Combination of arsenic trioxide and BCNU synergistically triggers redox-mediated autophagic cell death in human solid tumors

Arsenic trioxide (As ₂O ₃) is an effective treatment for relapsed or refractory acute promyelocytic leukemia (APL). After the discovery of As ₂O ₃ as a promising treatment for APL, several studies investigated the use of As ₂O ₃ as a single agent in the treatment of solid tumors; however, its therapeutic efficacy is limited. Thus, the systematic study of the combination of As ₂O ₃ with other clinically used chemotherapeutic drugs to improve its therapeutic efficacy in treating human solid tumors is merited. In this study, we demonstrate for the first time, using isobologram analysis, that As ₂O ₃ exhibits a synergistic interaction with N,N′-bis(2-chloroethyl)-N-nitrosourea (BCNU). The synergistic augmentation of the cytotoxicity of As ₂O ₃ with BCNU is in part through the autophagic cell death machinery in human solid tumor cells. As ₂O ₃ and BCNU in combination produce enhanced cytotoxicity via the depletion of reduced glutathione (GSH) and augmentation of reaction oxygen species (ROS) production. Further analysis indicated that the extension of GSH depletion by this combined regimen occurs through the inhibition of the catalytic activity of glutathione reductase. Blocking ROS production with antioxidants or ROS scavengers effectively inhibits cell death and autophagy formation, indicating that redox-mediated autophagic cell death involves the synergism of As ₂O ₃ with BCNU. Taken together, this is the first evidence that BCNU could help to extend the therapeutic spectrum of As ₂O ₃. These findings will be useful in designing future clinical trials of combination chemotherapy with As ₂O ₃ and BCNU, with the potential for broad use against a variety of solid tumors.