BACKGROUND: rDNA transcription is consistently dysregulated in cancer, mediated through oncogenic and tumour-suppressive pathways, and in particular by cMYC1. RNA polymerase I (Pol I) transcriptional hyperactivity is a consistent feature of many cancers, and has been shown to provide a survival checkpoint in haematological tumour cells2.
Our laboratory developed CX-5461, a highly selective small molecule inhibitor of Pol I transcription of ribosomal genes3, currently in a phase 1 clinical trial in relapsed/refractory haematological malignancies (Peter MacCallum Cancer Centre, Australia). 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 cells2,4. We demonstrated that single-agent treatment with CX-5461 provides a significant survival benefit in murine models of B-cell lymphoma, acute myeloid leukaemia and now multiple myeloma (MM)2,4-6. However, despite this improvement, drug resistance eventually occurs, indicating combination drug therapy is essential.
AIM: to examine the efficacy of CX-5461 in combination with other agents having proven clinical or promising preclinical efficacy in MM.
METHODS: we conducted a boutique, high-throughput screen in human myeloma cell lines of CX-5461 combined with a targeted range of agents. We measured the effect of CX-5461 on proliferation, cell death, cell cycle distribution, biomarkers of DDR and on-target effects. The effect of combining CX-5461 with one candidate, the histone deacetylase inhibitor (HDACi) panobinostat, was tested in vivo using both the tVk*MYC murine model of MM6 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. Using both the MYC-driven tVk*MYC model and the non-MYC-driven 5T33-KaLwRij models of MM, we showed that CX-5461 with panobinostat provides a significant survival advantage. Prolonged combination dosing in the slower tVk*MYC model did not cause haematological toxicity beyond that seen with single agents.
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, both in vitro and in vivo.
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 produces a survival advantage in both the tVk*MYC and the 5T33-KaLwRij models of MM. These results will direct the subsequent clinical trial using combination drug therapy.
Bywater et al., Nature Reviews Cancer 2013
Bywater et al., Cancer Cell 2012
Drygin et al., Cancer Research 2011
Quin et al, Oncotarget, 2016
Devlin et al., Cancer Discovery 2016
Hein et al., Blood 2017
Chesi et al., Blood 2012
Figure 1. Combination treatment with CX-5461 + panobinostat gives increased survival in the tVk*MYC model of MM. Mice were transplanted with 3.5x10^5 cells, then treatment was started 49 days post-transplant. Mice were treated with CX-5461 35mg/kg orally weekly and panobinostat 7.5mg/kg IP MWF.
Harrison: Celgene: Consultancy, Research Funding, Speakers Bureau.
Asterisk with author names denotes non-ASH members.