Nonperturbing test for cytotoxicity in isolated cells and spheroids, using electron paramagnetic resonance.

Electron paramagnetic resonance imaging (EPRI) and EPR spectroscopy of nitroxides have been applied to detect and quantify cytotoxic effects in mammalian cells in vitro. Images depicting microscopic viable and nonviable areas in the same multicellular spheroid (a model of solid tumors) have been acquired prior to and following the treatment with an antitumor drug, Adriamycin. The loss of viability in the inner region of the viable rim was detected. Such mapping of the time-course changes of the localization of viable and nonviable cells in the same intact biological object is not possible with routinely used methods to measure viability, such as histological examination. The experimental conditions required for high accuracy and sensitivity of the EPRI of spheroids have been evaluated and directions for further development of this approach are indicated.