Targeting ribosome biogenesis and metabolism in acute myeloid leukaemia

Abstract

Targeting ribosome biogenesis, a cellular process frequently upregulated in cancer, with the novel Pol I transcription inhibitor CX-5461 is highly efficacious in pre-clinical models of solid and haematological cancers, which lead to the commencement of clinical trials. However, as is common with single agent therapies in the mouse models (similar to challenges with treating human patients), the mice eventually succumb to disease, highlighting the need for a combination therapy approach. Based on the strong link between altered ribosome biogenesis and metabolism in cancer it was hypothesised that targeting these two processes in combination would prove efficacious in cancer, and acute myeloid leukaemia (AML) was chosen to test this as it is an aggressive malignancy with poor therapeutic options. In order to address this hypothesis In vitro drug synergy testing in AML cell lines was performed to identify promising combinations (Chapter 3), these were then tested for efficacy in in vivo transplant models of AML (Chapter 4). Finally, in vitro mechanistic analysis of the most promising drug combination was performed in order to understand the mechanisms of synergy (Chapter 5).

In vitro testing of CX-5461 in combination with 10 clinically-approved metabolism-modifying drugs confirmed that orlistat, dichloroacetate (DCA), ritonavir, omeprazole and chloroquine act synergistically with CX-5461 to reduce cell viability in multiple AML cell lines. Three such combination therapies were evaluated in a syngeneic mouse AML model. Neither orlistat nor DCA improved survival in combination with CX-5461 compared to CX-5461 alone, however synergy was observed with the autophagy inhibitor chloroquine. Interestingly, the combination of CX-5461 and chloroquine had limited efficacy in human cell line xenograft mouse models, despite strong in vitro results. As the dosing of CX-5461 and chloroquine could not be increased due to toxicity, mechanistic analysis was performed in order to identify an alternative to chloroquine with reduced toxicity, and potentially improved efficacy.

CX-5461 and chloroquine were found to synergise through cell cycle arrest and cell death in all four cell lines tested. Metabolic flux analysis revealed that the combination of drugs significantly affected mitochondrial activity, indicating that the combination of CX-5461 and chloroquine is placing the cells in mitochondrial stress. Therefore, direct targeting of the mitochondria was identified as a promising approach in combination with ribosome biogenesis inhibition with CX-5461, and various clinically-approved drugs that target mitochondria were identified for future combination testing.