Characterization of [ ^<b>18</b> F]PF-NB1 Enantiomers for Imaging NMDA GluN2B Receptors in Nonhuman Primates and Comparison with Analogous ^<b>18</b> F-labeled Radiotracers

The GluN2B subunit of <i>N</i>-methyl-d-aspartate receptors (NMDARs) plays a key role in regulating excitatory neurotransmission and synaptic plasticity. Aberrant GluN2B signaling contributes to glutamate-mediated excitotoxicity and has been implicated in neurodegenerative and neuropsychiatric disorders, making it a crucial target for therapeutic development and <i>in vivo</i> imaging. In this study, we systematically evaluated the pharmacokinetics and imaging characteristics of both (<i>R</i>)- and (<i>S</i>)-enantiomers of [¹⁸F]PF-NB1 PET tracers in nonhuman primates to determine their suitability for clinical translation. Dynamic PET scans, arterial input functions, kinetic modeling, and blocking studies were performed to assess brain uptake, regional distribution consistent with GluN2B expression, binding specificity, and metabolite profiles. In addition to direct comparison of the two enantiomers, we benchmarked their in vivo performance against analogous ¹⁸F-labeled GluN2B tracers, including OF-Me-NB1 and OF-NB1. This cross-evaluation revealed differences in brain uptake, regional distribution matching GluN2B expression, binding specificity, and metabolic stability. Regional time-activity curves were fitted with the one-tissue compartment (1TC) model to derive regional volume of distribution (<i>V</i>_T) values, which were higher for (<i>R</i>)-[¹⁸F]PF-NB1(61.5-169.4 mL/cm³) than (<i>S</i>)-[¹⁸F]PF-NB1 (48.6-88.9 mL/cm³) in the same monkey. A GluN2B selective antagonist, Co-101,244, substantially reduced the uptake and <i>V</i>_T values of both (<i>R</i>)- and (<i>S</i>)-[¹⁸F]PF-NB1 across brain regions, with calculated target occupancy of 96% and 90%, and nondisplaceable volume of distribution (<i>V</i>_ND, mL/cm³) values of 10.01 and 11.45, respectively. In the same monkey, regional nondisplaceable binding potential (<i>BP</i>_ND) values ranged from 3.24 to 6.76 for (<i>S</i>)-[¹⁸F]PF-NB1, and from 5.12 to 15.92 for (<i>R</i>)-[¹⁸F]PF-NB1.