Introduction: Although [ 18 F]FDG has been proven as the radioligand of choice for many extracranial tumors, radiolabeled amino acids are preferred for the study of intracerebral tumors with PET because transport mechanisms of large neutral amino acid (LNAA) are upregulated in those tumors. To overcome restrictions known for use of 11 C 18 F-labeled LNAA (e.g. L-FET) have been developed and used for this purpose. Recently experimental evidence has been obtained that D-enantiomers of some LNAA are superior to L-enantiomers with regard to the imaging of certain peripheral tumors. In this study, the transport of D-FET across the blood-brain barrier was studied with PET in piglets and compared with L-FET.
Experimental: Ten anesthetized piglets were intravenously infused with L-FET (98 Β± 11 MBq, n=4) or D-FET (128 Β± 34 MBq, n=6) in 10 ml saline. Compartmental modeling of PET data was used to estimate the rate constants for the blood-brain (K 1 ) and the brain-blood (k 2 ) transfer of both radiotracers. 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.
Results and Discussion: Our results reveal that the BBB transport of D-FET is significantly faster than that of L-FET. This finding is based on the following observations: (1) The normalized brain uptake in piglets of D-FET 5 min after injection is about 140% higher than that of L-FET. (2) 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. This stands in contrast with previous studies showing high and stereoselective uptake of L-FET in mouse brain. Thus, the transport kinetics for D-amino acids differs among species. The observed differences between 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.
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, Damino acid preferring transporter/transport system at the blood-brain barrier.