Purpose: Maintenance of telomeres by telomerase is critical for the continuing proliferation of most advanced cancer cells. Telomerase activity has been detected in the vast majority of cancer cells but not most normal cells, making the enzyme an attractive target for anticancer therapy. The aim of this study was to address the breast cancer translational potential of the novel telomerase inhibitor, GRN163L. Experimental Design: In the present study, we investigated the effects of GRN163L treatment on a panel of breast cancer cells representing different tumor subtypes with varying genetic backgrounds, including ER+, ERÀ, HER2+, BRCA1mutant breast tumor cells as well as doxorubicinresistant cancer cells.To investigate the in vivo effects of GRN163L, we employed a breast cancer xenograft and metastasis model that simulates a clinical situation in which a patient arrives with a primary tumor that may be then treated or surgically removed. Results: GRN163L effectively inhibited telomerase activity in a dose-dependent fashion in all breast cancer cell lines resulting in progressive telomere shortening. A mismatch control oligonucleotide showed no effect on telomerase activity and GRN163L did not significantly affect telomere shortening in normal human mammary epithelial cells or in endothelial cells. Breast cancer cells that exhibited telomerase inhibition also exhibited significant reduction in colony formation and tumorigenicity. Furthermore, GRN163L suppressed tumor growth and lung metastases (P = 0.017) of MDA-MB-231cells in vivo after 4 weeks of treatment. Conclusions:These results show in vivo effectiveness of GRN163L in breast cancer and support its promising clinical potential for breast cancer treatment.

RAD51 recombinase activity plays a critical role for cancer cell proliferation and survival, and often contributes to drug-resistance. Abnormally elevated RAD51 function and hyperactive homologous recombination (HR) rates have been found in a panel of cancers, including breast cancer and chronic myeloid leukaemia (CML). Directly targeting RAD51 and attenuating the deregulated RAD51 activity has therefore been proposed as an alternative and supplementary strategy for cancer treatment. Here we show that a newly identified small molecule, IBR2, disrupts RAD51 multimerization, accelerates proteasome-mediated RAD51 protein degradation, reduces ionizing radiation-induced RAD51 foci formation, impairs HR, inhibits cancer cell growth and induces apoptosis. In a murine imatinib-resistant CML model bearing the T315I Bcr-abl mutation, IBR2, but not imatinib, significantly prolonged animal survival. Moreover, IBR2 effectively inhibits the proliferation of CD34+ progenitor cells from CML patients resistant to known BCR-ABL inhibitors. Therefore, small molecule inhibitors of RAD51 may suggest a novel class of broad-spectrum therapeutics for difficult-to-treat cancers.

Telomeres are repetitive (TTAGGG) n DNA sequences found at the end of chromosomes that protect the ends from recombination, end to end fusions, and recognition as damaged DNA. Telomerase activity can be detected in 85% to 90% of human tumors, which stabilizes telomeres to prevent apoptosis or cellular senescence. Previous reports showed the efficacy of the novel telomerase template antagonist, GRN163L, as a potential anticancer agent. The objective of the present study was to elucidate the molecular effects of GRN163L in MDA-MB-231 breast cancer cells and to determine whether GRN163L could be used in mechanism-based combination therapy for breast cancer. We observed that GRN163L reduced MDA-MB-231 growth rates without a significant effect on breast cancer cell viability within the first 14 days in vitro. In addition, GRN163L altered cell morphology, actin filament organization, and focal adhesion formation in MDA-MB-231 cells. Importantly, the cellular response to GRN163L significantly augmented the effects of the microtubule stabilizer paclitaxel in MDA-MB-231 breast cancer cell growth in vitro and in vivo compared with paclitaxel alone or a mismatch control oligonucleotide plus paclitaxel. Furthermore, in vitro MDA-MB-231 invasive potential was significantly inhibited with GRN163L and paclitaxel.These data support a rationale for potentially combining GRN163L with paclitaxel for the treatment of breast cancer in the clinical setting.