Synthesis and Molecular Dynamic Simulation of Novel Cationic and Non-cationic Pyrimidine Derivatives as Potential G-quadruplex-ligands.

BACKGROUND: Drug resistance has been a problem in cancer chemotherapy, which often causes shortterm effectiveness. Further, the literature indicates that telomere G-quadruplex could be a promising anti-cancer target. OBJECTIVE: We synthesized and characterized two new pyrimidine derivatives as ligands for G-quadruplex DNA. METHODS: The interaction of novel non-cationic and cationic pyrimidine derivatives (3a, b) with G-quadruplex DNA (1k8p and 3qsc) was explored by circular dichroism (CD) and ultraviolet-visible spectroscopy and polyacrylamide gel electrophoresis (PAGE) methods. The antiproliferative activity of desired compounds was evaluated by the MTT assay. Apoptosis induction was assessed by Propidium iodide (P.I.) staining and flow cytometry. Computational molecular modeling (CMM) and molecular dynamics simulation (MD) were studied on the complexes of 1k8p and 3qsc with the compounds. The van der Waals, electrostatic, polar solvation, solventaccessible surface area (SASA), and binding energies were calculated and analyzed. RESULTS: The experimental results confirmed that both compounds 3a and 3b interacted with 1k8p and 3qsc and exerted cytotoxic and proapoptotic effects on cancer cells. The number of hydrogen bonds and the RMSD values increased in the presence of the ligands, indicating stronger binding and suggesting increased structural dynamics. The electrostatic contribution to binding energy was higher for the cationic pyrimidine 3b, indicating more negative binding energies. CONCLUSION: Both experimental and MD results confirmed that 3b was more prone to form a complex with DNA G-quadruplex (1k8p and 3qsc), inhibit cell growth, and induce apoptosis, compared to the non-cationic pyrimidine 3a.

Telomere-related DNA damage response pathways in cancer therapy: prospective targets.

Maintaining the structural integrity of genomic chromosomal DNA is an essential role of cellular life and requires two important biological mechanisms: the DNA damage response (DDR) mechanism and telomere protection mechanism at chromosome ends. Because abnormalities in telomeres and cellular DDR regulation are strongly associated with human aging and cancer, there is a reciprocal regulation of telomeres and cellular DDR. Moreover, several drug treatments for DDR are currently available. This paper reviews the progress in research on the interaction between telomeres and cellular DNA damage repair pathways. The research on the crosstalk between telomere damage and DDR is important for improving the efficacy of tumor treatment. However, further studies are required to confirm this hypothesis.

BIBR1532 combined with radiotherapy induces ferroptosis in NSCLC cells and activates cGAS-STING pathway to promote anti-tumor immunity.

BACKGROUND: Telomerase, by safeguarding damaged telomeres and bolstering DNA damage repair, has the capacity to heighten the radioresistance of tumour cells. Thus, in turn, can compromise the efficacy of radiotherapy (RT) and radioimmunotherapy. Our previous studies have revealed that the highly selective telomerase inhibitor, BIBR1532, possesses the potential to enhance the radiosensitivity of Non-small cell lung cancer (NSCLC). In this study, we delve further into the impact of BIBR1532 on the immune activation induced by RT and elucidate the underlying mechanisms. METHODS: Biological information analyses, immunofluorescence assays, western blot assays, flow cytometry analysis were conducted to elucidate the functions of the combination of BIBR1532 with radiotherapy in NSCLC. Intracellular levels of lipid peroxides, glutathione, malondialdehyde, and Fe2+ were measured as indicators of ferroptosis status. Both in vitro and in vivo studies were conducted to examine the antitumor effects. RESULTS: Our findings indicate that the confluence of BIBR1532 with RT significantly augments the activation of the cGAS-STING pathway in both in vivo and in vitro settings, thereby fostering an effective anti-tumoral immune response. The effects can be ascribed to two key processes. Firstly, ionizing radiation, in precipitating DNA double-strand breaks (DSBs), prompts the release of tumour-derived double-stranded DNA (dsDNA) into the cytoplasm. Subsequently, BIBR1532 amplifies the activation of antigen-presenting cells by dsDNA post-RT and instigates the cGAS-STING pathway. Secondly, BIBR1532 enhances the ferroptosis response in NSCLC following RT, thereby promoting unrestrained lipid peroxidation and elevated levels of reactive oxygen species (ROS) within tumour cells. This ultimately leads to mitochondrial stress and the release of endogenous mitochondrial DNA (mtDNA) into the cytoplasm, thus facilitating the activation of the STING pathway and the induction of a type I interferon (IFN)-linked adaptive immune response. CONCLUSION: This study underscores the potential of BIBR1532 as an efficacious and safe radiosensitizer and radioimmunotherapy synergist, providing robust preclinical research evidence for the treatment of NSCLC.

