Telomerase inhibitors--oligonucleotide phosphoramidates as potential therapeutic agents

Nanoscale octopus guiding telomere entanglement: An innovative strategy for inducing apoptosis in cancer cells

Telomere length plays a crucial role in cellular aging and the risk of diseases. Unlike normal cells, cancer cells can extend their own survival by maintaining telomere stability through telomere maintenance mechanism. Therefore, regulating the lengths of telomeres have emerged as a promising approach for anti-cancer treatment. In this study, we introduce a nanoscale octopus-like structure designed to induce physical entangling of telomere, thereby efficiently triggering telomere dysfunction. The nanoscale octopus, composed of eight-armed PEG (8-arm-PEG), are functionalized with cell penetrating peptide (TAT) to facilitate nuclear entry and are covalently bound to N-Methyl Mesoporphyrin IX (NMM) to target G-quadruplexes (G4s) present in telomeres. The multi-armed configuration of the nanoscale octopus enables targeted binding to multiple G4s, physically disrupting and entangling numerous telomeres, thereby triggering telomere dysfunction. Both in vitro and in vivo experiments indicate that the nanoscale octopus significantly inhibits cancer cell proliferation, induces apoptosis through telomere entanglement, and ultimately suppresses tumor growth. This research offers a novel perspective for the development of innovative anti-cancer interventions and provides potential therapeutic options for targeting telomeres.

Discovery of telomerase inhibitors: existing strategies and emerging innovations

Telomerase, crucial for maintaining telomere length, is an attractive target for cancer therapy due to its role in cellular immortality. Despite three decades of research efforts, no small-molecule telomerase inhibitors have been clinically approved, highlighting the extensive challenges in developing effective telomerase-based therapeutics. This review examines conventional and emerging methods to measure telomerase activity and discusses existing inhibitors, including oligonucleotides and small molecules. Furthermore, this review highlights recent breakthroughs in structural studies of telomerase using cryo-electron microscopy, which can facilitate improved structure-based drug design. Altogether, advancements in structural methodologies and high-throughput screening offer promising prospects for telomerase-based cancer therapeutic development.

Targeting telomeres: advances in telomere maintenance mechanism-specific cancer therapies

Cancer cells establish replicative immortality by activating a telomere-maintenance mechanism (TMM), be it telomerase or the alternative lengthening of telomeres (ALT) pathway. Targeting telomere maintenance represents an intriguing opportunity to treat the vast majority of all cancer types. Whilst telomerase inhibitors have historically been heralded as promising anticancer agents, the reality has been more challenging, and there are currently no therapeutic options for cancer types that use ALT despite their aggressive nature and poor prognosis. In this Review, we discuss the mechanistic differences between telomere maintenance by telomerase and ALT, the current methods used to detect each mechanism, the utility of these tests for clinical diagnosis, and recent developments in the therapeutic strategies being employed to target both telomerase and ALT. We present notable developments in repurposing established therapeutic agents and new avenues that are emerging to target cancer types according to which TMM they employ. These opportunities extend beyond inhibition of telomere maintenance, by finding and exploiting inherent weaknesses in the telomeres themselves to trigger rapid cellular effects that lead to cell death.

Modified internucleoside linkages for nuclease-resistant oligonucleotides

In the past few years, several drugs derived from nucleic acids have been approved for commercialization and many more are in clinical trials. The sensitivity of these molecules to nuclease digestion in vivo implies the need to exploit resistant non-natural nucleotides. Among all the possible modifications, the one concerning the internucleoside linkage is of particular interest. Indeed minor changes to the natural phosphodiester may result in major modifications of the physico-chemical properties of nucleic acids. As this linkage is a key element of nucleic acids' chemical structures, its alteration can strongly modulate the plasma stability, binding properties, solubility, cell penetration and ultimately biological activity of nucleic acids. Over the past few decades, many research groups have provided knowledge about non-natural internucleoside linkage properties and participated in building biologically active nucleic acid derivatives. The recent renewing interest in nucleic acids as drugs, demonstrated by the emergence of new antisense, siRNA, aptamer and cyclic dinucleotide molecules, justifies the review of all these studies in order to provide new perspectives in this field. Thus, in this review we aim at providing the reader insights into modified internucleoside linkages that have been described over the years whose impact on annealing properties and resistance to nucleases have been evaluated in order to assess their potential for biological applications. The syntheses of modified nucleotides as well as the protocols developed for their incorporation within oligonucleotides are described. Given the intended biological applications, the modifications described in the literature that have not been tested for their resistance to nucleases are not reported.

