Investigation of the Metabolites of (S,S)-[11C]MeNER in Humans, Monkeys and Rats

Introduction  (S,S)-¹¹C]MeNER ((S,S)-2-(α-(2-¹¹C]methoxyphenoxy)benzyl)morpholine) is a positron emission tomography (PET) radioligand recently applied in clinical studies of norepinephrine transporters (NETs) in the human brain in vivo. In view of further assessment of the suitability of (S,S)-¹¹C]MeNER as a NET radioligand, its metabolism and the identity of the in vivo radiometabolites of (S,S)-¹¹C]MeNER are of great interest. Materials and Methods  Thus, PET studies were used to measure brain dynamics of (S,S)-¹¹C]MeNER, and plasma reverse-phase radiochromatographic analysis was performed to monitor and quantify its rate of metabolism. Eighteen healthy human volunteers, five cynomolgus monkeys, and five rats were studied. Results and Discussion  In human subjects, the plasma radioactivity representing (S,S)-¹¹C]MeNER decreased from 88 ± 5% at 4 min after injection to 82 ± 7% at 40 min, while a polar radiometabolite increased from 3 ± 3% to 16 ± 7% at the same time-points, respectively. A more lipophilic radiometabolite than (S,S)-¹¹C]MeNER decreased from 9 ± 5% at 4 min to 1 ± 2% at 40 min. In monkeys, plasma radioactivity representing (S,S)-¹¹C]MeNER decreased from 97 ± 2% at 4 min to 74 ± 7% at 45 min, with a polar fraction as the major radiometabolite. A more lipophilic radiometabolite than (S,S)-¹¹C]MeNER, constituted 3 ± 2% of radioactivity at 4 min and was not detectable later on. In rats, 17 ± 4% of plasma radioactivity was parent radioligand at 30 min with the remainder comprising mainly a polar radiometabolite. (S,S)-¹¹C]MeNER in rat brain and urine at 30 min after injection were 90% and 4%, respectively. On a brain regional level, parent radioligand ranged from 87.5 ± 3.9% (57.2 ± 14.2% SUV standard uptake values, %injected radioactivity per mL multiplied with animal weight (in g)]; cerebellum) to 92.9 ± 1.8% (36.1 ± 4.7% SUV; striatum), with differential distribution of the radiometabolite in the cerebellum (6.7 ± 0.3% SUV) and the striatum (2.5 ± 0.3% SUV). Liquid chromatography-mass spectrometry analysis of rat urine identified a hydroxylation product of the methoxyphenoxy ring of (S,S)-MeNER as the main metabolite. In the brain, the corresponding main metabolite was the product from O-de-methylation of (S,S)-MeNER. PET measurements were performed in rats as well as in wild-type and P-gp-knock-out mice. In rats, the brain peak level of radioactivity was found to be very low (65%SUV). In mice, there was only a small difference in peak brain accumulation between P-gp knock-out and wild-type mice (145 vs. 125%SUV) with the following rank order of regional brain radioactivity: cerebellum × thalamus > cortical regions > striatum. Conclusion  It can be concluded that radiometabolites of (S,S)-¹¹C]MeNER are of minor importance in rat and monkey brain imaging. The presence of a transient lipophilic radiometabolite in peripheral human plasma may induce complications with brain imaging, but its kinetics appear favorable in relation to the slow kinetics of (S,S)-¹¹C]MeNER in humans.