Telomerase is required for the unlimited lifespan of cancer cells. The vast 
majority of pancreatic adenocarcinomas overexpress telomerase activity and 
blocking telomerase could limit their lifespan. GRN163L (Imetelstat) is a 
lipid-conjugated N3'→P5' thio-phosphoramidate oligonucleotide that blocks the 
template region of telomerase. The aim of this study was to define the effects 
of long-term GRN163L exposure on the maintenance of telomeres and lifespan of 
pancreatic cancer cells. Telomere size, telomerase activity, and telomerase 
inhibition response to GRN163L were measured in a panel of 10 pancreatic cancer 
cell lines. The cell lines exhibited large differences in levels of telomerase 
activity (46-fold variation), but most lines had very short telomeres (2-3 kb in 
size). GRN163L inhibited telomerase in all 10 pancreatic cancer cell lines, with 
IC50 ranging from 50 nM to 200 nM. Continuous GRN163L exposure of CAPAN1 
(IC50 = 75 nM) and CD18 cells (IC50 = 204 nM) resulted in an initial rapid 
shortening of the telomeres followed by the maintenance of extremely short but 
stable telomeres. Continuous exposure to the drug eventually led to crisis and 
to a complete loss of viability after 47 (CAPAN1) and 69 (CD18) doublings. 
Crisis In these cells was accompanied by activation of a DNA damage response 
(γ-H2AX) and evidence of both senescence (SA-β-galactosidase activity) and 
apoptosis (sub-G1 DNA content, PARP cleavage). Removal of the drug after 
long-term GRN163L exposure led to a reactivation of telomerase and re-elongation 
of telomeres in the third week of cultivation without GRN163L. These findings 
show that the lifespan of pancreatic cancer cells can be limited by continuous 
telomerase inhibition. These results should facilitate the design of future 
clinical trials of GRN163L in patients with pancreatic cancer.