Telomerase inhibition, telomere attrition and proliferation arrest of cancer cells induced by phosphorothioate ASO-NLS conjugates targeting hTERC and siRNAs targeting hTERT

Telomerase activity has been regarded as a critical step in cellular immortalization and carcinogenesis and because of this, regulation of telomerase represents an attractive target for anti-tumor specific therapeutics. Recently, one avenue of cancer research focuses on antisense strategy to target the oncogenes or cancer driver genes, in a sequence specific fashion to down-regulate the expression of the target gene. The protein catalytic subunit, human telomerase reverse transcriptase (hTERT) and the template RNA component (hTERC) are essential for telomerase function, thus theoretically, inhibition of telomerase activity can be achieved by interfering with either the gene expression of hTERT or the hTERC of the telomerase enzymatic complex. The present study showed that phosphorothioate antisense oligonucleotide (sASO)-nuclear localization signal (NLS) peptide conjugates targeting hTERC could inhibit telomerase activity very efficiently at 5 μM concentration but less efficiently at 1 μM concentration. On the other hand, siRNA targeting hTERT mRNA could strongly suppress hTERT expression at 200 nM concentration. It was also revealed that siRNA targeting hTERT could induce telomere attrition and then irreversible arrest of proliferation of cancer cells.

Targeting telomerase for its advent in cancer therapeutics

Telomerase has emerged as an important primary target in anticancer therapy. It is a distinctive reverse transcriptase enzyme, which extends the length of telomere at the 3' chromosomal end, and uses telomerase reverse transcriptase (TERT) and telomerase RNA template-containing domains. Telomerase has a vital role and is a contributing factor in human health, mainly affecting cell aging and cell proliferation. Due to its unique feature, it ensures unrestricted cell proliferation in malignancy and plays a major role in cancer disease. The development of telomerase inhibitors with increased specificity and better pharmacokinetics is being considered to design and develop newer potent anticancer agents. Use of natural and synthetic compounds for the inhibition of telomerase activity can lead to an opening of new vistas in cancer treatment. This review details about the telomerase biochemistry, use of natural and synthetic compounds; vaccines and oncolytic virus in therapy that suppress the telomerase activity. We have discussed structure-activity relationships of various natural and synthetic telomerase inhibitors to help medicinal chemists and chemical biology researchers with a ready reference and updated status of their clinical trials. Suppression of human TERT (hTERT) activity through inhibition of hTERT promoter is an important approach for telomerase inhibition.

DNA polymerase activity on synthetic N3'→P5' phosphoramidate DNA templates

Genetic polymers that could plausibly govern life in the universe might inhabit a broad swath of chemical space. A subset of these genetic systems can exchange information with RNA and DNA and could therefore form the basis for model protocells in the laboratory. N3'→P5' phosphoramidate (NP) DNA is defined by a conservative linkage substitution and has shown promise as a protocellular genetic material, but much remains unknown about its functionality and fidelity due to limited enzymatic tools. Conveniently, we find widespread NP-DNA-dependent DNA polymerase activity among reverse transcriptases, an observation consistent with structural studies of the RNA-like conformation of NP-DNA duplexes. Here, we analyze the consequences of this unnatural template linkage on the kinetics and fidelity of DNA polymerization activity catalyzed by wild-type and variant reverse transcriptases. Template-associated deficits in kinetics and fidelity suggest that even highly conservative template modifications give rise to error-prone DNA polymerase activity. Enzymatic copying of NP-DNA sequences is nevertheless an important step toward the future study and engineering of this synthetic genetic polymer.

Quantitative Biology of Human Shelterin and Telomerase: Searching for the Weakest Point

The repetitive telomeric DNA at chromosome ends is protected from unwanted repair by telomere-associated proteins, which form the shelterin complex in mammals. Recent works have provided new insights into the mechanisms of how human shelterin assembles and recruits telomerase to telomeres. Inhibition of telomerase activity and telomerase recruitment to chromosome ends is a promising target for anticancer therapy. Here, we summarize results of quantitative assessments and newly emerged structural information along with the status of the most promising approaches to telomerase inhibition in cancer cells. We focus on the mechanism of shelterin assembly and the mechanisms of how shelterin affects telomerase recruitment to telomeres, addressing the conceptual dilemma of how shelterin allows telomerase action and regulates other essential processes. We evaluate how the identified critical interactions of telomerase and shelterin might be elucidated in future research of new anticancer strategies.

Inhibitors of telomerase and poly(ADP-ribose) polymerases synergize to limit the lifespan of pancreatic cancer cells

Imetelstat (GRN163L) is a potent and selective inhibitor of telomerase. We have previously reported that GRN163L could shorten telomeres and limit the lifespan of CD18/HPAF and CAPAN1 pancreatic cancer cells. Here, we examined the effects of GRN163L on two other pancreatic cancer cell lines: AsPC1 and L3.6pl. In both lines, chronic exposure to GRN163L led to an initial shortening of telomeres followed by a stabilization of extremely short telomeres. In AsPC1 cells, telomere attrition eventually led to the induction of crisis and the loss of the treated population. In L3.6pl cells, crisis was transient and followed by the emergence of GRN163L-resistant cells, which could grow at increasing concentrations of GRN163L. The Shelterin complex is a telomere-associated complex that limits the access of telomerase to telomeres. The telomerase inhibitory function of this complex can be enhanced by drugs that block the poly(ADP-ribosyl)ation of its TRF1 and/or TRF2 subunits. Combined treatment of the GRN163L-resistant L3.6pl cells with GRN163L and 3-aminobenzamide (3AB), a general inhibitor of poly(ADP-ribose) polymerases, led to additional telomere shortening and limited the lifespan of the resistant cells. Results from this work suggest that inhibitors of telomerase and poly(ADP-ribose) polymerases can cooperate to limit the lifespan of pancreatic cancer cells.

