L-FBPA showed higher selectivity for L-type amino acid transporter 1: in vitro cellular uptake analysis compared to L-Methionine.

Objectives L isomeric form of 4-borono-2-(18) F-fluoro-phenylalanine (L-FBPA) has been reported to show high uptake in the malignant tumors and less physiological accumulation in the normal organs. In this work, we compared the accumulation of L-FBPA and L isomeric form of methionine (L-Met) to evaluate the contribution of L-type amino acid transporter (LAT), focusing on the selectivity between LAT1, which is highly expressed in the many types of malignant tumors, and LAT2, which is mainly expressed in the normal tissue.

Methods In vitro assay was performed using two cell lines: HEK293-hLAT1 (high expression of LAT1) and HEK293-hLAT2 (high expression of LAT2). We evaluated affinity of LATs by uptake inhibitory effects of ¹⁴C-leucine for LAT1 and ¹⁴C-alanine for LAT2 in the presence of L-FBPA (100 μmol/L), L-Met (100 μmol/L), and control, respectively. Uptake inhibition (%) was defined as the ratio of the test compound uptake to the control uptake, in which lower value indicated high affinity for the corresponding LAT. To evaluate transport capacity of LATs, the efflux analysis was performed by counting the extracellular 14C-leucine for LAT1 and 14C-alanine for LAT2 after the addition of L-FBPA (100 μmol/L), L-Met (100 μmol/L), and control, respectively. Radioactivity counts of ¹⁴C-leucine or ¹⁴C-alanine were compared by t-test and LAT1 selectivity was defined as the ratio of contribution of LAT1 to that of both LAT1 and LAT2.

Results In the uptake inhibitory tests, L-Met (63.0 ± 3.8%) showed significantly lower uptake compared to L-FBPA (74.9 ± 5.9%) for HEK293-hLAT1 (p < 0.01), and L-Met (50.1 ± 1.8%) showed markedly lower uptake compared to L-FBPA (81.3 ± 6.9%) for HEK293-hLAT2 (p<0.01). In the efflux analysis, no significant difference was observed between L-FBPA (25.3 ± 4.3%/min) and L-Met (25.8 ± 2.9%/min) for HEK293-LAT1, whereas L-Met (36.5 ± 3.4%/min) showed higher transport capacity compared to L-FBPA (24.7 ± 5.3%/min) for HEK293-hLAT2 (p < 0.01). These results suggested that L-Met had high affinity and transport capacity for LAT2, and L-FBPA had higher selectivity of affinity and transport capacity for LAT1 than L-Met.

Conclusions We showed that L-FBPA had higher selectivity for LAT1 compared to L-Met. On the other hand, L-Met had high affinity and transport capacity for LAT2 than L-FBPA, suggesting the possibility of more uptake of L-Met in the non-malignant cells than that of L-FBPA. This study indicated the clinical utility of ¹⁸F-FBPA, which can be used for tumor selective PET tracer compared to ¹¹C-Met.