**4. Conclusions**

The berberine and tetrahydroberberine sulfonates and their brominated analogues were tested to evaluate their Tdp1 inhibitory activity. To our knowledge, this is the first report that documents berberine-based compounds as inhibitors of this enzyme. The IC<sup>50</sup> values are in the 0.53 to 4 µM range. Of the alkyl sulfonates, only derivatives with bromine substitution on site 12 of tetrahydroberberrubine are active. While both tetrahydroberberrubine and 12-bromtetrahydroberberrubine derivatives containing polyfluoroaromatic substituents all have inhibitory activity with IC<sup>50</sup> values of ~1 µM. According to the inhibitory activity, toxicity data and ability to sensitize topotecan against the HeLa cancer cell line the two most promising derivatives were identified—**11g** and **12g**. These results indicate that sulfonates of tetrahydroberberine have potential to be developed as new agents for anticancer therapy due to their inhibitory activity and lack of toxicity.

**Supplementary Materials:** Supplementary Materials can be found at http://www.mdpi.com/1422-0067/21/19/ 7162/s1.

**Author Contributions:** Chemistry investigation, E.D.G., I.V.N., R.A.B. and O.A.L.; in vitro investigation, A.L.Z., A.A.C., E.S.I., N.S.D., E.M.M. and R.O.A.; modeling, J.R.; methodology, N.F.S. and O.I.L.; project administration, K.P.V.; supervision, K.P.V.; writing—original draft, E.D.G., A.L.Z. and O.A.L.; writing—review and editing, K.P.V., J.R., N.F.S. and O.I.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by the Russian Science Foundation grant № 19-13-00040.

**Acknowledgments:** Authors would like to acknowledge the Multi-Access Chemical Research Center SB RAS for spectral and analytical measurements.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
