Enhancing the efficacy of photodynamic cancer therapy by radicals from plant auxin (indole-3-acetic acid).

Abstract

Indole-3-acetic acid (plant auxin) has low toxicity but dramatically enhances the killing of mammalian cells on illuminating phenothiazinium dyes with red light. Suitable dyes include toluidine blue, used in cancer diagnosis because of localization in tumors, and methylene blue, used in experimental photodynamic therapy of cancer. The photosensitized oxidation of indole acetic acid forms a free radical that fragments in microseconds, forming reactive cytotoxins. Unlike conventional photodynamic therapy, requiring excitation of oxygen to the reactive singlet state, the treatment is effective even at the low oxygen levels common in tumors and with much lower light doses than normally used.