Chelation of intracellular iron with the antifungal agent ciclopirox olamine induces cell death in leukemia and myeloma cells.

Off-patent drugs with previously unrecognized anticancer activity could be rapidly repurposed for this new indication. To identify such compounds, we conducted 2 independent cell-based chemical screens and identified the antimicrobial ciclopirox olamine (CPX) in both screens. CPX decreased cell growth and viability of malignant leukemia, myeloma, and solid tumor cell lines as well as primary AML patient samples at low-micromolar concentrations that appear pharmacologically achievable. Furthermore, oral CPX decreased tumor weight and volume in 3 mouse models of leukemia by up to 65% compared with control without evidence of weight loss or gross organ toxicity. In addition, oral CPX prevented the engraftment of primary AML cells in nonobese diabetic/severe combined immunodeficiency mouse models, thereby establishing its ability to target leukemia stem cells. Mechanistically, CPX bound intracellular iron, and this intracellular iron chelation was functionally important for its cytotoxicity. By electron paramagnetic resonance, CPX inhibited the iron-dependent enzyme ribonucleotide reductase at concentrations associated with cell death. Thus, in summary, CPX has previously unrecognized anticancer activity at concentrations that are pharmacologically achievable. Therefore, CPX could be rapidly repurposed for the treatment of malignancies, including leukemia and myeloma.

Inhibition of the sodium potassium adenosine triphosphatase pump sensitizes cancer cells to anoikis and prevents distant tumor formation.

Normal epithelial cells undergo apoptosis upon detachment from the extracellular matrix, a process termed "anoikis." However, malignant epithelial cells with metastatic potential resist anoikis and can survive in an anchorage-independent fashion. Molecules that sensitize resistant cells to anoikis will be useful chemical probes to understand this pathway. To identify novel anoikis sensitizers in anoikis-resistant PPC-1 prostate adenocarcinoma cells, a library of 2,000 off-patent drugs and natural products was screened for their ability to preferentially induce cell death in suspension over adherent culture conditions. This screen identified five members of the family of cardiac glycosides as anoikis sensitizers, including ouabain, peruvoside, digoxin, digitoxin, and strophanthidin. We conducted further studies with ouabain to discern the mechanism of cardiac glycoside-induced anoikis sensitization. Ouabain initiated anoikis through the mitochondrial pathway of caspase activation. In addition, ouabain sensitized cells to anoikis by inhibiting its known target, the Na(+)/K(+) ATPase pump, and inducing hypoosmotic stress. Resistance to anoikis permits cancer cells to survive in the circulation and facilitates their metastasis to distant organs, so we tested the effects of Na(+)/K(+) ATPase inhibition on distant tumor formation in mouse models. In these mouse models, ouabain inhibited tumor metastases but did not alter the growth of subcutaneous tumors. Thus, we have identified a novel mechanism to sensitize resistant cells to anoikis and decrease tumor metastasis. These results suggest a potential mechanism for the observed clinical reduction in metastasis and relapse in breast cancer patients who have undergone treatments with cardiac glycosides.