Chimeric bifunctional oligonucleotides as a novel tool to invade telomerase assembly

Telomerase is a key participant in the telomere length maintaining system in eukaryotic cells. Telomerase RNA and protein reverse transcriptase subunits are essential for the appearance of active telomerase in vitro. Telomerase is active in many cancer types and is a potential target for anticancer drug development. Here we report a new approach for impairing telomerase function at the stage of human telomerase assembly. The approach is based on the application of chimeric bifunctional oligonucleotides that contain two oligonucleotide parts complementary to the functional domains of telomerase RNA connected with non-nucleotide linkers in different orientations (5'-3', 5'-5' or 3'-3'). Such chimeras inhibited telomerase in vitro in the nM range, but were effective in vivo in sub-nM concentrations, predominantly due to their effect on telomerase assembly and dimerization.

Telomerase inhibitor Imetelstat (GRN163L) limits the lifespan of human pancreatic cancer cells

Telomerase is required for the unlimited lifespan of cancer cells. The vast majority of pancreatic adenocarcinomas overexpress telomerase activity and blocking telomerase could limit their lifespan. GRN163L (Imetelstat) is a lipid-conjugated N3'→P5' thio-phosphoramidate oligonucleotide that blocks the template region of telomerase. The aim of this study was to define the effects of long-term GRN163L exposure on the maintenance of telomeres and lifespan of pancreatic cancer cells. Telomere size, telomerase activity, and telomerase inhibition response to GRN163L were measured in a panel of 10 pancreatic cancer cell lines. The cell lines exhibited large differences in levels of telomerase activity (46-fold variation), but most lines had very short telomeres (2-3 kb in size). GRN163L inhibited telomerase in all 10 pancreatic cancer cell lines, with IC50 ranging from 50 nM to 200 nM. Continuous GRN163L exposure of CAPAN1 (IC50 = 75 nM) and CD18 cells (IC50 = 204 nM) resulted in an initial rapid shortening of the telomeres followed by the maintenance of extremely short but stable telomeres. Continuous exposure to the drug eventually led to crisis and to a complete loss of viability after 47 (CAPAN1) and 69 (CD18) doublings. Crisis In these cells was accompanied by activation of a DNA damage response (γ-H2AX) and evidence of both senescence (SA-β-galactosidase activity) and apoptosis (sub-G1 DNA content, PARP cleavage). Removal of the drug after long-term GRN163L exposure led to a reactivation of telomerase and re-elongation of telomeres in the third week of cultivation without GRN163L. These findings show that the lifespan of pancreatic cancer cells can be limited by continuous telomerase inhibition. These results should facilitate the design of future clinical trials of GRN163L in patients with pancreatic cancer.

Clinical implications of antitelomeric drugs with respect to the nontelomeric functions of telomerase in cancer

Telomerase is responsible for maintaining the length of telomeres at the ends of chromosomes. Although most somatic cells do not exhibit telomerase activity, it is reactivated in approximately 85% of cancers. This simple and attractive phenomenon steers the development of anticancer drugs targeting telomeres and telomerase. Recent studies have been revealing extratelomeric roles of telomerase in normal tissues, affecting processes that are critical for survival and aging of organisms. In this review, we will discuss the current therapeutic strategies targeting telomeres and telomerase and evaluate their potential advantages and risks with respect to nontelomeric functions.

Targeting telomerase-expressing cancer cells

The role of telomeres and telomerase as a target for cancer therapeutics is an area of continuing interest. This review is intended to provide an update on the field, pointing to areas in which our knowledge remains deficient and exploring the details of the most promising areas being advanced into clinical trials. Topics that will be covered include the role of dysfunctional telomeres in cellular aging and how replicative senescence provides an initial barrier to the emergence of immortalized cells, a hallmark of cancer. As an important translational theme, this review will consider possibilities for selectively targeting telomeres and telomerase to enhance cancer therapy. The role of telomerase as an immunotherapy, as a gene therapy approach using telomerase promoter driven oncolytic viruses and as a small oligonucleotide targeted therapy (Imetelstat) will be discussed.

Is telomerase a viable target in cancer?

The ideal cancer treatment would specifically target cancer cells yet have minimal or no adverse effects on normal somatic cells. Telomerase, the ribonucleoprotein reverse transcriptase that maintains the ends of human chromosome, is an attractive cancer therapeutic target for exactly this reason [1]. Telomerase is expressed in more than 85% of cancer cells, making it a nearly universal cancer marker, while the majority of normal somatic cells are telomerase negative. Telomerase activity confers limitless replicative potential to cancer cells, a hallmark of cancer which must be attained for the continued growth that characterizes almost all advanced neoplasms [2]. In this review we will summarize the role of telomeres and telomerase in cancer cells, and how properties of telomerase are being exploited to create targeted cancer therapies including telomerase inhibitors, telomerase-targeted immunotherapies and telomerase-driven virotherapies. A frank and balanced assessment of the current state of telomerase inhibitors with caveats and potential limitations will be included.

