title: Simultaneous two-photon excitation of photofrin in relation to photodynamic therapy.
abstract: Photodynamic therapy (PDT), the use of light-activated drugs (photosensitizers), is an emerging treatment modality for tumors as well as various nononcologic conditions. Single-photon (1-gamma) PDT is limited by low specificity of the photosensitizer, leading to damage to healthy tissue adjacent to the diseased target tissue. One solution is to use simultaneous two-photon (2-gamma) excitation with ultrafast pulses of near-IR light. Due to the nonlinear interaction mechanism, 2-gamma excitation with a focused beam is localized in three dimensions, allowing treatment volumes on the order of femtoliters. We propose that this will be valuable in PDT of age-related macular degeneration (AMD), which causes blindness due to abnormal choroidal neovasculature and which is currently treated by 1-gamma PDT. Here, Photofrin has been used as the photosensitizer to demonstrate proof-of-principle of 2-gamma killing of vascular endothelial cells in vitro. The 2-gamma absorption properties of Photofrin were investigated in the 750-900 nm excitation wavelength range. It was shown that 2-gamma excitation dominates over 1-gamma excitation above 800 nm. The 2-gamma absorption spectrum of Photofrin in the 800-900 nm excitation wavelength range was measured. The 2-gamma cross section decreased from about 10 GM (1 GM = 10(-50) cm4 s/photon) at 800 nm to 5 GM at 900 nm. Adherent YPEN-1 endothelial cells were then incubated with Photofrin for 24 h and then treated by PDT at 850 nm where the 1-gamma contribution was negligible. Cell death was monitored with the use of 2-gamma scanning laser microscopy. The light doses required for killing were high (6300 J cm(-2) for approximately 50% killing), but 2-gamma cytotoxicity was unequivocally demonstrated. Although Photofrin is, per se, not a good choice for 2-gamma PDT due to its low 2-gamma cross section, this work provides baseline data to guide the development of novel photosensitizers with much higher 2-gamma cross sections (>100 GM), which will be required for 2-gamma PDT of AMD (and other conditions) to be clinically practical.
authors.affiliation: Division of Biophysics and Bioimaging, Ontario Cancer Institute, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Aliaksandr
authors.initials: A
authors.lastname: Karotki
authors.forename: Mamta
authors.initials: M
authors.lastname: Khurana
authors.forename: James R
authors.initials: JR
authors.lastname: Lepock
authors.forename: Brian C
authors.initials: BC
authors.lastname: Wilson
mesh_terms: D000375:Aging / Q000502:physiology; D017209:Apoptosis / Q000187:drug effects*; D001766:Blindness / Q000473:pathology / Q000517:prevention & control; D002460:Cell Line; D020256:Choroidal Neovascularization / Q000473:pathology; D017323:Dihematoporphyrin Ether / Q000627:therapeutic use*; D042783:Endothelial Cells / Q000473:pathology; D006801:Humans; D008268:Macular Degeneration / Q000188:drug therapy / Q000473:pathology / Q000517:prevention & control; D010778:Photochemotherapy / Q000379:methods*; D017785:Photons; D017319:Photosensitizing Agents / Q000627:therapeutic use*
chemical_list: D017319:Photosensitizing Agents; D017323:Dihematoporphyrin Ether