Glioblastoma is the most malignant and commonly diagnosed primary brain tumor in adults with
a dismal median overall survival of 14 to 16 months. In 2005 the Stupp regimen changed clinical
care with the discovery that the chemotherapeutic drug temozolomide (TMZ), with the addition of
surgery and radiation, could extend patient survival. However, new therapeutic avenues have
remained stagnant and with no second-line therapeutic options showing significant improvement
in recurrent GBM tumors, resistance to TMZ is uniformly fatal. To this end we sought to better
understand the molecular mechanisms of TMZ-resistant disease to provide patients a potential
second-line therapeutic option with a focus on cysteine depletion-induced ferroptosis in TMZ-
resistant GBM. Ferroptosis is an iron-dependent form of cell death which has recently gained
attention as an attractive avenue to eradicate otherwise drug resistant cancer cells. Our
preliminary data support the role of a previously uncharacterized metabolic enzyme in the
induction of cysteine depletion-induced ferroptosis. Our findings strongly suggest that the gamma
(¿)-glutamyl enzyme (¿-glutamylcyclotransferase; GGCT) recycles cysteine and prevents
glutathione (GSH) production – the main goal of the ¿-glutamyl pathway. We further show that
TMZ-resistant cells have changes consistent with a sensitivity to ferroptosis induction such as an
increase in reactive oxygen species (ROS), cysteine uptake and the cysteine/glutamate antiporter
– xCT, as well as mislocalized perinuclear mitochondria. Therefore, we sought to repurpose
ebselen, a previously characterized neuroprotective agent that was thought to be a glutathione
peroxidase 4 (GPX4) mimetic but has recently garnered attention for its selenium ion’s ability to
covalently bind cysteines. We show that the neuroprotective agent ebselen which has already
been shown to cross the blood brain barrier – a major hurdle for GBM treatment – specifically
targets TMZ-resistant GBM cells in vitro. Based on these results, we propose to further investigate
the role of cysteine recycling via GGCT and its therapeutic potential as a treatment vulnerability
in patient-derived xenograft (PDX) TMZ-resistant GBM orthotopic pre-clinical animal models.
Lastly, we seek to establish prognostic biomarkers of this aberrant cysteine recycling through a
metabolic byproduct of GGCTs enzymatic activity in clinical glioma specimens. Overall, this
proposal will give insight into a new avenue of ferroptosis induction in drug resistant GBM, and
potentially pave the way for cysteine deprivation-induced ferroptosis in other drug resistant
cancers.