Oligonucleotide n3'-->p5' phosphoramidates and thio-phoshoramidates as potential therapeutic agents

Nucleic acids analogues, i.e., oligonucleotide N3'-->P5' phosphoramidates and N3'-->P5' thio-phosphoramidates, containing 3'-amino-3'-deoxy nucleosides with various 2'-substituents were synthesized and extensively studied. These compounds resist nuclease hydrolysis and form stable duplexes with complementary native phosphodiester DNA and, particularly, RNA strands. An increase in duplexes' melting temperature, DeltaT(m), relative to their phosphodiester counterparts, reaches 2.2-4.0 degrees per modified nucleoside. 2'-OH- (RNA-like), 2'-O-Me-, and 2'-ribo-F-nucleoside substitutions result in the highest degree of duplex stabilization. Moreover, under close to physiological salt and pH conditions, the 2'-deoxy- and 2'-fluoro-phosphoramidate compounds form extremely stable triple-stranded complexes with either single- or double-stranded phosphodiester DNA oligonucleotides. Melting temperature, T(m), of these triplexes exceeds T(m) values for the isosequential phosphodiester counterparts by up to 35 degrees . 2'-Deoxy-N3'-->P5' phosphoramidates adopt RNA-like C3'-endo or N-type nucleoside sugar-ring conformations and hence can be used as stable RNA mimetics. Duplexes formed by 2'-deoxy phosphoramidates with complementary RNA strands are not substrates for RNase H-mediated cleavage in vitro. Oligonucleotide phosphoramidates and especially thio-phosphoramidates conjugated with lipid groups are cell-permeable and demonstrate high biological target specific activity in vitro. In vivo, these compounds show good bioavailability and efficient biodistribution to all major organs, while exerting acceptable toxicity at therapeutically relevant doses. Short oligonucleotide N3'-->P5' thio-phosphoramidate conjugated to 5'-palmitoyl group, designated as GRN163L (Imetelstat), was recently introduced as a potent human telomerase inhibitor. GRN163L is not an antisense agent; it is a direct competitive inhibitor of human telomerase, which directly binds to the active site of the enzyme and thus inhibits its activity. This compound is currently in multiple Phase-I and Phase-I/II clinical trials as potential broad-spectrum anticancer agent.

Telomerase structure paves the way for new cancer therapies

Inappropriate activation of a single enzyme, telomerase, is associated with the uncontrollable proliferation of cells observed in as many as 90% of all of human cancers. Since the mid-1990s, when telomerase activity was detected in human tumors, scientists have eyed the enzyme as an ideal target for developing broadly effective anticancer drugs. One of the missing links in the effort to identify such therapies has been the high-resolution structure of the enzyme, a powerful tool used for the identification and development of clinical drugs. A recent structure of the catalytic subunit of teleomerase from the Skordalakes laboratory, a major advancement in the field of telomeres, has opened the door to the development of new, broadly effective cancer drugs, as well as anti-aging therapies. Here we present a brief description of telomerase biology, current efforts to identify telomerase function modulators and the potential importance of the telomerase structure in future drug development.

Targeting critical steps of cancer metastasis and recurrence using telomerase template antagonists

Metastasis, tumor relapse, and drug resistance remain major obstacles in the treatment of cancer. Therefore, more research on the mechanisms of these processes in disease is warranted for improved treatment options. Recent evidence suggests that the capability to sustain tumor growth and metastasis resides in a subpopulation of cells, termed cancer stem cells or tumor-initiating cells. Continuous proliferation and self-renewal are characteristics of stem/progenitor cells. Telomerase and the maintenance of telomeres are key players in the ability of stem and cancer cells to bypass senescence and be immortal. Therefore, telomerase inhibitors have the therapeutic potential for reducing tumor relapse by targeting cancer stem cells and other processes involved in metastasis. Herein we review the role of telomerase in the immortal phenotype of cancer and cancer stem cells, targeting telomerase in cancer, and discuss other opportunities for telomerase inhibitors to target critical steps in cancer metastasis and recurrence.

Telomerase targeted oligonucleotide thio-phosphoramidates in T24-luc bladder cancer cells

Bladder carcinoma is the second most common genitourinary malignancy. Treatment options for bladder cancer include surgery, combined with chemotherapy, radiation, and/or immunotherapy. The adjuvant chemotherapy and immunotherapy regimen have been widely used in locally invasive as well as metastatic disease. The evaluation of new active agents with improved tolerability has been the focus of investigations over the past decade with minimal overall improvements in outcomes. Telomerase activity has been found in approximately 85-90% of all human tumors, but not in the majority of adjacent normal tissues. This suggests that telomerase may be an attractive target for the development of novel anticancer therapeutic agents. GRN163L is a lipid conjugated oligonucleotide N3' --> P5' thio-phosphoramidate, and is a potent telomerase RNA (hTR) template antagonist. In the present study, we show that the telomerase activity of T24-luc bladder cancer cells is inhibited by 1 microM GRN163L within 24 h of incubation. After two weeks of exposure to GRN163L, T24-luc cells became "clustered" whereas non-cancerous normal human uroepithelial cells were not morphologically affected. Moreover, in vitro GRN163L treated T24-luc bladder cancer cells entered G(0)/G(1) arrest following 2 weeks of continuous exposure and stopped dividing. Mismatch control compound had no effect on normal bladder epithelial cells or T24-luc cells. Additionally, a new generation of thio-phosphoramidate oligonucleotides were designed and tested in T24-luc cells and compared with GRN163L. The obtained results warrant further in vivo evaluation of GRN163L as a potential treatment for bladder cancer.

