The aim of this study was to demonstrate the feasibility of in vivo three-dimensional (3D) relaxation time mapping of a dicarboxy-PROXYL radical using continuous-wave electron paramagnetic resonance (CW-EPR) imaging.
Isotopically substituted dicarboxy-PROXYL radicals, 3,4-dicarboxy-2,2,5,5-tetra( 2H 3)methylpyrrolidin-(3,4- 2H 2)-(1- 15N)-1-oxyl ( 2H, 15N-DCP) and 3,4-dicarboxy-2,2,5,5-tetra( 2H 3)methylpyrrolidin-(3,4- 2H 2)-1-oxyl ( 2H-DCP), were used in the study. A clonogenic cell survival assay was performed with the 2H-DCP radical using squamous cell carcinoma (SCC VII) cells. The time course of EPR signal intensities of intravenously injected 2H, 15N-DCP and 2H-DCP radicals were determined in tumor-bearing hind legs of mice (C3H/HeJ, male, n = 5). CW-EPR-based single-point imaging (SPI) was performed for 3D mapping.
2H-DCP radical did not exhibit cytotoxicity at concentrations below 10 mM. The in vivo half-life of 2H, 15N-DCP in tumor tissues was 24.7 ± 2.9 min (mean ± standard deviation [SD], n = 5). The in vivo time course of the EPR signal intensity of the 2H, 15N-DCP radical showed a plateau of 10.2 ± 1.2 min (mean ± SD) where the EPR signal intensity remained at more than 90% of the maximum intensity. During the plateau, in vivo 3D maps with 2H, 15N-DCP were obtained from tumor-bearing hind legs, with a total acquisition time of 7.5 min.