Novel applications and translation of [18F]hGTS13, a system xc- specific radiopharmaceutical

ABSTRACT. Ferroptosis is a form of regulated cell death defined by the iron-dependent accumulation of membrane lipid peroxides that results in oxidative damage and plasma membrane disruption. Ferroptosis is a form of cell death that is mechanistically and morphologically distinct from other forms of cell death, such as necroptosis, apoptosis,

and pyroptosis. Targeting ferroptosis has garnered immense interest in the cancer research community owing to the potential to selectively active this mechanism of cell death in cancer cells. To realize the full potential of inducing ferroptosis in cancer, non-invasive imaging markers are needed. System xc-, the cystine-glutamate antiporter, is a plasma membrane transporter mediating the cellular uptake of

cystine in exchange for intracellular glutamate. System xc- plays a critical role in ferroptosis by regulating the availability of cystine, a crucial precursor for glutathione biosynthesis, thus impacting the cellular antioxidant defense system and ultimately influencing susceptibility to ferroptosis. Modulation of system xc- activity with

pharmacological inhibitors is a potential therapeutic strategy that regulates this form of cell death in anti-cancer therapy. Molecular imaging of system xc- therefore has the potential to identify cancers with appropriate transporter activity that would be suitable for ferroptosis induction, assess drug engagement and inhibition of

system xc- in living subjects, and monitor the efficacy of targeting this emerging mechanism of cell death in cancer. To the best of our knowledge, the investigation of system xc- radiopharmaceuticals in this context of ferroptosis, has yet to be reported. We have pioneered the development of [18F]hGTS13, a homo-glutamate radiotracer specific for system xc-.

Importantly, [18F]hGTS13 demonstrates several advantages over existing system xc- radiotracers including: 1) incorporation of a UV-active group, thus greatly facilitating radiosynthesis and quality control; 2) improved transporter specificity; and 3) reduced uptake in multiple immune cell types and improved cancer specificity. The

goals of this proposal are to establish [18F]hGTS13 PET as a non-invasive imaging marker of drug induced cancer ferroptosis in cell culture and pre-clinical models and to clinically translate [18F]hGTS13 by determining its pharmacokinetics in healthy human volunteers through an exploratory investigational new drug mechanism.

We will first establish the relationship between [18F]hGTS13 uptake and sensitivity to cancer ferroptosis in the in vitro and in vivo setting (SA1). We will then test the ability of [18F]hGTS13 to monitor the engagement of pro- ferroptotic drugs in vivo and predict treatment response (SA2). Lastly, we will translate [18F]hGTS13 for first-in-

human testing and determine its biodistribution and radiation dosimetry in healthy volunteers (SA3). Successful completion of this project will establish the foundation for employing [18F]hGTS13 to select patients that are likely to be sensitive to ferroptosis inducing therapies and monitor treatment effects. The findings from

this work will have high impact as inducers of ferroptosis advance to clinical investigation.