*Article* **New Hybrid Compounds Combining Fragments of Usnic Acid and Thioether Are Inhibitors of Human Enzymes TDP1, TDP2 and PARP1**

**Nadezhda S. Dyrkheeva <sup>1</sup> , Aleksandr S. Filimonov <sup>2</sup> , Olga A. Luzina <sup>2</sup> , Kristina A. Orlova <sup>3</sup> , Irina A. Chernyshova <sup>1</sup> , Tatyana E. Kornienko <sup>1</sup> , Anastasia A. Malakhova 1,4 , Sergey P. Medvedev 1,4 , Alexandra L. Zakharenko 1,5, Ekaterina S. Ilina <sup>1</sup> , Rashid O. Anarbaev <sup>1</sup> , Konstantin N. Naumenko <sup>1</sup> , Kristina V. Klabenkova 3,4, Ekaterina A. Burakova 3,4, Dmitry A. Stetsenko 3,4 , Suren M. Zakian 1,4 , Nariman F. Salakhutdinov <sup>2</sup> and Olga I. Lavrik 1,3,5,\***


**Abstract:** Tyrosyl-DNA phosphodiesterase 1 (TDP1) catalyzes the cleavage of the phosphodiester bond between the tyrosine residue of topoisomerase 1 (TOP1) and the 30 phosphate of DNA in the single-strand break generated by TOP1. TDP1 promotes the cleavage of the stable DNA–TOP1 complexes with the TOP1 inhibitor topotecan, which is a clinically used anticancer drug. This article reports the synthesis and study of usnic acid thioether and sulfoxide derivatives that efficiently suppress TDP1 activity, with IC<sup>50</sup> values in the 1.4–25.2 µM range. The structure of the heterocyclic substituent introduced into the dibenzofuran core affects the TDP1 inhibitory efficiency of the compounds. A five-membered heterocyclic fragment was shown to be most pharmacophoric among the others. Sulfoxide derivatives were less cytotoxic than their thioester analogs. We observed an uncompetitive type of inhibition for the four most effective inhibitors of TDP1. The anticancer effect of TOP1 inhibitors can be enhanced by the simultaneous inhibition of PARP1, TDP1, and TDP2. Some of the compounds inhibited not only TDP1 but also TDP2 and/or PARP1, but at significantly higher concentration ranges than TDP1. Leader compound **10a** showed promising synergy on HeLa cells in conjunction with the TOP1 inhibitor topotecan.

**Keywords:** usnic acid; thioether; tyrosyl-DNA phosphodiesterase 1; TDP1 inhibitor; inhibiting activity; TDP2; PARP1; topotecan; synergy; HEK293 knockout cell line
