Radiosynthesis and Preclinical Evaluation of m-[18F]FET and [18F]FET-OMe as Novel [18F]FET Analogs for Brain Tumor Imaging

O-([18F]Fluoroethyl)-l-tyrosine ([18F]FET) is actively transported into the brain and cancer cells by LAT1 and possibly other amino acid transporters, which enables brain tumor imaging by positron emission tomography (PET). However, tumor delivery of this probe in the presence of competing amino acids may be limited by a relatively low affinity for LAT1. The aim of the present work was to evaluate the meta-substituted [18F]FET analog m-[18F]FET and the methyl ester [18F]FET-OMe, which were designed to improve tumor delivery by altering the physicochemical, pharmacokinetic, and/or transport properties. Both tracers could be prepared with good radiochemical yields of 41–56% within 66–90 min. Preclinical evaluation with [18F]FET as a reference tracer demonstrated reduced in vitro uptake of [18F]FET-OMe by U87 glioblastoma cells and no advantage for in vivo tumor imaging. In contrast, m-[18F]FET showed significantly improved in vitro uptake and accelerated in vivo tumor accumulation in an orthotopic glioblastoma model. As such, our work identifies m-[18F]FET as a promising alternative to [18F]FET for brain tumor imaging that deserves further evaluation with regard to its transport properties and in vivo biodistribution.


Determination of molar activity of m-[ 18 F]FET and [ 18 F]FET-OMe
A sample of the purified tracer solution (20 µL) was analysed by analytical HPLC and the carrier amount was determined from the peak area according to the respective calibration curve.The molar activity (AM) was calculated according to following formula:  S1.S3.
Table S3: Calibration data (measured at 254 nm) for determination of carrier amount of [ 18 F]FET-OMe.

Figure S1 :
Figure S1: Calibration curve for determination of carrier amount of m-[ 18 F]FET.Raw data are shown in TableS1.

Figure S2 :
Figure S2: Calibration curve for determination of carrier amount of [ 18 F]FET-OMe.Raw data are shown in TableS3.

Figure S3 :
Figure S3: pH-values of aqueous Bu4NOTs or Bu4NOH solutions determined before and after elution of QMA anion exchange cartridges (pre-conditioned with 2 mL H2O or with 10 mL 0.05 M NaHCO3 followed by 10 mL H2O as indicated) and after heating of the eluates thus obtained at 85 °C for 5 min (n=3 per experimental condition).Statistically significant differences compared to the corresponding pH values before elution were identified by Welch's ANOVA with Games-Howell post-hoc test and are indicated by asterisks (*: p<0.05, ***: p≤0.001).

Figure S4 :
Figure S4: Elution profile of radioactivity from PD 10 cartridges loaded with the soluble fraction of homogenized U87 MG cells incubated with m-[ 18 F]FET for 1 h (orange) or with pure m-[ 18 F]FET (control experiments without incubation, turquoise).The cartridges were eluted with 20 × 1 mL of Earle's balanced salt solution (EBSS) and the radioactivity in each fraction was measured with a gamma counter, corrected for background activity and normalized by the maximum activity value.Note that the apparently negative values observed in some cases are an artefact introduced by the background subtraction that can be attributed to slight variations in background radioactivity.

Table S1 :
Calibration data (measured at 254 nm) for determination of carrier amount of m-[ 18 F]FET.

Table S5 :
pH value of Bu4NOH•30 H2O and Bu4NOTs solutions before and after elution of QMA cartridges (pre-conditioned with H2O) and after heating at 85 °C for 5 min.

Table S6 :
pH value of Bu4NOTs solutions before and after elution of QMA cartridges (preconditioned with 0.05 M NaHCO3 and H2O) and after heating at 85 °C for 5 min.