Photocytotoxicity and Intracellular Generation of Free Radicals by Tetrasulphonated Al- and Zn-phthalocyanines

Abstract

The photosensitizing properties of tetrasulphonated Al- and Zn-phthalocyanines (AlPcS₄ and ZnPcS₄) in lymphoma cells were studied as a function of the pre/post-illumination incubation time. Photocytotoxicity increased with incubation time, ranging from a transient cell-cycle arrest to cell killing. Under all experimental conditions, the phototoxicity of ZnPcS₄ was markedly higher than that of AlPcS₄. The primary photoprocesses initiated by metallo-phthalocyanines (MePcS₄) in the cells were probed with DMPO/esr spin-trapping techniques. Under all incubation conditions the intracellularly bound MePcS₄ sensitized formation of three different types of DMPO spin-adducts: DMPO/OH (hydroxyl radical), DMPO/R (organic carbon-centred radical(s)) and an unidentified simple nitroxyl, referred to as DMPO/ox. The yields of trapped radicals depended on the length of the incubation with the dyes prior to illumination and the formation of spin-adducts was shown to be intracellular. The ability of DMPO to protect cells from the photocytotoxic effects of Al- and ZnPcS₄, combined with the generation of carbon-centred spin-adducts is direct evidence for the involvement of free-radical-mediated damage of cellular constituents.