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

Abstract

The purinergic receptor P2Y₁₂ (P2Y₁₂R) has emerged as a promising biomarker for selectively imaging the anti-inflammatory phenotype of microglia. Developing positron emission tomography (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). Herein, the synthesis and evaluation of a series of fluorinated pyrazolidine-3,5-dione derivatives are conducted as potential P2Y₁₂R PET tracers, designed based on a literature compound with promising physicochemical properties for brain permeability. All synthesized derivatives exhibit strong affinity for the human P2Y₁₂R in vitro, with K_i values ranging from 1.21 to 5.66 nM. One candidate is selected for radiolabeling with fluorine-18 ([¹⁸F]8) and evaluated in healthy rats using dynamic PET imaging, as well as ex vivo biodistribution and metabolism studies. Unfortunately, [¹⁸F]8 shows low brain uptake, potentially due to deprotonation of the pyrazolidine-3,5-dione scaffold in vivo, in addition to a rapid metabolism. These findings highlight the need for novel chemical entities as starting points for tracer development to target the P2Y₁₂R in the CNS.