The BET bromodomain inhibitor exerts the most potent synergistic anticancer effects with quinone-containing compounds and anti-microtubule drugs

BET bromodomain inhibitors are very promising novel anticancer agents, however, single therapy does not cause tumor regression in mice, suggesting the need for combination therapy. After screening a library of 2697 small molecule compounds, we found that two classes of compounds, the quinone-containing compounds such as nanaomycin and anti-microtubule drugs such as vincristine, exerted the best synergistic anticancer effects with the BET bromodomain inhibitor JQ1 in neuroblastoma cells. Mechanistically, the quinone-containing compound nanaomycin induced neuroblastoma cell death but also activated the Nrf2-antioxidant signaling pathway, and the BET bromodomain proteins BRD3 and BRD4 formed a protein complex with Nrf2. Treatment with JQ1 blocked the recruitment of Nrf2 to the antioxidant responsive elements at Nrf2 target gene promoters, and JQ1 exerted synergistic anticancer effects with nanaomycin by blocking the Nrf2-antioxidant signaling pathway. JQ1 and vincristine synergistically induced neuroblastoma cell cycle arrest at the G₂/M phase, aberrant mitotic spindle assembly formation and apoptosis, but showed no effect on cell survival in normal non-malignant cells. Importantly, co-treatment with JQ1 and vincristine synergistically suppressed tumor progression in neuroblastoma-bearing mice. These results strongly suggest that patients treated with BET bromodomain inhibitors in clinical trials should be co-treated with vincristine.