CHARACTERIZATION OF PHOTODEGRADATION OF META-TETRA(HYDROXYPHENYL)CHLORIN (MTHPC) IN SOLUTION - BIOLOGICAL CONSEQUENCES IN HUMAN TUMOR-CELLS

The photobleaching of meta-tetra(hydroxyphenyl) chlorin (mTHPC) (irrad iation wavelength 413 mm) in protein-containing solution was evaluated by decay in absorbance in Soret band and in fluorescence (lambda(exc) = 423 mn, lambda(em) = 655 nm). Light exposure resulted in a decrease in absorption throughout the spectrum and simultaneous appearance of new absorption bands in the spectral region 325-450 mn, The rate of mT HPC photodegradation, followed by decay in absorbance, was 15-fold low er than that observed in fluorescence. This fact reflects the photoble aching of presumably monomeric, fluorescing species of mTHPC, In order to determine the consequences of photobleaching of fluorescing mTHPC material on cellular uptake and photocytotoxicity, human HT29 colon ad enocarcinoma cells were incubated with photobleached mTHPC during 5 h with or without following irradiation with the fixed fluence. Surprisi ngly, but up to the time when the fluorescence decreased by 50%, only a slight decrease in photocytotoxicity was detected. Either aggregated forms that have been taken up undergo intracellular monomerization (b ut we did not observe increase in fluorescence in living cells) or the photodynamic activity is mostly due to aggregates. The discrepancy of m-THPC-photodynamic therapy (PDT) effect and fluorescence measurement s may suggest that aggregated m-THPC plays an important role in mTHPC- PDT.