SLC7A11 is the transporter component of the membrane-bound cystine-glutamate antiporter system Xc-. SLC7A11 is essential for the production of glutathione, a major antioxidant that combats accumulation of toxic oxygen radicals. Inhibition of SLC7A11 induces an iron-dependent, non-apoptotic form of cell death called ferroptosis. Activating ferroptosis is a compelling strategy for cancer intervention due to the sensitivity of many cancer types to this form of cell death. In fact, we recently demonstrated that mutant p53 cancer cells are preferentially killed by inhibition of SLC7A11. However, current SLC7A11 small molecule inhibitors lack potency and/or specificity rendering them unfit for clinical use. This prompted us to undertake a high-throughput screen to identify novel compounds to target SLC7A11.
We conducted a phenotypic screen of ~25,000 compounds, measuring glutamate export by cells as a surrogate measure of SLC7A11 inhibitory activity. We identified 14 ‘top hits’ that best inhibited glutamate release. To validate the compounds’ activity, a secondary assay measuring uptake of cystine (C14-labelled cystine uptake) was performed and revealed 3 lead compounds. Notably, these 3 compounds share a common chemotype: Trifluoromethyl Thiadiazole. Here, we further characterise these compounds as specific SLC7A11 inhibitors using cell death assays in cell line models with different p53 status and SLC7A11 expression levels, with or without inhibitors of ferroptosis. It is most intriguing that whilst the lead compounds seemingly phenocopy SLC7A11 inhibition in their inhibition of glutamate release and cystine uptake, these studies have shown that the compounds do not activate ferroptotic cell death.
Aberrant SLC7A11 function is implicated in a number of cancers, provoking interest in this transporter system as a therapeutic target. Our screen has identified potential novel inhibitors of SLC7A11 function and work is ongoing to characterize these compounds further.