Tissue distribution of radioiodinated FAUC113

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 radioiodinated analogue of the D4-selective ligand FAUC 113 for tracer studies in vivo, we investigated the in-vivo stability, biodistribution and brain-uptake of 7-¹³¹I-FAUC 113 in Sprague-Dawley rats. Methods: Radiolabelling was carried 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 radiohplc and radio-tlc. For in-vivo evaluation, sagittal cryo-sections of the rat brain were investigated by in-vitro and exvivo autoradiography on a μ-Imager system. Results: 7-¹³¹I-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.; 7-¹³¹I-FAUC 113 displayed rapid uptake (0.21-0.26% ID/g) and fast clearance in various brain regions consistent with the determined logP-value of 2.36±0.15 (n=4). In-vivo stability of 7-¹³¹I-FAUC 113 was confirmed in the frontal cortex (>95%). Ex-vivo autoradiography 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 7-¹³¹I-FAUC 113. In-vitro autoradiography using authentic 7-iodo-FAUC 113 or L-750667 failed to cause significant displacement of the radioligand. Conclusions: Radioiodinated 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 affinity and subtype selectivity, specific activity or blood-brainbarrier permeability.

Zusammenfassung

Ziel: Für eine Vielzahl psychiatrischer Erkrankungen wird eine Implikation des Dopamin-D4-Rezeptorsubtyps diskutiert. Um die Eignung eines radioiodierten Analogons des D4-selektiven Liganden FAUC 113 für Tracerstudien in vivo zu beurteilen, haben wir die in-vivo-Stabilität, Bioverteilung sowie die Hirnaufnahme von 7-¹³¹I-FAUC 113 in Sprague-Dawley-Ratten untersucht. Methoden: Die Radiomarkierung lieferte den Tracer in hoher radiochemischer Ausbeute und spezifischer Aktivität. Nach intravenöser Injektion wurden die zu bestimmten Zeitpunkten entnommenen Blut- und Gewebeproben zur Radio-HPLC/Radio-DC Metabolitenanalyse gesammelt. Für die in-vivo Evaluierung wurden sagittale Kryo-Gewebeschnitte des Rattenhirns mit Hilfe der in-vitro- und ex-vivo-Autoradiographie an einem μ-Imager untersucht. Ergebnisse: 7-¹³¹I-FAUC 113 wurde schnell aus dem Blut ausgeschieden. Die höchste 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. 7-¹³¹I-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 7-¹³¹I-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 vermeintlich spezifische Bindung von 7-¹³¹I-FAUC 113 im Gehirn nicht unterdrücken. In der in-vitro-Autoradiographie unter Verwendung von 7-iodo-FAUC 113 oder L-750667 erfolgte keine signifikante Deplazierung des Radioliganden. Schlussfolgerung: Das radioiodierte FAUC 113 erlaubt in vivo sowie in vitro keine Darstellung des D4-Rezeptors im Rattenhirn. Weiterführende Studien sollten auf die Entwicklung von D4-selektiven Radioliganden mit verbesserten Eigenschaften, wie Rezeptoraffinität und Subtypselektivität, spezifische Aktivität oder Blut-Hirn-Schranken- Durchgängigkeit, abzielen.

Keywords

D4 receptor - radioligand - autoradiography

Schlüsselwörter

D4-Rezeptor - Radioligand - Autoradiographie

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