Synthesis of ([11C]carbonyl)raclopride and a comparison with ([11C]methyl)raclopride in a monkey PET study

IntroductionThe selective dopamine D2 receptor antagonist raclopride is usually labeled with carbon-11 using [11C]methyl iodide or [11C]methyl triflate for use in the quantification of dopamine D2 receptors in human brain. The aim of this work was to label raclopride at the carbonyl position using [11C]carbon monoxide chemistry and to compare ([11C]carbonyl)raclopride with ([11C]methyl)raclopride in non-human primate (NHP) using PET with regard to quantitative outcome measurement, metabolism of the labeled tracers and protein binding.MethodsPalladium-mediated carbonylation using [11C]carbon monoxide, 4,6-dichloro-2-iodo-3-methoxyphenol and (S)-(-)-2-aminomethyl-1-ethylpyrrolidine was applied in the synthesis of ([11C]carbonyl)raclopride. The reaction was performed at atmospheric pressure using xantphos as supporting phosphine ligand and palladium (π-cinnamyl) chloride dimer as the palladium source. ([11C]Methyl)raclopride was prepared by a previously published method. In the PET study, two female cynomolgus monkeys were used under gas anesthesia of sevoflurane. A dynamic PET measurement was performed for 63 min with an HRRT PET camera after intravenous injection of ([11C]carbonyl)raclopride and ([11C]methyl)raclopride, respectively, during the same day. The order of injection of the two PET radioligands was changed between the two monkeys. The venous blood sample for measurement of protein binding was taken 3 min prior to the PET scan. Binding potential (BPND) of the putamen and caudate was calculated with SRTM using the cerebellum as a reference region.ResultsThe target compound ([11C]carbonyl)raclopride was obtained with 50 ± 5% decay corrected radiochemical yield and 95% radiochemical purity. The trapping efficiency (TE) of [11C]carbon monoxide was 65 ± 5% and the specific radioactivity of the final product was 34 ± 1 GBq/μmol after a 50 min of synthesis time. The radiochemical yield of ([11C]methyl)raclopride was in the same range as published previously i. e. 50–60% and specific radioactivity of those two batches which were used in the present PET study were 192 GBq/μmol and 638 GBq/μmol respectively after a synthesis time of 32 min. In monkey PET studies, the percentage difference of BPND in putamen was < 3% and that in caudate was < 9% for the two radioligands. The plasma protein binding was 86.2 ± 0.3% and 85.7 ± 0.6% for ([11C]carbonyl)raclopride and ([11C]methyl)raclopride, respectively. The radiometabolite pattern was similar for both radioligands.ConclusionRaclopride was 11C-labeled at the carbonyl position using a palladium-mediated [11C]carbonylation reaction. A comparison between ([11C]carbonyl)raclopride and ([11C]methyl)raclopride with regard to quantitative PET outcome measurements, metabolism of radioligands and protein binding in monkey was performed. The monkey PET study with ([11C]carbonyl)raclopride showed similar results as for ([11C]methyl)raclopride. The PET studies were performed on 2 subjects.