New therapeutic approach for brain tumors: Intranasal delivery of telomerase inhibitor GRN163

The blood-brain barrier is a substantial obstacle for delivering anticancer agents to brain tumors, and new strategies for bypassing it are greatly needed for brain-tumor therapy. Intranasal delivery provides a practical, noninvasive method for delivering therapeutic agents to the brain and could provide an alternative to intravenous injection and convection-enhanced delivery. We treated rats bearing intracerebral human tumor xenografts intranasally with GRN163, an oligonucleotide N3'-->P5'thio-phosphoramidate telomerase inhibitor. 3'-Fuorescein isothiocyanate (FITC)-labeled GRN163 was administered intranasally every 2 min as 6 microl drops into alternating sides of the nasal cavity over 22 min. FITC-labeled GRN163 was present in tumor cells at all time points studied, and accumulation of GRN163 peaked at 4 h after delivery. Moreover, GRN163 delivered intranasally, daily for 12 days, significantly prolonged the median survival from 35 days in the control group to 75.5 days in the GRN163-treated group. Thus, intranasal delivery of GRN163 readily bypassed the blood-brain barrier, exhibited favorable tumor uptake, and inhibited tumor growth, leading to a prolonged lifespan for treated rats compared to controls. This delivery approach appears to kill tumor cells selectively, and no toxic effects were noted in normal brain tissue. These data support further development of intranasal delivery of tumor-specific therapeutic agents for brain tumor patients.

Comparison between properties of 2'-O,4'-C-ethylene-bridged nucleic acid (ENA) phosphorothioate oligonucleotides and N3'-P5' thiophosphoramidate oligonucleotides

Synthesis and properties of an oligonucleotide uniformly modified with 2'-O,4-C-ethylene-bridged nucleic acid (ENA) units were compared with those of GRN163, which is modified with N3'-P5' thiophosphoramidates, with the sequence targeting human telomerase RNA subunit. Although an ENA phosphorothioate oligonucleotide, ENA-13, could be synthesized using ENA phosphoramidites on a 100-mg scale, synthesis of GRN163 was very hard even on a 1-micomol scale. In view of both stability of the duplex formation with complementary RNA and the efficiency of cellular uptake by endocytosis, ENA-13 was superior to GRN163. These findings suggest that ENA-13 has useful properties for antisense therapeutic application.

Antiadhesive effects of GRN163L--an oligonucleotide N3'->P5' thio-phosphoramidate targeting telomerase

We determined previously that a novel human telomerase RNA (hTR) antagonist, GRN163L, inhibited the tumorigenic potential of A549-luciferase (A549-luc) lung cancer cells in vitro and in vivo. Further studies revealed that A549-luc cells were also morphologically altered by GRN163L. A549-luc cells treated before cell attachment with a single dose of GRN163L only weakly attached to the substrate and remained rounded, whereas control mismatch-treated cells exhibited typical epitheloid appearance and adhesion properties. These morphologic changes were independent of hTR expression and telomerase inhibition and were unrelated to telomere length. This effect is dependent on the molecular properties of the lipid moiety, the phosphorothioate backbone, and the presence of triplet-G sequences within the GRN163L structure. Altered adhesion was manifested by a 50% reduction in rapid cellular attachment and a 3-fold decrease in total cell spreading surface area. Administration of a single dose of GRN163L (15 mg/kg) at the time of cell inoculation, using an in vivo model of lung cancer metastasis, resulted in significant reductions in tumor burden at days 13, 20, and 27 of tumor progression. Thus, the potent antimetastatic effects of GRN163L may be related, in part, to the antiadhesive effects of this novel cancer therapeutic conferred via specific structural determinants and that these effects are independent of telomerase inhibition or telomere shortening.

Telomerase antisense inhibition for the proliferation of endometrial cancer in vitro and in vivo

The objective of this study was to investigate the antitumor effect of antisense telomerase oligodeoxynucleotides to endometrial cancer cells in vitro and in vivo. Antisense oligodeoxynucleotides (ODNs) against the human telomerase transcripatse (hTERT) synthesized to serve as telomerase inhibitors. Reverse transcription–polymerase chain reaction and 3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide (MTT) assay were used to test the expression of hTERT messengerRNA (mRNA) and inhibition of cell proliferation in vitro. In vivo, antitumor effects of ODNs or combined with cisplatin were evaluated in endometrial cancer xenograft. Telomerase activity was tested by telomeric repeat amplification protocol. Antisense ODNs could inhibit proliferation of human endometrial cancer cells (HEC-1-A) in vitro, and downregulate the expression hTRET mRNA in a dose- and period-dependent manner. The tumor growth inhibitory rate of low- and high-dose ODNs were 34.20% and 89.21%, and combined group was 75.30%. Telomerase activity was downregulated to 87.32% compared to the control in the ODNs-treated xenograft tumors. Antisense oligonucleotides of hTERT effectively inhibit the growth of endometrial cancer cell line. Telomerase inhibitor might be a new strategy for chemotherapy or chemoprevention in endometrial cancer.

