TARGETING MULTIDRUG RESISTANCE PROTEIN 1 (MDR1) POTENTIATES SMAC-MIMETIC THERAPY TO KILL LEUKEMIC STEM CELLS AND OVERCOME RESISTANCE IN ACUTE MYELOID LEUKEMIA

Acute myeloid leukemia (AML) is an aggressive disease with a current 5-year survival rate of only ∼25%, therefore development of effective treatments towards resistant AML is urgently needed. Potential mechanisms of therapy resistance include overexpression of members of the inhibitor of apoptosis protein (IAP) family. Natural IAP antagonists such as second-mitochondria-derived activator of caspases (Smac) exist, leading to the pharmaceutical development of Smac-mimetics (SMs). The clinical SM birinapant has been trialed in AML and solid cancers, as well as in hepatitis B virus (HBV), with variable and limited success. Using a high-throughput strategy, we screened 5,700 bioactive compounds and identified clinical drugs that overcome birinapant resistance through inhibition of multidrug resistance protein 1 (MDR1). We show that 3rd generation MDR1 inhibitors synergise with birinapant and other SMs to kill AMLs. We use mass spectrometry to show inhibition of MDR1 in SM resistant leukemias increased intracellular levels of birinapant, indicating birinapant is a novel substrate of MDR1. Genetic deletion of MDR1 confirmed combination synergy is mediated specifically by MDR1. Furthermore, we observed a strong correlation between MDR1 expression/activity and sensitivity to birinapant-based therapies in AML cells. Strikingly, combination treatment sensitised leukemic stem cells (LSCs) to birinapant killing and was well-tolerated and effective in treating leukemia in vivo. Moreover, MDR1 inhibitors potentiated birinapant-mediated killing in patient leukemia and ovarian cancer cells, and in HBV infected liver cells in vivo. This study identifies MDR1 as a biomarker of SM therapy and gives clear rationale for MDR1 screening, leading to informed SM clinical trials and a personalised therapy for AML patients.