Identification and regional distribution in rat brain of radiometabolites of the dopamine transporter PET radioligand [11C]PE2I

We aimed to determine the composition of radioactivity in rat brain after intravenous administration of the dopamine transporter radioligand, [11C]PE2I. PET time-activity curves (TACs) and regional brain distribution ex vivo were measured using no-carrier-added [11C]PE2I. Carrier-added [11C]PE2I was administered to identify metabolites with high-performance liquid radiochromatography (RC) or RC with mass spectrometry (LC-MS and MS-MS). The stability of [11C]PE2I was assessed in rat brain homogenates. After peak brain uptake of no-carrier-added [11C]PE2I, there was differential washout rate from striata and cerebellum. Thirty minutes after injection, [11C]PE2I represented 10.9 ± 2.9% of the radioactivity in plasma, 67.1 ± 11.0% in cerebellum, and 92.5 ± 3.2% in striata, and was accompanied by two less lipophilic radiometabolites. [11C]PE2I was stable in rat brain homogenate for at least 1 h at 37 C. LC-MS identified hydroxylated PE2I (1) (m/z 442) and carboxyl-desmethyl-PE2I (2) (m/z 456) in brain. MS-MS of 1 gave an m/z 442→424 transition due to H2O elimination, so verifying the presence of a benzyl alcohol group. Metabolite 2 was the benzoic acid derivative. Ratios of ex vivo measurements of [11C]PE2I, [11C]1, and [11C]2 in striata to their cognates in cerebellum were 6.1 ± 3.4, 3.7 ± 2.2 and 1.33 ± 0.38, respectively, showing binding selectivity of metabolite [11C]1 to striata. Radiometabolites [11C]1 and [11C]2 were characterized as the 4-hydroxymethyl and 4-carboxyl analogs of [11C]PE2I, respectively. The presence of the pharmacologically active [11C]1 and the inactive [11C]2 is a serious impediment to successful biomathematical analysis. (orig.)