Abstract Background: Tumor growth is sustained by cancer stem cells (CSC), a rare fraction of cells expressing high levels of drug efflux pumps responsible for therapy failure and tumor recurrence. The enzyme telomerase is critical to maintaining the self-renewal properties of CSC. We have recently found that sodium metaarsenite (KML001), a drug in phase I/II clinical trials in Europe and the US, can target both the catalytic subunit of telomerase and telomeres. PC cell lines appear to be among the most sensitive tumor cell types to KML001. In this study, we examined chemotherapy-naïve and taxane (paclitaxcel/Pac and docetaxel/Doc)-resistant DU145 PC cell lines for stem cell content, and the potential for KML001 to inhibit their mature and stem cell subpopulations. Methods: Methyltetrazolium (MTT)-based assays were used to determine the IC50 and IC100 values, and the relative resistance of DU145, DU145/Doc50 and DU145/Pac200 cells to KML001, docetaxel and paclitaxel. Fluorescence activated cell sorting (FACS) was employed to determine the prostate stem cell surface marker CD44, the side population and Pgp content in these cells. Telomerase activity was evaluated by a PCR ELISA assay and telomere length was assessed by Southern blots. Cells were treated with IC50 and IC100 concentrations of KML001 for 72 hrs to investigate its effects on the side population. Results: We found that while DU145/Doc50 and DU145/Pac200 cells were 2,000- and 400-fold resistant to docetaxel and paclitaxel, respectively, they retained sensitivity to KML001 (IC50 = 4µM). Resistance to taxanes was associated with expression of Pgp; parental DU145 cells expressed Pgp in 0.24% of the cell population, DU145/Doc50 in 47% and DU145/Pac200 in 79% of the cells. Moreover, Pgp expression correlated with the detection of a side population (SP), a cell fraction known to be enriched for CSC. The SP was 3.7% in DU145 cells and 56%–67% in the taxane-resistant cells. In contrast, telomerase activity and telomere lengths remained unchanged in all three cell lines. Exposure of the various DU145 cell lines to KML001 for 72 hrs resulted in a dramatic reduction in the SP. Similar to the Pgp inhibitor verapamil, KML001 reduced SP cells to ∼93% of controls in both parental and taxane-resistant cell lines. However, unlike verapamil, KML001 was able to reduce the mature cell population to the same extent as the SP. Conclusion: Overall, our data in the DU145 model demonstrate that KML001 can eradicate both mature drug-resistant and prostate cancer stem cells, which may offer a new treatment approach in this disease. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A57.