A CRISPR base editing approach for the functional assessment of telomere biology disorder-related genes in human health and aging.

Telomere Biology Disorders (TBDs) are a group of rare diseases characterized by the presence of short and/or dysfunctional telomeres. They comprise a group of bone marrow failure syndromes, idiopathic pulmonary fibrosis, and liver disease, among other diseases. Genetic alterations (variants) in the genes responsible for telomere homeostasis have been linked to TBDs. Despite the number of variants already identified as pathogenic, an even more significant number must be better understood. The study of TBDs is challenging since identifying these variants is difficult due to their rareness, it is hard to predict their impact on the disease onset, and there are not enough samples to study. Most of our knowledge about pathogenic variants comes from assessing telomerase activity from patients and their relatives affected by a TBD. However, we still lack a cell-based model to identify new variants and to study the long-term impact of such variants on the genes involved in TBDs. Herein, we present a cell-based model using CRISPR base editing to mutagenize the endogenous alleles of 21 genes involved in telomere biology. We identified key residues in the genes encoding 17 different proteins impacting cell growth. We provide functional evidence for variants of uncertain significance in patients with TBDs. We also identified variants resistant to telomerase inhibition that, similar to cells expressing wild-type telomerase, exhibited increased tumorigenic potential using an in vitro tumour growth assay. We believe that such cell-based approaches will significantly advance our understanding of the biology of TBDs and may contribute to the development of new therapies for this group of diseases.

BIBR1532 Affects Endometrial Cell Proliferation, Migration, and Invasion in Endometriosis via Telomerase Inhibition and MAPK Signaling.

OBJECTIVES: The effect of telomerase inhibitor BIBR1532 on endometriotic cells was investigated to explore the inhibitory effect of targeting telomerase on endometriosis. DESIGN: In vitro primary cell culture study. Participants/Materials: Primary endometrial cells derived from eutopic and ectopic endometrium in patients with endometriosis. SETTING: The study was conducted in the university hospital. METHODS: Paired eutopic and ectopic endometrial cells were collected from 6 patients from January 2018 to July 2021. A TRAP assay was performed to detect the telomerase activity of the cells. MTT, cell cycle, apoptosis, migration, and invasion assays were performed to study the inhibitory effect of BIBR1532. Enrichment analysis was performed to identify the key pathways involved in endometriosis progression and telomerase action. Then, Western blotting was used to investigate the expression of related proteins. RESULTS: BIBR1532 treatment significantly inhibited the growth of eutopic and ectopic endometrial cells, with apoptosis and cell cycle signaling involved. Migration and invasion, important characteristics for the establishment of ectopic lesions, were also inhibited by BIBR1532. The MAPK signaling cascade, related to telomerase and endometriosis, was decreased in eutopic and ectopic endometrial stromal cells with the treatment of BIBR1532. LIMITATIONS: The severe side effects of telomerase inhibitors might be the main obstacle to clinical application, so it is necessary to find better drug delivery methods in vivo. CONCLUSIONS: The telomerase inhibitor BIBR1532 affects endometrial cell proliferation, migration, and invasion in endometriosis.

