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Subcellular locations of free and liposome-bound phthalocyanine preparations in malignant cells in vitro. Roberto Giacominelli Stuffler, Daniela Barsacchi, Clotilde B. Angelucci, Enrico Dainese and Ivo Cozzani `Institute of Biological Chemistry and Molecular Biology, University of Teramo, Italy.` e-mail: cozzani@icbbm.vet.unite.it ABSTRACT The practical impacts of photodynamic reactions, sensitized by porphyrin and porphyrinoid compounds, on the biomedical fields stimulate investigations on the mechanisms connecting the primary photoreactions to modifications of the cellular and molecular structures, functions and morphology. The distribution of the sensitizer in intact tissues and in living cells is essential for identification of targets of the photodynamic response, mediated by singlet oxygen in vivo. Several lines of evidence indicated that the plasmamembranes play a major role in the binding of hematoporphyrin and its derivatives to living cells in the dark and in photodynamic mechanism triggered by light. We report here preliminary data on the binding of phthalocyanine to Yoshida ascites-hepatoma subcellular components, obtained by centrifugal fractionation, following incubation of the living cells with either water-soluble or liposome-bound Zn phthalocyanine preparations. The observed kinetics of distribution of Zn-phthalocyanine at the subcellular level in living isolated cells in the dark is quite different for the water insoluble Zn-phthalocyanine, bound to liposomes, and its highly soluble (tetrasulphonated) derivative. The latter diffuses readily and quickly through the cell membranes and compartments, without any evidence of affinity binding. In contrast Zn-phthalocyanine vehiculated by liposomes appears to bind slowly but firmely to the cell plasmamembranes; no detectable amount of the dye was found in the inner compartments for the whole interval of cell viability and metabolic activity (about 2h). Our results suggest the possibility of tailoring protocols of photodynamic therapy using both free and liposome bound phthalocyanine when the photodynamic in vivo targets will be completely defined.