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Photochem Photobiol. 1997 Jan;65(1):135-44.

The mechanism of Photofrin photobleaching and its consequences for photodynamic dosimetry.

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  • 1Department of Biophysics, University of Rochester Cancer Center, NY, USA.


We report experimental results that support a theory of self-sensitized singlet oxygen-mediated bleaching of the porphyrin photosensitizer Photofrin. Microelectrode measurements of photodynamic oxygen consumption were made near the surface of individual, Photofrin-sensitized EMT6 spheroids during laser irradiation. The progressive decrease in photochemical oxygen consumption with sustained irradiation is consistent with a theory in which bleaching occurs via self-sensitized singlet oxygen reaction with the photosensitizer ground state. A bleaching model based solely on absorbed optical energy density is inconsistent with the data. Photobleaching has a significant effect on calculated photodynamic dose distributions in 500 microns diameter spheroids. Dose distributions corrected for the effects of bleaching produce a new estimate (12.1 +/- 1.2 mM) for the threshold dose of reacting singlet oxygen in this system.

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