Magnetic resonance imaging (MRI) of pharmacological ascorbate-induced iron redox state as a biomarker in subjects undergoing radio-chemotherapy.

ABSTRACT

Pharmacological ascorbate (P-AscH-) combined with standard of care (SOC) radiation and temozolomide is being evaluated in a phase 2 clinical trial (NCT02344355) in the treatment of glioblastoma (GBM). Previously published data demonstrated that paramagnetic iron (Fe3+) catalyzes ascorbate's oxidation to form diamagnetic iron (Fe2+). Because paramagnetic Fe3+ may influence relaxation times observed in MR imaging, quantitative MR imaging of P-AscH--induced changes in redox-active Fe was assessed as a biomarker for therapy response. Gel phantoms containing either Fe3+ or Fe2+ were imaged with T2* and quantitative susceptibility mapping (QSM). Fifteen subjects receiving P-AscH- plus SOC underwent T2* and QSM imaging four weeks into treatment. Subjects were scanned pre-P-AscH- infusion, post-P-AscH- infusion, and post-radiation (3-4 h between scans). Changes in T2* and QSM relaxation times in tumor and normal tissue were calculated and compared to changes in Fe3+ and Fe2+ gel phantoms. A GBM mouse model was used to study the relationship between the imaging findings and the labile iron pool. Phantoms containing Fe3+ demonstrated detectable changes in T2* and QSM relaxation times relative to Fe2+ phantoms. Compared to pre-P-AscH-, GBM T2* and QSM imaging were significantly changed post-P-AscH- infusion consistent with conversion of Fe3+ to Fe2+. No significant changes in T2* or QSM were observed in normal brain tissue. There was moderate concordance between T2* and QSM changes in both progression free survival and overall survival. The GBM mouse model showed similar results with P-AscH- inducing greater changes in tumor labile iron pools compared to the normal tissue. CONCLUSIONS: T2* and QSM MR-imaging responses are consistent with P-AscH- reducing Fe3+ to Fe2+, selectively in GBM tumor volumes and represent a potential biomarker of response. This study is the first application using MR imaging in humans to measure P-AscH--induced changes in redox-active iron.