Zeolitic Imidazolate Framework-8 (ZIF-8) as a Drug Delivery Vehicle for the Transport and Release of Telomerase Inhibitor BIBR 1532.

Telomerase is constitutively overexpressed in the majority of human cancers and telomerase inhibition provides a promising broad-spectrum anticancer therapeutic strategy. BIBR 1532 is a well-known synthetic telomerase inhibitor that blocks the enzymatic activity of hTERT, the catalytic subunit of telomerase. However, water insolubility of BIBR 1532 leads to low cellular uptake and inadequate delivery and thus, limits its anti-tumor effects. Zeolitic imidazolate framework-8 (ZIF-8) is considered as an attractive drug delivery vehicle for improved transport, release and anti-tumor effects of BIBR 1532. Herein, ZIF-8 and BIBR 1532@ZIF-8 were synthesized, respectively, and the physicochemical characterizations confirmed the successful encapsulation of BIBR 1532 in ZIF-8 coupled with an improved stability of BIBR 1532. ZIF-8 could alter the permeability of lysosomal membrane probably by the imidazole ring-dependent protonation. Moreover, ZIF-8 encapsulation facilitated the cellular uptake and release of BIBR 1532 with more accumulation in the nucleus. BIBR 1532 encapsulation with ZIF-8 triggered a more obvious growth inhibition of cancer cells as compared with free BIBR 1532. A more potent inhibition on hTERT mRNA expression, aggravated G0/G1 arrest accompanied with an increased cellular senescence were detected in BIBR 1532@ZIF-8-treated cancer cells. Our work has provided preliminary information on improving the transport, release and efficacy of water-insoluble small molecule drugs by using ZIF-8 as a delivery vehicle.

The Effect of Telomerase Inhibition on NK Cell Activity in Acute Myeloid Leukemia.

Purpose: Acute myeloid leukemia (AML) is known to be an invasive and highly lethal hematological malignancy in adults and children. Resistance to the present treatments, including radiotherapy and chemotherapy with their side effects and telomere length shortening are the main cause of the mortality in AML patients. Telomeres sequence which are located at the end of eukaryotic chromosome play pivotal role in genomic stability. Recent studies have shown that apoptosis process is blocked in AML patient by the excessive telomerase activity in cancerous blasts. Therefore, the find of effective ways to prevent disease progression has been considered by the researchers. Natural killer (NK) cells as granular effector cells play a critical role in elimination of abnormal and tumor cells. Given that the cytotoxic function of NK cells is disrupted in the AML patients, we investigated the effect of telomerase inhibitors on NK cell differentiation. Methods: To evaluate telomerase inhibition on NK cell differentiation, the expression of CD105, CD56, CD57, and KIRs was evaluated in CD34+ derived NK cells after incubation of them with BIBR1532. Results: The results showed that the expression of CD105, CD56, CD57, and KIRs receptors reduces after telomerase inhibition. According to these findings, BIBR1532 affected the final differentiation of NK cells. Conclusion: The results revealed that telomerase inhibitor drugs suppress cancer cell progression in a NK cells-independent process.

Telomerase inhibitor MST-312 and quercetin synergistically inhibit cancer cell proliferation by promoting DNA damage.

Quercetin is a natural flavonoid with well-established anti-proliferative activities against a variety of cancers. Telomerase inhibitor MST-312 also exhibits anti-proliferative effect on various cancer cells independent of its effect on telomere shortening. However, due to their low absorption and toxicity at higher doses, their clinical development is limited. In the present study, we examine the synergistic potential of their combination in cancer cells, which may result in a decrease in the therapeutic dosage of these compounds. We report that MST-312 and quercetin exhibit strong synergism in ovarian cancer cells with combination index range from 0.2 to 0.7. Co-treatment with MST-312 and quercetin upregulates the DNA damage and augments apoptosis when compared to treatment with either compound alone or a vehicle. We also examined the effect of these compounds on the proliferation of normal ovarian surface epithelial cells (OSEs). MST-312 has a cytoprotective impact in OSEs at lower dosages, but is inhibitory at higher doses. Quercetin did not affect the OSEs proliferation at low concentrations while at higher concentrations it is inhibitory. Notably, combination of MST-312 and quercetin had no discernible impact on OSEs. These observations have significant implications for future efforts towards maximizing efficacy in cancer therapeutics as this co-treatment specifically affects cancer cells and reduces the effective dosage of both the compounds.

