DNA ligase I, III and IV inhibitor (IC50 values are 5, 9 and 5 μM respectively) that blocks DNA binding (Ki = 5 μM for DNA ligase I). Preferentially inhibits step 2 of the ligation reaction, and inhibits base excision repair (BER) and non-homologous end joining (NHEJ). Specifically sensitizes cancer cells to DNA damage and increases the cytotoxicity of DNA-damaging agents.