BACKGROUND: rDNA transcription is consistently dysregulated in cancer, with RNA polymerase I (Pol I) hyperactivity providing a survival checkpoint in haematological cancers1.
Our laboratory developed CX-5461, a highly selective small molecule inhibitor of Pol I transcription of ribosomal genes2, currently in early phase clinical trials. CX-5461 induces cell death via a p53-dependent nucleolar surveillance response, and a p53-independent nucleolar-specific DNA damage response (DDR), killing malignant cells while sparing normal cells1,3. CX-5461 increases survival in murine models of B-cell lymphoma, acute myeloid leukaemia and now multiple myeloma (MM)1,3-5. However, resistance eventually develops, indicating combination drug therapy is essential.
AIM: to examine the efficacy of CX-5461 in combination with agents having proven clinical or promising preclinical efficacy in MM.
METHODS: we conducted a boutique, high-throughput screen in myeloma cell lines using CX-5461 combined with a select panel of agents. We measured the effect of CX-5461 on proliferation, cell death, cell cycle distribution, biomarkers of DDR and on-target effects. CX-5461 and the histone deacetylase inhibitor (HDACi) panobinostat were combined in both the tVk*MYC6 model of MM, and in KaLwRij mice transplanted with 5T33 cells.
RESULTS: CX-5461 showed increased anti-proliferative effect and cell death in combination with multiple drug classes. The proteasome inhibitor carfilzomib and the HDACi panobinostat demonstrated the most impressive synergy with CX-5461. In 2 distinct immunocompetent models, CX-5461 with panobinostat provides a significant survival advantage. Prolonged combination dosing in the slower tVk*MYC model gave no increase in haematological toxicity.
We are currently interrogating the molecular synergistic response of this combination, including p53-dependent and –independent responses, and the ATM/ATR signalling pathway. Ongoing work is investigating the efficacy of the triplet drug combination of CX-5461, panobinostat and carfilzomib.
CONCLUSION: CX-5461 shows increased efficacy when combined with multiple drug classes, with panobinostat and carfilzomib showing the most promise in vitro. Combination of CX-5461 with panobinostat increases survival in 2 models of MM. These results will inform subsequent clinical trials utilising combination drug therapy.