We have synthesized a series of symmetrical phenothiazines in which the methyl groups of methylene blue have been substituted by longer alkyl chains. Intrinsic photosensitizing ability was not altered by increasing the chain length. However, in vitro phototoxicity after 2 h incubation of RIF‐1 murine fibrosarcoma cells followed the order n‐propyl > n‐pentyl > n‐butyl > n‐hexyl > ethyl > methyl, with ethyl and n‐propyl analogues being 14‐ and 130‐fold more phototoxic than methylene blue, respectively. All analogues also had an improved ratio of phototoxicity : dark toxicity (4:1 to 27:1) compared with methylene blue (3:1). Phototoxicity did not correlate with cellular phenothiazine levels, suggesting that the site of subcellular localization may be more important. After 2 h incubation of RIF‐1 cells with the phototoxicity LD50 concentration, methylene blue and all analogues were observed to be localized in the lysosomes by fluorescence microscopy. On exposure to light, methylene blue relocalized to the nucleus, the ethyl analogue did not relocalize, whereas the more phototoxic n‐propyl –n‐hexyl analogues relocalized to the mitochondria. Relocalization to the mitochondria was associated with an octanol : buffer partition coefficient ≥ 1. Therefore, the longer‐chain analogues of methylene blue show significantly improved phototoxicity in vitro and, in addition, are expected to avoid the problems of mutagenicity associated with the nuclear localization of methylene blue.
We have synthesized a series of symmetrical phenothiazines in which the methyl groups of methylene blue have been substituted by longer alkyl chains. Intrinsic photosensitizing ability was not altered by increasing the chain length. However, in vitro phototoxicity after 2 h incubation of RIF-1 murine fibrosarcoma cells followed the order n-propyl > n-pentyl > n-butyl > n-hexyl > ethyl > methyl, with ethyl and n-propyl analogues being 14- and 130-fold more phototoxic than methylene blue, respectively. All analogues also had an improved ratio of phototoxicity: dark toxicity (4:1 to 27:1) compared with methylene blue (3:1). Phototoxicity did not correlate with cellular phenothiazine levels, suggesting that the site of subcellular localization may be more important. After 2 h incubation of RIF-1 cells with the phototoxicity LD50 concentration, methylene blue and all analogues were observed to be localized in the lysosomes by fluorescence microscopy. On exposure to light, methylene blue relocalized to the nucleus, the ethyl analogue did not relocalize, whereas the more phototoxic n-propyl - n-hexyl analogues relocalized to the mitochondria. Relocalization to the mitochondria was associated with an octanol: buffer partition coefficient > or = 1. Therefore, the longer-chain analogues of methylene blue show significantly improved phototoxicity in vitro and, in addition, are expected to avoid the problems of mutagenicity associated with the nuclear localization of methylene blue.
Photodynamic therapy (PDT) is a treatment combining a photosensitiser, molecular oxygen and visible light of characteristic wavelength to produce cytotoxic reactive oxygen species (ROS). Within our centre, a series of phenothiazinium salts were synthesised and initial characterisation studies performed to determine any potential use for PDT. All photosensitisers within the series were shown to have useful spectral properties for PDT, with absorbance lambdamax above 667 nm. The Log P values of the compounds were shown to range from -0.9 to > +2.0. Furthermore, Log P values were shown to be important in determining the site of subcellular localisation and as such the site of photooxidative damage. Derivatives with a Log P value of greater than +1.0 were shown to initially localise to the lysosomes then relocalise throughout the cytoplasm following illumination, whereas compounds with intermediate Log P values (-0.7 to +1.0) all remained lysosomal. Only methylene blue (Log P-0.9) was shown to redistribute to the nucleus upon illumination. Following treatment of RIF-1 cells with each phenothiazinium salt for 1 h and subsequent exposure to 665 nm laser light at a fluence rate of 10 mW cm(-2)(18 J cm(-2)), it was determined that the most potent photosensitiser was 260-fold more potent than methylene blue. Furthermore, the PDT efficacy of the photosensitisers was shown to be related to the level of mitochondrial damage induced directly following illumination.
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