Background and aim: Acidosis and low-oxygen status are hallmarks of solid tumors. Acidification in extracellular space in solid tumors reflects a shift of cellular metabolism for tumors. Thus, visualization of extracellular pH (pHe) is useful to understand the pathophysiological status of tumors. A method of three-dimensional (3D) pHe mapping of murine tumors using electron paramagnetic resonance (EPR) is introduced in this talk.
Methods: pHe was measured with a 750-MHz home-built continuous-wave EPR spectrometer/imager using a pH-sensitive nitroxyl radical probe (dR-SG) [1,2]. pHe maps were obtained by four-dimensional spectral-spatial EPR imaging. For the proof-of-concept experiment, solution samples, adjusted to 6.60, 6.80, and 7.00 pH units, were visualized. Murine squamous cell carcinoma (SCC VII) cells were implanted into the right hind legs of mice. Tumor-bearing mouse legs were monitored 5 and 8 days after the implantation. Also, tumor xenograft mouse models of human-derived pancreatic ductal adenocarcinoma cells (MIA PaCa-2, SU.86.86, and Hs766t) were measured when the tumor volumes reached approximately 1 cm3.
Results: 3D pH maps were reconstructed for three tubes containing different pH solutions. The pH resolution was achieved at 0.078 pH units, and the trueness of pH values was 0.026 pH units . 3D pHe maps of SCC VII tumors on day 5 and 8 exhibited the progress of acidification. The tumor xenograft mouse models exhibited different levels of acidification and inhomogeneous spatial distribution of pHe in tumor tissues.
Conclusion: In vivo pHe mapping was demonstrated with tumor mouse models. The results suggest that the method of 3D pH mapping can be applied to future studies of tumor mouse models that involve tumor acidification.
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