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Biotech / Medical : QLT PhotoTherapeutics (QLTI)

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To: Zeev Hed who wrote (315)	4/11/1998 12:25:00 PM
From: SC	Read Replies (1) of 1321

Laser-tissue interactions may be categorized as follows: (1) 1)Photochemical effects a)Photoradiation i)Photosensitized cytotoxicity b)Photoablation i)Incision 2)Thermal effects a)Photocoagulation i)coagulation ii)cautery b)Photovaporization i)incision 3)Ionizing effects a)Photodisruption i)incision Currently available laser therapies for amd rely on thermal effects (photocoagulation) and typical wavelengths used are argon green (514.5nm), krypton red(647.1nm), and tunable dye lasers typically set in the yellow area of the color spectrum. Parameters that can be varied on the Argon Green laser include spot size(50-500 microns), Power (100-400 mW), and duration (0.05-0.2secs and possibly although rarely longer). Other lasers that produce light in the wavelenghts absorbed by the retina would work in a similar fasion. One application, that I am aware of, for photodynamic therapy (photosensitized cytotoxicity) that has been used on the eye since at least 1983 is photoradiation therapy for choroidal melanoma. This was described in 1983. The patient was given a Hematoporphyrin derivative which was selectively taken up and retained by metabolically active tumor tissue, then exposed to light from a rhodamine B dye laser(630nm)72 hours after being given the hematoporphyrin derivative. The irradiation produces an excited state of porphyrin which interacts with oxygen to produce oxygen free radicals which are cytotoxic. This sounds very similar to what is being attempted with Vertoporphyrin. I don't know what wavelength is being used or the parameters of treatment for the Vertoporphyrin study but evidently the power and wavelength are such that collateral damage from thermal effects is minimized if not eliminated entirely. Perhaps someone else could share the precise details with us. (1)Information obtained from "The Glaucomas/edited by Robert Ritch, M. Bruce Shields, and Theodore Krupin, St. Louis, 1989, The C.V. Mosby Company sc

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