title: The mechanism of Photofrin photobleaching and its consequences for photodynamic dosimetry.
abstract: 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.
authors.affiliation: Department of Biophysics, University of Rochester Cancer Center, NY, USA.
authors.forename: I
authors.initials: I
authors.lastname: Georgakoudi
authors.forename: M G
authors.initials: MG
authors.lastname: Nichols
authors.forename: T H
authors.initials: TH
authors.lastname: Foster
mesh_terms: D017323:Dihematoporphyrin Ether / Q000494:pharmacology*; D017324:Hematoporphyrin Derivative / Q000494:pharmacology*; D010101:Oxygen Consumption; D010777:Photochemistry; D010778:Photochemotherapy; D017319:Photosensitizing Agents / Q000494:pharmacology*; D011880:Radiotherapy Planning, Computer-Assisted
chemical_list: D017319:Photosensitizing Agents; D017324:Hematoporphyrin Derivative; D017323:Dihematoporphyrin Ether