A potential therapeutic application of an Nrf2 inhibitor such as Brusatol (NSC 172924) is the downregulation of Nrf2 pathway components in cells harboring constitutively high levels of the transcription factor. Brusatol (NSC 172924) provokes the depletion of Nrf2 via a mechanism that is not dependent on Keap1 and the proteasomal and autophagic protein degradation systems. Brusatol (NSC 172924) provokes a rapid and transient depletion of Nrf2 protein, through a posttranscriptional mechanism, in mouse Hepa-1c1c7 hepatoma cells. Brusatol (NSC 172924) also inhibits Nrf2 in freshly isolated primary human hepatocytes. To explore the possible synergistic cytotoxicity of Brusatol (NSC 172924) in combination with CDDP, the study investigates the effects of Brusatol and CDDP cotreatment on CT-26 cell viability using an MTT assay. CT-26 cells are treated with various concentrations of Brusatol (0.05, 0.15, 0.45, 1.35, 4.05 and 12.15 μg/mL) and CDDP (0.05, 0.15, 0.45, 1.35, 4.05 and 12.15 μg/mL) for 48 h, either alone or in combination. Following treatment with Brusatol (NSC 172924) and CDDP for 48 h, the viability of CT-26 cells is reduced in a dose-dependent manner, with IC 50 values of 0.27±0.01 and 1.44±0.22 μg/mL, respectively. When Brusatol (NSC 172924) is combined with CDDP at a constant concentration ratio of 1:1, cell growth inhibition is markedly enhanced compared with single-agent treatment; the IC 50 value of Brusatol (NSC 172924) and CDDP cotreatment is 0.19±0.02 μg/mL.
To explore the anticancer effect of Brusatol in vivo, A549 xenografts grown in nude mice are used as a model. Nude mice are injected with A549 cells to induce tumor growth, followed by a single i.p. injection of 2 mg/kg Brusatol. Tumors are isolated 24 h or 48 h postinjection. Nrf2 protein levels are significantly decreased at 24 h or 48 h postinjection, indicating that Brusatol (NSC 172924) is able to reach the tumor tissue and inhibit the Nrf2 pathway. To measure tumor growth, two different experiments are performed. In the first experiment, once the tumor size reaches an average of 230 mm 3 , DMSO, Brusatol (NSC 172924) (2 mg/kg), Cisplatin (2 mg/kg), or Cisplatin (2 mg/kg) and Brusatol (2 mg/kg) combined treatment is i.p. injected every other day for a total of five times. Cisplatin or Brusatol (NSC 172924) alone does not inhibit tumor growth significantly, whereas in the combination group, tumor size is significantly reduced.