Chemotherapeutic approaches for targeting cell death pathways

For several decades, apoptosis has taken center stage as the principal mechanism of programmed cell death in mammalian tissues. It also has been increasingly noted that conventional chemotherapeutic agents not only elicit apoptosis but other forms of nonapoptotic death such as necrosis, autophagy, mitotic catastrophe, and senescence. This review presents background on the signaling pathways involved in the different cell death outcomes. A re-examination of what we know about chemotherapy-induced death is vitally important in light of new understanding of nonapoptotic cell death signaling pathways. If we can precisely activate or inhibit molecules that mediate the diversity of cell death outcomes, perhaps we can succeed in more effective and less toxic chemotherapeutic regimens.

Telomerase inhibition in cancer therapeutics: molecular-based approaches

Current standard cancer therapies (chemotherapy and radiation) often cause serious adverse off-target effects. Drug design strategies are therefore being developed that will more precisely target cancer cells for destruction while leaving surrounding normal cells relatively unaffected. Telomerase, widely expressed in most human cancers but almost undetectable in normal somatic cells, provides an exciting drug target. This review focuses on recent pharmacogenomic approaches to telomerase inhibition. Antisense oligonucleotides, RNA interference, ribozymes, mutant expression, and the exploitation of differential telomerase expression as a strategy for targeted oncolysis are discussed here in the context of cancer therapeutics. Reports of synergism between telomerase inhibitors and traditional cancer therapeutic agents are also analyzed.

Effects of a novel telomerase inhibitor, GRN163L, in human breast cancer

Telomerase activity is undetectable in most normal tissues but the vast majorities of cancers express active telomerase. Therefore, telomerase serves as an attractive target for the treatment of cancers. GRN163L is a lipid-modified oligonucleotide N3'-->P5' thio-phosphoramidate complementary to the RNA template region of human telomerase. The anti-telomerase activity of GRN163L was evaluated using MDA-MB-231 and MDA-MB-435 human breast adenocarcinoma cell lines. Twice weekly administration of GRN163L resulted in the inhibition of telomerase activity and progressive telomere shortening. Cells treated with GRN163L did not demonstrate decreased cell proliferation for up to 2 weeks. However, after additional treatment, cell proliferation gradually decreased in GRN163L-treated cells compared to untreated or mismatch control oligoncleotide treated cells. Furthermore, anti-tumorigenic effects were seen in cells treated with GRN163L, as cells lose their ability to form colonies in soft agar and were unable to form colonies in the clonal efficiency assay upon incubation with GRN163L. Moreover, breast cancer cells that were treated with GRN163L for only 1 week prior to plating in invasion chambers, and when bulk telomere are still long, exhibit significantly diminished invasive potential. These results reveal critical information regarding the effectiveness of GRN163L as a potential therapeutic agent for the treatment of human breast cancer.

In vivo inhibition of lung cancer by GRN163L: a novel human telomerase inhibitor

Differential regulation of telomerase activity in normal and tumor cells provides a rationale for the design of new classes of telomerase inhibitors. The telomerase enzyme complex presents multiple potential sites for the development of inhibitors. GRN163L, a telomerase enzyme antagonist, is a lipid-modified 13-mer oligonucleotide N3' --> P5'-thio-phosphoramidate, complementary to the template region of telomerase RNA (hTR). We evaluated both the in vitro and in vivo effects of GRN163L using A549-luciferase (A549-Luc) human lung cancer cells expressing a luciferase reporter. GRN163L (1 micromol/L) effectively inhibits telomerase activity of A549-Luc cells, resulting in progressive telomere shortening. GRN163L treatment also reduces colony formation in soft agar assays. Surprisingly, after only 1 week of treatment with GRN163L, A549-Luc cells were unable to form robust colonies in the clonal efficiency assay, whereas the mismatch control compound had no effect. Finally, we show that in vivo treatment with GRN163L is effective in preventing lung metastases in xenograft animal models. These in vitro and in vivo data support the development of GRN163L as a therapeutic for the treatment of cancer.

