Cancer stem cells (CSCs) are thought to be responsible for tumor progression, 
metastasis, and recurrence. HER2 overexpression is associated with increased 
CSCs, which may explain the aggressive phenotype and increased likelihood of 
recurrence for HER2(+) breast cancers. Telomerase is reactivated in tumor cells, 
including CSCs, but has limited activity in normal tissues, providing potential 
for telomerase inhibition in anti-cancer therapy. The purpose of this study was 
to investigate the effects of a telomerase antagonistic oligonucleotide, 
imetelstat (GRN163L), on CSC and non-CSC populations of HER2(+) breast cancer 
cell lines. The effects of imetelstat on CSC populations of HER2(+) breast 
cancer cells were measured by ALDH activity and CD44/24 expression by flow 
cytometry as well as mammosphere assays for functionality. Combination studies 
in vitro and in vivo were utilized to test for synergism between imetelstat and 
trastuzumab. Imetelstat inhibited telomerase activity in both subpopulations. 
Moreover, imetelstat alone and in combination with trastuzumab reduced the CSC 
fraction and inhibited CSC functional ability, as shown by decreased mammosphere 
counts and invasive potential. Tumor growth rate was slower in 
combination-treated mice compared to either drug alone. Additionally, there was 
a trend toward decreased CSC marker expression in imetelstat-treated xenograft 
cells compared to vehicle control. Furthermore, the observed decrease in CSC 
marker expression occurred prior to and after telomere shortening, suggesting 
that imetelstat acts on the CSC subpopulation in telomere length-dependent and 
-independent mechanisms. Our study suggests addition of imetelstat to 
trastuzumab may enhance the effects of HER2 inhibition therapy, especially in 
the CSC population.