Several photosensitizers, when injected systemically, tend to localize at neoplastic loci. This can facilitate localization of tumors at remote sites, e.g., lung, via fluroscence endoscopy. Moreover, the ability of these agents to catalyze photodamage makes it feasible to carry out a selective form of tumor eradication, if an appropriate light flux can be provided. The affinity of one series of dyes for neoplastic tissue is based on the elevated number of tissue LDL receptors. These agents are derivatives of hematoporphyrin and show a strong affinity for LDL. Other tumor-localizing products bind to different plasma components and their precise mode(s) of selective tumor binding remains unknown. Photooxicity can derive from both direct cellular damage and photodamage to the tumor blood supply. The current limitations of ‘photodynamic therapy’ of neoplastic disease relate to light delivery systems, skin photosensitization and drug stability. The current product, HPD, requires 630 nm light, which is obtained only via complex laser/fiber-optic systems. And use of this agent leads to a 3-5 week photosensitization of skin. Drug development is being directed toward the synthesis of more efficient sensitizers which can utilize simpler light sources.
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