DNA-Photosensitization by New Drugs for Photoinactivation of Viruses and Phototherapy of Tumors
Doctoral thesis, 2001
Cyanines and pyropheophorbides are two classes of photoactivable dyes which are particularly interesting for photosterilization of blood used in transfusion and for treatment of localised cancers by photodynamic therapy (PDT).
After a bibliographic review about the principal mechanisms of DNA photosensitization, studies about the photosensitizing properties of these two families of compounds are presented.
First, photobleaching of the cyanine YO and its dimer YOYO has been studied in the absence and presence of DNA. Spectroscopic studies have shown that the nature of reactive species involved in this process depends on the DNA-binding mode of YO and YOYO, which can interact either by intercalation or, when all the intercalation sites are saturated, by external binding. Hydroxyl radicals generated via a Fenton reaction are responsible for photobleaching of the intercalated dye whereas photobleaching of the externally bound dye results from the action of singlet oxygen. Additionally, photocleavage of DNA by YO and YOYO has been studied using sequencing gels and a preference for 5'-G sites was identified. The two reactive species characterized previously in the bleaching studies are also active in producing both cleavage. We propose that they both generate 7,8-dihydro-8-oxo-deoxyguanosine which is then altered by an electron transfer with excited state YO to produce strand breaks, independent of the dye-binding mode to DNA.
In a second part, the mechanism of photocleavage and the site-specificity of damages induced on DNA by the chlorhydrate of aminopyropheophorbide have been determined. This drug has different binding modes to DNA and generates damage non-specifically on all guanines via formation of singlet oxygen. A frank break, specific to a 5'-CG sequence, appears when the compound is aggregated along the DNA helix.
Therefore, both classes of compounds studied here present a two-step photonuclease activity, which can be adjusted according to the dye-binding mode toDNA.