Evaluation of a deuterated triarylmethyl spin probe for in vivo R2∗‐based EPR oximetric imaging with enhanced dynamic range

Purpose In this study, we compared two triarylmethyl (TAM) spin probes, Ox071 and Ox063 for their efficacy in measuring tissue oxygen levels under hypoxic and normoxic conditions by R2*‐based EPR oximetry. Methods The R2* dependencies on the spin probe concentration and oxygen level were calibrated using deoxygenated 1, 2, 5, and 10 mM standard solutions and 2 mM solutions saturated at 0%, 2%, 5%, 10%, and 21% of oxygen. For the hypoxic model, in vivo imaging of a MIA PaCa‐2 tumor implanted in the hind leg of a mouse was performed on successive days by R2*‐based EPR oximetry using either Ox071 or Ox063. For the normoxic model, renal imaging of healthy athymic mice was performed using both spin probes. The 3D images were reconstructed by single point imaging and multi‐gradient technique was used to determine R2* maps. Results The signal intensities of Ox071 were approximately three times greater than that of Ox063 in the entire partial pressure of oxygen (pO2) range investigated. The histograms of the tumor pO2 images were skewed for both spin probes, and Ox071 showed more frequency counts at pO2 > 32 mm Hg. In the normoxic kidney model, there was a clear delineation between the high pO2 cortex and the low pO2 medulla regions. The histogram of high‐resolution kidney oximetry image using Ox071 was nearly symmetrical and frequency counts were seen up to 55 mm Hg, which were missed in Ox063 imaging. Conclusion As an oximetric probe, Ox071 has clear advantages over Ox063 in terms of sensitivity and the pO2 dynamic range.