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To date, various strategies have been proposed to increase the efficiency of cancer therapy. It is known that the action of DNA repair system can determine the resistance of cancer cells to DNA-damaging chemotherapy and radiotherapy, and one of these ways to increase therapeutic efficiency is the search for inhibitors of enzymes of the DNA repair system. Inhibition of the DNA repair enzyme tyrosyl-DNA phosphodiesterase1 (Tdp1) leads to an increase in the effectiveness of the topoisomerase 1 (Top1) inhibitor, the anticancer drug topotecan. Covalent complexes Top1-DNA, which are normally short-lived and are not a threat to the cell, are stabilized under the influence of topotecan and lead to cell death. Tdp1 eliminates such stabilized complexes and thus weaken the effect of topotecan therapy. We have previously shown that the use of the usnic acid hydrazonothiazole derivative OL9-119 in combination with topotecan increased the antitumor and antimetastatic efficacy of the latter in a mouse model of Lewis lung carcinoma. In this work, it was shown that the combined use of topotecan and Tdp1 inhibitor, the hydrazonothiazole derivative of usnic acid OL9-119, leads to an increase in the DNA-damaging effect of topotecan which is used in the clinic for the treatment of cancer. The study of the proapoptotic effect of the compound OL9-119 showed that the compound itself does not induce apoptosis, but increases the proapoptotic effect of topotecan. The results of the study could be used to improve the effectiveness of anticancer therapy and/or to reduce the therapeutic dose of topotecan and, therefore, the severity of side effects., (© 2023. The Author(s).)