Spectroscopic titrations and thermal denaturation experiments show that "acyclic" analogue 1 does not bind to ds-DNA, but under same conditions "cyclic" 2 strongly interacts with ds-DNA and ds-RNA by intercalation into the double helix. Besides, 2 is significantly more effective in inhibition of the tumor cell growth in vitro than 1. We have shown that it is possible to efficiently and irreversibly convert "DNA inactive" compound 1 into "DNA active" compound 2 by light irradiation of the aqueous solutions of the former. This strategy offers a new and attractive approach to photoinduced anticancer therapy.