In vivo tumour extracellular pH monitoring using electron paramagnetic resonance: the effect of X‐ray irradiation

Abstract

In vivo quantification of extracellular pH (pHe) in tumour may provide a useful biomarker for tumour cell metabolism. In this study, we assess the viability of continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy with a pH-sensitive nitroxide to measure extracellular tumour pH in the mouse model. 750 MHz CW-EPR spectroscopy of C3H HeJ mice hind leg squamous cell tumour was performed after intra-venous tail vein injection of pH-sensitive nitroxide (R-SG) during stages of normal tumour growth, and in response to a single 10 Gy dose of X-ray irradiation. An inverse relationship was observed between tumour volume and pHe value, whereby during normal tumour growth a constant reduction in pHse was observed. This relationship was disrupted by X-ray irradiation, and from 2–3 days post exposure, a transitory increase pHe was observed. In this study we demonstrated the viability of CW-EPR spectroscopy using R-SG nitroxide to obtain high sensitivity pH measurements in mouse tumour model with accuracy < 0.1 pH units. In addition, measured changes in pHe in response to X-ray irradiation suggest this may offer a useful method for assessing physiological change in response to existing and novel cancer therapies.

Functional electron paramagnetic resonance imaging of ischemic rat heart: Monitoring of tissue oxygenation and pH

“…The pH-sensitive nitroxide radical RSG has been used recently in several EPR studies and has proven itself as a useful probe for in vivo pH measurements (36,38,53,54). The radical has the ionization constant pK ¼ 6.6, which is perfectly suited for the measurements in the physiologically relevant pH range.…”

Molecular Probes for Evaluation of Oxidative Stress by In Vivo EPR Spectroscopy and Imaging: State-of-the-Art and Limitations

“…In recent years, major efforts have been dedicated to the development of extracellular dual probes for redox and pH, such as that displayed in Figure 10 [10,108], and of probes for GSH redox status (Figure 7) [51,52]. A multifunctional approach is indeed crucial for understanding tumor physiology and metabolism.…”

In vivo EPR extracellular pH-metry in tumors using a triphosphonated trityl radical

“…The principle of EPR pH-metry is to assess the change in the hyperfine splitting as a function of pH. Nitroxides are the most widely used EPR pH-sensitive probes (9)(10)(11)(12)(13)(14). However, the use of nitroxides in vivo may be impeded by their fast bioreduction into EPR-silent hydroxylamines.…”