Effect of bovine serum albumin on the photodynamic activity of sulfonated tetrapyrazinoporphyrazine

Abstract We synthesized water-soluble zinc(II) complex of octa( p -sulfonatophenyl)tetrapyrazinoporphyrazine (TPyzPz), which is monomeric above pH 2.5 up to micromolar concentration. Significant aggregation occurs at lower pH as a consequence of losing repulsion forces after protonation of sulfonate groups. Changes in both absorption and fluorescence spectra indicated strong interaction of TPyzPz and bovine serum albumin (BSA) with a maximum at 3:1 ratio (dye: BSA). Binding to the BSA leads to significant quenching of the singlet excited state of the photosensitizer and to longer triplet states lifetimes as a consequence of limited diffusion of oxygen to the excited TPyzPz. The photodynamic activity of this TPyzPz was also strongly influenced by binding to BSA; it was lower in the cell culture medium containing serum (EC 50  = 99.8 ± 23.5 μM) than in the medium without the proteins (EC 50  = 0.83 ± 0.31 μM). The compound was of minimal dark toxicity without irradiation (TC 50  > 1000 μM). Fluorescence microscopy revealed lysosomes as the localization site of the TPyzPz, their rupture after irradiation and subsequent relocalization of TPyzPz to the cytoplasm with predominantly necrotic mode of cell death.