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

[¹⁸F]Fluoroethoxy trimethoprim ([¹⁸F]FE-TMP, [¹⁸F]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, ¹⁸F-labeled 3 ([¹⁸F]3) underwent ¹⁸F-defluorination during PET imaging of ecDHFR-transfected mice. Subsequently, we evaluated FE-d₄ 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 ¹⁸F-labeled 5 ([¹⁸F]5) than [¹⁸F]2. PET imaging with [¹⁸F]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 [¹⁸F]5 as a PET probe for reporter gene imaging of the brain.