We synthesized a series of analogues of 5,20-diphenyl-10,15-bis(4-carboxylatomethoxy)phenyl-21,23-dithiaporphyrin (I) as potential
photosensitizers for
photodynamic therapy (
PDT). The
photosensitizers differ in the length of the side chains that bind the carboxyl to the
phenol at positions 10 and 15 of the thiaporphyrin. The spectroscopic, photophysical, and biophysical properties of these
photosensitizers are reported. The structural changes have almost no effect on the excitation/emission spectra with respect to I's spectra or on
singlet oxygen generation in MeOH. All of the
photosensitizers have a very high, close to 1.00,
singlet oxygen quantum yield in MeOH. On the contrary,
singlet oxygen generation in
liposomes was considerably affected by the structural change in the
photosensitizers. The
photosensitizers possessing short side chains (one and three carbons) showed high quantum yields of around 0.7, whereas the
photosensitizers possessing longer side chains showed smaller quantum yield, down to 0.14 for compound X (possessing side-chain length of 10 carbons), all at 1 microM. Moreover a self-quenching process of
singlet oxygen was observed, and the quantum yield decreased as the
photosensitizer's concentration increased. We measured the binding constant of I to
liposomes and found Kb = 23.3 +/- 1.6 (mg/mL)-1. All the other
photosensitizers with longer side chains exhibited very slow binding to
liposomes, which prevented us from assessing their Kb's. We carried out fluorescence resonance energy transfer (FRET) measurements to determine the relative depth in which each
photosensitizer is intercalated in the
liposome bilayer. We found that the longer the side chain the deeper the
photosensitizer core is embedded in the bilayer. This finding suggests that the
photosensitizers are bound to the bilayer with their
acid ends close to the aqueous medium interface and their core inside the bilayer. We performed
PDT with the dithiaporphyrins on U937 cells and R3230AC cells. We found that the dark toxicity of the
photosensitizers with the longer side chain (X, VI, V) is significantly higher than the dark toxicity of sensitizers with shorter side chains (I, III, IV).
Phototoxicity measurements showed the opposite direction; the
photosensitizers with shorter side chains were found to be more phototoxic than those with longer side chains. These differences are attributed to the relationship between diffusion and endocytosis in each
photosensitizer, which determines the location of the
photosensitizer in the cell and hence its
phototoxicity.