Methylene blue mediated photodynamic therapy in experimental colorectal tumors in mice

Methylene blue (MB+) is a well-known dye in medicine and has been discussed as an easily applicable drug for the topical treatment during photodynamic therapy (PDT). The therapeutic response of MB+ was investigated in vivo by local injection of MBC in a xenotransplanted subcutanous tumor (adeno-carc inoma, G-3) in female nude mice. MB+ in a concentration of 1% was applied b oth undiluted and diluted to 0.1 and 0.01% with isotonic sodium chloride. T reatment with 1% MB+ and subsequent irradiation at 662 nm with 100 J/cm(2) led to complete tumor destruction in 79% of the treated animals. A decrease of the fluence rate from 100 to 50 mW/cm(2) increased the phototoxic respo nse as well as fractionated light application. Small sensitizer concentrati ons reduced the PDT effect significantly. It seems that the light induced r eaction of MB+ could be correlated with the rapid production of reactive ox ygen species. Below a threshold dose of MB+ oxidative damage of the tissue is prevented. However, above this dose, as a point of no return, MB+ acts a s an extremely potent oxidant. (C) 2000 Elsevier Science S.A. All rights re served.