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Simultaneous Two‐photon Excitation of Photofrin in Relation to Photodynamic Therapy
Author(s) -
Karotki Aliaksandr,
Khurana Mamta,
Lepock James R.,
Wilson Brian C.
Publication year - 2006
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/2005-08-24-ra-657
Subject(s) - photodynamic therapy , photosensitizer , two photon excitation microscopy , excitation , laser , chemistry , absorption (acoustics) , biophysics , optics , optoelectronics , fluorescence , materials science , photochemistry , physics , biology , organic chemistry , quantum mechanics
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‐γ) PDT is limited by low specificity of the photo‐sensitizer, leading to damage to healthy tissue adjacent to the diseased target tissue. One solution is to use simultaneous two‐photon (2‐γ) excitation with ultrafast pulses of near‐IR light. Due to the nonlinear interaction mechanism, 2‐γ 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‐γ PDT. Here, Photofrin has been used as the photosensitizer to demonstrate proof‐of‐principle of 2‐γ killing of vascular endothelial cells in vitro . The 2‐γ absorption properties of Photofrin were investigated in the 750–900 nm excitation wavelength range. It was shown that 2‐γ excitation dominates over 1‐γ excitation above 800 nm. The 2‐γ absorption spectrum of Photofrin in the 800–900 nm excitation wavelength range was measured. The 2‐γ cross section decreased from about 10 GM (1 GM = 10 ‐5 cm 4 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‐γ contribution was negligible. Cell death was monitored with the use of 2‐γ scanning laser microscopy. The light doses required for killing were high (6300 J cm ‐2 for ∼50% killing), but 2‐γ cytotoxicity was unequivocally demonstrated. Although Photofrin is, per se , not a good choice for 2‐γ PDT due to its low 2‐γ cross section, this work provides baseline data to guide the development of novel photosensitizers with much higher 2‐γ cross sections (>100 GM), which will be required for 2‐γ PDT of AMD (and other conditions) to be clinically practical.