Tyrosyl-DNA Phosphodiesterase 1 Inhibitors: Usnic Acid Enamines Enhance the Cytotoxic Effect of Camptothecin.

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a repair enzyme for stalled DNA-topoisomerase 1 (Top1) cleavage complexes and other 3'-end DNA lesions. TDP1 is a perspective target for anticancer therapy based on Top1-poison-mediated DNA damage. Several novel usnic acid derivatives with an enamine moiety have been synthesized and tested as inhibitors of TDP1. The enamines of usnic acid showed IC50 values in the range of 0.16 to 2.0 μM. These compounds revealed moderate cytotoxicity against human tumor MCF-7 cells. These new compounds enhanced the cytotoxicity of the established Top1 poison camptothecin by an order of magnitude.