Characterization of C-11 or I-123 Labelled β-CIT-FP and β-CIT-FE Metabolism Measured in Monkey and Human Plasma. Identification of Two Labelled Metabolites with HPLC

β-CIT-FP and β-CIT-FE which are novel fluoroalkyl derivatives of 2β-carbomethoxy-3β-(4-iodophenyl)tropane (β-CIT), have recently been labelled with¹¹C or¹²³I and used for imaging the dopamine transporter with positron emission tomography (PET) or single photon emission tomography (SPET). The metabolite pattern of these novel tracers was studied with HPLC in plasma in monkeys and in urine and plasma in humans. Four labelled metabolites were found of which two were identified, the¹²³I-labelled free acid and¹¹C- or¹²³I-labelled nor-β-CIT. The latter was detected only in small amounts (<4 per cent). Furthermore, a polar metabolite and an unknown lipophilic¹¹C- or¹²³I-labelled metabolite were found. The amount of this unidentified labelled metabolite of [¹¹C]β-CIT-FP or [¹¹C]β-CIT-FE in monkey plasma was 21±8 per cent and 16±9 per cent at 60 min after injection, respectively. In conclusion, four types of labelled metabolites were found after injection of¹¹C- or¹²³I-labelled β-CIT-FP and β-CIT-FE in monkeys and humans of which one may hamper the quantitation of the dopamine transporter by PET or SPET.