Early Monitoring of Redox Status Based on Reduced Molecules Using In Vivo DNP-MRI for Cancer Treatment

Abstract

<h3>Purpose/Objective(s)</h3> Radiation treatment effectiveness is assessed through the observation of morphological changes with CT or MRI images after treatment. The treatment efficacy of radiotherapy depends on redox modulation in tumor tissue, such as hydroxyl radicals and of reactive oxygen species generated by irradiation. Therefore, redox status may be a prognostic factor response to radiotherapy. However, spatiotemporal response of the redox metabolism in tumor tissue post-irradiation remains unknown. The purpose of this study was to detect early changes in the redox status in tumor tissue after irradiation using <i>in vivo</i> dynamic nuclear polarization (DNP)-MRI, to clarify the key molecules that cause these changes, and to compare these changes with early changes in lactate metabolism using hyperpolarized ¹³C magnetic resonance spectroscopy (MRS). <h3>Materials/Methods</h3> To monitor the response of tumor redox status after irradiation, redox imaging using in vivo DNP MRI/ Carbamoyl PROXYL (CmP) as redox sensitive DNP probe was performed after 5 Gy irradiation. To clarify the relationship between redox status alteration and treatment efficacy, in vivo DNP MRI was performed before and after irradiation by 2 and 5 Gy. In addition, tissue GSH and AsA levels on day 1 post irradiation were determined by HPLC. To observe the Warburg effect after radiation treatment, dissolution DNP study using hyperpolarized ¹³C pyruvate was performed before and after irradiation. <h3>Results</h3> The DNP image intensity in the tumor demonstrated a maximum value approximately four minutes after CmP administration, thereafter the image intensity was decreased by the redox reaction. The tumor GSH and AsA level on day 1 post irradiation were decreased compared to the control group. Production of hyperpolarized ¹³C lactate in tumor tissue did not change before and after irradiation. <h3>Conclusion</h3> Our study highlights that redox status in tumor tissue is already significantly decreased on day 1 before the detection of any morphological changes, even after irradiation with a dose as low as 2 Gy. Early changes in this redox status may be related decreases in glutathione and ascorbic acid levels in the tumor tissue after irradiation, which may alter earlier than ¹³C pyruvate hyperpolarized MRS, based on lactate metabolism. Therefore, early assessment of change in decay rate using in vivo DNP-MRI/CmP probing may predict the subsequent radiotherapy effect.