Topoisomerase I inhibitor 2 (ZML-8) is a highly selective DNA topoisomerase I (Top1) inhibitor that inhibits Top1 activity and results DNA damage. Topoisomerase I inhibitor 2 blocks G2/M phase and induces apoptosis with anti-tumor effect [1].

Topoisomerase I inhibitor 2 demonstrates potent inhibitory effects on HepG2 cells with an IC50 of 0.58 μM and exhibits selectivity, with a selectivity index (SI) of 55.70% between HepG2 and normal human liver cells L-02 [1]. When administered at concentrations of 1.25 and 2.5 μM for 48 hours, it significantly suppresses tumor cell proliferation. This suppression is achieved through the downregulation of the anti-apoptotic protein Bcl-2 and the promotion of caspase-dependent apoptosis, alongside a decrease in Top1 activity leading to DNA damage via supercoiled DNA formation [1]. Additionally, cell cycle analysis reveals that at a concentration of 2.5 μM, there is a dose-dependent arrest in the G2/M phase within 24 hours. Further assays confirm that the apoptosis rate increases in a dose-dependent manner upon exposure for 48 hours, with rates of 65.0% and 77.6%. Moreover, Western blot analysis at concentrations of 0.625, 1.25, and 2.5 μM over 48 hours shows a significant decrease in Bcl-2 expression and an increase in pro-apoptotic proteins Bax, Bad, and p53, signifying the enhancement of caspase-dependent apoptosis and activation of Cleaved caspase-3 [1].