Telomerase: a good target in hepatocellular carcinoma? An overview of relevant preclinical data.

INTRODUCTION: The expression of telomerase reverse transcriptase (TERT) in liver is restricted to rare cells, that are able to replace senescent hepatocytes and regenerate tissue in response to hepatic damage, while becoming extinguished in differentiated progeny cells. TERT gene is permanently activated in liver neoplasms from the very early stage of the hepatocarcinogenesis mainly through the accumulation of genetic alterations, virus-related insertional mutagenesis and somatic mutations in the TERT promoter region. Several lines of evidence suggest that telomerase, beyond the canonical function of telomeres elongation, has multiple oncogenic activities in cancer cells and may represent a promising therapeutic target in hepatocellular carcinoma (HCC). AREAS COVERED: We review the mechanisms of activation of telomerase in HCC, the canonical and non-canonical functions of TERT as well as experimental strategies to directly target telomerase or to inhibit pathways associated with telomerase activity. EXPERT OPINION: TERT holoenzyme and telomerase components represent promising therapeutic targets in the treatment of liver malignancies. Several chemical agents and natural products known to alter telomerase activity are under evaluation for their potency to inhibit telomeres attrition in cirrhosis and TERT function in liver cancer. Therefore, this review outlines the current strategies pursued to suppress the multiple mechanisms of the major telomerase components in liver cancer.

PINX1 promotes malignant transformation of thyroid cancer through the activation of the AKT/MAPK/ß-catenin signaling pathway.

Although thyroid cancer is the most prevalent endocrine malignancy, overall patients with thyroid cancer have a good long-term survival. However, a small percentage of patients with progressive thyroid cancer have poor outcomes, and the genetic drivers playing a key role thyroid cancer progression are mostly unknown. Here, we investigated the role of the PINX1 in thyroid cancer progression. Interestingly, PINX1 expression was significantly higher in ATC than in PTC in both patients and cell lines. When PINX1 was knockdown in ATC cells, cell proliferation rates, colony formation capacity, and cell cycle progression were significantly reduced. Furthermore, cell motility and the expression of EMT drivers were reduced by PINX1 downregulation. In contrast, the overexpression of PINX1 in PTC cells significantly increased those phenotypes of tumor progression, which demonstrates that PINX1 could promote tumor proliferation and malignant transformation in both PTC and ATC cells. To further understand whether PINX1 is also involved in the progression of PTC to ATC, we examined PI3K/AKT, MAPK, and ß-catenin signaling activation after PINX1 modulation. Decreased PINX1 expression reduced the levels of p-AKT, p-ERK, p-p38, and ß-catenin in ATC cells, but the increase of PINX1 expression upregulated the phosphorylation of AKT, ERK, and p38 and the levels of ß-catenin in PTC cells. These results were all confirmed in xenograft mouse tumors. Our findings suggest that PINX1 regulates thyroid cancer progression by promoting cell proliferation, EMT, and signaling activation, and support the hypothesis that PINX1 could be a prognostic marker and a therapeutic target of thyroid cancer.

Antisense oligonucleotide repress telomerase activity via manipulating alternative splicing or translation.

