Abstract
We speculate that the mechanism of the accumulation of porphyrins in tumor tissue is the connection to proteins-because of their high πelectron content-and the amphipathicity nature of porphyrins, which causes a high affinity of lipoprotein and porphyrin. Cancer tissue takes up lipoprotein actively by endocytosis associated with the enhancement of LDL, trans-ferrin and hemopexin receptor activities. Cancer tissue can not exclude lipoprotein connected with porphyrin because of its immaturity or lack of lymphatic tissue.The photoreaction of porphyrins inducing phototoxicity may be divided into two major mechanisms : Type 1 mechanisms, in which the sensitizer molecules excited in the lowest triplet state react directly with biological substrates to lead to cell damage ; and Type 2 mechanism, in which the photogenerated triplet state of the sensitizer reacts with oxygen by an energy transfer process, to produce singlet molecular oxygen. Recently our studies have using a pulsed laser with high peak power revealed that the effective penetration depth of PDT is over 1.5 cm. PDT using new sensitizers with new devices may be useful for not only for the treatment of superficial tumors, but also of advanced, solid tumors.
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