Synthesis, Labeling, and Biological Evaluation of P2Y12 Receptor Radioligands for Positron Emission Tomography Imaging of Neuroinflammation

The P2Y₁₂ receptor (P2Y₁₂R) is a G-protein-coupled receptor whose expression level is directly correlated to microglial activation. Herein, we report on the design of a series of new P2Y₁₂R ligands and the radiolabeling and characterization of two positron emission tomography (PET) tracers, [¹¹C]37 and [¹⁸F]41. These compounds were evaluated by autoradiography studies on rat brain slices exhibiting overexpression of human P2Y₁₂Rs (AAV-hP2Y₁₂R). Metabolism and biodistribution of [¹⁸F]41 were evaluated ex vivo in healthy rats and indicated good metabolic stability with 41% of unchanged radioligand 1 h post injection and a limited crossing of the blood-brain barrier with a brain uptake of 0.02%ID/g 1 h post injection. In vivo PET imaging performed in the AAV-hP2Y₁₂R rat model confirmed this low brain uptake, and no significant difference was found in the transfected (SUV_mean 0.14 ± 0.01) versus contralateral (SUV_mean 0.13 ± 0.01) striatum of the AAV-hP2Y₁₂R model. Similar results were observed in healthy rats and in nonhuman primates. Additional studies in the presence of tariquidar led to a 3-4-fold increase in the [¹⁸F]41 brain concentration, suggesting that [¹⁸F]41 is a P-glycoprotein substrate. Future work will focus on improving radioligand design to enhance blood-brain barrier permeation and to reduce efflux transport.