Abstract Acute myeloid leukemia (AML) is characterized by a differentiation block in myeloid progenitors resulting in aberrant proliferation of immature myeloid cells. Therapeutics that induce differentiation of leukemic cells would overcome the high toxicity and low potency of conventional chemotherapy. Securinine, a plant-derived alkaloid from Securinega suffruticosa, has been shown to be a promising AML differentiation agent through an unclear mechanism, but is limited by its low potency (~20μM) and toxicity. We performed lead optimization chemistry on securinine that led to derivatives 250 and 317, with nanomolar efficacy and reduced toxicity. These derivatives effectively inhibit growth of AML cell lines and patient samples. Treatment of AML cell lines with compounds 250 and 317 induces the expression of differentiation markers CD11b and CD14c. Compounds 250 and 317 significantly reduced leukemic burden in a circulating model of AML in NSG mice. Additionally, these compounds were effective in inhibiting cell growth of HL60 cells that are resistant to a conventional chemotherapeutic, doxorubicin. By using the drug-affinity responsive target sensitivity (DARTS) assay, we identified that the redox mediator thioredoxin reductase (TrxR) is the primary target of these derivatives. 250 and 317 inhibit TrxR activity in vitro and in cells, and the resultant alterations in redox status are overcome by addition of a scavenger of reactive oxygen species (ROS), N-acetylcysteine. The ensuing oxidative metabolic stress rapidly alters the metabolite profile in AML cells including depletion of malate. Securinine derivatives decreased basal oxygen consumption rate and spare respiratory capacity, resulting in differentiation and cell death. Downregulating TrxR1 expression increased the sensitivity of OCI-AML3 cells to compounds 250 and 317. Together, we show here that these new derivatives of securinine exhibit preclinical efficacy in AML by decreasing mitochondrial spare respiratory capacity through the inhibition of TrxR. Citation Format: Sheela Karunanithi, Ruifu Liu, Anne Roe, Stephen Moreton, Yongchun Hou, Natasha Oldford, Juan Valentin Goyco, David Wald. Inhibition of mitochondrial spare respiratory capacity by new securinine derivatives as a novel differentiation therapy for acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 382.