title: Efficient tritiation of the translocator protein (18 kDa) selective ligand DPA-714.
abstract: DPA-714 (N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide) is a recently discovered fluorinated ligand of the translocator protein 18 kDa (TSPO). Labelled with the short-lived positron emitter fluorine-18, this structure is today the radioligand of reference for in vivo imaging of microglia activation and neuroinflammatory processes with positron emission tomography. In the present work, an isotopically tritium-labelled version was developed ([(3) H]DPA-714), in order to access high resolution in vitro and ex vivo microscopic autoradiography studies, repeated and long-lasting receptor binding studies and in vivo pharmacokinetic determination at late time points. Briefly, DPA-714 as reference, and its 3,5-dibrominated derivative as precursor for labelling, were both prepared from DPA-713 in nonoptimized 32% (two steps) and 10% (three steps) yields, respectively. Reductive debromination using deuterium gas and Pd/C as catalyst in methanol, performed at the micromolar scale, confirmed the regioselective introduction of two deuterium atoms at the meta positions of the phenyl ring. Tritiodebromination was analogously performed using no-carrier tritium gas. HPLC purification provided >96% radiochemically pure [(3) H]DPA-714 (7 GBq) with a 2.1 TBq/mmol specific radioactivity. Interestingly, additional hydrogen-for-tritium exchanges were also observed at the 5-methyl and 7-methyl positions of the pyrazolo[1,5-a]pyrimidine, opening novel perspectives in the labelling of compounds featuring this heterocyclic core.
authors.affiliation: CEA, DSV/I2BM, Service Hospitalier Frédéric Joliot, 91401, Orsay, France.
authors.forename: Annelaure
authors.initials: A
authors.lastname: Damont
authors.forename: Sébastien
authors.initials: S
authors.lastname: Garcia-Argote
authors.forename: David-Alexandre
authors.initials: DA
authors.lastname: Buisson
authors.forename: Bernard
authors.initials: B
authors.lastname: Rousseau
authors.forename: Frédéric
authors.initials: F
authors.lastname: Dollé
mesh_terms: D011720:Pyrazoles / Q000138:chemical synthesis*; D011743:Pyrimidines / Q000138:chemical synthesis*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis*; D014316:Tritium / Q000737:chemistry*
chemical_list: C530129:N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo(1,5-a)pyrimidin-3-yl)acetamide; D011720:Pyrazoles; D011743:Pyrimidines; D019275:Radiopharmaceuticals; D014316:Tritium
keywords: DPA-714; PBR; TSPO; hydrogen-for-tritium exchange; isotopic labelling; tritiodebromination; tritium