Development of a Novel Positron Emission Tomography Probe Deuterated [18F]FE-TMP ([18F]FE-TMP-d4), an Antagonist of Escherichia coli Dihydrofolate Reductase, for Reporter Gene Imaging of the Brain

[ 18 F]Fluoroethoxy trimethoprim ([ 18 F]FE-TMP, [ 18 F] 2 ), a unique antagonist of bacterial dihydrofolate reductase (ecDHFR), has been developed as a reporter gene imaging agent. Here, we developed new PET probes based on TMP. Simulations predicted that hydrophobic interactions around the benzene ring of 2 contributed to binding with ecDHFR. The more lipophilic fluoropropyl-TMP analog ( 3 ) showed a higher binding affinity for ecDHFR than 2 . However, 18 F-labeled 3 ([ 18 F] 3 ) underwent 18 F-defluorination during PET imaging of ecDHFR-transfected mice. Subsequently, we evaluated FE- d 4 analog ( 5 ) as a candidate exhibiting greater in vivo stability than 2 . Metabolite analysis showed a lower contamination of radiolabeled metabolites in mouse brains with 18 F-labeled 5 ([ 18 F] 5 ) than [ 18 F] 2 . PET imaging with [ 18 F] 5 of a non-human primate brain was characterized by a distinct signal/noise ratio, which allowed in vivo visualization of brain circuits expressing the reporter gene, demonstrating the superior potential of [ 18 F] 5 as a PET probe for reporter gene imaging of the brain.