Preferred blood-brain transfer of O-(2-[18F]fluoroethyl)-D-tyrosine (D-FET) into the porcine brain

Large neutral amino acids (LNAA) are transported by a variety of transporters, among which the SLC7A5/LAT1 transporter is regarded as the predominant LNAA transporter at the blood-brain barrier (BBB). LAT1 has broad substrate specificity, accepting even the D-isomers of amino acids as high affinity substrates. Also, LAT1 is frequently overexpressed in malignant tumors providing the rationale for the use of radiolabeled LNAA such as L-FET for imaging intracerebral tumors with PET. Recently, experimental evidence was obtained from various authors that D-enantiomers of some LNAA are superior to L-enantiomers with regard to the imaging of certain peripheral tumors. Therefore, in this study the transport of D-FET across the blood-brain barrier was studied with PET in anaesthetized piglets and patients after subtotal resection of brain tumors and compared with L-FET. Ten piglets of mixed German domestic breed were anesthetized with 0.25% isoflurane in 65% nitrous oxide and 35% oxygen. L-[18F[FET (98 ± 11 MBq, n=4) or D-[18F]FET (128 ± 34 MBq, n=6) in 10 ml saline was intravenously infused within 2 min. Compartmental modeling of PET data was used to estimate the rate constants for the blood-brain (K1) and the brain-blood (k2) transfer of both radiotracers. Six patients were investigated with L-FET-PET and D-FET-PET after subtotal resection of brain tumors. Standardized uptake values (SUV) were calculated in brain cortex and lesions. Additionally, two different approaches were used to assess the interaction of L-FET and D-FET with transporters known to have affinity for LNAA. First, we studied the hLAT1 transporter in HRPE cells (human retinal pigment epithelial cells). Second, hLAT1, LAT2, XPCT and PAT1 were studied in Xenopus laevis oocytes. The normalized brain uptake in piglets of D-FET 5 min after injection is about 140% higher than that of L-FET. The rate constants for the blood-brain and brain-blood transfer of D-FET in piglets are significantly higher (by factor 3-4) than those for L-FET. Both findings indicate that in piglets the BBB transport of D-FET is significantly faster than that of L-FET. This observation is in contrast with previous studies demonstrating high and stereoselective uptake of L-FET in mouse brain, and low uptake of D-FET into mouse brain (Wester et al., 1999). Also, the studies in humans revealed an initial uptake of D-FET which was similar to L-FET but followed by a rapid washout from brain resulting in a significantly higher uptake of L-FET at later time points. Thus, the transport kinetics for D-amino acids seems to differ among species. The differences in brain uptake of L-FET and D-FET can only be explained by differences in their affinity to specific transport systems. L-FET showed at least 20-fold higher in vitro affinity than D-FET towards LAT1 and LAT2 whereas both enantiomers have similar affinities to XPCT and PAT1 transporters. In conclusion, we have demonstrated specific transport of L-FET and D-FET across the BBB which cannot be fully explained by known transport systems. The study provides further evidence for the presence of an unknown, D- amino acid preferring transporter / transport system at the blood-brain barrier.