COMPARISON OF THE PHOTOBLEACHING EFFECT OF 3 PHOTOSENSITIZING AGENTS - MESO-TETRA(M-HYDROXYPHENYL)CHLORIN, MESO-TETRA(M-HYDROXYPHENYL)PORPHYRIN AND PHOTOFRIN DURING PHOTODYNAMIC THERAPY

V79 cells (Chinese hamster lung fibroblasts) were incubated with meso-tetra(m-hydroxyphenyl)chlorin (mTHPC), meso-tetra(m-hydroxyphenyl)porphyrin (mTHPP) or Photofrin and exposed to light. Light exposure resulted in the photobleaching of all three drugs and a change in the shape of the fluorescence emission spectrum of Photofrin, but no significant spectral change for mTHPC or mTHPP. The rate of photobleaching of mTHPC fluorescence was much higher than that of mTHPP and Photofrin. However, there was only a slight increase in the rate of photobleaching with increasing mTHPC concentration in the cells. It was proposed that the light-induced photobleaching of mTHPC may influence its ability in cell photoinactivation and change the shape of survival curve of the cells treated by photodynamic therapy (PDT). Photobleaching of mTHPC resulted in the need for increased light exposure for cell inactivation at low mTHPC concentrations. These results suggest that mTHPC photobleaching may play a role in protecting normal tissue from damage by PDT and enhancing the therapeutic ratio of PDT.