Telomerase is a reverse transcriptase that catalyzes the addition of telomeric repeated DNA onto the 3' ends of linear chromosomes. Telomerase inhibition was broadly used for cancer therapeutics. Here, six antisense oligonucleotides were designed to regulate TERT mRNA alternative splicing and protein translation. To pursue a better stability in vitro, we chemically modified the oligonucleotides into phosphorothioate (PS) backbone and 2'-O-methoxyethyl (2'-MOE PS) version and phosphoroamidate morpholino oligomer (PMO) version. The oligonucleotides were transfected into HEK 293T cells and HeLa cells, and the mRNA expression, protein level and catalytic activity of telomerase were determined. We found the Int8 notably promoted hTERT mRNA exon 7-8 skipping, which greatly reduced telomerase activity, and the 5'-UTR treatment led to an obvious protein translation barrier and telomerase inhibition. These results demonstrate the potential of antisense oligonucleotide drugs targeting hTERT for antitumor therapy. Moreover, two specific antisense oligonucleotides were identified to be effective in reducing telomerase activity.

Targeting Telomere Biology in Acute Lymphoblastic Leukemia.

Increased cell proliferation is a hallmark of acute lymphoblastic leukemia (ALL), and genetic alterations driving clonal proliferation have been identified as prognostic factors. To evaluate replicative history and its potential prognostic value, we determined telomere length (TL) in lymphoblasts, B-, and T-lymphocytes, and measured telomerase activity (TA) in leukocytes of patients with ALL. In addition, we evaluated the potential to suppress the in vitro growth of B-ALL cells by the telomerase inhibitor imetelstat. We found a significantly lower TL in lymphoblasts (4.3 kb in pediatric and 2.3 kb in adult patients with ALL) compared to B- and T-lymphocytes (8.0 kb and 8.2 kb in pediatric, and 6.4 kb and 5.5 kb in adult patients with ALL). TA in leukocytes was 3.2 TA/C for pediatric and 0.7 TA/C for adult patients. Notably, patients with high-risk pediatric ALL had a significantly higher TA of 6.6 TA/C compared to non-high-risk patients with 2.2 TA/C. The inhibition of telomerase with imetelstat ex vivo led to significant dose-dependent apoptosis of B-ALL cells. These results suggest that TL reflects clonal expansion and indicate that elevated TA correlates with high-risk pediatric ALL. In addition, telomerase inhibition induces apoptosis of B-ALL cells cultured in vitro. TL and TA might complement established markers for the identification of patients with high-risk ALL. Moreover, TA seems to be an effective therapeutic target; hence, telomerase inhibitors, such as imetelstat, may augment standard ALL treatment.

Characterization of In Vitro G-Quadruplex Formation of Imetelstat Telomerase Inhibitor.

Imetelstat (GRN163L) is a potent and specific telomerase inhibitor currently in clinical development for the treatment of hematological malignancies such as myelofibrosis and myelodysplastic syndrome. It is a 13-mer N3'-P5' thio-phosphoramidate oligonucleotide covalently functionalized at the 5'-end with a palmitoyl lipid moiety through an aminoglycerol linker. As a competitive inhibitor of human telomerase, imetelstat directly binds to the telomerase RNA component sequence (hTR) in the catalytic site of the enzyme and acts as a direct competitor of human telomere binding. Administration of imetelstat causes progressive shortening of the telomeres, thereby inhibiting malignant cells' proliferation. We report here the ability of imetelstat to form stable, parallel, intermolecular G-quadruplex structures in vitro. The impact of the ionic environment on the formation and stability of imetelstat higher-order structure was investigated through circular dichroism spectroscopy, thermal denaturation analysis, and size-exclusion chromatography. We demonstrated that different structural elements, such as the 5'-palmitoyl linker and the thio-phosphoramidate backbone, critically contribute to G-quadruplex stability. Experiments further showed that G-quadruplex formation does not hamper binding to the hTR oligonucleotide sequence in vitro.

Dual effect of Sapogenins extracted from Spirulina platensis on telomerase activity in two different cell lines.

