Exploration of Fluorinated Pyrazolidine-3,5-diones for Positron Emission Tomography Imaging of the P2Y12 Receptor in the Central Nervous System

The purinergic receptor P2Y12 (P2Y12R) has emerged as a promising biomarker for selectively imaging the anti-inflammatory phenotype of microglia. Developing PET tracers for this target is an active area of research, as imaging of specific microglial phenotypes can provide valuable insights into their dynamics in neuroinflammation. A key challenge is identifying high affinity P2Y12R ligands with optimal properties for targeting this receptor in the central nervous system (CNS). In this study, we report the synthesis and evaluation of a series of fluorinated pyrazolidine-3,5-dione derivatives as potential P2Y12R PET tracers, designed based on a lead structure with favorable physicochemical properties for brain permeability. All synthesized derivatives exhibited strong affinity for P2Y12R in vitro, with Ki values ranging from 1.21 to 5.66 nM. One candidate was selected for radiolabeling with fluorine-18 ([18F]6d) and evaluated in healthy rats using dynamic PET imaging under baseline conditions and efflux transporter-blocking, in addition to ex vivo biodistribution and metabolism studies. Unfortunately, [18F]6d showed low brain uptake, potentially due to ionization of the pyrazolidine-3,5-dione core in vivo or poor tracer stability. These findings highlight the need for novel chemical entities as starting points for targeting P2Y12R in the CNS.