(18)F-FBPA as a tumor specific tracer of L-type amino acid transporter 1 (LAT1): PET evaluation in tumor and inflammation compared to (18)F-FDG and (11)C-methionine.

OBJECTIVE

(18)F-FDG-PET is used worldwide for oncology patients. However, we sometimes encounter false positive cases of (18)F-FDG PET, such as moderate uptake in the inflammatory lesion, because (18)F-FDG accumulates not only in the cancer cells but also in the inflammatory cells (macrophage, granulation tissue, etc). To overcome this limitation of (18)F-FDG, we started to use (4-borono-2- [(18)F]fluoro-L-phenylalanine) (18)F-FBPA, an artificial amino acid tracer which is focusing attention as a tumor specific PET tracer. Physiological accumulation of (18)F-FBPA is limited in the kidney and urinary tract in humans, which enable preferable evaluation of uptake in the abdominal organs compared to (11)C-methionine ((11)C-MET). The purpose of this study was to evaluate (18)F-FBPA as a tumor specific tracer by in vitro cellular uptake analysis focusing on the selectivity of L-type amino acid transporter 1 (LAT1), which is specifically expressed in tumor cells, and in vivo PET analysis in rat xenograft and inflammation models compared to (18)F-FDG and (11)C-methionine.

SUBJECTS AND METHODS

Uptake inhibition and efflux experiments were performed in HEK293-LAT1 and LAT2 cells using cold BPA, cold (18)F-FBPA, and hot (18)F-FBPA to evaluate LAT affinity and transport capacity. Position emission tomography studies were performed in rat xenograft model of C6 glioma 2 weeks after the implantation (n=9, body weight=197±10.5g) and subcutaneous inflammation model 4 days after the injection of turpentine oil (n=9, body weight=197±14.4g). Uptake on static PET images were compared among (18)F-FBPA at 60-70min post injection, (18)F-FDG at 60-70min, and (11)C-MET at 20-30min in the tumors and the inflammatory lesions by maximum standardized uptake value (SUVmax).

RESULTS

Cellular uptake analysis showed no significant difference in inhibitory effect and efflux of LAT1 between cold (18)F-FBPA and cold BPA, suggesting the same affinity and transport capacity via LAT1. Uptake of (18)F-FBPA via LAT1 was superior to LAT2 by the concentration dependent uptake analysis. Position emission tomography analysis using SUVmax showed significantly higher accumulation of (18)F-FDG in the tumor and the inflammatory lesions (7.19±2.11 and 4.66±0.63, respectively) compared to (18)F-FBPA (3.23±0.40 and 1.86±0.19, respectively) and (11)C-MET (3.39±0.43 and 1.63±0.11, respectively) (P<0.01 by Tukey test). No significant difference was observed between (18)F-FBPA and (11)C-MET.

CONCLUSIONS

(18)F-FBPA showed high selectivity of LAT1 by in vitro cellular uptake analysis, suggesting the potential as a tumor-specific substrate. In vivo PET analysis showed significantly lower uptake of (18)F-FBPA and (11)C-MET in the inflammatory lesions compared to (18)F-FDG, suggesting comparable utility of (18)F-FBPA PET to (11)C-MET PET in differentiating between the tumor and the inflammation.