In vivo experimental studies on the role of free radicals in photodynamic therapy. II. Photodynamic effect on free radical concentration in mice tumors measured by ESR spectroscopy.

Changes in free radical concentrations of solid tumors of mice after photodynamic treatment using Photofrin II sensitizer has been measured by ESR spectroscopy. Decrease in the free radical concentrations by 46% in S180 and by 35% in P388 tumors induced photodynamically was found. As expected, the effect was proportional to the steady state concentrations of the free radicals in the malignant cells of mice prior to photodynamic treatment if comparing either different types of tumors or data referring to various stages of the development of identical types of tumors. Results support the contribution of the interactions between triplet excited sensitizer molecules and native free radicals to photodynamic effects as assumed earlier.