Stereotactic MCa4 and FSa tumor tissue biopsies registered with electron paramagnetic resonance (EPR) O2 imaging were used to identify sampling sites suitable to measure protein levels of three hypoxic cell biomarkers, hypoxia-induced factor 1-alpha (HIF-1α), carbonic anhydrase IX (CA9), and vascular endothelial growth factor (VEGF). EPR imaging provided quantitative localization of pO2 levels in tumors grown in the legs of C3H mice. Since hypoxic cells (pO2 <10 torr) are up to three times more resistant than normoxic cells to radiation lethality, identification of hypoxic populations to predict radiotherapeutic outcome is important. The oxygen broadening of narrow EPR spectral lines, or, equivalently, the increase in relaxation rates of electron magnetization, displays pO2 with 1-2 torr resolution in image voxels less than 1 mm3. The pO2 reporter molecule OX063 was used to image hypoxic areas in tumors. The molecule is selectively retained in tumors with a half-life of ~30 minutes. We used inversion recovery electron spin echo (IRESE) to measure the T1 rate of the trityl inside the tumor-bearing leg of mice. Voxel maps with O2 levels less than 10 torr (HF10) were used to guide biopsy cannulas to obtain tumor cell samples. Each of these biopsy cores were further subdivided into samples from which protein amounts could be determined via ELISA. Results from MCa4 and FSa tumor cells indicate a strong correlation between EPR pO2-identified hypoxic areas (<10 torr) and HIF-1α, CA9, and VEGF protein. The activation of various genes, including stem-cell activation, angiogenesis, vasodilation, glucose metabolism, reduced apoptotic activity, and cell cycle changes, contributes to poor radiotherapeutic outcome. EPR pO2 imaging may be used to precisely identify hypoxic areas harboring cells that protect tumor tissue from the toxicity of conventional radiation leading to radiotherapeutic failure.

Grant Support: This work was supported by grants from the NIH, including R01 CA98575 and P41 EB002034.

Citation Format: Richard Miller, Boris Epel, Martyna Elas, Martyna Krzykawska-Serda, Matthew Maggio, Eugene Barth, Mihai Giurcanu, Howard Halpern. Electron paramagnetic resonance (EPR) pO2 image-guided tumor biopsies to analyze hypoxia-induced proteins [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 668.