L-type amino acid transporter (LAT) 1 expression in F-FET-negative gliomas.

Background: O-(2-[F]-fluoroethyl)-L-tyrosine (F-FET) is a highly sensitive PET tracer for glioma imaging, and its uptake is suggested to be driven by an overexpression of the L-type amino-acid transporter 1 (LAT1). However, 30% of low- and 5% of high-grade gliomas do not present enhanced F-FET uptake at primary diagnosis ("F-FET-negative gliomas") and the pathophysiologic basis for this phenomenon remains unclear. The aim of this study was to determine the expression of LAT1 in a homogeneous group of newly diagnosed F-FET-negative gliomas and to compare them to a matched group of F-FET-positive gliomas. Forty newly diagnosed IDH-mutant astrocytomas without 1p/19q codeletion were evaluated (n = 20 F-FET-negative (tumour-to-background ratio (TBR) < 1.6), n = 20 F-FET-positive gliomas (TBR > 1.6)). LAT1 immunohistochemistry (IHC) was performed using SLC7A5/LAT1 antibody. The percentage of LAT1-positive tumour cells (%) and the staining intensity (range 0-2) were multiplied to an overall score (H-score; range 0-200) and correlated to PET findings as well as progression-free survival (PFS).

Results: IHC staining of LAT1 expression was positive in both, F-FET-positive as well as F-FET-negative gliomas. No differences were found between the F-FET-negative and F-FET-positive group with regard to percentage of LAT1-positive tumour cells, staining intensity or H-score. Interestingly, the LAT1 expression showed a significant negative correlation with the PFS (p = 0.031), whereas no significant correlation was found for TBR, neither in the overall group nor in the F-FET-positive group only (p = 0.651 and p = 0.140).

Conclusion: Although LAT1 is reported to mediate the uptake of F-FET into tumour cells, the levels of LAT1 expression do not correlate with the levels of F-FET uptake in IDH-mutant astrocytomas. In particular, the lack of tracer uptake in F-FET-negative gliomas cannot be explained by a reduced LAT1 expression. A higher LAT1 expression in IDH-mutant astrocytomas seems to be associated with a short PFS. Further studies regarding mechanisms influencing the uptake of F-FET are necessary.