Spirulina platensis is a photosynthetic filamentous, edible cyanobacterium that is known as a superfood. In this study, sapogenins were extracted from the spirulina and the effects of these compounds on telomerase activity were evaluated in MCF7 and HDF cell lines using Telomeric Repeat Amplification Protocol and ELIZA assay. The highest increase in telomerase activity was observed at 0.004 mg/ml of sapogenin by 26% ±20.5 in MCF7 cells, while in HDF cells in the same concentration telomerase activity decreased down to 47%±0.48 and the highest inhibition of telomerase activity was observed at 0.070 mg/ml of sapogenins from Spirulina by 68%±0.43. In conclusion, a compound could play a role as a telomerase activator in one cell line while it could play another role as a telomerase inhibitor in another cell line so introducing compounds as a telomerase inhibitor (anticancer) or as a telomerase activator (anti-aging) should be done with discreet.

A novel p53 regulator, C16ORF72/TAPR1, buffers against telomerase inhibition.

Telomere erosion in cells with insufficient levels of the telomerase reverse transcriptase (TERT), contributes to age-associated tissue dysfunction and senescence, and p53 plays a crucial role in this response. We undertook a genome-wide CRISPR screen to identify gene deletions that sensitized p53-positive human cells to telomerase inhibition. We uncovered a previously unannotated gene, C16ORF72, which we term Telomere Attrition and p53 Response 1 (TAPR1), that exhibited a synthetic-sick relationship with TERT loss. A subsequent genome-wide CRISPR screen in TAPR1-disrupted cells reciprocally identified TERT as a sensitizing gene deletion. Cells lacking TAPR1 or TERT possessed elevated p53 levels and transcriptional signatures consistent with p53 upregulation. The elevated p53 response in TERT- or TAPR1-deficient cells was exacerbated by treatment with the MDM2 inhibitor and p53 stabilizer nutlin-3a and coincided with a further reduction in cell fitness. Importantly, the sensitivity to treatment with nutlin-3a in TERT- or TAPR1-deficient cells was rescued by loss of p53. These data suggest that TAPR1 buffers against the deleterious consequences of telomere erosion or DNA damage by constraining p53. These findings identify C16ORF72/TAPR1 as new regulator at the nexus of telomere integrity and p53 regulation.

[Clinical Application and Challenges of Telomere and Telomerase Research 
in Lung Cancer].

Lung cancer is one of the malignant tumors with high incidence rate and high mortality worldwide. Telomere and telomerase are closely related to the occurrence and development of lung cancer. Although telomerase may not be the direct cause of carcinogenesis, it plays a key role in maintaining telomere length and tumor growth. The length of most tumors, including lung cancer, is shortened. The change of telomere length is related to the risk of lung cancer, and may become the therapeutic target and predictive index. Target drugs for telomere and telomerase signaling pathway are constantly being explored, and drugs represented by telomerase inhibitors are expected to be used in clinical treatment of lung cancer in the future. However, the research on telomere and telomerase is far from enough. The bypass mechanism of telomere length maintenance may be the direction of further research.
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Berberine Inhibits Telomerase Activity and Induces Cell Cycle Arrest and Telomere Erosion in Colorectal Cancer Cell Line, HCT 116.

Colorectal cancer (CRC) is the most common cancer among males and females, which is associated with the increment of telomerase level and activity. Some plant-derived compounds are telomerase inhibitors that have the potential to decrease telomerase activity and/or level in various cancer cell lines. Unfortunately, a deeper understanding of the effects of telomerase inhibitor compound(s) on CRC cells is still lacking. Therefore, in this study, the aspects of telomerase inhibitors on a CRC cell line (HCT 116) were investigated. Screening on HCT 116 at 48 h showed that berberine (10.30 ± 0.89 µg/mL) is the most effective (lowest IC50 value) telomerase inhibitor compared to boldine (37.87 ± 3.12 µg/mL) and silymarin (>200 µg/mL). Further analyses exhibited that berberine treatment caused G0/G1 phase arrest at 48 h due to high cyclin D1 (CCND1) and low cyclin-dependent kinase 4 (CDK4) protein and mRNA levels, simultaneous downregulation of human telomerase reverse transcriptase (TERT) mRNA and human telomerase RNA component (TERC) levels, as well as a decrease in the TERT protein level and telomerase activity. The effect of berberine treatment on the cell cycle was time dependent as it resulted in a delayed cell cycle and doubling time by 2.18-fold. Telomerase activity and level was significantly decreased, and telomere erosion followed suit. In summary, our findings suggested that berberine could decrease telomerase activity and level of HCT 116, which in turn inhibits the proliferative ability of the cells.

