PHOTODYNAMIC-ACTION OF ENDOGENOUSLY SYNTHESIZED PORPHYRINS FROM AMINOLEVULINIC ACID, USING A NEW MODEL FOR ASSAYING THE EFFECTIVENESS OF TUMORAL CELL-KILLING

1. The effectiveness of the photodynamic action of porphyrins, was studied by means of the tissue explant culture technique. A murine tumor tissue explant was incubated in a medium containing 0.6 mM of ALA for periods of 1 and 2 hr, total porphyrins synthesized under these conditions were of the same level as those found in our previous in vivo experiments. The explants were then irradiated for 30 min with He-Ne laser of 3.5 mW output power placed at a distance of 10 cm. Controls of non-irradiated tumor tissue slices incubated with and without ALA were performed. Immediately after irradiation, innocula of exactly 1 mm3 of the irradiated and non-irradiated tissue were subcutaneously injected under the right and left flanks of the same animal, respectively. The growth of the tumor was measured 15, 20 and 25 days after implantation. 2. Results obtained showed that the explants that were incubated for 1 hr with ALA and irradiated, reaching a concentration of 2.8 mug porphyrins/g tissue, produced a reduction of 50-70% of tumor size as compared with the non-irradiated controls incubated with ALA. Explants incubated for 2 hr, reaching a concentration of 4.6 mug porphyrins/g tissue, produced from 60% to complete lack of tumor growth. The effectiveness index (EI) of photoirradiation was calculated on the basis of the tumor growth in irradiated and non-irradiated tumors. El was nearly 100% showing almost complete tumor cell destruction for tumor irradiated for 2 hr with 0.6 mM ALA. 3. As indicators of cell injury and subsequent death and necrosis, LDH activity in the incubation medium and intracellular potassium content were measured. Results indicated that as a consequence of irradiation of porphyrin loaded tumor explants, significant release of LDH to the medium and loss of intracellular potassium occurred. These findings show great to complete tumor destruction by combination of porphyrins endogenously formed from ALA and low irradiance with laser.