RT Journal Article SR Electronic T1 DPA-714, a New Translocator Protein–Specific Ligand: Synthesis, Radiofluorination, and Pharmacologic Characterization JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 814 OP 822 DO 10.2967/jnumed.107.046151 VO 49 IS 5 A1 Michelle L. James A1 Roger R. Fulton A1 Johnny Vercoullie A1 David J. Henderson A1 Lucette Garreau A1 Sylvie Chalon A1 Frederic Dolle A1 Silvia Selleri A1 Denis Guilloteau A1 Michael Kassiou YR 2008 UL http://jnm.snmjournals.org/content/49/5/814.abstract AB The translocator protein (18 kDa) (TSPO), formerly known as the peripheral benzodiazepine receptor, is dramatically upregulated under pathologic conditions. Activated microglia are the main cell type expressing the TSPO at sites of central nervous system pathology. Radioligands for the TSPO can therefore measure active disease in the brain. This article details the synthesis, radiofluorination, and pharmacologic evaluation of a new TSPO-specific pyrazolopyrimidine, DPA-714. Methods: The affinity of DPA-714 for the TSPO was measured in rat kidney membranes with 3H-PK11195. The in vitro functional activity of DPA-714 was measured in a steroidogenic assay in which the ability of DPA-714 to increase pregnenolone synthesis was measured with rat C6 glioma cells. The radiofluorination of DPA-714 was achieved by nucleophilic 18F-fluoride displacement of the tosylate precursor. 18F-DPA-714 was assessed in rats harboring unilateral quinolinic acid (QA) lesions. In addition, pretreatment experiments were performed with PK11195 (5 mg/kg), DPA-714 (1 mg/kg), and DPA-713 (1 mg/kg). The in vivo binding and biodistribution of 18F-DPA-714 were determined in a baboon with PET. Experiments involving presaturation with PK11195 (1.5 mg/kg) and displacement with DPA-714 (1 mg/kg) were conducted to evaluate the specificity of radioligand binding. Results: In vitro binding studies revealed that DPA-714 displayed a high affinity for the TSPO (dissociation constant, 7.0 nM). DPA-714 stimulated pregnenolone synthesis at levels 80% above the baseline. 18F-DPA-714 was prepared at a 16% radiochemical yield and a specific activity of 270 GBq/μmol. In rats harboring unilateral QA lesions, an 8-fold-higher level of uptake of 18F-DPA-714 was observed in the ipsilateral striatum than in the contralateral striatum. Uptake in the ipsilateral striatum was shown to be selective because it was inhibited to the level in the contralateral striatum in the presence of PK11195, nonlabeled DPA-714, or DPA-713. PET studies demonstrated rapid penetration and good retention of 18F-DPA-714 in the baboon brain. Pretreatment with PK11195 effectively inhibited the uptake of 18F-DPA-714 in the whole brain, indicating its selective binding to the TSPO. The injection of nonlabeled DPA-714 20 min after the injection of 18F-DPA-714 resulted in radioligand washout, demonstrating the reversibility of 18F-DPA-714 binding. Conclusion: 18F-DPA-714 is a specific radioligand for the TSPO, displaying promising in vivo properties and thus warranting further investigation.