Medicinal applications of luminescent semiconductor quantum dots are of growing interest. In spite of the fact that their
fabrication and imaging applications have been extensively investigated for the last decade, very little is documented on
photodynamic action of quantum dots. In this study we demonstrate generation of singlet oxygen and other radical
species upon exposure of quantum dots to blue light and therapeutic red light. Extent of radical production can be
readily modified by antioxidants. Lay and scientific communities are two sites concerning potential hazards and
enthusiastic applications of nanotechnology. Synthesis of quantum dots composed of less toxic materials is of great
interest. A new candidate is a ubiquitous element carbon, which on nanoscale exhibits strong photoluminescence.
Photostability studies of porphyrin-type sensitizers performed in aqueous solutions, model systems (sensitizers bound to human serum albumin or in a suspension of released erythrocyte ghosts) and in tumor tissue reveal that similar photomodification takes place in all investigated environments: photobleaching during exposure to light is followed by the formation of red-absorbing photoproducts. Photoproducts are not formed in the solutions of the less photostable sensitizer chlorin e6 and the most photostable sensitizer aluminum phthalocyanine tetrasulphonate. Interaction with photooxidizable biomolecules increases the photobleaching rate of porphyrin-type sensitizers. This should be taken into account in the estimation of optimal photodynamic doses. The modified formula including photodestruction of sensitizer and formation of photoproduct is suggested.
Normal skin of nude mice (Balb/c) was treated topically with 5-aminolevulinic acid (ALA) and its methyl ester (ALA-Me) for 24 hours. Approximately 0.1 gram of freshly prepared cream was applied to a spot of 1 cm2 on the flank of the mice, which was then covered with a transparent dressing. The ALA induced protoporphyrin IX (PpIX) was studied by means of a noninvasive fiber-optic fluorescence probe connected to a luminescence spectrometer. The excitation wavelength was 407 nm, and the emission wavelength was 637 nm. For the first hour a slight lag in PpIX production was observed for the mice treated with ALA-Me compared to the mice treated with ALA. After approximately 12 hours the ALA and the ALA-Me treated mice showed the same PpIX fluorescence intensity. From 12 hours until 24 hours the PpIX fluorescence intensity decreased for both treatment modalities, even though ALA and ALA-Me were continuously present. At 24 hours ALA-Me-treated mice had less than half the amount of PpIX in their skin compared with ALA- treated mice.
The clearance of protoporphyrin IX (PpIX) from the skin of hairless BALB/c mice after topical application of 5- aminolevulinic acid (ALA) and its methyl ester (ALA-Me) was investigated. Creams containing 2 or 20% of ALA or ALA-Me were topically applied on spots of approximately 1 cm2 for 12 hours. The PpIX fluorescence was detected by the means of a Perkin Elmer LS50B luminescence spectrometer equipped with a fiber-optic probe. The emission spectrum was identical with that of cell-bound PpIX. After 12 hours application of ALA and ALA-Me similar amounts of PpIX were found. After creme removal the ALA-induced PpIX fluorescence decayed with a half-life of about 20 hours (20% ALA cream). The ALA-Me-induced PpIX was faster cleared from the skin than ALA-induced PpIX, and had a half-life of about 7 hours (20% ALA-Me cream).
The phototransformation of photodrugs during irradiation can influence the photosensitization reactions: (1) the photobleaching has been proposed to be taken advantage of to enhance the therapeutic ratio and to protect a normal tissue during photosensitized tumor therapy (PTT), (2) the formation of a photoproduct with absorption in the red spectral region can introduce new sensitization pathways. Therefore, the spectroscopic characteristics of the photodrugs in tumors and photostability under the irradiation by visible light were investigated. A few photoprocesses were studied under the irradiation of tumors ex vivo. Efficient photobleaching of porphyrin emission bands at 630 and 690 was detected in all cases. This process was accompanied by the formation of a broad emission band in the spectral region 660 - 680 nm. Careful examination of the emission spectra of tumors ex vivo (excitation at 488 nm) shows that the emission around 660 nm was related with photoproduct formation. The increase of emission intensity at around 680 nm and the observed redistribution of emission intensity after prolonged irradiation can be caused by photoinduced relocalization of the sensitizers in the tumor.
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