TMET-16. GABA PRODUCTION INDUCED BY IMIPRIDONES IS A TARGETABLE AND IMAGEABLE METABOLIC ALTERATION IN DIFFUSE MIDLINE GLIOMAS

Abstract Diffuse midline gliomas (DMGs) are lethal brain tumors in children. The imipridones ONC201 and ONC206 have emerged as promising therapies for DMG patients, but efficacy as monotherapy is limited. Another hurdle is the lack of biomarkers that report on drug-target engagement at an early timepoint after treatment. Here, using 1H-magnetic resonance spectroscopy (1H-MRS), which is a non-invasive method of quantifying metabolite pool sizes, we show that imipridones induce accumulation of gamma-aminobutyric acid (GABA) in patient-derived (BT245, SF8628) and syngeneic (26-B7, 26-C2) cells. Importantly, in vivo1H-MRS detects a significant increase in GABA within a week of ONC206 treatment, when anatomical alterations are absent, in mice bearing orthotopic BT245, SF8628 or 26-B7 xenografts. Mechanistic studies show that imipridones activate the mitochondrial protease ClpP and upregulate the stress-responsive transcription factor ATF4. ATF4, in turn, upregulates glutamate decarboxylase, the enzyme that synthesizes GABA, and downregulates GABA transaminase, which degrades GABA, leading to GABA accumulation in DMG cells and orthotopic tumors. Since GABA is known to regulate oxidative stress signaling and since imipridones induce oxidative stress, we assessed whether GABA influences oxidative stress in DMG cells. Our studies indicate that GABA induces expression of superoxide dismutase (SOD1), which scavenges superoxide radicals and, thereby, curbs apoptosis induced by imipridones. The biological effects of GABA are mediated via signaling cascades triggered by GABA receptors and we find that GABA acts in an autocrine manner via the GABAB receptor to upregulate SOD1. Importantly, the clinically translatable GABAB receptor antagonist SGS742 and the SOD1 inhibitor ATN-224 exacerbate oxidative stress and synergistically induce apoptosis in combination with imipridones in DMG cells. Furthermore, SGS742 is brain-penetrant and potentiates ONC206-induced apoptosis in mice bearing orthotopic SF8628 xenografts in vivo. Collectively, we identify GABA as a unique metabolic adaptation to imipridones that can be leveraged for imaging drug-target engagement and for enhancing response to therapy. Clinical translation of our studies will enable precision therapy and imaging for DMG patients.