Preliminary report on the protection from Ehrlich ascites tumor with tumor cells photoinactivated in the presence of psoralen or 8-methyl-psoralen.

In a previous paper 1, we reported that cells of the Ehrlich ascites tumor lose their in vivo ability to induce tumors when irradiated with long wave­ length ultraviolet (365 nm) in the presence of very small amounts of the so called “skin-photosensi­ tizing” furocoumarins (psoralens). Studies in this laboratory (Institute of Pharma­ ceutical Chemistry) have shown that these photo­ sensitizing substances lead to a photo-cyclo-addition to the pyrimidine bases of the nucleic acids 2-6. In order to demonstrate directly that the loss of the capacity to induce tumors is due to binding of the furocoumarin to DNA, we extracted DNA from Ehrlich ascites tumor cells previously irradiated in the presence of tritiated psoralen. We found that after irradiation, and only under this condition, psoralen was linked to DNA; moreover, after hydro­ lysis of the extracted DNA, we identified a sub­ stance which was identical to the fluorescent photoadduct already obtained from psoralen and thy­ mine ‘. We now want to report that the Ehrlich ascites tumor cells, when irradiated in the presence of psoralen, seem to behave like untreated tumor cells with regard to W r i g h t ’ s liquid and to the sopravital stain (trypan blue). Moreover, no difference