Pharmacological Inhibitory Effect of Imetelstat, a Novel Human Telomerase Inhibitor in Diffuse Large B-Cell Lymphoma and Peripheral T-Cell Lymphoma

Telomerase activity is present in some non-Hodgkin lymphomas (NHL), including diffuse large B-cell lymphoma (DLBCL) and peripheral T-cell lymphomas (PTCL), and correlated with proliferation index, suggesting anti-telomerase drugs may be efficacious in some types of NHL (Ely et al. 2000). Imetelstat, a telomerase inhibitor that specifically targets the RNA template of human telomerase, is a potent, first-in-class, competitive inhibitor of telomerase enzymatic activity and has demonstrated clinical activity in myeloid malignancies (Steensma et al. 2021; Mascarenhas et al. 2021). No studies have been conducted to demonstrate the effects of imetelstat in DLBCL and PTCL. In our study, we characterized telomerase activity and telomere length, as well as the in-vitro effects of imetelstat on cell viability and apoptosis of a panel of DLBCL and PTCL cell lines.

Six DLBCL cell lines, including germinal B cells (GCB: DOHH2, HT, SUDHL4), non-germinal B cells (HBL-1, TMD8), and one high-grade B cell lymphoma line RC with MYC and BCL2 gene rearrangements (double hit lymphomas; ), and five PTCL cell lines (Karpas 299, H9, MAC2A, SUDHL1, and SUPM2) were tested for imetelstat single agent activity as assessed by change of telomerase activity using ScienCell's Telomerase Activity Quantification qPCR Assay Kit, cell viability by Cell Titer-Glo Luminescent Cell Viability Assay and apoptosis by Annexin V/Propidium Iodide flow assay. Baseline telomerase activity and telomere length were measured for each cell line to correlate with imetelstat activity. In addition, PTCL cell lines were treated with Suberoylanilide hydroxamic acid (SAHA) and imetelstat to determine if the combination had an impact on cell viability over either of the single agents.

All DLBCL and PTCL cell lines have higher baseline telomerase activity with Ct 20.6-23.2 cycles than the telomerase-positive cell lysate control with Ct 30.6 cycles used in the assay. The dose and time-dependent effects of imetelstat were examined through cell viability and apoptosis assays. Imetelstat sensitivity was higher in DLBCL than in PTCL cell lines used in this study. Preliminary experiments determined that IC₅₀ 3-17.5 μM imetelstat markedly reduced cell viability and induced apoptosis in DLBCL cell lines. Specifically, non-GCB DLBCL cell lines (IC₅₀ = 3-4.1 μM) were more sensitive than the GCB-DLBCL (IC₅₀ = 6-8.2 μM). While the level of effects in cell viability and apoptosis by imetelstat varied in the DLBCL cell lines, the double hit DLBCL cell line RC was the least sensitive to imetelstat (IC₅₀ = 17.5 μM) after treatment for 8 days. In contrast, compared to DLBCL cell lines, imetelstat single-agent activity on cell viability was limited in PTCL cell lines, although a time and dose-dependent reduction of telomerase activity were noted. Additionally, the greater inhibitory effect in DLBCL than in PTCL cell lines by imetelstat treatment may be attributed to higher telomerase activity observed in DLBCL compared to PTCL cell lines. Furthermore, the PTCL cell lines had a ~7.3 fold longer telomere length than DLBCL cell lines, which was associated with less response to imetelstat than DLBCL lines. A combination of SAHA with imetelstat was more effective than a single agent alone, synergistic activity was observed in Karpas 299, while other PTCL lines had an additive effect.

Our findings indicate that imetelstat was more sensitive in non-GCB subtypes than in GCB-DLBCL. While in PTCL cell lines, the activity of imetelstat was enhanced when combined with SAHA. In the lymphoma landscape, telomerase may be a therapeutic target for ABC-type DLBCL; the novel telomerase inhibitor Imetelstat may present one of the potential treatment options.

Huang:Geron Corporation: Current Employment, Current equity holder in publicly-traded company. Vega:Crisp Therapeutics: Research Funding; Geron Corporation: Research Funding; Allogene: Research Funding.