Biodistribution studies of two 18 F-labeled pyridinylphenyl amides as subtype selective radioligands for the dopamine D3 receptor ☆ Carsten Hocke a, ⁎, Paul Cumming a , Simone Maschauer a , Torsten Kuwert a , Peter Gmeiner b , Olaf Prante a a Molecular Imaging and Radiochemistry, Nuclear Medicine Clinic, Friedrich-Alexander University, Schwabachanlage 6, D-91054 Erlangen, Germany b Department of Chemistry and Pharmacy, Emil Fischer Center, Friedrich-Alexander University, Schuhstrasse 19, D-91052 Erlangen, Germany abstract article info Article history: Received 25 September 2013 Received in revised form 3 December 2013 Accepted 12 December 2013 Keywords: Dopamine receptor D3 receptor Positron emission tomography PET Biodistribution studies Autoradiography Introduction: Dopamine D3 receptors are implicated in various neuropsychiatric diseases, drug abuse and alcoholism, but specific agents for D3 molecular imaging are lacking. We evaluated two in vitro selective fluorine-18-labeled radioligand candidates ([ 18 F]5 and [ 18 F]6) for positron emission tomography (PET) imaging of D3 receptor availability in the brain. Methods: Biodistribution was evaluated in Sprague–Dawley rats using ex vivo autoradiography and small- animal PET. Protein binding studies were conducted in human plasma and cerebrospinal fluid. Results: [ 18 F]5 showed rapid blood–brain barrier penetration and fast washout after intravenous injection, whereas the rat brain penetration of [ 18 F]6 was lower. The total distribution volume (V T ) of [ 18 F]5 was 20– 26 mL g -1 throughout brain. Co-injection with the D3 antagonist BP897 resulted in globally increased cerebral washout of [ 18 F]5 and [ 18 F]6, but SUV analysis and parametric mapping of binding potential (BP ND ) relative to the cerebellum did not reveal specific binding of either ligand in D3-rich brain regions, i.e. the ventral striatum. However, there was substantial displaceable binding of [ 18 F]5, and to a lesser extent [ 18 F]6, in the pituitary. Conclusion: These radioligands reveal dopamine D3 receptors in the pituitary, but are not suitable for PET imaging of in brain, possibly due to low specific signal relative to the globally high V T . © 2014 Elsevier Inc. All rights reserved. 1. Introduction Dysfunction of dopaminergic neurotransmission system is impli- cated in a number of neuropsychiatric diseases [1,2], and activation of specifically dopamine D3 receptors contributes to the reinforcing effects of drugs for substance abuse [3], and to the therapeutic effects of antipsychotic medications [4]. Molecular imaging of dopamine D3 receptors in vivo promises to improve our understanding of these issues, but available agents suffer from lack of specificity for the several subtypes of D2-like receptors (see Scheme 1 for some examples of dopaminergic ligands) [5]. In general, successful receptor radioligands for PET imaging studies must satisfy several require- ments, such as a high-yield radiosynthesis with high specific activity, good blood–brain barrier penetration, suitable affinity to the receptor, metabolic stability, and especially, a high pharmacological selectivity [6,7]. Autoradiographic studies in vitro with the D3-selective agonist [ 3 H]-7-hydroxy-DPAT [8–10] reveal abundant D3 binding in the nucleus accumbens and islands of Calleja, but lesser binding in the ventral caudate/putamen and cerebral cortex [11]. Among a variety of previously reported D3 radioligands [12,13], the dopamine D3- prefering agonist [ 11 C](+)PHNO ([ 11 C](+)-4-propyl-3,4,4a,5,6,10b- hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol) has been promoted as a selective D3 agent [14,15], but its distribution in brain fails to match the known pattern of D3 receptors [16], and furthermore suffers from the logistic limitations common to 11 C-labeled com- pounds. An 18 F-labeled PHNO derivative, potentially benefiting from the longer physical half-life of fluorine-18, did not show suitable properties for PET imaging [17]. We endeavoured in the present study to investigate the fitness of some novel 18 F-labeled D3-prefering partial agonists based on the arylpiperazine lead, which presents one of the most versatile structural templates for dopaminergic ligands [18,19], and has already served as the lead structure for a number of such investigations [20–23]. The 2-chloro- or 2,3-dichloro-substituted phenylpiperazine moiety represents an optimum balance between high D3 receptor affinity, and high selectivity for D3 over other dopamine receptor subtypes, as well as other classes of neuror- eceptors [3]. In our experience, the butylamide linking chain in these molecules imparts optimal selectivity for D3 binding sites. In a systematic search for new arylpiperazines to serve as selective D3 radioligands for PET, our group studied a series of compounds that have been modified at the amide and the aryl groups, and are labeled with [ 18 F]fluoride [24–27]. Based on this previous work, we Nuclear Medicine and Biology 41 (2014) 223–228 ☆ Contract/Grant sponsor: Deutsche Forschungsgemeinschaft (Contract/Grant No. PR 677/2-3). ⁎ Corresponding author at: Friedrich-Alexander University Erlangen-Nurnberg, Nuclear Medicine Clinic, Ulmenweg 18, D-91054 Erlangen, Germany. Tel.: +49 913 1853 9281; fax: +49 913 1853 9262. E-mail address: carsten.hocke@uk-erlangen.de (C. Hocke). 0969-8051/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.nucmedbio.2013.12.014 Contents lists available at ScienceDirect Nuclear Medicine and Biology journal homepage: www.elsevier.com/locate/nucmedbio