There are subtle physiological differences between normal tissue and tumour tissue. These differences mainly concern characteristics such as redox status and pO2 levels. Thus, imaging methods that enable detection of these biological variations may be valuable in developing treatment strategies. Electron Paramagnetic Resonance Imaging is capable of non-invasive measurements of redox state and pO2 level, involving proper nitroxide probes.
The study of the distribution and lifetime of nitroxides in normal and tumour tissue was the aim of Kuppusamy's research paper “In vivo Electron Paramagnetic Resonance Imaging of Tumour Heterogeneity and Oxygenation in a Murine Model”.
As has been pointed out, nitroxides occur in biological systems in two forms: as a free radical and/or as a diamagnetic hydroxylamine which is a product of one-electron reduction of the nitroxide free radical. The reduction rate of nitroxides is increasing during hypoxia conditions. A specificity of these compounds may indicate some of the physiological differences in tissues, which exist under various oxygenation levels.
In order to detect tissue heterogeneity, three nitroxides were used: TPL (4-OH, 2,2,6,6-tetramethylpiperidine-1-oxyl ), 3-CP (3-carbamoyl-proxyl) and PDT (4-oxo-L,2,6,6-tetramethylpiperadine-1-oxyl). TPL has been specified as an in vitro radioprotector, the 3-CP, due to greater biostability in tissues, was used in in vivo EPRI experiments and PDT was used as a probe for pO2 levels assessment. Mice bearing tumour in size approximately 10 – 15 mm in diameter were examined. Electron Paramagnetic Resonance Imaging was used to perform measurements of reduction rates, distribution and levels of nitroxides in normal and tumour tissues.
The obtained results have shown that the peak level of nitroxides was significantly lower in the tumour tissue. Moreover, the reduction rate of nitroxides was much higher in tumours, compared to normal tissues. 3D imaging has confirmed a significant heterogeneity in the distribution of spin probes and redox state in tumour tissue, whereas normal tissues has remained homogenous. With the use of PDT, a significant difference in pO2 level between normal tissue and tumour tissue have been shown. An average level of oxygenation in tumours was three times lower, then in normal tissue. A state of hypoxia in tumours is linked to lower nitroxides content and its greater reduction rate.
The study conducted by Kuppusamy’s group has confirmed, that EPRI is a technique that enables performing both the spatial bio-distribution and functional imaging as a result of spectral properties of nitroxides. EPRI may be a breakthrough approach to plan treatment of neoplastic diseases.
Kuppusamy P et al.;In vivo electron paramagnetic resonance imaging of tumor heterogeneity and oxygenation in a murine model; Cancer Res. 1998 Apr 1;58(7):1562-8.