The subcellular localization, efficacy and photooxidative mechanism of three new photosensitizing porphycenes (HexoTMPn, PeloTMPn, CpoTMPn) for photodynamic therapy with different substituents at position 9 of the tetrapyrrole macrocycle were investigated in vitro using different human skin-derived cell lines (HaCaT, SCL I, SCL II) with the aim of customizing the side-chain chemistry to accelerate cellular uptake and so enhance photodynamic activity. Cells were incubated with a porphycene and costained with organelle-specific markers. Subcellular localization was determined by fluorescence microscopy. Also, cells were incubated with different sensitizer concentrations (0-1000 nmol/l) and irradiated by an incoherent light source (lambda(em) = 600-750 nm, 40 mW/cm(2), 24 J/cm(2)) with/without quenchers or enhancers (NaN(3), histidine, mannitol or D(2)O). Cell viability was assessed. All porphycenes were localized in perinuclear lysosomes and induced a decrease in mitochondrial activity following irradiation. HexoTMPn was the most efficient in all three cell lines (EC(50) in HaCaT cells: HexoTMPn 14 nmol/l, CpoTMPn 62 nmol/l, PeloTMPn 89 nmol/l). Addition of either NaN(3) or histidine reduced the phototoxicity significantly. Due to the short lifetime of singlet oxygen, the sites of sensitizer localization are the initial subcellular targets. The cytotoxicity of each sensitizer varied depending on singlet oxygen quantum yield and cell line. Despite the different chemical structures, the biological effects were not very distinct, since they seemed to be mostly determined by the tetrapyrrole ring and only slightly modified by the substituent at position 9. Also, there was only a narrow margin between biological compatibility and efficacy.