GRN163L, a Novel and Potent Telomerase Inhibitor, Inhibits Myeloma Cell Growth In Vitro and In Vivo.

Telomerase activity is either low or completely absent in most normal somatic cells; while it is elevated in most cancer cells providing unlimited proliferative potential by preventing telomere shortening. The inhibitors of telomerase, therefore, induce telomere shortening leading to apoptotic cell death in tumor cells while having little or no effect on normal diploid cells. We have evaluated the in vitro and in vivo efficacy of thio-phosphoramidate oligonucleotide specifically targeting the RNA component of telomerase (GRN163L) with demonstrated nuclear uptake by >99% cells without the transfection enhancer. Delivery of GRN163L (1 μM) to MM cells (INA6 and ARP) was specifically associated with complete loss of telomerase activity as early as 6 hrs following exposure and was accompanied by a reduction in myeloma cell growth and survival. Treatment of INA6 cells with GRN163L for three weeks induced 96±4% and 100% cell death at 0.5 and 1 μM concentrations, respectively. ARP cells, which express higher levels of telomerase activity and have longer telomeres, showed 67±4% cell death at 5 weeks with 0.5 μM inhibitor and 82±3% and 100% cell death at 4 and 5 weeks, respectively, with 2 μM GRN163L. The apoptotic cell death was confirmed in 51% INA6 cells at two weeks and in >80% ARP cells at four weeks. Apoptosis was associated with reduction in mean Telomere Fluorescence Intensity (TFI) on interphase chromosomes from 87.1±6.2 in control oligo treated INA6 cells to 36.2±2 (2.4 fold) in GRN163L treated cells. Moreover, GRN163L treatment was also associated with a similar reduction in number of chromosomes with detectable telomeres, indicating development of telomere-free ends. We have confirmed in vivo efficacy of GRN163L in a SCID-hu murine model of multiple myeloma. Following growth of GFP-transduced myeloma cells in the fetal bone chip introduced into the mice, GRN163L was injected on alternate days. In two independent experiments significant reduction in tumor cell growth, as measured by reduction in human myeloma related protein, and better survival than mice injected with control oligo was observed. We have now evaluated efficacy of combination of GRN163L with other novel agents. We have observed synergistic activity with Hsp90 inhibitor 17AAG on myeloma cell death. Addition of 17AAG (0.05 μM) to myeloma cells pre-treated with GRN163L (1 μM) for one week led to complete growth arrest within four days compared to continued growth of cells not pre-treated with GRN-163. These data provide the preclinical rationale for clinical evaluation of GRN163L in myeloma and in combination with Hsp90 inhibitor.