[11C]MeRo25, a Unique Chemotype PET Radioligand, Shows Robust Brain Uptake but Insufficient Selectivity for GluN2B Imaging

Background/Objectives: GluN2B subunits of N-methyl-D-aspartate (NMDA) receptors are therapeutic targets for neuropsychiatric disorders, but an effective positron emission tomography (PET) radioligand for human GluN2B brain imaging is still needed. We developed [11C]MeRo25, a 11C-methylated derivative of the GluN2B-preferring ligand Ro 25-6981, and evaluated its suitability as a GluN2B PET radioligand. Methods: MeRo25 was synthesized through methylation of Ro 25-6981, and affinities for GluN2B and σ1 receptors were determined in vitro. [11C]MeRo25 was prepared using [11C]methyl iodide. PET studies were performed in rats and monkeys at baseline and after challenge with GluN2B or σ1 receptor-directed ligands. In monkeys, regional total distribution volume (VT) was estimated through kinetic modeling. Results: MeRo25 showed low nanomolar affinity for GluN2B and σ1 receptors. [11C]MeRo25 was obtained with a radiochemical yield of 20–30%, a purity of >99%, and high molar activity. [11C]MeRo25 showed rapid and high brain uptake in rats and monkeys. Pretreatment with either GluN2B or σ1 receptor-directed ligands in monkeys markedly reduced brain radioactivity and regional VT, demonstrating contributions from both receptor populations. Conclusions: Despite favorable brain uptake and high GluN2B affinity, [11C]MeRo25 exhibits substantial σ1 receptor binding, producing a mixed GluN2B/σ1 receptor PET signal that precludes its use as a selective GluN2B radioligand. These findings emphasize the importance of in vivo pharmacological validation and reveal limitations of the Ro 25-6981 scaffold for selective GluN2B PET radioligand development. This study also cautions against the use of Ro 25-6981 as a pharmacological tool where binding to σ1 receptors may be of concern, for example as a challenge agent in PET studies.