A screen of off-patent drugs identifies the antiparasitic agent, ivermectin, that reduces viability of leukemia cells. (A) OCI-AML2 cells were treated with aliquots of a small chemical library (n = 100) focused on antimicrobials and metabolic regulators. Seventy-two hours after incubation, cell growth and viability were measured by the MTS assay. Data represent the percentage of viable OCI-AML2 cells treated with the compounds (6μM) sorted in order of increasing activity. (B) Leukemia cell lines were treated with increasing concentrations of ivermectin. Seventy-two hours after incubation, cell growth and viability were measured by the MTS assay. Data represent the mean EC₅₀ and 95% CI from 3 independent experiments. (C) Primary normal hematopoietic cells (PBSC; n = 3), primary AML patient samples (AML; n = 3), and U937 leukemia cells were treated with increasing concentrations of ivermectin for 48 hours. After incubation, cell viability was measured by annexin V and PI staining. Data represent the mean ± SD percent viable cells from experiments performed in triplicate. (D) Primary AML cell samples (AML; n = 6) and normal hematopoietic peripheral blood stem cell samples (PBSC; n = 3) were treated with ivermectin (6μM) for 24 hours and then plated in a methylcellulose colony forming assay. Seven (AML) or 14 days (PBSCs) days after plating, the number of colonies was counted. Data represent the mean ± SD percent colony formation compared with control treated cells.