In vivo experimental studies on the role of free radicals in photodynamic therapy. I. Measurement of the steady state concentration of free radicals in tumor tissues of mice.

A method has been developed to determine the absolute values of steady state concentrations of free radicals in malignant cells. Measurements on P-388 tumor bearing mice revealed that these concentrations increase with the growth of the tumor, reach maximum and fall rapidly afterwards. In the vicinity of the maximum the corresponding values are in the range 1.2-1.5 x 10(-5) spin mol/l. Comparison with literature data on the concentration of oxygen at the fast growth of malignancy indicates the possibility of the competition between singlet oxygen and free radicals in their interactions with triplet photosensitizer during Photodynamic Therapy.