System x _c ⁻ imaging maps ferroptosis-linked redox remodeling in cancer

Ferroptosis is a regulated non-apoptotic form of programmed cell death that is implicated in tumor suppression and the normal tissue damage response. While the link between redox stress and ferroptosis is well established, no non-invasive methods exist to assess ferroptosis in vivo . Here, we demonstrate that the redox-sensitive positron emission tomography radiotracer and system x _c ⁻ substrate, ¹⁸ F-( S )-4-(3-fluoropropyl)-L-glutamic acid ([ ¹⁸ F]FSPG), serves as a non-invasive marker of tumor ferroptosis. Global changes in amino acids, glutathione, and system x _c ⁻ activity occurred before loss of membrane integrity in cells sensitive to ferroptosis, but not in resistant cells. Resistant cells sensitized to ferroptosis through nuclear factor erythroid 2-related factor 2 (NRF2) knockout had reduced glutathione and [ ¹⁸ F]FSPG retention, which were rescued by ferroptosis inhibitors. In vivo , immune checkpoint blockade decreased ferroptosis-specific [ ¹⁸ F]FSPG tumor retention prior to immune cell infiltration. Together, our data demonstrate that [ ¹⁸ F]FSPG can identify early redox changes that precede ferroptosis and enabled real-time monitoring of immunotherapeutic efficacy.