The DNA repair enzyme tyrosyl-DNA phosphodiesterase 1 (TDP1) removes various adducts from the 3'-end of DNA, including those induced by anticancer chemotherapeutics and is therefore considered an important therapeutic target. Previously, we investigated TDP1 inhibitors as sensitizers for the anticancer drug topotecan. Now, we have demonstrated that usnic acid derivatives containing a thiazole ring with an amide linker exhibited inhibitory activity against TDP1 at micromolar and submicromolar concentrations. Moreover, the lead compound OL11-119, (R)-N-(4-(8-acetyl-1,3,7-trihydroxy-2,9a-dimethyl-9-oxo-9,9a- dihydrodibenzo[b,d]furan-4-yl)thiazol-2-yl)-4-bromobenzamide, was found to enhance topotecan-induced tumor cell death at a nontoxic concentration. Molecular docking of OL11-119 and its analog with hydrazone linker, OL9-119, a previously identified potent TDP1 inhibitor, was performed. The binding energy of OL9- 119 to the enzyme active site was shown to be lower than that of OL11-119, which correlated with the higher inhibitory activity of OL9-119 against TDP1.