PET Imaging of L-Type Amino Acid Transporter (LAT1) and Cystine-Glutamate Antiporter (x _c^-) with [ ¹⁸ F]FDOPA and [ ¹⁸ F]FSPG in Breast Cancer Models

Purpose: The present study describes the analysis of amino acid transporters ASCT1, ASCT2, LAT1, and x_c^- in breast cancer under normoxic and hypoxic conditions. [¹⁸F]FDOPA-PET and [¹⁸F]FSPG-PET were used as imaging biomarkers to probe L-type amino acid transporter (LAT1) and cystine-glutamate antiporter (x_c^-) in breast cancer models.

Procedures: LAT1 and x_c^- transporters were studied under normoxic and hypoxic conditions with radiotracers [¹⁸F]FDOPA and [¹⁸F]FSPG in estrogen receptor-positive (ER+) MCF7 and triple-negative MDA-MB231 cells and in human mammary epithelial MCF10A control cells. Protein expression was analyzed using Western blot and immunohistochemistry.

Results: ASCT1 protein expression levels were comparable in all three cell lines, while noticeable ASCT2 expression levels were only found in MCF10A control cells. Higher LAT1 protein expression was detected in ER+ MCF7 cells. High x_c^- protein expression levels were detected in MDA-MB231 cells. Uptake of [¹⁸F]FDOPA through LAT1 was significantly higher in MCF7 versus MDA-MB231 cells, while the uptake of [¹⁸F]FSPG through x_c^- resulted in the opposite confirming expression and functional differences for both amino acid transporters in different breast cancer models. Hypoxia significantly increased [¹⁸F]FDOPA uptake in MCF7 cells and [¹⁸F]FSPG uptake in MDA-MB231 cells. In vivo PET imaging revealed substantially higher tumor uptake of [¹⁸F]FDOPA in MCF7 tumors as well as [¹⁸F]FSPG uptake in MDA-MB231 tumors confirming differences detected in vitro.

Conclusions: ER+ breast cancer cells express higher levels of amino acid transporter LAT1, whereas triple-negative breast cancer cells express more x_c^-. Cellular uptake and PET imaging experiments with [¹⁸F]FDOPA and [¹⁸F]FSPG confirmed functional LAT1 and x_c^- expression profiles. There was initial evidence that hypoxia regulates the function of both amino acid transporters in breast cancer. The results further indicated that [¹⁸F]FDOPA and [¹⁸F]FSPG are suitable radiotracer to distinguish between ER+ and triple-negative breast cancer types.

Keywords: Amino acid transporters; Breast cancer; PET; [18F]FDOPA; [18F]FSPG.