Telomerase reverse transcriptase (hTERT) mRNA and telomerase RNA (hTR) as targets for downregulation of telomerase activity

Telomerase is expressed in cancer cells but not in most normal cells, leading to the hypothesis that telomerase inhibitors may be a powerful approach to cancer therapy. It is possible that telomerase plays roles in the cell other than telomere elongation and that blocking telomerase expression may have consequences that differ from simply blocking the active site through competitive inhibition. Here, we test this hypothesis by comparing the effects of antisense oligonucleotides and small interfering RNAs (siRNAs) that target the telomerase reverse transcriptase (hTERT) mRNA with the effects of oligonucleotides that target the telomerase RNA component (hTR). We find that the use of anti-hTR oligomers is more effective in blocking telomerase expression than strategies that target hTERT mRNA. Anti-hTR compounds are active on addition to cells in the absence of lipid, whereas antisense oligonucleotides are not. The modest inhibition of hTERT expression caused by antisense oligonucleotides or siRNAs does not persist, suggesting development of resistance. These data suggest that strategies for telomerase inhibition that require downregulation of hTERT mRNA may be less straightforward than those that target hTR. In addition, we have not seen evidence for a role for hTERT other than in telomere maintenance.

Adding to the ends: what makes telomerase processive and how important is it?

Telomerase is a cellular reverse transcriptase responsible for telomere maintenance in most organisms. It does so by adding telomere repeats onto pre-existing ends using an integral RNA component as template. Compared to "prototypical" reverse transcriptases, telomerase is unique in being able to repetitively copy a short templating RNA segment, thus adding multiple copies of the repeat to the DNA substrate following a single binding event. This uniquely processive property hints at the intricate conformational alterations that the enzyme must choreograph during its reaction cycles. Recent studies have identified distinct structural elements within both the RNA and protein components of telomerase that modulate enzyme processivity. Pharmacological and genetic analysis suggest that telomerase processivity is a significant determinant of telomere length. Because telomere maintenance and the lack thereof have been linked to tumor progression and aging, further investigation of telomerase processivity may lead to novel medical intervention strategies.

Antitumor effects of specific telomerase inhibitor GRN163 in human glioblastoma xenografts

Telomerase is a ribonucleoprotein complex that elongates telomeric DNA and appears to play an important role in cellular immortalization of cancers. Because telomerase is expressed in the vast majority of malignant gliomas but not in normal brain tissues, it is a logical target for gliomaspecific therapy. The telomerase inhibitor GRN163, a 13-mer oligonucleotide N3'-->P5' thio-phosphoramidate (Geron Corporation, Menlo Park, Calif.), is complementary to the template region of the human telomerase RNA subunit hTR. When athymic mice bearing U-251 MG human brain tumor xenografts in their flanks were treated intratumorally with GRN163, a significant growth delay in tumor size was observed (P < 0.01 in all groups) as compared to the tumor size in mice receiving a mismatched oligonucleotide or the carrier alone. We also investigated biodistribution of the drug in vivo in an intracerebral rat brain-tumor model. Fluorescein-labeled GRN163 was loaded into an osmotic minipump and infused directly into U-251 MG brain tumors over 7 days. Examination of the brains revealed that GRN163 was present in tumor cells at all time points studied. When GRN163 was infused into intracerebral U-251 MG tumors shortly after their implantation, it prevented their establishment and growth. Lastly, when rats with larger intracerebral tumors were treated with the inhibitor, GRN163 increased animal survival times. Our results demonstrate that the antitelomerase agent GRN163 inhibits growth of glioblastoma in vivo, exhibits favorable intracerebral tumor uptake properties, and prevents the growth of intracerebral tumors. These findings support further development of this compound as a potential anticancer agent.

Telomerase inhibition with an oligonucleotide telomerase template antagonist: in vitro and in vivo studies in multiple myeloma and lymphoma

The effects of telomerase inhibition with an oligonucleotide N3' --> 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.

Oligonucleotide N3' --> P5' thio-phosphoramidate telomerase template antagonists as potential anticancer agents

Human telomerase is a reverse transcriptase that is expressed in essentially all cancer cells, but not in the vast majority of normal somatic cells. Therefore, the specific inhibition of telomerase activity in tumors might have significant beneficial therapeutic effects. We have designed and evaluated oligonucleotide N3' --> P5' thio-phosphoramidates as telomerase template antagonists. In biochemical cell-free assays 11-13-mer thio-phosphoramidate oligonucleotides demonstrated sequence specific and dose dependent inhibition of telomerase with pico-molar IC50 values. Optimization of the oligonucleotide sequence and length resulted in the identification of a 13-mer-oligonucleotide thio-phosphoramidate GRN163 as a drug development candidate. In cell cultures GRN163 was able to inhibit telomerase activity in the absence of cationic lipid with approximately 1 microM IC50 values. Telomerase inhibition by GRN163 produced gradual telomere shortening, followed by cellular senescence and/or apoptosis of cancer derived cell lines.

Telomere inhibition and telomere disruption as processes for drug targeting

The components and cofactors of the holoenzyme telomerase and its substrate telomeric DNA are attractive targets for anticancer agents that act by inhibiting the activity of telomerase. This review outlines recent advances in telomerase inhibition that have been achieved using antisense oligonucleotides and ribozymes that target the telomerase mRNA or its hTR RNA template. Although these are potent catalytic inhibitors of telomerase, they are challenging to implement in the clinic due to their delayed effectiveness. Drugs that directly bind to the telomeres, the complex structures that are associated at the telomeric ends, and stabilize secondary DNA structures such as G-quadruplexes are also potent inhibitors of telomerase. Special focus is given here to the telomeres, the biological machinery that works in tandem with telomerase to elongate telomeres, the causes of telomere disruption or dysfunction, and the consequences of disruption/dysfunction on the activity and design of anticancer agents.

