In vitro simultaneous mapping of the partial pressure of oxygen, pH and inorganic phosphate using electron paramagnetic resonance.

The partial pressure of oxygen (pO) and the extracellular pH in the tumour microenvironment are essential parameters for understanding the physiological state of a solid tumour. Also, phosphate-containing metabolites are involved in energy metabolism, and interstitial inorganic phosphate (Pi) is an informative marker for tumour growth. This article describes the simultaneous mapping of pO, pH and Pi using 750 MHz continuous-wave (CW) electron paramagnetic resonance (EPR) and a multifunctional probe, monophosphonated trityl radical pTAM-D. The concept was demonstrated by acquiring three-dimensional (3D) maps of pO, pH and Pi for multiple solution samples. This was made possible by combining a multifunctional radical probe, fast CW-EPR spectral acquisition, four-dimensional (4D) spectral-spatial image reconstruction, and spectral fitting. The experimental results of mapping pO, pH and Pi suggest that the concept of simultaneous mapping using EPR is potentially applicable for the multifunctional measurements of a mouse tumour model.