¹⁸F-5-Fluoroaminosuberic Acid as a Potential Tracer to Gauge Oxidative Stress in Breast Cancer Models

The cystine transporter (system x_C^-) is an antiporter of cystine and glutamate. It has relatively low basal expression in most tissues and becomes upregulated in cells under oxidative stress (OS) as one of the genes expressed in response to the antioxidant response element promoter. We have developed ¹⁸F-5-fluoroaminosuberic acid (FASu), a PET tracer that targets system x_C^- The goal of this study was to evaluate ¹⁸F-FASu as a specific gauge for system x_C^- activity in vivo and its potential for breast cancer imaging. Methods:¹⁸F-FASu specificity toward system x_C^- was studied by cell inhibition assay, cellular uptake after OS induction with diethyl maleate, with and without anti-xCT small interfering RNA knockdown, in vitro uptake studies, and in vivo uptake in a system x_C^--transduced xenograft model. In addition, radiotracer uptake was evaluated in 3 breast cancer models: MDA-MB-231, MCF-7, and ZR-75-1. Results: Reactive oxygen species-inducing diethyl maleate increased glutathione levels and ¹⁸F-FASu uptake, whereas gene knockdown with anti-xCT small interfering RNA led to decreased tracer uptake. ¹⁸F-FASu uptake was robustly inhibited by system x_C^- inhibitors or substrates, whereas uptake was significantly higher in transduced cells and tumors expressing xCT than in wild-type HEK293T cells and tumors (P < 0.0001 for cells, P = 0.0086 for tumors). ¹⁸F-FASu demonstrated tumor uptake in all 3 breast cancer cell lines studied. Among them, triple-negative breast cancer MDA-MB-231, which has the highest xCT messenger RNA level, had the highest tracer uptake (P = 0.0058 when compared with MCF-7; P < 0.0001 when compared with ZR-75-1). Conclusion:¹⁸F-FASu as a system x_C^- substrate is a specific PET tracer for functional monitoring of system x_C^- and OS imaging. By enabling noninvasive analysis of x_C^- responses in vivo, this biomarker may serve as a valuable target for the diagnosis and treatment monitoring of certain breast cancers.