AcTor, a novel mTOR stimulator, potentiates ixazomib for the treatment of acute myeloid leukemia

Background: mTORC1 activity is oncogenic. However, in the presence of chemotherapy, suppression of mTORC1 is cytoprotective. mTOR suppression requires an intact tuberous sclerosis complex (TSC), composed of TSC1, TSC2 and TBC1D7. Small molecules that activate mTOR by blocking the TSC are lacking. Methods: We applied in silico docking and medicinal chemistry to generate AcTor, a potential first-of-its-kind TSC2 inhibitor. Because inhibition of TSC2 results in increased sensitivity to proteasome inhibitors, we combined AcTor and the proteasome inhibitor ixazomib (IXZ) in various cancer cell types. Results: Potentiation of cytotoxic activity of IXZ by AcTor was observed across multiple acute myeloid leukemia (AML) cell lines and primary patient samples. The combination triggered a collapse of mitochondrial respiratory capacity, loss of mitochondrial membrane potential, accumulation of ROS and apoptosis. These attributes increased in drug-resistant AML. Transcriptomic profiling revealed that AcTor alone induced anabolic and oxidative phosphorylation programs, whereas AcTor/IXZ redirected the signaling towards stress-associated and pro-apoptotic transcriptional states, including a p53 pathway signature. In vivo studies revealed reduction in AML burden, depletion of blasts and of leukemic stem cells, and retention of activity upon relapse. AcTor/IXZ was equally potent in a TP53-mutated patient-derived xenograft model, exceeding the efficacy of standard-of-care. Conclusions: As a TSC2 inhibitor, AcTor should not be used alone in cancer. When combined with proteasome inhibitors, the pharmacodynamics of AcTor shifts towards the development of a mitochondrial catastrophe in AML, which is durable, broad range, agnostic to TP53 mutations and to the acquisition of resistance to common clinical anti-AML drugs.