The novel telomerase inhibitor GRN163L targets drug-resistant cancer stem cells in multiple myeloma

A97 The majority of chronic hematologic malignancies are responsive to initial treatment using standard agents, but most patients relapse and experience progressive disease. The factors responsible for disease relapse are poorly defined, but increasing evidence in a wide range of human cancers suggests that all tumor cells are not functionally equivalent. In these models, tumor cells are organized in a hierarchical fashion reminiscent of normal tissue systems in which rare populations of less mature, self-renewing cancer stem cells (CSC) give rise to differentiated progeny that constitute the bulk of the tumor and lack long-term replicative potential. The typical clinical pattern of relapse following treatment suggests that mature tumor cells are inhibited, but that CSC are spared and capable of tumor regrowth. We previously demonstrated that CSC in multiple myeloma (MM) do not express the characteristic plasma cell surface antigen CD138, but rather lack this antigen and phenotypically resemble B cells. We treated CD138+ plasma cells or CD138neg cells isolated by FACS from the human MM cell lines RPMI 8226 and NCI-H929 with the clinically utilized anti-MM agents dexamethasone (0.1μM), bortezomib (10nM), and lenalidomide (1μM) then evaluated their clonogenic growth. Similar to their clinical effects, each agent inhibited CD138+ plasma cells. In contrast, none of the agents significantly limited the clonogenic recovery of CD138neg MM precursors. We hypothesized that cellular processes involved in the self-renewal of normal stem cells are aberrantly regulated in CSC and could serve as effective targets. Telomerase activity (TA) is required by normal stem cells to slow the progressive shortening of telomeres during cellular replication that would ultimately result in cellular senescence and/or apoptosis. We examined TA in CD138+ and CD138neg cells from the RPMI 8225, NCI-H929, and U266 cell lines and found that both cellular compartments contained elevated TA compared to normal B cells using a PCR-based assay. GRN163L is a novel lipid-conjugated 13 nucleotide thio-phosphoramidate oligonucleotide that acts as a potent and specific active site inhibitor of telomerase. GRN163L (0.1-5μM) markedly reduced TA in both CD138+ and CD138neg cells. In order to examine the effects of TA inhibition on clonogenic MM growth, we treated CD138+ and CD138neg RPMI 8226 and NCI-H929 cells continuously with GRN163L (1μM) for up to 6 weeks. Clonogenic growth potential was assessed each week by plating cells in methylcellulose. GRN163L was active against both CD138+ and CD138neg cells from each cell line and eliminated the colony forming potential by 5 weeks. Similarly, GRN163L progressively inhibited CD138neg CSC isolated from primary clinical MM bone marrow specimens during long-term culture. These data demonstrate that MM CSC are relatively resistant to standard anti-MM therapies, but susceptible to TA inhibition, and clinical trials with GRN163L in MM patients are planned. In conclusion, active and specific telomerase inhibitors, such as GRN163L, may represent a novel therapeutic strategy with the potential to limit disease relapse in maligancies in which CSC play a critical pathogenic role.