Synthesis and preliminary biological evaluation of a novel P2X7R radioligand [¹⁸F]IUR-1601

Objectives: The purinergic receptor subtype 7 (P2X7R) is a novel molecular imaging target for neuroinflammation via PET. We have previously developed [¹¹C]GSK1482160 as a promising P2X7R radioligand for imaging neuroinflammation. Here we report the synthesis and preliminary biological evaluation of a new P2X7R radioligand [¹⁸F]IUR-1601.

Methods: The reference standard IUR-1601 ((S)-N-(2-chloro-3-(trifluoromethyl)benzyl)-1-(2-fluoroethyl)-5-oxopyrrolidine-2-carboxamide) and its corresponding precursor (S)-N-(2-chloro-3-(trifluoromethyl)benzyl)-1-(2-chloroethyl)-5-oxopyrrolidine-2-carboxamide were synthesized from tert-butyl (S)-5-oxopyrrolidine-2-carboxylate, fluoroethylbromide and chloroethylbromide, and 2-chloro-3-(trifluoromethyl)benzylamine. Another labeling precursor desmethyl-GSK1482160 was prepared from _L-pyroglutamic acid and 2-chloro-3-(trifluoromethyl)benzylamine. One-step radiosynthesis of [¹⁸F]IUR-1601 from chloroethyl- precursor was first conducted and failed. Therefore, [¹⁸F]IUR-1601 was synthesized from desmethyl-GSK1482160 with [¹⁸F]FCH₂CH₂OTs, prepared from TsOCH₂CH₂OTs and K[¹⁸F]F/Kryptofix2.2.2, in two steps. The potency of IUR-1601 in comparison with GSK1482160 was determined by a ligand competitive binding assay using [¹¹C]GSK1482160. Results : The overall chemical yields for IUR-1601 and its chloroethyl- precursor were 10-15% in 3 steps. The yield of desmethyl-GSK1482160 was 76% in 1 step. IUR-1601 and its chloroethyl- precursor are unstable under high temperature and strong base conditions, converting to (S)-N-(2-chloro-3-(trifluoromethyl)benzyl)-5-oxo-1-vinylpyrrolidine-2-carboxamide by elimination reaction, which caused one-step synthesis of [¹⁸F]IUR-1601 to fail. In addition, [¹⁸F]fluoroethylation of amide requires high reaction temperature and strong base, thus both result in that radiochemical yield for two-step synthesis of [¹⁸F]IUR-1601 was very low, 1-3%. The radiochemical purity was >99%, and the specific activity was >2 Ci/µmol at EOB determined by a “spike”

Methods: The binding affinity Ki for IUR-1601 and GSK1482160 is 4.3 and 5.1 nM, respectively. Conclusions : A new P2X7R radioligand [¹⁸F]IUR-1601 has been successfully radiosynthesized, and retains the P2X7R affinity of [¹¹C]GSK1482160. The radiosynthesis optimization, automation and in vivo biological evaluation of [¹⁸F]IUR-1601 are currently underway. Research Support : Showalter Young Investigator Award and Indiana University Department of Radiology and Imaging Sciences.

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