Abstract
An increased rate of cellular proliferation is a hallmark of cancer cells and may be accompanied by an increase in ribosome biogenesis and dysregulation in rRNA synthesis. In this regard CX-5461 has been developed as a novel RNA polymerase I inhibitor and is currently in Phase I/II clinical trials. The work described herein sets out to explore the use of this agent in combination with radiation therapy in solid cancer cell lines to understand the associated molecular mechanisms using this approach. Previous work has examined the CX-5461 in haematological malignancies and some solid cancers. Reports have shown that, treatment with CX-5461 induces potent anti-proliferative effects alongside induction of apoptosis in haematological malignancies. A panel of solid human tumour cell lines treated with CX-5461 indicated a broad spectrum of sensitivity (MTS Assay) with IC50 values ranging from 35nM to 1μM. The mode of cell death following CX-5461 exposure in the cancer cell lines studied was cell context dependent- either via autophagy (LC3I ® LC3II conversion alongside p62 sequestosome decrease) or via apoptosis (AnnexinV staining and late PARP cleavage). Treatment with CX-5461 also induced marked levels of DNA damage as detected through increased gH2AX foci staining. The presence of senescence marker β-galactosidase in CX-5461 treated cells indicates cell cycle arrest alongside the potent anti-proliferative effects of CX-5461 being highlighted through the clonogenic assays and decreased Ki-67 expression (flow cytometry). Through combination experiments involving CX-5461 and X-ray radiotherapy the current study has identified the highest levels of increased effectiveness in interactions between CX- 5461 and X-rays within the CaSki cervical cancer cell line. Highly significant effects were seen when low dose radiation and low dose CX-5461 were used together (combination index CI - < 0.2 with 6.25nM CX-5461 and 2Gy X-rays) through isobologram analysis. Cell cycle analysis of CaSki and A375 cells treated with single dose CX-5461 resulted in G2M cell cycle arrest and combination treatments with X-rays indicated cell cycle disruption with prolonged S-phase. The work described herein identifies the synergistic role of CX-5461 in combination with X- rays in solid cancers and may also inform the design of clinical trials of this novel agent.