradiography). Striatal binding potential (BPND) relative to cerebellum was 0.51–0.63 depending on the estimation method. Radiometabolites of [18F]FEt-AMC13 did not enter the brain. In vitro, application of 10 μmol/L raclopride reduced D2/3R-specific binding of [18F]FEt-AMC13 in the striatum by 81 %. In vivo, pre-treatment with 1 mg/kg (2.9 μmol/kg) raclopride led to 17–39 % decrease in D2/3R-specific binding in the striatum. Conclusions Varying the length of the [18F]fluoroalkyl chain helped improve the characteristics of the original candidate tracers. Further modifications of the current lead [18F]FEt-AMC13 can provide an agonist radiopharmaceutical suitable for D2/3R imaging by PET.

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Synthesis and evaluation in rats of homologous series of [18F]-labeled dopamine D2/3 receptor agonists based on the 2-aminomethylchroman scaffold as potential PET tracers

Shalgunov et al. EJNMMI Research Synthesis and evaluation in rats of 18 homologous series of [ F]-labeled dopamine D receptor agonists based on 2/3 the 2-aminomethylchroman scaffold as potential PET tracers Vladimir Shalgunov 0 Jan-Peter van Wieringen 2 Henk M. Janssen 1 P. Michel Fransen 1 Rudi A.J.O. Dierckx 0 Martin C. Michel 3 Jan Booij 2 Philip H. Elsinga 0 0 Department of Nuclear Medicine and Molecular Imaging, University Medical Center Groningen, University of Groningen , Groningen , The Netherlands 1 SyMO-Chem BV , Eindhoven, The 2 Department of Nuclear Medicine, Academic Medical Center, University of Amsterdam , Amsterdam , The Netherlands 3 Department of Pharmacology, Johannes Gutenberg University , Mainz , Germany Background: Agonist positron emission tomography (PET) tracers for dopamine D2/3 receptors (D2/3Rs) offer greater sensitivity to changes in endogenous dopamine levels than D2/3R antagonist tracers. D2/3R agonist tracers currently available for clinical research are labeled with the short-lived isotope carbon-11, which limits their use. We aimed to develop high-affinity D2R agonists amenable for labeling with the longer-living fluorine-18. Here, we report the evaluation as potential PET tracers of two homologous series of [18F]fluorinated tracers based on the 2-aminomethylchroman-7-ol (AMC) scaffold: (R)-2-((4-(2-fluoroalkoxy)benzylamino)methyl)chroman-7-ols (AMC13 homologues) and (R)-2-((2-(4(4-(fluoroalkoxy)phenyl)piperazin-1-yl)ethylamino)methyl)chroman-7-ols (AMC15 homologues). We varied the length of the 18F-fluoroalkyl chain in these structures to balance brain penetration and non-specific binding of the radioligands by adjusting their lipophilicity. Methods: The tracers were evaluated in brain slices of Sprague-Dawley rats by in vitro autoradiography and in living rats by microPET imaging and ex vivo autoradiography. PET data were analyzed with one- and two-tissue compartmental models (1TCM/2TCM), simplified reference tissue model (SRTM), and Logan graphical analysis. Specificity of binding was tested by blocking D2/3R with raclopride. Results: Homologues with a shorter fluoroalkyl chain consistently showed greater D2/3R-specific-to-total binding ratios in the striatum than those with longer chains. The fluoroethoxy homologue of AMC13 ([18F]FEt-AMC13) demonstrated the highest degree of D2/3R-specific binding among the evaluated tracers: mean striatum-to-cerebellum uptake ratio reached 4.4 in vitro and 2.1/2.8 in vivo/ex vivo (PET/autoradiography). Striatal binding potential (BPND) relative to cerebellum was 0.51-0.63 depending on the estimation method. Radiometabolites of [18F]FEt-AMC13 did not enter the brain. In vitro, application of 10 μmol/L raclopride reduced D2/3R-specific binding of [18F]FEt-AMC13 in the striatum by 81 %. In vivo, pre-treatment with 1 mg/kg (2.9 μmol/kg) raclopride led to 17-39 % decrease in D2/3R-specific binding in the striatum. Conclusions: Varying the length of the [18F]fluoroalkyl chain helped improve the characteristics of the original candidate tracers. Further modifications of the current lead [18F]FEt-AMC13 can provide an agonist radiopharmaceutical suitable for D2/3R imaging by PET. Agonist tracer; Dopamine receptor; PET; Fluorine-18 - Background Dysregulation of dopamine signaling through dopamine D2 and D3 receptors (D2/3Rs) is implicated in many neuropsychiatric disorders [1–4], making imaging of D2/3Rs by positron emission tomography (PET) highly relevant. Majority of PET tracers developed for D2/3R imaging are antagonists [5], but in the last two decades, agonists have attracted attention as potential PET ligands for G protein-coupled neurotransmitter receptors (GPCRs). In in vitro competition studies, agonists consistently show higher affinity for receptor molecules bound to G proteins (“high-affinity state”) than for free receptors (“low-affinity state”) [6–8]. Agonists are therefore expected to selectively recognize the “high-affinity” subset of receptor population in the imaging experiments, while antagonists bind to all receptors disregarding “affinity states” [9]. Though the existence of a separate “high-affinity state” subpopulation of D2/3R in the brain is still not conclusively demonstrated in vivo [10], radiolabeled D2/3R agonists indeed turned out to be more sensitive than the antagonist [11C]raclopride to amphetamine-induced release of dopamine (itself also an agonist) in rodents, cats, non-human primates, and humans [11–17]. Alteration of the relative abundance of “high-affinity state” D2/3Rs (D2/3R-high), detected by in vitro methods, is implied in dopamine supersensitivity, a state relevant to psychosis, Parkinsonism, and drug addiction [18]. Changes in this in vitro-based parameter are likely to be translated into effects observable in vivo, so agonist tracers might provide new insights for the research into, and (early) diagnosis of, these common neuropsychiatric disorders. Numerous scaffolds have been investigated i (...truncated)


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Vladimir Shalgunov, Jan-Peter van Wieringen, Henk M. Janssen, P. Michel Fransen, Rudi A.J.O. Dierckx, Martin C. Michel, Jan Booij, Philip H. Elsinga. Synthesis and evaluation in rats of homologous series of [18F]-labeled dopamine D2/3 receptor agonists based on the 2-aminomethylchroman scaffold as potential PET tracers, EJNMMI Research, 2015, pp. 41, Volume 5, Issue 1, DOI: 10.1186/s13550-015-0119-x