Received: May 12, 2005; in revised form: July 14, 2005 Nuklearmedizin 1/2006 41 © 2006 Schattauer GmbH Tissue distribution of radioiodinated FAUC113 Assessment of a pyrazolo(1,5-a)pyridine based dopamine D4 receptor radioligand candidate O. Prante 1 , C. Hocke 1 , S. Löber 2 , H. Hübner 2 , P. Gmeiner 2 , T. Kuwert 1 1 Laboratory of Molecular Imaging, Clinic of Nuclear Medicine (Head: Prof. Dr. T. Kuwert), 2 Department of Medicinal Chemistry (Head: Prof. Dr. P. Gmeiner), Friedrich-Alexander University Erlangen, Germany Keywords D4 receptor, radioligand, autoradiography Summary Aim: Disturbances of the D4 receptor subtype have been implicated in the genesis of a broad range of psychiatric disorders. In order to assess the suitability of a radioiodi- nated analogue of the D4-selective ligand FAUC 113 for tracer studies in vivo, we investigated the in-vivo stability, biodistribution and brain-uptake of 7- 131 I-FAUC 113 in Sprague-Dawley rats. Methods: Radiolabelling was car- ried out with high radiochemical yield and specific activity. After intravenous injection, blood and tissue samples, taken at designated time intervals, were collected for analysis. Analyses of metabolites were performed by radio- hplc and radio-tlc. For in-vivo evaluation, sagittal cryo-sec- tions of the rat brain were investigated by in-vitro and ex- vivo autoradiography on a µ-Imager system. Results: 7- 131 I-FAUC 113 was rapidly cleared from blood. Highest uptake was observed in kidney (0.603±0.047% ID/g, n=4) and liver (0.357±0.070% ID/g, n=4) at 10 min p.i.; 7- 131 I-FAUC 113 displayed rapid uptake (0.21-0.26% ID/g) and fast clearance in various brain re- gions consistent with the determined logP-value of 2.36±0.15 (n=4). In-vivo stability of 7- 131 I-FAUC 113 was confirmed in the frontal cortex (>95%). Ex-vivo auto- radiography revealed a frontal cortex-to-cerebellum ratio of 1.57±0.13 at 10 min p.i. (n=6). Coinjection with L-750667 could not suppress any putative specific binding of 7- 131 I-FAUC 113. In-vitro autoradiography using auth- entic 7-iodo-FAUC 113 or L-750667 failed to cause signifi- cant displacement of the radioligand. Conclusions: Radio- iodinated FAUC 113 does not allow imaging of D4 receptors in the rat brain in vivo nor in vitro. Further work should aim at the development of selective dopamine D4 radioligands with improved tracer characteristics, such as receptor affin- ity and subtype selectivity, specific activity or blood-brain- barrier permeability. Nuklearmedizin 2006; 45: 41–8 Schlüsselwörter D4-Rezeptor, Radioligand, Autoradiographie Zusammenfassung Ziel: Für eine Vielzahl psychiatrischer Erkrankungen wird eine Implikation des Dopamin-D4-Rezeptorsubtyps disku- tiert. Um die Eignung eines radioiodierten Analogons des D4-selektiven Liganden FAUC 113 für Tracerstudien in vivo zu beurteilen, haben wir die in-vivo-Stabilität, Biovertei- lung sowie die Hirnaufnahme von 7- 131 I-FAUC 113 in Sprague-Dawley-Ratten untersucht. Methoden: Die Ra- diomarkierung lieferte den Tracer in hoher radiochemischer Ausbeute und spezifischer Aktivität. Nach intravenöser In- jektion wurden die zu bestimmten Zeitpunkten entnomme- nen Blut- und Gewebeproben zur Radio-HPLC/Radio-DC Metabolitenanalyse gesammelt. Für die in-vivo Evaluie- rung wurden sagittale Kryo-Gewebeschnitte des Ratten- hirns mit Hilfe der in-vitro- und ex-vivo-Autoradiographie an einem µ-Imager untersucht. Ergebnisse: 7- 131 I-FAUC 113 wurde schnell aus dem Blut ausgeschieden. Die höchs- te Aufnahme wurde in der Niere (0.603±0.047% ID/g, n=4) und Leber (0.357±0.070% ID/g, n=4) 10 min p.i. beobachtet. 7- 131 I-FAUC 113 zeigte in verschiedenen Hirnregionen eine schnelle Aufnahme (0.21-0.26% ID/g) sowie Auswaschung. Dies stimmt mit dem ermittelten LogP-Wert von 2.36±0.15 (n=4) überein. Die in-vivo- Stabilität von 7- 131 I-FAUC 113 im frontalen Cortex wurde bestätigt (>95%). Die ex-vivo Autoradiographie zeigte ein Cortex/Cerebellum-Verhältnis von 1.57±0.13, 10 min p.i. (n=6). Eine Coinjektion mit L-750667 konnte die ver- meintlich spezifische Bindung von 7- 131 I-FAUC 113 im Ge- hirn nicht unterdrücken. In der in-vitro-Autoradiographie unter Verwendung von 7-iodo-FAUC 113 oder L-750667 erfolgte keine signifikante Deplazierung des Radioligan- den. Schlussfolgerung: Das radioiodierte FAUC 113 er- laubt in vivo sowie in vitro keine Darstellung des D4-Rezep- tors im Rattenhirn. Weiterführende Studien sollten auf die Entwicklung von D4-selektiven Radioliganden mit verbes- serten Eigenschaften, wie Rezeptoraffinität und Subtypse- lektivität, spezifische Aktivität oder Blut-Hirn-Schranken- Durchgängigkeit, abzielen. Bioverteilung von Radioiod- markiertem FAUC 113: Bewertung eines potenziellen Radioliganden für den Dopamin D4-Rezeptor mit Pyrazolo(1,5-a) -pyridin- Grundkörper T he dopamine D4 receptor subtype was cloned (34) and characterized by receptor autoradiography of knock-out mice (6) or immunohistochemis- try using receptor-specific antibodies (1, 7, 24). Its precise function and exact distribu- tion in the central nervous system are of great interest. Alterations and disturbances of the D4 receptor were implicated in the genesis and treatment of a broad range of neurobehavioral and psychiatric disorders such as novelty seeking, attention-deficit hyperactivity disorder and schizophrenia (26, 35). A higher density of D4 receptors was found in postmortem brain tissue of schizophrenic patients (32); however, this was not supported by other studies (9, 31). These discrepant findings could be ex- plained by the lack of a selective D4 recep- tor radioligand with high affinity and selec- tivity to other dopamine receptors. As yet, dopamine D4 receptor concentrations in vitro can only be determined by indirect binding studies, in which the number of binding sites defined with ³H-raclopride (D2/D3 antagonist) are substracted from total binding (³H-nemonapride) (13, 32). However, recent progress has been made by the synthesis of the first selective tritiated D4 agonist as a radioligand to more accu- rately characterize competitive binding for agonists at the D4 receptor (22). Moreover, the lack of D4 receptor selec- tive radioligands suitable for in-vivo im- aging techniques especially hampers the non-invasive investigation of neurotrans- mission by single photon emission to- mography (SPET) or positron emission to- mography (PET). These methodologies serve as high-performance imaging tools in nuclear medicine for understanding the