Evaluation of the amino acid transporter: comparison study between FBPA and BPA in vitro.

Objectives 4-borono-phenylalanine (BPA) has been used as carrier of ¹⁰B into the tumor for boron neutron capture therapy. The PET tracer ¹⁸F-FBPA is used to estimate uptake of BPA. In this work, we compared the accumulation FBPA and BPA to evaluate the contribution of L-type amino acid transporter (LAT), focusing on the LAT1 which is highly expressed in the many types of tumors.

Methods In vitro assay was performed using two cell lines: HEK293-LAT1 (high expression of LAT1) and HEK293-LAT2 (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 BCH (LAT inhibiter), BPA, and cold FBPA, respectively. 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 tyrosine, BPA, and cold FBPA, respectively.

Results Inhibitory effects were 49.5±1.0% with FBPA and 48.1±2.9% with BPA in HEK293-LAT1, 70.8±3.3% with FBPA and 37.4±2.2% with BPA in HEK293-LAT2. Effluxes were 28.9±1.7%/min with FBPA and 30.9%/min with BPA in HEK293-LAT1, 30.8%/min with FBPA and 42.7%/min with BPA in HEK293-LAT2. No significant difference was observed between FBPA and BPA in the affinities and transport capacities for LAT1, whereas those for LAT2 were significantly lower in FBPA than in BPA (p<0.01 and p<0.05, respectively).

Conclusions This study demonstrated both FBPA and BPA showed the same selectivity and transport property for LAT1, suggesting the effectiveness of ¹⁸F-FBPA-PET to estimate BPA accumulation in the tumor. However, attention must be paid for the ¹⁸F-FBPA-PET evaluation of normal organs rich in LAT2.