A novel organobismuth compound, 1-[(2-di-p-tolylbismuthanophenyl)diazenyl]pyrrolidine, induces apoptosis in the human acute promyelocytic leukemia cell line NB4 via reactive oxygen species.

A novel organobismuth compound, 1-[(2-di-p-tolylbismuthanophenyl)diazenyl]pyrrolidine (4), which has 1-(phenyldiazenyl)pyrrolidine (1) substituent in a benzene ring of tri(p-tolyl)bismuthane (2), was synthesized and tested for biological activity toward human tumor cell lines. 4 had a potent anti-proliferative effect on human cancer cell lines, although both 1 and 2 exhibited only weak activity. The sensitivity of leukemic cell lines to 4 was relatively high; IC(50) values for the human leukemia cell line NB4 and cervical cancer cell line HeLa were 0.88 μM and 5.36 μM, respectively. Treatment of NB4 cells with 4 induced apoptosis, loss of mitochondrial membrane potential (ΔΨ(mt)) and the generation of cellular reactive oxygen species (ROS). 1 and 2 did not induce apoptosis and had only a marginal effect on ΔΨ(mt) and the generation of ROS. N-acetyl cysteine (NAC) reduced the generation of ROS and conferred protection against 4-induced apoptosis, indicating a role for oxidative stress. 4 did not inhibit the polymerization of tubulin in vitro. 1-[2-(di-p-tolylstibanophenyl)diazenyl]pyrrolidine (3), which has the same chemical structure as 4 but contains antimony in place of bismuth, did not show any cytotoxic activity. The results suggest that the conjugated structure of the diazenylpyrrolidine moiety and bismuth center are key to the bioactivity of 4.