Application of lower fluence rate for less microvasculature damage and greater cell-killing during photodynamic therapy

Influence of light exposure on the kinetics of protoporphyrin IX formation in normal skin of hairless mice after application of 5-aminolevulinic acid methyl ester

The rates of protoporphyrin IX (PpIX) photodegradation and reappearance after light exposure at 420 and 632 nm were measured in mouse skin at different times after 1 h topical application of 5-aminolevulinic acid methyl ester (ALA-Me). After ALA-Me application (1 h) and removal, the fluorescence of PpIX increased for about 1 h, and then reached a maximum and started to decrease. Reappearance of PpIX fluorescence after exposures (degrading 60%-80% of the PpIX) was faster for exposures 0.5 h after ALA-Me application than for exposures 3 h. The bleaching rate was largest in the former case. This indicates that PpIX is located deeper in the skin after 3 h than after 0.5 h, whereas the pool of ALA-Me in the skin is largest at 0.5 h. In all cases, the reappearance was faster at a skin temperature of 35 degrees C than at 23 degrees C. Reappearance of PpIX fluorescence was faster after exposure to light at 420 nm than at 632 nm. The rate of elimination of PpIX from the volume of detection was faster after 420 nm light irradiation than that after 632 nm. These findings are discussed in view of penetration depths of light and ALA-Me, and diffusion of PpIX.

Mechanisms in photodynamic therapy: Part three-Photosensitizer pharmacokinetics, biodistribution, tumor localization and modes of tumor destruction

Photodynamic therapy (PDT) has been known for over a hundred years, but is only now becoming widely used. Originally developed as cancer therapy, some of its most successful applications are for non-malignant disease. The majority of mechanistic research into PDT, however, is still directed towards anti-cancer applications. In the final part of series of three reviews, we will cover the possible reasons for the well-known tumor localizing properties of photosensitizers (PS). When PS are injected into the bloodstream they bind to various serum proteins and this can affect their phamacokinetics and biodistribution. Different PS can have very different pharmacokinetics and this can directly affect the illumination parameters. Intravenously injected PS undergo a transition from being bound to serum proteins, then bound to endothelial cells, then bound to the adventitia of the vessels, then bound either to the extracellular matrix or to the cells within the tumor, and finally to being cleared from the tumor by lymphatics or blood vessels, and excreted either by the kidneys or the liver. The effect of PDT on the tumor largely depends at which stage of this continuous process light is delivered. The anti-tumor effects of PDT are divided into three main mechanisms. Powerful anti-vascular effects can lead to thrombosis and hemorrhage in tumor blood vessels that subsequently lead to tumor death via deprivation of oxygen and nutrients. Direct tumor cell death by apoptosis or necrosis can occur if the PS has been allowed to be taken up by tumor cells. Finally the acute inflammation and release of cytokines and stress response proteins induced in the tumor by PDT can lead to an influx of leukocytes that can both contribute to tumor destruction as well as to stimulate the immune system to recognize and destroy tumor cells even at distant locations.