Synthesis, telomerase inhibitory and anticancer activity of new 2-phenyl-4H-chromone derivatives containing 1,3,4-oxadiazole moiety.

Based on previous studies, 66 2-phenyl-4H-chromone derivatives containing amide and 1,3,4-oxadiazole moieties were prepared as potential telomerase inhibitors. The results showed most of the title compounds exhibited significantly inhibitory activity on telomerase. Among them, some compounds demonstrated the most potent telomerase inhibitory activity (IC50 < 1 µM), which was significantly superior to the staurosporine (IC50 = 6.41 µM). In addition, clear structure-activity relationships were summarised, indicating that the substitution of the methoxy group and the position, type and number of the substituents on the phenyl ring had significant effects on telomerase activity. Among them, compound A33 showed considerable inhibition against telomerase. Flow cytometric analysis showed that compound A33 could arrest MGC-803 cell cycle at G2/M phase and induce apoptosis in a concentration-dependent way. Meanwhile, Western blotting revealed that this compound could reduce the expression of dyskerin, which is a fragment of telomerase.

Clinical Application and Challenges of Telomere and Telomerase Research in Lung Cancer / 中国肺癌杂志

Lung cancer is one of the malignant tumors with high incidence rate and high mortality worldwide. Telomere and telomerase are closely related to the occurrence and development of lung cancer. Although telomerase may not be the direct cause of carcinogenesis, it plays a key role in maintaining telomere length and tumor growth. The length of most tumors, including lung cancer, is shortened. The change of telomere length is related to the risk of lung cancer, and may become the therapeutic target and predictive index. Target drugs for telomere and telomerase signaling pathway are constantly being explored, and drugs represented by telomerase inhibitors are expected to be used in clinical treatment of lung cancer in the future. However, the research on telomere and telomerase is far from enough. The bypass mechanism of telomere length maintenance may be the direction of further research.
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Myelosuppression in Patients Treated with the Telomerase Inhibitor Imetelstat Is Not Mediated through Activation of Toll-Like Receptors.

Imetelstat sodium (GRN163L; hereafter, imetelstat) is a first-in-class telomerase inhibitor that has demonstrated activity in patients with myeloproliferative neoplasms (MPNs). Treatment with imetelstat has been associated with thrombocytopenia and other hematologic adverse effects that were manageable and reversible. Toll-like receptors (TLRs) are proteins that recognize pathogen-associated molecular patterns and stimulate innate immune and pro-apoptotic responses. Because imetelstat is an oligonucleotide, and some oligonucleotides can activate TLRs, we conducted an in vitro study to rule out the possibility of imetelstat-associated thrombocytopenia by off-target effects through activation of TLRs. We used HEK293 cell lines stably co-expressing a human TLR gene and an NFκB-inducible reporter to investigate whether imetelstat can activate TLR signaling. We treated the cells with imetelstat or control oligonucleotides for 20 h, and used absorbance of the culture media to calculate the reporter activity. Treatment with imetelstat within or beyond the clinically relevant concentrations had no stimulatory effect on TLR2, TLR3, TLR4, TLR5, TLR7, or TLR9. This result was not surprising since the structure of imetelstat does not meet the reported minimal structural requirements for TLR9 activation. Furthermore, imetelstat treatment of the MPN cell line HEL did not impact the expression of TLR signaling pathway target genes that are commonly induced by activation of different TLRs, whereas it significantly reduced its target gene hTERT, human telomerase reverse transcriptase, in a dose- and time-dependent manner. Hence, cytopenias, especially thrombocytopenia observed in some patients treated with imetelstat, are not mediated by off-target interactions with TLRs.