The evolving role of telomerase inhibitors in the treatment of cancer

Telomerase is a ribonucleoprotein that maintains telomeres and is essential for cellular immortality and tumour growth. The differential expression of telomerase in cancer cells makes it an attractive therapeutic target. Anti-sense oligonucleotides directed against the RNA template of hTR and small molecules that can interact and stabilise the G-quadruplex represent promising therapeutic strategies. Human trials investigating the potential role of the catalytic subunit hTERT as a universal cancer vaccine have already commenced. Alternative lengthening of telomeres (ALT) and efficacy delay remain important limitations to anti-telomerase therapy.

Telomerase: a target for cancer therapeutics

The continuous growth of advanced malignancies almost universally correlates with the reactivation of telomerase. While there is still a great deal of basic and applied research to be done, telomerase remains a very attractive novel target for cancer therapeutics. In this review, we will discuss the challenges and the pros and cons of the most promising antitelomerase approaches currently being investigated.

Natural and pharmacological regulation of telomerase

The extremities of eukaryotic chromosomes are called telomeres. They have a structure unlike the bulk of the chromosome, which allows the cell DNA repair machinery to distinguish them from 'broken' DNA ends. But these specialised structures present a problem when it comes to replicating the DNA. Indeed, telomeric DNA progressively erodes with each round of cell division in cells that do not express telomerase, a specialised reverse transcriptase necessary to fully duplicate the telomeric DNA. Telomerase is expressed in tumour cells but not in most somatic cells and thus telomeres and telomerase may be proposed as attractive targets for the discovery of new anticancer agents.

Oligonucleotide N3'-->P5' phosphoramidates as efficient telomerase inhibitors

Human telomerase is a unique reverse transcriptase that is expressed in multiple cancers, but not in the vast majority of normal cells. The enzyme is responsible for telomere protection and maintenance, and supports the proliferative immortality of cancer cells. Thus, it has been proposed that the specific inhibition of telomerase activity in tumors might have significant and beneficial therapeutic effects. To this goal we have designed, synthesized, and evaluated several oligonucleotide N3'-->P5' phosphoramidates as telomerase inhibitors. These oligonucleotides are complementary to the template region of the RNA domain of telomerase (hTR). The prepared compounds were evaluated in HME50-5E breast epithelial cells, where their effects on telomerase activity were determined using a cell-based telomerase (TRAP) assay at 24 as well as 72 h after exposure to compounds. The oligo-N3'-->P5' phosphoramidate inhibited telomerase activity in cells in the presence of the cellular up-take enhancer (FuGENE6) in a dose- and sequence-dependent manner, with IC(50) values of approximately 1 nM. Inhibition of telomerase activity by this compound without the lipid carrier was not efficient. However, the isosequential oligonucleotide N3'-->P5' thio-phosphoramidate was able to inhibit telomerase activity with or without lipid carriers at nM, or low-microM concentrations, respectively. This inhibition of telomerase activity in HME50-5E cells by the oligonucleotide thio-phosphoramidates was also sequence specific. Long-term treatment of the cells with 0.5 microM of FuGENE6 formulated 13-mer thio-phosphoramidates, fully complementary to hTR, resulted in gradual telomere shortening, followed by cellular senescence and apoptosis, as would be predicted for a telomerase inhibitor. The mismatched control compound had no effect on cell proliferation. The results suggest that the oligonucleotide N3'-->P5' phosphoramidates, and particularly thio-phosphoramidates, might be further developed as selective anti-telomerase reagents.

Allosteric inhibitors of telomerase: oligonucleotide N3'-->P5' phosphoramidates

Telomerase is a ribonucleoprotein responsible for maintaining telomeres in nearly all eukaryotic cells. The enzyme is able to utilize a short segment of its RNA subunit as the template for the reverse transcription of d(TTAGGG) repeats onto the ends of human chromosomes. Transfection with telomerase was shown to confer immortality on several types of human cells. Moreover, telomerase activation appears to be one of the key events required for malignant transformation of normal cells. Inhibition of telomerase activity in transformed cells results in the cessation of cell proliferation in cultures and provides the rationale for the selection of telomerase as a target for anticancer therapy. Using oligonucleotide N3'-->P5' phosphoramidates (NPs) we have identified a region of the human telomerase RNA subunit (hTR) approximately 100 nt downstream from the template region whose structural integrity appears crucial for telomerase enzymatic activity. The oligonucleotides targeted to this segment of hTR are potent and specific inhibitors of telomerase activity in biochemical assays. Mutant telomerase, in which 3 nt of hTR were not complementary to a 15 nt NP, was found to be refractory to inhibition by that oligonucleotide. We also demonstrated that the binding of NP, oligonucleotides to this hTR allosteric site results in a marked decrease in the affinity of a telomerase substrate (single-stranded DNA primer) for the enzyme.