Evaluation of 3-l- and 3-d-[ 18 F]Fluorophenylalanines as PET Tracers for Tumor Imaging.

Purpose: The preclinical evaluation of 3-l- and 3-d-[ ¹⁸ F]FPhe in comparison to [ ¹⁸ F]FET, an established tracer for tumor imaging.
Methods: In vitro studies were conducted with MCF-7, PC-3, and U87 MG human tumor cell lines. In vivo µPET studies were conducted in healthy rats with/without the inhibition of peripheral aromatic l-amino acid decarboxylase by benserazide pretreatment ( n = 3 each), in mice bearing subcutaneous MCF-7 or PC-3 tumor xenografts ( n = 10), and in rats bearing orthotopic U87 MG tumor xenografts ( n = 14). Tracer accumulation was quantified by SUV _max , SUV _mean and tumor-to-brain ratios (TBrR).
Results: The uptake of 3-l-[ ¹⁸ F]FPhe in MCF-7 and PC-3 cells was significantly higher relative to [ ¹⁸ F]FET. The uptake of all three tracers was significantly reduced by the suppression of amino acid transport systems L or ASC. 3-l-[ ¹⁸ F]FPhe but not 3-d-[ ¹⁸ F]FPhe exhibited protein incorporation. In benserazide-treated healthy rats, brain uptake after 42-120 min was significantly higher for 3-d-[ ¹⁸ F]FPhe vs. 3-l-[ ¹⁸ F]FPhe. [ ¹⁸ F]FET showed significantly higher uptake into subcutaneous MCF-7 tumors (52-60 min p.i.), while early uptake into orthotopic U87 MG tumors was significantly higher for 3-l-[ ¹⁸ F]FPhe (SUV _max : 3-l-[ ¹⁸ F]FPhe, 107.6 ± 11.3; 3-d-[ ¹⁸ F]FPhe, 86.0 ± 4.3; [ ¹⁸ F]FET, 90.2 ± 7.7). Increased tumoral expression of LAT1 and ASCT2 was confirmed immunohistologically.
Conclusion: Both novel tracers enable accurate tumor delineation with an imaging quality comparable to [ ¹⁸ F]FET.