Dual RNA polymerase I inhibition with CX-5461 and BMH-21 synergizes in breast cancer by activating p53-dependent stress

Hyperactivated ribosomal RNA (rRNA) transcription by RNA polymerase I (Pol I) is a hallmark of cancer and drives elevated ribosome biogenesis required for rapid tumor growth. Several Pol I inhibitors have been identified that induce potent anti-cancer effects. However, clinical application of the first-in-class Pol I inhibitor, CX-5461, has been limited by patient toxicity, which is comparable to other chemotherapies. Identifying synergistic drug combinations offers a promising strategy to maintain on-target anti-cancer effects while minimizing adverse reactions. Synergistic drug combinations involve drugs that enhance each other’s effect, enabling dose reduction while preserving efficacy. Synergistic drug combinations of Pol I inhibitors and other anti-cancer agents have been reported; however, it remains unclear whether Pol I inhibitors can synergize with each other. We therefore explored whether two Pol I inhibitors synergize in cancer treatment. We found that CX-5461 and BMH-21 significantly reduced MCF-7 breast cancer cell viability at clinically relevant doses. Combined treatment with these inhibitors led to profound viability defects at sub-micromolar concentrations. Our biochemical analysis showed that CX-5461 and BMH-21 combination therapy enhanced Pol I inhibition and p53 activation compared to monotherapy, promoting growth arrest and apoptosis. Collectively, our findings demonstrate that CX-5461 and BMH-21 are complementary in inhibiting Pol I, activating p53, and suppressing cancer cell growth. Based on these pre-clinical findings, dual Pol I inhibition with CX-5461 and BMH-21 represents a promising therapeutic strategy for treating cancer that is potentially both broadly applicable and tolerable.