we have found that the great majority of pancreatic cancer cell lines have very short telomeres, in the range of 2 to 3 kb only. These very short telomeres were expected to make these cells particularly susceptible to the effects of chronic telomerase inhibition. We also have surveyed the panel for levels of telomerase activity, and an unexpected finding was the large differences in activities between the cell lines. The cell lines belonged to one of two groups, exhibiting either a higher or 10-fold lower level of activity, with the highest and lowest expressers exhibiting a 46-fold difference in activity. The significance of these large differences between the lines is unclear, as telomerase activity did not NS-187 correlate with either telomere size or DNA Ligase Inhibitor response to GRN163L. The lack of correlation with telomere size was no surprise since telomere length is regulated not just by the activity of telomerase but even more significantly by the Shelterin complex. All ten lines of the panel responded well to the telomerase inhibitory activity GRN163L. With IC50 in the nanomolar range, the response of pancreatic cancer cell lines to the drug was comparable if not better than previously reported for other cancer cell lines. Using CAPAN1 and CD18 as models, we examined the effects of continuous telomerase inhibition on pancreatic cancer cells. As expected for a telomerase inhibitor, GRN163L affected proliferation and survival only after a delay during which sufficient telomere shortening needed to occur. This telomere shortening was elicited by GRN163L but not by the mismatch oligo, which possesses the same lipidconjugated thio-phosphoramidate chemistry as GRN163L but is unable to hybridize to hTR. At the end of the lifespan of the GRN163L-treated CD18 cells, telomeres had become critically short to the point of undetectability. Only a few telomeres carried sufficient telomeric repeats to allow detection by the anti-TRF2 antibody. Southern blot analysis suggested that these telomeres still carried 1.8 kb of repeats. However, this method overestimates the length of telomeres. Recalculating median telomere size after correcting for signal intensity, as we have done before, returned a value of just 1.1 kb, with some of that still representing sub-telomeric DNA. Mos
