Tumour-localizing and -photosensitising properties of meso-tetra(4-nido-carboranylphenyl)porphyrin (H2TCP)

A water-soluble meso-substituted porphyrin (H2TCP) bearing 36 boron atoms, which appeared to be an efficient photodynamic sensitiser (singlet oxygen quantum yield = 0.44), was studied for its accumulation by murine melanotic melanoma cells (B16F1). The amount of H2TCP in the cells increased with the porphyrin dose in the incubation medium up to, and at least, 100 μM concentrations with no significant cytotoxic effect in the dark. Moreover, the H2TCP uptake increased with the incubation time reaching a plateau value corresponding with the recovery of 0.4 nmol of H2TCP per mg of cell proteins after 24 h incubation. Fluorescence microscopy observations showed that the porphyrin was largely localized intracellularly, exhibiting a discrete distribution in the cytoplasm with a pattern which was closely similar to that observed for the endosomal probe Lucifer yellow. The photosensitising efficiency of the H2TCP toward B16F1 cells was studied for different irradiation (1–15 min) and incubation (1–24 h) times. Nearly complete (>95%) cell mortality was obtained upon incubation with 20 μM H2TCP and 10 min irradiation with red light (600–700 nm, 20 mW/cm2). The porphyrin was also accumulated in appreciable amounts by the tumour tissue after intravenous injection to C57BL/6 mice bearing a subcutaneously transplanted melanotic melanoma. Maximum accumulation in the tumour was achieved by administration of H2TCP dissolved in the ternary mixture 20% dimethylsulfoxide (DMSO)–30% polyethyleneglycol (PEG 400)–50% water. Thus, this porphyrin could act as both a photodynamic therapy agent and a radiosensitising agent for boron neutron capture therapy.