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 TSPOspecific pyrazolopyrimidine, DPA-714. Methods: The affinity of DPA-714 for the TSPO was measured in rat kidney membranes with 3 H-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 18 F-fluoride displacement of the tosylate precursor. 18 F-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 18 F-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. 18 F-DPA-714 was prepared at a 16% radiochemical yield and a specific activity of 270 GBq/mmol. In rats harboring unilateral QA lesions, an 8-fold-higher level of uptake of 18 F-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 . PET studies demonstrated rapid penetration and good retention of 18 F-DPA-714 in the baboon brain. Pretreatment with PK11195 effectively inhibited the uptake of 18 F-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 18 F-DPA-714 resulted in radioligand washout, demonstrating the reversibility of 18 F-DPA-714 binding. Conclusion: 18 F-DPA-714 is a specific radioligand for the TSPO, displaying promising in vivo properties and thus warranting further investigation.

We report the synthesis and the affinity data at both the peripheral (PBR) and the central benzodiazepine receptors of a series of N,N-dialkyl-2-phenylindol-3-ylglyoxylamide derivatives III, designed as conformationally constrained analogues of 2-phenylindole-3-acetamides II such as FGIN-1-27. Most of the new compounds showed a high specificity and affinity for PBR, with K(i) in the nanomolar to subnanomolar range. The most potent ligands (4-7, 9, 13-27) stimulated steroid biosynthesis in rat C6 glioma cells with a potency similar to or higher than that of classical ligands. The SARs of this new class of compounds are discussed.

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Novel N,N-disubstituted indol-3-ylglyoxylamides (1-56), bearing different combinations of substituents R 1-R 5, were synthesized and evaluated as ligands of the translocator protein (TSPO), the 18 kDa protein representing the minimal functional unit of the "peripheral-type benzodiazepine receptor" (PBR). Most of the new compounds showed a nanomolar/subnanomolar affinity for TSPO and stimulated steroid biosynthesis in rat C6 glioma cells with a potency similar to or higher than that of classic TSPO ligands such as PK 11195. Moreover, when evaluated in vivo by means of the elevated-plus-maze (EPM) paradigm in the rat, compound 32, the best-performing derivative in terms of TSPO affinity and pregnenolone production, showed clear anxiolytic effects. The results of this study suggested that the novel N,N-disubstituted indol-3-ylglyoxylamides may represent a promising class of compounds potentially suited for the treatment of anxiety disorders.