Oxidative stress is the term used for a sudden and intense exposure of living tissue to reactive oxygen radicals. Tumor tissue response to oxidative stress, invoked in the action of photodynamic therapy (PDT) and some other modalities for cancer treatment, at the level of vascular endothelium has important therapeutic implications. Nitric oxide (NO), a transient radical species which is an important bioregulatory molecule involved in a diverse array of physiological events, has important functions in the regulation of progression of cancerous growth. Response to cancer therapies associated with the induction of oxidative stress was suggested to be amenable to NO mediation. Events involved in antitumor effects of PDT that can be markedly affected by changes in NO availability are listed. The correlation between endogenous NO production in tumors and the response of these lesions to PDT is discussed. Results of treatments aimed at modulating NO levels in PDT treated tumors are reviewed and evaluated.
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