Abstract A51: Identification of compounds targeting human leukemia stem cells
Authors: McDermott Sean P. , Eppert Kolja , Schimmer Aaron D. , Eberhard Yanina , Dick John E. 
Published: June 2012

Over the past 15 years, research has conclusively shown the existence of a rare population of cancer cells, termed cancer stem cells, in leukemia, brain, colon, and breast cancers. These cells are biologically distinct from bulk cells and are the only cells able to initiate and sustain the disease. Recent experiments indicate that standard chemotherapy for leukemia is less effective against leukemia stem cells (LSC) than bulk leukemia cells and typically does not spare normal hematopoietic stem cells (HSC) and progenitor cells leading to myelosuppression. Therefore, a new paradigm is needed to develop cancer therapeutics that effectively eradicate the disease by targeting the LSC. Here, we sought to identify compounds that selectively target LSC but not normal HSC by using populations enriched for LSC and HSC, instead of traditional cancer cell lines, in high-throughput screening of three libraries of small chemicals comprising over 4000 known bioactive, off-patent, and natural compounds. Recently, we generated human leukemia cells by direct transformation of normal human primitive hematopoietic cells (Lin- CB) with leukemogenic fusion oncogenes. These cells exhibit features of LSC, such as hierarchical organization, engraftment of NOD/SCID mice, and a differentiation block. As a proof of principle, we screened two leukemia cell lines using a simple cell-growth inhibition assay. 76 compounds were scored as positive against both lines. Numerous anti-cancer therapeutics (paclitaxel, etoposide, and vincristine), general cytotoxic agents (brefeldin), and the digitalis family of ion pump inhibitors (digoxin, ouabain) were identified. Since LSC share some pathways with normal HSC, we counter-screened potential hits on normal HSC and progenitor cells and identified only 10 compounds. Three of the 10 compounds targeted LSC, ciclopirox olamine, etoposide, and kinetin riboside (KR). KR was effective on primary AML and CML at levels similar to the AML chemotherapeutics cytarabine and mitoxantrone. Kinetin riboside induced apoptosis in phenotypic CD34+CD38- LSC, but not CD34+CD38- HSC similar to the anti-LSC compound parthenolide. In contrast to parthenolide, treatment of primary AML cells with kinetin riboside inhibited engraftment in NOD/SCID mice for 2 of 4 samples. The second compound, ciclopirox olamine, targeted LSC and not HSC, by chelating intracellular iron and inhibited the ribonucleotide reductase enzyme. Ciclopirox olamine is a clinically used antifungal and could be rapidly repurposed for treating leukemia. The third compound, etoposide, was effective in vitro on 29% (15 of 51) of primary AML and 67% (8 of 12) of CML patient cells. Etoposide inhibited NOD/SCID engraftment of three responsive AML samples, but not three non-responsive samples, indicating etoposide targeted the LSC in a subset of patients. Together, these screens have identified multiple anti-LSC compounds and represent a new paradigm for drug screening against LSC.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A51.