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.

Radiosynthesis and Preclinical Evaluation of [18F]F-DPA, A Novel Pyrazolo[1,5a]pyrimidine Acetamide TSPO Radioligand, in Healthy Sprague Dawley Rats

“…Table 1), thereby, suggesting the specificity of [ 18 F]F-FDA in vivo . Furthermore, both the affinity ( K i ) and selectivity ( K i CBR) of the noval tracer ( K i  = 1.7 nM, K i CBR > 1 μM)[25] are comparable with those of the routinely used [ 18 F]F-FDA-174 [17] ( K i  = 0.91 ± 0.08 nM, K i CBR > 1 μM) [24, 30, 31]. When this is combined with the specific binding shown for [ 18 F]F-FDA-174 [17], it gives promise to [ 18 F]F-FDA as a new TSPO ligand.…”

“…1a, compound 2 ) is a fluorine-containing pyrazolopyrimidine with a structure closely-related to DPA-714, also showing a good affinity ( K i  = 1.7 nM) and selectivity ( K i CBR > 1 μM) [25] towards the TSPO. In compound 2 , the alkoxy-chain bridging the phenyl group and the fluorine atom has been removed, thus positioning the fluorine directly on the phenyl moiety.…”

Extensive exploration of a novel rat model of Parkinson's disease using partial 6‐hydroxydopamine lesion of dopaminergic neurons suggests new therapeutic approaches

“…[ 125 I]PE2I was radiolabeled according to Chalon et al () and obtained with a specific activity of 95 TBq/mmol. [ 3 H]DPA‐714 was prepared according to Damont et al () and obtained with a specific activity of 2.01 TBq/mmol. Six brain sections per animal were studied.…”

Deuterium‐ and Tritium‐Labelled Compounds: Applications in the Life Sciences

“…This property makes tritium irreplaceable for the labelling of large molecules,particularly biomolecules such as peptides,p roteins,o ligonucleotides,a nd antibodies. [278] It should be noted that the kinetic isotope effect of tritium is significantly greater than that of deuterium.…”