Abstract 2118: Targeting telomerase in HER2 positive breast cancer: role of cancer stem cells.

Background: The reverse transcriptase enzyme telomerase is reactivated in 85-90% of malignancies and is a potential target for anti-cancer therapies due to its differential expression between normal and tumor cells. One such therapy involves targeting the template region of telomerase with an antagonistic oligonucleotide, imetelstat, which is currently in clinical trials for breast and other cancers. Imetelstat has been shown to be effective in inhibiting telomerase activity in breast cancer cell lines, as well as inhibiting breast tumor growth and decreasing lung metastases in an in vivo model. HER2 amplification (and/or overexpression) is associated with a more aggressive disease, a greater likelihood of recurrence, and decreased survival compared to breast cancers without HER2 amplification. Recent work has shown that HER2 overexpression increases both the normal and malignant stem/progenitor cell population. This increase in malignant stem cells, known as cancer stem cells (CSCs), drives tumorigenesis and invasion and may explain the aggressive phenotype and drug resistance of this disease. We hypothesize that CSCs have active telomerase and thus may be sensitive to imetelstat treatment, thereby enhancing the effects of therapy for HER2 positive cancers. Methods: HER2+/- breast cancer cells, grown as mammospheres in culture to enrich for CSCs, were analyzed by flow cytometry for CSC marker expression (CD49f+/EpCAM-, CD44+/CD24-, and ALDH1hi). Mammospheres were analyzed for spheroid forming efficiency, a functional measure of self-renewal potential, and telomerase activity in response to imetelstat treatment. Results: Imetelstat treatment resulted in inhibition of up to 90% of telomerase activity in mammospheres, similar to adherent cell cultures. The percentage of CSCs based upon analysis of marker expression decreased approximately 30% or 1.4-fold after 5 weeks of imetelstat treatment compared to untreated controls. Imetelstat pretreatment decreased spheroid counts by 84%. The spheroid formation efficiency decreased from 9.36% in the untreated to 1.5% after imetelstat pretreatment, a 6.24-fold decrease. Pretreatment with imetelstat also had a durable inhibitory effect on mammosphere telomerase activity, where over 60% of the enzyme9s activity remained inhibited after one week without additional treatment. Conclusions: CSCs have active telomerase that can be inhibited using imetelstat. The percentage of CSCs as well as spheroid formation efficiency decreases following imetelstat treatment. The addition of imetelstat to therapy regimens may help decrease metastases and reduce tumor recurrence. Citation Format: Jillian Koziel, Sergei Gryaznov, Brittney-Shea Herbert. Targeting telomerase in HER2 positive breast cancer: role of cancer stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2118. doi:10.1158/1538-7445.AM2013-2118