The effects of telomerase inhibition with an oligonucleotide N3 3 P5 thiophosphoramidate (GRN163) complementary to the telomerase template region were examined on human multiple myeloma (MM) and non-Hodgkin lymphoma (NHL) cell lines, primary MM cells, and tumor xenografts. GRN163 treatment reduced telomerase levels in all cells and induced more rapid telomeric shortening. Continuous GRN163 treatment for 7 to 14 days resulted in proliferative arrest, morphologic changes, and apoptosis characteristic of cell crisis in tumor cell lines with short (1.7-5.4 kb) but not long (9-11 kb) telomeres. Intratumoral administration of GRN163 also inhibited the growth of MM and NHL xenografts established from cell lines with short telomeres (Hs602 lymphoma, 2.7 kb; CAG myeloma, 2.7 kb) and increased tumor apoptosis. However, GRN163 therapy of NHL xenografts established from cells with long telomeres (11.0 kb) had equivocal effects on tumor growth and did not induce apoptosis during this time frame. Systemic daily intraperitoneal administration of GRN163 in myeloma xenografts with short telomere lengths also decreased tumor telomerase levels and reduced tumor volumes. These data demonstrate that telomerase is important for the replication of mature B-cell neoplasia by stabilizing short telomeres, and they suggest that telomerase inhibition represents a novel therapeutic approach to MM and NHL.
IntroductionThe preservation of telomeres, DNA-protein structures located at the ends of eukaryotic chromosomes, is essential to the immortalization process. In most normal somatic cells, telomeric DNA is progressively lost with each cell division until a critically short telomere length is obtained, and telomeres lose the ability to protect linear chromosomal ends. The resultant telomeric dysfunction induces cell "crisis" with chromosomal instability, end-to-end chromosome fusions, activation of DNA checkpoint responses, apoptosis, and cell death. 1 Human telomerase is a complex ribonucleoprotein reverse transcriptase composed of an RNA template (hTR) and a catalytic protein subunit (hTERT), which is responsible for synthesizing d-(TTAGGG) n telomeric repeats at chromosomal ends. Stabilizing telomeric DNA through telomerase up-regulation or activating alternative mechanisms of telomere maintenance is essential if cells are to survive and proliferate indefinitely. 2 Telomerase activity (TA) is not found in most normal somatic cells, which lack hTERT, but it has been detected in high levels in 85% to 90% of human cancers. 1 Although telomerase is not an oncogene, 3 transfecting hTERT into normal epithelial or endothelial cells transformed with the simian virus 40 large T-antigen and the N-ras oncogene allows cells to bypass crisis and ultimately to achieve malignancy, confirming the role of telomerase in cellular immortalization and tumorigenesis. 1 Telomerase inhibition by numerous strategies, including dominant-negative telomerase mutants, 4,5 small-molecular-weight compounds, 6 reversetranscriptase inhibitors, 7 G-quadruplex interactive...