In vivo evaluation of [18F]FEAnGA-Me: a PET tracer for imaging β-glucuronidase (β-GUS) activity in a tumor/inflammation rodent model

IntroductionThe PET tracer, 1-O-(4-(2-fluoroethyl-carbamoyloxymethyl)-2-nitrophenyl)-O-β-d-glucopyronuronate ([18F]FEAnGA), was recently developed for PET imaging of extracellular β-glucuronidase (β-GUS). However, [18F]FEAnGA exhibited rapid renal clearance, which resulted in a relatively low tracer uptake in the tumor. To improve the pharmacokinetics of [18F]FEAnGA, we developed its more lipophilic methyl ester analog, [18F]FEAnGA-Me.Methods[18F]FEAnGA-Me was obtained by alkylation of the O-protected glucuronide methyl ester precursor with [18F]-fluoroethylamine ([18F]FEA), followed by removal of the acetate protecting groups with NaOMe/MeOH. The PET tracer was evaluated by in vitro and in vivo studies.Results[18F]FEAnGA-Me was obtained in 5%–10% overall radiochemical yield. It is 10-fold less hydrophilic than [18F]FEAnGA and it is stable in PBS and in the presence of β-GUS for 1 h. However, in the presence of esterase or plasma [18F]FEAnGA-Me is converted to [18F]FEAnGA, and subsequently converted to [18F]FEA by β-GUS. MicroPET studies in Wistar rats bearing a C6 glioma and a sterile inflammation showed similar uptake in tumors after injection of either [18F]FEAnGA-Me or [18F]FEAnGA. Both tracers had a rapid two-phase clearance of total plasma radioactivity with a half-life of 1 and 8 min. The [18F]FEAnGA fraction generated from [18F]FEAnGA-Me by in vivo hydrolysis had a circulation half-life of 1 and 11 min in plasma. Similar distribution volume in the viable part of the tumor was found after injection of either [18F]FEAnGA-Me or [18F]FEAnGA.ConclusionThe imaging properties of [18F]FEAnGA-Me were not significantly better than those of [18F]FEAnGA. Therefore, other strategies should be applied in order to improve the kinetics of these tracers.