Photodynamic destruction of cultured human bladder tumor cells by hematoporphyrin derivative: effects of porphyrin molecular aggregation

A study was conducted to investigate the possible significance of molecular aggregation of intracellular porphyrins with regard to in vitro photosensitivity. Monolayer cultures of human bladder carcinoma cells (MGH-U1) were incubated with various concentrations of hematoporphyrin derivative (HpD) for 2, 8, or 21 h plus 8 h in serum-rich medium without HpD (efflux conditions) to yield constant intracellular porphyrin levels (2 mM). The photosensitivity of these cells was studied by the determination of relative cloning efficiency (i.e., survival) following illumination with red (> 580 nm) light. There was a correlation between photosensitivity and HpD/cell contact time; cells exposed to porphyrin efflux conditions exhibited the greatest sensitivity. Characterization of the spectral properties of intracellular porphyrins showed: two distinct, equivalent absorbance maxima at 365 and 400 nm after 2 h uptake; cells incubated with HpD for 8 h or 21 h - 8 h efflux showed single maxima at 403 and 405 nm, respectively. In addition, fluorescence emission measurements (X cx = 399 nm) showed increases in intensity, 1.9-fold at 8 h and 2.3-fold at 21 h + 8 h efflux, relative to 2 h uptake. These data suggest that porphyrin aggregation within the cell may be partially responsible for the observed differential photosensitivity following various HpD uptake conditions.